Control circuits and methods for switching mode power supplies

1. A switching mode power supply (SMPS), comprising:

a transformer having:

a primary winding;

a secondary winding for providing an output voltage of the SMPS; and

an auxiliary winding;

a power switch coupled to the primary winding;

a first control circuit coupled to the secondary winding, the first control circuit being configured to provide a first electrical signal to the secondary winding when the output voltage of the SMPS is less than a reference voltage, whereupon a second electrical signal is induced in the auxiliary winding; and

a second control circuit coupled to the auxiliary winding and the power switch, the second control circuit being configured to provide a control signal to the power switch in response to the second electrical signal; and

a rectifying circuit including a diode and an output capacitor, wherein the first control circuit comprises:

a first terminal directly connected to the cathode of the diode and the output capacitor to receive the output voltage of the SMPS;

a second terminal for directly connecting to a secondary side ground of the switching mode power supply; and

a third terminal directly connected to the anode of the diode and the secondary winding;

wherein the first control circuit is configured to provide a conducting path from the first terminal to the third terminal to transfer electric charges from the output capacitor to the second winding when the output voltage is less than the reference voltage.

2. The switching mode power supply of claim 1 wherein the first control circuit comprises:

a voltage divider coupled to the output voltage and configured to generated an attenuated voltage;

a first comparator having:

a first input terminal coupled to the attenuated voltage;

a second input terminal coupled to the reference voltage; and

an output terminal;

a pulse generator coupled to the comparator; and

a switch coupled to the pulse generator;

wherein the pulse generator is configured to turn on the switch when the attenuated voltage is less than the reference voltage.

3. The switching mode power supply of claim 2 wherein the switch is a PMOS or an NMOS transistor.

4. The switching mode power supply of claim 2 wherein the switch is connected between a terminal of the secondary winding and an output terminal of the SMPS.

5. The switching mode power supply of claim 1 wherein the secondary winding is in a discontinuous state substantially free from current flow, when the output voltage of the SMPS is less than a reference voltage.

6. The switching mode power supply of claim 1 wherein the second control circuit comprises:

a feedback terminal coupled to the auxiliary winding through a coupling network, the feedback terminal being configured to receive a feedback signal from the coupling network;

a driver unit configured to drive the power switch;

a sense current input terminal coupled to a terminal of the power switch;

a control unit coupled to the sense current input terminal and the feedback terminal; and

a dynamic event detection circuit coupled to the feedback terminal and configured to detect the electrical signal.

7. The switching mode power supply of claim 6 wherein the coupling network is a voltage divider network.

8. The switching mode power supply of claim 6 wherein the dynamic event detection circuit is configured to activate the driver unit for turning on the power switch for a predetermined time period upon detecting the electrical signal.

9. The switching mode power supply of claim 8 wherein the dynamic event detection circuit is configured to detect a ringing waveform in the auxiliary winding.

10. A three-terminal device for controlling a switching mode power supply (SMPS), the three-terminal device having only three terminals, comprising

a first terminal for directly connecting to the output of the switching mode power supply for providing power to the three-terminal device;

a second terminal for directly connecting to a secondary side ground of the switching mode power supply; and

a third terminal for directly connecting to a secondary winding;

wherein, in a discontinuous mode of the SMPS, the three-terminal device is configured to provide a conducting path from the first terminal to the third terminal and to provide an electric signal to a secondary winding in response to an output condition;

wherein, in a conduction mode of the SMPS, the three-terminal device is configured to provide a conduction path for the current in the secondary winding.

11. The three-terminal device of claim 10 , further comprising:

a MOSFET transistor having a drain for coupling to one terminal of the secondary winding and a source for coupling to the ground terminal or the voltage output terminal of the SMPS; and

a control unit configured to turn on and off the MOSFET.

12. The three-terminal device of claim 11 wherein the MOSFET is a PMOS transistor having a bulk substrate coupled to the source;

wherein the drain acts as a cathode of a rectifying element and the bulk substrate or source acts as an anode of the rectifying element.

13. The three-terminal device of claim 11 wherein the control unit comprises:

a first control circuit configured to turn on the MOSFET transistor for a time period when the output voltage is equal to or less than a reference voltage;

a second control circuit configured to turn on the MOSFET transistor during a conduction time of the second winding; and

a logic gate having a first input coupled to a first output of the first control circuit and a second input coupled to a second output of the second control circuit, and a first output coupled to a gate of the MOSFET transistor.

14. The three-terminal device of claim 13 wherein the first control circuit comprises:

a voltage divider coupled to a input terminal and configured to produce an attenuated voltage;

a first comparator having a third input coupled to the attenuated voltage, a fourth input coupled to the reference voltage, and a third output; and

a pulse generator coupled to the third output and configured to generate at least one pulse based on a comparison result of the first comparator;

wherein the at least one pulse turns on the MOSFET via the logic gate.

15. The three-terminal device of claim 13 wherein the second control circuit comprises:

an offset voltage source; and

a second comparator having a fifth input coupled to the drain of the MOSFET via the offset voltage source and a sixth input coupled to the source or bulk substrate of the MOSFET;

wherein the second comparator generates a control signal to turn on the MOSFET via the logic gate.