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Drive circuit for driving a piezoelectric transformer capable of decreasing heat developed from electronic parts composing an inverter circuit

  • US 5,894,184 A
  • Filed: 08/01/1997
  • Issued: 04/13/1999
  • Est. Priority Date: 08/01/1996
  • Status: Expired due to Term
First Claim
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1. A drive circuit for driving a piezoelectric transformer acting as a main transformer, said piezoelectric transformer having a resonance frequency, said piezoelectric transformer having first and second primary electrodes supplied with an input AC voltage, said piezoelectric transformer converting the input AC voltage into an output AC voltage by the use of piezoelectric effect, said piezoelectric transformer having a secondary electrode for supplying the output AC voltage to a load, said drive circuit comprising:

  • an inverter circuit, supplied with an input DC voltage having controllable input power and with a frequency control signal and connected to the first and the second primary electrodes of said piezoelectric transformer, for converting the input DC voltage into a main drive voltage in response to the frequency control signal, said inverter circuit supplying the first and the second primary electrodes of said piezoelectric transformer with the main drive voltage as the input AC voltage, said inverter circuit including first and second subsidiary transformers supplied with first and second subsidiary drive voltages, respectively;

    a frequency control circuit, supplied with a load current flowing in the load as a feedback current, for controlling a drive frequency for said piezoelectric transformer in response to the feedback current, said frequency control circuit having a normal frequency sweeping range defined between a normal upper limit frequency and a normal lower limit frequency, the normal frequency sweeping range including the resonance frequency, said frequency control circuit sweeping the drive frequency within the normal frequency sweeping range, said frequency control circuit being reset in response to a reset signal to shift the drive frequency from the normal lower limit frequency to the normal upper limit frequency, said frequency control circuit producing the frequency control signal and a chopping wave signal which indicate the drive frequency;

    a drive voltage control circuit, supplied with a DC power source voltage and connected to said inverter circuit and said frequency control circuit, for controlling, in response to the chopping wave signal and the first subsidiary drive voltage, the input power of the input DC voltage so as to keep the first subsidiary drive voltage at a predetermined peak value; and

    an overvoltage protection circuit, connected to the secondary electrode of said piezoelectric transformer and said frequency control circuit, for detecting overvoltage in the output AC voltage, said overvoltage protection circuit supplying said frequency control circuit with an additional reset signal and a switching signal on detection of the overvoltage in the output AC voltage, thereby making said frequency control circuit switch the normal frequency sweeping range to a provisional frequency sweeping range defined between a provisional lower limit frequency higher than the resonance frequency and a provisional upper limit frequency lower than the normal upper limit frequency.

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