NOVEL CONTROL METHOD TO REDUCE SWITCHING LOSS ON MOSFET

• US 20140043863A1
• Filed: 08/13/2012
• Published: 02/13/2014
• Est. Priority Date: 08/13/2012
• Status: Active Grant

• Current Assignee: Flextronics America LLC (Flextronics International Limited)
• Original Assignee: Flextronics America LLC (Flextronics International Limited)
• Inventors: Telefus, Mark, Li, Wei
• Granted Patent: US 9,287,792 B2
• Time in Patent Office: Days
• US Class Current: 363/17
• CPC Class Codes: H02M 1/0058 by employing soft switching techniques, i.e. commutation of transistors when applied voltage is zero or when current flow is zero using an auxiliary actively switched resonant commutation circuit connected to an intermediate DC voltage or between two push-pull branches of an inverter bridge H02M7/4811; in resonant inverters H02M7/4815; in inverters operating from a resonant DC source H02M7/4826
• CPC Class Codes: H02M 3/33576 having at least one active switching element at the secondary side of an isolation transformer
• CPC Class Codes: H02M 3/33592 having a synchronous rectifier circuit or a synchronous freewheeling circuit 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

A method of controlling a switching mode power converter enables zero voltage switching by forcing a voltage across the main switch to zero. This is accomplished by sensing when a current on the secondary side of the power converter drops to zero, or other threshold value, and then generating a negative current through the secondary winding in response. The negative secondary current results in a corresponding discharge current in the primary winding, which reduces the voltage across the main switch. The voltage across the main switch is monitored such that when the voltage reaches zero, or other threshold value, the main switch is turned ON. In this manner, the circuit functions as a bi-directional current circuit where a forward current delivers energy to a load and a reverse current provides control for reducing the voltage across the main switch to enable zero voltage switching.

Claims

1. A method of controlling a switching mode power converter comprising;

a. configuring a switching mode power converter having a transformer, an output circuit coupled to a secondary winding of the transformer, and a switch coupled to a primary winding of the transformer;

b. determining a threshold condition in the output circuit for which the switch is to be switched ON;

c. enabling a negative secondary current through the secondary winding of the transformer in response to determining the threshold condition, wherein the negative secondary current generates a corresponding discharge current through the primary winding, further wherein the discharge current reduces a voltage across the switch;

d. determining when the voltage across the switch drops to a threshold value; and

e. switching the switch ON once the voltage drops to the threshold value.

2. The method of claim 1 wherein the switch comprises a transistor and the discharge current discharges a parasitic capacitance of the transistor, thereby reducing the voltage across the transistor.

3. The method of claim 1 wherein the threshold value is substantially zero volts, thereby enabling zero voltage switching.

4. The method of claim 1 further comprising sensing a value of a secondary current through the secondary winding of the transformer, wherein the threshold condition in the output circuit corresponds to the value of the secondary current changing from a positive value to zero.

5. The method of claim 4 wherein enabling the negative secondary current comprises generating a drive signal to enable the negative secondary current, wherein the drive signal includes a delay after the secondary current changes from the positive value to zero.

6. The method of claim 1 wherein enabling the negative secondary current comprises enabling an alternative current path from an output capacitor in the output circuit to the secondary winding.

7. The method of claim 6 wherein the negative secondary current is generated as discharge from the output capacitor when the alternative current path is enabled.

8. The method of claim 6 wherein enabling the alternative current path comprises switching ON a second switch within the alternate current path.

9. The method of claim 1 wherein the switching mode power converter is configured as one of a flyback-type power converter circuit, a forward-type power converter circuit, a push-pull-type power converter circuit, a half-bridge-type power converter circuit, and a full-bridge-type power converter circuit.

10. A method of controlling a switching mode power converter comprising:

a. configuring a switching mode power converter having a transformer, an output circuit coupled to a secondary winding of the transformer, and a switch coupled to a primary winding of the transformer, wherein the power converter circuit is configured to store energy in the primary winding while the switch is ON and a positive primary current flows through the primary winding, and to deliver the stored energy from the primary winding to the output circuit while the switch is OFF and a positive secondary current flows through the secondary winding;

b. sensing the secondary current to determine when the secondary current drops from positive to zero while the switch is OFF;

c. enabling a negative secondary current through the secondary winding upon determining that the secondary current drops from positive to zero, wherein the negative secondary current generates a corresponding negative primary current through the primary winding, further wherein the negative primary current reduces a voltage across the switch;

d. determining when the voltage across the switch drops to a threshold value; and

e. switching the switch ON once the voltage drops to the threshold value.

11. The method of claim 10 wherein the switch comprises a transistor and the negative primary current discharges a parasitic capacitance of the transistor, thereby reducing the voltage across the transistor.

12. The method of claim 10 wherein the threshold value is substantially zero volts, thereby enabling zero voltage switching.

13. The method of claim 10 wherein enabling the negative secondary current comprises enabling an alternative current path from an output capacitor in the output circuit to the secondary winding.

14. The method of claim 13 wherein the negative secondary current is generated as discharge from the output capacitor when the alternative current path is enabled.

15. The method of claim 13 wherein enabling the alternative current path comprises switching ON a second switch within the alternate current path.

16. The method of claim 10 wherein the power converter circuit is configured as a flyback-type converter circuit.

17. The method of claim 10 wherein enabling the negative secondary current comprises generating a drive signal to enable the negative secondary current, wherein the drive signal includes a delay after the secondary current drops from the positive value to zero.

18. A switching mode power converter comprising:

a. a transformer having a primary winding coupled to an input supply voltage and a secondary winding;

b. a switch coupled in series to the primary winding;

c. a first controller coupled to the switch, wherein the first controller is configured to turn the switch ON and OFF; and

d. an output circuit coupled to the secondary winding, wherein the output circuit comprises a second controller configured to determine a threshold condition in the output circuit for which the switch is to be switched ON, the second controller is further configured to enable a negative secondary current through the secondary winding of the transformer in response to determining the threshold condition, wherein the negative secondary current generates a corresponding discharge current through the primary winding that reduces a voltage across the switch,

wherein the first controller is further configured to determine when the voltage across the switch drops to a threshold value, and to switch ON the switch once the voltage drops to the threshold value.

19. The power converter of claim 18 wherein the switch comprises a transistor and the discharge current discharges a parasitic capacitance of the transistor, thereby reducing the voltage across the transistor.

20. The power converter of claim 18 wherein the threshold value is substantially zero volts, thereby enabling zero voltage switching.

21. The power converter of claim 18 wherein the output circuit further comprises a sensing circuit coupled to the secondary winding and to the second controller, wherein the second controller is configured to determine a value of a secondary current through the secondary winding of the transformer according to data sensed by the sensing circuit, wherein the threshold condition in the output circuit corresponds to the value of the secondary current changing from a positive value to zero.

22. The power converter of claim 21 wherein the sensing circuit comprises a voltage divider circuit.

23. The power converter of claim 18 wherein the output circuit further comprises a diode coupled in series to the secondary winding of the transformer and an output capacitor coupled in series to the diode, wherein the diode is configured to enable current flow from the secondary winding to the output capacitor when forward-biased.

24. The power converter of claim 23 wherein the output circuit further comprises a second switch coupled in parallel to the diode, further wherein the second controller is coupled to the second switch, the second controller is configured to turn the second switch ON and OFF.

25. The power converter of claim 24 wherein the second controller is further configured to generate a drive signal for the second switch, wherein the drive signal includes a delay after the threshold condition in the output circuit is determined.

26. The power converter of claim 24 wherein when the second switch is ON, an alternative current path is formed between the output capacitor and the secondary winding of the transformer, further wherein the negative secondary current flows from the output capacitor to the secondary winding via the alternative current path.

27. The power converter of claim 26 wherein the negative secondary current is generated as discharge from the output capacitor when the alternative current path is formed.

28. The power converter of claim 26 wherein the output circuit is configured as a bi-directional current flow circuit, wherein a first current flow direction corresponds to positive secondary current from the secondary winding to the output capacitor and to an output of the output circuit to deliver power, and a second current flow direction corresponds to negative secondary current from the output capacitor to the secondary winding to provide control for switching the switch coupled to the primary winding.

29. The power converter of claim 24 wherein the second switch comprises a transistor.

30. The power converter of claim 18 further comprising a sensing circuit coupled to the switch and to the first controller, wherein the first controller is configured to determine the voltage across the switch according to data sensed by the sensing circuit.

31. The power converter of claim 18 wherein the power converter is configured as one of a flyback-type power converter circuit, a forward-type power converter circuit, a push-pull-type power converter circuit, a half-bridge-type power converter circuit, and a full-bridge-type power converter circuit.

32. A switching mode power converter comprising:

a. a transformer having a primary winding coupled to an input supply voltage and a secondary winding;

b. a first switch coupled in series to the primary winding;

c. a first controller coupled to the first switch, wherein the first controller is configured to turn the first switch ON and OFF;

d. a diode coupled in series to the secondary winding;

e. a second switch coupled in parallel to the diode;

f. a second controller coupled to the second switch, wherein the second controller is configured to turn the second switch ON and OFF;

g. an output capacitor coupled to the diode and to the second switch; and

h. a sensing circuit coupled to the secondary winding and to the second controller,

wherein the second controller is configured to determine a threshold condition according to data sensed by the sensing circuit and to switch ON the second switch when the threshold condition is achieved thereby forming an alternative current path for a negative secondary current to flow from the output capacitor to the secondary winding, the negative secondary current generates a corresponding discharge current through the primary winding that reduces a voltage across the first switch, wherein the first controller is further configured to determine when the voltage across the first switch drops to a threshold value, and to switch ON the first switch once the voltage drops to the threshold value.