Method for frequency control of a piezoelectric transformer and circuit arrangement comprising a piezoelectric transformer

1. A method for frequency regulation for a piezoelectric transformer, the method comprising:

exciting, by an AC voltage source, the piezoelectric transformer on an input side with an AC voltage of a predetermined frequency as an input voltage;

capturing, by a detector, a phase information for an input impedance of the piezoelectric transformer in a feedback path;

evaluating, by a regulator, the captured phase information with respect of a predetermined phase criterion; and

regulating, by the regulator, the frequency of the AC voltage based on the evaluated phase information.

2. The method according to claim 1 , wherein the predetermined phase criterion is chosen as a zero or a local extreme of a phase angle of the input impedance as a function of the frequency of the AC voltage, and wherein evaluating the captured phase information comprises a rating of an adequate satisfaction of the predetermined phase criterion.

3. The method according to claim 1 , wherein the detector in the feedback path compares a signal of the input voltage with a signal proportional to an input current on the input side of the piezoelectric transformer so that the detector generates an output signal that is proportional to an absolute value of an phase angle between the input voltage and the input current and serves as phase information for the input impedance of the piezoelectric transformer.

4. The method according to claim 1 , further comprising sampling a signal proportional to an input current on the input side of the piezoelectric transformer, and using a Fourier transformation to compute an phase angle of the signal from the samples, wherein the phase angle serves as phase information for the input impedance of the piezoelectric transformer.

5. The method according to claim 4 , wherein sampling the signal is performed by an impedance analyzer.

6. A method for operating a piezoelectric transformer as a plasma generator, the method comprising:

applying an input voltage at a frequency regulated in accordance with the method of claim 1 , on the input side; and

converting the input voltage to an output high voltage so that a plasma is produced on an output side on account of ionization of an operating gas flowing around the plasma generator.

7. A circuit arrangement comprising:

a piezoelectric transformer having an input side and an output side;

an AC voltage source configured to produce an input voltage of a predetermined frequency on the input side of the piezoelectric transformer;

a detector located in a feedback path between the input side of the piezoelectric transformer and the AC voltage source, wherein the detector is configured to capture a phase information for an input impedance of the piezoelectric transformer; and

a regulator configured to evaluate the captured phase information with respect to a predetermined phase criterion and to prescribe a frequency for producing the input voltage to the AC voltage source on a basis of the evaluated phase information.

8. The circuit arrangement according to claim 7 , wherein the detector comprises a phase detector arranged such that an signal of the input voltage is applied to a first input of the phase detector and a signal proportional to an input current on the input side of the piezoelectric transformer is applied to a second input of the phase detector, and wherein the phase detector is configured to output an output signal that comprises the phase information of the input impedance at an output.

9. The circuit arrangement according to claim 8 , wherein the phase detector comprises an XOR gate and a low-pass filter, which are connected in series, and wherein an arithmetic sign signal for the input voltage is applied to a first input of the XOR gate and an arithmetic sign signal proportional to the input current on the input side of the piezoelectric transformer is applied to a second input of the XOR gate such that an output signal is producible at the output of the XOR gate as a comparison signal that is to be averaged by the low-pass filter to produce a signal that is proportional to an absolute value of an phase angle of the input impedance of the piezoelectric transformer.

10. The circuit arrangement according to one of claim 9 , wherein the piezoelectric transformer is part of a piezoelectric plasma generator configured to produce an output high voltage on the output side from the input voltage on the input side so that a plasma is produced on the output side on account of ionization of an operating gas flowing around the plasma generator.

11. The circuit arrangement according to claim 7 , wherein the detector comprises an impedance analyzer arranged such that a signal proportional to an input current on the input side of the piezoelectric transformer is applied to the input of the impedance analyzer.

12. The circuit arrangement according to claim 11 , wherein the impedance analyzer is arranged to sample the signal and to supply samples to a Fourier transformation to compute an phase angle of the signal, which corresponds to the phase angle of the input impedance of the piezoelectric transformer.

13. The circuit arrangement according to claim 7 , wherein the piezoelectric transformer is a piezoelectric plasma generator configured to produce an output high voltage on the output side from the input voltage on the input side so that a plasma is to be produced on the output side on account of ionization of an operating gas flowing around the plasma generator.