Control circuit and system for switch mode power supply
A switch-mode power supply (SMPS) includes a transformer having a primary winding coupled to a power switch, a secondary winding for providing an output of the power supply, and a controller. The controller includes a first input terminal for receiving a current sensing signal related to a current in the primary winding, a second input terminal for receiving a feedback signal related to a current in the secondary winding, and an output terminal for providing a control signal to turn on and off the power switch. When the feedback signal is higher than a first reference voltage, the controller is configured to cause the SMPS to maintain a constant output current at a first current magnitude. When the feedback signal is lower than the first reference voltage, the controller is configured to cause the SMPS to provide a second output current at a second current magnitude higher than the first current magnitude.
1. A controller for a switched mode power supply (SMPS), wherein the SMPS includes a transformer having a primary winding coupled to a power switch and a secondary winding for providing an output of the power supply, the controller comprising:
a first input terminal for receiving a current sensing signal, which is related to a current in the primary winding;
a second input terminal for receiving a feedback signal related to a current in the secondary winding; and
an output terminal for providing a power switch control signal to turn on and off the power switch;
wherein:
when the feedback signal is higher than a first reference voltage, the controller is configured to cause the SMPS to maintain a constant output current at a first current magnitude; and
when the feedback signal is lower than the first reference voltage, the controller is configured to cause the SMPS to provide a second output current at a second current magnitude higher than the first current magnitude.
2. The controller of claim 1 , wherein, when the feedback signal is higher than a second reference voltage that is higher than the first reference voltage, the controller is configured to cause the SMPS to maintain a constant output voltage.
3. The controller of claim 1 , further comprising: a secondary conduction detection circuit configured to receive the feedback signal from the second input terminal and to generate a secondary conduction time signal, which is raised to a high value when a current is flowing in the secondary winding.
4. The controller of claim 3 , further comprising a constant-current control circuit configured to receive the secondary conduction time signal and produce a constant current control signal that causes the power switch control signal to be activated at fixed intervals.
5. The controller of claim 4 , wherein the constant-current control circuit comprises:
a first capacitor configured to be charged with a source current through a source current switch and discharged with a sink current through a sink current switch; and
a comparator coupled to the first capacitor and configured to produce the constant current control signal.
6. The controller of claim 3 , further comprising a critical conduction control circuit that is configured to trigger the power switch control signal to turn on the power switch on a falling edge of the secondary conduction time signal when the feedback signal is lower than the first reference voltage.
7. The controller of claim 6 , wherein the critical conduction control circuit comprises:
a comparator configured to receive the feedback signal and the first reference voltage, the comparator being enabled by the secondary conduction time signal;
a flip-flop coupled to the comparator; and
a D-trigger circuit coupled to the flip-flop through one or more logic gates.
8. The controller of claim 1 , further comprising a constant-voltage control circuit that is configured to trigger the power switch control signal when the feedback signal falls below a target output voltage.
9. The controller of claim 1 , further comprising an off time control circuit configured to deactivate the power switch control signal when the current sense signal reaches a preset value.
10. The controller of claim 1 , wherein the controller is configured to cause the power supply to provide a maximum output current at start up.
11. A switched mode power supply (SMPS), comprising:
a primary winding;
a power switch coupled to the primary winding;
a secondary winding for providing an output of the power supply; and
a controller coupled to the power switch, the controller having:
a first input terminal for receiving a current sensing signal, which is related to a current in the primary winding;
a second input terminal for receiving a feedback signal related to a current in the secondary winding; and
an output terminal for providing a power switch control signal to turn on and off the power switch;
wherein:
when the feedback signal is higher than a first reference voltage, the controller is configured to cause the SMPS to maintain a constant current at a first current; and
when the feedback signal is lower than the first reference voltage, the controller is configured to cause the SMPS to provide a second current that is higher than the first current.
12. The controller power supply of claim 11 , wherein, when the feedback signal is higher than a second reference voltage that is higher than the first reference voltage, the controller is configured to cause the SMPS to maintain a constant output voltage.
13. The controller of claim 11 , further comprising a secondary conduction detection circuit configured to receive the feedback signal from the second input terminal and generate a secondary conduction time signal, which is raised to a high value when a current is flowing in the secondary winding.
14. The controller of claim 13 , further comprising a constant-current control circuit configured to receive the secondary conduction time signal and produce a constant current control signal that causes the power switch control signal to be activated at fixed intervals.
15. The controller of claim 14 , wherein the constant-current control circuit comprises:
a first capacitor configured to be charged with a source current through a source current switch and discharged with a sink current through a sink current switch; and
a comparator coupled to the first capacitor and configured to produce the constant current control signal.
16. The controller of claim 13 , further comprising a critical conduction control circuit that is configured to trigger the power switch control signal to turn on the power switch on a falling edge of the secondary conduction time signal when the feedback signal is lower than the first reference voltage.
17. The controller of claim 16 , wherein the critical conduction control circuit comprises:
a comparator configured to receive the feedback signal and the first reference voltage, the comparator being enabled by the secondary conduction time signal;
a flip-flop coupled to the comparator; and
a D-trigger circuit coupled to the flip-flop through one or more logic gates.
18. The controller of claim 11 , further comprising a constant-voltage control circuit that is configured to trigger the power switch control signal when the feedback signal falls below a target output voltage.
19. The controller of claim 11 , further comprising an off time control circuit configured to deactivate the power switch control signal when the current sense signal reaches a preset value.
20. The controller of claim 11 , wherein the controller is configured to cause the power supply to provide a maximum output current at start up.