Double forward converter with soft-PWM switching

• US 5,973,939 A
• Filed: 08/29/1996
• Issued: 10/26/1999
• Est. Priority Date: 08/29/1996
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: TRW Limited (Zeppelin - Stiftung der Stadt Friedrichshafen)
• Inventors: Tan, F. Dong
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): Patel, Rajnikant B.
• Application Number:
  US08/705,577
• Time in Patent Office: 1,153 Days
• Field of Search: 363/16, 363/21, 363/20, 363/97, 363/89, 363/131, 323/224, 323/282
• US Class Current: 363/21.06
• CPC Class Codes: H02M 3/01 Resonant DC/DC converters
• CPC Class Codes: H02M 3/33569 having several active switching elements H02M3/3353 takes precedence
• 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

A double forward DC-to-DC converter with soft-PWM switching provides two voltage pulses to the output low-pass filter within one switching cycle (hence, the name double forward). The double forward DC to DC converter includes a transformer having a single primary winding and a two secondary windings. A main switch, under the control of a PWM control circuit, is connected is series with the primary winding. An auxiliary switch is coupled across with the main switch by way of a resonant capacitor. The resonant capacitor and auxiliary switch are used to automatically reset the transformer core while the main switch is on back to the voltage source connected to the transformer primary. A diode and snubber capacitor is connected across the drain-source terminals for both the main and auxiliary switches for minimizing switching losses. The current mirror used in prior art is abandoned in order to avoid the locked-up mode of operation.

The secondary windings of the transformer are connected to a pair of saturable reactors which, in turn, are connected to rectifier diodes in order to eliminate the reverse recovery losses of the rectifier diodes. A freewheeling diode is connected across the secondary winding and cooperates with the saturable reactors to maintain the load current to the secondary windings during the transitions of the main and auxiliary switches.

Claims

1. A DC-to-DC converter circuit comprising:

a transformer having a primary winding and a plurality of secondary windings, said primary winding adapted to be coupled to a DC voltage source;

a main switch serially coupled to said primary winding defining a node and electrically coupled to said DC voltage source, said main switch adapted to be controlled by a pulse width modulator circuit;

a pair of input terminals electrically coupled to said primary winding and said main switch;

a reset circuit electrically coupled to said primary winding and said DC voltage source for automatically returning magnetizing current to said DC voltage source when said main switch is turned off, said reset circuit including a first capacitor and an auxiliary switch, said auxiliary switch coupled across the main switch, said capacitor connected to said node and used to control said auxiliary switch;

a first diode connected in parallel across said main switch;

a second capacitor connected in parallel with said first diode;

a second diode electrically coupled across said auxiliary switch and said first capacitor;

a third diode, electrically coupled to one of said plurality of secondary windings;

a first saturable reactor, serially coupled to said third diode;

a fourth diode, electrically coupled to the other another of said plurality of secondary windings;

a second saturable reactor, serially coupled to said second rectifier diode;

a freewheeling fifth diode, electrically coupled between said third and fourth diode; and

a pair of output terminals defined across said freewheeling fifth diode.

2. The DC-to-DC converter circuit as recited in claim 1, wherein said main switch is a solid state switch.

3. The DC-to-DC converter circuit as recited in claim 2, wherein said solid state switch is a MOSFET having gate, drain and source terminals.

4. The DC-to-DC converter circuit as recited in claim 1, wherein said auxiliary switch is implemented as a MOSFET having gate, drain and source terminals.

5. The DC-to-DC converter circuit as recited in claim 1, further including a filter circuit which includes an inductor and a capacitor.

6. The DC-to-DC converter circuit as recited in claim 1, wherein said plurality of secondary windings have equal turns.

7. A DC-to-DC converter circuit comprising;

a transformer having a primary winding and a plurality of secondary windings, said primary winding adapted to be electrically coupled to a DC voltage source;

a main switch electrically coupled to said DC voltage source and serially coupled to said primary winding defining a node, said main switch for controlling the energy transfer from said DC voltage source and said transformer;

a first diode coupled in parallel across main switch;

a first capacitor coupled in parallel across said first diode and said main switch;

a pair of input terminals electrically coupled to said primary winding and said main switch;

a pulse width modulator circuit for controlling said main switch;

an auxiliary switch electrically coupled across said main switch;

a second capacitor electrically coupled across said auxiliary switch;

a second diode electrically coupled across said second capacitor and said auxiliary switch;

a third capacitor coupled to said node and used for automatically discharging the energy in said transformer when said main switch is turned off;

third and fourth diodes electrically coupled to said plurality of secondary windings;

a first saturable reactor, serially coupled to said third diode;

a second saturable reactor, serially coupled to said fourth diode; and

a freewheeling fifth diode, electrically coupled to said third and fourth diodes which conducts when the main and auxiliary switches are turned off to prevent reverse recovery losses of said third and fourth diodes; and

a pair of output terminals electrically coupled across said freewheeling fifth diode.

8. The DC-to-DC converter circuit as recited in claim 7, further including a filter circuit electrically coupled to said pair of output terminals.

9. The DC-to-DC converter circuit as recited in claim 8, wherein said filter circuit includes a capacitor and an inductor.

10. DC-to-DC converter circuit as recited in claim 7, wherein said main and auxiliary switches are MOSFETS and said first and second diodes are body diodes of said MOSFETS.

11. A DC-to-DC converter circuit comprising;

a transformer having a primary winding and a plurality of secondary windings, said primary winding adapted to be coupled to a DC voltage source;

a main switch serially coupled to said primary winding defining a node and electrically coupled to said DC voltage source, said main switch adapted to be controlled by a pulse width modulator circuit; and

a reset circuit electrically coupled to said primary winding and said DC voltage source for automatically returning magnetizing current to said DC voltage source when said main switch is turned off, said reset circuit including an auxiliary switch electrically coupled across said main switch, said reset circuit including a capacitor connected to said node and used to control said auxiliary switch;

a first rectifier diode, electrically coupled to one of said plurality of secondary windings;

a first saturable reactor, serially coupled to said first rectifier diode;

a second rectifier diode, electrically coupled to another of said plurality of secondary windings;

a second saturable reactor serially coupled to said second rectifier diode;

a freewheeling diode electrically coupled between first and second rectifier diodes which conducts when the main and auxiliary switches are turned off to prevent reverse discovery losses of said first and second rectifier diodes; and

a pair of output terminals electrically, coupled to said freewheeling diode.

12. A DC-to-DC converter circuit comprising:

a transformer having a primary winding and a plurality of secondary windings, said primary winding adapted to be coupled to a DC voltage source;

a main switch electrically coupled to said primary winding and said DC voltage source, said main switch adapted to be controlled by a pulse width modulator circuit;

a reset circuit electrically coupled to said primary winding and said DC voltage source for automatically returning magnetizing current to said DC voltage source when said main switch is turned off;

a first synchronous rectifier electrically coupled to one of said plurality of secondary windings;

a first saturable reactors, serially coupled to said first rectifier diodes;

a second synchronous rectifier electrically coupled to another of said plurality of secondary windings;

a second saturable reactor serially coupled to said second rectifier diode;

a freewheeling diode electrically coupled between said first and second rectifier diode; and

a pair of output terminals electrically coupled to said secondary winding.

13. A DC-to-DC converter circuit comprising:

a transformer having a primary winding and multiple independent secondary windings, said primary winding adapted to be coupled to a DC voltage source;

a main switch electrically coupled to said primary winding and said DC voltage source, said main switch adapted to be controlled by a pulse width modulator circuit;

a reset circuit electrically coupled to said primary winding and said DC voltage source for automatically returning magnetizing current to said DC voltage source when said main switch is turned off;

first and second rectifier diodes electrically coupled to each of said multiple independent secondary windings;

first saturable reactors, serially coupled to each of said first rectifier diodes;

second saturable reactors, serially coupled to each of said second rectifier diodes;

freewheeling diodes electrically coupled between said first and second rectifier diode; and

output terminals electrically coupled to each of said secondary windings.

14. A double forward DC-to-DC converter comprising:

a transformer having a primary winding and a plurality of secondary windings adapted to being connected to a source of DC voltage;

a main switch serially coupled to said primary winding defining a node and electrically coupled to said source of DC voltage, said main adapted switch to be controlled by a pulse width modulator circuit, each on and off cycle of said main switch defining a switching cycle;

a reset circuit electrically coupled to said primary winding for automatically returning magnetizing current to said voltage source each time said main switch is turned off, said reset circuit including a capacitor and an auxiliary switch, said auxiliary switch coupled across said main switch, said capacitor connected to said node and used to control said auxiliary switch;

means for providing at least two pulses to said output terminals during each switching cycle; and

a pair of output terminals electrically coupled to said secondary windings.

15. A DC-to DC- converter circuit comprising;

a transformer having a primary winding and predetermined secondary windings, said primary winding adapted to be coupled to a DC voltage source;

a main switch serially coupled to said primary winding defining a node and electrically coupled to said DC voltage source, said main switch adapted to be controlled by a pulse width modulator circuit, each on and off cycle defining a switching cycle;

a reset circuit electrically coupled to said primary winding and said DC voltage source for automatically returning magnetizing current to said DC voltage source when said main switch is turned off, said reset circuit including a capacitor and an auxiliary switch, said auxiliary switch coupled across said main switch, said capacitor connected to said node and used to control said auxiliary switch;

a first rectifier device electrically coupled to said secondary windings;

a first saturable reactor, serially coupled to said first rectifier device;

a second rectifier device electrically coupled to said secondary windings;

a freewheeling diode electrically coupled between said first and second rectifier devices; and

output terminals electrically coupled to said freewheeling diode.

16. The DC-to-DC converter as recited in claim 15, wherein said first and second rectifier devices are synchronous rectifiers.

17. The DC-to-DC converter as recited in claim 15, wherein said first and second rectifier devices are rectifier diodes.

18. The DC-to-DC converter as recited in claim 15, wherein said predetermined secondary windings.

19. The DC-to-DC converter as recited in claim 15, wherein said reset circuit includes means for providing a plurality of pulses to said output terminals every switching cycle.