JP2006085962A - Cold-cathode tube lighting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lighting device of a cold-cathode tube which prevents the heating and the expansion of the burning of a substrate caused by arc discharge by stopping arc discharge at the initial time of the occurrence of arc discharge caused by disconnection or the like caused by a connection failure. <P>SOLUTION: This cold-cathode tube lighting device has an inverter circuit which supplies a voltage of a prescribed frequency by converting a DC voltage into an AC voltage. In order to stop an initial arc discharge state, a starting period for impressing a high frequency voltage and a stopping period for not impressing the high frequency to the cold-cathode tube 20 are provided additionally, and the stopping period is set for a prescribed period at prescribed cycles. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a cold-cathode tube lighting device, and more particularly to safe and stable lighting of a cold-cathode tube lighting device.

  As a conventional cold cathode type discharge tube lighting device, for example, the one shown in FIG. 5 is used. In this circuit, an inverter circuit 22 composed of a modified Royer circuit is provided at the subsequent stage of the step-down DC-DC converter 21 connected to the DC power source DC. Self-oscillation is performed in the modified Royer circuit, and the resonance voltage is boosted according to the turns ratio of the transformer 5 and appears on the secondary side. CP is a current limiting ballast capacitor. Dimming of the discharge tube 20 is performed by controlling the output voltage of the DC-DC converter 21. The current detector 7 detects the tube current, feeds back a DC voltage corresponding to the value of the tube current to the protection circuit 23, and controls the output voltage of the DC-DC converter 21 in the previous stage so that the tube current becomes constant. Yes.

  As described above, the discharge tube lighting device is usually provided with a protection circuit that stops the operation when the output voltage becomes abnormally high. For example, when the output current does not flow because the high voltage side of the transformer is disconnected or the circuit element is broken, the protection circuit detects these and stops the operation.

Further, a discharge lighting device for supplying a current of a predetermined oscillation frequency to a discharge tube against a case where arc discharge occurs in a poor connection state is provided with a protection circuit including a current detector. There is a configuration in which when a discharge noise frequency component higher than a predetermined oscillation frequency is mixed in the detected current, the protection circuit stops the voltage supply to the discharge tube (for example, Patent Document 1).
Japanese Patent Laid-Open No. 2002-151287

  However, in the conventional configuration, even when a high-voltage component such as a transformer, a capacitor, or a connector becomes defective in connection, the protection circuit may not work depending on the state of the connection failure. This is because when the interruption distance at the location where the conduction is interrupted is short, arc discharge is instantaneously started, so that current continues to flow and the output voltage does not increase. As a result, when a disconnection occurs in the discharge tube or circuit board in continuous discharge using a cold cathode tube, the current value of continuous arc discharge from there may not be much different from the current value during normal operation. . Therefore, even if the current value is detected, the accident is not noticed, and there is a possibility that heat generation and substrate burns are enlarged. Even when a discharge noise frequency component higher than a predetermined oscillation frequency is detected from the current detected by the current detector, external noise other than the noise component from the arc discharge occurrence point is detected from a transformer or a coil. And there is a problem that it takes time and effort to actually create a circuit for detecting the noise.

  The present invention solves the above-described conventional problems, and when a heat generation or a burnout accident occurs due to arc discharge, the protection circuit regardless of fluctuations in the current value of the arc discharge flowing in the cold-cathode tube lighting device or the state of noise. An object of the present invention is to provide a safe cold-cathode tube lighting device that easily stops arc discharge without using a current detector and prevents the substrate from burning out.

  In order to solve the above-described conventional problems, a cold-cathode tube lighting device according to the present invention has a cold-cathode tube having an inverter circuit that converts a DC voltage into an AC voltage and supplies a predetermined high-frequency voltage to a plurality of cold-cathode tubes. In the lighting device, in order to stop the arc discharge, a starting period for applying the high-frequency voltage to the cold cathode tube and a stopping period for not applying the high-frequency voltage are additionally provided, and the stopping period is provided at a predetermined period for a predetermined period. It is characterized by.

  That is, unlike the conventional PWM dimming that does not provide a stop period when the brightness is maximum, the stop period is always provided even when the brightness is maximum, so that continuous discharge of arc discharge can be reliably prevented. Here, dimming refers to adjusting the brightness of a lighting device such as a backlight, and the dimming frequency refers to a frequency at which an applied voltage repeatedly starts and stops during the dimming. In addition, PWM dimming refers to adjusting the brightness while changing the ratio of the start / stop period by adding a pulse (PWM) that repeats start / stop for a predetermined period at a predetermined cycle as necessary.

  According to the cold-cathode tube lighting device of the present invention, when an arc discharge occurs due to a disconnection accident such as a poor solder connection of a lead wire or a poor connection of a high voltage component of a transformer / capacitor / connector, an arc is generated when the output voltage is stopped. Discharging also stops. Then, arc discharge occurs again when the output voltage is started. As described above, while the arc discharge is repeatedly stopped and started, the substrate or the like is burned out. However, when the burnout of the substrate or the like due to the arc discharge proceeds to a cutoff distance of about 2 mm to 3 mm, even if a voltage is applied again, that 2 mm. An arc discharge that destroys the dielectric strength at a breaking distance of ˜3 mm does not occur, and a continuous arc discharge does not occur. In other words, it is possible to prevent the heat generated by the arc discharge and the substrate burnout from expanding.

  Furthermore, it is possible to provide a cold-cathode tube lighting device that can be stably lit even in a dark state where the surroundings are shielded from light and at a low temperature of −10 ° C. to 0 ° C.

  Embodiments of a cold cathode tube lighting device according to the present invention will be described below in detail with reference to the drawings.

  The voltage waveform applied to the cold cathode tube 20 is shown in FIG. There are a start-up period in which the high-frequency voltage is applied and a stop period in which the high-frequency voltage is not applied. The applied high-frequency voltage is 40 KHz to 100 KHz, and the stop period in which the high-frequency voltage is not applied is about 1 to 10 cycles of the high-frequency voltage. The repetition period of the stop period is 2 msec to 33 msec (30 Hz to 500 Hz).

When the high-frequency voltage is 45 KHz, the period from the 1st period to the 10th period of the stop period is 22.22 μsec to 222.22 μsec as shown in FIG.
FIG. 2 shows a waveform diagram when the intermittent pulse (PWM pulse) in the start period and the stop period is 200 Hz and the high-frequency voltage is 45 KHz. The LOW period of the PWM pulse is the stop period of the cold cathode tube 20.

  FIG. 3 is a configuration circuit diagram of the cold cathode tube lighting device according to the first embodiment of the present invention.

  In FIG. 3, the first block 1 has a high-frequency oscillation circuit 4 and a step-up transformer 5 and is configured as a parallel resonant push-pull inverter.

  The high frequency oscillation circuit 4 applies a pulse wave OS1 that is an oscillation output from the first oscillator 10 to the transistor Q1 via the gate circuit 11, and outputs the output of the oscillation output OS1 of the first oscillator 10 to the inverter 13. The inverted pulse wave is applied to the transistor Q2 through the gate circuit 12, and the output with the phase inverted is applied to the step-up transformer 5. The step-up transformer 5 includes two primary windings 51A and 51B and a secondary winding 52 separated by a neutral point M1.

  The DC power supply DC maintains the output voltage Vin at a constant value (for example, 16V). The first capacitor C1 stably maintains the input voltage Vin from the DC power source DC. This prevents a voltage drop due to the internal impedance of the power source and serves as a ripple filter that supplies a stable voltage.

  The repetition frequency of the pulse wave of the oscillation output OS1 of the first oscillator 10 is selected to be 45 kHz, for example. This frequency is selected such that the lamp efficiency is suitable, but a range of 40 KHz to 100 KHz is suitable. This frequency range can make the luminance unevenness such as a phenomenon that the luminance is biased toward the high-voltage terminal side of the cold cathode tube (single lighting) become inconspicuous. In particular, when used in a backlight of a liquid crystal display device, uneven brightness becomes a problem. This frequency range is also preferable from the viewpoint of low temperature startup and lamp light emission efficiency.

  A pulse wave of the oscillation output OS1 of the first oscillator 10 is applied to the control terminals of the two transistors Q1 and Q2 via the gate circuits 11 and 12. The inverter 13 reverses the polarity of the pulse wave output from the transistor Q2 and applied to the primary winding 51B of the transformer 5, and the polarity of the pulse wave output from the transistor Q1 and applied to the primary winding 51A of the transformer 5. Make it polar.

  That is, the output voltage waveforms at terminals A and B in FIG. 3 are as shown in FIG. Furthermore, the pulse wave OS2 whose pulse wave LOW level period from the second transmitter 14 is a predetermined PWM wave is always input to the AND gates 11 and 12. The frequency of OS2 is, for example, 200 Hz, and the stop period in one cycle is selected to be 25 μsec of one cycle of the high-frequency voltage (45 KHz). The frequency of the OS2 may be a frequency at which flicker is not noticeable on the display video screen when used as a backlight of a liquid crystal display device or the like.

  That is, the stop period is set to 1 to 10 periods (22.22 μsec to 222.22 μsec) of the high frequency voltage of 45 KHz so as to prevent the arc discharge from continuing and to prevent uneven brightness, and a pulse that is a PWM wave The frequency of the wave OS2 is selected to be about 30 Hz to 500 Hz so that flicker can be prevented. The output waveforms of the gate circuits 11 and 12 (the waveforms of the terminals C and D in FIG. 4) are such that the pulse wave of the oscillation output OS1 of the first oscillator 10 is output intermittently.

  Here, if the stop period is set to less than one cycle of the high frequency voltage of 45 KHz, the initial arc discharge state may or may not stop, which is not suitable for the stop period. Similarly, if it is set in a range exceeding 10 cycles of a high frequency voltage of 45 KHz, the initial arc discharge state is stopped, but the brightness of the backlight is visibly darker than that at the time of continuous discharge, so it is not suitable for the stop period. .

  That is, the initial arc discharge state can be stopped by setting it to one cycle or more of the high-frequency voltage. The frequency of the PWM wave is selected to be about 30 Hz to 500 Hz in order to prevent flicker. However, if the initial arc discharge is stopped, it can be stopped even if the repetition period of the PWM wave is selected longer.

  The pulse waves output to the terminals C and D are input to the transistors Q1 and Q2 to switch the transistors Q1 and Q2, and the switching outputs thereof are input voltages to the primary windings 51A and 51B of the step-up transformer 5. Vi is applied with reverse polarity.

  That is, during the HIGH period of the 200 Hz pulse wave OS2, the 45 KHz pulse wave OS1 of OS1 having a frequency higher than the frequency of the pulse wave OS2 is output, and the LOW period is not output.

  In the high frequency pulse wave of 45 KHz output during the HIGH period of the pulse wave OS2, the inductor L and the second capacitor C2 resonate, and the secondary voltage V of the step-up transformer 5 is the same as the output voltage of the terminals C and D. The cold cathode discharge tube 20 is driven at a cycle. That is, a high-frequency drive voltage of OS1 is applied during the HIGH period of the PWM wave of the pulse wave OS2, the PWM wave of the pulse wave OS2 is activated during the HIGH period, and during the LOW period, the PWM wave of the pulse wave OS2 is repeated. Applied in synchronization with the frequency. That is, the output voltage waveform of the secondary voltage V of the transformer 5 has a start period in which a voltage is applied and a stop period in which the voltage is not applied as shown in FIG.

  As described above, in the first embodiment, the PWM wave of the pulse wave OS2 is always applied as shown in FIG. 3, so that the secondary voltage V of the transformer 5 becomes the HIGH period and the LOW period of the PWM wave of the pulse wave OS2. In response to, start / stop is repeated in the same cycle.

  Here, when arc discharge occurs due to poor solder connection of lead wires or disconnection accident due to poor connection of high voltage parts such as connectors, capacitors and transformers, the arc discharge is also stopped when the secondary voltage V is stopped. When the secondary voltage V is started, arc discharge occurs again. As described above, while the arc discharge is repeatedly stopped and started, the substrate is burned out. However, when the substrate is burned out by arc discharge to about 2 mm to 3 mm, even if the secondary voltage V is started again, 2 mm to The electric discharge which destroys the dielectric strength of the interruption | blocking distance of 3 mm does not occur, and arc discharge stops completely. In other words, even if an arc discharge occurs due to a disconnection accident or the like, the continuation of the arc discharge can be stopped, so that the heat generated by the arc discharge and the expansion of substrate burnout can be prevented.

The current detector 7 outputs a signal to stop the OS 2 for safety when an abnormal current is detected in a predetermined current fluctuation with respect to the rated current to the plurality of cold cathode tubes 20. That is, at this time, since the PWM pulse wave OS2 continues to be in the LOW state, the output of the secondary voltage V is stopped.
For example, when 16 cold-cathode tubes are used in the backlight of a liquid crystal display device, the rated current of all the discharge currents is about 100 mA, and when the fluctuation of the discharge current of 20% or more is detected, Stop application of drive voltage.

  Here, the relationship between the start period of the output voltage and the arc discharge due to the stop period will be described. If the ratio of the stop period to the start period is too small, the lighting brightness will be stable, but if arc discharge occurs, it will be difficult to stop. . Similarly, if the ratio of the stop period to the start period is too large, the lighting brightness becomes unstable, but the arc discharge tends to stop. Therefore, it is necessary to set the optimum frequency of the pulse OS2 for the high-frequency pulse OS1 and the stop period in one cycle to the optimum predetermined period so that the lighting brightness is stable and the arc discharge is stopped. Naturally, the stop period is set shorter than the start period.

  In the first embodiment, when the frequency of OS1 is set to 45 KHz and the PWM wave whose frequency of OS2 is 200 Hz (period 5 msec) is used, the start period is set to 4.975 msec and the stop period is set to 25 μsec. When the arc discharge occurs and damage to the substrate progresses and spreads to a cut-off distance of about 2 mm to 3 mm, the arc discharge stops. The range of the stop period is suitably 22.22 μsec to 222.22 μsec (1 cycle to 10 cycles of 45 KHz) when the frequency of OS1 is 45 KHz. In other words, in the present embodiment where the frequency of OS1 is 45 KHz, about 0.44% to 4.4% with a repetition period of 5 msec is appropriate.

  In this stop period, even if the high frequency voltage is in the range of 40 KHz to 100 KHz, the arc discharge can be stopped if it is set to about 1 cycle to 10 cycles.

In addition, when one cycle of the PWM wave is used as a backlight of a liquid crystal display screen, it is usually selected to be 30 Hz or more (repetition cycle is 33 msec or less) in a range where the flicker of the display screen is not noticeable. Even if it is selected to be longer than several hours, the initial arc discharge state can be stopped.
In general, the brightness unevenness tends to become unstable when the fluorescent lamp or the backlight is cut off from the surrounding brightness in a dark state and started in a low temperature environment, but in this embodiment, it is stable even at a low temperature. Works. In the present embodiment, when operating in a dark state and in an environment of −10 ° C. to 0 ° C., luminance unevenness (single lighting) occurs during continuous discharge, but when a pulse wave OS2 that is a predetermined PWM wave is used. Confirms that stable luminance can be obtained.

  The present invention can be applied as a safe and stable driving device for a backlight of a liquid crystal television that avoids the danger caused by continuous arc discharge from a damaged portion and has sufficient image brightness even at low temperatures.

Applied voltage waveform diagram of cold cathode tube lighting device in Example 1 of the present invention The figure which shows an example of the applied voltage waveform of the cold cathode tube lighting device in Example 1 of this invention 1 is a circuit diagram showing a configuration of a cold cathode tube lighting device according to a first embodiment of the present invention. FIG. 3 is a waveform diagram of output voltages at terminals of the main part of the cold cathode tube lighting device according to the first embodiment of the present invention. Circuit diagram of conventional cold cathode tube lighting device

Explanation of symbols

DC DC power supply 1 First block 4 High frequency oscillation circuit 10 First oscillator 14 Second oscillator (PWM wave oscillator)
Q1, Q2 Transistor 13 Inverter 11, 12 AND gate 5 Step-up transformer 51A, 51B Primary winding 52 Secondary winding 6, 20 Cold cathode tube 7 Current detector

Claims (3)

 In a cold-cathode tube lighting device having an inverter circuit that converts a direct-current voltage into an alternating-current voltage and supplies a predetermined high-frequency voltage to a plurality of cold-cathode tubes, in order to stop arc discharge, an additional connection to the cold-cathode tube A cold-cathode tube lighting device comprising a start period during which the high-frequency voltage is applied and a stop period during which the high-frequency voltage is not applied, and the stop period is provided at a predetermined period. The cold-cathode tube lighting device according to claim 1, wherein the stop period is 1 to 10 periods of the high-frequency voltage. The cold-cathode tube lighting device according to claim 1, wherein a repetition period of the stop period is 2 msec to 33 msec.

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