Power supply circuit with secondary side to primary side communication

1. An electrical circuit for a power supply, the electrical circuit comprising:

a primary-side switch on a primary side of the power supply, the primary-side switch being connected to a primary winding of a flyback transformer;

a secondary-side controller integrated circuit (IC) that is configured to monitor an output voltage on a secondary side of the power supply and to induce a detection signal from a secondary winding of the flyback transformer to the primary winding of the flyback transformer in response to the output voltage of the power supply dropping below a threshold; and

a primary-side controller IC comprising:

a first pin that is connected to the primary-side switch;

the primary-side controller IC configured to control a switching operation of the primary-side switch to regulate the output voltage of the power supply, and to turn on the primary-side switch in response to sensing the detection signal at the first pin;

a high-side switch and a low-side switch that are configured to drive a gate of the primary-side switch through the first pin, wherein the first pin is at high impedance when the high-side switch and the low-side switch are both off, and the primary-side controller IC senses the detection signal while the first pin is at high impedance.

2. The electrical circuit of claim 1 , wherein the detection signal is a resonant signal that is induced by the secondary-side controller IC by turning on a secondary-side switch on the secondary side of the power supply.

3. The electrical circuit of claim 2 , wherein the first pin is connected to a gate of the primary-side switch, and the resonant signal is detected by the primary-side controller IC from the gate of the primary-side switch while the first pin is at high impedance.

4. The electrical circuit of claim 3 , wherein the resonant signal is detected at the first pin through a parasitic capacitance between a drain and the gate of the primary-side switch.

5. The electrical circuit of claim 1 , wherein the flyback transformer further comprises

an auxiliary winding, and the primary-side controller IC samples a sense voltage from the auxiliary winding to sense the output voltage of the power supply.

6. The electrical circuit of claim 1 , wherein the primary-side switch is internal to the primary-side controller IC.

7. The electrical circuit of claim 1 , further comprising:

an auxiliary switch that is connected to a source of the primary-side switch, wherein the source of the primary-side switch is connected to the first pin.

8. The electrical circuit of claim 1 , wherein a gate of the primary-side switch is connected to the first pin.

9. The electrical circuit of claim 1 wherein the primary-side controller IC further comprises:

an oscillator that has a trigger input and a pulse output;

a SR flip-flop that has a Set input, a Reset input, and a SR output, the Set input coupled to the pulse output of the oscillator;

a driver circuit that has a drive input, a high-gate output, and a low-gate output, the drive input coupled to the SR output, the high-gate output coupled to a gate of the high-side switch, and a low-gate output coupled to the gate of the low-side switch;

a voltage sense circuit that has a sense input, a high-gate input, a low-gate input, and a trigger output, the sense input coupled to the first pin, the high-gate input coupled to the high-gate output of the driver circuit, the low-gate input coupled to the low-gate output of the driver circuit, and the trigger output coupled to the oscillator;

and wherein when the primary-side controller IC senses the detection signal, the voltage sense circuit is configured to assert the sense output to the oscillator when the high-gate input and low-gate input are both de-asserted and a detection signal is detected on the sense input; and

the oscillator configured to assert the pulse output responsive to assertion of the trigger input.

10. An electrical circuit for a power supply, the electrical circuit comprising:

a primary-side controller integrated circuit (IC) that is configured to receive a sense voltage that is indicative of an output voltage of the power supply, to control a switching operation of a first switch on a primary side of the power supply, to regulate the output voltage of the power supply, to sample the sense voltage after the first switch is turned off, to place a switch pin at high impedance, and to turn on the first switch in response to detecting a dynamic detection signal on the switch pin while the switch pin is at high impedance,

wherein the dynamic detection signal is induced from a secondary side of the power supply;

the primary-side controller IC comprises:

a high-side switch and a low-side switch that are coupled to the switch pin; and

a driver circuit that is configured to turn on the high-side switch to turn on the first switch, to turn on the low-side switch to pull down the switch pin, and to turn off the high-side switch and the low-side switch to place the switch pin at high impedance.

11. The electrical circuit of claim 10 , further comprising:

a secondary-side controller IC that is configured to control a second switch that is on the secondary side of the power supply.

12. The electrical circuit of claim 11 , wherein the secondary-side controller IC controls the second switch to induce the dynamic detection signal when an output voltage of the power supply drops below a threshold.

13. The electrical circuit of claim 10 , wherein the first switch comprises a metal-oxide semiconductor (MOS) transistor and the dynamic detection signal is a resonant drain voltage of the MOS transistor.

14. The electrical circuit of claim 10 , wherein the first switch comprises a metal-oxide semiconductor (MOS) transistor and the dynamic detection signal is a resonant source voltage of the MOStransistor.

15. The electrical circuit of claim 10 wherein the primary-side controller IC further comprises:

an oscillator that has a trigger input and a pulse output;

a SR flip-flop that has a Set input, a Reset input, and a SR output, the Set input coupled to the pulse output of the oscillator;

a driver circuit that has a drive input, a high-gate output, and a low-gate output, the drive input coupled to the SR output, the high-gate output coupled to a gate of the high-side switch, and a low-gate output coupled to the gate of the low-side switch;

a voltage sense circuit that has a sense input, a high-gate input, a low-gate input, and a trigger output, the sense input coupled to the first pin, the high-gate input coupled to the high-gate output of the driver circuit, the low-gate input coupled to the low-gate output of the driver circuit, and the trigger output coupled to the oscillator;

and wherein when the primary-side controller IC senses the detection signal, the voltage sense circuit is configured to assert the sense output to the oscillator when the high-gate input and low-gate input are both de-asserted and a detection signal is detected on the sense input; and

the oscillator configured to assert the pulse output responsive to assertion of the trigger input.

16. A method of operating an electrical circuit for a power supply, the method comprising:

sampling, from a primary side of the power supply, a sense voltage that is indicative of an output voltage of the power supply;

controlling a switching operation of a primary-side switch on the primary side of the power supply to regulate the output voltage of the power supply based on the sense voltage;

inducing a detection signal from a secondary side of the power supply;

coupling the detection signal from a secondary winding of a flyback transformer to a primary winding of the flyback transformer;

placing a pin of a primary-side controller integrated circuit (IC) at high impedance;

sensing the detection signal at the pin of the primary-side controller IC while the pin is at high impedance, the sensing at the pin through a parasitic capacitance between a drain and a gate of the primary-side switch;

in response to sensing the detection signal, turning on the primary-side switch.

17. The method of claim 16 , wherein the detection signal is a resonant signal.

18. The method of claim 17 , wherein the resonant signal is induced by turning on a secondary-side switch on the secondary side of the power supply in response to detecting the output voltage of the power supply dropping below a threshold.

19. The method of claim 16 ,

wherein placing a pin of a primary-side controller integrated circuit (IC) at high impedance further comprises:

making a high-side switch, coupled to the gate of the primary-side switch, non-conductive;

making a low-side switch, coupled to the gate of the primary-side switch, non-conductive;

wherein sensing the detection signal at the pin of the primary-side controller IC further comprises:

detecting, by a voltage sense circuit, that both the high-side switch and low-side switch are non-conductive; and

sensing, by the voltage sense circuit, the detection signal on the gate of the primary-side switch;

wherein turning on the primary-side switch further comprises commanding, by the voltage sense circuit, assertion of a pulse signal from an oscillator circuit.