JP2012174488A - Light emission control device, display device and light emission control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrict temperature unevenness between one LED control block and another when emission luminance is controlled per control area by using a plurality of LEDs.SOLUTION: An emission control device for LEDs 11, 17 includes PWM control units 12, 18 which can change emission luminance every control area. An oscillator 14 generates a high frequency signal, and amplifiers 15, 20 amplify the output of the oscillator 14. Switching elements 16, 21 choose whether or not to supply drive signal waveforms output by the amplifiers 15, 20 to the cathodes of the LEDs 11, 17, respectively. When the switching element 16 is open, the LED 11 is turned on at the luminance set by the PWM control unit 12. When the switching element 21 is closed, a high frequency drive signal equal to or greater than a reverse breakdown voltage and less than a forward voltage at turn-on time is applied between the anode and the cathode of the LED 17 to heat the LED 17.

Description

  The present invention relates to a light emission control technique for a device using a plurality of light emitting diodes (hereinafter referred to as LEDs).

LEDs are widely used as light source devices for transmissive displays such as liquid crystal display devices. As a driving method of the LED, a method of adjusting a forward current flowing through the LED and a pulse width modulation (PWM) driving method of repeating blinking of the LED in a short cycle that cannot be recognized by a person are generally used. The LED driving method disclosed in Patent Document 1 uses light modulated at a sufficiently high frequency that humans cannot substantially perceive.
In addition, the luminance and chromaticity of the LED change due to changes in internal temperature and environmental temperature due to energization. In order to compensate for the change in the light emission characteristics, there is a technique for controlling the temperature of the LED by causing a reverse current to flow when no forward current flows through the LED (see Patent Document 2).
In recent years, local dimming control for the purpose of improving contrast and improving power saving performance is becoming common in liquid crystal display devices. This control dynamically adjusts the light amount of the light source device according to the input image signal, and adjusts the light emission luminance of the plurality of light sources for each control block according to the feature amount of the input image signal. For this reason, there are control blocks that emit bright light, control blocks that emit dark light, and control blocks that turn off.

JP 2009-040404 A JP-A-2005-236190

Since the internal temperature of the LED rises during light emission, when performing local dimming control with a light source device using the LED, the light emission brightness and the light emission time are different for each control block of the LED. This may cause unevenness in the temperature distribution.
In the technique of Patent Document 2, changes in luminance and chromaticity can be suppressed by applying a reverse voltage to the LED and controlling its temperature. However, when a reverse voltage exceeding the reverse withstand voltage is applied to the LED, there is a risk of deterioration or damage. Further, when a voltage less than the reverse withstand voltage is applied, a sufficient current does not flow through the LED. Therefore, when performing local dimming control in still image display or the like, there is a concern that the unevenness of heat generation is insufficiently corrected and uneven brightness or chromaticity occurs. In addition, the modulation driving technique at a high frequency disclosed in Patent Document 1 is intended to achieve both illumination and communication, and thus cannot improve temperature unevenness.
Accordingly, an object of the present invention is to suppress temperature unevenness for each control block of the LED when the light emission luminance is controlled for each control area using a plurality of LEDs.

  In order to solve the above-described problems, an apparatus according to the present invention is a light emission control apparatus that controls light emission luminance for each control area using a plurality of light emitting diodes, and includes an anode and a cathode in a non-lighting period of the light emitting diodes. A drive circuit for driving the light emitting diode at a high frequency is provided such that a drive signal applied therebetween is equal to or higher than a reverse withstand voltage of the light emitting diode and less than a forward voltage at the time of lighting.

  ADVANTAGE OF THE INVENTION According to this invention, when controlling light emission brightness for every control area using several LED, the temperature nonuniformity for every control block of LED can be suppressed.

It is a block diagram which shows the structural example of the apparatus which concerns on 1st Embodiment of this invention. It is a figure which illustrates the output waveform of the amplifier of FIG. It is explanatory drawing which illustrates the light emission state of the input image and light source device at the time of performing local dimming control. It is a figure which illustrates the drive signal waveform of LED which concerns on 1st Embodiment of this invention. FIG. 7 is a diagram illustrating an output waveform of an amplifier in order to explain the second embodiment of the present invention in conjunction with FIG. 6. It is a figure which illustrates the drive signal waveform of LED.

  Each embodiment to which the light emission control device according to the present invention is applied will be described.

[First Embodiment]
FIG. 1 is a block diagram illustrating a configuration example of a backlight device (light source device) for a display device. FIG. 1 shows only a plurality of LEDs and a high-frequency driving circuit used for controlling the brightness thereof, and an image display unit using a display device such as a liquid crystal display panel is not shown.
In the local dimming control, the contrast is improved by locally adjusting the luminance of the light source device according to the feature amount of the image signal input to the display device. In the first embodiment, in order to locally adjust the luminance of the backlight unit for the display device, the control blocks of the LEDs arranged in the backlight unit are arranged in horizontal 5 × vertical 3 as shown in FIG. The case where it is divided will be described.

In FIG. 1, only the 1st LED11 and 2nd LED17 from which a light emission state differs are illustrated, and the light emission control apparatus is shown. The LEDs 11 and 17 in this example are white LEDs. The anodes of the LEDs 11 and 17 are connected to a “+3 V” power supply terminal, and the cathode of the LED 11 is connected to the constant current source 13 and the switch element 16. The cathode of the LED 17 is connected to the constant current source 19 and the switch element 21. In FIG. 1, both switch elements 16 and 21 are indicated by switch symbols. In this example, the forward voltage (denoted as VF) of the LEDs 11 and 17 is 3V, and the reverse voltage (denoted as VR) is −4V. Note that the forward voltage VF and the reverse voltage VR may be set to optimum values for each LED type or individual because the VF characteristic and the VR characteristic differ depending on the emission color of the LED and individual differences.
The pulse width modulation control units (PWM control units) 12 and 18 are circuit units for controlling current values determined by the constant current sources 13 and 19 respectively, and change the light emission luminance of the LEDs 11 and 17. When the PWM values of these PWM controllers 12 and 18 are set to the maximum values, the LEDs 11 and 17 are lit at the maximum brightness. When the PWM values of the PWM control units 12 and 18 are set to the minimum value, the LEDs 11 and 17 are turned off. The PWM controllers 12 and 18 are independent circuits, and different PWM values can be set for each.

  The oscillator 14 generates a high frequency signal of 1 MHz to 1 GHz and outputs it to the amplifiers 15 and 20, respectively. The amplifiers 15 and 20 amplify the high frequency signal from the oscillator 14 to a predetermined voltage. The switch element 16 selects whether to supply the high frequency signal amplified by the amplifier 15 to the cathode of the LED 11. The switch element 21 selects whether or not to supply the high-frequency signal amplified by the amplifier 20 to the cathode of the LED 17. When the LED 11 is turned on by light emission control, the switching element 16 is opened (turned off) as shown in the figure, so that the drive frequency set by the PWM control unit 12 is +3 V between the anode and the cathode of the LED 11. A drive signal having a peak value is applied. On the other hand, when the LED 17 is caused to generate heat, during the non-lighting period of the LED 17, the switch element 21 is closed (turned on) as shown in the figure to switch to the heat generation driving of the LED 17.

FIG. 2 illustrates the output waveform of the amplifier 20.
When the switching element 21 is closed by a switching control unit (not shown) during the turn-off period of the LED 17, a drive signal of 100 MHz is applied to the cathode of the LED 17 with a voltage amplitude of 4 V (3 to 7 V).

FIG. 3 shows an input image and a light emission state of the backlight unit when local dimming control is performed in a backlight unit of a transmissive display such as a liquid crystal display device. The control block of the LED arranged in the backlight unit is divided into 15 blocks of 5 × 3. Numbers 1 to 15 attached to each control block indicate identification numbers for distinguishing the blocks.
For example, in the case of local dimming control in which the light emission luminance is changed for each LED control area, the PWM value of the PWM control unit 12 is set to the maximum, and the PWM value of the PWM control unit 18 is set to the minimum. The light emission state of the control area shown can be realized. When the input image of FIG. 3 is continuously displayed, the LED 11 corresponding to the eighth block of the display image continues to be lit and the LED 17 corresponding to the ninth block is continuously turned off. When the input image of the display device changes, the lighting states of the LEDs 11 and 17 change according to the feature amount of the input image.

FIG. 4 illustrates drive signal waveforms applied between the anode and cathode of each LED when the LED 11 is turned on and the LED 17 is turned off.
FIG. 4A shows a drive signal waveform applied between the anode and the cathode of the LED 11, and the frequency thereof is 120 Hz. During a period in which 3 V is applied as the forward voltage VF, the LEDs 11 arranged in the eighth block in FIG. 3 are lit.
FIG. 4B shows a drive signal waveform applied between the anode and the cathode during the non-lighting period of the LED 17, and a reverse voltage VR having an amplitude of 4 V (0 V to −4 V) is applied at a frequency of 100 MHz. . If the magnitude of the reverse voltage VR is set higher than the withstand voltage, the LED may be deteriorated or damaged. However, in this example, the LED 17 is driven within the withstand voltage range, and therefore the LED 17 is not deteriorated or damaged. There is no concern. During the non-lighting period of the LED 17, the forward voltage VF is set to 0 V, so that the LED 17 is not lit.

When a reverse voltage of 0 V to −4 V is applied between the anode and cathode of the LED 17 at a frequency of 100 MHz, the PN junction of the LED 17 generates heat. This is because a junction capacitance exists at the PN junction of the LED. In other words, when the LED PN junction is driven by applying a high frequency signal above a certain level, free discharge occurs and current is consumed at the LED PN junction. As a result, the PN junction of the LED 17 generates heat.
If the reverse voltage VR applied between the anode and cathode of the LED is set low, the LED cannot sufficiently generate heat. Therefore, there is a possibility that the temperature distribution is uneven between the control area where the LED is lit and the control area where the LED is not lit.

  In the first embodiment, in the control for changing the light emission luminance of the light emitting diode for each control area, the drive circuit defines the drive signal voltage and the frequency applied during the non-lighting period of the LED 17. That is, the LED 17 is driven by a high frequency signal that is equal to or higher than the reverse withstand voltage of the LED 17 and less than the forward voltage at the time of lighting and is 100 MHz or higher. Therefore, even when local dimming control of the light source device is performed, temperature unevenness for each control block can be reduced.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. In addition, since the structure of the light emission control apparatus which concerns on 2nd Embodiment is the same as that of the case of 1st Embodiment shown in FIG. 1, those description is abbreviate | omitted by using the code | symbol already used. Below, the drive method of LED17 is demonstrated using the wave form diagram of FIG.5 and 6. FIG.

  FIG. 5 illustrates the output waveform of the amplifier 20. During the non-lighting period of the LED 17, a driving voltage for generating heat is supplied from the amplifier 20 to the cathode of the LED 17 through the switch element 21. In this example, a drive signal waveform of 100 MHz with an amplitude of 5 V (2 to 7 V) is shown.

FIG. 6 shows drive signal waveforms applied between the anode and cathode of each LED when the LED 11 is turned on and the LED 17 is turned off.
The difference from the first embodiment is the voltage value of the drive signal shown in FIG. That is, FIG. 6 (A) shows the same drive signal waveform as FIG. 4 (A), and the LED 11 is turned on by applying a forward voltage of 3V. FIG. 6B shows a drive signal waveform applied between the anode and the cathode of the LED 17, the forward voltage VF is + 1V, the reverse voltage VR is −4V, and a frequency signal of 100 MHz with a voltage amplitude of 5V. Is applied to the LED 17. Note that a forward voltage of +1 V is applied between the anode and the cathode of the LED 17, but this is less than the forward voltage at the time of lighting, so the LED 17 does not light up.

In the second embodiment, by setting the forward voltage to 1 V in a range where the LED 17 does not emit light, the applied voltage between the anode and the cathode increases by 1 V compared to the case of the first embodiment. For this reason, the calorific value in the PN junction part of LED17 further increases.
In this example, the drive frequency of the output signal of the amplifier 20 is 100 MHz, but a configuration in which the drive frequency is variably controlled in the range of 1 MHz to 1 GHz according to the light emission luminance of the LED may be employed. Note that when the drive frequency is less than 1 MHz, a sufficient amount of heat is not obtained at the PN junction of the LED 17, so that the stable luminance control and temperature unevenness suppressing effect of the LED is reduced. Further, when the driving frequency exceeds 1 GHz, the driving waveform applied between the anode and the cathode is attenuated, so that the stable luminance control and temperature unevenness suppressing effect of the LED are reduced. Therefore, the drive frequency is more preferably in the range of 1 MHz or more and 1 GHz or less (however, the characteristics may vary depending on the material and structure of the LED, and the drive frequency is not limited to this range).

  In the second embodiment, the drive signal applied during the non-lighting period of the LED is a voltage range that is equal to or higher than the reverse withstand voltage of the LED and less than the forward voltage (positive voltage) during lighting. The LED is driven at a high frequency of 1 GHz or less. Thereby, even when local dimming control of the light source device is performed, temperature unevenness for each control block of the LED can be reduced. In other words, the LED changes in luminance and chromaticity due to changes in internal temperature due to energization and changes in environmental temperature, but it is controlled by flowing a reverse current between the anode and cathode of the LED during the non-lighting period of the LED. The temperature distribution of the block is made uniform. In this way, it is possible to reduce luminance unevenness and chromaticity unevenness of the light source device due to temperature unevenness of each LED.

11, 17 LED
12, 18 PWM controller 13, 19 Constant current source 14 Oscillator 15, 20 Amplifier 16, 21 Switch element

Claims (6)

A light emission control device that controls light emission luminance for each control area using a plurality of light emitting diodes,
The drive signal applied between the anode and the cathode during the non-lighting period of the light emitting diode is set to a voltage range that is equal to or higher than the reverse withstand voltage of the light emitting diode and less than the forward voltage at the time of lighting. A light emission control device comprising a drive circuit for driving a high frequency signal. The drive circuit is
A constant current source connected to the cathode of the light emitting diode;
A pulse width modulation control unit for controlling the constant current source;
An oscillator for generating a frequency signal;
An amplifier that amplifies and outputs a frequency signal from the oscillator;
A switching element connected to the cathode of the amplifier and the light emitting diode;
When the switch element is opened, the light emitting diode is turned on with the brightness set by the pulse width modulation control unit,
2. The driving signal output from the amplifier is applied between the anode and the cathode of the light emitting diode to generate heat when the switch element is closed during the non-lighting period of the light emitting diode. Light emission control device.   The light emission control device according to claim 1, wherein the drive circuit controls a drive frequency of the light emitting diode at 1 MHz or more in the non-lighting period.   The light emission control device according to claim 3, wherein the drive circuit controls the drive frequency of the light emitting diode in a range of 1 MHz to 1 GHz in the non-lighting period. A backlight unit whose brightness is controlled for each control area using a plurality of light emitting diodes;
An image display unit for displaying an image using light from the backlight unit;
A display device comprising the light emission control device according to claim 1, wherein the light emission diode of the backlight unit is controlled for each control area. A light emission control method for controlling light emission luminance for each control area using a plurality of light emitting diodes,
The drive signal applied between the anode and the cathode during the non-lighting period of the light emitting diode is set to a voltage range that is equal to or higher than the reverse withstand voltage of the light emitting diode and less than the forward voltage at the time of lighting. A light emission control method characterized by driving a high frequency.

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