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For engineers designing LED power supplies, electromagnetic interference problems should be a key issue that has always existed in design. However, those who are familiar with the design of power supply circuits know that electromagnetic interference EMI is a big problem in the design process of LED power supply. How can we solve this problem?
First, let's look at several factors that can affect EMI/EMC: the circuit structure of the drive power; switching frequency, grounding, PCB design, reset circuit design of intelligent LED power supply. Since the original LED power supply is a linear power supply, the linear power supply consumes a large amount of energy in the form of heat when it is operating. The linear power supply works in such a way that he must have a pressure device from high voltage to low voltage, which is generally a transformer, and then rectified to output DC voltage. Although it is bulky and has a large amount of heat, the advantage is that the external interference is small, the electromagnetic interference is small, and it is easy to solve. Nowadays, more LED switching power supplies are used, and the LED driving power supply in the form of PWM is to make the power transistor work in the on and off states. When turned on, the voltage is low and the current is large; when turned off, the voltage is high and the current is small, so the loss generated on the power semiconductor device is also small. The disadvantage is that electromagnetic interference (EMI) is also more serious.
The problem of electromagnetic compatibility of the LED power supply is generally in the power supply of the switching circuit. The switching circuit is one of the main sources of interference for the switching power supply. The switching circuit is the core of the LED driving power supply, and the switching circuit is mainly composed of a switching tube and a high frequency transformer. It produces a du/dt with a large amplitude pulse, a wide frequency band and a rich harmonic. The main reason for this high-frequency pulse interference is that the switching tube load is the primary coil of the high-frequency transformer and is an inductive load. At the instant of conduction, the primary coil generates a large inrush current, and a high surge spike voltage appears at both ends of the primary coil; at the moment of disconnection, due to the leakage flux of the primary coil, part of the energy is not transmitted from the primary coil to the second In the secondary coil, a decaying oscillation with a spike is formed in the circuit, superimposed on the turn-off voltage to form a turn-off voltage spike. High frequency pulses produce more emissions and periodic signals produce more emissions. In an LED power system, the switching circuit produces a current spike, which also produces a current spike when the load current changes. This is one of the root causes of electromagnetic interference.
Basically in all the problems of electromagnetic interference problems, mainly due to improper grounding. There are three methods of signal grounding: single point, multiple points, and hybrid. When the switching circuit frequency is lower than 1MHz, a single-point grounding method can be used, but it is not suitable for high frequency; in high-frequency applications, multi-point grounding is preferred. Hybrid grounding is a method in which the low frequency is grounded at a single point and the high frequency is grounded at multiple points. The ground layout is the key, and the grounding circuit of the high-frequency digital circuit and the low-level analog circuit cannot be mixed. It can be said that proper printed circuit board (PCB) wiring is critical to preventing EMI. In the LED power supply, there are many intelligent LED power supplies that are controlled by single-chip microcomputers, and some LED power supplies use a single-chip microcomputer to control the duty cycle of the switching circuit. The watchdog system of the single-chip microcomputer plays a particularly important role in the operation of the entire LED power supply, due to All sources of interference cannot be isolated or removed. Once the CPU interferes with the normal operation of the program, the reset system combined with the software processing becomes an effective barrier to correct the defense. There are two commonly used reset systems: 1 external reset system. The external "watchdog" circuit can be designed by itself or by a special "watchdog" chip. Thus, if the program system is stuck in an infinite loop, and the loop happens to have a "feed dog" signal, then the reset circuit cannot achieve its proper function. 2 Now more and more LED power supplies have their own on-chip reset system, so users can easily use their internal reset timer, but some intelligent LED power supply control circuit reset instructions are too simple, so There will also be a "feeding dog" command like the above-mentioned infinite loop, which will cause it to lose its monitoring role.
To solve the electromagnetic interference problem of the LED driving power supply, the following aspects can be started from the hardware:
1. Reduce the interference of the switching power supply itself: soft switching technology, adding inductance and capacitance components in the original hard switching circuit, using the resonance of the inductor and capacitor to reduce the du/dt and di/dt in the switching process, so that the switching device is turned on. The voltage drop is prior to the rise of the current, or the fall of the current before the turn-off precedes the rise of the voltage to eliminate the overlap of the voltage and current. The switching frequency modulation technique disperses the energy concentrated on fc and its harmonics 2fc, 3fc... into the frequency bands around them by modulating the switching frequency fc to reduce the EMI amplitude at each frequency point. The choice of components, select components that are not easy to produce noise, difficult to conduct and radiate noise. Of particular note is the selection of winding components such as diodes and transformers. The fast recovery diode with low reverse recovery current and short recovery time is an ideal device for the high-frequency rectification part of the switching power supply. Reasonable use of electromagnetic interference filter, one of the main purposes of EMI filter, grid noise is a kind of electromagnetic interference, it belongs to radio frequency interference (RFI), its conducted noise spectrum is roughly 10KHz~30MHz, up to 150MHz. Under normal circumstances, the differential mode interference amplitude is small, the frequency is low, and the interference caused by the difference is small; the common mode interference amplitude is large, the frequency is high, and the radiation can be generated by the wire, and the interference caused by the interference is large. The most effective way to reduce conducted interference is to install an electromagnetic interference filter in the switching power supply input and output circuits. LED power supplies generally use a simple single-stage EMI filter, which mainly includes a common mode choke and a filter capacitor. The EMI filter effectively suppresses electromagnetic interference from the switching power supply adapter.
2. Reduce the electromagnetic interference problem by cutting off the propagation path of the interference signal: In the first case, the power line interference can be filtered out using the power line filter. A reasonable and effective switching power supply EMI filter should have strong suppression of differential mode and common mode interference on the power line. Improving Electromagnetic Compatibility of PCB Boards Design PCBs are the support for circuit components and devices in LED power systems that provide electrical connections between circuit components and devices. With the rapid development of electronic technology, the density of PCBs is getting higher and higher. The quality of the PCB design has a great impact on the electromagnetic compatibility of the LED power system. Practice has confirmed that even if the schematic design of the circuit is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the LED power system. The PCB anti-interference design mainly includes PCB layout, wiring and grounding, and its purpose is to reduce the electromagnetic interference between the PCB and the crosstalk between the circuits on the PCB. In addition, the frequency of the ac vibration caused by the electromagnetic interference of the general transformer is generally about 50 Hz, and the humming sound caused by the improper wiring of the grounding wire is discernible because the frequency of the frequency doubling of the rectifier circuit is about 100 Hz. Therefore, when designing a printed circuit board, you should pay attention to the correct method, follow the general principles of PCB design, and should meet the anti-interference design requirements.
3. Actively greatly enhance the anti-jamming capability of the interfered body: In the LED power system, the input/output is also the transmission line of the interference source, and the source of the receiving RF interference signal. We generally take effective measures when designing: The necessary common mode/differential mode suppression circuit must also take certain filtering and anti-electromagnetic shielding measures to reduce the interference. Whenever possible, take various isolation measures (such as optical isolation or magnetoelectric isolation) to block the spread of interference. Lightning protection measures, outdoor use of the LED power system or the introduction of indoor power lines, signal lines from the outside, to consider the system's lightning protection. Commonly used lightning protection devices are: gas discharge tube, TVS (Transient Voltage Suppression) and the like. The gas discharge tube is a gas breakdown discharge when the voltage of the power source is greater than a certain value, usually tens of V or hundreds of V, and the strong shock pulse on the power line is guided into the earth. The TVS can be viewed as two parallel and opposite-direction Zener diodes that conduct when the voltage across the terminals is above a certain value. It is characterized by the ability to transiently pass hundreds of thousands of currents.
Through this paper, we can summarize the main control technologies for LED power supply EMC/EMI: circuit measures, EMI filtering, component selection, shielding and anti-interference design of printed circuit boards. If you can solve these problems correctly and reasonably, it is not a problem to successfully pass the 3C certification!
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