JPH0864885A - Piezoelectric transformer drive circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] A resonance circuit is formed by the inductance of the secondary side of the drive transformer T and the capacitor of the piezoelectric transformer 1, and the primary side circuit of the drive transformer T is turned on and off to the secondary side. In a piezoelectric transformer drive circuit that obtains a high voltage by applying a generated voltage to the piezoelectric transformer 1, an increase in loss due to a change in resonance point caused by a change in capacitance of the piezoelectric transformer 1 due to a temperature change is prevented. [Structure] A temperature compensating capacitor Cx having a negative temperature characteristic is inserted in parallel with a resonance circuit formed by a secondary side of a drive transformer T and a piezoelectric transformer 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a drive circuit for a cold cathode tube,
In addition, a piezoelectric transformer drive circuit used in a DC high voltage generator such as a television receiver and an electronic copying machine, in particular, a resonance circuit is formed by the inductance of the secondary side of the drive transformer and the capacitor of the piezoelectric transformer. The present invention relates to a piezoelectric transformer drive circuit that obtains a high voltage by turning on / off a secondary circuit and applying a voltage generated on the secondary side to a piezoelectric transformer.

[0002]

2. Description of the Related Art As a conventional piezoelectric transformer drive circuit,
For example, there is a circuit shown in FIG. In the figure, 1 is a piezoelectric transformer, 2 is a drive circuit, 3 is a current detector, and 5 and 6 are DC.
Input terminal, T is drive transformer, Q is transistor or FE
A switching element, which is a semiconductor element that performs a switching operation such as T, RL is a piezoelectric transformer 1 such as a cold cathode tube.
It is a load that receives high voltage from

Therefore, one end 5 of the DC input terminals 5 and 6 is connected to the plus side of a DC power source (not shown), and the other end 6 is connected to the minus side. One end 5 of the DC input terminals 5 and 6 is connected to one of the primary sides of the drive transformer T, and the other end is connected to the collector (drain) of the switching element Q. The other ends 6 of the DC input terminals 5 and 6 are connected to the emitter (source) of the switching element Q and also to one of the secondary side of the drive transformer T and the current detection unit 3. The secondary side of the drive transformer T is connected to the drive section of the piezoelectric transformer 1, the output section of the piezoelectric transformer 1 is connected to one end of the load RL, and the other end is connected to the current detection section 3
It is connected to the. The current detector 3 is connected to the drive circuit 2, and the drive circuit 2 is further connected to the base (gate) of the switching element Q.

In the piezoelectric transformer drive circuit having such a configuration, the current supplied from the DC power source connected to the DC input terminals 5 and 6 is turned on / off by the operation of the switching element Q, and the drive transformer T A predetermined voltage is generated on the secondary side of and is applied to the drive unit of the piezoelectric transformer 1. The inductance of the secondary side of the drive transformer T and the capacitor of the piezoelectric transformer 1 are set so as to form a resonance circuit, so that the piezoelectric transformer 1 is efficiently driven. Then, a high voltage corresponding to the voltage applied to the drive section is generated at the output section of the piezoelectric transformer 1, and a load current flows through the load RL. This load current is converted into a voltage by the detection resistor R of the current detection unit 3 and input to the drive circuit 2 via the diode D. The drive circuit 2 controls the switching element Q so that the piezoelectric transformer 1 generates a voltage at which the load current becomes a predetermined value from the detected voltage.

The drive circuit 2 is a circuit which is composed of, for example, an integrating circuit, a VF converter, an amplifying circuit, etc., and which gives an output of a predetermined frequency to an input voltage.
Is a circuit that controls the output voltage of the piezoelectric transformer 1 by utilizing the fact that the output voltage changes according to the input frequency.

[0006]

In the drive circuit for the piezoelectric transformer described above, the capacitor of the piezoelectric transformer 1 and the inductance on the secondary side of the drive transformer T form a resonant circuit. As shown in FIG. 2, it has a characteristic that it fluctuates depending on the temperature and the capacity increases as the temperature rises.

However, since the capacitance of the piezoelectric transformer 1 is normally designed with reference to the value at 25 ° C., as is clear from the waveform of each part of the switching element Q shown in FIG. 3, for example, at room temperature, (b) Even if it is optimally adjusted like
At low temperature, the capacitance of the piezoelectric transformer 1 is reduced as shown in (a), the flyback of V _CE is accelerated, and the zero cross occurs before the switching element Q is turned on. On the other hand, at high temperature, as shown in (c), the capacitance of the piezoelectric transformer 1 increases, the flyback of V _CE is delayed, and the switching element Q is turned on before the zero cross, and in any case, the resonance operation is performed. This leads to an increase in loss.

The present invention has been made in view of the above points, and an object of the present invention is to correct the capacitance variation of the piezoelectric transformer 1 due to a temperature change to keep the resonance operation proper and to eliminate the loss due to the temperature variation. It is to provide a piezoelectric transformer drive circuit with a small increase in power consumption.

[0009]

In order to achieve the above object, the present invention sets a resonance circuit in which the inductance of the secondary side of the drive transformer T and the capacitor of the piezoelectric transformer 1 resonate to set the drive transformer T. In the piezoelectric transformer drive circuit that generates a high voltage by generating a voltage on the secondary side by turning on / off the primary side circuit of the piezoelectric transformer 1 and the capacitance with respect to the temperature of the piezoelectric transformer 1. A temperature compensating capacitor Cx for correcting the change is connected in parallel to the resonance circuit of the drive transformer T and the piezoelectric transformer 1,

Further, the temperature compensating capacitor Cx has a temperature characteristic of F characteristic, and its peak is set to about -10 ° C.

[0011]

As shown in FIG. 2, the internal capacity of the piezoelectric transformer 1 changes with a change in temperature, and the capacity increases as the temperature rises. Therefore, by inserting the temperature compensating capacitor Cx exhibiting the characteristic opposite to the temperature characteristic in parallel with the resonance circuit of the drive transformer T and the piezoelectric transformer 1, it is possible to correct the capacitance variation due to the temperature change.

Further, the temperature characteristic of the piezoelectric transformer 1 shows a characteristic almost opposite to the F characteristic of the capacitor whose capacitance changes with temperature, and the region from the peak of the change is about -10.
When the temperature is set to ℃, it is convenient for correcting the temperature change of the piezoelectric transformer.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, one embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a basic configuration of a circuit as a typical embodiment of the present invention. In the figure, Cx is a temperature compensating capacitor, which is the same as the circuit of FIG. 7 except that it is connected in parallel to the resonance circuit formed by the secondary side of the drive transformer T and the piezoelectric transformer 1, and the same components are not shown. The same reference numerals are given and the description is omitted.

The secondary side of the drive transformer T and the piezoelectric transformer 1
Since the resonance frequency f ₀ of the resonance circuit is determined by the inductance L _S on the secondary side of the drive transformer T and the capacitor C ₀ of the piezoelectric transformer 1, if the capacitor C ₀ of the piezoelectric transformer 1 changes, the resonance frequency f ₀ also changes. To do. Therefore, by connecting a temperature compensating capacitor Cx in parallel with the piezoelectric transformer 1, the fluctuation of the capacitor C ₀ of the piezoelectric transformer 1 is corrected. That is, this temperature compensating capacitor C
As shown in FIG. 2, x has a negative temperature characteristic opposite to that of the piezoelectric transformer 1, and the fluctuation of the capacitor C ₀ of the piezoelectric transformer 1 is absorbed by the temperature compensating capacitor Cx having a negative temperature characteristic, so that the resonance frequency The fluctuation of f ₀ is prevented.

Further, the temperature compensating capacitor Cx
In general, several types of capacitors are specified as the temperature characteristics of the capacitor, but as shown in FIG. 4, the type specified as the so-called F characteristics is exactly the same as the temperature characteristics of the piezoelectric transformer 1. Therefore, it is preferable.

In the case of a normal F-characteristic capacitor, its peak is around 25 ° C. as shown in FIG. 4, and therefore, there is a drawback that it cannot be used for temperature compensation of the piezoelectric transformer 1 below this temperature. For this reason, in the present embodiment, as shown in FIG. 5, a capacitor having a normal F-characteristic peak was set at around −10 ° C., and a capacitor was manufactured and used as a temperature compensating capacitor.

FIG. 6 is a plot of the combined capacitance of the capacitor of the piezoelectric transformer 1 and the temperature compensating capacitor Cx while changing the temperature. In the figure, A, B and C are piezoelectric transformers 1 having different capacities, and A is 7
85 PF, B is 807 PF, C is 852 PF, and the capacitance of the temperature compensating capacitor Cx for this is 36 respectively.
They are 2 PF, 275 PF and 216 PF. The characteristics of A, B, and C are different from each other because of the difference in capacitance. Therefore, the temperature characteristic of the piezoelectric transformer 1 has a smaller rate of change in temperature than the characteristic of the temperature compensating capacitor Cx. It is due to the appearance.

As described above, although there is some variation, the capacitance variation due to the temperature change is caused by the temperature compensating capacitor Cx.
Approximately 6% compared to 18% without
It can be seen that the fluctuation has decreased.

[0019]

As described above, according to the present invention, since the temperature compensating capacitor Cx is connected in parallel to the resonance circuit formed by the secondary side of the drive transformer T and the piezoelectric transformer 1, the temperature of the piezoelectric transformer 1 is changed. By correcting the variation of the capacitance, it is possible to provide a piezoelectric transformer drive circuit which has a proper resonance operation and has a small increase in loss due to temperature variation.

Further, according to the present invention, since the temperature characteristic of the temperature compensating capacitor Cx is the F characteristic and its peak is set to about −10 ° C., the temperature variation is well adapted to the temperature characteristic of the piezoelectric transformer 1. Can be corrected, and increase in loss due to temperature change can be suppressed to a minimum.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration of a circuit that is an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between changes in temperature and capacity of a piezoelectric transformer and a temperature compensating capacitor.

3A and 3B are diagrams showing changes in waveforms at various parts of the switching element due to changes in temperature. FIG. 3A shows a low temperature, FIG. 3B shows a normal temperature, and FIG. 3C shows a high temperature. .

FIG. 4 is a diagram showing temperature characteristics of a normal F characteristic capacitor and a piezoelectric transformer.

FIG. 5 is a diagram showing temperature characteristics of a capacitor and a piezoelectric transformer having a peak of −10 ° C.

FIG. 6 is a diagram showing a state in which a temperature compensating capacitor corrects a change in capacitance of the piezoelectric transformer.

FIG. 7 is a diagram showing a conventional piezoelectric transformer drive circuit.

[Explanation of symbols]

 1 Piezoelectric transformer 2 Drive circuit 3 Current detector 5, 6 DC input terminal Q Switching element T Drive transformer Cx Temperature compensation capacitor R Detection resistor D Diode RL Load

Claims (2)

[Claims] 1. A secondary circuit of a drive transformer (T) and a capacitor of a piezoelectric transformer (1) form a resonance circuit, and a primary circuit of the drive transformer (T) is turned on and off to form a secondary circuit. In a piezoelectric transformer drive circuit that obtains a high voltage by applying a voltage generated on the side to the piezoelectric transformer (1), a temperature compensating capacitor (Cx) that corrects a capacitance change with temperature of the piezoelectric transformer (1) is driven by a drive transformer (Tx). ) And the resonance circuit of the piezoelectric transformer (1) are connected in parallel with each other. 2. The piezoelectric transformer drive circuit according to claim 1, wherein the temperature compensating capacitor (Cx) has a temperature characteristic of F characteristic, and the peak thereof is set to approximately -10 ° C.