Control circuit and method for driving and controlling parasitic vibration of a piezoelectric transformer-inverter

• US 5,923,546 A
• Filed: 08/25/1997
• Issued: 07/13/1999
• Est. Priority Date: 08/23/1996
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Kawashima, Shingo, Shimada, Yasuhei
• Primary Examiner(s): STERRETT, JEFFREY L
• Application Number:
  US08/917,108
• Time in Patent Office: 687 Days
• Field of Search: 310/318, 363/15, 363/16, 363/39, 363/40, 363/95, 363/97, 363/131
• US Class Current: 363/40
• CPC Class Codes: H02M 7/48 using discharge tubes with control electrode or semiconductor devices with control electrode
• CPC Class Codes: H10N 30/804 for piezoelectric transformers

Abstract

A piezoelectric transformer is driven with the PWM driving method by means of a driving voltage with a waveform obtained by time-dividing a first frequency with a binary pulse of a second frequency lower than the first frequency. A harmonic component of said second frequency component contained in said first frequency component can be suppressed by decreasing a slope of an envelope of said driving voltage waveform. A parasitic vibration of the piezoelectric transformer in the piezoelectric transformer-inverter controlled with the PWM system can be suppressed by controlling such inverter as described above and construction of the piezoelectric transformer support mechanism can be simplified.

Claims

1. A method for driving a piezoelectric transformer-inverter with a pulse width modulation system, the method comprising the steps of:

time-dividing a waveform having a first frequency with a binary pulse of a second frequency which is lower than the first frequency to generate a driving voltage waveform for driving a piezoelectric transformer; and

modulating said driving voltage waveform to suppress a harmonic component of said second frequency component contained in said first frequency component by decreasing a slope of an envelope of said driving voltage waveform to thereby control parasitic oscillation of the piezoelectric transformer.

2. A method for driving a piezoelectric transformer-inverter according to claim 1, wherein a trapezoidal wave is used as an envelope waveform of said driving voltage.

3. A method for driving a piezoelectric transformer-inverter according to claim 1, wherein an exponential function wave is used as an envelope waveform of said driving voltage.

4. A control circuit of a piezoelectric transformer-inverter, comprising:

a driving circuit for driving a piezoelectric transformer with an AC voltage of a first frequency derived from a DC voltage externally supplied;

a frequency sweep oscillator for generating a driving frequency for said driving circuit;

a load current comparing circuit for comparing an output current from said piezoelectric transformer with a reference value to increase or decrease the driving frequency generated by said frequency sweep oscillator based on the comparison; and

a duty variable pulse oscillator for time-division driving said transformer with a time-divided driving voltage waveform having a second frequency, wherein said driving circuit comprises an envelope generating circuit for decreasing the slope of an envelope of the time-divided driving voltage waveform to suppress a harmonic component of said second frequency component contained in said first frequency to thereby control parasitic oscillation of the piezoelectric transformer, and said piezoelectric transformer is driven by the driving voltage modulated in said envelope generating circuit.

5. A circuit for controlling a piezoelectric transformer-inverter according to claim 4, wherein a trapezoidal wave is used as an envelope waveform of the driving voltage of the piezoelectric transformer generated by said envelope generating circuit.

6. A circuit for controlling a piezoelectric transformer-inverter according to claim 5, wherein said envelope generating circuit comprises a waveform shaping circuit for generating a trapezoidal wave by a constant current charge and discharge of a capacitor and an integrating circuit, wherein a trapezoidal wave generated by said waveform shaping circuit is modulated in said integrating circuit into said driving voltage of the first frequency.

7. A circuit for controlling a piezoelectric transformer-inverter according to claim 6, wherein a current mirror circuit is used as a constant current source for charging and discharging said capacitor in said envelope generating circuit.

8. A circuit for controlling a piezoelectric transformer-inverter according to claim 4, wherein an exponential function wave is used as an envelope waveform of the driving voltage of the piezoelectric transformer generated by said envelope generating circuit.

9. A circuit for controlling a piezoelectric transformer-inverter according to claim 8, wherein said envelope generating circuit comprises a waveform shaping circuit for generating an exponential wave by a resistor and a capacitor and an integrating circuit, wherein an exponential wave generated by said waveform shaping circuit is modulated in said integrating circuit into said driving voltage of the first frequency.

10. A piezoelectric transformer-inverter comprising:

a piezoelectric transformer which outputs an AC voltage applied to a primary side to a secondary side using piezoelectric effect; and

a control circuit comprising;

a driving circuit for driving a piezoelectric transformer with an AC voltage of a first frequency derived from a DC voltage externally supplied;

a frequency sweep oscillator for generating a driving frequency for said driving circuit;

a load current comparing circuit for comparing an output current from said piezoelectric transformer with a reference value to increase or decrease the driving frequency generated by said frequency sweep oscillator based on the comparison; and

a duty variable pulse oscillator for time-division driving said transformer with a time-divided driving voltage waveform having a second frequency, wherein said driving circuit comprises an envelope generating circuit for decreasing the slope of an envelope of the time-divided driving voltage waveform to suppress a harmonic component of said second frequency component contained in said first frequency to thereby control parasitic oscillation of the piezoelectric transformer, and said piezoelectric transformer is driven by the driving voltage modulated in said envelope generating circuit.

11. A piezoelectric transformer-inverter according to claim 10, wherein said piezoelectric transformer comprises a support mechanism for supporting vibration nodes of said piezoelectric transformer, wherein said support mechanism has a resonance frequency apart from integral multiples and integral quotients of a first frequency for driving said piezoelectric transformer.