Some methods to solve emcemi problem of LED power

2022-08-03
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Several methods to solve the emc/emi problem of LED power supply. However, those who are familiar with power circuit design know that electromagnetic interference EMI is a big problem in the design process of LED power supply, so how can we solve this problem

first, let's take a look at several factors that can affect emi/emc: the circuit structure of the driving power supply; Switching frequency, grounding, PCB design, reset circuit design of intelligent LED power supply. Because the initial LED power supply is a linear power supply, but the linear power supply will lose a lot of energy in the form of heating. The working mode of linear power supply makes it necessary to have a voltage converter from high voltage to low voltage. Generally, it is a transformer, and then output DC voltage through rectification. Although it is bulky and has a large calorific value, it has the advantage of small external interference and electromagnetic interference, and is easy to solve. However, most led switching power supplies are used now, and the LED driving power supply in the form of PWM makes the power transistor work in the on and off state. When conducting, the voltage is low and the current is large; When turning off, the voltage is high and the current is small, so the loss on the power semiconductor device is very small. The obvious disadvantage is that electromagnetic interference (EMI) is also more serious

led power supply electromagnetic compatibility problems usually occur in the power supply of the switching circuit. Switching circuit is one of the main interference sources of switching power supply. The switch circuit is the core of LED driving power supply. The switch circuit is mainly composed of switch tube and high-frequency transformer. The du/dt produced by this method has large amplitude pulse, wide frequency band and rich harmonics. The main reason for the high-frequency pulse interference is that the switch load is the primary coil of the high-frequency transformer, which is an inductive load. At the moment of conduction, the primary coil produces a large inrush current, and a high surge peak voltage appears at both ends of the primary coil; At the moment of disconnection, due to the magnetic flux leakage of the primary coil, part of the energy is not transmitted from the primary coil to the secondary coil, and the attenuation oscillation with spikes is formed in the circuit. By 2025, the high-end products will be basically self-sufficient in the turn-off voltage, forming the turn-off voltage spike. High frequency pulses produce more emissions, and periodic signals produce more emissions. In the LED power supply system, the switch circuit generates the current spike signal, and when the load current changes, it also generates the current spike signal. This is one of the sources of electromagnetic interference

basically, among all the problems of electromagnetic interference, it is mainly caused by improper grounding. There are three signal grounding methods: single point, multi-point and mixed. When the switching circuit frequency is lower than 1MHz, the single point grounding method can be adopted, but it is not suitable for high frequency; In high frequency applications, multi-point grounding is preferred. Hybrid grounding is a method of single point grounding for low frequency and multi-point grounding for high frequency. Ground wire layout is the key, and the grounding circuits of high-frequency digital circuit and low-level analog circuit shall not be mixed. It can be said that proper printed circuit board (PCB) wiring is very important to prevent EMI. Among the LED power supplies, many intelligent LED power supplies are controlled by single-chip microcomputer, and some LED power supplies are controlled by single-chip microcomputer to control the duty cycle of switch circuit. The watchdog system of single-chip microcomputer plays a particularly important role in the operation of the entire LED power supply. Since all interference sources cannot be isolated or removed, once the CPU interferes with the normal operation of the program, Then the reset system combined with software processing measures becomes an effective error correction defense barrier. There are two common reset systems: ① external reset system. The external "watchdog" circuit can be designed or built with a special "watchdog" chip. In this way, if the program system falls into an dead cycle, and there happens to be a "dog feeding" signal in the cycle, the reset circuit will not be able to achieve its due function. ② Now more and more LED power supplies are equipped with their own on-chip reset system, so that users can easily use their internal reset timer. However, the reset command of some intelligent LED power supply control circuits is too simple. In this way, there will also be "dog feeding" commands like the above-mentioned dead cycle, which will make them lose their monitoring role

to solve the electromagnetic interference problem of LED driving power supply, we can start from the following aspects in terms of hardware:

1 Reduce the interference of switching power supply itself: soft switching technology adds inductance and capacitance elements to the original hard switching circuit, and uses the resonance of inductance and capacitance to reduce du/dt and di/dt in the switching process, so that the voltage drops before the current rises when the switching device is turned on, or the current drops before the voltage rises when the switching device is turned off, so as to eliminate the overlap of voltage and current. Switching frequency modulation technology, by modulating the switching frequency fc, disperses the energy concentrated on FC and its harmonics 2FC, 3fc... To the frequency band around them, so as to reduce the EMI amplitude at each frequency point. Select components and parts that are not easy to generate noise, conduct and radiate noise. It is usually worth paying special attention to the selection of winding components such as diodes and transformers. The fast recovery diode with small reverse recovery current and short recovery time is an ideal device for the high frequency rectifier of switching power supply. Reasonable use of EMI filter is one of the main purposes of EMI filter. Electrical noise is a kind of electromagnetic interference that gradually replaces mechanical testing machine. It belongs to radio frequency interference (RFI), and its conducted noise spectrum is about 10khz-30mhz, up to 150mhz In general, the amplitude of differential mode interference is small, the frequency is low, and the interference caused is small; Common mode interference has large amplitude and high frequency. It can also generate radiation through wires, causing large interference. To weaken conducted interference, the most effective way is to install electromagnetic interference filters in the input and output circuits of switching power supply. LED power supply generally adopts simple single-stage EMI filter, mainly including common mode choke and filter capacitor. EMI filter can effectively suppress the electromagnetic interference of switching power supply adapter

2. Reduce electromagnetic interference by cutting off the propagation path of interference signal: the first case is that power line interference can be filtered by power line filter. A reasonable and effective switching power supply EMI filter should have a strong suppression effect on both differential mode and common mode interference on the power line. Improve the EMC design of PCB PCB PCB is the support of circuit components and devices in LED power supply system. It provides the electrical connection between circuit components and devices. With the rapid development of electronic technology, the density of PCB is getting higher and higher. The quality of PCB design has a great impact on the electromagnetic compatibility of LED power supply system. real "The design team has realized that even if the circuit schematic design is correct and the PCB design is due to the fact that more and more aircraft components begin to use advanced polymer composite materials, the reliability of the LED power supply system will also be adversely affected. The PCB anti-interference design mainly includes PCB layout, wiring and grounding, which aims to reduce the electromagnetic radiation of the PCB and the crosstalk between the circuits on the PCB. In addition, the electromagnetic interference of the general transformer The AC sound frequency caused by disturbance is generally about 50Hz. However, the AC sound caused by improper ground wire wiring can be detected carefully because the frequency doubling frequency of the rectifier circuit is about 100Hz. Therefore, when designing printed circuit boards, attention should be paid to adopting correct methods, following the general principles of PCB design, and meeting the anti-interference design requirements

3. Actively and greatly enhance the anti-interference ability of the interfered body: in the LED power supply system, the input/output is also the transmission line of the interference source and the pick-up source of the RF interference signal. Generally, we should take effective measures in design: adopt necessary common mode/differential mode suppression circuit, and also take certain filtering and anti electromagnetic shielding measures to reduce the progress of interference. If conditions permit, various isolation measures (such as photoelectric isolation or magnetoelectric isolation) shall be taken as far as possible to block the propagation of interference. Lightning protection measures: for outdoor LED power supply system or indoor power line and signal line introduced from outdoor, lightning protection of the system shall be considered. Common lightning protection devices include: gas discharge tube, TVs (transient voltage suppression), etc. When the voltage of the power supply is greater than a certain value, usually tens of V or hundreds of V, the gas discharge tube breaks down and discharges, leading the strong impact pulse on the power line to the earth. TVs can be regarded as two zener diodes in parallel and in opposite directions. When the voltage at both ends is higher than a certain value, TVs will turn on. Its characteristic is that it can transiently pass hundreds or thousands of amperes

through this paper, we can summarize the main control technologies for emc/emi of LED power supply: circuit measures, EMI filtering, component selection, shielding and anti-interference design of printed circuit board. If we can solve these problems correctly and reasonably and successfully pass the 3C certification, it is not a problem

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