Drive circuit for piezoelectric high voltage generating device

In a high voltage generating device comprising a piezoelectric transformer having two input drive electrodes on its driver portion and an output electrode on its generator portion, a rectifying and smoothing means connected to said output electrode for supplying a DC output voltage, and a variable frequency drive voltage signal source for driving said transformer, said device exhibiting a frequency versus DC output voltage characteristic having a peak-portion including a maximum voltage generated substantially at a natural resonant frequency of the transformer when said transformer is driven over a predetermined frequency range from said voltage signal source;

• the improvement comprising:
• an impedance conversion circuit comprising a series resonant circuit having first and second elements each having first and second terminals, the first terminals of said first and second elements being coupled to one of the input drive electrodes on the driver portion of said piezoelectric transformer, the second terminals of said first and second elements being coupled across the output of said voltage signal source and the other input drive electrode of said piezoelectric transformer being coupled to the second terminal of said second element, one of said first and second elements being an inductor and the other an independent capacitor, the series resonant circuit formed by said first and second elements resonanting substantially at said natural resonant frequency thereby producing a frequency versus input driving voltage characteristic for the voltage between said drive electrodes having a double-humped shape over said predetermined frequency range, said double-humped characteristic having a valley-portion including a minimum voltage at about said resonant frequency to compensate for the peak-portion of said DC output voltage at said natural resonant frequency, whereby the resultant frequency versus effective DC output voltage characteristic of said device is flattened.

A high voltage generator comprising:

• a piezoelectric transformer having first and second input electrodes on its driver portion and an output electrode on its generator portion;
• a variable frequency control voltage source having first and second output terminals,
• an inductor having a self-inductance L.sub.s coupling the first input electrode of said piezoelectric transformer to the first output terminal of said control voltage source, said self-inductance causing a resonance in said transformer,
• a capacitor having a high capacitance C relative to the input interelectrode capacitance C.sub.o of said transformer connected between the first and second input electrodes of said transformer, the capacitance C of said capacitor and the self-inductance L.sub.s of said inductor being related by the relation ##EQU13## where L.sub.e is the equivalent resonant inductance of said piezoelectric transformer and C.sub.e is the equivalent resonant capacitance of said transformer, and
• an output circuit for generating a high DC output voltage coupled between the output and second input electrodes of said piezoelectric transformer, the second input electrode of said transformer and the second output terminal of said control voltage source being connected together, the relative values of said self-inductance L.sub.s and capacitance C being such as to flatten the resultant frequency versus effective DC output voltage characteristic of said high voltage generator.