Power conversion apparatus utilizing soft switching resonant snubber circuitry

• US 6,438,004 B1
• Filed: 12/26/2001
• Issued: 08/20/2002
• Est. Priority Date: 04/28/2000
• Status: Expired due to Fees

• Current Assignee: TDK Corporation
• Original Assignee: TDK Corporation
• Inventors: Tanaka, Katsuaki, Itoh, Kazuyuki, Okita, Yoshihisa
• Primary Examiner(s): Vu, Bao Q.
• Application Number:
  US10/033,412
• Publication Number:
  US 20020054499A1
• Time in Patent Office: 237 Days
• Field of Search: 363/56.02, 363/56.03, 363/56.04, 363/56.05, 363/95, 363/98, 363/131, 363/132
• US Class Current: 363/56.02
• CPC Class Codes: H02M 1/0048 Circuits or arrangements for reducing losses using snubbers H02M1/34
• CPC Class Codes: H02M 1/12 Arrangements for reducing harmonics from ac input or output
• CPC Class Codes: H02M 7/4826 operating from a resonant DC source, i.e. the DC input voltage varies periodically, e.g. resonant DC-link inverters
• CPC Class Codes: H02M 7/5233 the commutation elements being in a push-pull arrangement
• 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

The present invention discloses a power conversion apparatus comprising a control circuit for generating a switching signal at the timing allowing soft-switching to be achieved, and free from any occurrence of ripple.

The power conversion apparatus includes a first main switch (Q1) and a second main switch (Q2) which are connected in series with each other. One of the ends of the first main switch is connected with the positive side of a DC power supply, and one of the ends of the second main switch is connected to the negative side of the DC power supply. A diode (D1, D2) is connected in parallel with each of the main switches so as to become reverse biased with respect to the DC power supply. A main-switch snubber capacitor (C1, C2) is connected in parallel with each of the main switches. A load is connected with the junction between the pair of main switches, and the main switches are controllably switched according a switching signal from a control circuit to generate an output. A first auxiliary resonant circuit including serial-connected first and second auxiliary switches (Q3, Q4, Q5, Q6) and a resonant inductor (L1) connected in series with the second auxiliary switch is connected with each of the positive side of the DC power supply and the junction between the two main switches. A diode is connected to each of the first and second auxiliary switches so as to become reverse biased with respect to the DC power supply. The control circuit provides a turn-on signal to the first and second auxiliary switches according to a voltage signal as an input representing the voltage across each of the main switches and auxiliary switches from voltage detecting means before a turn-on signal as the switching signal is provided to the first main switch.

Claims

1. A power conversion apparatus including:

at least a pair of main switches composed of serial-connected first and second main switches, one of the ends of said first main switch being connected with the positive side of a DC power supply, one of the ends of said second main switch being connected to the negative side of said DC power supply;

a diode connected in parallel with each of said main switches so as to become reverse biased with respect to said DC power supply;

a main-switch snubber capacitor connected in parallel with each of said main switches;

a load connected with the junction between said pair of main switches; and

a control circuit for forming a switching signal for controlling the switching operation of said main switches by using a load voltage and/or a load current as an input thereof, wherein said main switches are controllably switched according said switching signal from said control circuit so as to generate an output, said power conversion apparatus comprising;

a first auxiliary resonant circuit including serial-connected first and second auxiliary switches and a resonant inductor connected in series with said second auxiliary switch, said first auxiliary resonant circuit being connected with each of the positive side of said DC power supply and the junction between said pair of main switches;

a diode connected to each of said first and second auxiliary switches so as to become reverse biased with respect to said DC power supply; and

voltage detecting means for detecting the voltage across each of said main switches and auxiliary switches;

wherein

said control circuit is applied with a voltage signal as an input representing said voltage across each of said main switches and auxiliary switches from said voltage detecting means;

said control circuit being adapted to provide a turn-on signal to said first and second auxiliary switches according to said input before a turn-on signal as the switching signal is provided to said first main switch;

said control circuit being adapted to provide the turn-on signal to said first and second auxiliary switches when the load current passes through said diode connected in parallel with said second main switch, so as to turn on said first and second auxiliary switches to direct the current from said DC power supply to said resonant inductor;

whereby

a resonant circuit is formed by said resonant inductor and said snubber capacitors connected in parallel with said main switches when the current of said resonant inductor goes up approximately to the load current;

wherein

said control circuit is adapted to output a signal for turning on said first main switch when the voltage across said first main switch goes down approximately to zero through the resonance in said resonance circuit.

2. A power conversion apparatus as defined in claim 1, which further includes:

serial-connected third and fourth auxiliary switches which are connected between the negative side of said DC power supply and said inductor so as to form a second auxiliary resonant circuit;

an auxiliary-switch snubber capacitor connected between the junction between said first and second auxiliary switches and the junction between said third and fourth auxiliary switches; and

a diode connected to each of said third and fourth auxiliary switches so as to become reverse biased with respect to said DC power supply;

wherein

said control circuit is adapted to provide a turn-off signal to said first auxiliary switch when the charged voltage of said auxiliary-switch snubber capacitor is approximately equal to the voltage of said DC power supply after said first main switch is turned on, and to provide the turn-off signal to said second auxiliary switch when the charged voltage of said auxiliary-switch snubber capacitor is approximately equal to zero after said first main switch is turned on, so as to achieve soft-switching of said first and second auxiliary switches.

3. A power conversion apparatus including:

at least a pair of main switches composed of serial-connected first and second main switches, one of the ends of said first main switch being connected with the positive side of a DC power supply, one of the ends of said second main switch being connected to the negative side of said DC power supply;

a diode connected in parallel with each of said main switches so as to become reverse biased with respect to said DC power supply;

a main-switch snubber capacitor connected in parallel with each of said main switches;

a load connected with the junction between said pair of main switches; and

a control circuit for forming a switching signal for controlling the switching operation of said main switches by using a load voltage and/or a load current as an input thereof, wherein said main switches are controllably switched according said switching signal from said control circuit so as to generate an output, said power conversion apparatus comprising;

a second auxiliary resonant circuit including serial-connected third and fourth auxiliary switches and a resonant inductor connected in series with said fourth auxiliary switch, said second auxiliary resonant circuit being connected with each of the negative side of said DC power supply and the junction between said pair of main switches;

a diode connected to each of said third and fourth auxiliary switches so as to become reverse biased with respect to said DC power supply; and

voltage detecting means for detecting the voltage across each of said main switches and auxiliary switches;

wherein

said control circuit is applied with a voltage signal as an input representing said voltage across each of said main switches and auxiliary switches from said voltage detecting means,

said control circuit being adapted to provide a turn-on signal to said third and fourth auxiliary switches according to said input, before a turn-on signal as the switching signal is provided to said second main switch, when said first main switch is in ON-state to allow the load current to pass through said first main switch and said load current is less than a threshold associated with the product of multiplying the capacity of said main-switch snubber capacitor by the power supply voltage of said DC power supply, so as to turn on said third and fourth auxiliary switches to direct the current from said DC power supply to said resonant inductor;

wherein

said control circuit is adapted to provide a turn-off signal to said first main switch when the current of said resonant inductor goes up approximately to said threshold, so as to turn off said first main switch.

4. A power conversion apparatus as defined in claim 3, wherein said control circuit is adapted to provide the turn-on signal to said second main switch, when said third and fourth auxiliary switches are in ON-state and the current passing through said resonant inductor is refluxed from said third and fourth auxiliary switches through said diode connected in parallel with said second main switch.

5. A power conversion apparatus as defined in claim 4, wherein said control circuit is adapted to provide the turn-off signal to said third auxiliary switch after said second main switch is turned on, when the initial voltage of said auxiliary-switch snubber capacitor is approximately equal to the voltage of said DC power supply, and to provide the turn-off signal to said fourth auxiliary switch after said second main switch is turned on, when the initial voltage of said auxiliary-switch snubber capacitor is approximately equal to zero.

6. A power conversion apparatus as defined in claim 3, wherein said control circuit is adapted to provide the turn-off signal to said first main switch without providing any turn-on signal to said third and fourth auxiliary switches, when said load current is larger than said threshold.

7. A power conversion apparatus as defined in claim 3, wherein said threshold is defined by the following formula;

8. A power conversion apparatus including:

at least a pair of main switches composed of serial-connected first and second main switches, one of the ends of said first main switch being connected with the positive side of a DC power supply, one of the ends of said second main switch being connected to the negative side of said DC power supply;

a diode connected in parallel with each of said main switches so as to become reverse biased with respect to said DC power supply;

a main-switch snubber capacitor connected in parallel with each of said main switches;

a load connected with the junction between said pair of main switches; and

a control circuit for forming a switching signal for controlling the switching operation of said main switches by using a load voltage and/or a load current as an input thereof, wherein said main switches are controllably switched according said switching signal from said control circuit so as to generate an output, said power conversion apparatus comprising;

a second auxiliary resonant circuit including serial-connected third and fourth auxiliary switches and a resonant inductor connected in series with said fourth auxiliary switch, said second auxiliary resonant circuit being connected with each of the negative side of said DC power supply and the junction between said pair of main switches;

a diode connected to each of said third and fourth auxiliary switches so as to become reverse biased with respect to said DC power supply; and

voltage detecting means for detecting the voltage across each of said main switches and auxiliary switches;

wherein

said control circuit is applied with a voltage signal as an input representing said voltage across each of said main switches and auxiliary switches from said voltage detecting means,

said control circuit being adapted to provide a turn-on signal to said third and fourth auxiliary switches and provide a turn-off signal to said first main switch according to said input, before a turn-on signal as the switching signal is provided to said second main switch, when said first main switch is in ON-state to allow the load current to pass through said first main switch and said load current is less than a threshold associated with the product of multiplying the capacity of said main-switch snubber capacitor by the power supply voltage of said DC power supply, so as to turn on said third and fourth auxiliary switches to generate a resonance between said resonant inductor and said snubber capacitors connected in parallel with said main switches.

9. A power conversion apparatus including:

at least a pair of main switches composed of serial-connected first and second main switches, one of the ends of said first main switch being connected with the positive side of a DC power supply, one of the ends of said second main switch being connected to the negative side of said DC power supply;

a diode connected in parallel with each of said main switches so as to become reverse biased with respect to said DC power supply;

a main-switch snubber capacitor connected in parallel with each of said main switches;

a load connected with the junction between said pair of main switches; and

a control circuit for forming a switching signal for controlling the switching operation of said main switches by using a load voltage and/or a load current as an input thereof, wherein said main switches are controllably switched according said switching signal from said control circuit so as to generate an output, said power conversion apparatus comprising;

a first auxiliary resonant circuit including serial-connected first and second auxiliary switches and a resonant inductor connected in series with said second auxiliary switch, said first auxiliary resonant circuit being connected with each of the positive side of said DC power supply and the junction between said pair of main switches;

a diode connected to each of said first and second auxiliary switches so as to become reverse biased with respect to said DC power supply;

a second auxiliary resonant circuit formed by connecting serial-connected third and fourth auxiliary switches between the negative side of said DC power supply and said inductor;

an auxiliary-switch snubber capacitor connected between the junction between said first and second auxiliary switches and the junction between said third and fourth auxiliary switches;

a diode connected to each of said third and fourth auxiliary switches so as to become reverse biased with respect to said DC power supply; and

voltage detecting means for detecting the voltage across each of said main switches and auxiliary switches;

wherein

said control circuit is applied with a voltage signal as an input representing said voltage across each of said main switches and auxiliary switches from said voltage detecting means,

said control circuit being adapted to provide a turn-on signal to said first and second auxiliary switches according to said input before a turn-on signal as the switching signal is provided to said first main switch,

said control circuit being adapted to provide the turn-on signal to said first and second auxiliary switches when the load current passes through said diode connected in parallel with said second main switch, so as to turn on said first and second auxiliary switches to direct the current from said DC power supply to said resonant inductor;

wherein

said control circuit is adapted to output a signal for turning on said first main switch when the voltage across said first main switch goes down approximately to zero through the resonance in a resonance circuit formed by said resonant inductor and said snubber capacitors connected in parallel with said main switches when the current of said resonant inductor goes up approximately to the load current.

10. A power conversion apparatus as defined in claim 9, wherein said control circuit is adapted to provide a turn-off signal to said first auxiliary switch when the charged voltage of said auxiliary-switch snubber capacitor is approximately equal to the voltage of said DC power supply after said first main switch is turned on, and to provide the turn-off signal to said second auxiliary switch when the charged voltage of said auxiliary-switch snubber capacitor is approximately equal to zero after said first main switch is turned on, so as to achieve soft-switching of said first and second auxiliary switches.

11. A power conversion apparatus as defined in claim 9, wherein said control circuit is adapted to provide a turn-on signal to said third and fourth auxiliary switches, before a turn-on signal as the switching signal is provided to said second main switch, when said first main switch is in ON-state to allow the load current to pass through said first main switch and said load current is less than a threshold associated with the product of multiplying the capacity of said main-switch snubber capacitor by the power supply voltage of said DC power supply, so as to turn on said third and fourth auxiliary switches to direct the current from said DC power supply to said resonant inductor;

wherein

said control circuit is adapted to provide a turn-off signal to said first main switch when the current of said resonant inductor goes up approximately to said threshold, so as to turn off said first main switch.

12. A power conversion apparatus as defined in claim 11, wherein said control circuit is adapted to provide the turn-on signal to said second main switch, when said third and fourth auxiliary switches are in ON-state and the current passing through said resonant inductor is refluxed from said third and fourth auxiliary switches through said diode connected in parallel with said second main switch.

13. A power conversion apparatus as defined in claim 9, wherein said control circuit is adapted to provide a turn-on signal to said third and fourth auxiliary switches and provide a turn-off signal to said first main switch, before a turn-on signal as the switching signal is provided to said second main switch, when said first main switch is in ON-state to allow the load current to pass through said first main switch and said load current is less than a threshold associated with the product of multiplying the capacity of said main-switch snubber capacitor by the power supply voltage of said DC power supply, so as to turn on said third and fourth auxiliary switches to generate a resonance between said resonant inductor and said snubber capacitor connected in parallel with said first main switch.

14. A power conversion apparatus including:

at least a pair of main switches composed of serial-connected first and second main switches, one of the ends of said first main switch being connected with the positive side of a DC power supply, one of the ends of said second main switch being connected to the negative side of said DC power supply;

a diode connected in parallel with each of said main switches so as to become reverse biased with respect to said DC power supply;

a main-switch snubber capacitor connected in parallel with each of said main switches;

a load connected with the junction between said pair of main switches; and

a control circuit for forming a switching signal for controlling the switching operation of said main switches by using a load voltage and/or a load current as an input thereof, wherein said main switches are controllably switched according said switching signal from said control circuit so as to generate an output, said power conversion apparatus comprising;

a second auxiliary resonant circuit including serial-connected third and fourth auxiliary switches and a resonant inductor connected in series with said fourth auxiliary switch, said second auxiliary resonant circuit being connected with each of the negative side of said DC power supply and the junction between said pair of main switches;

a diode connected to each of said third and fourth auxiliary switches so as to become reverse biased with respect to said DC power supply; and

voltage detecting means for detecting the voltage across each of said main switches and auxiliary switches;

wherein

said control circuit is applied with a voltage signal as an input representing said voltage across each of said main switches and auxiliary switches from said voltage detecting means,

said control circuit being adapted to provide a turn-on signal to said third and fourth auxiliary switches according to said input before a turn-on signal as the switching signal is provided to said second main switch,

said control circuit being adapted to provide the turn-on signal to said third and fourth auxiliary switches when the load current passes through said diode connected in parallel with said first main switch, so as to turn on said third and fourth auxiliary switches to direct the current from said DC power supply to said resonant inductor, whereby

a resonant circuit is formed by said resonant inductor and said snubber capacitors connected in parallel with said main switches when the current of said resonant inductor goes up approximately to the load current, wherein

said control circuit is adapted to output a signal for turning on said second main switch when the voltage across said second main switch goes down approximately to zero through the resonance in said resonance circuit.

15. A power conversion apparatus as defined in claim 14, which further includes:

serial-connected first and second auxiliary switches which are connected between the positive side of said DC power supply and said inductor so as to form a first auxiliary resonant circuit;

an auxiliary-switch snubber capacitor connected between the junction between said first and second auxiliary switches and the junction between said third and fourth auxiliary switches; and

a diode connected to each of said first and second auxiliary switches so as to become reverse biased with respect to said DC power supply;

wherein

said control circuit is adapted to provide a turn-off signal to said third auxiliary switch when the charged voltage of said auxiliary-switch snubber capacitor is approximately equal to the voltage of said DC power supply after said second main switch is turned on, and to provide the turn-off signal to said fourth auxiliary switch when the charged voltage of said auxiliary-switch snubber capacitor is approximately equal to zero after said second main switch is turned on, so as to achieve soft-switching of said third and fourth auxiliary switches.

16. A power conversion apparatus including:

at least a pair of main switches composed of serial-connected first and second main switches, one of the ends of said first main switch being connected with the positive side of a DC power supply, one of the ends of said second main switch being connected to the negative side of said DC power supply;

a diode connected in parallel with each of said main switches so as to become reverse biased with respect to said DC power supply;

a main-switch snubber capacitor connected in parallel with each of said main switches;

a load connected with the junction between said pair of main switches; and

a control circuit for forming a switching signal for controlling the switching operation of said main switches by using a load voltage and/or a load current as an input thereof, wherein said main switches are controllably switched according said switching signal from said control circuit so as to generate an output, said power conversion apparatus comprising;

a first auxiliary resonant circuit including serial-connected first and second auxiliary switches and a resonant inductor connected in series with said second auxiliary switch, said first auxiliary resonant circuit being connected with each of the negative side of said DC power supply and the junction between said pair of main switches;

a diode connected to each of said first and second auxiliary switches so as to become reverse biased with respect to said DC power supply; and

voltage detecting means for detecting the voltage across each of said main switches and auxiliary switches;

wherein

said control circuit is applied with a voltage signal as an input representing said voltage across each of said main switches and auxiliary switches from said voltage detecting means,

said control circuit being adapted to provide a turn-on signal to said first and second auxiliary switches according to said input, before a turn-on signal as the switching signal is provided to said first main switch, when said second main switch is in ON-state to allow the load current to pass through said second main switch and said load current is less than a threshold associated with the product of multiplying the capacity of said main-switch snubber capacitor by the power supply voltage of said DC power supply, so as to turn on said first and second auxiliary switches to direct the current from said DC power supply to said resonant inductor;

wherein

said control circuit is adapted to provide a turn-off signal to said second main switch when the current of said resonant inductor goes up approximately to said threshold, so as to turn off said second main switch.

17. A power conversion apparatus as defined in claim 16, wherein said control circuit is adapted to provide the turn-on signal to said first main switch, when said first and second auxiliary switches are in ON-state and the current passing through said resonant inductor is refluxed from said first and second auxiliary switches through said diode connected in parallel with said first main switch.

18. A power conversion apparatus as defined in claim 17, wherein said control circuit is adapted to provide the turn-off signal to said first auxiliary switch after said first main switch is turned on, when the initial voltage of said auxiliary-switch snubber capacitor is approximately equal to the voltage of said DC power supply, and to provide the turn-off signal to said second auxiliary switch after said first main switch is turned on, when the initial voltage of said auxiliary-switch snubber capacitor is approximately equal to zero.

19. A power conversion apparatus as defined in claim 16, wherein said control circuit is adapted to provide the turn-off signal to said second main switch without providing any turn-on signal to said first and second auxiliary switches, when said load current is larger than said threshold.

20. A power conversion apparatus as defined in claim 16, wherein said threshold is defined by the following formula;

21. A power conversion apparatus including:

at least a pair of main switches composed of serial-connected first and second main switches, one of the ends of said first main switch being connected with the positive side of a DC power supply, one of the ends of said second main switch being connected to the negative side of said DC power supply;

a diode connected in parallel with each of said main switches so as to become reverse biased with respect to said DC power supply;

a main-switch snubber capacitor connected in parallel with each of said main switches;

a load connected with the junction between said pair of main switches; and

a control circuit for forming a switching signal for controlling the switching operation of said main switches by using a load voltage and/or a load current as an input thereof, wherein said main switches are controllably switched according said switching signal from said control circuit so as to generate an output, said power conversion apparatus comprising;

a first auxiliary resonant circuit including serial-connected first and second auxiliary switches and a resonant inductor connected in series with said second auxiliary switch, said first auxiliary resonant circuit being connected with each of the negative side of said DC power supply and the junction between said pair of main switches;

a diode connected to each of said first and second auxiliary switches so as to become reverse biased with respect to said DC power supply; and

voltage detecting means for detecting the voltage across each of said main switches and auxiliary switches;

wherein

said control circuit is applied with a voltage signal as an input representing said voltage across each of said main switches and auxiliary switches from said voltage detecting means,

said control circuit being adapted to provide a turn-on signal to said first and second auxiliary switches and provide a turn-off signal to said second main switch according to said input, before a turn-on signal as the switching signal is provided to said first main switch, when said second main switch is in ON-state to allow the load current to pass through said second main switch and said load current is less than a threshold associated with the product of multiplying the capacity of said main-switch snubber capacitor by the power supply voltage of said DC power supply, so as to turn on said first and second auxiliary switches to generate a resonance between said resonant inductor and said snubber capacitor connected in parallel with said second main switch.

22. A power conversion apparatus including:

at least a pair of main switches composed of serial-connected first and second main switches, one of the ends of said first main switch being connected with the positive side of a DC power supply, one of the ends of said second main switch being connected to the negative side of said DC power supply;

a diode connected in parallel with each of said main switches so as to become reverse biased with respect to said DC power supply;

a main-switch snubber capacitor connected in parallel with each of said main switches;

a load connected with the junction between said pair of main switches; and

a control circuit for forming a -switching signal for controlling the switching operation of said main switches by using a load voltage and/or a load current as an input thereof, wherein said main switches are controllably switched according said switching signal from said control circuit so as to generate an output, said power conversion apparatus comprising;

a first auxiliary resonant circuit including serial-connected first and second auxiliary switches and a resonant inductor connected in series with said second auxiliary switch, said first auxiliary resonant circuit being connected with each of the positive side of said DC power supply and the junction between said pair of main switches;

a diode connected to each of said first and second auxiliary switches so as to become reverse biased with respect to said DC power supply;

a second auxiliary resonant circuit formed by connecting serial-connected third and fourth auxiliary switches between the negative side of said DC power supply and said inductor;

an auxiliary-switch snubber capacitor connected between the junction between said first and second auxiliary switches and the junction between said third and fourth auxiliary switches;

a diode connected to each of said third and fourth auxiliary switches so as to become reverse biased with respect to said DC power supply; and

voltage detecting means for detecting the voltage across each of said main switches and auxiliary switches;

wherein

said control circuit is applied with a voltage signal as an input representing said voltage across each of said main switches and auxiliary switches from said voltage detecting means;

said control circuit being adapted to provide a turn-on signal to said third and fourth auxiliary switches according to said input before a turn-on signal as the switching signal is provided to said second main switch;

said control circuit being adapted to provide the turn-on signal to said third and fourth auxiliary switches when the load current passes through said diode connected in parallel with said first main switch, so as to turn on said third and fourth auxiliary switches to direct the current from said DC power supply to said resonant inductor;

wherein

said control circuit is adapted to output a signal for turning on said second main switch when the voltage across said second main switch goes down approximately to zero through the resonance in a resonance circuit formed by said resonant inductor and said snubber capacitors connected in parallel with said main switches when the current of said resonant inductor goes up approximately to the load current.

23. A power conversion apparatus as defined in claim 22, wherein said control circuit is adapted to provide a turn-off signal to said third auxiliary switch when the charged voltage of said auxiliary-switch snubber capacitor is approximately equal to the voltage of said DC power supply after said second main switch is turned on, and to provide the turn-off signal to said fourth auxiliary switch when the charged voltage of said auxiliary-switch snubber capacitor is approximately equal to zero after said second main switch is turned on, so as to achieve soft-switching of said third and fourth auxiliary switches.

24. A power conversion apparatus as defined in claim 22, wherein said control circuit is adapted to provide a turn-on signal to said first and second auxiliary switches, before a turn-on signal as the switching signal is provided to said first main switch, when said second main switch is in ON-state to allow the load current to pass through said second main switch and said load current is less than a threshold associated with the product of multiplying the capacity of said main-switch snubber capacitor by the power supply voltage of said DC power supply, so as to turn on said first and second auxiliary switches to direct the current from said DC power supply to said resonant inductor;

wherein

said control circuit is adapted to provide a turn-off signal to said second main switch when the current of said resonant inductor goes up approximately to said threshold, so as to turn off said second main switch.

25. A power conversion apparatus as defined in claim 24, wherein said control circuit is adapted to provide the turn-on signal to said first main switch, when said first and second auxiliary switches are in ON-state and the current passing through said resonant inductor is refluxed from said first and second auxiliary switches through said diode connected in parallel with said first main switch.

26. A power conversion apparatus as defined in claim 22, wherein said control circuit is adapted to provide a turn-on signal to said first and second auxiliary switches and provide a turn-off signal to said second main switch, before a turn-on signal as the switching signal is provided to said first main switch, when said second main switch is in ON-state to allow the load current to pass through said second main switch and said load current is less than a threshold associated with the product of multiplying the capacity of said main-switch snubber capacitor by the power supply voltage of said DC power supply, so as to turn on said first and second auxiliary switches to generate a resonance between said resonant inductor and said snubber capacitor connected in parallel with said second main switch.