JP5035422B2 - Discharge tube lighting device - Google Patents

Description

  The present invention relates to a lighting device for a discharge tube such as a cold cathode tube for a backlight of a liquid crystal display.

  Cold cathode tubes are used for backlights of liquid crystal displays. The cold cathode tube is lit with an alternating voltage of several hundred volts because of its lamp characteristics, and the light emission luminance is determined by the tube current. Therefore, the tube current is controlled by controlling the voltage applied to the cold cathode tube, thereby adjusting the luminance.

  However, even if the applied voltage is lowered to reduce the luminance after the cold cathode tube is turned on, the discharge state cannot be maintained and the lamp is extinguished if the voltage value falls below the voltage value for maintaining the arc discharge. Therefore, an intermittent driving method called burst control has been adopted for dimming control in the conventional cold cathode tube lighting device.

  Since the drive circuit of the cold cathode tube inverter constitutes an inverter circuit that generates an alternating voltage from a direct current input voltage, the conventional lighting device such as a dimmable cold cathode tube drives the inverter intermittently. Is. This makes it possible to perform dimming control while reducing the average value of the tube current while maintaining the applied voltage to the cold cathode tube at the required voltage, and preventing the light from being turned off.

  However, intermittent drive control has a low frequency band of about several tens to several hundreds of Hz, and in some cases, the screen of the liquid crystal display flickers or audible sound is generated from the transformer. was there.

  In order to suppress this problem, the driving frequency is controlled so that a voltage lower than the lighting voltage of the cold-cathode tube is emitted during a period of “intermittent” that is normally intermittent (and therefore, it is not substantially intermittent oscillation). An apparatus is disclosed in Patent Document 1. Patent Document 2 discloses a technique for reducing the peak of the noise level by changing the intermittent drive frequency itself of the intermittent drive control.

  FIG. 1 is a circuit diagram showing a configuration of a lighting device of Patent Document 1, and FIG. 2 is a waveform diagram of an applied voltage to a cold cathode tube. This lighting device includes an inverter circuit 1, a lighting tube 2, a variable DC voltage supply means 3, an input power supply 4, a DC pulse voltage supply means 7, and an OR circuit 8. A variable DC voltage supply means 3 and a DC pulse voltage supply means 7 are provided in parallel between the input power supply 4 and the OR circuit 8. Further, the output of the OR circuit 8 is configured to be input to the inverter circuit 1. With this configuration, the applied voltage is switched between the period T1 and the period T2 shown in FIG.

JP 11-87085 A JP 2000-123994 A

However, in the lighting devices shown in Patent Documents 1 and 2, since the transformer is always driven, power consumption per luminance is large. Further, in Patent Document 2, since the intermittent drive frequency fluctuates, there is a problem that the screen flickers when used for a backlight of a liquid crystal display.
The above-mentioned problems are not limited to cold cathode tubes but apply to discharge tubes in general.

  Accordingly, an object of the present invention is to provide a discharge tube lighting device that suppresses the generation of audible sound and reduces power consumption while avoiding the problem of flickering of light emission luminance when performing dimming control by intermittent drive. It is in.

In order to solve the above problems, the present invention is configured as follows.
(1) AC voltage generating means for converting a DC voltage into an AC voltage having a first frequency and generating a driving voltage applied to the discharge tube; and an AC voltage having the first frequency lower than the first frequency. In a discharge tube lighting device comprising intermittent driving means for performing light control by intermittent driving at a frequency of 2,
During the off period of the drive voltage at the second frequency by the intermittent drive of the intermittent drive means, the first frequency at a third frequency lower than the first frequency and higher than the second frequency. An auxiliary intermittent driving means for intermittently driving an AC voltage is provided.

  With this configuration, even during the “missing” period of the intermittent drive cycle, the AC voltage of the first frequency is inserted for a very short time based on the third frequency, and vibration due to the intermittent drive is reduced in a short ON period. Since it is diffused by the vibration of another period that occurs, the spectrum of vibration is spread and noise is reduced.

(2) In addition, auxiliary intermittent drive frequency control means for controlling the variation of the third frequency is provided.
Thereby, the periodicity of the vibration generated by the short ON period can be made random, the spectrum spread effect of the vibration due to the intermittent drive period is further increased, and the noise reduction effect is increased.

(3) In addition, it is assumed that a plurality of AC voltage waves of the first frequency that are driven on / off at the third frequency exist during an off period at the second frequency.
As a result, the spread spectrum effect of vibration due to the intermittent drive cycle is further increased, and the noise reduction effect is increased.

  According to the present invention, dimming is performed by intermittent driving of the applied voltage to the discharge tube at the second frequency, and the vibration by the intermittent driving is diffused by the vibration by the driving at the third frequency, so that the vibration spectrum is It is diffused and noise is reduced. Moreover, since the discharge tube is always driven intermittently, an increase in power consumption can be suppressed.

It is a circuit diagram which shows the structure of the lighting device of patent document 1. FIG. It is a wave form diagram of the applied voltage with respect to a discharge tube. It is a block diagram which shows the structure of the discharge tube lighting device 100 which concerns on 1st Embodiment. It is a specific circuit diagram of the discharge tube lighting device 100 according to the first embodiment. It is a wave form diagram of signals S1-S4 in FIG. It is a specific circuit diagram of the discharge tube lighting device 101 according to the second embodiment.

<< First Embodiment >>
A discharge tube lighting device according to a first embodiment of the present invention will be described with reference to FIGS.
FIG. 3 is a block diagram showing the configuration of the discharge tube lighting device 100. The discharge tube lighting device 100 includes a drive circuit 21 that drives the high-voltage transformer 22 and the discharge tube 23 based on an input voltage, an output current detection circuit 24 that detects an output current flowing through the discharge tube 23, and the output current detection circuit 24. And an inverter control circuit 25 for driving the drive circuit 21 based on the signal from the dimming signal generation circuit 10 in accordance with the detection signal.

  The dimming signal generation circuit 10 includes a dimming burst control circuit 12 that generates a rectangular wave signal according to a voltage generated by the dimming voltage generation circuit 11 and a logic circuit 13 that generates a control signal for the inverter control circuit 25 from the signal. It has.

  In FIG. 3, the inverter control circuit 25 controls the drive circuit 21 so that an AC voltage is applied to the high-voltage transformer 22 and the discharge tube 23, and the dimming signal generation circuit 10 intermittently drives the inverter control circuit 25. Dimming by

  FIG. 4 is a specific circuit diagram of the discharge tube lighting device 100 shown in FIG. The drive circuit 21 includes two transistors Q1 and Q2 and capacitors C1 and C2 connected in series between a power supply terminal to which an input voltage is input and the ground. The primary windings of a plurality of high-voltage transformers 22a, 22b,... 22n are connected in parallel to the connection point of Q1, Q2 and the connection point of C1, C2. The secondary windings of these high-voltage transformers 22a, 22b... 22n are connected to discharge tubes 23a, 23b. Also, output current detection circuits 24a, 24b... 24n are connected to these current paths.

  The inverter control circuit 25 complementarily turns on / off the transistors Q1 and Q2 of the drive circuit 21 at the first frequency f1. The inverter control circuit 25 corresponds to “AC voltage generating means” according to the present invention. The inverter control circuit 25 intermittently controls the on / off of the transistors Q1 and Q2 in accordance with the logic level of the signal from the dimming signal generation circuit 10.

  The output current detection circuit 24 detects the peak current value of the tube current flowing through the discharge tubes 23a, 23b,. The inverter control circuit 25 performs feedback control by adjusting the on-duty ratios of the transistors Q1 and Q2 in order to stabilize the peak current value of the tube current detected by the output current detection circuit 24.

  In the dimming signal generation circuit 10, the triangular wave generation circuit 12b generates a triangular wave signal of the second frequency f2 that intermittently drives the driving at the first frequency f1. The dimming voltage generation circuit 11 generates a DC dimming voltage according to the user's operation. The comparator 12d compares the output voltage of the dimming voltage generation circuit 11 with the output signal of the triangular wave generation circuit 12b, and generates a rectangular wave signal S2 that is PWM-modulated according to the dimming voltage. The dimming voltage generation circuit 11, the triangular wave generation circuit 12b, and the comparator 12d correspond to the “intermittent drive means” according to the present invention.

  The rectangular wave generation circuit 12c generates a rectangular wave signal S3 having a third frequency f3 that is lower than the first frequency f1 and higher than the second frequency f2.

  The logic circuit 13 is a circuit that outputs a logical sum signal S4 of the signals S2 and S3.

  FIG. 5 is a waveform diagram of the above signals. Here, the signal S1 is a signal of frequency f1 due to self-oscillation of the inverter control circuit 25, and S2, S3, S4 are signals of each part of the dimming signal generation circuit 10. Dimming of the discharge tube 23 is performed by the dimming voltage generated by the dimming voltage generation circuit 11. That is, the frequency f2 of the signal S2 is constant according to the dimming voltage, but the on-duty ratio changes and the on-period Ton changes. As a result, the average value of the tube current flowing through the discharge tube 23 changes, thereby controlling the light emission luminance.

The frequency f1 of the signal S1 is 20 to 50 kHz, and the frequency f2 of the signal S2 is about 60 to 300 Hz. The frequency f3 of the signal S3 is about 2 to 20 times the frequency f2. Therefore, the auxiliary intermittent driving is performed at least once in the OFF period Toff of the signal S2 (“missing period of intermittent driving”).
The rectangular wave generating circuit 12c and the logic circuit 13 correspond to “auxiliary intermittent driving means” according to the present invention.

  In the example shown in FIG. 5, since the on-duty ratio of the rectangular wave signal S3 is less than 50%, even if auxiliary intermittent driving is performed with this signal S3, the average value of the tube current flowing in the discharge tube is hardly increased. There is almost no increase in light emission brightness or power consumption.

  Thus, since the inverter is intermittently driven with the repetition period of the signal S3 even in the off period Toff of the signal S2, the vibration due to the frequency f2 is diffused and the spectrum of the vibration is diffused, and the audible noise is reduced. Even if auxiliary intermittent driving is performed, the period during which the tube current flows is not particularly long, and an increase in power consumption can be suppressed.

  In the example shown in FIG. 4, a half-bridge type inverter circuit is configured. However, other circuit configurations such as a full-bridge type and a push-pull type can be similarly applied. Further, the high voltage transformer 22 may be a non-insulated type as long as the side that supplies the input power supply voltage is insulated from the commercial power supply.

  In the example shown in FIG. 4, a rectangular wave signal is generated by a circuit, and a signal that finally determines the on / off timing of the transistors Q1 and Q2 of the drive circuit 21 is generated by a logic circuit. However, an on / off timing signal for the transistors Q1 and Q2 of the drive circuit 21 may be generated by program processing.

<< Second Embodiment >>
FIG. 6 shows a circuit of a discharge tube lighting device according to the second embodiment.
The first embodiment is different from the circuit shown in FIG. 4 in that the rectangular wave generating circuit 12c is a circuit in which the frequency generated by a modulation signal given from the outside changes, and the rectangular wave generating circuit 12c is modulated. A modulation circuit 14 for supplying a signal is provided.

The modulation circuit 14 is a circuit that generates a sine wave signal or a triangular wave signal having a frequency lower than the frequency f3 of the signal S3 as a modulation signal. The rectangular wave generating circuit 12c modulates the frequency of the rectangular wave in the frequency range of ± Δf with f3 as the center, according to the modulation signal.
The modulation circuit 14, the rectangular wave generation circuit 12c, and the logic circuit 13 correspond to “auxiliary intermittent drive frequency control means” according to the present invention.

  As a result, the number of auxiliary intermittent drives during the Toff period of the signal S4 shown in FIG. 5 varies, and the spectrum spread effect of vibration due to the intermittent drive period becomes larger, and the noise reduction effect is enhanced.

DESCRIPTION OF SYMBOLS 10 ... Dimming signal generation circuit 11 ... Dimming voltage generation circuit 12 ... Dimming burst control circuit 12d ... Comparator 13 ... Logic circuit ... Modulation circuit 21 ... Drive circuit 22 ... High voltage transformer 22a, 22b ... 22n ... High voltage transformer 23 ... Discharge tube 23a, 23b ... 23n ... Discharge tube 24 ... Output current detection circuit 24a, 24b ... 24n ... Output current detection circuit 25 ... Inverter control circuit 100 ... Discharge tube lighting device 101 ... Discharge tube lighting device C1 , C2 ... capacitors Q1, Q2 ... transistors

Claims (3)

AC voltage generating means for generating a drive voltage to be applied to the discharge tube by converting a DC voltage into an AC voltage having a first frequency, and an AC voltage having the first frequency being lower than the first frequency. In the discharge tube lighting device provided with intermittent drive means that performs dimming by intermittently driving,
During the OFF period of the drive voltage at the second frequency by the intermittent drive of the intermittent drive means, the first frequency at a third frequency lower than the first frequency and higher than the second frequency. A discharge tube lighting device comprising auxiliary intermittent driving means for intermittently driving an AC voltage.   The discharge tube lighting device according to claim 1, further comprising auxiliary intermittent drive frequency control means for performing fluctuation control on the third frequency.   3. The discharge tube according to claim 1, wherein a plurality of waves of the alternating-current voltage of the first frequency driven on / off at the third frequency are present during an off period at the second frequency. Lighting device.

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