As the main component of LED display screen, the quality and performance of switching power supply have a direct bearing on the quality and life of the display screen. Because the characteristics of optoelectronic devices in led outdoor full-color display are different from those of general displays, the cost is very expensive, and the performance and quality of switching power supply are very high, a special switching power supply for outdoor full-color screen is developed in this paper. The characteristics of switching power supply for outdoor LED display screen used in LED outdoor full color display shall have the following characteristics. The main results are as follows: 1) keeping the LED pixel driven by constant current under certain conditions will affect the pixel current and cause the brightness change because of the change of the voltage of the LED group caused by the load change of the display screen. In order to ensure the brightness uniformity of the whole screen module and reduce the brightness change, the sampling point of the output sampling circuit of the switching power supply is moved from the power supply port to a certain point in the display screen, so that the voltage of this point remains unchanged. The voltage change caused by the load change is adjusted by the switching power supply, that is, when the load is small, the output voltage is low, and when the load is large, the output voltage is high. Therefore, the output voltage to the LED module is constant and the pixel driving current is constant under the condition of constant temperature and brightness. 2) the change of LED forward pressure drop VF caused by compensating ambient temperature the forward pressure drop VF of LED decreases with the increase of ambient temperature. The temperature variation coefficients of various LED are different. In the normal working temperature range, the change of VF can reach 20. Under constant voltage driving, the decrease of VF will cause the LED current to rise and the whole temperature to rise. In the process of feedback, the working condition of LED will deteriorate more and more, which will seriously affect the quality and life of outdoor screen. Therefore, the switching power supply of outdoor screen is required to have temperature compensation function to ensure that the LED drive current does not rise with the increase of temperature. A simple temperature compensation method is to connect a negative temperature coefficient thermistor with a temperature change rate close to that of LEDVF in the switching power supply, and to introduce a switching power supply PWM chip to compare the reference voltage Vi (Vo=KVi). When the temperature rises, the Vi decreases and the output voltage decreases, which inhibits the rise of the LED working current. Due to the wide dispersion of thermistor, to ensure the consistency of the product, it is necessary to select. In addition, each rate of temperature change has an independent resistance combination, poor interchangeability. In order to overcome these shortcomings, the output digital signal controlled NC switching power supply is adopted. According to the temperature drift curves of various LED, the driving voltage needed to be supplied to LED at different temperatures is made into a table, which is stored in a single chip computer for inquiry. The output voltage of the NC switching power supply can be obtained according to the temperature look-up table measured by the single chip microcomputer of the temperature sensor. 3) the brightness of display screen is controlled by adjusting the voltage of switching power supply. In order to make full use of the limited resources in the full color screen control system, the switching power supply is required to adjust the brightness of the full color screen by adjusting the output voltage. Due to the great variation of outdoor lighting, it is necessary to make the voltage value corresponding to the brightness of the full color screen into a table according to different weather and time periods, and store it in a single chip microcomputer. The output voltage of the NC switching power supply is obtained by looking up the signal sent by the illumination sensor to control the brightness of the display screen. The working principle of general switching power supply is shown in figure 1. The 3525 pulse width modulation (PWM) module is used as the control circuit of switching power supply with complete function and convenient application. The error amplifier amplifies the error signal from the output of the power supply and sends the sampling circuit to the reverse input of the comparator to compare with the oblique wave voltage output from the oscillator in the reference circuit. The duty cycle of 3525 internal output power transistor is controlled by square wave pulse output by comparator. Different voltage values can be obtained by changing the reference voltage and the output sampling circuit.

Working principle of NC switching Power supply CNC switching Power supply is a switching power supply whose output voltage is controlled by digital signal. Its working principle is shown in figure 2.

　　The X9313 non-volatile numerical control potentiometer is chosen as the controllable device for comparing the reference voltage of the PWM chip, which has the advantages of low power consumption, convenient operation and good reliability. X9313 is a resistor array with 31 resistance units. There are tap points for sliding units to access at the endpoints between each cell. The position of the sliding unit is controlled by the three input ports of CS.U/D and INC, and the position of the sliding end is stored in a non-volatile memory, which can be reinvoked at the next power up, and its function is shown in figure 3.

　　The output voltage range of switching power supply is determined by VL,VH. From the calculation formula of output voltage in Fig. 2, it can be seen that the consistency of output voltage of power supply can be ensured as long as the consistency of VL,VH is guaranteed. Assuming that the voltage adjustment range is 4.5V ~ 5.2V, the output voltage deviation is ±20mV, which can fully meet the power supply requirements of LED. In order to improve the anti-interference performance, the digital signal entering the switching power supply is optoelectronic isolated. The signal generator of NC switching power supply is a signal processing and generating system composed of single chip computer, as shown in figure 4. It stores the output voltage values at different temperatures to compensate for the LED temperature drift and the output voltage values determined by the brightness of the LED display screen under different illumination conditions. CNC switching power supply has three control modes: automatic, manual and program.

　　The main contents are as follows: 1) temperature sensor and illumination sensor are automatically controlled to collect the temperature and intensity of illumination in the field and sent to the signal generator for data processing. Corresponding to the control signal generated by the exact position in the 32 file of the NC potentiometer, the signal driver is amplified separately to meet the needs of signal transmission of m × n switching power supply. After receiving the digital control signal, the output voltage is adjusted and then sent to the LED display. 2) manually controlling the keyboard of the signal generator and inputting a certain value. After the signal generator is processed, the digital control signal is generated, which is amplified by the driver and sent to each NC switching power supply, and the output voltage is sent to the LED display screen. 3) the program control is sent to the signal generator by the broadcasting machine through the serial interface. After processing, the control signal is generated, amplified by the driver, and sent to each NC switching power supply, and the output voltage is sent to the LED display screen. The special switching power supply for outdoor full-color LED display has been widely used in many outdoor full-color screens. The use is satisfactory. With the continuous development of outdoor full-color LED display technology, switching power supply needs to be further developed to meet all kinds of requirements of full-color screen.