JP2012238447A - Backlight drive circuit and display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a backlight drive circuit and a display device that reconcile overvoltage protection and undervoltage protection.SOLUTION: The backlight drive circuit comprises: a transformer comprising a first transformer and a second transformer for supplying an AC voltage to a backlight; an inverter circuit for generating AC voltages opposite in phase in the first transformer and the second transformer, respectively; a clamping circuit connected to secondary coils of the first transformer and second transformer; an addition circuit connected to the secondary coils of the respective transformers to combine voltages generated in the respective secondary coils; a smoothing circuit for smoothing the combined voltage to acquire a voltage; a comparison circuit for comparing the smoothed voltage with a predetermined threshold voltage, and if the voltage is equal to or lower than the threshold voltage, outputting an error signal; and a control circuit for stopping driving the inverter circuit if the error signal is output.

Description

  The present invention relates to a backlight driving circuit for driving a backlight, and more particularly to a backlight driving circuit including a circuit for detecting an abnormality and a display device including the backlight driving circuit.

  Conventionally, a liquid crystal display device uses a backlight as a light source for displaying an image. For example, a cold cathode tube or an LED (Light Emitting Diode) is used as the backlight. When an AC drive type cold cathode tube is used as a backlight, a drive voltage for driving the backlight is generated using an inverter circuit that converts a DC voltage into an AC voltage.

  The voltage generated by the inverter circuit is a high voltage and includes various abnormality detection circuits from the viewpoint of safety. For example, as an example of the abnormality detection circuit, there are an overvoltage protection circuit that detects an overvoltage inside the inverter circuit, a voltage drop protection circuit that monitors a waveform abnormality of a voltage generated in a transformer, and the like (see, for example, Patent Documents 1-4). ).

  The voltage drop protection circuit detects this state as abnormal when the voltage generated in the transformer is lower than that of other transformers. Therefore, the voltages generated in the plurality of transformers are added by the adder circuit, and the voltages are compared. Here, when the voltage generated in the predetermined transformer is in the reduced voltage state, the voltage added by the adder circuit is also lowered. Therefore, the reduced voltage is detected by comparing the voltage at this time with the predetermined threshold voltage. Can do.

JP 2009-004194 A JP 2007-294368 A JP 2006-155943 A JP 2005-005059 A

  By the way, when an overvoltage protection circuit is incorporated in an inverter circuit, a clamp circuit may be connected to the transformer to facilitate detection of a voltage change. For example, by connecting the clamp circuit to the secondary coil of the transformer, the lower limit value of the detection voltage can be shifted upward, and overvoltage detection can be facilitated. On the other hand, in the conventional voltage reducing circuit, the voltage generated in a plurality of transformers is added using an adder circuit. Therefore, when a clamp circuit is connected to the transformer, the voltage added by the adder circuit is also a value of 0 V or more. Therefore, there is a case where the reduced voltage cannot be detected.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a backlight driving circuit and a display device capable of achieving both overvoltage protection and undervoltage protection.

  In order to solve the above problems, in the present invention, a backlight driving circuit for driving a backlight using a cold cathode tube as a light source includes a first transformer and a second transformer for supplying an alternating voltage to the backlight. A transformer, an inverter circuit for generating AC voltages of opposite phases in the first transformer and the second transformer, a clamp circuit connected to a secondary coil of the first transformer and the second transformer, An addition circuit that is connected to the secondary coil of each transformer and synthesizes the voltage generated in each secondary coil, a smoothing circuit that obtains a voltage obtained by smoothing the synthesized voltage, and the smoothed voltage Is compared with a predetermined threshold voltage, and if the voltage is equal to or lower than the threshold voltage, a comparison circuit that outputs an error signal and the error signal that is output when the error signal is output. There the drive of the inverter circuit as a configuration and a control circuit for stopping.

  In the invention configured as described above, a clamp circuit is connected to the secondary coils of the first transformer and the second transformer that supply voltage to the backlight, and the lower limit of the voltage generated in each transformer is shifted by the clamp circuit. Has been. In addition, since the comparison circuit is supplied with a voltage that is smoothed by adding a voltage whose lower limit is shifted, the voltage obtained when either the voltage of the first transformer or the second transformer is reduced is acquired. The voltage value falls below the threshold voltage, and an error signal is output. Therefore, the control circuit can detect a voltage drop abnormality of each transformer by inputting an error signal, and can detect the voltage drop even when a clamp circuit is used for the transformer.

  As a preferred example of the control circuit, the control circuit detects a voltage value generated in the first transformer and the second transformer, and performs overvoltage protection for the inverter circuit according to the detected voltage. It is good.

  As a preferred example of the clamp circuit, the clamp circuit may be a diode connected to the secondary coil of the first transformer and the second transformer at the cathode and grounded at the anode.

  As described above, in the present invention, even when a clamp circuit is used for the transformer, it is possible to detect a reduced voltage, and thus it is possible to achieve both overvoltage protection and reduced voltage protection.

2 is a block configuration diagram for explaining a configuration of a display device 1; FIG. It is a block block diagram for demonstrating the structure of a backlight drive circuit. 6 is a diagram for explaining a function of a clamp circuit 65. FIG. FIG. 6 is a diagram for explaining voltage drop protection for an inverter circuit 61. FIG. 6 is a diagram for explaining voltage drop protection for an inverter circuit 61.

Hereinafter, embodiments of the present invention will be described in the following order.
1. First embodiment:
2. Other embodiments:

1. First embodiment:
Hereinafter, a first embodiment of a backlight driving circuit according to the present invention will be described with reference to the drawings. In the first embodiment, description will be given based on the display device 1 including a backlight drive circuit. FIG. 1 is a block configuration diagram for explaining the configuration of the display device 1. FIG. 2 is a block diagram for explaining the configuration of the backlight drive circuit.

  The display device 1 according to this embodiment includes a display panel 2, a backlight 3, a power supply circuit 4, a panel drive circuit 5, and a backlight drive circuit 6. The panel drive circuit 5 is connected to the display panel 2 and the power supply circuit 4, and is driven by the power supplied from the power supply circuit 4 to drive the display panel 2. The backlight drive circuit 6 is connected to the backlight 3 and the power supply circuit 4, and is driven by the power supplied from the power supply circuit 4 to drive the backlight 3.

  The power supply circuit 4 includes a plurality of switching circuits (not shown). The switching circuit generates AC power or DC power for driving each part of the display device 1 from commercial power (AC 100 V). In the present embodiment, the power supply circuit 4 generates a backlight driving voltage of DC voltage from AC100V.

  The panel drive circuit 5 is constituted by a well-known integrated circuit, and generates a drive voltage for driving the display panel 2 by converting the video signal from digital to analog. That is, the video signal input to the panel drive circuit 5 is subjected to pixel number conversion processing, gamma correction, and the like according to the resolution of the display panel 2, and then converted to an analog voltage according to the gradation value of the video signal. Output to the display panel 2.

  The display panel 2 is an active matrix type liquid crystal panel, and is configured by arranging pixels of R (red), G (green), and B (blue) colors vertically and horizontally in accordance with the resolution. Each pixel includes a liquid crystal layer and an electrode (pixel electrode, common electrode) surrounding the liquid crystal layer. When a driving voltage is supplied to the pixel electrode, the liquid crystal layer corresponds to a potential difference between the pixel electrode and the counter electrode. Is driven, and the image is displayed by changing the transmittance of the light emitted from the backlight 3.

  In the present embodiment, the backlight 3 is configured by arranging a first cold cathode tube 31 driven by an AC voltage and a second cold cathode tube 32 in the sub-scanning direction. In FIG. 2, a straight tube is used as the backlight 3, but other than this, an S-shaped tube or a U-shaped tube may be used.

  The backlight drive circuit 6 generates an AC voltage (backlight drive voltage) from the DC voltage supplied from the power supply circuit 4 and supplies the AC voltage to the backlight 3. The backlight driving circuit 6 according to the present embodiment includes an inverter circuit 61, a transformer (first transformer 62, second transformer 63), a PWM control IC (control circuit) 64, and elements that connect the circuits. It is configured.

  The inverter circuit 61 is a conversion circuit that converts a DC voltage output from the power supply circuit 4 into an AC voltage, and includes an FET (Field Effect Transistor) (not shown) therein. That is, the inverter circuit 61 includes a pair of FETs (a high-side FET, a low-side FET) and a control transistor that controls on / off of each FET, and the high-side FET and the low-side FET are arranged in a predetermined cycle. And an AC voltage is generated from the DC voltage. The inverter circuit 61 is connected to the primary coils T1 and T1 'of the first transformer 62 and the second transformer 63, respectively, and generates voltages having different phases in the first transformer 62 and the second transformer 63, respectively.

  The first transformer 62 and the second transformer 63 are connected so as to be interposed between the inverter circuit 61 and the backlight 3, and voltages having a phase difference of 180 degrees (reverse phase) are respectively applied to the secondary coils due to the oscillation of the inverter circuit 61. Is generated. One end of the secondary coil T2 of the first transformer 62 is connected to a connector Co1, and is connected to one end of the first cold cathode tube 31 via the connector Co1. A connector Co2 is connected to one end of the secondary coil T2 'of the second transformer 63, and is connected to one end of the second cold cathode tube 32 via the connector Co2. Therefore, the inverter circuit 61 is supplied with AC voltages in opposite phases through the connector Co1 and the connector Co2, respectively, to drive the first cold cathode tube 31 and the second cold cathode tube 32, respectively.

  The PWM control IC 64 controls driving of the inverter circuit 61. The PWM control IC 64 is connected to each control transistor of the inverter circuit 61 through the drive terminal 64a, and supplies a drive signal with a predetermined duty to the gate electrode of each control transistor of the inverter circuit 61. Turn the FET on and off. As a result, a backlight drive voltage having a phase difference of 180 degrees is generated in the first transformer 62 and the second transformer 63 by the on / off control of each FET.

  Further, the PWM control IC 64 monitors the voltage value and waveform of the backlight drive voltage generated by the inverter circuit 61 through each terminal. When an abnormality is detected, the drive of the inverter circuit 61 is stopped. The backlight drive circuit 6 is protected. In the present embodiment, the PWM control IC 64 functions as an overvoltage protection circuit that protects the inverter circuit 61 from an overvoltage and a voltage reduction protection circuit that monitors the balance of the waveform of the backlight drive voltage generated in the first and second transformers. It has the function of. Here, the voltage drop protection is protection when the voltage generated in one transformer does not occur in a balanced manner with respect to the voltage generated in the other transformer. In the present embodiment, the case where the phase of the voltage generated in each transformer is deviated from the specified value or the amplitude of the voltage generated in one transformer is lower than the amplitude of the voltage generated in the other transformer is regarded as abnormal. Yes.

  In order for the PWM control IC 64 to function as a voltage drop protection circuit, the voltage drop protection terminal 64 b of the PWM control IC 64 is connected to the first transformer 62 and the second transformer 63 through the detection circuit 66. Here, the detection circuit 66 includes capacitors C1 to C4, an adder circuit 70, a smoothing circuit 71, and a comparator (comparison circuit) 72. The voltage drop protection terminal 64b of the PWM control IC 64 is connected to an internal block for performing voltage drop protection inside the PWM control IC 64. When an error signal is input from the detection circuit 66, the internal block stops the drive signal and stops driving the inverter circuit 61.

  In the detection circuit 66 according to this embodiment, the capacitors C1 and C2 constituting the series circuit are connected to both ends of the secondary coil T2 of the first transformer 62. Similarly, capacitors C3 and C4 constituting a series circuit are also connected to both ends of the secondary coil T2 'of the second transformer 63.

  The adder circuit 70 includes resistors R1 and R2. The resistor R1 is connected to the connection point between the capacitors C1 and C2 at one end, and is connected to the resistor R2 and the smoothing circuit 71 at the other end. Similarly, the resistor R2 of the adder circuit 70 is connected to the connection point between the capacitors C3 and C4 at one end, and is connected to the resistor R1 and the smoothing circuit 71 at the other end.

  The smoothing circuit 71 includes a diode D1 connected to the adder circuit 70 at the cathode, a resistor R3 connected to the anode of the diode D1 at one end and the other end grounded, and connected to the anode of the diode D1 at one end. And a capacitor C5 whose end is grounded.

  Further, the comparator 72 is connected to the comparison voltage (threshold voltage) Vref at the non-inverting input terminal, is connected to the smoothing circuit 71 at the inverting input terminal, and is connected to the reduced voltage protection terminal 64b of the PWM control IC 64 at the output terminal. ing.

  Further, in order for the PWM control IC 64 to function as an overvoltage protection circuit, the overvoltage protection terminal 64 c of the PWM control IC 64 is connected to the connection point between the capacitors C 1 and C 2 via the smoothing circuit 73. The overvoltage protection terminal 64c of the PWM control IC 64 is connected to an internal block for performing overvoltage protection inside the PWM control IC 64. The internal block performs overvoltage protection of the inverter circuit 61 by changing the duty ratio of the drive signal when the voltage generated in the secondary coil of each transformer exceeds a predetermined threshold. Similarly, the connection point between the capacitor C3 and the capacitor C4 is also connected to the overvoltage protection terminal 64c via the smoothing circuit 74.

  Further, a clamp circuit 65 is connected to the secondary coils T2, T2 ′ of the first transformer 62 and the second transformer 63, and the lower limit of the voltage value detected by the secondary coils T2, T2 ′ is shifted upward. ing. The clamp circuit 65 according to the present embodiment is connected to one end of the secondary coil T2 of the first transformer 62 and the anode, and connected to the diode D2 whose cathode is grounded, and the secondary coil T2 ′ of the second transformer and the anode. And a diode D3 whose cathode is grounded.

  FIG. 3 is a diagram for explaining the function of the clamp circuit 65. Here, FIG. 3A shows a voltage generated in the secondary coil T <b> 2 when the clamp circuit 65 is not connected to the first transformer 62. 3B shows a voltage generated in the secondary coil T2 when the clamp circuit 65 is connected to the first transformer 62.

  As shown in FIG. 3A, when the clamp circuit 65 is not connected to the transformer, the smoothing circuit 73 generates an AC voltage that changes from the positive side to the negative side with reference to 0 V. Therefore, after smoothing by the smoothing circuit 73 The voltage is averaged and has a small amount of change. On the other hand, as shown in FIG. 3B, when the clamp circuit 65 is connected to the transformer, an AC voltage whose lower limit is shifted to 0 V is generated in the secondary coil T2. For this reason, the voltage after smoothing by the smoothing circuit 3 is also a voltage having 0 V as a lower limit, so that the amount of change is large and the voltage after smoothing also has a predetermined voltage value. For the above reason, by connecting the clamp circuit 65 to the secondary coil T2, the PWM control IC 64 can monitor the voltage whose lower limit is 0V, and can easily detect overvoltage.

  4 and 5 are diagrams for explaining the reduced voltage protection for the inverter circuit 61. FIG. FIG. 4 shows each waveform when the drive of the inverter circuit 61 is normal. That is, FIG. 4A shows waveforms of the voltage generated in the first transformer 62 and the second transformer 63 and the voltage output from the adder circuit 70, and FIG. 4B shows the voltage supplied to the inverting input terminal of the comparator 72. Indicates. On the other hand, FIG. 5 shows each waveform when the drive of the inverter circuit 61 is abnormal. That is, FIG. 5A shows waveforms of the voltage generated in the first transformer 62 and the second transformer 63 and the voltage output from the adder circuit 70, and FIG. 5B shows the voltage supplied to the inverting input terminal of the comparator 72. Indicates.

  As shown in FIG. 4A, when the drive of the inverter circuit 61 is normal, an AC voltage with an amplitude Am1 is generated in the secondary coil T2 of the first transformer 62, and the secondary coil T2 ′ of the second transformer 63 is generated in the secondary coil T2 ′. It is assumed that an alternating voltage that is 180 degrees different in phase from the voltage generated in the first transformer 62 with the amplitude Am2 is generated. At this time, the amplitude Am1 of the voltage generated in the first transformer 62 and the amplitude Am2 of the voltage generated in the second transformer 63 are substantially the same value and have a phase difference of 180 degrees, so that the adding circuit 70 synthesizes each waveform. Thus, a voltage (waveform indicated by a dotted line in the figure) having a substantially constant voltage is generated. The smoothing circuit 71 smoothes this voltage and outputs it to the comparator 72 (hereinafter, this voltage is referred to as a normal voltage Vn).

  Therefore, a voltage corresponding to the normal voltage Vn is generated at the inverting input terminal of the comparator 72 (FIG. 4B), and a voltage corresponding to the voltage difference from the comparison voltage Vref is output from the output terminal as an error signal. Here, if the comparison voltage Vref is set to the same value as the normal voltage Vn or within a predetermined error, an error signal is output from the comparator 72 in the normal state of the voltage generated in the first transformer 62 and the second transformer 63. Is not output. Therefore, an error signal is not input to the reduced voltage protection terminal 64b of the PWM control IC 64, and the PWM control IC 64 does not stop driving the inverter circuit 61.

  On the other hand, as shown in FIG. 5A, when the amplitude Am2 of the voltage generated in the secondary coil T2 ′ decreases due to an abnormality (that is, when Am1> Am2), the voltage added by the adding circuit 70 is normally The value is lower than that of the time (waveform shown by the dotted line in the figure). Therefore, the voltage smoothed by the smoothing circuit 71 (hereinafter, this voltage is referred to as an abnormal voltage Verr) is equal to or lower than the normal voltage Vn (FIG. 5B), and the comparator 72 differentially amplifies the output. An error signal is output from the terminal. When the PWM control IC 64 receives this error signal via the reduced voltage protection terminal 64b, the PWM control IC 64 stops the drive signal output from the drive terminal 64a. As a result, the inverter circuit 61 stops driving and the reduced voltage protection is performed.

  Similarly, even when the phase of the voltage generated in the second transformer 63 is shifted from the normal state (opposite phase with the voltage generated in the first transformer 62) and the amplitude is small, the voltage input to the comparator 72 is the comparison voltage Vref. Since it becomes smaller than that, an error signal is output from the output terminal of the comparator 72.

  As described above, in the backlight drive circuit 6 according to the present embodiment, even if the secondary coil of the transformer is provided with a clamp circuit, it is possible to perform the voltage drop protection, and the overvoltage protection function and the voltage drop It is possible to achieve both a protective function.

2. Other embodiments:
The configuration in which the first transformer 62 and the second transformer 63 are connected to separate cold cathode tubes is an example, and other configurations may be used. For example, in a double-sided drive system in which voltage is supplied from both sides of the cold cathode tube, connectors Co1 and Co2 may be connected to both sides of the cold cathode tube, respectively, and AC voltages in opposite phases may be supplied to the cold cathode tube. .

  The use of a display device as a product including the backlight driving circuit according to the present embodiment is an example, and other products may be used as long as the product includes a backlight.

Needless to say, the present invention is not limited to the above embodiments. It goes without saying for those skilled in the art,
・ Applying mutually interchangeable members and configurations disclosed in the above embodiments by appropriately changing the combination thereof.− Although not disclosed in the above embodiments, it is a publicly known technique and the above embodiments. The members and configurations that can be mutually replaced with the members and configurations disclosed in the above are appropriately replaced, and the combination is changed and applied. It is an embodiment of the present invention that a person skilled in the art can appropriately replace the members and configurations that can be assumed as substitutes for the members and configurations disclosed in the above-described embodiments, and change the combinations and apply them. It is disclosed as.

  DESCRIPTION OF SYMBOLS 1 ... Display apparatus, 2 ... Display panel, 3 ... Backlight, 4 ... Power supply circuit, 5 ... Panel drive circuit, 6 ... Backlight drive circuit, 31 ... 1st cold cathode tube, 32 ... 2nd cold cathode tube, 61 ... inverter circuit, 62 ... first transformer, 63 ... second transformer, 64 ... PWM control IC, 65 ... clamp circuit, 66 ... detection circuit, 70 ... addition circuit, 71 ... smoothing circuit, 72 ... comparator, 73, 74: Smoothing circuit

Claims (4)

In a backlight driving circuit for driving a backlight using a cold cathode tube as a light source,
A transformer composed of a first transformer and a second transformer for supplying an AC voltage to the backlight;
An inverter circuit for generating AC voltages of opposite phases in the first transformer and the second transformer,
A clamp circuit connected to the secondary coil of the first transformer and the second transformer;
An adder circuit connected to the secondary coil of each transformer to synthesize a voltage generated in each secondary coil;
A smoothing circuit for obtaining a voltage obtained by smoothing the synthesized voltage;
A comparison circuit that compares the smoothed voltage with a predetermined threshold voltage and outputs an error signal if the voltage is less than or equal to the threshold voltage;
And a control circuit that stops driving the inverter circuit when the error signal is output.   The said control circuit detects the voltage value which generate | occur | produces in said 1st transformer and said 2nd transformer, and performs the overvoltage protection with respect to the said inverter circuit according to the said detected voltage. Backlight drive circuit.   3. The diode according to claim 1, wherein the clamp circuit is a diode connected to a secondary coil of the first transformer and the second transformer at a cathode and grounded at an anode. 4. Backlight drive circuit. The backlight driving circuit according to claim 1;
A liquid crystal display panel;
And a panel driving circuit for driving the liquid crystal display panel based on a video signal.

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