JP2003033046A - Control circuit of piezoelectric transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control circuit of a piezoelectric transformer which can practice a stable step-up operation even if a waveform distortion is produced in an input voltage of a piezoelectric transformer by the adjustment of a duty ratio of an oscillation signal corresponding to a fluctuation of a DC power supply voltage. SOLUTION: In a control circuit which generates an AC high voltage for driving a cold cathode tube 2, a notch filter 9 whose central frequency is twice a drive frequency of a piezoelectric transformer 1 is inserted in a line for applying an input voltage Vi to the piezoelectric transformer 1, and a distortion having a frequency component with the twice frequency and superposed upon the input voltage is eliminated by the notch filter. The control circuit drives a Rosen type laminated piezoelectric transformer by a half-bridge type drive circuit 6 with a λ/2 mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric transformer control circuit, for example, to a piezoelectric transformer control circuit suitable for use in a cold cathode fluorescent lamp driving device.

[0002]

2. Description of the Related Art In recent years, a liquid crystal display has been widely used as a display device for a notebook personal computer or the like which is easy to carry. Inside this liquid crystal display device, a cold cathode tube is provided as a light source of an optical system for illuminating the liquid crystal display panel from the front side or the back side, and a step-up inverter for lighting the cold cathode tube is provided with a step-up transformer. Piezoelectric transformers are becoming popular.

As an example of a control circuit for such a piezoelectric transformer, a prior Japanese Patent Application No. 9-2604 filed by the applicant of the present application is proposed.
No. 24 discloses a control circuit capable of controlling the load current flowing in the cold cathode tube to a predetermined value regardless of the change in the DC power supply voltage that drives the control circuit of the piezoelectric transformer. Here, the control circuit will be outlined.

FIG. 6 is a block diagram of a piezoelectric transformer control circuit previously proposed by the applicant of the present application.

In the figure, 1 is a piezoelectric transformer, 2 is a load such as a cold cathode tube connected to the output side of the piezoelectric transformer 1, 3 is a detection resistor for detecting a current flowing through the load, and 4 is a detection resistor. A rectifier circuit for converting the AC voltage generated in 3 into a DC voltage, and 5 is an output voltage (load current detection voltage) V of the rectifier circuit 4.
An error amplifier circuit that compares ri with the reference voltage Vref1 and amplifies the difference, and 7 is a bridge type (half bridge, full bridge) drive circuit including a plurality of switching elements and the like. L1 is a coil that shapes the rectangular-wave voltage waveform output from the drive circuit 7.

Further, 8a and 8b are detection resistors for detecting the magnitude of the input voltage Vi of the piezoelectric transformer 1, 9 is a rectifier circuit for converting the AC voltage generated in the detection resistor 8a into a DC voltage, and 10 is a rectifier. Output voltage Vti of circuit 9 and reference voltage Vre
An error amplification circuit that compares f2 and amplifies the difference.

Reference numeral 11 denotes a voltage controlled oscillator circuit which outputs an oscillation signal having a frequency according to the output voltage of the error amplifier circuit 5 and a duty ratio according to the output voltage of the error amplifier circuit 10.

According to the above control circuit, the DC power supply voltage V
When a change occurs in d, the voltage-controlled oscillation circuit 11 switches the inside of the drive circuit 7 according to the adjustment of the frequency and the duty ratio of the rectangular-wave oscillation signal supplied to the drive circuit 7. Since the operating state of the element can be changed, the load current can be controlled to a predetermined value, and the input voltage Vi applied to the piezoelectric transformer 1 can be controlled to a predetermined value.

[0009]

However, in the above-mentioned conventional control circuit, a Rosen type piezoelectric transformer is adopted as the piezoelectric transformer 1, and two switching elements (field effect transistor (FET)) are used as the drive circuit 7.
When a so-called half-bridge type drive circuit that alternately performs switching operation is driven in the λ / 2 mode, when the power supply voltage Vd fluctuates, the load current flowing through the cold cathode tube 2 becomes substantially constant. It has been found that there are cases in which control that is maintained at the (predetermined value) is hindered. Here, this problem will be described with reference to FIG. 7.

FIG. 7 is a piezoelectric circuit having waveform distortion caused by the switching operation of the half-bridge type drive circuit 7 in the control circuit shown in FIG. 6 and the duty ratio adjustment of the oscillation signal according to the fluctuation of the power supply voltage Vd. Input voltage V of transformer 1
It is a figure which shows i and the output voltage Vo.

The plurality of types of waveforms shown in the figure are gate signals supplied from above to the switching element on the upper side (high side) of the drive circuit 7 from the voltage controlled oscillator circuit 11, and also from the lower side (low side). Gate signals supplied to the switching elements, and the drive circuit 7 according to those gate signals
Shows the output voltage output by the piezoelectric transformer 1, the input voltage Vi of the piezoelectric transformer 1 based on the output voltage, and the output voltage Vo output by the piezoelectric transformer 1 driven by the input voltage Vi. In this example, voltage control is performed. The case where the pulse width of only the switching element on the high side is adjusted by controlling the duty ratio in the oscillation circuit 11 is shown.

Here, the power source voltage V
Consider the case where d increases from the predetermined voltage in the + direction. In this case, as shown in FIG. 7, the gate signal of the switching element on the high side is controlled by the duty ratio control performed by the voltage controlled oscillation circuit 11 according to the degree of voltage rise of the power supply voltage Vd. The pulse width shown by the broken line changes to the narrow pulse width shown by the solid line. Therefore, the rectangular-wave-shaped output voltage of the power supply voltage Vd that is switched according to the operation of each of the high-side and low-side switching elements also has a waveform in which the fluctuation of the pulse width of the high-side switching element is reflected.

The piezoelectric transformer 1 has a resonance characteristic,
The boost ratio and boost efficiency change according to the drive frequency. In general, in a piezoelectric transformer, even if the input voltage applied from the outside is not a sinusoidal waveform but a distorted waveform due to the resonance characteristic, the waveform of the output voltage of the piezoelectric transformer is approximately proportional. It becomes sinusoidal.

However, the Q (degree of sharpness of resonance characteristics) of the Rosen type piezoelectric transformer 1 driven in the λ / 2 mode is considerably lower than that in the λ mode. Therefore, the AC high voltage (the input voltage Vi of the piezoelectric transformer 1) obtained by shaping the output voltage in which the pulse width variation of the switching element on the high side is reflected by the coil L1.
Shows that as the pulse width of the gate signal of the switching element on the high side becomes narrower, the voltage waveform is distorted from the pulse width shown by the broken line in FIG. 7 as shown by the solid line. Also, substantially the same distortion occurs in the voltage waveform of the output voltage Vo of the piezoelectric transformer 1.

Further, in the control circuit shown in FIG. 6, the control for keeping the load current (output current) flowing through the cold cathode tubes 2 substantially constant is performed based on the result of detecting the effective value of the load current. There is. In this case, when the circuit configuration adopted in the rectifier circuit 4 is a simple half-wave rectifier type circuit configuration in terms of cost, the voltage component of the output voltage Vo detected by the rectifier circuit is shown in FIG. This is the portion near the peak of the voltage waveform in which distortion has occurred as indicated by the solid line. For this reason,
When control is performed in the circuit shown in FIG. 6 based on the effective value (effective current) of the voltage component detected in the half-wave rectification circuit, the detected effective value is the output voltage V.
As the waveform distortion of o becomes more severe, it is detected as an apparently larger value.

Therefore, when the waveform distortion of the output voltage Vo occurs as described above, the control function for keeping the load current (output current) originally held by the control circuit shown in FIG. 6 as a result results. As the waveform distortion of the output voltage Vo becomes more severe, the load current is controlled so as to be smaller, and the lighting state of the cold cathode fluorescent lamp 2 which should be lighted at the same brightness when the load current (output current) is essentially constant. But it gets dark. That is, the power supply voltage Vd in the control circuit shown in FIG.
It can be seen that the fluctuations in 1 h hinder the control for maintaining the load current flowing through the cold cathode fluorescent lamp 2 at a substantially constant value (predetermined value).

The reason why the brightness of the cold cathode fluorescent lamp 2 becomes dark as described above is that the rectifier circuit has a positive side (+ side) of the waveform.
This is because it is a simple half-wave rectification type circuit configuration that detects the magnitude of, and, in the case of a simple half-wave rectification type circuit configuration that detects the magnitude of the negative side (-side) of the waveform. Causes a problem that the brightness of the cold cathode tube 2 becomes brighter because the operation is the opposite of the above case.

Therefore, the present invention relates to a piezoelectric transformer that performs stable boosting operation even when waveform distortion occurs in the input voltage of the piezoelectric transformer due to the adjustment of the duty ratio of the oscillation signal according to the fluctuation of the DC power supply voltage. The purpose is to provide a control circuit.

[0019]

In order to achieve the above object, a piezoelectric transformer control circuit according to the present invention is characterized by the following configuration. This control circuit corresponds to, for example, a first embodiment (FIG. 1) described later.

That is, an AC high voltage is generated based on a rectangular wave voltage obtained by switching a DC power supply voltage based on an oscillation signal, and the AC high voltage is used as an input voltage to drive a Rosen type piezoelectric transformer into a λ / When driving in two modes, the frequency of the oscillation signal is adjusted according to the result of comparison between the input voltage and the first predetermined value, and
A control circuit for a piezoelectric transformer that adjusts a duty ratio of the oscillation signal according to a result of comparing a load current flowing through a cold cathode tube connected to the piezoelectric transformer with a second predetermined value. The line for applying the input voltage is provided with a notch filter, and distortion of the voltage waveform generated in the input voltage is caused by the duty ratio adjustment of the oscillation signal according to the variation of the DC power supply voltage. It is characterized in that it is removed by a filter.

In the above circuit configuration, preferably, the center frequency of the notch filter is set to a second harmonic of the driving frequency of the piezoelectric transformer, so that the second harmonic has a frequency component superimposed on the input voltage. It is good to remove the distortion.

Further, a control circuit of a piezoelectric transformer having another circuit configuration for achieving the above-mentioned same purpose is characterized by the following configuration. This control circuit corresponds to, for example, a second embodiment (FIG. 4) described later.

That is, an AC high voltage is generated based on a rectangular wave voltage obtained by switching a DC power supply voltage based on an oscillation signal, and the AC high voltage is used as an input voltage to drive a Rosen type piezoelectric transformer with a λ / When driving in two modes, the frequency of the oscillation signal is adjusted according to the result of comparison between the input voltage and the first predetermined value, and
A control circuit of a piezoelectric transformer that adjusts the duty ratio of the oscillation signal according to the result of comparing a load current flowing in a cold cathode tube connected to the piezoelectric transformer and a second predetermined value, A capacitor is provided as a detecting means for detecting, and is not affected by the distortion of the voltage waveform generated in the output voltage of the piezoelectric transformer due to the duty ratio adjustment of the oscillation signal according to the fluctuation of the DC power supply voltage. The load current is detected.

Alternatively, a control circuit for a piezoelectric transformer having another circuit configuration for achieving the above-mentioned object is characterized by the following configuration. This control circuit is, for example, a third
Corresponding to the embodiment (FIG. 5).

That is, an AC high voltage is generated based on a rectangular wave voltage obtained by switching a DC power supply voltage based on an oscillation signal, and the AC high voltage is used as an input voltage to drive a Rosen type piezoelectric transformer into a λ / When driving in two modes, the frequency of the oscillation signal is adjusted according to the result of comparison between the input voltage and the first predetermined value, and
A control circuit of a piezoelectric transformer that adjusts the duty ratio of the oscillation signal according to the result of comparing a load current flowing in a cold cathode tube connected to the piezoelectric transformer and a second predetermined value, A low-pass filter is provided in the subsequent stage of the detecting means for detecting, due to the duty ratio adjustment of the oscillation signal according to the fluctuation of the DC power supply voltage, the distortion of the voltage waveform generated in the output voltage of the piezoelectric transformer, It is characterized in that it is removed by the low-pass filter.

In any of the above circuit configurations, a Rosen type laminated piezoelectric transformer is preferably used as the piezoelectric transformer.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a piezoelectric transformer control circuit according to the present invention will be described below with reference to the drawings.

[First Embodiment] FIG. 1 is a block diagram of a piezoelectric transformer control circuit according to the first embodiment.

The control circuit described above is based on the control circuit described with reference to FIG. 6, and the basic driving method of the piezoelectric transformer 1 is the same. 1 is a so-called Rosen type piezoelectric transformer element, and a laminated type piezoelectric transformer element in which a plurality of internal electrodes are fired inside the primary side is adopted, and the circuit configuration shown in FIG. In addition, a notch filter 9 is provided between one terminal of the coil L1 and one input electrode of the piezoelectric transformer 1 in the line for applying the input voltage Vi to the piezoelectric transformer 1. Therefore, in the following explanation,
The control operation achieved by this device will be mainly described.

In the present embodiment, in the control circuit shown in FIG. 1, the drive frequency fd is a resonance frequency of about 100 kHz,
Moreover, it is driven in the λ / 2 mode.

FIG. 2 is a diagram showing an extracted circuit configuration of the drive circuit 7, the coil L1, and the notch filter 9 in the control circuit of the piezoelectric transformer shown in FIG.

In the drawing, in the drive circuit 7, switching elements 7a and 7b such as FETs (field effect transistors) alternately perform switching operation according to the frequency f of the oscillation signal fosc output from the voltage controlled oscillation circuit 11. This is a half-bridge type drive circuit that is performed, and the operation itself of this drive circuit is common nowadays, and thus detailed description in this embodiment will be omitted.

In the notch filter 9, the coil L2 and the capacitor C1 connected in series are connected in the line for applying the input voltage Vi to the Rosen type laminated piezoelectric transformer 1.
It is connected in parallel to the reference potential (GND) of the control circuit.

In this embodiment, the inductance capacitance of the coil L2 and the capacitance capacitance of the capacitor C1 are matched to cause series resonance, and C1≈1 /
The relation (4π ² fo ² L2) is satisfied, and the vertical axis represents the combined impedance Z of the coil L2 and the capacitor C1.
In the frequency characteristic of the notch filter 9 shown in FIG. 3, which is an absolute value of, the center frequency fo of the notch filter 9 is the second harmonic of the drive frequency fd of the piezoelectric transformer 1 (fo = 2fd≈200 kHz
z).

The coil L1 for shaping the rectangular high voltage output from the drive circuit 7 and the coil L2 are L1.
<< There is a magnitude relationship of L2 (for example, L2 = 47 μH with respect to L1 = 4.7 μH).

The inductance capacitance of the coil L1 is matched with the capacitance C of the primary side region of the piezoelectric transformer 1 in order to realize an efficient boosting operation by the piezoelectric transformer 1, and L1≈ 1 / (8π ² fd
² C) to 1 / (4π ² fd ² C) are satisfied.

In the control circuit of the present embodiment, due to the duty ratio adjustment of the oscillation signal by the voltage controlled oscillation circuit 11 according to the fluctuation of the power supply voltage Vd described in the above-mentioned "Problems to be solved by the invention". Piezoelectric transformer 1
When the distortion having the frequency component of the second harmonic of the driving frequency fd is superposed on the input voltage Vi of, the distortion is efficiently absorbed by the notch filter 9 having the center frequency 2fd, and therefore, after the distortion is removed. An AC high voltage can be applied to the piezoelectric transformer 1, and the control circuit shown in FIG. 1 makes the load current flowing through the cold cathode tube 2 substantially constant (predetermined value), as in the case where no waveform distortion occurs. The held control operation can be continued.

That is, according to the present embodiment described above, stable boosting is performed even when waveform distortion occurs in the input voltage of the piezoelectric transformer due to the adjustment of the duty ratio of the oscillation signal according to the fluctuation of the DC power supply voltage. Can perform actions.

[Second Embodiment] FIG. 4 is a block diagram of a piezoelectric transformer control circuit according to the second embodiment.

The above-mentioned control circuit is based on the control circuit described with reference to FIG. 6, and the basic driving method of the piezoelectric transformer 1 is the same, so that the duplicate description will be omitted, but FIG. In the present embodiment, a capacitor 13 for detecting the load current is connected instead of the detection resistor 3 for detecting the load current shown in FIG. Therefore, in the following description, the control operation achieved by this device will be mainly described.

In the present embodiment, in the control circuit shown in FIG. 4, the drive frequency fd is a resonance frequency of about 100 kHz,
In addition, the Rosen type laminated piezoelectric transformer 1 is driven in the λ / 2 mode. The drive circuit 7 is a half-bridge type drive circuit in which the switching elements 7a and 7b alternately perform the switching operation as in the first embodiment.

In the control circuit of the present embodiment, due to the duty ratio adjustment of the oscillation signal by the voltage controlled oscillation circuit 11 according to the fluctuation of the power supply voltage Vd described in the above-mentioned "Problems to be solved by the invention". Piezoelectric transformer 1
When the input voltage Vi is distorted, waveform distortion is also generated in the output voltage Vo according to the distortion. In the present embodiment, since the capacitor 13 is provided as shown in FIG. The waveform distortion superimposed as the high frequency component of the voltage Vo is not detected by the half-wave rectification rectifier circuit 4 in the present embodiment, and the rectifier circuit detects only the fundamental wave component of the output voltage Vo.

Therefore, in the control circuit shown in FIG. 4, even when the input voltage Vi of the piezoelectric transformer 1 is distorted,
Since the rectifier circuit 4 does not have to be affected by the output voltage Vo affected by the waveform distortion, the fundamental wave component of the output voltage Vo of the piezoelectric transformer 1 is almost the same as when the waveform distortion is not generated. Based on this, it is possible to continue the control operation for maintaining the load current flowing through the cold cathode fluorescent lamp 2 at a substantially constant value (predetermined value).

That is, according to the present embodiment described above, stable boosting is performed even when waveform distortion occurs in the input voltage of the piezoelectric transformer due to the duty ratio adjustment of the oscillation signal according to the fluctuation of the DC power supply voltage. Can perform actions.

[Third Embodiment] FIG. 5 is a block diagram of a piezoelectric transformer control circuit according to a third embodiment.

The above-mentioned control circuit is based on the control circuit described with reference to FIG. 6, and the basic driving method of the piezoelectric transformer 1 is the same, so that a duplicate description will be omitted, but FIG. In addition to the circuit configuration shown in, a low pass filter (LPF) is provided between the rectifier circuit 4 and the error amplifier 5.
Are connected. Therefore, in the following explanation,
The control operation achieved by this device will be mainly described.

In the present embodiment, in the control circuit shown in FIG. 4, the drive frequency fd is a resonance frequency of about 100 kHz,
In addition, the Rosen type laminated piezoelectric transformer 1 is driven in the λ / 2 mode. The drive circuit 7 is a half-bridge type drive circuit in which the switching elements 7a and 7b alternately perform the switching operation as in the first embodiment.

In the control circuit of the present embodiment, due to the duty ratio adjustment of the oscillation signal by the voltage controlled oscillation circuit 11 according to the fluctuation of the power supply voltage Vd described in the above-mentioned “Problems to be solved by the invention”. Piezoelectric transformer 1
When a distortion occurs in the input voltage Vi of, the waveform distortion also occurs in the output voltage Vo according to the distortion, and the output voltage Vo
Unlike the case of the second embodiment described above, the waveform distortion superposed as the high frequency component of is detected by the detection resistor 3 as in the conventional technique shown in FIG. However, in this embodiment, as shown in FIG.
14 is provided, and the LPF 14 is applied to the voltage detected by the detection resistor 3, so that the distortion component (harmonic component) included in the detected voltage is removed.

Therefore, the rectifier circuit 4 can rectify the voltage that does not include the distortion component, and therefore the error amplifier 5
Then, as the load current detection voltage Vri representing the magnitude of the output current (load current), the fundamental wave component of the detection voltage can be compared with the reference voltage Vref1.

That is, when the input voltage Vi of the piezoelectric transformer 1 is distorted, the control circuit shown in FIG. 5 causes the detection resistor 3 to be affected by the output voltage Vo affected by the waveform distortion. Although the waveform distortion caused by the detected voltage is removed by the LPF 14, the fundamental voltage component of the output voltage Vo of the piezoelectric transformer 1 is substantially the same as when the input voltage Vi has no waveform distortion. Based on the above, the control operation for maintaining the load current flowing through the cold cathode tube 2 at a substantially constant value (predetermined value) can be continued.

As described above, according to the above-described embodiment, even if the input voltage of the piezoelectric transformer is distorted in waveform due to the adjustment of the duty ratio of the oscillation signal according to the fluctuation of the DC power supply voltage, it is stable. The boosting operation can be performed.

In each of the embodiments described above, the rectifying circuit 4
Was described as a half-wave rectification type rectifier circuit,
Similar effects can be obtained when a full-wave rectification circuit is applied.

[0053]

According to the present invention described above, stable boosting is performed even when waveform distortion occurs in the input voltage of the piezoelectric transformer due to the duty ratio adjustment of the oscillation signal according to the fluctuation of the DC power supply voltage. It is possible to provide a control circuit of a piezoelectric transformer that operates.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of a piezoelectric transformer control circuit according to a first embodiment.

2 is a diagram showing an extracted circuit configuration of a drive circuit 7, a coil L1, and a notch filter 9 in the control circuit of the piezoelectric transformer shown in FIG.

FIG. 3 is a diagram showing frequency characteristics of a notch filter 9 according to this embodiment.

FIG. 4 is a block configuration diagram of a piezoelectric transformer control circuit according to a second embodiment.

FIG. 5 is a block configuration diagram of a piezoelectric transformer control circuit according to a third embodiment.

FIG. 6 is a block configuration diagram of a control circuit for a piezoelectric transformer that the applicant of the present application has previously proposed.

7 shows a switching operation of a half-bridge type drive circuit 7 in the control circuit shown in FIG. 6 and a piezoelectric transformer 1 in which waveform distortion has occurred due to duty ratio adjustment of an oscillation signal according to fluctuations in a power supply voltage Vd. It is a figure which shows the input voltage Vi and the output voltage Vo.

[Explanation of symbols]

1: Piezoelectric transformer, 2: Cold cathode tube, 3: Detection resistor, 4, 9: rectifier circuit, 5, 10: error amplifier circuit, 7: drive circuit, 7a, 7b: switching element, 8a, 8b: drive voltage detection resistor, 11: voltage controlled oscillator circuit, 12: Notch filter, 13: capacitor, 14: LPF (low pass filter),

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H05B 41/24 H01L 41/08 A F term (reference) 3K072 AA01 AC02 AC11 BA03 BC07 CB08 CB10 FA05 GA02 GB12 HA04 HA10 HB03 5H007 AA02 BB03 CB09 DA03 DA06 DB01 DC02 DC05 EA13

Claims (5)

[Claims] 1. An AC high voltage is generated based on a rectangular wave voltage obtained by switching a DC power supply voltage based on an oscillation signal, and the AC high voltage is used as an input voltage to drive a Rosen type piezoelectric transformer into a λ / When driving in the two modes, the frequency of the oscillation signal is adjusted according to the result of comparison between the input voltage and the first predetermined value, and the load current flowing through the cold cathode tube connected to the piezoelectric transformer and the first 2 is a control circuit of a piezoelectric transformer that adjusts the duty ratio of the oscillation signal according to the result of comparison with a predetermined value of 2, wherein a notch filter is provided in a line that applies the input voltage to the piezoelectric transformer, The notch filter removes the distortion of the voltage waveform generated in the input voltage due to the adjustment of the duty ratio of the oscillation signal according to the fluctuation of the voltage. Piezoelectric transformer control circuit, characterized in that. 2. The center frequency of the notch filter is set to the second harmonic of the drive frequency of the piezoelectric transformer, so that distortion having a frequency component of the second harmonic superimposed on the input voltage is removed. Claim 1 characterized by the above.
A piezoelectric transformer control circuit described. 3. An AC high voltage is generated based on a rectangular wave voltage obtained by switching a DC power supply voltage based on an oscillation signal, and the AC high voltage is used as an input voltage to drive a Rosen type piezoelectric transformer with a λ / When driving in the two modes, the frequency of the oscillation signal is adjusted according to the result of comparison between the input voltage and the first predetermined value, and the load current flowing through the cold cathode tube connected to the piezoelectric transformer and the first 2 is a control circuit of a piezoelectric transformer that adjusts the duty ratio of the oscillation signal according to the result of comparison with a predetermined value of 2, wherein a capacitor is provided as a detection unit that detects the load current, and Due to the duty ratio adjustment of the oscillation signal according to, without being affected by the distortion of the voltage waveform generated in the output voltage of the piezoelectric transformer,
A control circuit for a piezoelectric transformer, which detects the load current. 4. An AC high voltage is generated based on a rectangular wave voltage obtained by switching a DC power supply voltage based on an oscillation signal, and the AC high voltage is used as an input voltage to drive a Rosen type piezoelectric transformer with a λ / When driving in the two modes, the frequency of the oscillation signal is adjusted according to the result of comparison between the input voltage and the first predetermined value, and the load current flowing through the cold cathode tube connected to the piezoelectric transformer and the first A control circuit of a piezoelectric transformer that adjusts the duty ratio of the oscillation signal according to a result of comparison with a predetermined value of 2, wherein a low-pass filter is provided at a stage subsequent to a detection unit that detects the load current, The distortion of the voltage waveform generated in the output voltage of the piezoelectric transformer due to the duty ratio adjustment of the oscillation signal according to the fluctuation of Piezoelectric transformer control circuit, characterized in that the removal. 5. The piezoelectric transformer control circuit according to claim 1, wherein the piezoelectric transformer is a Rosen type laminated piezoelectric transformer.