JPH09232073A - Driving device of capacitive load - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a driving device of a capacitive load by which generation of a noise is reduced and whose size reduction is easy. SOLUTION: One ends of choke coils 1 and 4 and anodes of diodes 2 and 5 are connected to each other, and cathodes of the diodes 2 and 5 and drains of FETs 3 and 6 are connected to each other, and series circuits of the choke coils 1 and 4 where sources of the FETs 3 and 6 are grounded, the diodes 2 and 5 and the FETs 3 and 6 are arranged by two sets in parallel to each other, and a DC power source 7 is supplied to a connecting point to which the mutual tail connecting ends of the choke coils 1 and 4 are connected. Connecting points A and B of the choke coils 1 and 4 and the diodes 2 and 5 are respectively connected to an electrode of a capacitive load, and driving signals on which phases are different by 180 degrees from each other and the duty ratio is almost 50% are impressed on gates of the two FETs 3 and 6. Here, a constant is determined so that a resoance frequency by equivalent capacity of the capacitive load and an inductance of the choke coils and a frequency of the driving signals become almost equal to each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive device for a capacitive load, which receives a low DC voltage as an input and converts it into an AC voltage to drive a capacitive load. For example, it is mainly used as a light source for illumination of a liquid crystal display device. It is suitable as a power source for lighting the organic dispersion type EL used.

[0002]

2. Description of the Related Art An organic dispersed EL (hereinafter abbreviated as EL) lighting power source will be described as an example of the prior art. EL is often used as a backlight light source of a liquid crystal display device that is often used as a display device of portable audio equipment and the like. An alternating voltage of about 50 V at a frequency of 5 KHz or less is required for lighting the EL. Therefore, a power supply that returns a power supply voltage, which is generally a low DC voltage, to a desired AC voltage is used. As a power source for EL lighting, a method of oscillating and boosting using a transistor 31 and an electromagnetic transformer 35 has been conventionally used as shown in FIG. In FIG.
Is DC power supply, 10 is EL, 32, 33 are resistors, 34, 3
6 is a capacitor. As another conventional example, there is a circuit shown in FIG. 4, which operates as follows. Choke coil 44
The current flowing through the first transistor 41 is interrupted,
EL is generated by the high voltage pulse generated at the moment when the current is cut off.
Charge directly to an equivalent capacity of 10. EL every time a pulse is generated
The voltage across the terminals of rises. After the interruption of the current is repeated a certain number of times, the voltage charged in the EL 10 is discharged to the ground by the second transistor 42. First transistor 4
The first and second transistors 42 are controlled by the control circuit 43. By repeating the above operation, a sawtooth or square voltage is applied to both ends of the EL 10
10 emits light.

[0003]

The conventional driving device shown in FIG. 3, for example, has a drawback that it is difficult to miniaturize it because it uses a transformer, and the EL device shown in FIG.
In the power source for lighting, a high voltage pulse is generated by rapidly cutting off the current flowing in the choke coil 44, which causes a large amount of noise, which may cause malfunctions or may cause annoyance when incorporated into a radio or cassette tape reproducing device. It had the drawback of producing a lot of audible noise.

In view of the above-mentioned drawbacks of the prior art, an object of the present invention is to realize a drive circuit for a capacitive load which can be easily downsized and which can be applied to an EL lighting power source with less noise.

[0005]

As a technical means for achieving the above object, the present invention connects one end of a choke coil and an anode of a diode, and connects the cathode of the diode and one terminal of a switch element. Two sets of series circuits connected to each other and having the other terminal of the switch element grounded are arranged in parallel, a power supply voltage is supplied to the unconnected terminals of each choke coil, and a connection point between each choke coil and a diode is connected. The capacitive load is connected to a capacitive load, and the capacitive load is AC-driven by applying a drive signal having a phase difference of 180 degrees and a duty ratio of about 50% to the control terminals of the switch elements. Characterize.

In the drive device for the capacitive load according to the present invention,
When the current flowing through the choke coil becomes 0, the switch element opens and closes, so that no surge current that causes noise or lowers reliability is not generated. Further, according to the circuit of the present invention, without using a transformer that is difficult to miniaturize,
Since it can be configured by using small choke coils and parts such as FETs, it is possible to realize a drive device of a capacitive load that can be easily miniaturized.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of an EL lighting power source showing an embodiment of the present invention. One end of the choke coil 1 is connected to the anode of the diode 2, the cathode of the diode 2 is connected to the drain of the FET 3, and the source of the FET 3 is grounded. Similarly, choke coil 4 and diode 5
And FET 6 are arranged in series, and the DC power supply 7 is supplied to the connection point where the unconnected ends of the choke coils 1 and 4 are connected.
Further, the connection point between the choke coil 1 and the diode 2 is connected to one electrode of the EL 10 which is a load, and the connection point between the choke coil 4 and the diode 5 is connected to the other electrode of the EL 10. A drive signal having a duty ratio of 180 degrees different from each other and a duty ratio of about 50% (preferably 45 to 55%) is applied to the gates of the FETs 3 and 6 by the signal source 8 and the inverting circuit 9. Here, in order to obtain a high boosting ratio and a high conversion efficiency, the equivalent capacitance of the EL 10 and the choke coil 1 or 4 are used.
The constant is determined so that the resonance frequency due to the inductance of and the frequency of the drive signal become substantially equal.

The operation of the circuit configured as described above will be described below. The magnetic energy stored in the choke coil 1 while the FET 3 is closed causes the FET 3 to open and the magnetic energy
In the period in which 6 is closed, the choke coil 1 and the series resonance circuit of the equivalent capacitance of EL produce a substantially sinusoidal voltage / current. Further, the magnetic energy accumulated in the choke coil 4 while the FET 6 is closed causes the F 6 to open when the FET 6 is opened.
While the ET3 is closed, the choke coil 4 and the series resonance circuit of the equivalent capacitance of the EL cause a voltage of a substantially sinusoidal waveform.
It appears as an electric current. The voltage and current waveforms that appear at this time are shown in FIG.

Here, the electrode voltage of the EL connected to the anode of the diode 2, that is, the voltage VA at the point A is the FET 3
When VA is about to become positive during the closing period, the diode 2 in the forward direction conducts the current, so that the influence of the parasitic diode 3A built in the FET 3 is eliminated, and when VA is a positive voltage, it is fixed at about 0V. . On the other hand, when VA is a negative voltage, the diode 2 is in the reverse direction, so that a negative VA can appear. As for the current flowing through the drain terminal of the FET 3, only the current flowing in the forward direction of the diode 2 can flow. Similarly, the voltage of the other electrode of EL connected to the anode of the diode 5, that is, the voltage VB at the point B, is conducted by the forward diode 5 while the FET 6 is closed, so that the influence of the parasitic diode 6A incorporated in the FET 6 is exerted. Is eliminated and VB is fixed at approximately 0V when the voltage is positive, but a voltage with negative VB can appear. Only the current flowing through the drain terminal of the FET 6 in the forward direction of the diode 5 can flow.

As a result, an oscillating current IA that starts from 0 and returns to 0 during the period when FET 3 is closed and FET 6 is open is equivalent capacitance of choke coil 4-EL10-diode 2-FET.
As shown by the solid line, VA becomes approximately 0 V, while the electrode of EL10 (corresponding to point B) connected to the anode of diode 2 starts from a negative maximum value and ends with a positive maximum value. A voltage VB indicated by a broken line is generated. As a result, a sinusoidal voltage VA-VB is applied to both ends AB of EL. On the other hand, an oscillating current IB that starts from 0 and returns to 0 also flows while the FET 6 is closed and the FET 3 is open, flows through the equivalent capacitance of the choke coil 1-EL10-diode 5-FET6, and a sinusoidal voltage VA is generated at the point A. Then B
The point becomes approximately 0 V as indicated by the broken line.

By repeating the above operation, the EL
A sine wave AC voltage is continuously applied between both electrodes of the EL element 10, and the EL emits light. Here, the duty ratio of the drive signal when the switching elements are alternately opened and closed is preferably about 50%, preferably 45 to 55% in order to suppress the surge current at the time of switching and prevent loss such as heat generation.

The operation has been described above by taking the case where the load is the EL as an example, but the present invention is not limited to the use only for the EL, and the piezoelectric vibrating element and other capacitive elements require an AC voltage for driving. It can be applied to any load as long as it does. Further, although the description has been given using the MOS-FET having a high switching speed and a small switching loss as the switch element, the same effect can be obtained by using the bipolar transistor.

[0013]

According to the present invention, since the switching element is always opened and closed when the current (in the case of FET, the drain current) is 0, a surge current does not flow through the switching element and the switching element may deteriorate. No noise is generated on top. Further, it is possible to realize a small-capacity-load drive device by using only small parts without the need for parts such as transformers that are difficult to miniaturize.

[Brief description of drawings]

FIG. 1 is a circuit diagram applied to an EL lighting power source as an example of a drive circuit for a capacitive load according to the present invention.

FIG. 2 is an explanatory diagram showing the relationship between voltage and current in the circuit shown in FIG.

FIG. 3 Drive circuit for conventional capacitive load (eg, EL)

FIG. 4 Drive circuit for conventional capacitive load (eg EL)

[Explanation of symbols]

1,4 Choke coil 2,5 Diode 3,6 FET (switch element) 7 DC power supply 3A, 6A Parasitic diode built in between FET drain and source 8 Signal source 9 Inversion circuit 10 EL (load)

Claims (5)

[Claims] 1. Two sets of parallel series circuits in which one end of a choke coil is connected to the anode of a diode, the cathode of the diode is connected to one terminal of a switch element, and the other terminal of the switch element is grounded. Power source voltage is supplied to the unconnected terminals of each choke coil, and the connection point between each choke coil of the series circuit and the diode is connected as an output terminal to a capacitive load to control the switch element. A device for driving a capacitive load by an alternating current that applies a drive signal having a phase difference of 180 degrees to each other and a duty ratio of about 50%. 2. The drive device for a capacitive load according to claim 1, wherein the resonance frequency due to the equivalent capacitance of the capacitive load and the inductance of the choke coil are substantially equal to the frequency of the drive signal. 3. The drive device for a capacitive load according to claim 1, wherein the switch element is a FET. 4. The capacitive load driving device according to claim 1, wherein the load is an organic dispersion type EL. 5. The capacitive load driving device according to claim 1, wherein the load is a piezoelectric element.