Control circuit and method therefor
In one embodiment, a power supply controller, or alternately a semiconductor device having a power supply controller, may have a first circuit configured to form a sense signal that is representative of a signal from an auxiliary winding of a transformer. A feedback circuit may be configured to allow the sense signal to increase in response to a turn-off of the power switch, to subsequently detect a second increase of the sense signal prior to subsequently turning on the power switch, and to form a feedback signal as a value of the sense signal responsively to the second increase of the sense signal.
1. A control circuit for a power supply controller circuit comprising:
a PWM circuit of the control circuit configured to control a power switch to regulate an output voltage formed from a secondary winding of a transformer;
a feedback circuit configured to receive a sense signal from an auxiliary winding of the transformer;
a first detection circuit having a delay circuit to form a delay signal that is representative of the sense signal delayed by a time interval, a first comparator configured to compare the delay signal to a first signal that is representative of the sense signal offset by first value;
a second detection circuit having a storage circuit that stores a first value of the sense signal as a stored signal responsively to the power switch being disabled, the storage circuit having an offset circuit that offsets the stored signal by an offset value to form a second signal, the second detection circuit having a second comparator configured to compare the sense signal to the second signal;
the feedback circuit configured to store a second value of the sense signal as a feedback signal that is representative of the output voltage, wherein the feedback circuit stores the second value of the sense signal in response to the first comparator compare operation and also stores the second value of the sense signal in response to the second comparator compare operation; and
the PWM circuit configured to adjust an on-time of a subsequent enabling of the power switch according to a value of the feedback signal.
2. The control circuit of claim 1 wherein a synchronous rectifier is configured to be coupled to the secondary winding and wherein the first detection circuit is configured to detect the synchronous rectifier turning off no later than a secondary current becoming substantially zero.
3. The control circuit of claim 1 wherein the first detection circuit includes an offset circuit configured to add the first value to the sense signal to form the first signal.
4. The control circuit of claim 3 wherein the first comparator has a first input coupled to receive the first signal and a second input coupled to receive the delay signal.
5. The control circuit of claim 1 wherein the first detection circuit includes a flip-flop that is reset by an output of the first comparator.
6. The control circuit of claim 1 wherein a synchronous rectifier is configured to be coupled to the secondary winding and wherein the second detection circuit is configured to detect the synchronous rectifier turning off prior to a secondary current becoming substantially zero.
7. The control circuit of claim 1 wherein the storage circuit includes a capacitor that is selectively coupled to receive the sense signal in response to disabling the power switch.
8. The control circuit of claim 7 wherein the storage circuit includes a switch that couples the capacitor to receive the sense signal in response to disabling the power switch.
9. The control circuit of claim 7 wherein the storage circuit includes an offset circuit configured to add the offset value to the stored signal to form the second signal.
10. The control circuit of claim 1 wherein the second detection circuit includes a flip-flop that is reset by an output of the second comparator.
11. A control circuit comprising:
a PWM circuit configured to control a power switch to regulate an output voltage formed from a secondary winding of a transformer wherein a synchronous rectifier is configured to be coupled to the secondary winding;
a feedback circuit configured to receive a sense signal that is representative of the output voltage;
a first detection circuit configured to detect a decrease in the sense signal during a time interval that the power switch is disabled;
a second detection circuit configured to detect the synchronous rectifier turning off prior to a secondary current becoming substantially zero during the time interval that the power switch is disabled;
the feedback circuit configured to store the sense signal as a feedback signal that is representative of the output voltage in response to the first detection circuit detecting the decrease in the sense signal and the feedback circuit configured to store the sense signal in response to the second detection circuit detecting the synchronous rectifier turning off prior to the secondary current becoming substantially zero; and
the control circuit configured to adjust an on-time of a subsequent enabling of the power switch according to a value of the feedback signal.
12. The control circuit of claim 11 wherein the second detection circuit includes a first storage circuit to store the sense signal as a stored signal responsively to the power switch being disabled, a first offset circuit configured to add a first offset value to the stored signal to form an first offset signal, a first comparator configured to compare the sense signal to the first offset signal.
13. The control circuit of claim 12 wherein the first detection circuit includes a second comparator to detect the decrease in the sense signal after the power switch is disabled, the first detection circuit having a delay circuit to form a delayed signal that is representative of the sense signal delayed by a delay time interval wherein the first comparator compares the delayed signal to a first signal that is representative of the sense signal.
14. The control circuit of claim 13 wherein the first detection circuit includes a second offset circuit configured to add a second offset value to the sense signal to form the first signal.
15. The control circuit of claim 11 wherein the feedback circuit is configured to store the sense signal on a capacitor as a stored value in response to the second detection circuit detecting the synchronous rectifier turning off prior to the secondary current becoming substantially zero, and to subsequently transfer the stored value to a second capacitor as the feedback signal.
16. A method of forming a control circuit for a power supply controller comprising:
configuring a PWM circuit of the control circuit to control a power switch to regulate an output voltage formed from a secondary winding of a transformer wherein a synchronous rectifier is configured to be coupled to the secondary winding;
configuring a feedback circuit to receive a sense signal that includes information relating to the output voltage; and
configuring the feedback circuit to store a value of the sense signal as a feedback signal in response to detecting a change in the sense signal wherein the change is formed in response to disabling the synchronous rectifier.
17. The method of claim 16 including configuring the PWM circuit to adjust an on-time of a subsequent enabling of the power switch according to a value of the feedback signal.
18. The method of claim 16 including configuring the feedback circuit to store the value of the sense signal responsively to detecting an increase of the sense signal wherein the increase is in response to disabling the synchronous rectifier.
19. The method of claim 18 including configuring the feedback circuit to store the value of the sense signal responsively to detecting a decrease of the sense signal wherein the decrease is in response to disabling the synchronous rectifier.
20. The method of claim 18 including configuring a comparator to detect the change in the sense signal.