Backlight using multi-color LED color complement

1 Introduction

As LED technology matures, LEDs of various colors have been developed, and the cost has also dropped significantly. LED drive circuits with an output current of more than 300mA are commonly driven by inductive switching power supplies and linear power supplies, and there are also combined driving methods of switching power supplies and linear power supplies. There is almost no ground bounce and ripple in the linear power supply drive circuit, and the interference to the circuit is small. It is often used in occasions with high requirements for electromagnetic compatibility.

The backlight technology of multi-color LED color mixing and color complementation is a hot topic in current research. As early as 2000, Japan had already produced multi-chip LED backlight products. Subsequently, major companies successively launched high-performance LCDs with multi-chip LED backlights. However, there are still few public documents on how to mix and complement colors.

Although the backlight color gamut of the red, green and blue LED lights is wide, there is a red edge phenomenon. This paper uses white LED lights as the main light source, and then uses orange and blue lights as complementary light sources to obtain a standard white light with a white field color temperature within (6500±100)K, color coordinates u′ within 0.198±0.01, and v′ within 0.468±0.01.

2 Complementary color circuit diagram

White LED uses blue light to excite phosphor to produce white light. Since the LCD screen attenuates short-wave components more than long-wave components, the light emitted by the white LED is yellowish after leaving the screen. It is necessary to supplement both blue light and red light.

When a typical current of 20mA flows through, the voltage of the orange light is 2V, the voltage of the blue light is 3V, and the voltage of the white light is 3V at room temperature.

All three types of lights are in a string of 8. The orange light only needs about 16V to work under typical current conditions, and the linear constant current source efficiency of its driving circuit is less than 60%. Since the orange light is a complementary color, the maximum power loss is less than 1W; the linear power supply efficiency of the white and blue lights exceeds 85%.

Each of the three types of LED lamps is driven by a linear constant current source, and the total current of each channel is set to 200mA to ensure that the current of each string of LED lamps is its typical current of 20mA, as shown in Figure 1. Each linear constant current source independently uses a PWM dimming signal of the same frequency. Their frequency is fixed, and the duty time changes with the dimming brightness.

Using FPGA as the controller, the PWM duty cycle is adjusted according to the dimming instructions of the host computer to control the backlight brightness; the data of the color sensor is sampled to fine-tune the PWM duty cycle of the orange and blue lights to ensure that the color temperature remains unchanged when the temperature changes.

Figure 1 Block diagram of LED orange and blue complementary color circuit.

3 PWM dimming and complementary colors

3.1 Preparation before using orange and blue light for color matching

Figure 2 shows the PWM dimming waveforms of white lamps and complementary color lamps. The drive circuit should meet the following conditions:

(1) The driving voltage should be able to ensure the voltage output required for the constant current of the LED lamp; (2) The output current of the driving circuit should have consistency in the on-time, so that the brightness can be linearly adjusted; (3) In daylight mode, the white light should gradually increase, while the complementary blue and orange lights should gradually increase in proportion to the change of the white light.

Figure 2 PWM dimming waveforms of white light and complementary color light.

3.2 Light mixing on the SS-230 flat panel display system integrated analysis instrument

Figure 3 is a photo of the SS-230 flat panel display comprehensive analysis system. During the experiment, the white light was adjusted from low brightness to high brightness, and the orange and blue lights were gradually increased in proportion to ensure that the color temperature was (6500±30)K, and the color coordinates u′=0.198, v′=0.468. The data of the high level duration of the dimming PWM waveform of the white light, orange light and blue light were recorded during the dimming process from low brightness to high brightness, and a total of 40 sets of data were recorded.

Figure 3 SS-230 flat panel display integrated analysis system.

3.3 Data fitting

Based on the 40 sets of data recorded, the dimming PWM high-level time of the three lights under different brightness conditions can be obtained. The relationship between the dimming high-level time and brightness of the three lights is fitted below, as shown in Figure 4.

The expressions of the three uninterrupted curves in Figure 3 are as follows:

Figure 4 Relationship between brightness and dimming levels of three types of lamps.

In formula (1), L is the brightness, W, O and B are the high-level time series of the PWM waveform of the white light, orange light and blue light, the dimming cycle is 500μs, and the high-level time series is (7FFF)16=(32 767)10 levels.

If the high level time series of the white light is 7EE0, then the high level time in its dimming PWM waveform is:

3.4 Color Closed-Loop Control

According to the obtained fitting curve, when the backlight temperature is constant and consistent with the backlight temperature when the fitting curve was made, a standard white light source can be obtained by dimming in the full brightness range. However, it is impossible to ensure that the backlight LED temperature remains unchanged. As the LED temperature changes, the attenuation of white light, orange light and blue light is different, and the color temperature will drift.

The number of pulses output by the color sensor for the three color components of red, green and blue is recorded within 200 dimming cycles, that is, within 100ms. These data are used as the brightness values ​​of the three color components.

When the temperature changes, the high level time of the PWM waveform of the orange light and the blue light is fine-tuned according to the red, green and blue brightness data measured by the color sensor to make the brightness ratio of the red, green and blue colors constant, and the color temperature remains unchanged.

4 Conclusion

On the SS-230 flat panel display system comprehensive analysis instrument, 40 points were measured from low brightness to high brightness, and the PWM pulse high level time of the three lamps was adjusted to obtain a standard white light source. Then, the linear fitting method was used to obtain the proportional relationship of the high level duration of the PWM waveform of the three lamps under different brightness conditions. According to this relationship, dimming can ensure that the backlight source is standard white light in the entire brightness range. According to the red, green and blue three color channel data output by the color sensor, the PWM high level time of the orange lamp and the blue lamp was fine-tuned. The proportional relationship of the red, green and blue three color channel data was maintained unchanged to keep the color temperature constant. The above method was used to obtain a standard white light with a white field color temperature within (6 500±100)K, a color coordinate u′ within 0.198±0.01, and a v′ within 0.468±0.01.