AC/DC PFC converter using a half bridge resonant converter, and corresponding conversion method

1. An AC/DC PFC converter, comprising:

a PFC pre-regulator, comprising:

an AC input;

a rectifier for rectifying a mains voltage;

a half bridge inverter comprising a high side switch and a low side switch, wherein an output is defined from a node between the switches;

an LLC circuit coupled to the output; and

a control circuit for generating a gate drive signal for controlling the switching of the high side and low side switches thereby to generate a controlled output voltage (vbus), wherein a high gate drive signal turns on one switch and turns off the other switch and a low gate drive signal turns off the one switch and turns on the other switch; and

an output stage, comprising:

a DC/DC converter;

a feedback unit adapted to provide a feedback signal to the control circuit of the PFC pre-regulator for modulating the controlled output voltage (vbus) of the PFC pre-regulator in dependence on at least the output power of the output stage, wherein a set-point for the controlled output voltage is varied synchronously with the mains voltage, such that an imposed bus voltage variation is synchronous with the rectified input mains voltage.

2. A converter as claimed in claim 1 , wherein the control circuit is for generating a gate drive signal in dependence on an electrical feedback parameter from the LLC circuit.

3. A converter as claimed in claim 2 , wherein the control circuit is adapted to set a threshold level for the electrical feedback parameter in dependence on the feedback signal and the rectified input voltage without measurement of the input current.

4. A converter as claimed in claim 3 , wherein the electrical feedback parameter comprises a voltage (vC) across a capacitor of the LLC circuit, which is representative of the input current.

5. A converter as claimed in claim 4 , wherein the control circuit is adapted to compare the voltage across the capacitor of the LLC circuit with a threshold at a switching instant of the gate drive signal.

6. A converter as claimed in claim 1 , wherein the output stage is a switch mode power converter.

7. A converter as claimed in claim 6 , wherein the output stage is a buck converter.

8. A converter as claimed in claim 1 , wherein the feedback unit is adapted to provide a feedback signal to the control circuit of the PFC pre-regulator for modulating the controlled output voltage of the PFC pre-regulator in dependence on the amplitude and phase of the input voltage and the output power.

9. A converter as claimed in claim 8 , wherein the amplitude and phase of the input voltage are derived by the feedback unit from the frequency of a high frequency ripple on the controlled output voltage (vbus).

10. A converter as claimed in claim 8 , wherein the amplitude and phase of the input voltage are derived by the feedback unit based on a feedforward signal from the PFC pre-regulator.

11. A converter as claimed in claim 1 , wherein the control unit comprises an oscillator and controls the oscillation frequency or the control unit comprises a latch and controls the timing of the latch switching.

12. An LED driver, comprising:

an AC/DC PFC converter as claimed in claim 1 .

13. An AC/DC PFC conversion method, comprising:

operating a PFC pre-regulator by:

rectifying a mains voltage;

controlling a half bridge inverter comprising a high side switch and a low side switch by generating a gate drive signal and providing an output from a node between the switches to an LLC circuit thereby to generate a controlled output voltage, wherein a high gate drive signal turns on one switch and turns off the other and a low gate drive signal turns off the one switch and turns on the other switch; and

providing an output from the LLC circuit to an output stage; and

operating the output stage by:

implementing DC/DC conversion; and

providing a feedback signal from the output stage to the PFC pre-regulator for modulating the controlled output voltage of the PFC pre-regulator in dependence on at least the output power of the output stage, wherein a set-point for the controlled output voltage is varied synchronously with the mains voltage, such that an imposed bus voltage variation is synchronous with the rectified input mains voltage.

14. A method as claimed in claim 13 , comprising providing a feedback signal to the PFC pre-regulator for modulating the controlled output voltage of the PFC pre-regulator also in dependence on amplitude and phase of the input voltage.

15. A method as claimed in claim 14 , comprising deriving the amplitude and phase of the AC input:

from the frequency of a high frequency ripple on the controlled output voltage; or

from a feedforward signal from the PFC pre-regulator.