JP2012133937A - Led backlight device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an LED backlight device capable of making DC currents passing through a plurality of LED strings constant, and thus supplying identical currents even if each of the LED strings has different forward voltage.SOLUTION: An LED backlight device comprises: a converter which outputs an AC voltage; a plurality of transformers which have a plurality of primary winding disposed in parallel, and having one ends of the plurality of primary winding connected in parallel on the output stage of the inverter, and a plurality of secondary winding connected in series in a loop-like form, and which output AC currents; a plurality of rectification circuits which are connected to the other ends of each of the primary winding of the plurality of transformers, rectify the AC currents output by the plurality of transformers, and output the rectified currents; a plurality of smoothing circuits which are connected to the output stages of the plurality of rectification circuits respectively, and smooth the respective rectified currents to output DC currents; and a plurality of LED strings having one ends respectively connected with each of the output stages of the plurality of smoothing circuits, and each having a plurality of LEDs.

Description

  The present invention relates to an LED backlight device that lights a plurality of LEDs used as a light source of a liquid crystal display device.

  Conventionally, liquid crystal display devices (LCDs) have been required to have functions such as light weight, thinness, and low power consumption driving. Since a liquid crystal display device is not a self-luminous display device, a light source is required. Recently, LEDs have been used as light sources. For example, there is an LED backlight device 100 shown in FIG. The LED backlight device 100 includes a DC power supply 101, a DC converter 102, a bottom detection circuit 103, LED strings 104 to 107, and current drive circuits 108 to 111.

  The DC converter 102 converts a DC voltage supplied from the DC power supply 101 into a DC voltage necessary for lighting the LED strings 104 to 106 and supplies the converted DC voltage to the LED strings 104 to 106. As shown in FIG. 9B, the current driving circuit 108 includes an error amplifier 108a, an FET 108b, and a resistor R. Similarly, the current driver circuits 109 to 111 have the circuit configuration shown in FIG. The current driving circuits 108 to 111 detect the current flowing from the LED strings 104 to 107 to the source electrode of the FET 108b with the resistor R, compare the detected voltage with the reference voltage REF with the error amplifier 108a, and supply the detected voltage to the gate electrode of the FET 108b. To control the voltage. By controlling the gate electrode, the current flowing through the resistor R is controlled to be constant, and the current flowing through the drain electrode of the FET 108b is controlled to be constant, whereby the current flowing through the LED strings 104 to 107 is controlled to be constant.

  The bottom detection circuit 103 detects the minimum value among the lowermost cathode voltages of the LED strings 104 to 107 and outputs the minimum value detection signal to the DC converter 102. Each LED that constitutes the LED strings 104 to 107 has different forward voltages due to individual differences. For this reason, even if the currents flowing through the LED strings 104 to 107 are controlled to be constant by the current driving circuits 108 to 111, the forward voltages of the LED strings 104 to 107 are different. For this reason, it is necessary for the DC converter 102 to supply a direct-current voltage that has a maximum forward voltage among the LED strings 104 to 107. The variation in the forward voltage of the LED also occurs due to a difference in ambient temperature between the LED strings 104 to 107. Therefore, the bottom detection circuit 103 detects the minimum value among the lowermost cathode voltages of the LED strings 104 to 107, and outputs the minimum value detection signal to the DC converter 102, thereby supplying the direct current supplied to the LED strings 104 to 107. The voltage is adjusted to the minimum necessary value. That is, the DC converter 102 adjusts the DC voltage level supplied to the LED strings 104 to 107 so that the voltage level of the minimum value detection signal input from the bottom detection circuit 103 is constant.

  Moreover, in the light emitting diode lighting device described in Patent Document 1, current is alternately output to a series LED group from a plurality of booster circuits connected in parallel, and the series LED group is grounded via a resistor and generated in the resistor. Feedback control is performed so as to keep the voltage to be constant.

  Moreover, in the LED drive circuit described in Patent Document 2, a shunt coil in which one end portions of two coils are connected to each other via a tap, and a backflow prevention diode are respectively connected to one end portion and the other end portion of the shunt coil. In this way, series LED groups are connected to suppress the occurrence of temperature rise and life difference due to uneven light quantity in each LED circuit and differences in current values.

Table 2007/0769371 JP 2006-319221 A

  However, in the LED backlight device 100 described above, the current drive circuits 108 to 111 are constant current circuits that control the current flowing through the drain electrode of the FET 108b to be constant. In the LED backlight device 100, the loss is determined by the product of the output voltage of the constant current circuit and the constant current value. When the output voltage of the constant current circuit increases, power loss increases and heat is generated. When the variation in the forward voltage of the LED strings 104 to 107 is large, the DC voltage supplied from the DC converter 102 becomes excessive, the output voltage of the current drive circuits 108 to 111 becomes high, and there is a problem that heat is generated. Such a problem cannot be solved even if each of the devices disclosed in Patent Documents 1 and 2 is used.

  In addition, the LED backlight device 100 described above requires the bottom detection circuit 103 as a feedback circuit, and each of the devices disclosed in Patent Documents 1 to 3 also requires a feedback circuit that detects the terminal voltage and current of the LED group. There is also a problem that the control system becomes complicated and parameter adjustment in the circuit becomes complicated.

  An object of the present invention is to provide an LED backlight device in which direct currents flowing through a plurality of LED strings are made substantially the same, and the same current is supplied even if the forward voltages of the LED strings are different.

  An LED backlight device according to an embodiment of the present invention includes a converter in which an input stage is connected to a DC power source and outputs an AC voltage, and a plurality of primary windings arranged in parallel, and the plurality of primary windings One end of each wire is connected in parallel to the output stage of the inverter, a plurality of secondary windings are connected in series in a loop, and a plurality of transformers that output alternating current, and a primary side of the plurality of transformers A plurality of rectifier circuits each connected to each other end of the winding and rectifying an alternating current output from each of the plurality of transformers, and connected to output stages of the plurality of rectifier circuits; A plurality of smoothing circuits each for smoothing the rectified current and outputting a direct current; and a plurality of LED strings each having one end connected to each output stage of the plurality of smoothing circuits and each having a plurality of LEDs. Specially To. According to this LED backlight device, the currents flowing through the plurality of LED strings can be made substantially the same, and the currents can be equalized even if the forward voltages of the plurality of LED strings are different.

  Further, the other end portions of the plurality of LED strings may be connected to a common ground portion. According to this LED backlight device, the configuration related to the ground wiring can be simplified.

  Further, the plurality of transformers adjust inductances of the primary side winding and the secondary side winding so that the alternating currents flowing through the primary side winding and the secondary side winding are the same. Also good. According to this LED backlight device, the currents flowing through the plurality of LED strings can be made substantially the same, and the currents can be equalized even if the forward voltages of the plurality of LED strings are different.

  According to the present invention, the direct currents flowing through the plurality of LED strings can be made substantially the same without using a feedback circuit, and the current can be made uniform even if the forward voltages of the plurality of LED strings are different. it can.

It is a figure which shows the circuit structure of the LED backlight apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows the structural example of the simulation circuit for simulating operation | movement of the LED backlight apparatus of FIG. It is a figure which shows the signal of each part of the simulation circuit of FIG. It is a figure which shows the circuit structure of the LED backlight apparatus which concerns on Embodiment 2 of this invention. It is a figure which shows the structural example of the simulation circuit for simulating operation | movement of the LED backlight apparatus of FIG. It is a figure which shows the signal of each part of the simulation circuit of FIG. It is a block diagram which shows the principal part structure of the liquid crystal display device which concerns on Embodiment 3 of this invention. It is a disassembled perspective view which shows the structure of the liquid crystal display device which concerns on Embodiment 3 of this invention. (A) is a figure which shows the circuit structure of the conventional LED backlight apparatus, (B) is a figure which shows the circuit structure of the current drive circuit of (A).

(Embodiment 1)
Hereinafter, the LED backlight device according to Embodiment 1 of the present invention will be described in detail with reference to the drawings.

<Circuit configuration>
FIG. 1 is a diagram illustrating a circuit configuration of an LED backlight device 200 according to the first embodiment. In FIG. 1, an LED backlight device 200 includes a DC power supply 201, a boost converter 202, transformers 203 to 206, LED strings 207 to 211, rectifying diodes D1 to D4, and capacitors C1 to C4. . Note that the LED backlight device 200 has a circuit configuration for driving the four LED strings 207 to 210, but the number of LED strings may be more or less than four. That is, since the number of LED strings is appropriately changed according to the size of the liquid crystal display device, the number of transformers, rectifier circuits, and capacitors in the LED backlight device 200 is appropriately changed.

  The boost converter 202 has a DC power supply 201 connected to the input stage, and one end of each primary winding of the transformers 203 to 206 connected in parallel to the output stage. The boost converter 202 converts a DC voltage supplied from the DC power supply 201 into an AC voltage having a predetermined frequency and a predetermined amplitude, and supplies the AC voltage to the transformers 203 to 206.

  In each of the transformers 203 to 206, the other end of the primary side winding is connected to each anode electrode of the rectifying diodes D1 to D4, and the secondary side windings are connected in series in a loop. The transformers 203 to 206 have the same turn ratio between the primary side winding and the secondary side winding. Since the secondary side windings of the transformers 203 to 206 are connected in series in a loop shape, the alternating current flowing through each primary side winding is the same. Therefore, the AC current flowing through the primary windings of transformers 203 to 206 that are AC driven by boost converter 202 is also controlled in the same manner. The transformers 203 to 206 output the same alternating current to the rectifying diodes D1 to D4, respectively. The turns ratio of the transformers 203 to 206 is set in consideration of the DC voltage value for lighting the LED strings 207 to 210.

  In the rectifying diodes (rectifier circuits) D1 to D4, each anode electrode is connected to the other end of the primary winding of each of the transformers 203 to 206, and each cathode electrode is connected to the capacitors C1 to C4. The rectifying diodes D1 to D4 rectify alternating currents input from the transformers 203 to 206 into positive currents.

  Capacitors (smoothing circuits) C1 to C4 smooth the rectified positive current and supply it to the LED strings 207 to 210 as a DC positive current.

  The LED strings 207 to 210 are each configured by connecting 12 LEDs in series, and each anode electrode is connected to a connection node between the cathode electrode of the rectifying diodes D1 to D4 and the capacitors C1 to C4. The cathode electrodes of the LED strings 207 to 210 are connected to a common ground portion. The LED strings 207 to 210 each have a configuration in which twelve LEDs are connected in series, but the number of LEDs connected in series may be more or less than twelve. That is, the number of LEDs connected in series is appropriately changed according to the size of the liquid crystal display device.

<Circuit operation>
First, in the boost converter 202, the DC voltage VIN supplied from the DC power source 201 is converted into an AC voltage having a predetermined frequency and a predetermined amplitude and supplied to one end of each primary winding of the transformers 203 to 206. Each primary side winding of the transformers 203 to 206 is supplied and driven by an alternating voltage, and the same alternating current flows through each primary side winding, and each secondary side connected in series in a loop shape of the transformers 203 to 206 The same alternating current flows through the winding.

  In the transformers 203 to 206, the same alternating current is output to the rectifying diodes D1 to D4 connected to the other end of each primary winding. In the rectifying diodes D1 to D4, the alternating currents input from the transformers 203 to 206 are rectified into positive currents. The capacitors C1 to C4 smooth the positive current rectified by the rectifying diodes D1 to D4 and supply the smoothed positive current to the LED strings 207 to 210. Therefore, the same DC positive current is supplied to the LED strings 207 to 210, and the LED strings 207 to 210 are turned on.

  As described above, in the LED backlight device 200 according to Embodiment 1 of the present invention, a constant current circuit is used without using the current driving circuits 108 to 111 as in the conventional LED backlight device 100 shown in FIG. Therefore, the circuit configuration can be simplified, and substantially the same current can be supplied even if the forward voltages of the LED strings 207 to 210 are different. Further, in the LED backlight device 200 according to Embodiment 1 of the present invention, the bottom detection circuit 103 is not required unlike the conventional LED backlight device 100 illustrated in FIG. 9, and the alternating current output from the boost converter 202 is not necessary. The DC current flowing through each of the LED strings 207 to 210 can be controlled to be constant only by stabilizing the. Furthermore, in the LED backlight device 200 according to the first embodiment of the present invention, the constant current outputs from the transformers 203 to 206, the rectifying diodes D1 to D4, and the capacitors C1 to C4 are connected to the respective anode electrodes of the LED strings 207 to 210. Therefore, each cathode electrode of the LED strings 207 to 210 can be grounded. For this reason, the cathode electrodes of the LED strings 207 to 210 can be easily connected to, for example, the casing of the liquid crystal display device as a common ground, and there is no need to route the ground wiring. In the conventional LED backlight device 100 shown in FIG. 9, each circuit unit must be individually connected from the ground terminal to the ground terminal via a cable. However, by using the LED backlight device 200 according to Embodiment 1 of the present invention for a liquid crystal display device, it is possible to simplify the configuration related to the ground wiring in the liquid crystal display device. In the LED backlight device 200 according to the first embodiment of the present invention, the LED strings 207 to 210 are connected to a constant current circuit including the transformers 203 to 206, the rectifying diodes D1 to D4, and the capacitors C1 to C4. Therefore, even if the impedances on the ground side of the LED strings 207 to 210 are different, a constant current can be passed, and impedance adjustment is not necessary. Furthermore, in the LED backlight device 200 according to Embodiment 1 of the present invention, the inductances of the primary side windings and the secondary side windings of the transformers 203 to 206 are adjusted so that the same alternating current flows, whereby the LED strings. Even if the forward voltages of 207 to 210 are different, substantially the same current can be supplied.

  Next, a circuit configuration for simulating the operation of the LED backlight device 200 shown in FIG. 1 will be described with reference to FIG. 2 replaces the DC power supply 201 of the LED backlight device 200 shown in FIG. 1 with a DC power supply 301, replaces the boost converter 202 with a circuit 302 composed of the DC power supply V2 and the switching element S, and replaces the LED strings 207 to 210 with resistors. It is a figure which shows the structure of the simulation circuit 300 at the time of replacing with load R1-R4.

<Circuit configuration>
In FIG. 2, a simulation circuit 300 includes a circuit 302 as a boost converter, transformers 303 to 306, rectifying diodes D1 to D4, resistance loads R1 to R4 as LED strings, and capacitors C1 to C6. .

  The circuit 302 supplies an AC voltage having a predetermined frequency and a predetermined amplitude to one end of each primary side winding of the transformers 303 to 306. The transformers 303 to 306 have the same turn ratio as the transformers 203 to 206 shown in FIG. 1, and output the same alternating current to the rectifying diodes D1 to D4 by the supplied alternating voltage. The rectifying diodes D1 to D4 rectify the alternating currents input from the transformers 303 to 306 into positive currents. Capacitors C1 to C6 each smooth the rectified positive current and supply it to the resistance loads R1 to R4 as a DC positive current.

<Circuit operation>
Next, the operation of the simulation circuit 300 will be described with reference to FIG. 3A shows currents −i (R1) to −i (R4) flowing through resistance loads R1 to R4 as LED strings, and FIG. 3B shows output voltages v (O1) of the capacitors C1 to C4. ˜v (O4), and (C) shows currents i (L5) and i (L6) flowing through the coils L5 and L6 of FIG.

  In the simulation operation of the LED backlight device 300, the forward voltage of the LED strings is set to the maximum, minimum, and standard in consideration of variations in the forward voltage due to variations in the LED strings and temperature changes. Further, the boost converter is configured as a circuit 302 as an ideal sine wave voltage generator, and an AC voltage is supplied from the circuit 302 to each primary winding of the transformers 303 to 306.

  3A to 3C, the horizontal axis indicates the measurement time, and the change of each signal in the period of 200 μs to 400 μs.

  In FIG. 3, a circuit 302 as a boost converter generates, for example, a sinusoidal AC voltage having an amplitude of 24V. Currents i (L5) and i (L6) flowing through the coils L5 and L6 by this sine wave AC voltage are as shown in FIG. Then, the transformers 303 to 306 are AC driven by this sine wave AC voltage, and the same AC current is applied to the rectifying diodes D1 to D4 to be rectified into positive currents, smoothed by the capacitors C1 to C4, and DC positive. A current is applied to resistance loads R1 to R4 as LED strings. At this time, the output voltages v (O1) to v (O4) of the capacitors C1 to C4 are as shown in FIG. In addition, as shown in FIG. 3A, each current -i (R1) to -i (R4) flowing through the resistance loads R1 to R4 converged to 130 mA.

  As described above, in the simulation circuit 300 for simulating the operation of the LED backlight device 200 shown in FIG. 1, the resistive loads R1 to R4 as the LED strings have different power in order to simulate variations in the forward voltage. When the loss was set, it was confirmed that the currents flowing through the resistive loads R1 to R4 were almost the same.

  Therefore, according to the simulation operation of the LED backlight device 200 by the simulation circuit 300, the same alternating current is output from the transformers 303 to 306 driven by the circuit 302 to the rectifier diodes D1 to D4. It was confirmed that the currents -i (R1) to -i (R4) flowing through the resistance loads R1 to R4 are constant.

(Embodiment 2)
Next, an LED backlight device according to Embodiment 2 of the present invention will be described with reference to FIG.

<Circuit configuration>
FIG. 4 is a diagram illustrating a circuit configuration of the LED backlight device according to the second embodiment. 4, the LED backlight device 400 includes a DC power supply 401, a boost converter 402, transformers 403 and 404, rectifying diodes D1 and D2, LED strings 405 and 406, and capacitors C1 and C2. . In addition, although the circuit structure for driving the two LED strings 405 and 406 is shown in the LED backlight device 400, the number of LED strings may be more or less than two. That is, since the number of LED strings is appropriately changed according to the size of the liquid crystal display device, the number of transformers, rectifying diodes, and capacitors in the LED backlight device 400 is appropriately changed.

  In the boost converter 402, a DC power supply 401 is connected to the input stage, and one end of each primary winding of the transformers 403 and 404 is connected in parallel to the output stage. The boost converter 402 converts the DC voltage supplied from the DC power supply 401 into an AC voltage having a predetermined frequency and a predetermined amplitude, and supplies the AC voltage to the transformers 403 and 404.

  In the transformers 403 and 404, the other end of the primary side winding is connected to each anode electrode of the rectifying diodes D1 and D2, and the secondary side windings are connected in series in a loop shape. In the transformers 403 and 404, the turns ratio of the primary side winding and the secondary side winding is the same. Since the secondary side windings of the transformers 403 and 404 are connected in series in a loop shape, the alternating current flowing in each primary side winding is the same. Therefore, the AC current flowing through the primary windings of transformers 403 and 404 that are AC driven by boost converter 402 is also controlled in the same manner. The transformers 403 and 404 output the same alternating current to the rectifying diodes D1 and D2, respectively. The turns ratio of the transformers 403 and 404 is set in consideration of a DC voltage value for lighting the LED strings 405 and 406.

  In the rectifying diodes (rectifier circuits) D1 and D2, each anode electrode is connected to the other end of the primary winding of each of the transformers 403 and 404, and each cathode electrode is connected to the capacitors C1 and C2. The rectifying diodes D1 and D2 rectify the alternating current input from the transformers 403 and 404 to a positive current.

  Capacitors (smoothing circuits) C1 and C2 smooth the rectified positive current and supply it to the LED strings 405 and 406 as a DC positive current.

  Each of the LED strings 405 and 406 is configured by connecting 12 LEDs in series, and each anode electrode is connected to a connection node between the cathode electrodes of the rectifying diodes D1 and D2 and the capacitors C1 and C2. The cathode electrodes of the LED strings 405 and 406 are connected to a common ground portion. The LED strings 405 and 406 each show a configuration in which 12 LEDs are connected in series, but the number of LEDs connected in series may be more than 12 or less. That is, the number of LEDs connected in series is appropriately changed according to the size of the liquid crystal display device.

<Circuit operation>
First, in the boost converter 402, the DC voltage VIN supplied from the DC power supply 401 is converted into an AC voltage having a predetermined frequency and a predetermined amplitude and supplied to one end of each primary winding of the transformers 403 and 404. Each primary side winding of the transformers 403 and 404 is supplied and driven by an AC voltage, the same AC current flows through each primary side winding, and each secondary side connected in series in a loop shape of the transformers 403 and 404 The same alternating current flows through the winding.

  In the transformers 403 and 404, the same alternating current is output to the rectifying diodes D1 and D2 connected to the other end of each primary winding. In the rectifying diodes D1 and D2, the alternating currents input from the transformers 403 and 404 are rectified into positive currents. The capacitors C1 and C2 smooth the positive current rectified by the rectifying diodes D1 and D2 and supply the smoothed positive current to the LED strings 405 and 406. Therefore, substantially the same DC positive current is supplied to the LED strings 405 and 406, and the LED strings 405 and 406 are turned on.

  As described above, in the LED backlight device 400 according to Embodiment 2 of the present invention, a constant current circuit is used without using the current drive circuits 107 to 109 as in the conventional LED backlight device 100 shown in FIG. As a result, the circuit configuration can be simplified, and substantially the same current can be supplied even if the forward voltages of the LED strings 405 and 406 are different. Further, in the LED backlight device 400 according to the second embodiment of the present invention, the bottom detection circuit 103 is unnecessary as in the conventional LED backlight device 100 shown in FIG. The DC currents flowing through the LED strings 405 and 406 can be controlled almost the same by simply stabilizing the. Further, in the LED backlight device 400 according to the second embodiment of the present invention, the constant current outputs from the transformers 403 and 404, the rectifying diodes D1 and D2, and the capacitors C1 and C2 are connected to the anode electrodes of the LED strings 405 and 406, respectively. Therefore, the cathode electrodes of the LED strings 405 and 406 can be grounded. Therefore, it becomes easy to connect the cathode electrodes of the LED strings 405 and 406 as a common ground, for example, to the casing of the liquid crystal display device, and there is no need to route the ground wiring. In the conventional LED backlight device 100 shown in FIG. 9, each circuit unit must be individually connected from the ground terminal to the ground terminal via a cable. However, by using the LED backlight device 400 according to Embodiment 2 of the present invention for a liquid crystal display device, it is possible to simplify the configuration related to the ground wiring in the liquid crystal display device. In the LED backlight device 400 according to the second embodiment of the present invention, the LED strings 405 and 406 are connected to a constant current circuit including transformers 403 and 404, rectifying diodes D1 and D2, and capacitors C1 and C2. Therefore, even if the impedances on the ground side of the LED strings 405 and 406 are different, a constant current can be passed, and impedance adjustment becomes unnecessary. Furthermore, in the LED backlight device 400 according to Embodiment 2 of the present invention, the LED strings are adjusted by adjusting the inductances of the primary and secondary windings of the transformers 403 and 404 so that the same alternating current flows. Even if the forward voltages of 405 and 4060 are different, substantially the same current can be supplied.

  Next, a circuit configuration for simulating the operation of the LED backlight device 400 shown in FIG. 4 will be described with reference to FIG. 5 replaces the DC power supply 401 of the LED backlight device 400 shown in FIG. 4 with a DC power supply 501, replaces the boost converter 402 with a circuit 502 composed of the DC power supply V2 and the switching element S, and replaces the LED strings 405 and 406 with resistors. It is a figure which shows the structure of the simulation circuit 500 at the time of replacing with load R1 and R2.

<Circuit configuration>
In FIG. 5, a simulation circuit 500 includes a circuit 502 as a boost converter, transformers 503 and 504, rectifying diodes D1 and D2, resistive loads R1 and R2 as LED strings, and capacitors C1 and C2. .

  The circuit 502 supplies an alternating voltage having a predetermined frequency and a predetermined amplitude to one end of each primary winding of the transformers 503 and 504. The transformers 503 and 504 have the same turn ratio as the transformers 403 and 404 shown in FIG. 4, and output the same alternating current to the rectifying diodes D1 and D2 by the supplied alternating voltage, respectively. The rectifying diodes D1 and D2 rectify the alternating current input from the transformers 503 and 504, respectively, into a positive current. Capacitors C1 and C2 respectively smooth the rectified positive current and supply it to the resistance loads R1 and R2 as a DC positive current.

<Circuit operation>
Next, the operation of the simulation circuit 400 will be described with reference to FIG. 6A shows currents −i (R1) and −i (R2) flowing through resistance loads R1 and R2 as LED strings, and FIG. 6B shows output voltages v (O1) of the capacitors C1 and C2. And v (O2), and (C) shows currents i (L5) and i (L6) flowing through the coils L5 and L6 of FIG.

  In the simulation operation of the LED backlight device 500, the forward voltage of the LED strings is set to the maximum, minimum, and standard in consideration of variations in the forward voltage due to variations in the LED strings and temperature changes. The boost converter is configured as a circuit 502 as an ideal sine wave voltage generator, and an AC voltage is supplied from the circuit 502 to the primary windings of the transformers 503 and 504.

  6A to 6C, the horizontal axis represents the measurement time, and shows the change of each signal during the period of 400 μs to 600 μs.

  In FIG. 5, a circuit 502 as a boost converter generates, for example, a sinusoidal AC voltage having an amplitude of 24V. Currents i (L5) and i (L6) flowing through the coils L5 and L6 by this sine wave AC voltage are as shown in FIG. 6 (C). Then, the transformers 503 and 504 are AC driven by the sine wave AC voltage, and the same AC current is applied to the rectifying diodes D1 and D2 to be rectified into positive currents, smoothed by the capacitors C1 and C2, and DC positive Current is applied to resistive loads R1 and R2 as LED strings. At this time, the output voltages v (O1) and v (O2) of the capacitors C1 and C2 are as shown in FIG. 6B. In addition, as shown in FIG. 6 (A), each current −i (R1) and −i (R2) flowing through the resistance loads R1 and R2 converged to 127 mA.

  As described above, in the simulation circuit 500 for simulating the operation of the LED backlight device 400 shown in FIG. 4, the resistive loads R1 and R2 as the LED strings have different power in order to simulate the variation in the forward voltage. When the loss was set, it was confirmed that the currents flowing through the resistive loads R1 and R2 were almost the same.

  Therefore, according to the simulation operation of the LED backlight device 400 by the simulation circuit 500, the same alternating current is output from the transformers 503 and 504 driven by the circuit 502 to the rectifier diodes D1 and D2, so that the LED strings It was confirmed that the currents -i (R1) and -i (R2) flowing through the resistance loads R1 and R2 are constant.

(Embodiment 3)
Next, a liquid crystal display device including the LED backlight device 200 shown in FIG. 1 will be described with reference to the block diagram shown in FIG. As shown in FIG. 7, the liquid crystal display device 900 includes an AC / DC power supply device 910, an LCD module unit 920, and a backlight device 930.

  The AC / DC power supply device 910 includes an outlet 911, an AC / DC rectifying unit 912, and a DC / DC converter 913. The AC / DC power supply device 910 converts an external commercial AC power supply voltage 100V or 240V into a DC power supply voltage and supplies it to the LCD module unit 920. Output.

  The LCD module unit 920 includes a DC / DC converter 921, a common electrode voltage generation unit (Vcom generation unit) 922, a γ voltage generation unit 923, an LCD panel unit 924, and a backlight device 930. An external graphic controller (see FIG. An image corresponding to the image data input from (not shown) is displayed. In the LCD panel unit 924, a plurality of liquid crystal elements are connected to portions where a plurality of data lines and a plurality of gate lines extend from the gate driver unit and the data driver unit, respectively. The plurality of liquid crystal elements are divided for each display image area, and the gradation is controlled for each display image area.

  The common electrode voltage generation unit 922 generates a common electrode voltage Vcom based on the DC voltage that is level-converted by the DC / DC converter 921 and supplies the generated voltage to the LCD panel unit 924.

  The γ voltage generation unit 923 generates a γ voltage Vdd based on the DC voltage level-converted by the DC / DC converter 921 and supplies it to the LCD panel unit 924. Although FIG. 7 shows an example in which the common electrode voltage generation unit 922 and the γ voltage generation unit 923 are separated from the LCD panel unit 924, they may be configured to be included in the LCD panel unit 924.

  The backlight device 930 includes a backlight driving unit 931 and a backlight unit 932. The backlight drive unit 931 includes the inverter 202, transformers 203 to 205, rectifier circuits 206 to 208, and capacitors C1 to C6 shown in FIG. The backlight unit 932 includes the plurality of LED strings 209 to 214 shown in FIG. When the backlight unit 932 displays an input image on the LCD panel unit 924, the lighting unit is controlled to control the gradation in accordance with the luminance of the input image. Further, the backlight drive unit 931 and the backlight unit 932 are connected to a common ground GND. The ground GND may be connected to a housing such as the storage container 1081 shown in FIG.

  Since the liquid crystal display device 900 includes the above-described inverter 202, transformers 203 to 205, rectifier circuits 206 to 208, and capacitors C1 to C6 in the backlight driving unit 931 in the backlight device 930, the backlight device 930 consumes power. Electric power can be reduced. Note that the AC / DC power supply device 910 may be built in the LCD module unit 920.

  FIG. 8 is an exploded perspective view showing the structure of the liquid crystal display device according to the fifth embodiment. FIG. 8 illustrates the mechanism, not the circuit configuration of the liquid crystal display device. As shown in FIG. 8, the liquid crystal display device 1000 includes a backlight assembly 1010, a display unit 1070, and a storage container 1080.

  The display unit 1070 includes a liquid crystal display panel 1071 that displays an image, a data printing circuit 1072 that outputs a driving signal for driving the liquid crystal display panel 1071, and a gate printing circuit 1073. The data printing circuit 1072 and the gate printing circuit 1073 are electrically connected to the liquid crystal display panel 1071 through a data tape carrier package (hereinafter referred to as TCP) 1074 and a gate TCP 1075, respectively.

  The liquid crystal display panel 1071 includes a thin film transistor (hereinafter referred to as TFT) substrate 1076, a color filter substrate 1077 coupled to face the TFT substrate 1076, and liquid crystal 1078 interposed between the substrates 1076 and 1077.

  The TFT substrate 1076 is, for example, a transparent glass substrate on which TFTs (not shown) as switching elements are formed in a matrix. Data and gate lines are connected to the source and gate terminals of the TFT, respectively, and a common electrode (not shown) made of a transparent conductive material is formed at the drain terminal.

  The color filter substrate 1077 is, for example, a substrate on which RGB pixels (not shown) as color pixels are formed by a thin film process. The color filter substrate 1077 is formed with a common electrode (not shown) made of a transparent conductive material.

  The storage container 1080 includes a bottom surface 1081 and a side wall 1082 formed to form a storage space at the edge portion of the bottom surface 1081. The container 1080 is fixed so that the backlight assembly 1010 and the liquid crystal display panel 1071 do not move.

  The bottom surface 1081 preferably has a bottom surface area sufficient for mounting the backlight assembly 1010 (backlight portion 932 in FIG. 7) and has the same configuration as the backlight assembly 1010. In this example, the bottom surface 1081 and the backlight assembly 1010 have a square plate shape. The side wall 1082 extends substantially vertically from the edge of the bottom surface 1081 so that the backlight assembly 1010 does not leave the outside.

  The liquid crystal display device 1000 in this example further includes a backlight driving unit 1060 and a top chassis 1090.

  The backlight driving unit 1060 is disposed inside the receiving container 1080 and generates a DC voltage for driving the backlight assembly 1010. The DC voltage generated from the backlight driving unit 1060 is applied to the backlight assembly 1010 through the first power supply line 1063 and the second power supply line 1064. The first power supply line 1063 and the second power supply line 1064 may be directly connected to the first electrode 1040a and the second electrode 1040b formed on both sides of the backlight assembly 1010, or other members (not shown). May be connected to the first electrode 1040a and the second electrode 1040b. Further, the backlight assembly 1010 is grounded to the receiving container 1080 by a ground wiring 1041. The backlight driving unit 1060 is grounded to the receiving container 1080 similarly to the backlight assembly 1010 by being stored in the receiving container 1080. That is, the backlight assembly 1010 and the backlight driving unit 1060 are grounded to the common container 1080.

  The top chassis 1090 is coupled to the receiving container 1080 while surrounding the edge portion of the liquid crystal display panel 1071. By providing the top chassis 1090, the liquid crystal display panel 1071 can be prevented from being damaged by an external impact, and the liquid crystal display panel 1071 can be prevented from being detached from the storage container 1080.

  The liquid crystal display device 1000 may further include at least one optical sheet 1095 for improving characteristics of light emitted from the backlight assembly 1010. The optical sheet 1095 may include a diffusion sheet for diffusing light or a prism sheet for collecting light.

  Accordingly, in the liquid crystal display device 1000 including the LED backlight device 200, the backlight driving unit 1060 includes the above-described inverter 202, transformers 203 to 205, rectifier circuits 206 to 208, and capacitors C1 to C6. It becomes possible to achieve power saving of the apparatus. Further, the LED backlight device 300 of the second embodiment may be applied to the liquid crystal display device 1000.

  In addition, it is preferable to apply the LED backlight apparatus shown in the said Embodiment 1-Embodiment 2 to a comparatively large liquid crystal panel instead of a small size or medium size liquid crystal panel.

  200, 400 ... LED backlight device, 202, 402 ... boost converter, 203-206, 403, 404 ... transformer, 207-210, 405, 406 ... LED strings, 300, 500 ... simulation circuit, C1-C4 ... capacitor, D1 to D4: rectifying diode, 900: liquid crystal display device.

Claims (4)

A converter whose input stage is connected to a DC power source and outputs an AC voltage;
A plurality of primary windings are arranged in parallel, one end of each of the plurality of primary windings is connected in parallel to the output stage of the inverter, and a plurality of secondary windings are connected in series in a loop. A plurality of transformers that output alternating current;
A plurality of rectifier circuits connected to the other ends of the primary windings of the plurality of transformers, respectively, for rectifying alternating currents output from the transformers and outputting the rectified currents;
A plurality of smoothing circuits respectively connected to output stages of the plurality of rectifier circuits, each of which smoothes the rectified current and outputs a direct current;
An LED backlight device comprising: a plurality of LED strings each having one end connected to each output stage of the plurality of smoothing circuits and each having a plurality of LEDs.   The LED backlight device according to claim 1, wherein the other end portions of the plurality of LED strings are connected to a common ground portion.   The plurality of transformers adjust respective inductances of the primary side winding and the secondary side winding so that alternating currents flowing through the primary side winding and the secondary side winding are the same. The LED backlight device according to claim 1.   A liquid crystal display device comprising the LED backlight device according to claim 1.