Switching power supply solution for outdoor LED display

1 Introduction

Since the characteristics of optoelectronic devices in LED outdoor full-color display screens are different from those of general display screens, the cost is very expensive and there are high requirements for the performance and quality of the switching power supply. Therefore, this paper develops a special switching power supply for outdoor full-color screens.

2 Characteristics of outdoor LED display switching power supply

The power supply used for LED outdoor full-color display screen should have the following characteristics.

1) Keep the LED pixel driven by constant current under certain conditions. The voltage change of the LED group caused by the load change of the display screen will affect the pixel current and cause the brightness change. In order to ensure the uniform brightness of the entire screen module and reduce the brightness change, the sampling point of the switching power supply output sampling circuit is moved from the power port to a certain point in the display screen so that the voltage at 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; when the load is large, the output voltage is high. Therefore, it is ensured that the voltage output to the LED module remains unchanged and the pixel drive current is constant when the temperature and brightness remain unchanged.

2) Compensate for the change in LED forward voltage drop VF caused by ambient temperature. The forward voltage drop VF of LED decreases as the ambient temperature rises. The temperature change coefficients of various LEDs are different. Within the normal operating temperature range, the change in VF can reach 20%. Under constant voltage drive, the decrease 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 increasingly 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 drive current does not increase with the increase in temperature.

A simple temperature compensation method is to connect a negative temperature coefficient thermistor whose temperature change rate is close to 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 decreases, the output voltage decreases, and the increase of the LED working current is suppressed. Due to the large discreteness of thermistors, they need to be selected to ensure the consistency of the product. In addition, there is an independent resistor combination 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 curves of various LEDs, the drive voltage to be provided to the LED at different temperatures is tabulated and stored in the single-chip microcomputer for query. According to the temperature measured by the temperature sensor single-chip microcomputer, the corresponding output voltage of the digitally controlled switching power supply can be obtained by looking up the table.

3) Control the brightness of the display screen 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 is required to adjust the brightness of the full-color screen by adjusting the output voltage. Due to the large changes in outdoor light, it is necessary to make a table of the voltage values ​​corresponding to the brightness of the full-color screen in advance according to different climates and time periods and store them in the single-chip computer. According to the signal sent by the light sensor, the table is looked up to obtain the output voltage value of the CNC switching power supply to control the brightness of the display screen.

3 Working principle of universal switching power supply

The working principle of the universal switching power supply is shown in Figure 1. The 3525 pulse width modulation (PWM) component is used as the control circuit of the switching power supply, which is complete in function and easy to use. The error amplifier amplifies the error signal at the output end of the power supply, and sends it to the reverse input end of the comparator through the sampling circuit to compare with the ramp voltage output from the oscillator in the reference circuit. The square wave pulse output by the comparator is used to control the duty cycle of the output power tube inside the 3525. Different voltage values ​​can be obtained by changing the reference voltage and the output sampling circuit.

4 Working principle of CNC switching power supply

The digital control switching power supply is a switching power supply whose output voltage is controlled by a digital signal. Its working principle is shown in Figure 2.

The X9313 non-volatile digitally controlled potentiometer is selected as the controllable device for the PWM chip to compare the reference voltage. It has low power consumption, is easy to use, and has good reliability. The X9313 is a resistor array containing 31 resistor units. There are tap points for the sliding unit to access at the endpoints between each unit. The position of the sliding unit is controlled by the three input terminals CS.U/D and INC. The position of the sliding end is stored in a non-volatile memory and can be recalled when the power is turned on next time. Its function is shown in Figure 3.

The output voltage range of the switching power supply is determined by VL and VH. From the calculation formula of the output voltage in Figure 2, it can be seen that as long as the consistency of VL and VH is guaranteed, the consistency of the power supply output voltage can be guaranteed. Assuming that the voltage adjustment range is 4.5V to 5.2V, the output voltage deviation is ±20mV, which can fully meet the power supply requirements of the LED. In order to improve the anti-interference performance, the digital signal entering the switching power supply is optically isolated.

5 Application of CNC switching power supply

The signal generator of the digital control switching power supply is a signal processing system composed of a single chip microcomputer, as shown in Figure 4. It stores the output voltage value to compensate for the LED temperature drift at different temperatures, as well as the output voltage value determined by the brightness of the LED display under different lighting conditions. The digital control switching power supply has three control modes: automatic, manual and program.

1) Automatic control

The temperature sensor and light sensor collect the temperature and light intensity of the scene regularly and send them to the signal generator for data processing. The control signal corresponding to the exact position of the 32 gears of the digital control potentiometer is amplified by the signal driver to meet the signal transmission requirements of m×n switching power supplies. After receiving the digital control signal, the digital control switching power supply adjusts the output voltage and then sends it to the LED display.

2) Manual control

Use the keyboard of the signal generator to input a certain value. After being processed by the signal generator, a digital control signal is generated, amplified by the driver, and sent to each CNC switch power supply, which outputs different voltages to the LED display.

3) Program control

The broadcaster sends instructions through the serial interface to the signal generator, which generates control signals after processing, amplifies them by the driver, and sends them to each digital control switch power supply, which outputs different voltages to the LED display.

6 Conclusion

The dedicated switching power supply for outdoor full-color LED display has been widely used in many outdoor full-color screens.