Multi-mode operation and control of a resonant converter
In accordance with an embodiment, a method of controlling a switched-mode power includes generating a feedback signal proportional to an output of the switched-mode power supply, and operating the switched-mode power supply in a normal mode. If the feedback signal crosses a first threshold, the switched-mode power operates in a second operating mode. In the first operating mode the pulse modulated signal is adjusted to regulate a feedback signal to a first signal level, and in the second operating mode, a dead-time of the pulse modulated signal is adjusted to signal to regulate a feedback signal to a second signal level different from the first signal level. The method further includes driving a switch of the switched-mode power supply with the pulse modulated signal.
1. A method of controlling a switched-mode power supply, the method comprising:
generating a feedback signal proportional to an output of the switched-mode power supply;
operating the switched-mode power supply in a normal mode comprising adjusting a pulse modulated signal to regulate a feedback signal to a first signal level, wherein the pulse modulated signal comprises a high-side switch signal and a low-side switch signal;
when the feedback signal crosses a first threshold, operating the switched-mode power supply in a second operating mode comprising
periodically asserting the high-side switch signal and the low-side switch signal, wherein assertions of the high-side switch signal alternate with assertions of the low-side switch signal with a dead-time between each assertion of the high-side switch signal and each assertion of the low-side switch signal, and
adjusting the dead-time between each assertion of the high-side switch signal and each assertion of the low-side switch signal to regulate a feedback signal to a second signal level different from the first signal level; and
driving a switch of the switched-mode power supply with the pulse modulated signal.
2. The method of claim 1 , wherein generating the feedback signal comprises generating the feedback signal based on a load requirement.
3. The method of claim 1 , further comprising, changing an operating mode of the switched-mode power supply from the second operating mode to the normal operating mode when the feedback signal crosses a second threshold.
4. The method of claim 3 , wherein the second threshold is between the first threshold and the first signal level.
5. The method of claim 1 , further comprising turning off the pulse modulated signal when the feedback signal crosses a third threshold.
6. The method of claim 1 , wherein:
in the normal operating mode, the time during which the high-side switch signal or the low-side switch signal is asserted is greater than the dead-time between assertions of the high-side switch signal and the low-side switch signal; and
in the second operating mode, the dead-time between assertions of the high-side switch signal and the low-side switch signal is greater than the time during which the high-side signal or the low-side signal as asserted.
7. The method of claim 6 , wherein the time during which the high-side switch signal or the low-side switch signal is asserted is about 1.5 times greater than the dead-time between assertions of the high-side switch signal and the low-side switch signal.
8. The method of claim 1 , wherein an on-time of the pulse modulated signal is constant in the second operating mode.
9. The method of claim 1 , wherein:
the switched-mode power supply is a resonant power supply; and
operating the switched-mode power supply in the normal mode comprising adjusting a frequency of the pulse modulated signal to regulate the feedback signal to the first signal level.
10. A switch controller for a switched-mode power supply, the controller comprising:
a first threshold detector coupled to a feedback signal of the switched-mode power supply; and
a switching signal generator coupled to a feedback signal input and to an output of the first threshold detector, the switching signal generator configured to:
generate a first pulse modulated signal in a normal operating mode to regulate a feedback signal to a first signal level,
transition to a second operating mode when the output of the first threshold detector indicates that the feedback signal has crossed the first threshold, and
generate a second pulse modulated signal in a second operating mode to regulate a feedback signal to a second signal level by adjusting a dead-time of the pulse modulated signal, wherein the second signal level is different from the first signal level, the second pulse modulated signal comprises a high-side switch signal and a low-side switch signal, the switching signal generator is configured to generate the second pulse modulated signal by periodically asserting the high-side switch signal and the low-side switch signal, assertions of the high-side switch signal alternate with assertions of the low-side switch signal with the adjusted dead-time between each assertion of the high-side switch signal and each assertion of the low-side switch signal.
11. The switch controller of claim 10 , further comprising:
an error amplifier comprising
a first input coupled to the feedback signal,
a second input coupled to a second mode reference voltage corresponding to the second signal level, and
an output coupled to a dead-time control input of the switching generator.
12. The switch controller of claim 10 , wherein the first threshold detector comprises a first comparator comprising a first input coupled to the feedback signal, a second input coupled to a first threshold voltage corresponding to the first threshold signal, and an output coupled to a second mode control input of the switching signal generator, wherein the switching signal generator is configured to transition to the second mode when the second mode control input is asserted.
13. The switch controller of claim 12 , further comprising:
a second comparator comprising a first input coupled to the feedback signal, a second input coupled to a second threshold voltage, and an output coupled to a second mode to normal mode transition input of the switching signal generator, wherein the switching signal generator is configured to transition from the second mode to the normal mode when the second mode to normal mode transition input is asserted.
14. The switch controller of claim 10 , wherein the switching signal generator further comprises an on-time setting input configured to be coupled to a reference capacitor, wherein an on-time of the second pulse modulated signal in the second mode depends on a capacitance of the reference capacitor.
15. The switch controller of claim 10 , wherein the switching signal generator generates a high-side switch signal and a low-side switch signal.
16. The switch controller of claim 10 , wherein the switch controller is disposed on an integrated circuit.
17. The switch controller of claim 10 , wherein:
switched-mode power supply comprises a resonant power supply; and
the switching signal generator is configured to generate a first pulse modulated signal in the normal operating mode by adjusting a frequency of the first pulse modulated signal to regulate the feedback signal to the first signal level.
18. A switched-mode power supply system comprising:
a switching controller comprising:
a first threshold detector coupled to a feedback signal of a switched-mode power supply; and
a switching signal generator coupled to a feedback signal input and to an output of the first threshold detector, the switching signal generator configured to:
generate a first pulse modulated signal in a normal operating mode to regulate a feedback signal to a first signal level,
transition to a second operating mode when the output of the first threshold detector indicates that the feedback signal is has crossed the first threshold, and
generate a second pulse modulated signal in a second operating mode to regulate a feedback signal to a second signal level by adjusting a dead-time of the pulse modulated signal, wherein the second signal level is different from the first signal level, the second pulse modulated signal comprises a high-side switch signal and a low-side switch signal, the switching signal generator is configured to generate the second pulse modulated signal by periodically asserting the high-side switch signal and the low-side switch signal, assertions of the high-side switch signal alternate with assertions of the low-side switch signal with the adjusted dead-time between each assertion of the high-side switch signal and each assertion of the low-side switch signal.
19. The switched-mode power supply system of claim 18 , further comprising:
a switch driver coupled to the switching signal generator;
a plurality of power switches coupled to the switch driver;
a power transformer coupled to the plurality of power switches; and
a feedback interface circuit coupled between an output of the switched-mode power supply and a feedback input of the switching controller.
20. The switched-mode power supply system of claim 19 , further comprising an inductor and a capacitor coupled between the plurality of power switches and the power transformer.
21. The switched-mode power supply of claim 19 , wherein the switched-mode power supply comprises a resonant converter.
22. The switched-mode power supply of claim 21 , wherein the switching signal generator is configured to generate the second pulse modulated signal having a fixed switch-on time, wherein the fixed switch-on time is between ¼ and ¾ of a resonant period of the resonant converter.