JPH1169826A - Piezoelectric transformer drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric transformer drive circuit which stably drives a piezoelectric transformer at its resonance point, so as to supply, high output voltage. SOLUTION: A piezoelectric transformer 5 is integrated into the closed loop of a self-excited oscillating circuit. An output voltage 16 of the piezoelectric transformer 5 is fed back to a synchronous circuit 1 from a feedback circuit 7, since a feedback voltage 9 has a certain phase difference with respect to this. The synchronous circuit 1 generates rectangular waves 10 synchronized with the feedback voltage 9, and a saw-toothed wave generating circuit 2 generates saw-toothed waves 11 which are synchronized with the rectangular waves 10. A duty control circuit 3 generates duty waves 13 by comparing the saw-toothed waves 11 with the DC control voltage 12 from a control circuit 8. A piezoelectric transformer driver 4 generates a piezoelectric transformer drive voltage 15 synchronized with the duty waves 13, so as to drive the piezoelectric transformer 5. The piezoelectric transformer 5 operates at a resonance point at all times, since the phase difference generated in the feedback circuit 7 is set so that the phase difference between the input voltage 15 and the output voltage of the piezoelectric transformer 5 conforms to a specified phase difference between the input and output voltages at the time of the piezoelectric transformer 5 operating at the resonance point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for a cold cathode tube, and a driving circuit for a piezoelectric transformer used in a high voltage generating circuit such as an air purifier or an ion generator. And a driving circuit for performing the operation.

[0002]

2. Description of the Related Art Hitherto, piezoelectric transformers have been favorably used as high voltage generating elements in driving circuits for cold cathode tubes, high voltage generating circuits such as ion generators, and the like. As a driving method of the piezoelectric transformer, for example, a frequency control method using a frequency characteristic of a piezoelectric transformer as disclosed in Nikkei Electronics (P147 to 157) of November 7, 1994 is known.

The frequency control method is a drive method in which the load current is fed back and the drive frequency of the piezoelectric transformer is tracked so that the output voltage becomes a set value, thereby controlling the boost ratio. In other words, as shown in FIG. 2, the output voltage is controlled by using the driving circuit shown in the block diagram of FIG. 1 and performing frequency control using the frequency characteristics of the piezoelectric transformer. That is, the oscillation circuit is started at a frequency higher than the resonance frequency f0 of the piezoelectric transformer to drive the piezoelectric transformer, and the secondary voltage is supplied to the load. The difference voltage between the voltage detected on the load side and the reference voltage is input to the integrator. The output voltage of the integrator changes until the difference voltage becomes zero, and adjusts the oscillation frequency of the VF converter. As a result, the piezoelectric transformer is driven at the drive frequency f1 and outputs a set voltage. As described above, in the frequency control method, the output voltage is controlled using the slope portion of the frequency characteristic shown in FIG.

Another driving method is disclosed in Japanese Patent Laid-Open No.
A self-excited oscillation system as disclosed in Japanese Patent Publication No. 3 (Kokai) No. 3 is also known. In this method, the output voltage is controlled by adjusting the power supply voltage or adjusting the input current of the piezoelectric transformer with a variable resistor.

[0005]

In the frequency control method described above, control is performed using a slope portion of the frequency characteristic.
If the drive frequency jumps over the peak of the frequency characteristic (resonance point f0) due to turbulence or the like, the control becomes ineffective and the drive frequency runs away from the resonance point f0 to a lower frequency side, which may cause a circuit abnormality or component breakdown. There is.

In order to avoid this, it is necessary to limit the frequency range to be used to a region sufficiently higher than the resonance point. As a result, the peak of the frequency characteristic cannot be used, and only the region with a much lower boost ratio can be used, so that it is difficult to obtain a high voltage or a large power from the piezoelectric transformer. In addition, since driving is performed in a frequency region far away from the resonance point, the output voltage is not a clean sine wave but a distorted waveform, which causes noise and is undesirable for a cold cathode tube of a load.

In the case of the self-excited oscillation system, it is difficult to stabilize the output with respect to power supply voltage fluctuations and load fluctuations because the oscillation state becomes unstable.

Accordingly, an object of the present invention is to provide a driving circuit for a piezoelectric transformer, which always drives the piezoelectric transformer at the resonance point of its frequency characteristic, thereby stably obtaining a high output voltage and a large electric power, and a sinusoidal signal having little distortion. To get the wave output.

[0009]

According to the present invention, a piezoelectric transformer is encased in a closed loop of a self-excited oscillation circuit.
This self-excited oscillation circuit has means for generating a voltage having a certain phase difference with respect to the output voltage of the piezoelectric transformer and inputting the voltage to the piezoelectric transformer. By the above means, the phase difference between the input voltage and the output voltage of the piezoelectric transformer is controlled when the piezoelectric transformer operates at the resonance point, and the circuit constant of the self-excited oscillation circuit is controlled when the piezoelectric transformer operates at the resonance point. Is set so that the oscillation condition is satisfied, the piezoelectric transformer is driven stably at the resonance point, that is, at the point where the output voltage reaches a peak.

In the present invention, preferably, the duty ratio of the input voltage to the piezoelectric transformer is controlled to adjust the voltage applied to the piezoelectric transformer, so that the output voltage can be controlled while keeping the operating point at the peak voltage.

In one configuration example of the present invention, a reference waveform for generating a duty waveform is generated so as to have a predetermined phase difference with respect to the output voltage in synchronization with the output voltage, and the duty waveform is used to generate the reference waveform. A duty-controlled input voltage is generated and applied to the piezoelectric transformer. As a result, the input voltage of the piezoelectric transformer has a predetermined phase difference from the output voltage, the resonance point driving is performed, and the output voltage control by the peak voltage driving becomes possible.

A more specific configuration example includes a feedback circuit for feeding back the output voltage of the piezoelectric transformer, a synchronization circuit for generating a synchronization waveform synchronized with the feedback voltage fed back by the feedback circuit, and a duty control synchronized with the synchronization waveform. A reference waveform generating circuit for generating a reference waveform, a duty control circuit for comparing a duty control reference waveform with a predetermined signal level to generate a duty waveform corresponding to the signal level, and a duty waveform generated by the duty control circuit. A piezoelectric transformer driving unit for generating a driving voltage for driving the piezoelectric transformer based on the driving voltage. The synchronous waveform generated by the synchronous circuit is, for example, a rectangular wave having a duty of 50% ± 10%, and the duty control reference waveform generated by the reference voltage generating circuit starts charging at the rising point of the rectangular wave synchronous waveform and falls, for example. Is a saw-tooth wave that rapidly discharges. Any of the feedback voltage, the output voltage, the output current, and the power supply voltage of the driving circuit can be used as the predetermined signal level. The piezoelectric transformer drive unit is, for example, a class-E switching circuit that stores energy in the inductor during the ON period of the duty waveform, and releases the energy stored in the inductor to the piezoelectric transformer when the OFF period starts.

[0013] Preferably, the synchronization circuit may include a starting circuit that generates a starting waveform that oscillates at a specific frequency when there is no feedback voltage. The oscillation frequency of the start-up waveform generated by the start-up circuit is, for example, 110% or more of the resonance frequency of the piezoelectric transformer. When the load returns to normal, the synchronization circuit operates to generate a synchronization waveform, but when the load on the piezoelectric transformer is abnormal, the startup circuit operates to generate the startup waveform, thereby providing a safety protection function. Can be. Alternatively, the oscillation frequency of the start-up waveform generated by the start-up circuit is set to, for example, 50% or less of the resonance frequency of the piezoelectric transformer, and when the load is abnormal, the start-up circuit operates to generate the start-up waveform and generate an overcurrent in the drive circuit. And an overcurrent protection element may be provided at the input of the drive circuit to activate the same.

The piezoelectric transformer drive circuit of the present invention can be entirely constituted by a dedicated individual hardware circuit as in the preferred embodiment, but this is not necessarily required, and at least some of the functions ( For example, a function of controlling a phase difference between an input voltage and an output voltage, a function of performing duty control, and the like) can be implemented by software using a computer.

[0015]

3 and 4 show an embodiment of the present invention.
This will be described in detail with reference to FIG. FIG. 3 is a block diagram showing a basic configuration of an embodiment of the drive circuit of the present invention. FIG. 4 is an operation waveform of each block shown in FIG. In this embodiment, a self-excited oscillation circuit is composed of eight blocks.

As shown in FIG. 3, the output voltage 16 of the piezoelectric transformer 5 is fed back to the synchronous circuit 1 by the feedback circuit 7 as a sine-wave feedback voltage 9 having a predetermined phase difference from the voltage 16. . The synchronous circuit 1 has a sine-wave-like feedback voltage 9.
, A synchronous rectangular wave 10 having a duty of 50% synchronized with the zero-cross point is generated, and the synchronous rectangular wave 10 is applied to the sawtooth wave generating circuit 2 at the next stage. Note that the synchronization circuit 1 also functions as a start-up circuit when the power is turned on.

The sawtooth wave generating circuit 2 generates a sawtooth wave 11 synchronized with the synchronous rectangular wave 10. That is, the saw-tooth wave generation circuit 2 detects the rising point 10 a of the synchronous rectangular wave 10.
To form a linear rising waveform 11a and rapidly discharge at a falling point 10b of the synchronous rectangular wave 10 to form a sharp falling waveform 11b. As a result, a sawtooth synchronized with the synchronous rectangular wave 10 A wave 11 is created.

The duty control circuit 3 compares the sawtooth wave 11 from the sawtooth wave generation circuit 2 with the DC control voltage 12 from the control circuit 8 to generate a duty wave 13. That is, the duty control circuit 3 is ON during a period when the voltage level of the sawtooth wave 11 is higher than the voltage level of the DC control voltage 12.
A duty wave 13 in which a low period is an OFF period is generated. Due to the duty wave 13, the start 13 a of the ON period moves due to the voltage level of the DC control voltage 12, and O
The length of the N period is controlled.
b always coincides with the falling edge 11b of the sawtooth wave 11 in time.

The duty wave 13 is applied to the next stage piezoelectric transformer drive unit 4. The piezoelectric transformer driving section 4 is configured by an inductor L, a transistor Q, and the like. The transistor Q is turned on only during the on-period of the duty wave 13, and an inductor current (transistor current) 14 that rises linearly flows through the inductor L, and the current energy is stored in the inductor L. Then, the transistor Q is turned off at the turn-off time 13 b of the duty wave 13, and the energy stored in the inductor L starts to be released to the piezoelectric transformer 5. Continued duty wave 13 OFF
During the period, the emitted energy applies a sinusoidal voltage to the piezoelectric transformer 5. As a result, the piezoelectric transformer 5
A half-wave sine-wave-shaped piezoelectric transformer drive voltage 15 is supplied with a rising point at the turn-off point 13b of the duty wave 13.

Therefore, the input voltage 15 of the piezoelectric transformer 5 has a waveform synchronized with the output voltage 16. The output voltage 16 generated by the feedback circuit 7 and the output voltage 16 are set so that the phase difference between the input voltage 15 and the output voltage 16 becomes equal to a predetermined phase difference between the voltages 15 and 16 when the piezoelectric transformer 5 operates at the resonance point. The phase difference between the feedback voltages 9 is set. The circuit constant of each part of the self-excited oscillation circuit is set so as to satisfy the oscillation condition (the original gain of the closed loop is 1 and the phase transition of the closed loop is 360 degrees) at the resonance frequency of the piezoelectric transformer 5. As a result, the piezoelectric transformer 5 stably operates at the resonance point.

The control circuit 8 can supply the duty control circuit 8 with a DC control voltage 12 controlled in a form corresponding to the control purpose. For example, the output feedback voltage 9 is supplied to the control circuit 8 so that the DC control voltage
By adjusting the level 2 and controlling the duty ratio, the output voltage can be stabilized. Or,
By supplying the power supply voltage to the control circuit 8 and controlling the duty ratio by adjusting the level of the DC control voltage 12 according to the power supply voltage fluctuation, the output voltage can be stabilized against the power supply voltage fluctuation. it can.

After all, according to the present embodiment, the piezoelectric transformer 5 operates at the resonance point with the input voltage 15 and the output voltage 16 always maintaining a constant phase difference, and detects the power supply voltage fluctuation and the load fluctuation. The compensated and stable output voltage is supplied to the load 6. Further, while the piezoelectric transformer 5 is stably operated at the resonance point of the frequency characteristic, a wide range of output voltage control can be performed by the duty control of the input voltage.

Further, at the time of load opening or load short-circuit failure,
Since the feedback voltage 9 disappears, the synchronous circuit 1 operates at the oscillation frequency at the time of starting. At this time, if the oscillation frequency at the time of starting is set to 110% or more of the resonance frequency, the piezoelectric transformer 5 is driven in a region away from the resonance point. The side is safe, and since the inductor L of the piezoelectric transformer drive unit 4 does not cause magnetic saturation, there is no danger of breaking down circuit components, and automatic recovery is possible when the load is normal.

Alternatively, the oscillation frequency at the time of startup is set to 50% or less of the resonance point so that an overcurrent flows when the load is opened or the load is short-circuited, and an overcurrent protection element is provided in the input section or the like. By operating this to shut off the circuit, safety may be ensured. However, in this case, even if the load returns to normal, automatic recovery cannot be performed.

Next, a specific example of the above-described piezoelectric transformer drive circuit will be described with reference to FIG. FIG. 6 shows output voltage characteristics with respect to power supply voltage fluctuation in this circuit example.

In the circuit example shown in FIG. 5, since the control circuit 8 detects the feedback voltage 9 and adjusts the level of the DC control voltage 12, the stabilization control of the output voltage 16 is performed. An output having a constant voltage characteristic as shown can be obtained. That is, as shown in FIG.
Even if cc varied between 4.5 V and 7 V, the output voltage could be kept constant at about 1.5 kVP-P. In the region where the power supply voltage is 4.5 V or less, duty control becomes impossible due to insufficient input voltage, and the output voltage drops. However, since this region is outside the practical range, there is no problem.

In FIG. 5, the synchronizing circuit 1 includes a starting circuit, which comprises a comparator IC1, resistors R3 to R6, and capacitors C4 and C5.
And a CR oscillation circuit having R3 and R3. C4 and R3 at startup
Vibrates in the piezoelectric transformer 5 at an oscillation frequency determined by the time constant of the above, and the maximum output voltage (output voltage at the resonance point) of the frequency component is fed to the feedback circuit 7 (capacitors C11 and C11).
5 and the resistor R5). The circuit constant of the feedback circuit 9 is set such that the feedback voltage 9 has a predetermined phase difference with respect to the output voltage and is higher than the voltage of the capacitor C4 of the starting circuit. Therefore, after the feedback voltage 9 is input to the synchronous circuit 1, the feedback voltage 9 has priority over the voltage (voltage of C4) of the activation circuit, and the activation circuit switches its function to the synchronous circuit and operates. And
The synchronization circuit 1 performs a synchronization operation based on the feedback voltage 9 and generates a synchronization rectangular wave 10.

The synchronous rectangular wave 10 is sent to the sawtooth wave generating circuit 2 and turns on / off the transistor Q1 through the resistor R7 and the capacitor C6. Thereby, charging and discharging of the capacitor C7 are performed, and the sawtooth wave 11 is generated.

The duty control circuit 3 is a comparator IC
The comparator IC 2 compares the sawtooth wave 11 and the DC control voltage 12 from the control circuit 8 to generate a duty wave 13.

The piezoelectric transformer driving section 4 includes transistors Q2 and Q3 formed of a complementary circuit, an inductor L1 formed of a class E switching circuit, and a transistor Q.
4 series circuits. Duty wave 1
3, when the transistor Q4 is switched through the transistors Q2 and Q3, the transistor Q4 is turned on.
The current energy stored in the inductor L1 during the period is
It is supplied to the piezoelectric transformer 5 during the OFF period, and the piezoelectric transformer 5 is driven.

As described above, since the phase difference between the input and output voltages of the piezoelectric transformer 5 is controlled to a predetermined value and does not fluctuate, stable operation by resonance point driving is continued. Further, since the duty is adjusted by feeding back the output voltage, the output voltage is stable even if there is a load fluctuation or an input voltage fluctuation.

According to the present embodiment, by driving the piezoelectric transformer at the resonance point, the boosting capability of the piezoelectric transformer can be utilized to the maximum. Therefore, the size of the piezoelectric transformer can be reduced, and the number of output rectification stages can be reduced. This is advantageous for miniaturization.

Further, since the piezoelectric transformer can be driven stably at the resonance point, when the piezoelectric transformer receives an impact and causes abnormal vibration as in the conventional frequency control method, it runs away to the low frequency side and is not controlled. And the operating state automatically returns to the resonance point, so that the permissible conditions for stable operation are widened.

Further, by driving the piezoelectric transformer at the resonance point, the output voltage of the piezoelectric transformer becomes a sine wave with less distortion, which is advantageous for reducing noise and improving the lighting life of the cold-cathode tube.

Further, since the energy stored in the inductor is controlled to be constant irrespective of fluctuations in the power supply voltage, there is a problem that the magnetic saturation occurs in the inductor when the power supply voltage increases as in the conventional frequency control method. Therefore, circuit design is easy.

The above description relates to one embodiment of the present invention, and it goes without saying that the present invention is not limited only to the above content. The functions substantially equivalent to part or all of the synchronization circuit 1, the sawtooth wave generation circuit 2, the duty control circuit 3, the feedback period circuit 7, and the control circuit 8 shown in FIG. The present invention can also be realized, and a driving circuit implemented using a computer in such a manner is also included in the scope of the present invention.

[0037]

According to the present invention, the piezoelectric transformer can be driven at the resonance point.

[Brief description of the drawings]

FIG. 1 is a block diagram of a conventional piezoelectric transformer driving circuit based on a frequency control method.

FIG. 2 is a frequency characteristic diagram showing a control principle in a frequency control method.

FIG. 3 is a block diagram showing a basic configuration of one embodiment of a piezoelectric transformer drive circuit of the present invention.

FIG. 4 is an operation waveform diagram of each unit in the block diagram of FIG. 3;

FIG. 5 is a circuit diagram showing a specific circuit example of the block diagram of FIG. 3;

6 is a characteristic diagram showing output voltage characteristics with respect to power supply voltage fluctuation in the circuit example of FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 synchronization circuit 2 sawtooth wave generation circuit 3 duty control circuit 4 piezoelectric transformer drive unit 5 piezoelectric transformer 6 load 7 feedback circuit 8 control circuit 9 feedback voltage 10 synchronous rectangular wave 11 sawtooth wave 12 control voltage 13 duty waveform (transistor drive waveform 14) Inductor current (transistor current waveform) 15 Piezo transformer driving voltage F1 Fuse R1 to R16 Resistance C1 to C10 Capacitor Q1 to Q5 Transistor L1 Inductor D1, D2 Diode ZD1 Zener diode IC1, IC2 Comparator IC3 Operational amplifier

Claims (11)

[Claims] 1. A self-excited oscillation circuit comprising: a piezoelectric transformer; and a phase difference control means for generating an input voltage having a certain phase difference with respect to an output voltage of the piezoelectric transformer and inputting the input voltage to the piezoelectric transformer. Drive circuit for piezoelectric transformer. 2. The drive circuit according to claim 1, wherein the phase difference control means determines a phase difference between an input voltage and an output voltage of the piezoelectric transformer by controlling a phase difference of the piezoelectric transformer when the piezoelectric transformer operates at a resonance point. A drive circuit for a piezoelectric transformer that controls the phase difference between input and output voltages to be equal. 3. The piezoelectric transformer drive circuit according to claim 1, wherein the self-excited oscillation circuit further includes duty control means for adjusting a level of an input voltage of the piezoelectric transformer by duty control. 4. The drive circuit according to claim 1, wherein said phase difference control means generates a feedback voltage having a predetermined phase difference with respect to an output voltage of said piezoelectric transformer, and said feedback means. Synchronizing means for generating a synchronous waveform synchronized with the feedback voltage from the means, reference waveform generating means for generating a duty control reference waveform synchronized with the synchronous waveform, and a signal level according to the duty control reference waveform and control purpose. A piezoelectric control unit that generates a duty waveform corresponding to the signal level, and a piezoelectric transformer driving unit that generates an input voltage of the piezoelectric transformer whose duty is controlled in synchronization with the duty waveform. Transformer drive circuit. 5. The driving circuit according to claim 4, wherein the phase control unit includes a starting unit that generates a starting waveform that oscillates at a specific frequency when there is no feedback voltage. 6. The drive circuit according to claim 4, wherein the synchronization circuit has a duty of 50% as the synchronization waveform.
Drive circuit for piezoelectric transformer that generates ± 10% square wave. 7. The driving circuit according to claim 4, wherein the reference voltage generation circuit generates a sawtooth wave synchronized with the synchronization waveform as the duty control reference waveform. 8. The driving circuit according to claim 4, further comprising control means for adjusting a signal level according to the control purpose. 9. The driving circuit according to claim 5, wherein an oscillation frequency of a starting waveform generated by the starting circuit is a value equal to or higher than 110% of a resonance frequency of the piezoelectric transformer,
A driving circuit for a piezoelectric transformer, wherein the starting circuit generates the starting waveform when the load of the piezoelectric transformer is abnormal, and the synchronizing circuit generates the synchronizing waveform again when the load returns to normal. 10. The drive circuit according to claim 5, wherein an oscillation frequency of a start-up waveform generated by the start-up circuit has a value of 50% or less of a resonance frequency of the piezoelectric transformer, and a load on the piezoelectric transformer is abnormal. And a drive circuit for the piezoelectric transformer, further comprising: an overcurrent protection element that shuts off the drive circuit when an overcurrent flows in the drive circuit. 11. The driving circuit according to claim 1, wherein the phase difference control means includes a computer for executing at least a part of a function of generating the input voltage.