Adaptive dead time control apparatus and method for switching power converters

1. An apparatus comprising:

a secondary synchronous rectifier coupled to a secondary side of a power converter comprising a forward switch and a freewheeling switch; and

a secondary gate drive controller coupled to a transformer winding of the power converter to provide a forward gate drive signal for the forward switch and a freewheeling gate drive signal for the freewheeling switch, wherein the secondary gate drive controller generates a dead time between the forward gate drive signal and the freewheeling gate drive signal, and wherein the secondary gate drive controller generates a turn-off threshold of the forward switch, and wherein the turn-off threshold is generated by applying a voltage across the transformer winding to a first capacitor through a signal shaping circuit and a half wave rectifier, wherein the signal shaping circuit comprises a second capacitor, a third capacitor, a first resistor between the second capacitor and the third capacitor and a first diode in parallel with the third capacitor, and wherein an input of the signal shaping circuit is connected to the transformer winding, an output of the signal shaping circuit is connected to a diode of the half wave rectifier, and an output of the half wave rectifier is coupled to the first capacitor, and wherein the first capacitor is discharged by a resistor divider connected in parallel with the first capacitor, and wherein, after passing the signal shaping circuit, the voltage across the transformer winding is directly compared with the turn-off threshold to determine a turn-off of the forward switch.

2. The apparatus of claim 1 , wherein the transformer winding is a power transformer winding coupled to the forward switch and the freewheeling switch.

3. The apparatus of claim 1 , wherein the transformer winding is an auxiliary transformer winding magnetically coupled to a transformer of the power converter.

4. The apparatus of claim 1 , wherein the signal shaping circuit is coupled between the transformer winding and the secondary gate drive controller.

5. A system comprising:

a transformer of a power converter having a primary winding coupled to a dc power source through a primary switch and a secondary winding coupled to an output filter;

a secondary synchronous rectifier coupled between the secondary winding and the output filter comprising a forward switch and a freewheeling switch; and

a secondary gate drive controller coupled to a transformer winding to provide a forward gate drive signal for the forward switch and a freewheeling gate drive signal for the freewheeling switch, wherein the secondary gate drive controller generates a dead time between the forward gate drive signal and the freewheeling gate drive signal, and wherein the secondary gate drive controller generates a turn-off threshold of the forward switch, and wherein the turn-off threshold is generated by applying a voltage across the transformer winding to a first capacitor through a signal shaping circuit and a half wave rectifier, wherein the signal shaping circuit comprises a second capacitor, a third capacitor, a first resistor connected between the second capacitor and the third capacitor and a first diode in parallel with the third capacitor, and wherein an input of the signal shaping circuit is connected to the transformer winding, an output of the signal shaping circuit is connected to a diode of the half wave rectifier, and an output of the half wave rectifier is coupled to the first capacitor, and wherein the diode of the half wave rectifier is connected to the first capacitor through a resistor, and a resistor divider is connected in parallel with the first capacitor, and wherein a midpoint of the resistor divider is configured to generate the turn-off threshold, and wherein a voltage at the output of the signal shaping circuit is directly compared with the turn-off threshold to determine a turn-off of the forward switch.

6. The system of claim 5 , wherein the power converter is selected from the group consisting of a forward converter, a flyback converter, a half bridge converter, a full bridge converter and a push-pull converter.

7. The system of claim 5 , wherein the power converter adaptively adjusts the dead time between the forward gate drive signal and the freewheeling gate drive signal.

8. The system of claim 5 , further comprising an active reset device connected in parallel with the primary switch, wherein the active reset device comprises:

an auxiliary switch; and

a reset capacitor connected in series with the auxiliary switch.

9. The system of claim 8 , further comprising a primary side controller configured to:

generate a primary switch gate drive signal; and

generate an auxiliary switch gate drive signal, wherein the auxiliary switch gate drive signal is approximately complementary to the primary switch gate drive signal.

10. A method comprising:

detecting a switching pulse from a transforming winding of a power converter;

generating a forward switch gate drive signal for the power converter, wherein a turn-off threshold of the forward switch gate drive signal is generated by applying the switching pulse of the transformer winding to a capacitor through a signal shaping circuit and a half wave rectifier, wherein an input of the signal shaping circuit is coupled to the transformer winding, an output of the signal shaping circuit is coupled to an input of the half wave rectifier, and an output of the half wave rectifier is coupled to the capacitor;

generating a freewheeling switch gate drive signal complementary to the forward switch gate drive signal for the power converter;

generating a dead time between the forward switch gate drive signal and the freewheeling switch gate drive signal;

generating a first blanking period after a leading edge of a drain-to-source voltage of a freewheeling switch; and

generating a second blanking period after a falling edge of an internal PWM signal.

11. The method of claim 10 , further comprising:

comparing a detected signal from the transformer winding with a first predetermined threshold;

generating a leading edge of the internal PWM signal after a first delay;

setting the freewheeling gate drive signal to a logic low state; and

setting the forward switch gate drive signal to a logic high state after a second delay.

12. The method of claim 10 , further comprising:

generating an internal waveform from a detected signal from the transformer winding using a rectifier;

comparing the detected signal from the transformer winding with the internal waveform;

generating the falling edge of the internal PWM signal after a third delay;

setting the forward gate drive signal to a logic low state;

comparing the drain-to-source voltage of the freewheeling switch with a second predetermined threshold; and

setting the freewheeling switch gate drive signal to a logic high state after a fourth delay.

13. The method of claim 10 , further comprising:

detecting the switching pulse from a transforming power winding coupled to a forward switch and a freewheeling switch of the power converter.

14. The method of claim 10 , further comprising:

detecting the switching pulse from a transforming auxiliary winding magnetically coupled to a transformer of the power converter.

15. The method of claim 10 , further comprising:

configuring a secondary side gate drive controller such that the power converter adaptively adjusts the dead time between the forward switch gate drive signal and the freewheeling switch gate drive signal.

16. The method of claim 10 , further comprising:

generating a first predetermined threshold indicating a leading edge of a detected signal from the transformer winding;

generating a second predetermined threshold indicating a falling edge of the detected signal from the transformer winding; and

generating a third predetermined threshold indicating a falling edge of a drain-to-source voltage of the freewheeling switch.

17. The method of claim 16 , further comprising:

adjusting the first predetermined threshold by using a resistor coupled to a secondary gate drive controller.