Switching DC-to-DC converter utilizing a soft switching technique

• US 6,304,460 B1
• Filed: 05/05/2000
• Issued: 10/16/2001
• Est. Priority Date: 05/05/2000
• Status: Expired due to Fees

• Current Assignee: Slobodan Cuk
• Original Assignee: Slobodan Cuk
• Inventors: Cuk, Slobodan
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): Vu, Bao Q.
• Application Number:
  US09/566,045
• Time in Patent Office: 529 Days
• Field of Search: 363/16, 363/40, 363/55, 363/95, 363/97, 363/131, 363/41
• US Class Current: 363/16
• CPC Class Codes: H02M 1/0064 Magnetic structures combining different functions, e.g. storage, filtering or transformation
• CPC Class Codes: H02M 1/34 Snubber circuits
• CPC Class Codes: H02M 1/342 Active non-dissipative snubbers
• CPC Class Codes: H02M 3/158 including plural semiconductor devices as final control devices for a single load
• CPC Class Codes: H02M 3/33576 having at least one active switching element at the secondary side of an isolation transformer
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

Soft switching DC-to-DC converter achieves simultaneously ultra high efficiency and very small size owing to a new magnetic circuit structure and a corresponding novel converter circuit configuration with special properties. Unique magnetic design also provides an overload current capability of several times the nominal load current. Despite its simple implementation requiring only proper drive timing of the switching devices, the unique soft-switching methods result in nearly complete elimination of switching losses over the entire operating range. This, in turn, permits operation at even higher switching frequencies, and leads to further reduction in size and weight.

Claims

1. A switching DC-to-DC converter for providing power from a DC voltage source connected between an input terminal and a common input terminal to a DC load connected between an output terminal and a common output terminal, said converter comprising:

an input inductor winding, a middle inductor winding, and an output inductor winding, placed on a common magnetic core to form an effective DC Transformer, and each winding having one dot-marked end and another unmarked end whereby any AC voltage applied to said middle inductor winding induces AC voltages in said input and output inductor windings so that all three AC voltages are in phase at dot-marked ends of said input, output and middle inductor windings;

said input inductor winding connected at a dot-marked end thereof to said input terminal to form an input winding of a DC Transformer;

said output inductor winding connected at a dot-marked end thereof to said output terminal to form an output winding of said DC Transformer;

said middle inductor winding connected at a dot-marked end thereof to said common input terminal and said common output terminal to enable said DC Transformer operation;

an input capacitor, having one end connected to an unmarked end of said input inductor winding and another end of said input capacitor connected to an unmarked end of said middle inductor winding;

an input switch with one end connected to said common input terminal and another end connected to said unmarked end of said input inductor;

an output switch with one end connected to said common output terminal and another end connected to an unmarked end of said output inductor winding;

a complementary output switch, having one end connected to said unmarked end of said output inductor winding and another end of said complementary output switch connected to said unmarked end of said middle inductor winding;

a branch comprised of a complementary input switch and an auxiliary capacitor connected in series;

switching means for keeping both said input switch and said output switch closed for a duration of time interval DT_S and, keeping both said complementary input switch and said complementary output switch closed for a duration of complementary time interval D′

T_S=(1−

D)T_S, where D is a duty ratio and D′

is a complementary duty ratio within one complete and controlled switch operating cycle T_S;

means for connecting the ends of said branch to said converter whereby during said complementary time interval, current through said branch is equal to the sum of input inductor current flowing into said dot-marked end of said input inductor winding and middle inductor current flowing into said dot-marked end of said middle inductor winding reduced by output inductor current flowing out of said dot-marked end of said output inductor winding;

wherein said input switch, said complementary input switch, and said complementary output switch are semiconductor current bidirectional switching devices, capable of conducting the current in both directions while in an ON state, and sustaining voltage in one direction while in an OFF state;

wherein said output switch is a semiconductor voltage bidirectional switching device, capable of conducting the current while in an ON state, and sustaining voltage in both directions, while in an OFF state;

wherein said switching devices turn ON and OFF at high switching frequency;

wherein a DC-to-DC voltage conversion ratio of said converter depends linearly on said operating duty ratio D;

wherein at any duty ratio D, both said input inductor DC current and said middle inductor DC current flow into said dot-marked ends of their respective windings, whereas said output inductor DC current flows out of said dot-marked end of said output inductor winding;

wherein at any duty ratio D, the sum of said DC currents of said input inductor and said middle inductor is equal to the magnitude of said DC current of said output inductor;

wherein said DC Transformer includes an equal number of turns for said input, output, and middle inductors and, at any operating duty ratio D, DC ampere-turns of said input inductor current and said middle inductor current are positive and generate positive DC fluxes which add together, while DC ampere-turns of said output inductor current are negative and generate negative DC flux to result in net zero DC flux in said common magnetic core, and

wherein said common magnetic core has no air-gap;

whereby said net zero DC flux in said common magnetic core enables full utilization of the magnetic core material to generate maximum inductances of said input, middle, and output inductors and said converter has current overload capability several times higher than nominal load current, and

whereby said DC Transformer combines said input inductor winding, said output inductor winding, and said middle inductor winding to obtain unique and effective DC-to-DC power transfer from said input inductor to said output inductor with substantially reduced energy storage within said common magnetic core of said DC Transformer, thereby reducing size and weight of said converter by reducing said common magnetic core size and weight while simultaneously increasing efficiency and overload capability of said converter, and providing reduction of electromagnetic interference.

2. A switching converter as defined in claim 1,

wherein said switching means includes electronically controlling operation of said semiconductor switches whereby two transitions D to D′ and D′

to D are obtained during each successive switch operating cycle T_S, in which said output switch and said complementary input switch are both OFF during said transition intervals wherein said transition intervals are short compared to said switch operating cycle;

wherein switch timing by said switching means of respective switches is as follows;

said first transition D to D′

is initiated by turning said output switch OFF and, when voltage on said complementary output switch is reduced to zero, said complementary output switch is by said switching means turned ON at zero voltage for zero switching losses while said input switch is simultaneously turned OFF, and said first transition continues until the voltage on said complementary input switch reduces to zero, at which instant said complementary input switch is also turned ON by said switching means at zero voltage for zero switching losses, and

said second transition D′

to D is initiated by turning said complementary input switch OFF and when voltage on said input switch is reduced to zero, said input switch is by said switching means turned ON at zero voltage for zero switching losses while said complementary output switch is simultaneously turned OFF, and then with significantly reduced voltage on the said output switch, the said output switch is turned ON for much reduced switching losses for accomplishing a full soft switching cycle, and

whereby this soft switching cycle is based on the fundamental property of said converter configuration in which said auxiliary capacitor during said complementary time interval D′

T_s carries only AC current necessary and sufficient to complete both soft-switching transitions with zero switching losses on said input switch, complementary input switch, and complementary output switch and much reduced switching losses on said output switch, for any operating duty ratio D, and

whereby both soft switching transitions are independent of the DC load current and dependent only on said auxiliary capacitor AC current, thus resulting in equal transition intervals and symmetrical voltage waveforms across said output switch hence termed “

symmetrical”

soft switching, and

whereby voltage stress on said semiconductor devices is significantly reduced and efficiency of said converter is significantly increased while electromagnetic interference is reduced, and

whereby switching frequency can be significantly increased for further reduction of the converter'"'"'s size and weight without negative impact on converter'"'"'s overall efficiency.

3. A soft-switching converter as defined in claim 2,

wherein said DC Transformer has a single-loop, UU-like magnetic core;

wherein said input inductor and said middle inductor are placed side-by-side on one leg of said single-loop, UU-like magnetic core;

wherein said output inductor is placed on the opposite leg of said single-loop, UU-like magnetic core;

whereby leakage inductance between said input inductor winding and said middle inductor winding caused by deliberate side-by-side positioning of said windings along one leg of said single-loop, UU-like magnetic core provides substantially reduced and near zero-ripple current in said input inductor, and

whereby leakage inductance between said output inductor winding and said middle inductor winding caused by deliberate positioning of said windings on opposite legs of said single-loop, UU-like magnetic core provides substantially reduced ripple current in said output inductor.

4. A soft-switching converter as defined in claim 3,

wherein a small air-gap is positioned on said one leg of said single-loop, UU-like magnetic core with said output inductor winding to provide an effective AC voltage divider between leakage and magnetizing inductances of said output inductor, and

wherein said effective AC voltage divider enables better matching of AC voltages on said input inductor, said middle inductor, and said output inductor,

whereby, for one particular duty ratio D, said output inductor DC current has a negligible current ripple.

5. A soft-switching converter as defined in claim 4,

including a separate external inductor connected in series with said output inductor to reduce ripple in current from said output inductor and said output inductor number of turns is adjusted for better matching of AC voltages of said output inductor, said input inductor, and said middle inductor,

whereby said external inductor has inductance an order of magnitude smaller than inductance of said output inductor and has to support an order of magnitude lower AC voltage than said output inductor, thereby resulting in an order of magnitude smaller core size and weight, and substantially lower power losses than said DC Transformer.

6. A soft-switching converter as defined in claim 4,

including a separate leakage magnetic leg with no windings and a large air-gap in a magnetic flux path with said single-loop magnetic core, wherein said large air-gap is adjusted to provide said output inductor current with a negligible ripple at a particular duty ratio D,

whereby said leakage magnetic leg substantially increases the leakage inductance between said output inductor winding and said input and middle inductor windings, and provides reduction of ripple in said output inductor current, and

by adjusting said output inductor number of turns for better matching of AC voltages on said input, middle and output inductor windings, said output inductor ripple current is reduced.

7. A soft-switching converter as defined in claim 2,

wherein said DC Transformer has a multiple-loop, EE-like magnetic core;

wherein said input inductor and said middle inductor are placed side-by-side on the center leg of said multiple-loop, EE-like magnetic core;

wherein said output inductor winding is split into two windings connected in series so that their respective AC voltages are in phase and add, and each winding of said split output inductor has the same number of turns as said output inductor;

wherein each winding of said split output inductor winding is placed on a separate outer magnetic leg of said multiple-loop, EE-like magnetic core, and

whereby leakage inductance between said input inductor winding and said middle inductor winding caused by deliberate side-by-side positioning of said windings along said center leg of said multiple-loop, EE-like magnetic core provides substantially reduced and near zero-ripple current in said input inductor, and

whereby leakage inductances between said split output inductor windings and said middle inductor winding caused by deliberate positioning of said split output inductor windings on separate outer legs of said multiple-loop, EE-like magnetic core provide substantially reduced ripple current in said output inductor.

8. A soft-switching converter as defined in claim 7,

wherein small air-gaps are positioned on each of said outer legs of said multiple-loop, EE-like magnetic core with said split output inductor windings to provide the effective AC voltage dividers between leakage and magnetizing inductances of respective said split output inductor windings;

wherein said AC voltage dividers enable better matching of AC voltages on said input, said middle, and said output inductor, and

whereby, for one particular duty ratio D, said output inductor DC current has a negligible current ripple.

9. A switching converter as defined in claim 1,

wherein said switching means includes electronically controlling operation of said semiconductor switches whereby two transitions D to D′

and D′

to D are obtained during each successive switch operating cycle T_S, wherein said transition intervals are short compared to said switch operating cycle;

wherein switch timing by said switching means of respective switches is as follows;

said first transition D to D′

is initiated by turning said input switch OFF and, when voltage on said complementary output switch is reduced to zero, said complementary output switch is by said switching means turned ON at zero voltage for zero switching losses while said output switch is simultaneously turned OFF and said first transition continues until the voltage on said complementary input switch reduces to zero, at which instant said complementary input switch is also turned ON by said switching means at zero voltage for zero switching losses, and

said second transition D′

to D is initiated by turning said complementary input switch OFF, and when voltage on said input switch is reduced to zero, said input switch is by said switching means turned ON at zero voltage for zero switching losses while said complementary output switch is simultaneously turned OFF, and then with significantly reduced voltage on the said output switch, the said output switch is turned ON for much reduced switching losses for accomplishing a full soft switching cycle, and

whereby this soft switching cycle is based on the fundamental property of said converter configuration in which said auxiliary capacitor during said complementary time interval D′

T_S carries only AC current necessary and sufficient to complete both soft-switching transitions with zero switching losses on said input switch, complementary input switch, and complementary output switch and much reduced switching losses on said output switch, for any operating duty ratio D, and

whereby D to D′

transition is dependent on both the DC load current and said auxiliary capacitor AC current, while the D′

to D transition is dependent only on said auxiliary capacitor AC current, thus resulting in unequal transition intervals with asymmetrical voltage waveform across said output switch and hence termed “

asymmetrical”

soft switching, and

whereby voltage stress on said semiconductor devices is significantly reduced and efficiency of said converter is significantly increased while electromagnetic interference is reduced, and

whereby switching frequency can be significantly increased for further reduction of the converter'"'"'s size and weight without negative impact on converter'"'"'s overall efficiency.

10. A soft-switching converter as defined in claim 9,

wherein said DC Transformer has a single-loop, UU-like magnetic core;

wherein said input inductor and said middle inductor are placed side-by-side on one leg of said single-loop, UU-like magnetic core;

wherein said output inductor is placed on the opposite leg of said single-loop, UU-like magnetic core;

whereby leakage inductance between said input inductor winding and said middle inductor winding caused by deliberate side-by-side positioning of said windings along one leg of said single-loop, UU-like magnetic core provides substantially reduced and near zero-ripple current in said input inductor, and

whereby leakage inductance between said output inductor winding and said middle inductor winding caused by deliberate positioning of said windings on opposite legs of said single-loop, UU-like magnetic core provides substantially reduced ripple current in said output inductor.

11. A soft-switching converter as defined in claim 10,

wherein a small air-gap is positioned on said one leg of said single-loop, UU-like magnetic core with said output inductor winding to provide an effective AC voltage divider between leakage and magnetizing inductances of said output inductor, and

wherein said effective AC voltage divider enables better matching of AC voltages on said input inductor, said middle inductor, and said output inductor,

whereby, for one particular duty ratio D, said output inductor DC current has a negligible current ripple.

12. A soft-switching converter as defined in claim 11,

including a separate external inductor connected in series with said output inductor to reduce ripple in current from said output inductor and said output inductor number of turns is adjusted for better matching of AC voltages of said output inductor, said input inductor, and said middle inductor,

whereby said external inductor has inductance an order of magnitude smaller than inductance of said output inductor and has to support an order of magnitude lower AC voltage than said output inductor, thereby resulting in an order of magnitude smaller core size and weight, and substantially lower power losses than said DC Transformer.

13. A soft-switching converter as defined in claim 11,

including a separate leakage magnetic leg with no windings and a large air-gap in a magnetic flux path with said single-loop magnetic core, wherein said large air-gap is adjusted to provide said output inductor current with a negligible ripple at a particular duty ratio D,

whereby said leakage magnetic leg substantially increases the leakage inductance between said output inductor winding and said input and middle inductor windings, and provides reduction of ripple in said output inductor current, and

by adjusting said output inductor number of turns for better matching of AC voltages on said input, middle and output inductor windings, said output inductor ripple current is reduced.

14. A soft-switching converter as defined in claim 9,

wherein said DC Transformer has a multiple-loop, EE-like magnetic core;

wherein said input inductor and said middle inductor are placed side-by-side on the center leg of said multiple-loop, EE-like magnetic core;

wherein said output inductor winding is split into two windings connected in series so that their respective AC voltages are in phase and add, and each winding of said split output inductor has the same number of turns as said output inductor;

wherein each winding of said split output inductor winding is placed on a separate outer magnetic leg of said multiple-loop, EE-like magnetic core, and

whereby leakage inductance between said input inductor winding and said middle inductor winding caused by deliberate side-by-side positioning of said windings along said center leg of said multiple-loop, EE-like magnetic core provides substantially reduced and near zero-ripple current in said input inductor, and

whereby leakage inductances between said split output inductor windings and said middle inductor winding caused by deliberate positioning of said split output inductor windings on separate outer legs of said multiple-loop, EE-like magnetic core provide substantially reduced ripple current in said output inductor.

15. A soft-switching converter as defined in claim 14,

wherein small air-gaps are positioned on each of said outer legs of said multiple-loop, EE-like magnetic core with said split output inductor windings to provide the effective AC voltage dividers between leakage and magnetizing inductances of respective said split output inductor windings;

wherein said AC voltage dividers enable better matching of AC voltages on said input, said middle, and said output inductor, and

whereby, for one particular duty ratio D, said output inductor DC current has a negligible current ripple.

16. A switching DC-to-DC converter as defined in claim 1,

wherein said middle inductor comprises an autotransformer;

wherein a dot-marked end of a winding of said autotransformer is connected to said common input terminal and said common output terminal, an unmarked end of said winding of said autotransformer is connected to said another end of said complementary output switch, and a tapped end of said winding of said autotransformer is connected to said another end of said input capacitor;

wherein said winding of said autotransformer is placed on said common magnetic core with said input inductor winding and said output inductor winding to form a DC Transformer;

wherein ratio of number of turns of said winding of said autotransformer to number of turns between dot-marked end and tapped end of said winding of said autotransformer provides additional scaling of DC-to-DC voltage conversion ratio of said converter;

wherein said input inductor winding has the same number of turns as said number of turns between dot-marked end and tapped end of said winding of said autotransformer, and said output inductor winding has the same number of turns as said winding of said autotransformer, whereby at any operating duty ratio D, zero DC flux is obtained in said common magnetic core;

wherein said common magnetic core has no air-gap, whereby said zero DC flux in said common magnetic core enables full utilization of the magnetic core material to generate maximum inductances of said input and output inductors and said autotransformer thereby providing said converter with current overload capability several times higher than nominal load current, and

whereby said DC Transformer combines said input inductor winding, said output inductor winding, and said winding of said autotransformer to provide increased conversion ratio between said input DC voltage source and said DC load and unique and effective DC-to-DC power transfer from said input inductor to said output inductor with substantially reduced energy storage within said common magnetic core of said DC Transformer, thereby reducing size and weight of said converter by reducing said common magnetic core size and weight while simultaneously increasing efficiency and converter overload capability, and providing reduction of electromagnetic interference.

17. A switching converter as defined in claim 1,

wherein one end of said input capacitor is connected to said common input terminal and another end of said input capacitor is connected to said common output terminal;

wherein said dot-marked end of said middle inductor winding is connected to said common output terminal, and

wherein said unmarked end of said middle inductor winding is connected to said unmarked end of said input inductor.

18. A switching converter as defined in claim 1,

wherein said output switch is comprised of a series connection of a MOSFET-like device and a Current Rectifier (diode);

whereby said output switch is a voltage bidirectional switch which operates as a two-quadrant switch and has reduced conduction losses due to the low conduction losses of said MOSFET-like device.

19. A switching converter as defined in claim 1,

wherein said output switch is comprised of two N-channel MOSFET-like devices having a source terminal of one MOSFET-like device connected to a source terminal of another MOSFET-like device (back-to-back connection) and gate terminals of said MOSFET-like devices connected together;

wherein a drain terminal of one MOSFET-like device is connected to said common output terminal and a drain terminal of another MOSFET-like device is connected to said unmarked end of said output inductor winding;

whereby said output switch is both voltage bidirectional and current bidirectional and operates as a four-quadrant switch with significantly reduced current conduction losses compared to two-quadrant voltage bidirectional switch implementation due to the low conduction losses of said two MOSFET-like devices.

20. An isolated switching DC-to-DC converter for providing power from a DC voltage source connected between an input terminal and a common input terminal to a DC load connected between an output terminal and a common output terminal, said converter comprising:

an input inductor winding, an isolation transformer with primary and secondary windings, and an output inductor winding, placed on a common magnetic core to form an effective Isolated DC Transformer, and each winding having one dot-marked end and another unmarked end whereby any AC voltage applied to said primary winding of said isolation transformer induces AC voltages in said secondary winding of said isolation transformer and said input and output inductor windings so that all four AC voltages are in phase at dot-marked ends of said input inductor winding, said output inductor winding and said primary and secondary windings of said isolation transformer;

said input inductor winding connected at a dot-marked end thereof to said input terminal to form an input winding of said Isolated DC Transformer;

said output inductor winding connected at a dot-marked end thereof to said output terminal to form an output winding of said Isolated DC Transformer;

said primary winding of said isolation transformer connected at a dot-marked end thereof to said common input terminal to enable said Isolated DC Transformer operation;

said secondary winding of said isolation transformer connected at a dot-marked end thereof to said common output terminal to enable said Isolated DC Transformer operation;

an input capacitor connected between an unmarked end of said input inductor winding and an unmarked end of said primary winding of said isolation transformer;

an input switch with one end connected to said common input terminal and another end connected to said unmarked end of said input inductor;

an output switch with one end connected to said common output terminal and another end connected to an unmarked end of said output inductor winding;

a complementary output switch, having one end connected to said unmarked end of said output inductor winding and another end of said complementary output switch connected to an unmarked end of said secondary winding of said isolation transformer;

a branch comprised of a complementary input switch and an auxiliary capacitor connected in series, wherein one end of said branch is connected to dot-marked end of said primary winding of said isolation transformer and another end of said branch is connected to said unmarked end of said input inductor;

switching means for keeping both said input switch and said output switch closed for a duration of time interval DT_S and, keeping both said complementary input switch and said complementary output switch closed for a duration of complementary time interval D′

T_S=(1−

D)T_S, where D is a duty ratio and D′

is a complementary duty ratio within one complete and controlled switch operating cycle T_S;

wherein said input switch, said complementary input switch, and said complementary output switch are semiconductor current bidirectional switching devices, capable of conducting the current in both directions while in an ON state, and sustaining voltage in one direction while in an OFF state;

wherein said output switch is a semiconductor voltage bidirectional switching device, capable of conducting the current while in an ON state, and sustaining voltage in both directions, while in an OFF state;

wherein said switching devices turn ON and OFF at high switching frequency;

wherein said primary winding and said secondary winding are tightly coupled for reduced leakage between said primary winding and said secondary winding;

wherein a DC-to-DC voltage conversion ratio of said converter depends linearly on said operating duty ratio D;

wherein turns ratio of said secondary winding to said primary winding of said isolation transformer provides additional scaling of DC-to-DC voltage conversion ratio of said converter;

wherein at any duty ratio D, said input inductor DC current and said primary and secondary windings DC currents flow into said dot-marked ends of their respective windings, whereas said output inductor DC current flows out of said dot-marked end of said output inductor winding;

wherein said input inductor winding has the same number of turns as said primary winding of said isolation transformer, and said output inductor winding has the same number of turns as said secondary winding of said isolation transformer, whereby at any operating duty ratio D, net zero DC flux is obtained in said common magnetic core;

wherein said common magnetic core has no air-gap;

whereby said net zero DC flux in said common magnetic core enables full utilization of the magnetic core material to generate maximum inductances of said input and output inductors and said isolation transformer thereby providing said converter with current overload capability several times higher than nominal load current, and

whereby said Isolated DC Transformer combines said input inductor winding, said output inductor winding, and said primary and secondary windings of said isolation transformer to provide a galvanic isolation between said input DC voltage source and said DC load and unique and effective DC-to-DC power transfer from said input inductor to said output inductor with substantially reduced energy storage within said common magnetic core of said Isolated DC Transformer, thereby reducing size and weight of said converter by reducing said common magnetic core size and weight while simultaneously increasing efficiency and converter overload capability, and providing reduction of electromagnetic interference.

21. An isolated switching converter as defined in claim 20, further including a leakage inductance of said isolation transformer effectively connected in series with said input capacitor,

wherein said switching means includes electronically controlling operation of said semiconductor switches whereby two transitions D to D′ and D′

to D are obtained during each successive switch operating cycle T_S, wherein said transition intervals are short compared to said switch operating cycle;

wherein switch timing by said switching means of respective switches is as follows;

said first transition D to D′

is initiated by turning said input switch OFF and, when voltage on said complementary output switch is reduced to zero, said complementary output switch is by said switching means turned ON at zero voltage for zero switching losses, and said first transition continues while the voltage on said complementary input switch reduces to zero, at which instant said complementary input switch is also turned ON by said switching means at zero voltage for zero switching losses, and said first transition continues until the current of said output switch is reduced to zero at which instant said output switch is turned OFF, and

said second transition D′

to D is initiated by turning said complementary input switch OFF and, when voltage on said input switch is reduced to zero, said input switch is by said switching means turned ON at zero voltage for zero switching losses, while said output switch is simultaneously turned ON with much reduced switching losses accomplishing a soft switching cycle, and said second transition continues until the current of said complementary output switch is reduced to zero at which instant said complementary output switch is turned OFF, and

whereby both soft switching transitions result in zero switching losses on said input switch, said complementary input switch, and said complementary output switch and much reduced switching losses on said output switch, for any operating duty ratio D, and

whereby D to D′

transition is dependent only on DC load current while D′

to D transition is dependent only on said auxiliary capacitor AC current, thus resulting in the D to D′

transition much shorter than D′

to D transition, and

whereby said leakage inductance substantially reduces duration of said D to D′

transition and does not effect said D′

to D transition, and

whereby voltage stress on said semiconductor devices is significantly reduced and efficiency of said converter is significantly increased while electromagnetic interference is reduced, and

whereby switching frequency can be significantly increased for further reduction of the converter'"'"'s size and weight without negative impact on converter'"'"'s overall efficiency.

22. An isolated soft-switching converter as defined in claim 21,

wherein said Isolated DC Transformer has a single-loop, UU-like magnetic core;

wherein said input inductor and said isolation transformer are placed side-by-side on one leg of said single-loop, UU-like magnetic core;

wherein said output inductor is placed on the opposite leg of said single-loop, UU-like magnetic core;

whereby leakage inductance between said input inductor winding and said isolation transformer windings caused by deliberate side-by-side positioning of said windings along one leg of said single-loop, UU-like magnetic core provides substantially reduced and near zero-ripple current in said input inductor, and

whereby leakage inductance between said output inductor winding and said isolation transformer windings caused by deliberate positioning of said windings on opposite legs of said single-loop, UU-like magnetic core provides substantially reduced ripple current in said output inductor.

23. An isolated soft-switching converter as defined in claim 22,

wherein a small air-gap is positioned on said one leg of said single-loop, UU-like magnetic core with said output inductor winding to provide an effective AC voltage divider between leakage and magnetizing inductances of said output inductor, and

wherein said effective AC voltage divider enables better matching of AC voltages on said input inductor, said isolation transformer, and said output inductor,

whereby, for one particular duty ratio D, said output inductor DC current has a negligible current ripple.

24. An isolated soft-switching converter as defined in claim 23,

including a separate external inductor connected in series with said output inductor to reduce ripple in current from said output inductor and said output inductor number of turns is adjusted for better matching of AC voltages of said output inductor, said input inductor, and said isolation transformer,

whereby said external inductor has inductance an order of magnitude smaller than inductance of said output inductor and has to support an order of magnitude lower AC voltage than said output inductor, thereby resulting in an order of magnitude smaller core size and weight, and substantially lower power losses than said Isolated DC Transformer.

25. An isolated soft-switching converter as defined in claim 23,

including a separate leakage magnetic leg with no windings and a large air-gap in a magnetic flux path with said single-loop magnetic core, wherein said large air-gap is adjusted to provide said output inductor current with a negligible ripple at a particular duty ratio D,

whereby said leakage magnetic leg substantially increases the leakage inductance between said output inductor winding, said input inductor winding, and said isolation transformer windings, and provides reduction of ripple in said output inductor current, and

by adjusting said output inductor number of turns for better matching of AC voltages on said input inductor, said output inductor, and said isolation transformer, said output inductor ripple current is reduced.

26. An isolated soft-switching converter as defined in claim 21,

wherein said Isolated DC Transformer has a multiple-loop, EE-like magnetic core;

wherein said input inductor and said isolation transformer are placed side-by-side on the center leg of said multiple-loop, EE-like magnetic core;

wherein said output inductor winding is split into two windings connected in series so that their respective AC voltages are in phase and add, and each winding of said split output inductor has the same number of turns as said output inductor;

wherein each winding of said split output inductor winding is placed on a separate outer magnetic leg of said multiple-loop, EE-like magnetic core, and

whereby leakage inductance between said input inductor winding and said isolation transformer windings caused by deliberate side-by-side positioning of said windings along said center leg of said multiple-loop, EE-like magnetic core provides substantially reduced and near zero-ripple current in said input inductor, and

whereby leakage inductances between said split output inductor windings and said isolation transformer windings caused by deliberate positioning of said split output inductor windings on separate outer legs of said multiple-loop, EE-like magnetic core provide substantially reduced ripple current in said output inductor.

27. An isolated soft-switching converter as defined in claim 26,

wherein small air-gaps are positioned on each of said outer legs of said multiple-loop, EE-like magnetic core with said split output inductor windings to provide the effective AC voltage dividers between leakage and magnetizing inductances of respective said split output inductor windings;

wherein said AC voltage dividers enable better matching of AC voltages on said input inductor, said output inductor, and said isolation transformer, and

whereby, for one particular duty ratio D, said output inductor DC current has a negligible current ripple.

28. An isolated soft-switching DC-to-DC converter as defined in claim 21, further including additional secondary windings and separate output circuits for separate DC load outputs;

wherein said isolation transformer includes said additional secondary windings for said additional DC load outputs;

wherein each of said separate output circuits have a configuration with connections identical to the configuration and connections of an output circuit of said isolated switching DC-to-DC converter;

wherein each of said separate output circuits is connected between said additional secondary windings of said isolation transformer and said DC load outputs in an identical way as said output circuit of said isolated switching DC-to-DC converter is connected between said secondary winding of said isolation transformer and said output DC load;

wherein said primary winding, said secondary winding, and said additional secondary windings of multiple-output isolation transformer are placed on said common magnetic core with said input inductor winding, said output inductor winding, and output inductor windings of said separate output circuits for said DC load outputs to form a multiple-output Isolated DC Transformer;

wherein said switching means keeps said input switch, said output switch, and each output switch of said separate output circuits closed for a duration of time interval DT_S and keeps said complementary input switch, said complementary output switch, and each complementary output switch of said separate output circuits closed for a duration of complementary time interval D′

T_S=(1−

D)T_S, where D is a duty ratio and D′

is a complementary duty ratio within one complete and controlled switch operating cycle T_S;

wherein said switching means includes electronically controlling operation of said semiconductor switches whereby two transitions D to D′ and

D′

to D are obtained during each successive switch operating cycle T_S, wherein said transition intervals are short compared to said switch operating cycle;

wherein switch timing by said switching means of respective switches is as follows;

said first transition D to D′

is initiated by turning said input switch OFF and, when voltages on said complementary output switches are reduced to zero, said complementary output switches are by said switching means turned ON at zero voltage for zero switching losses, and said first transition continues while the voltage on said complementary input switch reduces to zero, at which instant said complementary input switch is also turned ON by said switching means at zero voltage for zero switching losses, and said first transition continues until the currents of said output switches are reduced to zero at which instant said output switches are turned OFF, and

said second transition D′

to D is initiated by turning said complementary input switch OFF and, when voltage on said input switch is reduced to zero, said input switch is by said switching means turned ON at zero voltage for zero switching losses, while said output switches are simultaneously turned ON with much reduced switching losses accomplishing a soft switching cycle, and said second transition continues until the currents of said complementary output switches are reduced to zero at which instant said complementary output switches are turned OFF, and

whereby both soft switching transitions result in zero switching losses on said input switch, said complementary input switch, and said complementary output switches and much reduced switching losses on said output switches, for any operating duty ratio D, and

whereby D to D′

transition is dependent only on DC load currents while D′

to D transition is dependent only on said auxiliary capacitor AC current, thus resulting in the D to D′

transition much shorter than D′

to D transition, and

whereby said leakage inductance substantially reduces duration of said D to D′

transition and does not effect said D′

to D transition, and

whereby voltage stress on said semiconductor devices is significantly reduced and efficiency of said converter is significantly increased while electromagnetic interference is reduced;

wherein said primary winding, said secondary winding, and said additional secondary windings on said common magnetic core are tightly coupled for reduced leakage between said primary winding, said secondary winding, and said additional secondary windings;

wherein turns ratios of said additional secondary windings to said primary winding of said multiple-output isolation transformer provide additional scaling of DC-to-DC voltage conversion ratio of said converter for each said additional DC load respectively;

wherein each additional output inductor winding for each said additional DC load has the same number of turns as respective said additional secondary winding of said multiple-output isolation transformer, whereby at any operating duty ratio D, zero DC flux is obtained in said common magnetic core;

wherein said common magnetic core has no air-gap, whereby said zero DC flux in said common magnetic core enables full utilization of the magnetic core material to generate maximum inductances of said input and output inductors and said multiple-output isolation transformer, thereby providing said converter with current overload capability several times higher than sum of nominal load currents of said DC load output and each said additional DC load output, and

whereby said multiple-output Isolated DC Transformer combines said input inductor winding, said output inductor winding, and said primary and secondary windings of said multiple-output isolation transformer to provide galvanic isolation between said input DC voltage source, said DC load, and said additional DC loads and in addition an unique and effective DC-to-DC power transfer from said input inductor to said output inductor and said additional output inductors with substantially reduced energy storage within said common magnetic core of said multiple-output Isolated DC Transformer, thereby reducing size and weight of said converter by reducing said common magnetic core size and weight while simultaneously increasing efficiency and converter overload capability, and providing reduction of electromagnetic interference.

29. An isolated switching converter as defined in claim 20,

wherein one end of said branch is connected to said dot-marked end of said primary winding of said isolation transformer and another end of said branch is connected to said unmarked end of said primary winding of said isolation transformer.

30. An isolated switching converter as defined in claim 20,

wherein one end of said branch is connected to said dot-marked end of said input inductor and another end of said branch is connected to said unmarked end of said input inductor.

31. An isolated switching converter as defined in claim 20,

wherein one end of said branch is connected to said dot-marked end of said input inductor and another end of said branch is connected to said unmarked end of said primary winding of said isolation transformer.

32. An isolated switching converter as defined in claim 20,

wherein one end of said input capacitor is connected to said common input terminal;

wherein said dot-marked end of said primary winding of said isolation transformer is connected to another end of said input capacitor;

wherein said unmarked end of said primary winding of said isolation transformer is connected to said unmarked end of said input inductor, and

wherein one end of said branch is connected to said dot-marked end of said primary winding of said isolation transformer and another end of said branch is connected to said unmarked end of said primary winding of said isolation transformer.

33. An isolated switching converter as defined in claim 20,

wherein one end of said branch is connected to said unmarked end of said secondary winding of said isolation transformer and another end of said branch is connected to said output terminal.

34. An isolated switching converter as defined in claim 20,

wherein one end of said complementary output switch is connected to said common terminal;

wherein said dot-marked end of said secondary winding of said isolation transformer is connected to another end of said complementary output switch;

wherein said unmarked end of said secondary winding of said isolation transformer is connected to another end of said unmarked end of said output inductor, and

wherein one end of said branch is connected to said unmarked end of said secondary winding of said isolation transformer and another end of said branch is connected to said dot-marked end of said secondary winding of said isolation transformer.

35. An isolated switching DC-to-DC converter as defined in claim 20,

wherein said output switch is comprised of a series connection of a MOSFET-like device and a Current Rectifier (diode);

whereby said output switch is a voltage bidirectional switch which operates as a two-quadrant switch and has reduced conduction losses due to the low conduction losses of said MOSFET-like device.

36. An isolated switching DC-to-DC converter as defined in claim 20,

wherein said output switch is comprised of two N-channel MOSFET-like devices having a source terminal of one MOSFET-like device connected to a source terminal of another MOSFET-like device (back-to-back connection) and gate terminals of said MOSFET-like devices connected together;

wherein a drain terminal of one MOSFET-like device is connected to said common output terminal and a drain terminal of another MOSFET-like device is connected to said unmarked end of said output inductor winding;

whereby said output switch is both voltage bidirectional and current bidirectional and operates as a four-quadrant switch with significantly reduced current conduction losses compared to two-quadrant voltage bidirectional switch implementation due to the low conduction losses of said two MOSFET-like devices.

37. An isolated switching converter as defined in claim 20, further comprising means for connecting the ends of said branch to said converter preserving galvanic isolation whereby during said complementary time interval, current through said branch is AC current;

whereby substantially zero DC flux in said common magnetic core enables full utilization of the magnetic core material providing said converter with current overload capability several times higher than nominal load current, and

whereby said Isolated DC Transformer combines said input inductor winding, said output inductor winding, and said primary and secondary windings of said isolation transformer to provide a galvanic isolation between said input DC voltage source and said DC load and unique and effective DC-to-DC power transfer from said input inductor to said output inductor with substantially obviated DC energy storage within said common magnetic core of said Isolated DC Transformer, thereby reducing size and weight of said converter by reducing said common magnetic core size and weight while simultaneously increasing efficiency and converter overload capability, and providing reduction of electromagnetic interference.

38. An isolated switching converter as defined in claim 37, further including a leakage inductance of said isolation transformer effectively connected in series with said input capacitor,

wherein said switching means includes electronically controlling operation of said semiconductor switches whereby two transitions D to D′

and D′

to D are obtained during each successive switch operating cycle T_S, wherein said transition intervals are short compared to said switch operating cycle;

wherein switch sequence and timing by said switching means of respective switches is as follows;

said first transition D to D′

is initiated by turning said input switch OFF and, when voltage on said complementary output switch is reduced to substantially zero, said complementary output switch is by said switching means turned ON at substantially zero voltage for substantially zero switching losses, and said first transition continues while the voltage on said complementary input switch reduces to substantially zero, at which instant said complementary input switch is also turned ON by said switching means at substantially zero voltage for substantially zero switching losses, and said first transition continues until the current of said output switch is reduced to substantially zero at which instant said output switch is turned OFF, and

said second transition D′

to D is initiated by turning said complementary input switch OFF and, when voltage on said input switch is reduced to substantially zero, said input switch is by said switching means turned ON at substantially zero voltage for substantially zero switching losses, while said output switch is simultaneously turned ON with much reduced switching losses accomplishing a lossless switching cycle, and said second transition continues until the current of said complementary output switch is reduced to substantially zero at which instant said complementary output switch is turned OFF, and

whereby both lossless switching transitions result in substantially zero switching losses on said input switch, said complementary input switch, and said complementary output switch and much reduced switching losses on said output switch, for any operating duty ratio D, and

whereby D to D′

transition is dependent only on DC load current while D′

to D transition is dependent only on said branch AC current, thus resulting in the D to D′

transition much shorter than D′

to D transition, and

whereby said leakage inductance substantially reduces duration of said D to D′

transition and does not effect said D′

to D transition, and

whereby voltage stress on said semiconductor devices is significantly reduced and efficiency of said converter is significantly increased while electromagnetic interference is reduced, and

whereby switching frequency can be significantly increased for further reduction of the converter'"'"'s size and weight without negative impact on converter'"'"'s overall efficiency.

39. An isolated switching converter as defined in claim 38,

wherein said Isolated DC Transformer has a single-loop, UU-like magnetic core;

wherein said input inductor and said isolation transformer are integrated side-by-side on one leg of said single-loop, UU-like magnetic core;

wherein said output inductor is integrated on the opposite leg of said single-loop, UU-like magnetic core;

whereby leakage inductance between said input inductor winding and said isolation transformer windings caused by deliberate side-by-side positioning of said windings along one leg of said single-loop, UU-like magnetic core reduces current ripple in said input inductor substantially to zero, and

whereby leakage inductance between said output inductor winding and said isolation transformer windings caused by deliberate positioning of said windings on opposite legs of said single-loop, UU-like magnetic core reduces current ripple in said output inductor substantially to zero.

40. An isolated switching converter as defined in claim 39,

wherein a small air-gap is positioned on said one leg of said single-loop, UU-like magnetic core with said output inductor winding to provide an effective AC voltage divider between leakage and magnetizing inductances of said output inductor, and

whereby, for one particular duty ratio D, said output inductor DC current has a substantially zero current ripple.

41. An isolated switching converter as defined in claim 40,

including a separate external inductor connected in series with said output inductor to reduce current ripple in said output inductor, and

whereby said external inductor has inductance an order of magnitude smaller than inductance of said output inductor and has to support an order of magnitude lower AC voltage than said output inductor, thereby resulting in an order of magnitude smaller core size and weight, and substantially lower power losses than said Isolated DC Transformer.

42. An isolated switching converter as defined in claim 40,

including a separate leakage magnetic leg with no windings and another air-gap in its magnetic flux path, wherein said another air-gap is adjusted to provide said output inductor current with a substantially zero current ripple at a particular duty ratio D,

whereby said leakage magnetic leg substantially increases the leakage inductance between said output inductor winding, said input inductor winding, and said isolation transformer windings, and provides reduction of current ripple in said output inductor current over the range of said input source voltage,

by adjusting said output inductor number of turns for better matching of AC voltages on said input inductor, said output inductor, and said isolation transformer, said output inductor current ripple is reduced.

43. An isolated switching converter as defined in claim 38,

wherein said Isolated DC Transformer has a multiple-loop, EE-like magnetic core;

wherein said input inductor and said isolation transformer are integrated side-by-side on the center leg of said multiple-loop, EE-like magnetic core;

wherein said output inductor winding is split into two windings connected in series so that their respective AC voltages are in phase at dot-marked ends and add, and each winding of said split output inductor has the same number of turns as said output inductor;

wherein each winding of said split output inductor winding is placed on a separate outer magnetic leg of said multiple-loop, EE-like magnetic core, and

whereby leakage inductance between said input inductor winding and said isolation transformer windings caused by deliberate side-by-side positioning of said windings along said center leg of said multiple-loop, EE-like magnetic core reduces current ripple in said input inductor to substantially zero over the range of said input source voltage;

whereby leakage inductances between said split output inductor windings and said isolation transformer windings caused by deliberate positioning of said split output inductor windings on separate outer legs of said multiple-loop, EE-like magnetic core reduces current ripple in said output inductor.

44. An isolated switching converter as defined in claim 43,

wherein small air-gaps are positioned on each of said outer legs of said multiple-loop, EE-like magnetic core with said split output inductor windings to provide the effective AC voltage dividers between leakage and magnetizing inductances of respective said split output inductor windings;

whereby, for one particular duty ratio D, said output inductor DC current has a substantially zero current ripple.

45. An isolated switching DC-to-DC converter as defined in claim 38, wherein said isolation transformer is a multiple-output isolation transformer integrated on said common magnetic core to form a multiple-output Isolated DC Transformer;

wherein said means for connecting the ends of said branch to said converter preserves galvanic isolation of said multiple-output isolation transformer whereby during said complementary time interval, current through said branch is AC current;

wherein turns ratios of said secondary windings to said primary winding of said multiple-output isolation transformer provide additional scaling of DC-to-DC voltage conversion ratio of said converter;

wherein each output inductor winding for each of multiple DC loads has the same number of turns as respective secondary winding of said multiple-output isolation transformer, whereby for range of operating duty ratio D, substantially zero DC flux is obtained in said common magnetic core;

whereby said substantially zero DC flux in said common magnetic core provides said converter with substantial current overload capability.

46. A method for power conversion comprising:

providing an input and complementary input switch being controllable semiconductor CBS (Current Bidirectional Switch) switches, each said CBS switch having an anti-parallel diode and parallel capacitance;

providing an output switch being controllable semiconductor VBS (Voltage Bidirectional Switch) switch having a parallel capacitance, and a complementary output switch being a semiconductor current rectifier having a parallel capacitance;

controlling an ON-time and an OFF-time of said controllable semiconductor switches regulating an output load voltage, each said controllable semiconductor switch being turned ON or OFF during natural and forced transition intervals which are short relative to said ON-time and OFF-time;

providing a negative current in a branch with said complementary input controllable semiconductor CBS switch during said forced transition interval;

controlling sequence and timing of turn-ON and turn-OFF signals for said controllable semiconductor switches during said natural transition interval, recycling charge among said capacitances of said controllable semiconductor switches and turning ON said complementary input controllable semiconductor CBS switch substantially losslessly at substantially zero voltage for any range of source voltages and load currents;

controlling sequence and timing of turn-ON and turn-OFF signals for said controllable semiconductor switches over said forced transition interval, said negative current recycling charge among said capacitances of said controllable semiconductor switches, turning ON said input controllable semiconductor CBS switch substantially losslessly at substantially zero voltage and simultaneously turning ON said output controllable semiconductor VBS switch at negative voltage across its capacitance for any range of source voltages and load currents;

integrating three inductor windings on a common magnetic core into a DC Transformer;

subjecting said inductor windings to AC voltages in phase at dot-marked ends of said inductor windings;

subjecting two of said inductor windings to DC currents flowing into said dot-marked ends thereof, thereby to generate a DC flux in one direction in said common magnetic core;

subjecting a third of said inductor windings to DC current flowing out of said dot-marked end thereof, thereby to generate a DC flux in opposite direction in said common magnetic core;

providing a selected number of turns for each of said inductor windings to produce substantially equal volts-per-turn and substantially zero total DC flux in said common magnetic core for a range of magnitudes of said three DC currents and for a range of magnitudes of said substantially equal volts-per-turn, whereby substantially obviating DC energy storage in said DC Transformer.

47. A method for power conversion as defined in claim 46 wherein said complementary output switch is a controllable semiconductor CBS switch having an anti-parallel diode and a parallel capacitance, said complementary output controllable semiconductor CBS switch being turned ON and OFF as a synchronous rectifier to reduce conduction losses by bypassing said anti-parallel diode.

48. A method as defined in claim 47 wherein said common magnetic core is a single-loop, UU-like magnetic core further comprising:

integrating said two inductor windings on one leg of said UU-like magnetic core side-by-side to provide substantially higher leakage inductance between said two inductor windings, reducing ripple current in one of said two inductor windings substantially to zero over the range of said substantially equal volts-per-turn;

integrating said third inductor winding on the opposite leg of said UU-like magnetic core to provide substantially higher leakage inductance between said third inductor winding and said two inductor windings reducing ripple current in said third inductor winding substantially to zero over the range of said substantially equal volts-per-turn.

49. A method as defined in claim 48 further including a small air-gap in said opposite leg of said single-loop, UU-like magnetic core, providing an effective AC voltage divider between leakage and magnetizing inductances of said third inductor winding further reducing current ripple in said third inductor winding.

50. A method as defined in claim 49 further including a separate small external inductor connected in series with said third inductor winding further reducing current ripple in said third inductor winding.

51. A method as defined in claim 49 further including a separate leakage magnetic leg with an additional air-gap in its magnetic flux path further reducing current ripple in said third inductor winding.

52. A method as defined in claim 47 wherein said common magnetic core is a multiple-loop, EE-like magnetic core further comprising:

integrating said two inductor windings side-by-side on the center leg of said multiple-loop, EE-like magnetic core;

splitting said third inductor winding into two windings having the same number of turns as said third inductor winding;

integrating said split inductor windings onto two outer magnetic legs of said multiple-loop, EE-like magnetic core;

connecting said split inductor windings in series having their respective AC voltages in phase at dot-marked ends of said split inductor windings;

reducing current ripples in said split inductor windings and in one of said two inductor windings over the range of said substantially equal volts-per-turn.

53. A method as defined in claim 52 further including small air-gaps in each of said outer legs of said multiple-loop, EE-like magnetic core, having effective AC voltage dividers to reduce current ripple in said split inductor windings.

54. A method as defined in claim 47 wherein one of said two inductor windings is replaced with an autotransformer windings, substantially obviating DC energy storage in a DC Transformer.

55. A method for power conversion as defined in claim 46 wherein one of said two inductor windings is replaced with an isolation transformer windings, substantially obviating DC energy storage in an Isolated DC Transformer.

56. A method for power conversion as defined in claim 55 wherein said complementary output switch is a controllable semiconductor CBS switch having an anti-parallel diode and a parallel capacitance, said complementary output controllable semiconductor CBS switch being turned ON and OFF as a synchronous rectifier to reduce conduction losses by bypassing said anti-parallel diode.

57. A method as defined in claim 56 wherein said common magnetic core is a single-loop, UU-like magnetic core further comprising:

integrating said two inductor windings on one leg of said UU-like magnetic core side-by-side to provide substantially higher leakage inductance between said two inductor windings, reducing ripple current in one of said two inductor windings substantially to zero over the range of said substantially equal volts-per-turn;

integrating said third inductor winding on the opposite leg of said UU-like magnetic core to provide substantially higher leakage inductance between said third inductor winding and said two inductor windings reducing ripple current in said third inductor winding substantially to zero over the range of said substantially equal volts-per-turn.

58. A method as defined in claim 57 further including a small air-gap in said opposite leg of said single-loop, UU-like magnetic core, providing an effective AC voltage divider between leakage and magnetizing inductances of said third inductor winding further reducing current ripple in said third inductor winding.

59. A method as defined in claim 58 further including a separate small external inductor connected in series with said third inductor winding further reducing current ripple in said third inductor winding.

60. A method as defined in claim 58 further including a separate leakage magnetic leg with an additional air-gap in its magnetic flux path further reducing current ripple in said third inductor winding.

61. A method as defined in claim 56 wherein said common magnetic core is a multiple-loop, EE-like magnetic core further comprising;

integrating said two inductor windings side-by-side on the center leg of said multiple-loop, EE-like magnetic core;

splitting said third inductor winding into two windings having the same number of turns as said third inductor winding;

integrating said split inductor windings onto two outer magnetic legs of said multiple-loop, EE-like magnetic core;

connecting said split inductor windings in series having their respective AC voltages in phase at dot-marked ends of said split inductor windings;

reducing current ripples in said split inductor windings and in one of said two inductor windings over the range of said substantially equal volts-per-turn.

62. A method as defined in claim 61 further including small air-gaps in each of said outer legs of said multiple-loop, EE-like magnetic core, having effective AC voltage dividers to reduce current ripple in said split inductor windings.

63. A method as defined in claim 56 wherein said isolation transformer is a multiple-output isolation transformer, whereby substantially obviating DC energy storage in a multiple-output Isolated DC Transformer.

64. A method for substantially obviating DC energy storage in a common magnetic core resulting in a DC Transformer, comprising:

integrating three inductor windings on said common magnetic core;

subjecting said inductor windings to AC voltages in phase at dot-marked ends of said inductor windings;

subjecting two of said inductor windings to DC currents flowing into said dot-marked ends thereof, thereby to generate a DC flux in one direction in said common magnetic core, and subjecting a third of said inductor windings to DC current flowing out of said dot-marked end thereof, thereby to generate a DC flux in opposite direction in said common magnetic core;

selecting turn numbers for each of said inductor windings to produce substantially equal volts-per-turn and substantially zero total DC flux in said common magnetic core for a range of magnitudes of said three DC currents and for a range of magnitudes of said substantially equal volts-per-turn.

65. A method as defined in claim 64 wherein said common magnetic core is a single-loop, UU-Like magnetic core further comprising:

integrating said two inductor windings on one leg of said UU-like magnetic core side-by-side to provide substantially higher leakage inductance between said two inductor windings, reducing ripple current in one of said two inductor windings substantially to zero over the range of said substantially equal volts-per-turn;

integrating said third inductor winding on the opposite leg of said UU-like magnetic core to provide substantially higher leakage inductance between said third inductor winding and said two inductor windings reducing ripple current in said third inductor winding substantially to zero over the range of said substantially equal volts-per-turn.

66. A method as defined in claim 65 further including a small air-gap in said opposite leg of said single-loop, UU-like magnetic core, providing an effective AC voltage divider between leakage and magnetizing inductances of said third inductor winding further reducing current ripple in said third inductor winding.

67. A method as defined in claim 66 further including a separate small external inductor connected in series with said third inductor winding further reducing current ripple in said third inductor winding.

68. A method as defined in claim 66 further including a separate leakage magnetic leg with an additional air-gap in its magnetic flux path further reducing current ripple in said third inductor winding.

69. A method as defined in claim 64 wherein said common magnetic core is a multiple-loop, EE-like magnetic core further comprising:

integrating said two inductor windings side-by-side on the center leg of said multiple-loop, EE-like magnetic core;

splitting said third inductor winding into two windings having the same number of turns as said third inductor winding;

integrating said split inductor windings onto two outer magnetic legs of said multiple-loop, EE-like magnetic core;

connecting said split inductor windings in series having their respective AC voltages in phase at dot-marked ends of said split inductor windings;

reducing current ripples in said split inductor windings and in one of said two inductor windings over the range of said substantially equal volts-per-turn.

70. A method as defined in claim 69 further including small air-gaps in each of said outer legs of said multiple-loop, EE-like magnetic core, having effective AC voltage dividers to reduce current ripple in said split inductor windings.

71. A method as defined in claim 64 wherein one of said two inductor windings is replaced with an autotransformer windings, substantially obviating DC energy storage in a DC Transformer.

72. A method as defined in claim 64 wherein one of said two inductor windings is replaced with an isolation transformer windings, substantially obviating DC energy storage in an Isolated DC Transformer.

73. A method as defined in claim 72 wherein said common magnetic core is a single-loop, UU-like magnetic core further comprising:

integrating said two inductor windings on one leg of said UU-like magnetic core side-by-side to provide substantially higher leakage inductance between said two inductor windings, reducing ripple current in one of said two inductor windings substantially to zero over the range of said substantially equal volts-per-turn;

integrating said third inductor winding on the opposite leg of said UU-like magnetic core to provide substantially higher leakage inductance between said third inductor winding and said two inductor windings reducing ripple current in said third inductor winding over the range of said substantially equal volts-per-turn.

74. A method as defined in claim 73 further including a small air-gap in said opposite leg of said single-loop, UU-like magnetic core, providing an effective AC voltage divider between leakage and magnetizing inductances of said third inductor winding further reducing current ripple in said third inductor winding.

75. A method as defined in claim 74 further including a separate small external inductor connected in series with said third inductor winding further reducing current ripple in said third inductor winding.

76. A method as defined in claim 74 further including a separate leakage magnetic leg with an additional air-gap in its magnetic flux path further reducing current ripple in said third inductor winding.

77. A method as defined in claim 72 wherein said common magnetic core is a multiple-loop, EE-like magnetic core further comprising;

integrating said two inductor windings side-by-side on the center leg of said multiple-loop, EE-like magnetic core;

splitting said third inductor winding into two windings having the same number of turns as said third inductor winding;

integrating said split inductor windings onto two outer magnetic legs of said multiple-loop, EE-like magnetic core;

connecting said split inductor windings in series having their respective AC voltages in phase at dot-marked ends of said split inductor windings;

reducing current ripples in said split inductor windings and in one of said two inductor windings over the range of said substantially equal volts-per-turn.

78. A method as defined in claim 77 further including small air-gaps in each of said outer legs of said multiple-loop, EE-like magnetic core, having effective AC voltage dividers to reduce current ripple in said split inductor windings.

79. A method as defined in claim 72 wherein said isolation transformer is a multiple-output isolation transformer, whereby substantially obviating DC energy storage in a multiple-output Isolated DC Transformer.

80. A method for substantially lossless switching comprising:

providing an input and complementary input switch being controllable semiconductor CBS switches, each said CBS switch having an anti-parallel diode and parallel capacitance;

providing an output switch being controllable semiconductor VBS switch having a parallel capacitance, and a complementary output switch being a semiconductor current rectifier having a parallel capacitance;

controlling an ON-time and an OFF-time of said controllable semiconductor switches regulating an output load voltage, each said controllable semiconductor switch being turned ON and OFF during natural and forced transition intervals which are short relative to said ON-time and OFF-time;

providing a negative current in a branch with said complementary input controllable semiconductor CBS switch during said forced transition interval;

controlling sequence and timing of turn-ON and turn-OFF signals for said controllable semiconductor switches during said natural transition interval, recycling charge among said capacitances of said controllable semiconductor switches and turning ON said complementary input controllable semiconductor CBS switch substantially losslessly at substantially zero voltage for any range of source voltages and load currents;

controlling sequence and timing of turn-ON and turn-OFF signals for said controllable semiconductor switches over said forced transition interval, said negative current recycling charge among said capacitances of said controllable semiconductor switches, turning ON said input controllable semiconductor CBS switch substantially losslessly at substantially zero voltage and simultaneously turning ON said output controllable semiconductor VBS switch at negative voltage across its capacitance for any range of source voltages and load currents.

81. A method for substantially lossless switching as defined in claim 80 wherein said complementary output switch is a controllable semiconductor CBS switch having an anti-parallel diode and parallel capacitance, said complementary output CBS switch being turned ON and OFF as a synchronous rectifier to reduce conduction losses by bypassing said anti-parallel diode.

82. A method for substantially obviating DC energy storage in a common magnetic core resulting in a DC Transformer, comprising:

integrating three inductor windings on said common magnetic core;

subjecting said three inductor windings to AC voltages in phase at dot-marked ends of said inductor windings;

subjecting two of said three inductor windings to DC currents flowing into said dot-marked ends thereof, thereby to generate a DC flux in one direction in said common magnetic core, and subjecting a third of said three inductor windings to DC current flowing out of said dot-marked end thereof, thereby to generate a DC flux in opposite direction in said common magnetic core, resulting in subtraction of said DC fluxes in said common magnetic core;

providing a selected number of turns for each of said three inductor windings to produce substantially equal volts-per-turn and substantially reduce total DC flux in said common magnetic core for a range of magnitudes of said three DC currents and for a range of magnitudes of said substantially equal volts-per-turn.