JPH11195496A - Cold-cathode tube lighting inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the mobility in power source input of a cold-cathode tube and the lighting property at low temperature thereof by detecting the current carried to the cold-cathode tube to which a lighting voltage is inputted, controlling the oscillating frequency of a voltage control oscillating circuit, and protecting a piezoelectric transformer for lighting voltage in conformation to the change of load impedance. SOLUTION: When a DC voltage Vcc is inputted to an inverter 10, the secondary output of a piezoelectric transformer 11 is applied to a cold-cathode tube 50, and a cold-cathode tube current detecting circuit 15 detects the current as voltage signal. The detected current is fed back to a voltage control oscillating circuit, in which the oscillating frequency is controlled so as to be close to the resonance frequency of the piezoelectric transformer 11. When the step-up ratio of the piezoelectric transformer 11 is raised, and the cold-cathode tube 50 starts to discharge, the current is suddenly increased, and the oscillating frequency of the voltage control oscillating circuit 12 is stabilized in the resonance frequency of the piezoelectric transformer 11. A primary current of the piezoelectric transformer 11 is monitored by a protecting circuit 30, and when an excessive current is detected, a stop signal is outputted to temporarily stop the output of the voltage control oscillating circuit 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an inverter for lighting a cold cathode tube, and more particularly to an inverter for lighting a cold cathode tube using a piezoelectric transformer.

[0002]

2. Description of the Related Art As is well known, an inverter includes a transformer and a switching circuit, and switches a DC input by the switching circuit and supplies the switching to a transformer to obtain an AC output from the transformer. Under control, the AC output is adjusted. Here, the transformer used is called an inverter transformer.

On the other hand, a cold cathode tube is used as a backlight of a liquid crystal display used as a display device in a device such as a notebook personal computer or a word processor. Due to the demand for reduction in size and weight of these devices, piezoelectric transformers that are much smaller, lighter, and lower in height than conventional electromagnetic transformers are used in inverters as lighting circuits for backlight cold cathode tubes. .

FIG. 3 shows a conventional inverter circuit for lighting a cold-cathode tube using a piezoelectric transformer as an inverter transformer. Referring to FIG. 3, when DC voltage + Vc is input from the input terminal of inverter circuit 1, drive transistor 5 is turned on, and the output voltage of drive transistor 5 is applied to piezoelectric transformer 1.
1 is applied to the primary side via the input terminals 2 and 3, and as a result, a current flows to the primary side through the output detecting voltage dividing resistor 6.

After the voltage across the output detection voltage dividing resistor 6 due to the current is amplified by the detection signal amplifying transistor 7, the switching of the drive transistor 5 is controlled. In this way, the switching frequency of the driving transistor 5 follows the resonance frequency of the piezoelectric transformer 11 to maintain the self-excited oscillation, and turns on the cold cathode tube 50 connected to the output terminal 4 of the piezoelectric transformer 11. be able to.

[0006]

However, due to the dark effect of the cold-cathode tube, the current hardly flows when the ambient temperature of the cold-cathode tube is low. There is a problem that the cold-cathode tube is difficult to light immediately after turning on the power. When the cold cathode fluorescent lamp does not light, the output side of the piezoelectric transformer is open,
In the worst case, there was a fatal defect that the piezoelectric transformer was damaged.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned deficiencies, and an object of the present invention is to provide a cold-cathode tube lighting inverter having excellent start-up properties at power-on, particularly excellent low-temperature lighting properties. .

[0008]

According to the present invention, there is provided a piezoelectric transformer having a predetermined resonance frequency for outputting a lighting voltage of a cold-cathode tube, and a voltage-controlled oscillation circuit oscillating at a frequency near the resonance frequency. A driving circuit for receiving the output of the voltage-controlled oscillation circuit to drive the piezoelectric transformer, and detecting a current flowing through the cold-cathode tube connected to the piezoelectric transformer and receiving the lighting voltage; A piezoelectric transformer-type cold-cathode tube lighting inverter for controlling the oscillation frequency of the voltage-controlled oscillation circuit in accordance with the current detected by the cold-cathode tube current detection circuit, which responds to changes in load impedance. And a protection means for protecting the piezoelectric transformer.

Further, the protection circuit monitors the input current of the piezoelectric transformer, and turns on the cold-cathode tube by using a circuit that stops the voltage-controlled oscillation circuit only while the input current is excessive. At this time, the lighting voltage can be intermittently applied to the cold cathode tube.

[0010] Alternatively, the protection circuit monitors the output voltage of the piezoelectric transformer and stops the voltage control oscillation circuit only while the output voltage is excessive, thereby lighting the cold cathode tube. At this time, the lighting voltage can be intermittently applied to the cold cathode tube.

Further, a timer circuit is provided which is activated when the power of the inverter is turned on and operates for a predetermined time and stops the voltage controlled oscillation circuit after the predetermined time, and the cold cathode tube current detection is performed within the operation time of the timer circuit. It is preferable that the operation of the timer circuit is canceled when the detection current is output from the circuit.

Further, there is provided a dimming circuit for generating a dimming signal, the dimming signal having a predetermined dimming frequency and a duty ratio corresponding to a desired luminance of the cold-cathode tube, and The control oscillation circuit is controlled by the dimming signal so that it is intermittently operated only during each ON period of the dimming signal.

[0013]

According to the piezoelectric transformer type cold-cathode tube lighting inverter of the present invention, when the cold-cathode tube starts lighting, when the impedance on the secondary side of the piezoelectric transformer becomes excessively large and the input current is excessively large, the protection circuit supplies a voltage. Stop the control oscillation circuit,
As a result, when the input current to the piezoelectric transformer stops,
The voltage controlled oscillation circuit operates again, and the piezoelectric transformer is driven again. In this way, the piezoelectric transformer can be intermittently driven at the time of starting the cold cathode tube lighting, and the lighting voltage can be applied intermittently to the cold cathode tube. The cold-cathode tube, which has become difficult to light, can be lighted smoothly in a short time.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

Referring to FIG. 1, a cold cathode tube lighting inverter 10 according to an embodiment of the present invention is shown as a lighting circuit for lighting a cold cathode tube 50 as a backlight of a liquid crystal display 40. ing. The illustrated inverter 10 includes a piezoelectric transformer 11 and a voltage-controlled oscillation circuit 12.
An oscillation frequency control circuit 13 for controlling an oscillation frequency in the voltage controlled oscillation circuit 12, a driving circuit 14 having a switching circuit for driving the piezoelectric transformer 11, and a cold cathode tube 5
The circuit comprises a cold cathode tube current detection circuit 15 for detecting a zero current, a protection circuit 30 for protecting the piezoelectric transformer 11 from a change in load impedance, and a timer circuit 31. The inverter 10 is connected to a dimming circuit 20 having an adjusting knob 21 so that the brightness of the cold cathode tube 50 can be controlled.

When a DC voltage Vcc is input to the inverter 10 by turning on the power, a voltage (oscillation signal) having a predetermined oscillation frequency is output from the voltage control oscillation circuit 12 and supplied to the drive circuit 14, where the piezoelectric transformer is used as a drive voltage. The secondary output of the piezoelectric transformer 11 is applied to the cold cathode tube 50. The current flowing through the CCFL 50 is detected by the CCFL current detection circuit 15 as a voltage signal.

More specifically, the cold-cathode tube current detection circuit 15
It has a resistor connected between the cold cathode tube 50 and the ground, and a rectifying and smoothing circuit connected to the resistor. When a cold-cathode tube current flows through this resistor, A is generated at both ends of the resistor.
By rectifying the C voltage with a rectifier circuit and smoothing with a smoothing circuit, a DC voltage proportional to the cold cathode tube current is obtained as a detection voltage signal.

This detection voltage signal is supplied to the oscillation frequency control circuit 1
The control voltage is fed back to the voltage-controlled oscillation circuit 12 as a control voltage via 3 to control the oscillation frequency of the voltage-controlled oscillation circuit 12. Thus, the oscillation frequency of the voltage controlled oscillation circuit 12 is controlled so as to approach the resonance frequency of the piezoelectric transformer 11. As a result, the step-up ratio of the piezoelectric transformer 11 increases, and the cold cathode tube 50 starts discharging.

As a result, the current flowing through the cold-cathode tube 50 sharply increases, so that the oscillation frequency of the voltage-controlled oscillation circuit 12 controlled by the cold-cathode tube current detection circuit 15 and the oscillation frequency control circuit 13 And the light emission of the cold cathode tube 50 is also stabilized.

Incidentally, the DC voltage Vc is supplied to the inverter 10.
is input, the inverter starts operating and the cold cathode fluorescent lamp 50
If the cold cathode tube 50 is not turned on at the start of lighting due to its dark effect or left at a low temperature, the secondary side of the piezoelectric transformer 11 is open and the piezoelectric transformer 11
Excessive electric power is input to the piezoelectric transformer 11, and the piezoelectric transformer 11 is damaged.

Therefore, in the present inverter 10, in order to protect the piezoelectric transformer 11, the current flowing to the primary side of the piezoelectric transformer 11 is monitored by the protection circuit 30, and when an excessive current is detected, the protection circuit 30 is stopped. The voltage-controlled oscillation circuit 12 outputs a signal, and upon receiving the stop signal, temporarily stops the output.

Specifically, the protection circuit 30 includes the driving circuit 14
And a voltage comparator having an input terminal connected to the output terminal of the drive circuit, another input terminal connected to the reference voltage source, and an output terminal. ing. Part of the primary current supplied from the drive circuit 14 to the piezoelectric transformer 11 flows through this resistor, and generates a voltage drop proportional to the primary current. The potential with respect to the ground due to this voltage drop is supplied to one input terminal of the voltage comparator,
The voltage is compared with the voltage of the reference voltage source, and the difference is output from the output terminal as a detection voltage. A detection voltage when the voltage drop is higher than the potential of the reference voltage source is supplied to the voltage control oscillation circuit 12 as a stop signal. The voltage controlled oscillation circuit 12 has, for example, a switch circuit in its output circuit, and the supply of the drive voltage to the drive circuit 14 is stopped when the switch circuit is turned off by a stop signal.

The stop signal from the protection circuit 30 is
Instead of being directly supplied to the voltage controlled oscillation circuit 12, it may be supplied to the oscillation frequency control circuit 13 to control the supply of the control voltage to the voltage controlled oscillation circuit 12.

As a result, since no drive voltage is applied to the primary side of the piezoelectric transformer 11, no current flows to the primary side of the piezoelectric transformer 11, and the protection circuit 30 does not operate.
The drive voltage of the voltage control oscillation circuit 12 to is supplied to the piezoelectric transformer 11 again.

The start, stop, and restart of the voltage controlled oscillation circuit 12 under the control of the protection circuit 30 described above (when a switch circuit is used as the output circuit as in the above-described example, the output transmission, stop, and Re-sending))
The process is repeated until the cold cathode tube 50 is turned on and the cold cathode tube current detection circuit 15 detects the cold cathode tube current. During this repetition, an AC voltage is intermittently input to the piezoelectric transformer 11. That is, a burst voltage is input. The burst cycle is desirably 20 ms or less in consideration of ensuring lighting performance and protecting the piezoelectric transformer 11.

If the secondary side of the piezoelectric transformer 11 is kept open due to breakage of the cold-cathode tube 50 or the like, no cold-cathode tube current flows even if the above operation is repeated. Therefore, the repetitive operation should be stopped after continuing for several seconds. For that, for example,
A timer circuit 31 that operates for several seconds when the power is turned on is provided. When the cold cathode tube current detection circuit 15 detects the cold cathode tube current while the timer circuit 31 is operating, the operation of the timer circuit 31 is canceled. On the other hand, if the cold cathode tube current is not detected within the operation period of the timer circuit 31, the timer circuit 31 generates a stop signal after the elapse of the timer operation time. This stop signal is supplied to the voltage-controlled oscillation circuit 12 to stop the voltage-controlled oscillation circuit 12, similarly to the stop signal from the protection circuit 30. However, the timer circuit 31
Even after the elapse of the timer operation time, the stop signal is continuously output, so that the output from the voltage control oscillation circuit 12 is not supplied to the piezoelectric transformer 11 until the timer circuit is restarted.

The stop signal of the timer circuit 31 is supplied directly to the oscillation frequency control circuit 13 without being directly supplied to the voltage control oscillation circuit 12, and is supplied to the voltage control oscillation circuit 12.
The supply of the control voltage to the power supply may be stopped.

The dimming circuit 20 is for adjusting the light emission intensity (brightness) of the cold cathode fluorescent lamp 50.
A dimming signal is output as a pulse signal having a duty ratio set at 1 and corresponding to the luminance. Here, the frequency of the dimming signal (dimming frequency) is much lower than the oscillation frequency of the voltage controlled oscillation circuit 12. This dimming signal is applied to the oscillation frequency control circuit 13 and controls the supply of a control voltage from the oscillation frequency control circuit 13 to the voltage controlled oscillation circuit 12.

For example, it is assumed that the oscillation frequency control circuit 13 has an AND gate in its output circuit and supplies a dimming signal and a control voltage signal to two inputs. As a result,
During the ON period of the light control signal, the control voltage
2 and not supplied during the OFF period. As a result, the voltage controlled oscillation circuit 12 is operated only during the ON period of the dimming signal. That is, the piezoelectric transformer 11 is operated intermittently, so that the AC signal is intermittently supplied to the primary side of the piezoelectric transformer 11, and the secondary side output is intermittently obtained in response thereto, and the cold cathode tube 50 is intermittently operated. It will be lit.

That is, the cold-cathode tube 50 supplies the O
Lighting is repeated during the N period and turned off during the OFF period. By choosing the dimming frequency so large that the blinks are not visually perceptible (usually,
(From 100 Hz to 1 kHz), visually, the cold-cathode tube 50 is maintained in the lighting state. However, since the actual lighting time changes depending on the duty ratio of the dimming signal, the lighting time integrated within a certain time much longer than the cycle of the dimming signal changes, so that the visual brightness at the time of lighting is changed. (Brightness) also changes. That is, dimming can be achieved by adjusting the duty ratio of the dimming signal.

FIG. 2 is a diagram showing an inverter circuit according to another embodiment, which is the same as that of FIG. 1 except for a protection circuit. Therefore, the same components are denoted by the same reference numerals, and the description thereof will be omitted.

In FIG. 2, the protection circuit is shown as 30 ', monitors the output voltage of the piezoelectric transformer 11, generates a stop signal when the output voltage is excessive, and turns off the voltage control oscillator circuit 12. It is to stop.

The protection circuit 30 'comprises a voltage comparator,
The output voltage of the piezoelectric transformer is compared with a reference voltage to generate an error voltage. The error voltage when the former is higher than the latter is supplied to the voltage controlled oscillator circuit 12 as a stop signal.

The protection circuit 30 'can protect the piezoelectric transformer 11 as in the protection circuit 30 of FIG.

[0035]

As described above, in the inverter according to the present invention, the protection circuit detects when the impedance on the output side of the piezoelectric transformer is excessive, and stops the supply of the driving voltage to the piezoelectric transformer during that time. By applying a lighting voltage intermittently to the cold-cathode tube at the start of lighting of the cold-cathode tube, it is possible to quickly and smoothly turn on the cold-cathode tube which has become difficult to light due to the dark effect and low-temperature storage. As a result, the startability of the cold cathode fluorescent lamp is improved.

In addition, since the driving voltage is intermittently applied to the piezoelectric transformer and the lighting voltage is intermittently supplied to the cold-cathode tube, the load on the piezoelectric transformer is small and no damage occurs.

Further, when the cold-cathode tube is not lit even when the lighting voltage is repeatedly applied for a certain period of time, the voltage control oscillation circuit is stopped by the timer circuit, and the primary side input of the piezoelectric transformer is stopped. Transformer damage can be prevented.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a cold cathode tube lighting inverter according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a cold cathode tube lighting inverter according to another embodiment of the present invention.

FIG. 3 is a circuit diagram showing a conventional cold-cathode tube lighting inverter using a piezoelectric transformer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Inverter for lighting a cold cathode tube 11 Piezoelectric transformer 12 Voltage controlled oscillation circuit 13 Oscillation frequency control circuit 14 Drive circuit 15 Cold cathode tube current detection circuit 20 Dimming circuit 21 Adjustment knob 30, 30 'Protection circuit 31 Timer circuit 40 Liquid crystal display 50 cold cathode tube

Claims (5)

[Claims] 1. A piezoelectric transformer having a predetermined resonance frequency for outputting a lighting voltage of a cold-cathode tube, a voltage-controlled oscillation circuit oscillating at a frequency near the resonance frequency, and receiving an output of the voltage-controlled oscillation circuit. A driving circuit for driving the piezoelectric transformer, and a cold-cathode tube current detection circuit connected to the piezoelectric transformer and detecting a current flowing through the cold-cathode tube to which the lighting voltage has been input, A piezoelectric transformer-type cold-cathode tube lighting inverter that controls the oscillation frequency of the voltage-controlled oscillation circuit according to the current detected by the detection circuit.
An inverter for lighting a cold-cathode tube, comprising protection means for protecting the piezoelectric transformer. 2. The protection circuit is a circuit that monitors an input current of the piezoelectric transformer and stops the voltage-controlled oscillation circuit only while the input current is excessive. The lighting voltage is intermittently applied to the cold-cathode tube during lighting.
Cold cathode tube lighting inverter. 3. The protection circuit is a circuit that monitors an output voltage of the piezoelectric transformer and stops the voltage-controlled oscillation circuit only while the output voltage is excessive. The lighting voltage is intermittently applied to the cold-cathode tube during lighting.
Cold cathode tube lighting inverter. 4. The cold-cathode tube lighting inverter according to any one of claims 1 to 3, which is activated when the inverter is powered on and operates for a predetermined time, and after the predetermined time, the voltage-controlled oscillation circuit is activated. A timer circuit for stopping, and when the detection current is output from the cold-cathode tube current detection circuit within the operation time of the timer circuit, the operation of the timer circuit is canceled, and Inverter. 5. The cold-cathode tube lighting inverter according to claim 1, further comprising a dimming circuit for generating a dimming signal, wherein the dimming signal is a predetermined dimming signal. Having a light frequency and a duty ratio corresponding to a desired luminance of the cold-cathode tube, wherein the voltage-controlled oscillation circuit is intermittently operated only during each ON period of the light-control signal by the light-control signal. An inverter for lighting a cold-cathode tube, which is controlled.