For the design of LED power supply engineers, electromagnetic interference problem should always exist in the design of a key issue. But familiar with the power supply circuit design friends know that the design process in the LED power supply, EMI EMI is not a small problem, so how can solve this problem?
First we look at several factors that can affect the EMI/EMC: the circuit structure of the driver power supply; switching frequency, grounding, PCB design, intelligent LED power supply reset circuit design. As the original LED power supply is a linear power supply, but the linear power supply will work in the form of heat loss of a lot of energy. The linear power supply works in such a way that he must have a voltage device from high voltage to low voltage, the general ones are transformers, and then after rectification output DC voltage. Although bulky, heat generation, the advantage is that the external interference is small, electromagnetic interference is small, but also easy to solve. And now use more LED switching power supply, are in the form of PWM LED drive power supply is to let the power transistor work in the on and off state. In the on, the voltage is low, the current is high; off, the voltage is high, the current is small, so the power semiconductor devices generated on the loss is also very small. The disadvantage is more obvious, electromagnetic interference (EMI) is also more serious.
LED power supply electromagnetic compatibility problems are generally switching circuit in the power supply. And the switching circuit is one of the main sources of interference in the switching power supply. Switching circuit is the core of the LED driver power supply, the switching circuit mainly consists of switching tubes and high-frequency transformers. It produces du / dt with a larger pulse, a wider band and rich harmonics. The main reason for this high-frequency pulse interference is: the switching tube load is the primary coil of the high-frequency transformer, which is an inductive load. On instant, the primary coil generates a large inrush current and a high surge spike voltage at both ends of the primary coil; off instant, due to the leakage flux of the primary coil, resulting in part of the energy not transferred from the primary coil to the secondary coil, the circuit forms a decay oscillation with spikes, superimposed on the turn-off voltage, forming a turn-off voltage spike. High frequency pulses produce more emissions and periodic signals produce more emissions. In LED power systems, the switching circuit generates current spike signals, and when the load current changes, it also generates current spike signals. This is one of the root causes of electromagnetic interference.
Basically, in all topics of electromagnetic interference problems, it is mainly caused by improper grounding. There are three signal grounding methods: single-point, multi-point and mixed. In the switching circuit frequency below 1MHz, single point grounding method can be used, but it is not suitable for high frequency; in high frequency applications, it is better to use multi-point grounding. Hybrid grounding is a single-point grounding method for low frequencies and a multi-point grounding method for high frequencies. Ground layout is the key, high-frequency digital circuits and low-level analog circuit grounding circuit as far as possible can not be mixed. It can be said that the appropriate printed circuit board (PCB) wiring to prevent EMI is critical. In the LED power supply, there are many intelligent LED power supply using a single-chip control, and some LED power supply using a single-chip control of the duty cycle of the switching circuit, the single-chip watchdog system plays a particularly important role in the operation of the entire LED power supply, as all sources of interference can not be isolated or to remove, once into the normal operation of the CPU interference program, then the reset system combined with software processing Measures have become an effective error correction defense barrier. Commonly used reset system has the following two kinds: ① external reset system. External “watchdog” circuit can be designed by themselves or can use a special “watchdog” chip to build. In this way, if the program system is trapped in a dead loop, and the loop happens to have a “feed dog” signal, then the reset circuit will not be able to achieve its proper function. ② Now more and more LED power supply with their own on-chip reset system, so that users can easily use its internal reset timer, but some intelligent LED power control circuit reset instructions are too simple, so there will be like the above dead loop “feed the dog” instructions, so that it is lost to monitor role.
To solve the problem of electromagnetic interference in the LED driver power supply, from the hardware can start from the following aspects:
1、Reduce the interference of the switching power supply itself:
soft-switching technology, in the original hard-switching circuit to add inductors and capacitive components, the use of inductors and capacitors resonance, reduce the switching process du / dt and di / dt, so that the switching device opens when the voltage drop before the current rise, or off when the current drop before the voltage rise, to eliminate the overlap of voltage and current. Switching frequency modulation technique, by modulating the switching frequency fc, the energy concentrated in fc and its harmonics 2fc, 3fc … on the frequency band around them to reduce the EMI amplitude at each frequency point. The choice of components, choose components that are not easy to generate noise, not easy to conduct and radiation noise. Usually particularly noteworthy is the choice of winding components such as diodes and transformers. Reverse recovery current is small, short recovery time of the fast recovery diode is the ideal device for high-frequency rectification part of the switching power supply. Reasonable use of electromagnetic interference filters, EMI filters, one of the main purposes of the grid noise is a kind of electromagnetic interference, it belongs to the radio frequency interference (RFI), the spectrum of its conducted noise is roughly 10KHz ~ 30MHz, up to 150MHz. In general, the differential mode interference amplitude is small, low frequency, the interference caused by the smaller; common mode interference amplitude is large, high frequency, but also Through the wire to generate radiation, resulting in greater interference. To weaken the conducted interference, the most effective way is to add electromagnetic interference filter in the switching power supply input and output circuit. LED power supply generally uses a simple single-stage EMI filter, including common mode choke and filter capacitors. EMI filter can effectively suppress the switching power adapter electromagnetic interference.
2、Reduce electromagnetic interference problems by cutting off the propagation path of interference signals: the first case is the power line interference can be filtered using the power line filter.
A reasonable and effective switching power supply EMI filter should have a strong suppression of both differential mode and common mode interference on the power line. Improve the EMC design of the PCB board PCB is the support of the circuit components and devices in the LED power supply system, which provides the electrical connection between the circuit components and devices. With the rapid development of electronic technology, the density of PCB is getting higher and higher. the good or bad design of PCB has a great impact on the EMC of LED power supply system. Practice has confirmed that even if the circuit schematic design is correct, improper design of the printed circuit board will have a negative impact on the reliability of the LED power system. PCB anti-interference design mainly includes PCB layout, wiring and grounding, the purpose of which is to reduce the electromagnetic radiation from the PCB and crosstalk between the circuits on the PCB. Also, the general transformer electromagnetic interference triggered by AC sound frequency is generally about 50HZ, and ground wiring improperly caused by AC sound, due to the frequency of the rectifier circuit multiplier effect of about 100HZ, careful distinction can still be detected. Therefore, in the design of printed circuit boards, attention should be paid to the use of the correct method to comply with the general principles of PCB design, and should meet the design requirements of anti-interference.
3、Active and substantially enhance the anti-interference ability of the interfered body:
in the LED power supply system into / output is also the source of interference in the transmission line, and receive RF interference signal pickup source, we design to generally take effective measures: the use of the necessary common mode / differential mode suppression circuit, but also to take certain filtering and anti-electromagnetic shielding measures to reduce the interference into. As far as possible under the conditions allowed to take a variety of isolation measures (such as photoelectric isolation or magnetic isolation), thereby blocking the spread of interference. Lightning protection measures, outdoor use of LED power system or from the outdoor crowded introduction of indoor power lines, signal lines, to consider the system’s lightning strike topics. Commonly used lightning protection devices are: gas discharge tube, TVS (Transient Voltage Suppression), etc.. Gas discharge tube is when the voltage of the power supply is greater than a certain value, usually tens of V or hundreds of V, gas breakdown discharge, the power line on the strong shock pulse into the earth. TVS can be seen as two parallel and opposite direction of the Zener diode, when the voltage at both ends is higher than a certain value on. It is characterized by the transient passage of hundreds of thousands of A currents.
Through this paper we can summarize the main control techniques for the LED power EMC/EMI are: circuit measures, EMI filtering, component selection, shielding and printed circuit board anti-interference design, etc.. If you can correctly and reasonably solve these problems, smoothly through the 3C certification, not a problem!
