JP2013057881A - Power supply circuit for backlight - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem associated with a power supply circuit for backlight of a liquid crystal display device in which, when any abnormality occurs in a driver circuit of an LED unit and output voltage thereof is not generated, LED is put into the unlighted state and the liquid crystal display device is put into the non-display state, and thus a user cannot view the display screen.SOLUTION: When abnormality of an LED driver circuit (1) causes no generation of output voltage, the fact that the output voltage becomes equal to or less than a predetermined value is detected, and liquid crystal panel driving voltage (VDDG) is supplied from a liquid crystal panel driving power supply circuit unit (5) of the liquid crystal display device through a switching circuit (4) to an LED unit (2).

Description

  The present invention relates to a power supply circuit for a light source device, and in particular, can be suitably used for a liquid crystal display device using an LED as a light source.

There is a planar light source device that illuminates a display panel such as a liquid crystal panel with uniform light over the entire display area. This planar light source device includes a direct type in which a light emitting element is disposed on the back surface of a light guide plate and an edge light type in which a light emitting element is disposed on a side surface side of the light guide plate. Further, as light emitting elements of these planar light source devices, a linear light source such as a CCFL (Cold Cathode Fluorescent Lamp) and a point light source such as an LED (Light Emitting Diode) are generally used. .
CCFLs contain environmentally hazardous substances such as mercury in the discharge gas, and the applied voltage to the CCFL is as high as several hundred to 1000 V, and the drive circuit becomes complicated. What is hereinafter referred to as LED) has become the mainstream of planar light source devices (hereinafter, planar light source devices using LEDs as light sources are referred to as LED backlights).

  Since LEDs are point light sources, it is necessary to connect a plurality of LEDs in series, and depending on the size of the display panel, a plurality of LEDs need to be connected in parallel. However, the drive circuit is small and the board space is greatly reduced. It is not difficult to mount a display panel drive circuit and an LED drive circuit (hereinafter referred to as an LED driver circuit) on the same substrate.

  As the LED driver circuit configuration in which a plurality of LEDs connected in series are arranged in parallel, a method in which a driver circuit is configured independently for each of a plurality of parallel LED series and a plurality of parallel anodes are connected in common There is a method to do. Independently configuring the circuits and arranging them in parallel increases the board space, resulting in increased costs. On the other hand, an example in which a plurality of series LEDs are arranged in one line, and the common anode method have the advantage that both the board space and the cost can be significantly reduced.

However, in the case of a circuit configuration in which a plurality of series LEDs are arranged in one series or a system in which a plurality of parallel anodes are connected in common, when an abnormality occurs in the LED driver circuit and no output is generated, etc. There is a problem in that the current does not flow through the LED, the liquid crystal display device is turned off, and the user cannot visually recognize the display screen.
In addition, in a liquid crystal display device using an LED backlight in which a plurality of LEDs are connected in series, when any one of the plurality of LEDs is damaged and becomes electrically open, all the LEDs connected in series are all There is a problem that the current does not flow and the light is turned off.

  Regarding such a conventional problem, for example, Patent Document 1 includes a means for bypassing a current in parallel to LEDs connected in series, thereby lighting other LEDs other than the failed LED. A technique for reducing non-lighting or a decrease in luminance due to an open failure is described. Further, Patent Document 2 discloses a technique for bypassing the element current of a part when the OPEN mode of the element is defective and automatically avoiding the disconnection state of the part when driving a plurality of LEDs connected in series at a constant current. Have been described.

  In the above-described technology, when the LED on the load side of the LED driver circuit is damaged and the open mode is defective, all of the LED units connected in series do not light up. In some cases, the liquid crystal display device is not displayed. Is a workaround for the problem.

JP 2007-165161 A JP 2005-310999 A

  As described above, as an LED driver circuit configuration for driving the LED backlight of a conventional liquid crystal display device (entire configuration including an LED unit on the load side), an example in which a plurality of series-connected LEDs are arranged in one line In the example of a system in which a plurality of parallel anodes are connected as a common, when an abnormality occurs in the LED driver circuit and the output voltage is no longer generated, the current does not flow through the LED and the liquid crystal display device is in a non-display state. There is a problem in that the user cannot see the display screen.

  A backlight power supply circuit for a liquid crystal display device according to the present invention includes a LED unit portion in which a plurality of LEDs are connected in series and an LED driver circuit portion for driving the LED unit portion. A liquid crystal panel driving power supply circuit portion of the liquid crystal display device, a voltage abnormality detection circuit portion for detecting that the output voltage of the LED driver circuit portion is a predetermined value or less, and a switch circuit portion, When the output voltage of the LED driver circuit unit is detected to be equal to or lower than a predetermined value by the voltage abnormality detection circuit unit, the liquid crystal panel driving power source circuit unit drives the LED unit via the switch circuit unit. It is characterized by supplying a working voltage.

  According to the present invention, the current is supplied to the LED from the power supply circuit unit for driving the liquid crystal panel provided in the liquid crystal display device, and the lighting state is continued even if the current supply from the LED driver circuit is lost. The user can continue to use it without interrupting the work.

It is a schematic block diagram of the power supply circuit for LED backlight which concerns on Embodiment 1 of this invention. It is a schematic block diagram which shows the modification of the power supply circuit for LED backlight which concerns on Embodiment 1 of this invention. It is a schematic block diagram of the power supply circuit for LED backlight which concerns on Embodiment 2 of this invention. It is a schematic block diagram which shows the other modification of the power supply circuit for LED backlight which concerns on Embodiment 2 of this invention. It is the schematic of a power supply sequence. It is a schematic block diagram which shows the other modification of the power supply circuit for LED backlight which concerns on Embodiment 1 of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In order to avoid redundant descriptions, the same reference numerals are given to elements having the same or corresponding functions in each drawing.

Embodiment 1 FIG.
FIG. 1 is a schematic configuration diagram of an LED backlight power supply circuit 10 of the liquid crystal display device according to Embodiment 1 of the present invention.
This circuit configuration includes an LED driver circuit 1, an LED unit 2 in which a plurality of LEDs are connected in series, an output voltage abnormality detection circuit 3, a switch circuit (hereinafter referred to as SW circuit) 4, and a DC / DC for generating power for display panel drive. The circuit 5 includes a current backflow prevention diode 6, a current limiting resistor 7, and a current backflow prevention diode 9.

The LED driver circuit 1 is a DC / DC converter for driving and lighting an LED of an LED backlight, which is a light source of a liquid crystal display device, at a constant current, and is connected to the anode side of the LED unit 2 via a current backflow prevention diode 9. Connected.
The voltage abnormality detection circuit 3 is connected to the rectified output of the LED driver circuit 1, the liquid crystal panel gate-on voltage VDDG of the DC / DC circuit 5, and the LED driver circuit input voltage Vin (LED) as its inputs. The voltage abnormality detection circuit 3 determines whether or not there is an abnormality in the output voltage of the LED driver circuit 1 through a voltage processing circuit (exception processing circuit, not shown) based on the three voltages, and controls input (control) of the SW circuit 4 Switch ON / OFF control signal is output to the terminal.

The input (one terminal) of the SW circuit 4 is connected to the liquid crystal panel gate-on voltage VDDG of the display panel driving power generation DC / DC circuit 5.
On the other hand, the output (the other terminal) of the SW circuit 4 is connected to the anode of the LED unit 2 via the current backflow prevention diode 6 and the current limiting resistor 7.

  Here, the voltage processing circuit (exception processing circuit) is the power supply startup (T2 + T6) and power supply shutdown (T3 + T5) in the power supply startup / shutdown sequence in the schematic diagram of the power supply sequence of FIG. This is a circuit for setting the abnormal state to be an exception process so that the SW circuit 4 is erroneously closed during an unstable period (unsteady period) and the LED is not turned on erroneously, and stopping the abnormality determination.

  Next, the operation of the LED backlight power supply circuit 10 configured as described above will be described. The voltage abnormality detection circuit 3 monitors the output voltage of the LED driver circuit 1 in a steady state other than the rising / falling sequence of each power source shown in FIG. A switch ON signal is output to the control input of the SW circuit 4 so that the driver circuit 1 is judged to be faulty (at the time of abnormality) so that the SW circuit 4 is closed. On the other hand, if the output voltage of the LED driver circuit 1 is equal to or higher than the predetermined voltage, the LED driver circuit 1 is determined to be in a normal state (normal state), and the switch OFF signal is sent to the SW circuit so that the SW circuit 4 maintains the open circuit state. 4 output to the control input.

  As described above, the power supply circuit 10 for the LED backlight is configured, and the voltage abnormality detection circuit 3 monitors the output voltage of the LED driver circuit 1 and controls the SW circuit 4 to close when there is an abnormality. The panel gate-on voltage VDDG is applied to the LED unit 2 and the lighting state of the LED is continued.

The current backflow prevention diode 9 is for preventing current from flowing into the output voltage abnormality detection circuit 3 when current is supplied through the SW circuit 4.
Further, the current flowing through the LED unit 2 is adjusted by the current limiting resistor 7.

  Here, in general, the display panel driving power generation DC / DC circuit 5 has a limit in the current supply capability of its booster circuit (charge pump), so that the LED driver circuit 1 is operating normally. In comparison, when the LED driver circuit 1 fails, the display becomes dark. However, since the non-display state of the liquid crystal display device due to the failure of the LED driver circuit 1 can be avoided, the display device can be visually recognized continuously.

Next, FIG. 2 shows a schematic configuration diagram illustrating a modification of the LED backlight power supply circuit 10 according to Embodiment 1 of the present invention. This circuit configuration includes an LED driver circuit 1, an LED unit 2 in which a plurality of LEDs are connected in series, an output voltage abnormality detection circuit 3, an SW circuit 4, a display panel driving power generation DC / DC circuit 5, and a current backflow prevention diode 6. , A current limiting resistor 7, a booster circuit 8, and a current backflow prevention diode 9.
In this modification, the switching rectangular wave voltage of the DC / DC circuit 5 is provided with a booster circuit 8 (configured by a charge pump circuit or the like) separately from the booster circuit in the DC / DC circuit 5. VDD is boosted and supplied to the input (one terminal) of the SW circuit 4 in the same manner as in the first embodiment, and other configurations and operations are the same as those in the first embodiment described above. Description is omitted.

In the first embodiment described above, since the current supply capability of the DC / DC circuit 5 is limited, the display brightness of the liquid crystal display device becomes dark when the LED driver circuit 1 fails, but in this modification, the booster circuit 8 Since (added with a charge pump circuit or the like) is added, the current supply capability to the LED unit 2 is improved to some extent.
Further, the boosting factor of the booster circuit 8 is appropriately set according to the number of LEDs in the LED unit 2 in series, the boosted voltage is adjusted, and the value of the current limiting resistor 7 is appropriately selected to Even if there is a relatively large amount, it can be handled.
As described above, the same effect as in the first embodiment can be obtained.

Embodiment 2. FIG.
FIG. 3 is a schematic configuration diagram of the LED backlight power supply circuit 10 according to Embodiment 2 of the present invention. The LED backlight power supply circuit 10 having this circuit configuration includes an LED driver circuit 1, an LED unit 2 in which a plurality of LEDs are connected in series, an output voltage abnormality detection circuit 3, a SW circuit 4, a display panel drive power generation DC / The circuit includes a DC circuit 5, a current backflow prevention diode 6, and a current limiting resistor 7.

The LED driver circuit 1 is a DC / DC converter for driving and lighting an LED of an LED backlight, which is a light source of a liquid crystal display device, with a constant current, and is connected to the anode side of the LED unit 2.
The voltage abnormality detection circuit 3 is connected to the rectified output of the LED driver circuit 1, the liquid crystal panel analog voltage VDDA of the DC / DC circuit 5, and the LED driver circuit input voltage Vin (LED) as its inputs. The voltage abnormality detection circuit 3 determines whether or not there is an abnormality in the output voltage of the LED driver circuit 1 through the above-described voltage processing circuit (not shown) based on the three voltages, and a control input (control terminal) of the SW circuit 4 Output a switch ON / OFF control signal.

  In the second embodiment of the present invention, a liquid crystal panel driving analog voltage VDDA (for example, 12 VDC) generated by the DC / DC circuit 5 is connected to an input (one terminal) of the SW circuit 4.

When the analog voltage VDDA generated by the DC / DC circuit 5 is lower than the total forward voltage drop (Vf) of all LEDs connected in series in the LED unit 2, the LED unit 2 does not light up, A point (node) that supplies power to the LED unit 2 through the backflow prevention diode 6 and the current limiting resistor 7 is defined as a predetermined connection wiring portion between the LEDs in the LED unit 2.
For example, as illustrated in FIG. 3, when five LEDs are connected in series and only three of them are lit, the connection wiring between the second and third LEDs counted from the LED driver circuit 1 side is used. A power supply line from the SW circuit (the other terminal) is connected via a current backflow prevention diode 6 and a current limiting resistor 7.

  Next, the operation of the LED backlight power supply circuit 10 configured as described above will be described. The voltage abnormality detection circuit 3 monitors the output voltage of the LED driver circuit 1 in a steady state other than the rising / falling sequence of each power source shown in FIG. A switch ON signal is output to the control input of the SW circuit 4 so that the driver circuit 1 is judged to be faulty (at the time of abnormality) so that the SW circuit 4 is closed. On the other hand, if the output voltage of the LED driver circuit 1 is equal to or higher than the predetermined voltage, the LED driver circuit 1 is determined to be in a normal state (normal state), and the switch OFF signal is sent to the SW circuit so that the SW circuit 4 maintains the open circuit state. 4 output to the control input.

  As described above, the power supply circuit 10 for the LED backlight is configured, and the voltage abnormality detection circuit 3 monitors the output voltage of the LED driver circuit 1 and controls the SW circuit 4 to close when there is an abnormality. The panel drive analog voltage VDDA is applied to some of the LEDs of the LED unit 2 (in FIG. 3, only 3 of the 5 LEDs are lit), and the LED is kept on.

  Depending on whether the LED unit 2 is mounted in the up / down / left / right direction of the liquid crystal display device, for example, only the right half or the left half of the display screen, only the upper half or the lower half of the LED is turned on. Since the screen is displayed due to the diffusion effect, the operation itself can be continued.

  Depending on the screen size of the liquid crystal display device, a device having a small number of LED series connections between the LED driver circuit 1 and GND generally has a plurality of LED units 2 (n ≧ 2). In this case, the entire screen is displayed by reversing the mechanical arrangement direction of the LEDs for each system of the LED unit 2 (not shown), and the user can work without feeling inconvenience. Can continue.

  In this example, when a current is supplied via the SW circuit 4 (at the time of abnormality), a current backflow prevention diode that prevents the current from flowing into the output voltage abnormality detection circuit 3 (to the diode 9 of the first embodiment). Is not necessary because an upper LED in the LED unit can be substituted.

FIG. 4 is a schematic configuration diagram showing a modification of the LED backlight power supply circuit 10 according to Embodiment 2 of the present invention.
This circuit configuration includes an LED driver circuit 1, a plurality of LED units 2 connected in series, an output voltage abnormality detection circuit 3, a SW circuit 4, a current backflow prevention diode 6, and a current limiting resistor 7.
The LED driver circuit input power supply Vin (LED) (for example, 12 VDC) is used as an input (one terminal) of the SW circuit 4. Other configurations are the same as those in the second embodiment, and detailed description thereof is omitted. However, when the voltage value of the input power supply Vin (LED) is high to some extent such as 12 V, the above-described second embodiment is realized by a simple circuit configuration. Equivalent effect is obtained.

Other modified examples.
In the first embodiment described above that although the case of one system as a system number of LED units, as shown in FIG. 6, the LED unit 2 _-1, the anode of _2-2 are commonly connected, 2 Similar effects can be obtained when the systems are driven in parallel.
Furthermore, it is the same even if the number of LEDs is increased in order to increase the luminance of the display device, and the number of systems in the LED unit is increased as the arrangement method.

  Similarly, in the second embodiment, when the anode side of the LED units is connected in common and a plurality of units are driven in parallel, the current backflow prevention diode 6 and the current limiting resistor 7 are provided corresponding to each unit. The effect can be obtained in the same manner as in the second embodiment by arranging and connecting to the midway connection wiring of the LEDs connected in series.

1 LED driver circuit _{2,2 -1,} 2 -2 LED unit 3 output voltage abnormality detection circuit 4 switch circuit (SW circuit)
5 Display panel drive power generation DC / DC circuit 6, 9 Current backflow prevention diode 7 Current limiting resistor 8 Boost circuit 10 LED backlight power supply circuit VDD Input voltage Vin (LED) LED driver circuit input voltage VDDA Liquid crystal panel drive Analog voltage VDDG Gate-on voltage for LCD panel

Claims (3)

In a backlight power supply circuit of a liquid crystal display device comprising an LED unit part in which a plurality of LEDs are connected in series, and an LED driver circuit part for driving the LED unit part,
A power supply circuit unit for driving a liquid crystal panel of the liquid crystal display device;
A voltage abnormality detection circuit unit for detecting that the output voltage of the LED driver circuit unit has become a predetermined value or less, and a switch circuit unit;
When the output voltage of the LED driver circuit unit is detected to be equal to or lower than a predetermined value by the voltage abnormality detection circuit unit, the liquid crystal panel driving power source circuit unit drives the LED unit via the switch circuit unit. A power supply circuit for a backlight of a liquid crystal display device, characterized by supplying a working voltage. When the output voltage of the LED driver circuit unit is detected to be a predetermined value or less in the voltage abnormality detection circuit unit, the switch is connected to a predetermined connection wiring unit between the LEDs in the LED unit units connected in series. 2. The backlight power supply circuit for a liquid crystal display device according to claim 1, wherein a liquid crystal panel drive voltage is supplied from the liquid crystal panel drive power supply circuit section through a circuit section. 3. The backlight power supply circuit for a liquid crystal display device according to claim 1, wherein a current backflow prevention diode and a current limiting resistor are connected in series between the switch circuit portion and the LED unit portion.

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