Secondary side controller and method therefor

1. A secondary side power supply controller for regulating a secondary voltage of a secondary winding of a power supply comprising:

a synchronizing circuit configured to receive an input signal representative of a primary side switching drive signal used to control a power switch of a primary side of the power supply and responsively provide a sync signal having an active state representative of the primary side switching drive signal;

a secondary side switching control section configured to receive a feedback signal that is representative of the secondary voltage and receive the sync signal and responsively form a switching control signal operable to control a secondary side power switch for regulating the secondary voltage, the secondary side power switch coupled in series with the secondary winding; and

control logic configured to receive the sync signal and the switching control signal and responsively form a switching drive signal operable to control the secondary side power switch wherein the control logic is configured to enable the secondary side power switch independently of a state of the primary side switching drive signal at least after the secondary voltage is less than a first value.

2. The secondary side power supply controller of claim 1 wherein the control logic is also configured to disable the secondary side power switch independently of the primary side switching drive signal at least after the secondary voltage is greater than a second value, and configured to operate the secondary side power switch responsively to the feedback signal at least after the secondary voltage is between the first value and the second value.

3. The secondary side power supply controller of claim 2 wherein the control logic also configured to regulate the secondary voltage substantially synchronously with the primary side switching drive signal at least after the secondary voltage is between the first value and the second value.

4. The secondary side power supply controller of claim 3 wherein the control logic also configured to regulate the secondary voltage substantially synchronously with the primary side switching drive signal at least after the secondary voltage is between the first value and the second value includes the control logic configured to forma an active state of the switching control signal substantially synchronously with the primary side switching drive signal.

5. The secondary side power supply controller of claim 1 wherein the control logic includes a first latch having a set input coupled to receive the sync signal from the synchronizing circuit and a first edge detector coupled to receive the switching control signal and responsively reset the first latch responsively to a negative transition of the switching control signal when the sync signal is disabled.

6. The secondary side power supply controller of claim 5 wherein the control logic includes a second latch having a set input coupled to receive the switching control signal and a second edge detector coupled to receive the sync signal from the synchronizing circuit and responsively reset the second latch responsively to a negative transition of the sync signal when the switching control signal is disabled.

7. The secondary side power supply controller of claim 1 further including the control logic configured to enable the secondary side power switch with a substantially one hundred per cent duty cycle at least after the secondary voltage is less than the first value.

8. The secondary side power supply controller of claim 7 wherein the control logic overrides the sync signal at least after the secondary voltage is less than the first value.

9. A method of forming a secondary side controller of a power supply system comprising:

forming the secondary side controller to enable a secondary side power switch independently of a state of a primary side power switch at least after a secondary voltage of a secondary winding of the power supply system is no greater than a first value.

10. The method of claim 9 wherein forming the secondary side controller to enable the secondary side power switch independently of the state of the primary side power switch includes configuring the secondary side controller to enable the secondary side power switch with substantially one hundred percent duty cycle.

11. The method of claim 9 wherein forming the secondary side controller to enable the secondary side power switch independently of the state of the primary side power switch includes configuring the secondary side controller to form a sync signal that is representative of a primary drive control signal used to control the primary side power switch and to override the sync signal responsively to the secondary voltage being no greater than the first value.

12. The method of claim 11 further including configuring the secondary side controller to enable secondary side power switch responsively to the sync signal responsively to the secondary voltage being greater than the first value.

13. The method of claim 9 further including configuring the secondary side controller to enable the secondary side power switch substantially synchronously with the primary side power switch responsively to the secondary voltage being greater than the first value.

14. The method of claim 9 further including disabling the secondary side power switch independently of the state of the primary side power switch responsively to the secondary voltage being greater than a second value.

15. The method of claim 14 further including configuring the secondary side controller to enable the secondary side power switch substantially synchronously with the primary side power switch and to disable the secondary side power switch responsively to a value of the secondary voltage that is between the first value and the second value.

16. The method of claim 15 wherein configuring the secondary side controller to enable the secondary side power switch substantially synchronously with the primary side power switch includes coupling an output of a synchronizing circuit to enable the secondary side power switch without a delay from a capacitor used to generate a ramp signal used to control an inactive state of the secondary side power switch.

17. A secondary side power supply controller for regulating a secondary voltage of a secondary winding of a power supply comprising:

a primary side switching control section having a ramp generator, an error amplifier and a PWM comparator;

a secondary side synchronizing circuit configured to receive a signal from the secondary winding and responsively form a sync signal that is representative of a primary side switching drive signal used to control a primary winding of the power supply; and

a secondary side control section configured to form a switching drive signal operable to control a secondary side power switch, the secondary side control section configured to form an active state of the switching drive signal independently of a state of the sync signal responsively to the secondary voltage being no greater than a first value.

18. The secondary side power supply controller of claim 17 wherein the secondary side power supply controller is configured to form the active state of the switching drive signal with a substantially one hundred percent duty cycle.

19. The secondary side power supply controller of claim 17 further including the secondary side power supply controller configured to form the active state of the switching drive signal substantially synchronously with the primary side switching drive signal at least after the secondary voltage is greater than the first value.

20. The secondary side power supply controller of claim 19 wherein the sync signal is coupled to form the active state of the switching drive signal without a capacitive delay from a capacitor used by the ramp generator.