JP2010040209A - Backlight device for liquid crystal panel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To structure a driving circuit of a backlight device for a liquid crystal panel with the use of low-cost general-purpose control ICs in place of high-cost exclusive inverter control ICs. <P>SOLUTION: The driving circuit 3 is provided with a driving-pulse generating part 32 (switching elements 32a, 32b) connected to a drive-use direct-current power source Vin of a power source part 4 for generating driving pulses, of which voltages periodically change, due to repetition of ON/OFF switching, an inverter transformer 33, of which a primary winding is connected to the driving-pulse generating part 32 and a secondary winding is connected to a cold-cathode fluorescent lamp 2, a current detecting part 35 (a resistor R1) detecting changes of drive currents flowing between the drive-use direct-current power source Vin and the driving-pulse generating part 32, and a control part 34 (a control IC 34a) carrying feed-back control so that an input power is to be constant based on changes of the drive currents detected by the current detecting part 35. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a backlight device for a liquid crystal panel used for a liquid crystal television or the like.

  For example, as described in Patent Document 1, a cold cathode fluorescent lamp (hereinafter referred to as a fluorescent lamp or CCFL: Cold Cathode Fluorescent Lamp) is widely used as a light source of a backlight device for a liquid crystal panel. It is used.

  By the way, when the screen size is large, such as a liquid crystal television, a plurality of fluorescent lamps are used. In order to make the brightness of each fluorescent lamp uniform and to reduce the fluctuation of the brightness over time, each fluorescent lamp The tube current and tube voltage flowing through the tube are detected and feedback control is performed (see Patent Document 2 or Patent Document 3).

  In a backlight device for a liquid crystal panel using a fluorescent lamp as a light source, a driving DC power supply is boosted and increased in frequency by an inverter circuit and applied between the electrodes of each fluorescent lamp. FIG. 7 shows a configuration of a driving circuit of a conventional backlight device for a liquid crystal panel.

  The drive circuit 100 of the conventional backlight device for a liquid crystal panel shown in FIG. 7 uses a dedicated inverter control IC 101 as a drive IC, and the switching element 102 connected to the DC drive power supply by the inverter control IC 101 is used. Drive to generate a drive pulse of a predetermined frequency, and let it flow through the primary winding of the inverter transformer 103. The secondary winding of the inverter transformer 103 is connected to the fluorescent lamp 106 via the tube current detection circuit 104 and the tube voltage detection circuit 105. The tube current signal and the tube voltage signal detected by the tube current detection circuit 104 and the tube voltage detection circuit 105 are input to the inverter control IC 101, respectively.

  When the fluorescent lamp 106 is activated, the inverter control IC 101 determines whether or not the level of the tube current signal has reached a predetermined first set level (first current threshold) within a predetermined time from the start of activation. Here, when the level of the tube current signal reaches a predetermined first set level within a predetermined time, it is determined that the fluorescent lamp 106 is normally lit, and the inverter control IC 101 determines the tube current signal and the drive pulse. Are compared, and a predetermined second set level (second current threshold value) and a third set level (second set value) in which the level of the tube current signal is higher than the first set level are compared. The duty width of the drive pulse is controlled so as to be maintained within a predetermined range defined by (3 current threshold). On the other hand, when the level of the tube current signal does not reach the predetermined first set level within the predetermined time, the inverter control IC 101 determines that the fluorescent lamp 106 is not lit and shifts to the drive stop mode. When the level of the tube voltage signal exceeds a predetermined set level (second voltage threshold), the inverter control IC 101 determines that the fluorescent lamp 106 is abnormal and shifts to the drive stop mode. Further, when the tube current signal level does not reach the second set level (second current threshold) even when the duty width of the drive current is increased, the inverter control IC 101 determines that the fluorescent lamp 106 is abnormal and stops driving. Enter mode.

With such a configuration, the level of the tube current flowing through the fluorescent lamp 106 can be maintained within a predetermined range. However, the tube current flowing through the secondary side of the inverter transformer 103 which is the high voltage side is detected and feedback control is performed. In this case, an expensive dedicated inverter control IC 101 with a high withstand voltage is required, which causes an increase in cost. In particular, when the lighting of one fluorescent lamp 106 is controlled by one drive circuit 100, the inverter control ICs 101 are required as many as the number of fluorescent lamps 106, which further increases the cost. Furthermore, since the oscillation waveform generated by the oscillation circuit is compared with the detected waveform of the tube current, the feedback control itself is complicated.
JP-A-6-60988 JP-A-8-55691 JP 2007-5005 A

  The present invention has been made to solve the above-described problems of the conventional example. A drive circuit is configured using an inexpensive general-purpose control IC instead of an expensive dedicated inverter control IC, and a tube that flows through a fluorescent lamp. An object of the present invention is to provide a backlight device for a liquid crystal panel capable of maintaining a current level within a predetermined range.

In order to achieve the above object, the invention of claim 1 is directed to a liquid crystal panel backlight device comprising at least one cold cathode fluorescent lamp and a drive circuit for controlling lighting of the cold cathode fluorescent lamp.
The drive circuit is
A driving pulse generator that is connected to a driving DC power source and generates a driving pulse whose voltage periodically changes by repeatedly turning on and off;
An inverter transformer in which a primary winding is connected to the drive pulse generator, and a secondary winding is connected to the cold cathode fluorescent lamp;
A current detector that detects a change in drive current flowing between the driving DC power supply and the drive pulse generator;
A control unit is further provided that performs feedback control based on a change in the drive current detected by the current detection unit so that the input power becomes constant.

  According to a second aspect of the present invention, in the backlight device for a liquid crystal panel according to the first aspect, the control unit controls a timing at which the drive pulse generating unit is turned on and off, and controls a duty width of the drive pulse. Thus, feedback control is performed.

  According to a third aspect of the present invention, in the backlight device for a liquid crystal panel according to the first or second aspect, the current detection unit includes a resistor connected between the driving DC power source and the driving pulse generation unit. The voltage between the terminals of the resistor is detected.

The invention of claim 4 is the backlight device for a liquid crystal panel according to any one of claims 1 to 3,
A tube current detection circuit and a tube voltage detection circuit connected between the secondary winding of the inverter transformer and the cold cathode fluorescent lamp;
A comparison circuit for comparing the tube current and the tube voltage detected by the tube current detection circuit and the tube voltage detection circuit with a predetermined threshold value, respectively;
The drive circuit is out of a range where the value of the tube current is less than a first current threshold when a predetermined time has elapsed after the start of activation or the value of the tube voltage is greater than or equal to a first voltage threshold and less than a second voltage threshold. In this case, the control power supply to the control unit is shut off.

  According to the present invention, a current flowing between the driving DC power supply and the driving pulse generator, that is, a driving current flowing on the primary winding side of the inverter transformer is detected, and feedback control is performed using a signal related to the detected current. Since the input power is controlled to be constant by the orthodox feedback control by the drive current, the drive circuit configuration is simple and the feedback control itself is easy. In addition, since a change in the drive current flowing to the primary winding side of the inverter transformer, which is the low-voltage side, is detected, the detected signal voltage is low, the withstand voltage is low, and an inexpensive general-purpose control IC is used. A driving circuit of a backlight device for a liquid crystal panel can be realized. As a result, the cost of the liquid crystal panel backlight device can be reduced. As a specific example of the feedback control, for example, the timing at which the drive pulse generator is turned on and off is controlled to control the duty width of the drive pulse, thereby facilitating the feedback control. Furthermore, the configuration of the drive circuit can be simplified by connecting a resistor between the drive DC power supply and the drive pulse generator and detecting the voltage across the terminals of the resistor. Further, a tube current detection circuit and a tube voltage detection circuit are provided between the secondary winding of the inverter transformer and the cold cathode fluorescent lamp, and the tube current and the tube voltage detected by these are respectively compared with a predetermined threshold value, By shutting off the control power supply to the control unit when it can be determined that the cathode fluorescent lamp is abnormal, it is possible to provide a safe and reliable backlight device for a liquid crystal panel. At this time, since the tube current and tube voltage on the high voltage side are not directly input to the control IC, an inexpensive general-purpose control IC can be used as the control unit.

  A backlight device for a liquid crystal panel according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a block configuration of a backlight device 1 for a liquid crystal panel according to the present embodiment.

  The liquid crystal panel backlight device 1 includes at least one cold cathode fluorescent lamp (hereinafter referred to as a fluorescent lamp) 2, a drive circuit 3 for controlling lighting of the fluorescent lamp 2, and a power supply unit 4. One drive circuit 3 corresponds to one fluorescent lamp 2. Therefore, when a plurality of fluorescent lamps 2 are provided, it is assumed that the same number of drive circuits 3 are provided in parallel for one power supply unit 4.

  The power supply unit 4 is a power supply circuit for supplying direct current power to the control circuit unit, and includes a control power supply for outputting a constant voltage direct current for IC control and a direct current used for lighting the fluorescent lamp 2. Drive DC power supply.

  The drive circuit 3 is connected to a timer circuit 31 such as a CR timing circuit composed of a resistor and a capacitor, for example, and a DC power source for driving the power supply unit 4, and the voltage changes periodically by repeatedly turning on and off. A drive pulse generating section 32 for generating a driving pulse, an inverter transformer 33 in which a primary winding is connected to the driving pulse generating section 32 and a secondary winding is connected to the fluorescent lamp 2, and an oscillation circuit 38. The generated clock signal is used to control the on / off timing of the drive pulse generator 32, and the controller 34 performs feedback control of input power, which will be described later, and the DC power source for driving the drive unit 4 and the drive pulse generation A current detector 35 for detecting a change in drive current flowing between the inverter 32 and the secondary winding of the inverter transformer 33 and the fluorescent lamp 2. And a tube current detecting circuit 36 and the tube voltage detecting circuit 37.

  FIG. 2 shows specific circuit configurations of the inverter transformer 33, the tube current detection circuit 36 and the tube voltage detection circuit 37. Specific circuit configurations of the inverter transformer 33, the tube current detection circuit 36, and the tube voltage detection circuit 37 are the same as the configuration of the conventional example shown in FIG. 7, and the current / current generated in the secondary winding of the inverter transformer 33 is as follows. A tube current detection circuit 36 and a tube voltage detection circuit 37 for detecting the voltage are provided between the secondary winding of the inverter transformer 33 and the fluorescent lamp 2.

  Each tube current detection circuit 36 includes a diode, a resistor, and the like. As information about the tube current, a voltage between terminals of a resistor inserted in series between the secondary winding of the inverter transformer 33 and the fluorescent lamp 2 is used. Output. Each tube voltage detection circuit 37 is composed of a diode, a capacitor, and the like. As information about the tube voltage, the voltage generated in the secondary winding of the inverter transformer 33 is divided by the capacitor to reduce the voltage, and further rectified and smoothed. Output. 2 are connected to the terminals indicated by the same reference numerals in FIG. 3, respectively. The terminals indicated by reference signs A, B, c, d, e, and f in FIG.

  FIG. 3 shows specific circuit configurations of the timer circuit 31, the drive pulse generator 32, and the controller 34. The drive pulse generator 32 includes two sets of switching elements 32a and 32b that are turned on and off at different timings. Each switching element drive pulse generator 32 is connected to the driving DC power source Vin of the power source unit 4. Further, a resistor R1 for detecting a current flowing between them, that is, a current flowing in the primary winding of the inverter transformer 33, is provided between the driving DC power supply Vin of the power supply unit 4 and the switching elements 32a and 32b. Connected in series. Both terminals of the resistor R1 are respectively input to the OP amplifier 35a, and the output of the OP amplifier 35a, that is, the voltage between the terminals of the resistor R1, is input to a control IC 34a, which will be described later, as current information flowing through the primary winding of the inverter transformer 33. Input to terminal EA1. The resistor R1 and the OP amplifier 35a constitute a part of the current detector 35.

  The control unit 34 includes a general-purpose control IC 34a and comparison circuits 34b to 34d. In FIG. 3, OP amplifiers 34e and 34g depict two OP amplifiers provided in parallel as one OP amplifier for convenience, and the input voltage is compared with two threshold values. Details will be described later. The timer circuit 31 is a CR timing circuit composed of a capacitor, a resistor, and the like, and is connected to the control IC 34a and the comparison circuits 34b to 34d via a switching transistor or the like. Further, an oscillation circuit 38 composed of a switching transistor, a resistor, a capacitor and the like connected to the timer circuit 31 is connected to the input terminal OSC of the control IC 34a, and the oscillation frequency varies depending on whether the switching transistor is on or off. Can be switched. The OP amplifiers 34b to 34f are connected to a switching transistor provided between the control power supply Vc of the power supply unit 4 and the control Ic 34a.

  FIG. 4 is a timing chart showing a change in voltage of each part after the drive circuit 3 is started. When the drive circuit 3 is activated, the oscillation circuit 38 connected to the control IC 34a starts oscillating at a predetermined first oscillation frequency (see OSC), and at the same time, the timer circuit 31 starts measuring a predetermined activation timer time. (See TM). The control IC 34a turns on and off the switching elements 32a and 32b at the first oscillation frequency, and a drive pulse having a predetermined frequency is applied to the primary winding of the inverter transformer 33 (see DR1 / 3). Along with this, high voltage and high frequency AC power generated in the secondary winding of the inverter transformer 33 is applied between the terminals of the fluorescent lamp 2, and the fluorescent lamp 2 is turned on.

  FIG. 5 shows the relationship between the output from the tube current detection circuit 36 and a predetermined threshold value. When the predetermined start timer time is counted by the timer circuit 31, the switching transistor is turned on, and information (voltage value) corresponding to the output from the tube current detection circuit 36, that is, the current flowing through the secondary winding of the inverter transformer 33. ) Is compared with predetermined first current threshold value JPA and third current threshold value JPC by OP amplifier 34e and OP amplifier 34f. Here, when the output from the tube current detection circuit 36 is less than the first current threshold value JPA, it is considered that the fluorescent lamp 2 is not lit. Therefore, a signal is output from the OP amplifier 34e, and the power supply unit 4 is controlled. The switching transistor provided between the power source Vc and the control Ic 34a is turned off, the control power source Vc of the control Ic 34a is shut off, and the lighting of the fluorescent lamp 2 is stopped. Similarly, when the output from the tube current detection circuit 36 is equal to or greater than the third current threshold JPC, it is considered that the fluorescent lamp 2 is abnormal, so that the control power supply Vc of the control Ic 34a is cut off and the fluorescent lamp 2 stops lighting. Is done.

  FIG. 6 shows the relationship between the output from the tube voltage detection circuit 37 and a predetermined threshold value. When the predetermined start timer time is counted by the timer circuit 31, the output from the tube voltage detection circuit 37, that is, the voltage between the terminals of the fluorescent lamp 2, is generated by the OP amplifier 34g by the predetermined first voltage threshold value JPE and second voltage threshold value. Compared with JPF. Here, when the output from the tube voltage detection circuit 37 is less than the first voltage threshold value JPE, it is considered that the abnormality is caused by the consumption of the electrodes of the fluorescent lamp 2, and therefore a signal is output from the OP amplifier 34g. The switching transistor provided between the control power supply Vc and the control Ic 34a of the power supply unit 4 is turned off, the control power supply Vc of the control Ic 34a is cut off, and the lighting of the fluorescent lamp 2 is stopped. Further, when the output from the tube voltage detection circuit 37 is equal to or higher than the second voltage threshold value JPF, it is considered that the abnormality is caused by out-of-gassing of the fluorescent lamp 2, so the control power supply Vc of the control Ic 34a is cut off, and the fluorescent lamp The lighting of 2 is stopped.

  On the other hand, when the output from the tube current detection circuit 36 is not less than the first current threshold JPA and less than the third current threshold JPC, and the output from the tube voltage detection circuit 37 is not less than the first voltage threshold JPE and less than the second voltage threshold JPF, Since the fluorescent lamp 2 is normally lit, the switching transistor connected to the oscillation circuit 38 is turned on, and the oscillation circuit 38 starts oscillating at a predetermined second oscillation frequency (see OSC). Further, the control IC 34a performs feedback control so that the input power becomes constant based on the change in the drive current detected by the current detection unit 35. Specifically, when the amount of current flowing through the resistor R1 decreases (current value decreases), the voltage across the resistor R1 decreases, so that the control IC 34a increases the on-time of the switching elements 32a and 32b. Adjust the on / off timing. Thereby, the duty width of the drive pulse applied to the primary winding of the inverter transformer 33 is expanded. Conversely, when the amount of current flowing through the resistor R1 increases (current value increases), the voltage across the resistor R1 increases, so that the control IC 34a turns on / off so that the on-time of the switching elements 32a and 32b is shortened. Adjust the off timing. As a result, the duty width of the drive pulse applied to the primary winding of the inverter transformer 33 is reduced. As a result, feedback control is performed so that the input power on the primary winding side of the inverter transformer 33 is constant.

  As described above, according to the backlight device 1 for a liquid crystal panel according to the present embodiment, the current flowing between the driving DC power source Vin of the power source unit 4 and the driving pulse generator 32 (switching elements 32a and 32b). That is, the driving current flowing in the primary winding side of the inverter transformer 33 is detected by the resistor R1, and the control IC 34a of the control unit 34 performs feedback control using a signal related to the detected current, so that orthodox feedback by the driving current is performed. Since the input power is controlled to be constant by the control, the configuration of the drive circuit 3 is simple and the feedback control itself can be easily performed. Further, since a change in the drive current flowing through the primary winding of the inverter transformer 33 on the low voltage side is detected, the detected signal voltage is low, the withstand voltage is low, and the general-purpose control IC is inexpensive as the control Ic 34a. Can be used. As a result, the cost of the liquid crystal panel backlight device 1 can be reduced.

  As a specific example of the feedback control, for example, the timing at which the drive pulse generator is turned on and off is controlled, and the duty width of the drive pulse is controlled (PWM control), thereby facilitating the feedback control. Further, as the current detection unit 35, the resistor R1 is connected between the driving DC power source Vin of the power source unit 4 and the driving pulse generator 32, and the voltage across the resistor R1 is detected, whereby the driving circuit 3 and the current are detected. The configuration of the detection unit 35 can be simplified. Further, a tube current detection circuit 36 and a tube voltage detection circuit 37 are provided between the secondary winding of the inverter transformer 33 and the fluorescent lamp 2, and the tube current and the tube voltage detected thereby are respectively obtained by OP amplifiers 34e to 34g. Since the control power supply Vc for the control unit 34 is cut off when it can be determined that the fluorescent lamp 2 is abnormal as compared with a predetermined threshold value, the liquid crystal panel backlight device 1 that is safe and highly reliable can be provided. . At that time, since the high-voltage side tube current and tube voltage are not directly input to the control IC 34a, an inexpensive general-purpose control IC can be used as the control IC 34a.

1 is a block diagram showing a configuration of a drive circuit of a backlight device for a liquid crystal panel according to an embodiment of the present invention. FIG. 3 is a circuit diagram showing specific configurations of an inverter transformer, a tube current detection circuit, and a tube voltage detection circuit in the drive circuit of the backlight device for a liquid crystal panel. FIG. 3 is a circuit diagram showing a specific configuration of a timer circuit, a drive pulse generation unit, and a control unit in the drive circuit of the backlight device for a liquid crystal panel. The timing chart which shows the change of the voltage of each part after starting of the said drive circuit. The graph which shows the relationship between the output from the said tube current detection circuit, and a predetermined threshold value. The graph which shows the relationship between the output from the said tube voltage detection circuit, and a predetermined threshold value. The circuit diagram which shows the structure of the drive circuit of the backlight apparatus for conventional liquid crystal panels.

Explanation of symbols

1 Backlight device for liquid crystal panel 2 Cold cathode fluorescent lamp (fluorescent lamp)
DESCRIPTION OF SYMBOLS 3 Drive circuit 4 Power supply part 31 Timer circuit 32 Drive pulse generation part 32a, 32b Switching element 33 Inverter transformer 34 Control part 34a Control IC
34b to 34d Comparison circuit 34e to 34g OP amplifier 35 Current detection unit 36 Tube current detection circuit 37 Tube voltage detection circuit 38 Transmitter circuit R1 resistance JPA first current threshold value JPB second current threshold value JPC third current threshold value JPE first voltage threshold value JPF Second voltage threshold Vin Driving DC power supply Vc Control power supply

Claims (4)

In a backlight device for a liquid crystal panel comprising at least one cold cathode fluorescent lamp and a drive circuit for controlling lighting of the cold cathode fluorescent lamp,
The drive circuit is
A driving pulse generator that is connected to a driving DC power source and generates a driving pulse whose voltage periodically changes by repeatedly turning on and off;
An inverter transformer in which a primary winding is connected to the drive pulse generator, and a secondary winding is connected to the cold cathode fluorescent lamp;
A current detector that detects a change in drive current flowing between the driving DC power supply and the drive pulse generator;
A backlight device for a liquid crystal panel, further comprising a control unit that performs feedback control based on a change in drive current detected by the current detection unit so that input power is constant.   The liquid crystal panel back according to claim 1, wherein the control unit performs feedback control by controlling a timing at which the drive pulse generating unit is turned on and off, and controlling a duty width of the drive pulse. Light equipment.   The current detection unit includes a resistor connected between the driving DC power source and the driving pulse generation unit, and detects a voltage between terminals of the resistor. Backlight device for liquid crystal panel. A tube current detection circuit and a tube voltage detection circuit connected between the secondary winding of the inverter transformer and the cold cathode fluorescent lamp;
A comparison circuit for comparing the tube current and the tube voltage detected by the tube current detection circuit and the tube voltage detection circuit with a predetermined threshold value, respectively;
The drive circuit is out of a range where the value of the tube current is less than a first current threshold when a predetermined time has elapsed after the start of activation or the value of the tube voltage is greater than or equal to a first voltage threshold and less than a second voltage threshold. 4. The backlight device for a liquid crystal panel according to claim 1, wherein the control power supply to the control unit is cut off when the power is.

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