Circuits and methods for reducing output voltage overshoot of switch mode power supply
A switching mode power supply (SMPS) includes a transformer having a primary winding for coupling to an input power source, a secondary winding for providing an output voltage of the SMPS, and an auxiliary winding. The SMPS also has a power transistor coupled to the primary winding and a primary side control circuit coupled to the auxiliary winding and the power transistor. The primary side control circuit is configured to regulate the output of the SMPS by controlling the power switch in response to a feedback voltage signal that is representative of an output of the SMPS. The SMPs also has a secondary-side control circuit coupled to the secondary winding and being configured to cause the output voltage of the SMPS to discharge when the output voltage of the SMPS is higher than a first reference voltage.
1. A switching mode power supply (SMPS), comprising:
a transformer having:
a primary winding for coupling to an input power source;
a secondary winding for providing an output voltage of the SMPS; and
an auxiliary winding;
a power transistor coupled to the primary winding;
a primary side control circuit coupled to the auxiliary winding and the power transistor, the primary side control circuit being configured to regulate the output of the SMPS by controlling the power switch in response to a feedback voltage signal that is representative of an output of the SMPS; and
a secondary-side control circuit coupled to the secondary winding and being configured to cause the output voltage of the SMPS to discharge when the output voltage of the SMPS is higher than a first overvoltage reference voltage;
wherein the secondary-side control circuit comprises a discharge control circuit having a power supply terminal coupled to the output voltage of the SMPS, the discharge control circuit being configured to issue a discharge signal;
wherein the discharge control circuit comprises a comparator having a negative input coupled to the second reference voltage and a positive input coupled to a third reference voltage that is lower than the second reference voltage, the comparator also having a power supply terminal coupled to the output voltage of the SMPS.
2. The switching mode power supply of claim 1 , wherein the discharge control circuit is configured to provide a signal to cause the primary side control circuit to temporarily stop regulating the output of the SMPS when discharge is occurring on the secondary side.
3. The switching mode power supply of claim 1 , wherein the discharge control circuit further comprises a first transistor coupling the power supply terminal of the discharge control circuit to a ground terminal, the first transistor also having a control terminal coupled to an output of the comparator.
4. The switching mode power supply of claim 1 , wherein the discharge control circuit further comprises a second transistor coupling the power supply terminal of the discharge control circuit to the output of the comparator.
5. The switching mode power supply of claim 4 , wherein the second transistor of the discharge control circuit is controlled by a first and a second flip-flops, the first flip-flop being coupled to a signal indicating that the output voltage of the SMPS is higher than the first overvoltage reference voltage and the second flip-flop being coupled to a delay signal.
6. The switching mode power supply of claim 1 , wherein the discharge control circuit is configured such that the output voltage prevented from discharging to below the second reference voltage.
7. The switching mode power supply of claim 1 , wherein the primary side control circuit is a pulse width modulated (PWM) controller.
8. The switching mode power supply of claim 1 , wherein the primary side control circuit is a pulse frequency modulated (PFM) controller.
9. A three-terminal device for controlling a switching mode power supply (SMPS), comprising
a first terminal for coupling to the output of the switching mode power supply;
a second terminal for coupling to a secondary side ground of the SMPS;
a third terminal for coupling to a secondary winding of the SMPS;
a discharge control circuit coupled to the first terminal, the discharge control circuit being configured to issue a discharge signal to cause a voltage at the first terminal to discharge to the second terminal when the voltage at the first terminal is higher than a first overvoltage reference voltage;
wherein the discharge control circuit includes a comparator having a negative input coupled to the second reference voltage and a positive input coupled to a third reference voltage that is lower than the second reference voltage, the comparator also having a power supply terminal coupled to the output voltage of the SMPS.
10. The device of claim 9 , wherein the discharge control circuit further comprising a first transistor coupling the first terminal to the second terminal, the first transistor also having a control terminal coupled to an output of the comparator.
11. The device of claim 9 , wherein the discharge control circuit further comprises a second transistor coupling the first terminal to the output of the comparator.
12. The device of claim 11 , wherein the second transistor is controlled by a first and a second flip-flops, the first flip-flop being coupled to a signal indicating that the voltage at the first terminal is higher than the first overvoltne reference voltage and the second flip-flop being coupled to a delay signal.
13. A method of controlling the output of a switching mode power supply (SMPS), the SMPS including a transformer having a primary winding coupled to a power switch, a secondary winding for providing an output, and an auxiliary winding, the method comprising:
on the primary side of the transformer, regulating the output of the SMPS by controlling the power switch in response to a feedback voltage signal that is representative of an output of the SMPS; and
on the secondary side of the transformer, causing the output voltage of the SMPS to discharge when the output voltage of the SMPS is higher than a first overvoltage reference voltage;
generating a pulsed discharge signal to cause the discharge;
wherein generating the pulsed discharge signal comprises detection when the output voltage of the SMPS is higher than the first overvoltage reference voltage and combining the result of the detection with a delay signal and a clock signal.
14. The method of claim 13 , further comprising providing a discharge control circuit that includes a comparator having a negative input coupled to the second reference voltage and a positive input coupled to a third reference voltage that is lower than the second reference voltage, the comparator also having a power supply terminal coupled to the output voltage of the SMPS.
15. The method of claim 14 , the discharge control circuit further comprising a first transistor coupling the power supply terminal of the discharge control terminal to a ground terminal, the first transistor also having a control terminal coupled to an output of the comparator.
16. The method of claim 13 , further comprising temporarily halting the regulating of the output of the SMPS from the primary side, when discharge is occurring on the secondary side.