Switching mode power supply with selectable constant-voltage constant-current control

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

a transformer having a primary winding and a secondary winding, the primary winding for coupling to a power source and the secondary winding for providing an output to a load device;

a power switch coupled to the primary winding of the transformer;

a primary-side controller coupled to the power switch, the primary-side controller being configured to control the power switch for turning on and turning off a current flow in the primary winding, and

a secondary-side controller coupled to the secondary winding for providing constant voltage (CV) and constant current (CC) control of the SMPS, the secondary-side controller configured to:

receive an output selection signal at an output selection terminal, the output selection signal configured for selecting one of a plurality of values for the output;

based on the output selection signal, select a voltage reference signal from a plurality of voltage reference signals and select a current reference signal from a plurality of current reference signals;

monitor an output voltage and an output current of the SMPS to determine if the output voltage is below the selected voltage reference signal and if the output current is below the selected current reference signal; and

provide a turn-on signal to the primary-side controller for turning on the power switch upon determining that the output voltage is below the selected voltage reference signal and the output current is below the selected current reference signal;

wherein the primary-side controller is configured to turn on the power switch in response to the turn-on signal from the secondary side controller, and the primary-side controller is configured to turn off the power switch when a current in the primary winding reaches a primary current reference level.

2. The switch mode power supply of claim 1 , wherein the secondary-side controller is configured to provide the turn-on signal to the primary-side controller through optical coupling.

3. The switch mode power supply of claim 2 , wherein the secondary-side controller is configured to provide the turn-on signal to the primary-side controller through an opto-coupler.

4. The switch mode power supply of claim 1 , wherein the secondary-side controller is configured to provide the turn-on signal to the primary-side controller through inductive coupling.

5. The switch mode power supply of claim 4 , wherein the secondary-side controller is configured to provide the turn-on signal to the primary-side controller through inductive coupling between windings in the transformer.

6. The switch mode power supply of claim 5 , wherein the secondary-side controller is configured to provide the turn-on signal to the primary-side controller when no current is flowing in either the primary winding or the secondary winding.

7. The switch mode power supply of claim 4 , wherein the secondary-side controller is configured to provide the turn-on signal to the primary-side controller through inductive coupling provided by a second transformer.

8. The switch mode power supply of claim 7 , wherein the secondary-side controller is configured to provide the turn-on signal to the primary-side controller when a current is flowing in either the primary winding or the secondary winding.

9. The switch mode power supply of claim 1 , wherein the secondary-side controller further comprises:

one or more input terminals for receiving the output selection signal;

a decoder circuit for decoding the output selection signal;

a reference signal generation circuit for generating a voltage reference signal and a current reference signal based on the output selection signal;

a voltage sense terminal for providing information about the output voltage of the secondary winding;

a current sense terminal for providing information about the current in the secondary winding;

a control circuit including:

a first comparator coupled to the voltage sense terminal and the voltage reference signal;

a second comparator coupled to the current sense terminal and the current reference signal; and

a logic circuit coupled to the first comparator and the second comparator and being configured to provide a the turn-on signal;

a driver circuit configured to provide the turn-on signal to the primary-side controller for turning on the power switch.

10. The switch mode power supply of claim 1 , wherein the turn-on signal from the secondary-side controller is a pulsed signal, and wherein the primary-side controller is configured to detect a fall edge or a rising edge of the pulsed signal.

11. The switch mode power supply of claim 1 , wherein the primary-side controller further comprises:

a comparator circuit with one input for receiving the turn-on signal from the secondary-side controller, and another input coupled to a reference signal; and

a D-latch having a clock input terminal coupled to an output of the comparator circuit.

12. A switch mode power supply (SMPS), comprising:

a transformer having a primary winding and a secondary winding, the primary winding for coupling to a power source and the secondary winding for providing an output to a load device;

a power switch coupled to the primary winding of the transformer;

a primary-side controller coupled to the power switch, the primary-side controller being configured to control the power switch for turning on and turning off a current flow in the primary winding, and

a secondary-side controller coupled to the secondary winding for providing constant voltage (CV) and constant current (CC) control of the SMPS, the secondary-side controller configured to:

receive an output selection signal at an output selection terminal, the output selection signal configured for selecting one of a plurality of values for the output;

based on decoding the output selection signal at a decoder circuit, select a voltage reference signal from a plurality of voltage reference signals;

monitor an output voltage and an output current of the SMPS to determine if the output voltage is below the selected voltage reference signal and if the output current is below a current reference signal; and

provide a turn-on signal to the primary-side controller for turning on the power switch upon determining that the output voltage is below the selected voltage reference signal and the output current is below a current reference signal;

wherein the primary-side controller is configured to turn on the power switch in response to the turn-on signal from the secondary side controller, and the primary-side controller is configured to turn off the power switch when a current in the primary winding reaches a primary current reference level.

13. The switch mode power supply of claim 12 , wherein the secondary-side controller is further configured to, based on the output selection signal, select the current reference signal from a plurality of current reference signals.

14. The switch mode power supply of claim 12 , wherein secondary-side controller is configured to provide the turn-on signal to the primary-side controller through an opto-coupler.

15. The switch mode power supply of claim 12 , wherein the secondary-side controller is configured to provide the turn-on signal to the primary-side controller through inductive coupling provided by a second transformer.

16. A method for providing constant voltage (CV) and constant current (CC) control in a switch mode power supply (SMPS), which has a transformer including a primary winding and a secondary winding, a power switch coupled to the primary winding, a primary-side controller, and a secondary-side controller, the method comprising:

in the secondary-side controller,

receiving an output selection signal, the output selection signal configured for selecting one of a plurality of values for the output;

based on decoding the output selection signal at a decoder circuit, selecting a target voltage reference signal from a plurality of voltage reference signals and selecting a target current reference signal from a plurality of current reference signals;

monitoring an output voltage and an output current of the SMPS to determine if the output voltage is below the target voltage reference signal and if the output current is below the target current reference signal; and

providing a turn-on signal to the primary-side controller for turning on the power switch upon determining that the output voltage is below the target voltage reference signal and the output current is below the target current reference signal,

and

in the primary-side controller,

providing a control signal to turn on the power switch in response to the turn-on signal from the secondary side.

17. The method of claim 16 , further comprising:

in the secondary controller,

receiving an output selection signal;

based on the output selection signal, selecting the target voltage reference signal from a plurality of voltage reference signals and selecting the target current reference signal from a plurality of current reference signals.

18. The method of claim 16 , further comprising, in the primary controller, turning off the power switch when a current in the primary winding reaches a peak current reference level.

19. The method of claim 16 , further comprising the secondary-side controller providing the turn-on signal to the primary-side controller through an opto-coupler.

20. The method of claim 16 , further comprising the secondary-side controller providing the turn-on signal to the primary-side controller through a second transformer.

21. The method of claim 16 , further comprising the primary-side controller detecting a fall edge or a rising edge of the turn-on signal from the secondary-side controller.