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Switching power supply is the main component of LED display. Its quality and performance are directly related to the quality and life of the display. Since the characteristics of the optoelectronic device in the full-color LED display are different from those of the general display, the cost is very expensive, and the performance and quality of the switching power supply are very high. What characteristics of the power supply should be able to adapt to the high load of the LED display? How is it working? The following is a simple arrangement of related information for LED industry insiders.
1. Under certain conditions, keeping the LED pixels driven by the constant current is caused by the change of the LED group voltage caused by the change of the display load, which will affect the pixel current and cause the brightness to change. In order to ensure uniform brightness and reduce brightness variation of the whole screen module, the sampling point of the switching power supply output sampling circuit is moved from the power port to a point in the display screen, so that the voltage at the point remains unchanged. The voltage change caused by the load change is adjusted by the switching power supply, that is, the load is small and the output voltage is low; when the load is large, the output voltage is high. Therefore, it is ensured that the voltage output to the LED module does not change under the condition of constant temperature and brightness, and the pixel driving current is constant.
2. Control the brightness of the LED display by adjusting the voltage of the switching power supply In order to make full use of the limited resources in the full color screen control system, the switching power supply can adjust the brightness of the full color screen by adjusting the output voltage. Due to the large change of outdoor illumination, it is necessary to pre-set the voltage value corresponding to the brightness of the full color screen according to different climates and time periods, and store it in the single chip microcomputer. According to the signal sent by the illumination sensor, the output voltage value of the digitally controlled switching power supply is obtained to control the brightness of the display screen.
3. Compensation for LED forward voltage drop VF caused by ambient temperature The forward voltage drop VF of the LED decreases with increasing ambient temperature. The temperature variation coefficients of various LEDs vary. The VF can vary by up to 20% over the normal operating temperature range. Under constant voltage drive, a drop in VF will cause the LED current to rise, causing the overall temperature to rise. In this feedback process, the working condition of the LED will become worse and worse, seriously affecting the quality and life of the outdoor screen. Therefore, the switching power supply of the outdoor screen is required to have a temperature compensation function to ensure that the LED driving current does not rise with an increase in temperature.
The simple temperature compensation method is to connect the negative temperature coefficient thermistor with the temperature change rate close to the LEDVF in the switching power supply, and introduce the switching power supply PWM chip to compare the reference voltage Vi (Vo=KVi). When the temperature rises, Vi drops and the output voltage drops, which suppresses the rise of the LED operating current. Due to the large dispersion of the thermistors, in order to ensure product consistency, it is necessary to select. In addition, there is an independent combination of resistances for each temperature change rate, and the interchangeability is poor. In order to overcome these shortcomings, a digitally controlled switching power supply whose output is controlled by a digital signal is used. According to the temperature drift curve of various LEDs, the driving voltages to be supplied to the LEDs at different temperatures are tabulated and stored in the MCU for inquiry. According to the temperature check table measured by the temperature sensor, the output voltage of the corresponding digitally controlled switching power supply can be obtained.