Hybrid control of switching power converters

1. A method of operating a switching power converter, the method comprising:

operating the switching power converter such that a charge control switch has a conduction time in each switching cycle;

making adjustments to the conduction time, the adjustments dominated by voltage-mode control and having a current-mode control contribution when the switching power converter is in a first operational state, and the adjustments dominated by the current-mode control and having the voltage-mode control contribution when the switching power converter is in a second operational state distinct from the first operational state, the making adjustments by:

creating a ramp signal with a slope, the slope proportional to an output voltage error;

creating a hybrid signal by combining the ramp signal and a signal indicative of primary current; and

setting the conduction time in each switching cycle based on the hybrid signal.

2. The method of claim 1 wherein the first operational state is providing a first output current, and wherein the second operational state is a second output current higher than the first output current.

3. The method of claim 1 wherein the first operational state is providing a first power level, and wherein the second operational state is a second power level higher than the first power level.

4. The method of claim 1 wherein increasing slope increases contribution of the voltage-mode control.

5. The method of claim 1 wherein decreasing slope decreases contribution of the voltage-mode control.

6. The method of claim 1 wherein setting the conduction time in each switching cycle further comprises ending a conduction mode of the charge control switch when a magnitude of the hybrid signal crosses a predetermined threshold.

7. The method of claim 6 :

wherein ending the conduction mode further comprises ending the conduction mode of a high-side switch, the ending of the conduction mode defines an on time;

and the method further comprises, after ending the conduction mode of the high-side switch, making a low-side switch conductive for the on time.

8. The method of claim 1 wherein operating the switching power converter further comprises operating an inductor-inductor-capacitor (LLC) resonant converter.

9. A primary-side controller for a switching power converter comprising:

a first gate terminal, a current sense terminal, and a voltage feedback terminal;

a cycle control logic defining a first gate output and a trigger input, the first gate output coupled to the first gate terminal, the cycle control logic configured to de-assert the first gate output based on assertion of the trigger input;

an on-time control logic coupled to the trigger input, the current sense terminal, and the voltage feedback terminal, the on-time control logic configured to:

create a voltage error signal based on a signal indicative of output voltage sensed on the voltage feedback terminal;

create a ramp signal with a slope, the slope proportional to the voltage error signal;

create a hybrid signal by combining the ramp signal and a signal indicative of current in a primary winding sensed on the current sense terminal; and

assert the trigger input of the cycle control logic when a magnitude of the hybrid signal crosses a predetermined threshold.

10. The primary-side controller of claim 9 wherein the cycle control logic further comprises:

a start control circuit defining a start output, wherein the start control circuit asserts the start output a switching frequency; and

a latch defining a start input, a stop input, and a latch output, the start input coupled to the start output, and the stop input coupled to the trigger input, wherein the latch asserts the latch output based on assertion of the start input, and the latch de-asserts the latch output based on assertion of the stop input.

11. The primary-side controller of claim 10 further comprising the start control circuit configured to assert the start output at the switching frequency that is fixed.

12. The primary-side controller of claim 10 further comprising the start control circuit is configured to assert the start output at the switching frequency that is variable and based on an asserted time of the latch output.

13. The primary-side controller of claim 9 further comprising:

a second gate terminal;

the cycle control logic defining a second gate output coupled to the second gate terminal;

wherein the cycle control logic is configured to:

assert the first gate terminal, and de-assert the first gate terminal responsive to assertion of the trigger input;

measure an on time of the first gate terminal;

assert the second gate terminal for the on time.

14. A switching power converter comprising:

a first switch associated with a first inductance;

a current sensor associated with the first inductance;

an output voltage node defined by the switching power converter;

a converter controller comprising:

a cycle control logic defining a first gate output coupled to a control input of the first switch, and a trigger input, the cycle control logic configured to de-assert the control input of the first switch based on assertion of the trigger input;

an on-time control logic coupled to the trigger input, the current sensor, and the output voltage node, the on-time control logic configured to:

create a voltage error signal based on a signal indicative of output voltage sensed from the output voltage node;

create a ramp signal with a slope, the slope proportional to the voltage error signal;

create a hybrid signal by combining the ramp signal and a signal indicative of current sensed from the current sensor; and

assert the trigger input of the cycle control logic when a magnitude of the hybrid signal crosses a predetermined threshold.

15. The switching power converter of claim 14 wherein the cycle control logic further comprises:

a start control circuit defining a start output, wherein the start control circuit asserts the start output a switching frequency; and

a latch defining a start input, a stop input, and a latch output, the start input coupled to the start output, and the stop input coupled to the trigger input, wherein the latch asserts the latch output based on assertion of the start input, and the latch de-asserts the latch output based on assertion of the stop input.

16. The switching power converter of claim 15 further comprising the start control circuit configured to assert the start output at the switching frequency that is fixed.

17. The switching power converter of claim 15 further comprising the start control circuit is configured to assert the start output at the switching frequency that is variable and based on an asserted time of the latch output.

18. The switching power converter of claim 14 further comprising:

a primary winding of a transformer that defines the first inductance;

a second switch associated with the primary winding;

the cycle control logic defining a second gate output coupled to a control input of the second switch;

wherein the cycle control logic is configured to:

assert the control input of the first switch, and de-assert the control input of the first switch responsive to assertion of the trigger input;

measure a conduction time of the first switch; and

assert the control input of the second switch for the conduction time.

19. The switching power converter of claim 18 wherein the primary winding is arranged as an inductor-inductor-capacitor (LLC) primary.