JPH0956175A - Power supply circuit apparatus containing piezoelectric transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to perform the control of a drive frequency of a piezoelectric transformer by a simple circuit in a power supply circuit apparatus containing a piezoelectric transformer. SOLUTION: A series circuit of a primary winding of an auxiliary transformer 14 and an FET 17 is connected to a DC power supply 12. A piezoelectric transformer 19 is connected to the second winding of the auxiliary transformer. A gate voltage of the FET 17 and a drain to source voltage are detected for detecting the resonance state of the piezoelectric transformer 19, and these are mixed together in a mixing circuit 34. A mixed output voltage and a reference voltage are compared by a comparator 36, and an output containing the information about resonance state of the piezoelectric transformer 19 is obtained. An output from a comparator 36 is integrated by VCO 39, and a drive frequency of the piezoelectric transformer 19 is made closer to this resonance frequency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit device including a piezoelectric transformer suitable for lighting a CCFL (cold cathode fluorescent tube).

[0002]

2. Description of the Related Art A piezoelectric transformer of a type in which a drive voltage is applied to a pair of electrodes formed in the thickness direction of a piezoelectric ceramic plate and an output voltage is taken out from the electrodes formed in the length direction is known. Since this type of piezoelectric transformer uses mechanical vibration, it has features that unnecessary radiation of electromagnetic waves does not occur, waveform distortion and noise are small, thin and highly efficient, nonflammable and safe. CC utilizing these features
The use of a piezoelectric transformer in the FL power supply circuit is described in, for example, "Nikkei Electronics," published on Nov. 7, 1994, pages 147 to 157, "Liquid Crystal Backlight Inverter, Piezoelectric Transformer: Thickness 4". .5 mm "
And so on.

By the way, since the resonance frequency of the piezoelectric transformer changes due to changes in load current, changes in input voltage, changes in temperature, etc., in order to drive the piezoelectric transformer efficiently under all conditions, the resonance frequency of the piezoelectric transformer is always maintained. You have to excite following the change of. The following three methods are known as methods for controlling the excitation voltage of the piezoelectric transformer, that is, the frequency of the drive voltage. (1) The piezoelectric transformer is driven at a fixed frequency slightly higher than the natural resonance frequency of the piezoelectric transformer. (2) Since the resonance frequency of the piezoelectric transformer also changes depending on the level of the drive voltage of the piezoelectric transformer, the input voltage is detected and the frequency of the drive voltage is detected as described in the above-mentioned article of "Nikkei Electronics". Change. (3) For example, as disclosed in JP-A-61-220386, the phase relationship between the input voltage of the piezoelectric transformer and the output current or the input current is detected by comparison, and the drive voltage is made to follow the frequency change of the piezoelectric transformer. Change the frequency of.

FIG. 1 shows a power supply circuit of a CCFL according to the above method (3). In this power supply circuit, the piezoelectric transformer 1 is driven by an output of a VCO (voltage controlled oscillator) amplified by a drive amplifier 3. In order to control the frequency of the VCO2, the input voltage to the piezoelectric transformer 1 and the voltage detected by the current detector 4 are phase-compared by the phase comparator 5, and this output is smoothed by the low frequency filter 6 to control the VCO2 control voltage. Becomes The piezoelectric transformer 1 has an input electrode 7, an output electrode 8 and a common electrode 9, and a load 10 is connected to the output electrode 8.

[0005]

However, the above (1)
The conventional methods (2) and (3) have the following problems. First, in the frequency fixed system of (1), the power conversion efficiency is limited to only one place under the condition that there is the best point. Also,
It is necessary to adjust the frequency to match the frequency variation of each piezoelectric transformer. Further, the influence of the shift of the resonance frequency of the piezoelectric transformer due to the change of the ambient temperature is large.
In the supply voltage detection method of (2), the change of the resonance frequency due to the change of the input voltage of the piezoelectric transformer has an effect of improving if the tracking is appropriate, but the load change and the temperature change are the same as the above (1). . In the phase detection method of (3), since the resonance state of the piezoelectric transformer due to changes in supply voltage, load, and temperature is constantly detected and frequency is controlled,
Although it is the most stable against a change factor, it uses a phase comparator to complicate the circuit configuration, has many components, and is not suitable for downsizing and cost reduction.

Therefore, an object of the present invention is to provide a power supply circuit device capable of controlling the drive frequency of a piezoelectric transformer to an optimum value or in the vicinity thereof with a relatively simple structure.

[0007]

The present invention for achieving the above object comprises a DC power supply, an auxiliary transformer, a switching element, a piezoelectric transformer, and a control circuit for the switching element, wherein the auxiliary transformer is a primary winding. And a secondary winding, one end of the primary winding is connected to one end of the DC power supply, the switching element has first and second main terminals and a control terminal, and The first main terminal is connected to the other end of the primary winding, the second main terminal of the switching element is connected to the other end of the DC power supply, and the piezoelectric transformer has an input electrode and an output electrode. A common electrode, wherein the input electrode of the piezoelectric transformer is 2
The common electrode of the piezoelectric transformer is connected to one end of a secondary winding, the common electrode of the piezoelectric transformer is connected to the other end of the secondary winding, and a load is connected between the output electrode and the common electrode of the piezoelectric transformer. The control circuit is for supplying a pulse for controlling ON / OFF of the switching element to the control electrode of the switching element, and between the control electrode of the switching element and the second main electrode. First of
First voltage detecting means for detecting the voltage of the second element, second voltage detecting means for detecting the second voltage between the first and second main electrodes of the switching element, and the first and second A mixing circuit that mixes voltages, a reference voltage source, a voltage comparator that generates a comparison output of the reference voltage of the reference voltage source and the mixed voltage obtained from the mixing circuit, and the output of the voltage comparator is integrated. Power supply having an integrating circuit for controlling the voltage, a voltage controlled oscillator controlled by an output voltage of the integrating circuit, and a means for supplying the output of the voltage controlled oscillator to the control terminal of the switching element. It relates to a circuit device. As described in claim 2, instead of the transformer of claim 1, a reactor can be connected in series with the switching element. Further, as shown in claim 3, the transformer of claim 1 can be a single-winding transformer. Further, as shown in claim 4, a reactor, that is, an inductance element can be added in series to the primary winding of claim 1. Further, as described in claim 5, it is desirable to connect a resonance capacitor in parallel with the switching element. Further, as described in claim 6, it is desirable to provide a pulse width modulation circuit and a load current detection circuit and control the pulse width for turning on / off the switching element by the output of the load current detection circuit.

[0008]

According to the invention of each claim, the first voltage between the control electrode of the switching element and the second main electrode and the second voltage between the first and second main electrodes are set. After mixing, the resonance state of the piezoelectric transformer is detected by comparing this with a reference voltage, and the voltage controlled oscillator is controlled based on this detection. As a result, when the resonance frequency of the piezoelectric transformer changes, the driving frequency of the piezoelectric transformer changes accordingly. As a result, the piezoelectric transformer can be operated with the best conversion efficiency with respect to changes in load current, changes in input voltage, and changes in operating temperature. Further, according to the invention of each claim, it is possible to know the resonance state of the piezoelectric transformer without current detection, and it is possible to reduce the power loss. Further, when the resonance capacitor is provided as described in the fifth aspect, it becomes easy to detect the resonance state of the piezoelectric transformer based on the output of the mixing circuit. According to claim 6,
Current control can be easily achieved by pulse width modulation.

[0009]

[First Embodiment] A CCFL power supply circuit device according to a first embodiment of the present invention will now be described with reference to FIGS. In FIG. 2, the DC power supply circuit 11 includes a DC power supply 12 composed of a rectifier or a battery and a smoothing or stabilizing capacitor 13 connected in parallel with the DC power supply 12. The transformer 14 includes a primary winding 15 and a secondary winding 16, and one end of the primary winding 15 having a leakage inductance is connected to one end of the power supply 12. A FET (field effect transistor) 17 as a switching element has a drain as a first main electrode, a source as a second main electrode, and a gate as a control electrode, and the drain is the other end of the primary winding 15. It is connected to the. The FET 17 is an insulated gate (MOS) type FET having a source connected to the substrate, and has a built-in diode connected in parallel to the source / drain. In addition, it has a stray capacitance between the drain and the source. The capacitor 18 connected in parallel to the primary winding 15 is a resonance capacitor, and the power supply circuit 11
Is sufficiently smaller than the condenser 13 of FIG.

The piezoelectric transformer 19 is a piezoelectric ceramic element 2
The input electrode 21, the output electrode 22, and the common electrode 23 are provided at 0, the input electrode 21 is connected to one end of the secondary winding 16, and the common electrode 23 is connected to the other end of the secondary winding 16 and It is connected to the other end of the power source 12, that is, the ground. One end of the CCFL 24 as a load is connected to the output electrode 22 of the piezoelectric transformer 19, and the other end is connected to the ground via the dimming current detection circuit 25.

The current detection circuit 25 includes first and second diodes 26 and 27, first and second resistors 28 and 29, and
It is composed of one variable resistor 30 for dimming. First resistor 28
Is connected between the CCFL 24 and the ground through the first diode 26, and the second diode 28 has a direction opposite to that of the first diode 26.
6 and the resistor 28 are connected in parallel to the series circuit. The second resistor 29 is connected in parallel to the first resistor 28 via the variable resistor 30. First and second resistors 2
A positive half-wave load current flows through the first and second diodes 26 and 29 and the variable resistor 30. Therefore, the current detection circuit 25 is detecting the positive half-wave load current. The second diode 27 is for passing a negative half-wave load current. An AC voltage is generated at the output terminal of the piezoelectric transformer 19.

The capacitor 31 connected between the output line of the current detecting circuit 25 and the ground is provided as an integrating circuit, that is, a smoothing circuit. Therefore, a smoothed current detection voltage is obtained from both ends of the capacitor 31.

The control circuit of the FET 17 includes a first voltage detection line 32 as first voltage detection means for detecting the gate-source voltage (first voltage) of the FET 17, and the FET.
Second voltage detection line 3 as second voltage detection means for detecting the drain-source voltage (second voltage) 17
3, and a mixing circuit 34 that mixes the first and second voltages,
Reference voltage circuit 35, voltage comparator 36, integration circuit 37
An amplifier circuit 38 and a VCO (voltage controlled oscillator) 39
And a PWM (pulse width modulation) circuit 40.

The mixing circuit 34 includes a first circuit, a second circuit, a third circuit, and a fourth circuit.
Of mixing resistors 41, 42, 43, 44, a waveform delaying capacitor 45, and a diode 46. The first mixing resistor 41 is connected between the first voltage detection line 32 and the ground via a capacitor 45. The second mixing resistor 42 is connected between one end of the capacitor 45 and the output line 47 of the mixing circuit 34.
The third mixing resistor 43 is connected between the first voltage detection line 32 and the mixing output line 47 via a diode 46. That is, the third mixing resistor 43 is connected in parallel to the series circuit of the first and second mixing resistors 41 and 42. The fourth mixing resistor 44 is connected between the second voltage detection line 33 and the mixing output line 47. The output line 47 of the mixing circuit 34 is connected to one input terminal (positive input terminal) of the comparator 36. The constant of each part of the mixing circuit 34 is obtained by combining the gate voltage Vg as the first voltage shown in FIG. 4B and the drain-source voltage Vds as the second voltage shown in FIG. 4A. Figure 4
It is set so that the mixed output voltage Vm of the waveform of (C) can be obtained.

The reference voltage circuit 35 includes voltage dividing resistors 48 and 49.
Is connected to the DC power supply terminal 50, and the voltage dividing point is connected to the other input terminal (negative input terminal) of the comparator 36. The reference voltage Vr at the voltage dividing point of the reference voltage circuit 35 has the level shown by the dotted line in FIG.

The comparator 36 is formed to compare the mixed output voltage Vm and the reference voltage Vr and generate a binary comparison output voltage shown in FIG. 4 (D). The waveform of FIG. 4D changes with a response delay as compared with the waveform of FIG. 4C.

The integrating circuit 37 is connected to the output terminal of the comparator 36, and is formed so as to smooth the comparison output of FIG. The output of the integrating circuit 37 is V through the amplifier 38.
It is supplied to the voltage control terminal of CO39.

The VCO 39 is a well-known oscillator whose output frequency changes according to the change of the control voltage.

The PWM circuit 40 includes a VCO 39 and a FET 17
Connected to the gate of the VCO 39 and is configured to generate a PWM pulse at a frequency that matches the output frequency of the VCO 39. The PWM circuit 40 is formed so that the output pulse width changes in response to the voltage of the line 51 connected to the output stage capacitor 31 of the current detection circuit 25.

FIG. 3 shows the PWM circuit 40 in detail, which comprises a triangular wave generating circuit 52, a reference voltage source 53, an error amplifier 54, and a voltage comparator (comparator) 55. The triangular wave generation circuit 52 generates a triangular wave voltage in synchronization with the output signal of the VCO 39. The error amplifier 54 forms a signal indicating the difference between the current detection voltage of the line 51 and the reference voltage of the reference voltage source 53. The comparator 55 compares the triangular wave voltage with the output voltage of the error amplifier 54 to form a PWM pulse, and supplies this to the gate of the FET 17.

[0021]

[Operation] The voltage of the power supply 12 is applied to the primary winding 15 while the FET 17 is on, and thereby the auxiliary boosted voltage is induced in the secondary winding 16 while the FET 17 is off. The voltage of the secondary winding 16 is supplied between the input electrode 21 and the common electrode 23 of the piezoelectric transformer 19. Since the FET 17 is intermittently turned on / off, the piezoelectric transformer 19 is also intermittently driven. The piezoelectric transformer 19 causes the output electrode 22 to have a high alternating voltage (actual value of 1500
V, 500 V) is generated in a steady state. The voltage of the piezoelectric transformer 19 is applied to the CCFL 24, and a current flows there. The current of the CCFL 24 is detected by the current detection circuit 25, smoothed by the capacitor 31, and becomes a control signal of the PWM circuit 40. The variable resistor 30 included in the current detection circuit 25 is used to adjust the brightness of the CCFL 24. That is, when the value of the variable resistor 30 is changed, the width of the PWM pulse generated from the PWM circuit 40 changes and the brightness of the CCFL 24 changes.

In order to drive the piezoelectric transformer 19 efficiently, it is necessary to control the frequency of this drive voltage in accordance with the resonance frequency. For this purpose, the resonance state of the piezoelectric transformer 19 is detected and the output frequency of the VCO 39 is controlled. In the present invention, the gate voltage and drain
The resonance state of the piezoelectric transformer 19 is detected based on the voltage between the sources. In Figure 4, the current of CCFL24 is 4.5m
The voltage of each part in the case of A is shown. In the period from t1 to t2 in which the gate voltage Vg of FIG. 4B is at a low level, the inductance of the primary winding 15 and the resonance of the capacitor 18 occur,
The voltage of the capacitor 18 changes sinusoidally. Since the resonance capacitor 18 is connected in parallel to the FET 17 via the capacitor 13 having a larger capacity than this, the FET 1
The drain-source voltage Vds of 7 becomes a half-wave of a sine wave like the voltage change of the resonance capacitor 18, as shown in FIG. Drain-source voltage Vds of FIG.
Is adjusted in level by a resistor 44, and the gate voltage Vg in FIG. 4B is adjusted in level by resistors 41 and 42 to be mixed or synthesized. The capacitor 4 in the mixing circuit 34
Reference numeral 5 serves as a hold capacitor for preventing the fall of the waveform of the gate voltage Vg and the fall of the waveform of the drain-source voltage Vds. Further, the resistor 43 and the diode 46 have a function of preventing the rising of the rising waveform of the gate voltage Vg. By mixing the drain-source voltage Vds and the gate voltage Vg, the mixed voltage Vm of FIG. 4C is obtained. The waveform of the mixed voltage Vm has a depression near the rising time t2 of the gate voltage Vg. This hollow is
Half-wave resonant voltage waveform V generated in the OFF period of the FET 17
This occurs because the generation period of ds is shorter than the off period of the gate voltage Vg. 4 (B1) and (B2) show resistors 41, 42, 43 and a capacitor 4 at the gate voltage Vg of FIG. 4 (B).
5 shows a state in which a delay is given by the circuit of 5 and the diode 46.
The waveform of FIG. 4 (B1) is the waveform of the voltage V45 of the capacitor 45, and the waveform of FIG. 4 (B2) is the connection point of the resistor 42 and the diode 46 before mixing the drain-source voltage Vds. It is the waveform of the obtained voltage Vg2. Therefore, FIG.
Is obtained by mixing the drain-source voltage Vds of FIG. 4 (A) and the delayed gate voltage Vg2 of FIG. 4 (B2). The size of this recess is the piezoelectric transformer 19
It changes according to the change of the input impedance of. That is,
The input impedance of the piezoelectric transformer 19 changes according to the change of the input frequency (driving frequency) of the piezoelectric transformer 19 and the change of the output current, and becomes the smallest value when the piezoelectric transformer 19 is in the resonance state and the load is large. Piezoelectric transformer 1
The change of the input impedance of 9 is the auxiliary transformer 1 of the previous stage.
4 is converted to the primary side, and the voltage waveform on the primary side, that is, the drain-source voltage Vds is affected. The rise of the drain-source voltage Vds of the FET 17 coincides with the fall of the gate voltage Vg, but the fall position of the drain-source voltage Vds changes with the change of the input impedance of the piezoelectric transformer 19.

In the comparator 36, the reference voltage V shown in FIG.
Comparing r with the mixed voltage Vm, the negative pulse (low level pulse) shown in FIG. 4D is obtained corresponding to the period when the mixed voltage Vm becomes lower than the reference voltage Vr. Since the length of the period t2 to t3 of the negative pulse includes the information of the input impedance of the piezoelectric transformer 19, that is, the information of the resonance state,
When this is integrated by the integrating circuit 37, inverted by the inverting amplifier 38 and added to the VCO 39, the output frequency of the VCO 39 and the output frequency of the PWM circuit 40 change, and the piezoelectric transformer 1
The drive frequency of 9 follows this resonance frequency.

The CCF is adjusted by adjusting the variable resistor 30 for brightness adjustment.
When the current of L24 is set to 6 mA, the voltage Vds, Vg of each part
, Vm, and Vc change as shown in FIGS. That is, the width of the PWM pulse shown in FIG. 5B is widened by adjusting the variable resistor 30. On the other hand, the width of the waveform of the drain-source voltage Vds is narrowed under the influence of the change in the impedance of the piezoelectric transformer 19. This allows
The width of the recess in the negative direction of the mixed voltage Vm shown in FIG. 5 (C) is also narrowed, and the width of the negative pulse of the output of the comparator 36 is also narrowed. As a result, the control voltage of the VCO 39 drops and the VCO 39
The output frequency of 39 also decreases. Therefore, when the load current increases, the output frequency of the VCO 39 and the drive frequency of the piezoelectric transformer 19 decrease.

When the current of the CCFL 24 is reduced to 3 mA, the voltages Vds, Vg, Vm and Vc of the respective parts are shown in FIG.
It changes as shown in (A) to (D). That is, in this case, the operation is the reverse of the case where the current of the CCFL 24 is increased from 4.5 mA to 6 mA, the width of the PWM pulse in FIG. 6 (B) becomes narrower, and the drain / source in FIG. The width of the waveform of the inter-voltage Vds becomes wider, and the mixed voltage Vds of FIG.
The width of the negative depression of m becomes wider, and the negative pulse output from the comparator 36 becomes wider.

As is apparent from the above, according to the present embodiment, the output frequency of the VCO 39 is controlled by the piezoelectric transformer with a relatively simple circuit configuration including the mixing circuit 34, the voltage comparator 36, the integrating circuit 37, and the inverting amplifier 38. It can be changed by following the change of the resonance frequency of 19. Further, in the present embodiment, without detecting the current of the piezoelectric transformer 19,
Since the resonance state of the piezoelectric transformer 19 can be detected, power loss is reduced.

[0027]

Second Embodiment Next, a CCFL power supply circuit device of a second embodiment will be described with reference to FIG. However, in FIG. 7 and FIGS. 8 and 9 to be described later, the substantially same parts as those in FIG. The circuit of FIG. 7 has a reactor 60 instead of the auxiliary transformer 14 of FIG.
2 and the capacitor 18a is directly connected to the FET 17 in parallel, which is the same as the circuit of FIG. That is, one end of the reactor 60 in FIG. 7 is connected to one end of the power supply 12, and the other end is connected to the drain of the FET 17 and the input electrode 21 of the piezoelectric transformer 19.

In the circuit of FIG. 7, energy is stored in the reactor 60 while the FET 17 is on. FET17
While is off, a voltage obtained by adding the voltage of the reactor 60 to the voltage of the power supply 12 is applied to the piezoelectric transformer 19. Other operations of the circuit of FIG. 7 are the same as those of FIG. 2, and the same effects can be obtained.

[0029]

[Third Embodiment] The circuit of FIG. 8 corresponds to the auxiliary transformer 1 of FIG.
The configuration is the same as that of FIG. 2 except that a single-winding transformer (auto transformer) 14a is provided in place of 4, and a capacitor 18a is connected in parallel to the FET 17. That is, one end or the input terminal of the primary and secondary common winding 15a is connected to the power source 12, and the other end or the common terminal is connected to the drain of the FET 17. The boost output winding 16a is connected between one end of the common winding 15a and the input electrode 21 of the piezoelectric transformer 19. That is, the output terminal of the autotransformer 14a is connected to the input electrode 21 of the piezoelectric transformer 19. Since the embodiment of FIG. 8 is the same as that of FIG. 2 except that the form of the auxiliary transformer is changed, it has the same operation and effect as the circuit of FIG.

[0030]

[Fourth Embodiment] The circuit of FIG. 9 has the same configuration as that of FIG. 2 except that an inductance element 61 is added to the circuit of FIG. The inductance element 61 is connected in series to the primary winding 15, and the sum of the inductance of the primary winding 15 and the inductance of the inductance element 61 contributes to resonance. 9 is the same as FIG. 2 except for this, and therefore, FIG.
2 has the same effect as that of FIG.

[0031]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) The capacitance corresponding to this can be obtained by the stray capacitance (stray capacitance) of the winding 15 without connecting individual capacitors as the capacitors 18 and 18a. (2) The configuration of the PWM circuit 40 can be variously modified. (3) Instead of the FET 17, a semiconductor switching element such as a bipolar transistor can be used. (4) Gate voltage Vg of FET 17 and drain source
The configuration for obtaining the frequency control voltage by comparing the inter-cell voltage Vds can be variously modified.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a power supply circuit device having a conventional piezoelectric transformer.

FIG. 2 is a circuit diagram showing a power supply circuit device according to a first embodiment.

FIG. 3 is a circuit diagram showing the PWM circuit of FIG.

FIG. 4 is a waveform diagram showing the voltage of each part of the circuit of FIG. 2 when the load current is standard.

5 is a waveform diagram showing the voltage of each part of the circuit of FIG. 2 when the load current is larger than the standard.

FIG. 6 is a waveform diagram showing the voltage of each part of the circuit of FIG. 2 when the load current is smaller than the standard.

FIG. 7 is a circuit diagram showing a power supply circuit device according to a second embodiment.

FIG. 8 is a circuit diagram showing a power supply circuit device according to a third embodiment.

FIG. 9 is a circuit diagram showing a power supply circuit device according to a fourth embodiment.

[Explanation of symbols]

 14 Auxiliary transformer 17 FET 19 Piezoelectric transformer 34 Mixed circuit

Claims (6)

[Claims] 1. A direct current power supply, an auxiliary transformer, a switching element, a piezoelectric transformer, and a control circuit for the switching element, wherein the auxiliary transformer has a primary winding and a secondary winding. One end is connected to one end of the DC power supply, the switching element has first and second main terminals and a control terminal, and the first main terminal of the switching element is the other end of the primary winding. The second main terminal of the switching element is connected to the other end of the DC power supply, the piezoelectric transformer has an input electrode, an output electrode, and a common electrode, and the input electrode of the piezoelectric transformer is Connected to one end of the secondary winding, the common electrode of the piezoelectric transformer connected to the other end of the secondary winding, and a load connected between the output electrode and the common electrode of the piezoelectric transformer. , The system A circuit is for supplying a pulse for controlling ON / OFF of the switching element to the control electrode of the switching element, and between the control electrode of the switching element and the second main electrode. First voltage detecting means for detecting a first voltage, second voltage detecting means for detecting a second voltage between the first and second main electrodes of the switching element, and the first and the second A mixing circuit for mixing two voltages, a reference voltage source, a voltage comparator for generating a comparison output of the reference voltage of the reference voltage source and the mixed voltage obtained from the mixing circuit, and an output of the voltage comparator A voltage-controlled oscillator controlled by the output voltage of the integration circuit, and means for supplying the output of the voltage-controlled oscillator to the control terminal of the switching element. You Power supply circuit device. 2. A direct current power supply, a reactor, a switching element, a piezoelectric transformer, and a control circuit for the switching element, wherein one end of the reactor is connected to one end of the direct current power supply, and the switching element has first and second switching elements. A main terminal and a control terminal, the first main terminal of the switching element is connected to the other end of the reactor, and the second main terminal of the switching element is connected to the other end of the DC power supply. The piezoelectric transformer has an input electrode, an output electrode, and a common electrode, the input electrode of the piezoelectric transformer is connected to the other end of the reactor, and the common electrode of the piezoelectric transformer is the second electrode of the switching element. A load is connected between the output electrode and the common electrode of the piezoelectric transformer, and the control circuit is connected to the switch terminal. A pulse for controlling ON / OFF of the switching element to the control electrode of the switching element, the first electrode between the control electrode of the switching element and the second main electrode. First voltage detecting means for detecting a voltage, second voltage detecting means for detecting a second voltage between the first and second main electrodes of the switching element, and the first and second voltages And a reference voltage source, a voltage comparator that generates a comparison output of the reference voltage of the reference voltage source and the mixed voltage obtained from the mixing circuit, and the output of the voltage comparator is integrated. A power supply circuit having an integrating circuit, a voltage controlled oscillator controlled by an output voltage of the integrating circuit, and means for supplying an output of the voltage controlled oscillator to the control terminal of the switching element. apparatus. 3. A DC power supply, an auxiliary transformer, a switching element, a piezoelectric transformer, and a control circuit for the switching element, wherein the auxiliary transformer is a single-winding transformer and has a winding having an input terminal, a common terminal, and an output terminal. The input terminal of the winding is connected to one end of the DC power supply, the switching element has a first and second main terminals and a control terminal, the first main terminal of the switching element is The piezoelectric transformer is connected to a common terminal of the winding, the second main terminal of the switching element is connected to the other end of the DC power source, the piezoelectric transformer has an input electrode, an output electrode, and a common electrode, The input electrode of the transformer is connected to the output terminal of the winding, the common electrode of the piezoelectric transformer is connected to the second terminal of the switching element, the piezoelectric transformer A load is connected between the output electrode and the common electrode of the switching element, and the control circuit is for supplying a pulse for on / off controlling the switching element to the control electrode of the switching element. Between a first voltage detecting means for detecting a first voltage between the control electrode of the switching element and the second main electrode, and between the first and second main electrodes of the switching element. Second voltage detection means for detecting a second voltage, a mixing circuit for mixing the first and second voltages, a reference voltage source, a reference voltage of the reference voltage source and the mixing circuit are obtained. A voltage comparator that generates a comparison output with a mixed voltage, an integrating circuit that integrates the output of the voltage comparator, a voltage controlled oscillator controlled by the output voltage of the integrating circuit, and an output of the voltage controlled oscillator Switchon And a means for supplying to the control terminal of the switching element. 4. The power supply circuit device according to claim 1, further comprising an inductance element connected in series to the primary winding. 5. The power supply circuit device according to claim 1, further comprising a resonance capacitor connected in parallel to the switching element. 6. A load current detection circuit, a current detection integration circuit, and a pulse width modulation circuit, the load current detection circuit detecting a current of the load connected to the output electrode of the piezoelectric transformer. The current detection integrating circuit is formed to smooth and output a voltage corresponding to the load current obtained from the load current detection circuit, and the pulse width modulation circuit is configured to output the voltage. A triangular wave generation circuit connected to the controlled oscillator, a triangular wave voltage obtained from the triangular wave generation circuit and a voltage obtained from the current detection integration circuit or a voltage corresponding thereto are compared to form a pulse, and 6. A power supply circuit device according to claim 1, further comprising a voltage comparator for applying a pulse to the control terminal of the switching element.