Circuit arrangement and method for controlling power converters

1. A circuit arrangement for controlling power converters with a microcontroller comprising a digitally controlled pulse width modulation unit having an output, at which a switching signal for switching a power converter switch is present, wherein the circuit arrangement includes an analog circuit for switching on a power converter switch if the voltage across the power converter switch is substantially zero, wherein the power converter switch is switched on by a first pulse generated by the analog circuit part and the first pulse is combined with the switching signal of the pulse width modulation unit, said switching signal being present at the output of the pulse width modulation unit, by an OR logic combination.

2. The circuit arrangement as claimed in claim 1 , wherein the power converter switch is switched on by a first switching edge, which sets a bistable multivibrator, the output of which switches the power switch.

3. The circuit arrangement as claimed in claim 1 , wherein the first pulse or the first switching edge for switching on the power converter switch is generated by the analog circuit part only in the quasi resonant mode of the power converter.

4. The circuit arrangement as claimed in claim 1 , wherein a second pulse or a second switching edge, which indicates to the pulse width modulation unit of the microcontroller the zero crossing of the voltage across the power converter switch, is generated substantially simultaneously with the first pulse or the first switching edge by the analog circuit part.

5. The circuit arrangement as claimed in claim 4 , wherein the outputting of the first and of the second pulse or of the first and of the second switching edge is suppressed if the power converter switch is switched on.

6. The circuit arrangement as claimed in claim 1 , wherein the length of the first pulse is configured in such a way as to reliably bridge a delay brought about by the pulse width modulation unit of the switching signal at the output with respect to the zero crossing of the voltage across the power converter switch.

7. The circuit arrangement as claimed in claim 4 , wherein the length of the second pulse is configured in such a way that said pulse is reliably detected by the pulse width modulation unit of the microcontroller.

8. The circuit arrangement as claimed in claim 1 , wherein the generation of the first pulse or of the first switching edge for switching on a power converter switch is suppressed when a parameter of the power converter leaves a predetermined range.

9. The circuit arrangement as claimed in claim 8 , wherein the parameter of the power converter is its output voltage.

10. The circuit arrangement as claimed in claim 9 , wherein the predetermined output voltage range extends from 0 V to 300 V.

11. The circuit arrangement as claimed in claim 9 , wherein the predetermined output voltage range extends from 0 V to 200 V.

12. A method for controlling a power converter with a microcontroller comprising a digitally operating pulse width modulation unit for regulating the power converter, said unit having an output, at which a switching signal for switching a power converter switch is present, wherein the power converter switch is switched on in the operating state of the quasi resonant mode of the power converter by means of a fast analog circuit, wherein the output of the pulse width modulation unit is connected to an output of the fast analog circuit by an OR logic combination in order to drive the power converter switch.