Soft-burst circuit for switched-mode power supplies

1. A soft-burst circuit for a switched-mode power supply, the circuit comprising:

an integrated circuit configured to control switching of an output transistor to deliver a regulated output voltage to a load, the integrated circuit having a control pin for receiving a control signal for setting an operating frequency of the switched-mode power supply;

a capacitor coupled to the control pin through a first resistor, the capacitor having a first terminal coupled in series to a first terminal of the first resistor;

a first transistor having a first terminal coupled to receive the control signal; and

a second transistor having a first terminal coupled to a second terminal of the first transistor, the first terminal of the second transistor being coupled to a reference signal, the second transistor having a second terminal coupled to the first terminal of the capacitor, the second transistor being configured to discharge the capacitor based on the control signal, a switching of the second transistor being controlled by the first transistor.

2. The circuit of claim 1 wherein the first transistor and the second transistor comprise bipolar transistors, a base of the first transistor is coupled to the control signal, a collector of the first transistor is coupled to a base of the second transistor, and a collector of the second transistor is coupled to the first terminal of the capacitor.

3. The circuit of claim 1 wherein the integrated circuit controls switching of the output transistor by pulse frequency modulation (PFM).

4. The circuit of claim 1 wherein a second terminal of the capacitor is coupled to ground.

5. The circuit of claim 1 wherein the integrated circuit is configured to synchronously switch the output transistor and another output transistor.

6. The circuit of claim 5 wherein the output transistor comprises metal oxide semiconductor field effect transistors.

7. The circuit of claim 1 wherein the control signal is indicative of a level of the output voltage.

8. A method of controlling a switched-mode power supply, the method comprising:

generating an output voltage of a switched-mode power supply;

monitoring the output voltage to generate a frequency control signal that controls an operating frequency of the switched-mode power supply;

charging a storage device using the frequency control signal; and

after the charging of the storage device, discharging the storage device when the frequency control signal reduces to a particular level, wherein the particular level is a voltage level that turns OFF a first transistor for turning ON a second transistor that is coupled across the storage device.

9. The method of claim 8 wherein the storage device comprises a capacitor.

10. The method of claim 8 wherein the frequency control signal comprises a voltage corresponding to an impedance of an opto-coupler transistor.

11. The method of claim 8 further comprising:

using the frequency control signal to enable or disable protection features of an integrated circuit configured to control the operating frequency of the switched-mode power supply.

12. A switched-mode power supply comprising:

an output transistor coupled to a primary winding of a transformer;

a control circuit configured to control switching of the output transistor to deliver a regulated output voltage to a load of the switched-mode power supply;

a storage device coupled to a pin of the control circuit, the pin of the control circuit being configured to receive a control signal for adjusting an operating frequency of the switched-mode power supply, wherein the storage device comprises a capacitor; and

a soft-burst circuit configured to discharge the storage device based on the control signal, wherein the soft bust circuit comprises:

a first transistor having a first terminal coupled to receive the control signal; and

a second transistor having a first terminal coupled to a second terminal of the first transistor, the first terminal of the second transistor being coupled to a reference signal, the second transistor having a second terminal coupled to a first terminal of the capacitor, the second transistor being configured to discharge the capacitor based on the control signal, a switching of the second transistor being controlled by the first transistor.

13. The switched-mode power supply of claim 12 wherein the first and second transistors comprise bipolar transistors, a base of the first transistor is coupled to the control signal, a collector of the first transistor is coupled to a base of the second transistor, and a collector of the second transistor is coupled to the first terminal of the capacitor.

14. The switched-mode power supply of claim 12 wherein the control circuit synchronously switches the output transistor and another output transistor.

15. The switched-mode power supply of claim 14 wherein the output transistor and the other output transistor comprise metal oxide semiconductor field effect transistors.

16. The switched-mode power supply of claim 12 wherein the control signal is indicative of a level of the output voltage.

17. The switched-mode power supply of claim 12 wherein the switched-mode power supply comprises an LLC resonant half-bridge converter.