Methods and apparatus for constant power/current control for switch-mode power converters
In a first aspect, a control circuit is provided for use with a switch-mode power stage that provides an output voltage signal and an output current signal at an output power. The switch-mode power stage has a nominal voltage, a nominal current, a maximum current, an output power and a maximum power. The control circuit includes a voltage control loop and a current control loop, and the control circuit uses the voltage control loop to provide voltage mode control if the output current signal is greater than or equal to the nominal current and less than the maximum current, wherein the output power is substantially constant. Numerous other aspects are also provided.
1. A control circuit for use with a switch-mode power stage that provides an output voltage signal and an output current signal at an output power, the switch-mode power stage comprising a nominal voltage, a nominal current, a maximum current, an output power and a maximum power, the control circuit comprising: a voltage control loop; and a current control loop; wherein the control circuit only uses the voltage control loop to provide voltage mode control if the output current signal is greater than or equal to the nominal current and less than the maximum current, wherein the output power is constant, further comprising a reference ramp control circuit that provides a reference voltage signal to the voltage control loop, wherein the reference voltage has a value that ramps from a voltage less than the nominal output voltage to the nominal output voltage over a ramp time interval.
2. The control circuit of claim 1 , wherein the control circuit switches from voltage mode control to current mode control when the output current signal equals the maximum current.
3. The control circuit of claim 2 , wherein switching occurs at an output voltage equal to the maximum power divided by the maximum current.
4. The control circuit of claim 2 , wherein the switch-mode power stage is coupled to a load, and wherein the control circuit switches from current mode control to voltage mode control when the load is released from a maximum current operation of the switch-mode power stage.
5. The control circuit of claim 1 , wherein the control circuit causes an operating mode of the switch-mode power stage to switch from constant voltage mode to constant current mode at an output voltage less than the nominal output voltage.
6. The control circuit of claim 1 , wherein the ramp time interval is set to reduce voltage overshoot on the output voltage.
7. A control circuit for use with a switch-mode power stage that provides an output voltage signal and an output current signal at an output power, the switch-mode power stage comprising a nominal voltage, a nominal current, a maximum current, an output power and a maximum power, the control circuit comprising: a voltage control loop; and a current control loop; wherein the control circuit only uses the current control loop to provide average current mode control if the output current signal is greater than or equal to the nominal current and less than the maximum current, wherein the output power is constant, further comprising a reference ramp control circuit that provides a reference voltage signal to the voltage control loop, wherein the reference voltage has a value that ramps from a voltage less than the nominal output voltage to the nominal output voltage over a ramp time interval.
8. The control circuit of claim 7 , wherein the control circuit switches from constant power mode control to constant current mode control when the output current signal substantially equals the maximum current.
9. The control circuit of claim 8 , wherein switching occurs at an output voltage equal to the maximum power divided by the maximum current.
10. The control circuit of claim 8 , wherein the switch-mode power stage is coupled to a load, and wherein the control circuit switches from constant current mode control to constant power mode control or constant voltage mode control when the load is released from the switch-mode power stage.
11. The control circuit of claim 7 , wherein the control circuit causes an operating mode of the switch-mode power stage to switch from constant voltage mode to constant power mode at an output voltage less than the nominal output voltage.
12. The control circuit of claim 7 , wherein the ramp time interval is set to reduce voltage overshoot on the output voltage.
13. A method for controlling a switch-mode power stage that provides an output voltage signal and an output current signal at an output power, the switch-mode power stage comprising a nominal voltage, a nominal current, a maximum current, an output power and a maximum power, the method comprising:
using only voltage mode control to provide a constant output power if the output current signal is greater than or equal to the nominal current and less than the maximum current, further comprising:
using a voltage control loop to provide voltage mode control; and
providing a reference voltage signal to the voltage control loop, wherein the reference voltage has a value that ramps from a voltage less than the nominal output voltage to the nominal output voltage over a ramp time interval, wherein the ramp time interval is set to reduce voltage overshoot on the output voltage.
14. The method of claim 13 , further comprising switching from voltage mode control to current mode control when the output current signal equals the maximum current.
15. The method of claim 14 , wherein switching from constant voltage mode to constant power mode occurs at an output voltage less than the nominal output voltage.
16. The method of claim 14 , wherein switching from constant power mode to constant current mode occurs at an output voltage equal to the maximum power divided by the maximum current.