JPH10223388A - Control circuit of piezoelectric transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control circuit capable of reducing the influence of switching noises generated from a switching element. SOLUTION: An intermittent pulse signal based on a pulse signal from a pulse oscillation circuit 6 and an oscillation signal from an oscillation circuit 3 is input from an AND circuit 7 to a transistor 4a to be intermittently turned on/off. The oscillation signal output from the oscillation circuit 3 is input to a transistor 4b to be always turned on/off. Since a drive circuit 4 switches input voltages Vi through these two transistors 4a and 4b, a piezoelectric transformer 1 is intermittently drived.

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 suitable for use in a driving device for a cold cathode tube.

[0002]

2. Description of the Related Art In recent years, a liquid crystal display has been widely used as a display device of a portable personal computer or the like which is easy to carry. Inside the liquid crystal display device, a cold cathode tube is provided as a so-called backlight in order to illuminate the liquid crystal display panel. In order to turn on the cold cathode tube, start lighting from a DC low voltage of a battery or the like. Hour 1000
A step-up inverter capable of converting to an AC high voltage of not less than Vrms and about 500 Vrms during steady lighting is required.
Conventionally, a so-called winding transformer has been used as a step-up transformer of the step-up inverter. Recently, however, a piezoelectric transformer that performs electric conversion through mechanical energy to perform step-up has been used. This piezoelectric transformer generally has an undesirable characteristic that the step-up ratio greatly changes depending on the magnitude of the output load (load resistance). On the other hand, the dependence on the load resistance is due to the inverter power supply of the cold cathode tube. It has attracted attention as a compact high-voltage power supply that meets the demand for thinner and more efficient liquid crystal displays. An example of such a piezoelectric transformer control circuit will be described with reference to FIG.

FIG. 1 is a block diagram of a control circuit of a conventional piezoelectric transformer.

In the figure, 101 is a piezoelectric transformer, 102 is a load such as a cold cathode tube connected to the output side of the piezoelectric transformer 101, 103 is an oscillation circuit that oscillates an AC signal such as a square wave,
Reference numeral 104 denotes a signal oscillated by the oscillation circuit 103 (hereinafter, referred to as a signal).
This is a driving circuit for driving the piezoelectric transformer 101 by an oscillation signal. The drive circuit 104 includes, for example, a P-type transistor (FET: field effect transistor) 104 a and an N-type transistor (FET) 1 connected in a half-bridge type.
04b, and the two transistors (104a, 104b) alternately switch according to the state of the oscillation signal output from the oscillation circuit 103.
By the switching operation of the drive circuit 104, a drive voltage (AC voltage) having an amplitude of the input voltage Vi is applied to the piezoelectric transformer 101. The output voltage of the piezoelectric transformer 101 greatly changes in a mountain-like manner in accordance with the frequency of the AC voltage obtained from the drive circuit 104. When the piezoelectric transformer 101 is driven at the resonance frequency of the piezoelectric transformer 101, the output voltage takes a maximum value. It is generally known that the resonance frequency varies depending on the temperature and the magnitude of the load of the output (load resistance). Therefore, an oscillation signal that is substantially equal to the resonance frequency is output from the oscillation circuit 103, and the driving circuit 104 drives the piezoelectric transformer 101 based on the state of the oscillation signal to generate a high voltage on the output side of the piezoelectric transformer 101. General.

The brightness of the cold-cathode tube connected as the load 102 is adjusted by changing the tube current (load current) flowing through the cold-cathode tube. Method 1
For example, for example, Japanese Patent Application No.
Japanese Patent No. 228458 proposes a method for adjusting the average tube current of a cold cathode tube by driving the piezoelectric transformer by applying an intermittent pulse voltage. Here, the outline of the method will be described by applying to the control circuit having the configuration of FIG. 2 described above.

FIG. 2 is a block diagram of a conventional piezoelectric transformer control circuit capable of adjusting the brightness of a cold cathode tube. 1 is basically the same as the circuit configuration of FIG. 1 except that a pulse power supply circuit 105 capable of intermittently outputting an input voltage Vi as a drive power supply for a piezoelectric transformer 101 in a desired period. It is to have. FIG. 3 shows a circuit configuration of the pulse power supply circuit 105.

FIG. 3 is a diagram for explaining a configuration example of a pulse power supply circuit as a conventional example.

In FIG. 1, a pulse power supply circuit 105 converts an input voltage Vi, which is a DC voltage, into, for example, a MOS-FET (MOS
Switching element 105 such as a type field effect transistor)
By turning on / off according to the signal output from the pulse oscillation circuit 105a by b, a pulse voltage is output to the drive circuit 104. That is, the pulse oscillation circuit 105a
The switching speed of the switching element 105b is changed by changing the signal output from the switching element 105b. The operation of such a control circuit will be described with reference to FIGS.

FIGS. 4 and 5 are diagrams for explaining a control operation of a control circuit as a conventional example. FIG. 4 shows a case of maximum luminance, and FIG. 5 shows a dimming state.

4 and 5, the horizontal axis represents time, and the vertical axis represents pulse voltage supplied from the pulse power supply circuit 105 to the drive circuit 104 and output from the oscillation circuit 103 in order from the top. An oscillation signal and an output voltage of the piezoelectric transformer 101 are shown.

FIG. 4 shows a state in which the cold-cathode tube is lit at the maximum luminance. The pulse voltage supplied from the pulse power supply circuit 105 to the drive circuit 104 is set to a DC state with a pulse interval of zero. As a result, the drive circuit 104 is always in an operating state, so that the piezoelectric transformer 101 is always driven according to the oscillation signal obtained from the oscillation circuit 103, and the output voltage is continuously output.

On the other hand, as shown in FIG. 5, when the brightness of the cold-cathode tube is adjusted to be small, a pulse-like power supply voltage having a certain pulse interval is supplied from the pulse power supply circuit 105 to the drive circuit 1.
04. Here, the High period (T high) of the pulse voltage supplied from the pulse power supply circuit 105 to the drive circuit 104 is sufficiently longer than the period of the oscillation signal obtained from the oscillation circuit 103 (for example, the frequency of the pulse voltage is 100 kHz). In contrast, the frequency of the pulse voltage is about several hundred Hz.) As illustrated, when the pulsed power supply voltage is in the Low period (Tlow), the driving circuit 104
Does not operate because no voltage is applied to the
01 is not driven and the output voltage is zero. On the other hand, when the pulse voltage is in the High period (Thigh), the driving circuit 104
Operates when a voltage is applied to the piezoelectric transformer 101.
Is also driven to generate an output voltage. By repeating this operation, the average value of the tube current (load current) flowing through the cold-cathode tube (load 102) becomes smaller than in the continuous output state of FIG. 4, and the luminance can be reduced. In this case, if the Low period in the pulse voltage is further lengthened,
Since the average tube current flowing through the cold-cathode tube becomes smaller, the brightness decreases. This is because afterimages during the High period remain for human vision. Therefore, if the duty ratio of the pulse voltage is adjusted in the control circuit as shown in FIG. 2, while maintaining the output voltage of the piezoelectric transformer at a voltage higher than the voltage required for the cold-cathode tube to maintain the discharge,
A wide range of dimming is possible with the luminance stabilized.

A conventional example using a driving circuit for a half-bridge type piezoelectric transformer is disclosed in, for example, JP-A-8-1.
No. 39382 discloses a method of intermittently driving a half-bridge type driving circuit by an oscillation signal using an AND gate circuit.

[0014]

However, in the above-described conventional example (FIG. 2), the switching noise generated by the switching element inside the pulse power supply circuit 105 is directly superimposed on the pulse voltage for driving the piezoelectric transformer 101. Therefore, there is a problem that the switching noise is transmitted to the piezoelectric transformer 101 via the drive circuit 104. In general, on / off characteristics of a transistor (FET) are different from on to off and from off to on. Therefore, when a circuit configuration such as the drive circuit 104 is employed, a temporal shift in the ON / OFF operation of each of the transistors used causes a temporary short-circuit state of the drive circuit 104 and causes the piezoelectric transformer 101 to generate a short circuit. However, there is a possibility that a decrease in efficiency due to the generation of an unsteady large current (noise) may cause a problem.

In the latter conventional example, the two transistors configured in a half-bridge type are turned on / off by the same pulse signal output from the AND gate element. Are both turned off. That is, during the intermittent period of the pulse signal, the driving voltage of the piezoelectric transformer floats with respect to zero potential, resulting in an unstable state. Not necessarily.

Accordingly, an object of the present invention is to provide a control circuit for a piezoelectric transformer that can reduce the influence of switching noise generated by a switching element to be used.

[0017]

In order to achieve the above object,
A control circuit for a piezoelectric transformer according to the present invention has the following configuration.

That is, there is provided a driving means for generating an AC voltage by switching an input DC voltage by a transistor configured in a half-bridge type or a full-bridge type, and driving a piezoelectric transformer by the AC voltage to obtain an AC high voltage. A control circuit for the piezoelectric transformer comprising: an oscillating means for generating an oscillating signal for driving the transistor of the driving means; an intermittent oscillating means for generating a pulse signal; and a pulse signal from the intermittent oscillating means and the oscillating means. And a logical product calculating means for calculating a logical product based on the oscillating signal of the above, and each of the transistors of the driving means is driven by an oscillating signal from the oscillating means or an output signal from the logical product calculating means. Thus, the piezoelectric transformer is intermittently driven.

Preferably, the intermittent oscillation means is capable of adjusting a duty ratio of a pulse signal to be generated, and the piezoelectric transformer intermittently outputs an AC high voltage according to the duty ratio of the pulse signal. And

With the above configuration, the influence of switching noise is reduced, the voltage transformer is driven intermittently, and the brightness of the cold-cathode tube connected to its output is adjusted in a stable lighting state.

[0021]

An embodiment of the present invention will be described below with reference to the drawings. First, the configuration of a control circuit for a piezoelectric transformer to which the present invention is applied will be described.

FIG. 6 is a block diagram of a piezoelectric transformer control circuit according to an embodiment of the present invention.

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 an oscillating circuit for oscillating an AC signal such as a rectangular wave, and 4 is an oscillating circuit 3. This is a half-bridge type driving circuit that drives the piezoelectric transformer 1 by the oscillation signal of FIG. Since these configurations are known from the related art, detailed description will be omitted.

Reference numeral 6 denotes a pulse oscillating circuit which generates a pulse signal serving as a basis for changing the luminance of the cold-cathode tube and controls the pulse width and interval. Reference numeral 7 denotes an AND circuit that generates a gate signal of the high-side transistor 4a from the logical product of the pulse signal from the pulse oscillation circuit 6 and the oscillation signal from the oscillation circuit 3.

Next, the operation of the control circuit having the above configuration will be described with reference to FIG.

FIG. 7 is a diagram for explaining the operation of the control circuit of the piezoelectric transformer as one embodiment of the present invention.

In the figure, the horizontal axis represents time, respectively.
The vertical axes indicate, in order from the top, a pulse signal output from the pulse oscillation circuit 6, an oscillation signal output from the oscillation circuit 3, a gate signal of the high-side transistor 4a, a gate signal of the low-side transistor 4b, and a high side. 3 shows the operating states of the side and low side transistors and the output voltage of the piezoelectric transformer 1. Note that the frequency of the oscillation signal is sufficiently lower than the frequency of the pulse signal.

Since the oscillation signal output from the oscillation circuit 3 is always input to the low-side transistor 4b as a gate signal, the low-side transistor 4b always turns on / off in accordance with the cycle of the oscillation signal from the oscillation circuit 3. . On the other hand, the inverted signal of the pulse signal from the pulse oscillation circuit 6 and the oscillation circuit 3
The inverted signal of the comb-shaped signal, which is the logical product with the oscillation signal from the input terminal, is input as the gate signal. Therefore, the high-side transistor 4a performs an operation such that only the low period of the oscillation signal in the high period of the pulse signal is on, and the other period is off. That is, the high-side transistor 4a performs a switching operation of the drive circuit 4 for generating an AC voltage for driving the piezoelectric transformer 1 and an operation of intermittently turning on / off the AC voltage output from the drive circuit 4. And have used it for both operations. Accordingly, the piezoelectric transformer 1 is intermittently driven as in the prior art, and the brightness of the cold-cathode tube (load 2) is stabilized by controlling the length (duty ratio) of the high period or the low period of the pulse signal. Can be adjusted.

<Modification 1 of Embodiment> FIG. 8 is a block diagram of a control circuit of a piezoelectric transformer according to a modification 1 of the embodiment of the present invention.

6 differs from FIG. 6 in that the pulse signal from the pulse oscillation circuit 6 and the oscillation circuit 3
And that the logical product signal with the oscillation signal from is input as the gate signal of the transistor 4b on the low side.
Only the operations of the high-side and low-side transistors are opposite to those described above, but the point that the piezoelectric transformer 1 can be intermittently driven and the cold-cathode tube can be dimmed is the same. Detailed description is omitted.

[Effect of this Embodiment] The logical product signal of the pulse signal from the pulse oscillation circuit 6 and the oscillation signal from the oscillation circuit 3 is transferred to the high side (FIG. 6) or the low side (FIG. 8) inside the drive circuit 4. ), A switching element for generating an AC voltage for driving a piezoelectric transformer in the pulse power supply circuit 105 shown in FIG. A switching transistor;
It is used for both functions. Therefore, since the intermittent operation is realized by the switching transistor line of the drive circuit 4 in which only a much smaller current flows than the input voltage line, the factor of the switching noise to the piezoelectric transformer 1 can be reduced. Further, since the piezoelectric transformer 1 can be driven intermittently as in the prior art shown in FIG. 2, the brightness of the cold cathode tube (load 2) can be adjusted in a stable lighting state.

Further, instead of operating the high-side and low-side transistors with a common gate signal as in the conventional example, the transistors 4a and 4b are operated with individual gate signals, respectively, and the period of Tlow of the pulse signal is reduced. In either case, one of the transistors is always turned on /
The transistor is turned off (low-side transistor 4b in FIG. 6). Accordingly, the time during which both the transistors are turned off during the period of Tlow of the pulse signal is reduced to one half of that in the conventional example, so that the driving voltage of the piezoelectric transformer floats with respect to zero potential. That unstable time can be reduced.

<Modification 2 of Embodiment> FIG. 9 is a block diagram of a control circuit of a piezoelectric transformer according to a modification 2 of the embodiment of the present invention. 6 is different from the circuit configuration of FIG. 6 in that the drive circuit 4A includes transistors 4a to 4d.
To form a full-bridge type as shown in the figure, and that a NOT (NOT) is provided before the transistors 4c and 4d.
That is, a circuit (inverter) 8 is provided. Full-bridge drive circuit 4 in which transistors 4a and 4d and transistors 4b and 4d alternately turn on and off
The switching operation of B is well-known and will not be described. In this modification, the transistors 4b and 4d
Is turned on / off constantly by the oscillation signal from the oscillation circuit 3, and the transistors 4a and 4c are turned on / off intermittently.
The point that the cold cathode tube can be dimmed is the same, and the same reference number is assigned and the detailed description is omitted.

<Third Modification of the Embodiment> FIG. 10 is a block diagram of a piezoelectric transformer control circuit as a third modification of the embodiment of the present invention. 8 shows a half-bridge type driving circuit of FIG. 8 in which a full-bridge type driving circuit is used. Transistors 4a and 4c are constantly turned on / off by an oscillation signal from an oscillation circuit 3, and transistors 4b and 4d are intermittently driven. Will be turned on / off
Other operations are the same as those of the above-described modification 2 (FIG. 9). Therefore, the piezoelectric transformer 1 is intermittently driven and the cold cathode fluorescent lamp can be dimmed in the same manner, and the same reference numerals are given and the detailed description is omitted.

[0035]

As described above, according to the present invention,
Thus, it is possible to provide a piezoelectric transformer control circuit capable of reducing the influence of switching noise generated by a switching element used.

[0036]

[Brief description of the drawings]

FIG. 1 is a block diagram of a control circuit of a conventional piezoelectric transformer.

FIG. 2 is a block diagram of a control circuit of a conventional piezoelectric transformer capable of adjusting the brightness of a cold cathode tube.

FIG. 3 is a diagram illustrating a configuration example of a pulse power supply circuit as a conventional example.

FIG. 4 is a diagram illustrating a control operation of a control circuit as a conventional example.

FIG. 5 is a diagram illustrating a control operation of a control circuit as a conventional example.

FIG. 6 is a block diagram of a control circuit of a piezoelectric transformer according to an embodiment of the present invention.

FIG. 7 is a diagram for explaining the operation of the control circuit of the piezoelectric transformer as one embodiment of the present invention.

FIG. 8 is a block diagram of a control circuit of a piezoelectric transformer according to a first modification of the embodiment of the present invention.

FIG. 9 is a block diagram of a control circuit of a piezoelectric transformer according to a second modification of the embodiment of the present invention.

FIG. 10 is a block diagram of a control circuit of a piezoelectric transformer according to a third modification of the embodiment of the present invention.

[Explanation of symbols]

 1,101 Piezoelectric transformer 2,102 Load 3,103 Oscillation circuit 4,4A, 4B, 104 Driving circuit 4a, 4c, 104a P-type transistor 4b, 4d, 104b N-type transistor 6,105a Pulse oscillation circuit 7 AND (AND) Circuit 8 Knot (NOT) circuit 105b Switching element 105 Pulse power supply circuit

Claims (5)

[Claims] 1. A driving means for generating an AC voltage by switching an input DC voltage by a transistor configured in a half-bridge type or a full-bridge type, and driving a piezoelectric transformer with the AC voltage to obtain an AC high voltage. A control circuit for the piezoelectric transformer comprising: an oscillating means for generating an oscillating signal for driving the transistor of the driving means; an intermittent oscillating means for generating a pulse signal; and a pulse signal from the intermittent oscillating means and the oscillating means. And a logical product calculating means for calculating a logical product based on the oscillating signal of the above, and each of the transistors of the driving means is driven by an oscillating signal from the oscillating means or an output signal from the logical product calculating means. A piezoelectric transformer control circuit for intermittently driving the piezoelectric transformer. 2. The intermittent oscillation means is capable of adjusting a duty ratio of a pulse signal to be generated, and the piezoelectric transformer intermittently outputs an AC high voltage according to a duty ratio of the pulse signal. The control circuit for a piezoelectric transformer according to claim 1, wherein 3. An output signal from the logical product calculating means,
2. The piezoelectric transformer intermittently outputs an AC high voltage by inputting only to a high-side or low-side transistor of the driving unit.
A control circuit for a piezoelectric transformer according to claim 2. 4. The transistor according to claim 1, wherein an oscillating signal from said oscillating means is input only to a low-side or high-side transistor of said driving means, so that said transistor is always turned on / off. A control circuit for a piezoelectric transformer according to claim 3. 5. The control circuit for a piezoelectric transformer according to claim 1, wherein said control circuit is used for a driving circuit of a cold cathode tube.