The interference problem of LED driver power supply is not easy to solve, the main reason is that the impact of interference is often impossible to prevent, and difficult to predict, in the face of different sizes of interference how to do this problem well, you need to find the root cause of the interference problem.

1, LED driver power interference causes
  LED drive power supply first power frequency AC rectifier to DC, then inverter to high frequency, and finally through the rectifier filter circuit output, to obtain a stable DC voltage, so it contains a lot of harmonic interference. At the same time, due to the leakage inductance of the transformer and the peak caused by the reverse recovery current of the output diode, electromagnetic interference is formed. The interference source in the switching power supply is mainly concentrated in the components with large voltage and current changes, especially in the switching tubes, diodes, and high-frequency transformers.
  ① Electromagnetic interference generated by switching circuits
  Switching circuit is one of the main interference sources of switching power supply. The switching circuit is the core of the switching power supply (the same is true of the LED street light power supply and the LED tunnel light driver power supply), which is mainly composed of switching tubes and high-frequency transformers. The du/dt generated by it has a large pulse, a wide frequency band and rich harmonics. The main reason for this pulse interference is that the switching tube load is the primary coil of the high-frequency transformer, which is the inductive load. When the switching tube is on, the primary coil produces a large surge current, and a high surge peak voltage appears at both ends of the primary coil. At the moment when the switching tube is disconnected, due to the leakage flux of the primary coil, a part of the energy is not transmitted from the primary coil to the secondary coil, and this part of the energy stored in the inductor will form attenuation oscillation with a peak and the capacitor and resistance in the collector circuit, superimposed on the turn-off voltage, forming a turn-off voltage peak. Power supply voltage interruption will produce the same magnetization impulse current transient as when the primary coil is switched on, this transient is a conducted electromagnetic interference, both affecting the transformer primary, but also make the conducted interference back to the distribution system, causing harmonic electromagnetic interference in the power grid, thus affecting the safe and economic operation of other equipment.
  ② Electromagnetic interference caused by rectifier circuit
  In the rectifier circuit, there is a reverse current at the cut-off of the output rectifier diode, and the time of its recovery to zero is related to the junction capacitance and other factors. Among them, the diode that can quickly restore the reverse current to zero is called the hard recovery characteristic diode, which will produce strong high-frequency interference under the influence of the transformer leakage induction and other distribution parameters, and its frequency can reach tens of MHz. The rectifier diode in the high frequency rectifier circuit has a large forward current when it passes through the positive pilot. When it is turned off by the reverse bias voltage, due to the accumulation of more carriers in the PN junction, the current will reverse flow for a period of time before the carrier disappears, resulting in a sharp reduction of the reverse recovery current of the carrier disappearance and a great current change.
  ③ High frequency transformer
  The high frequency switching current loop composed of the primary coil, switching tube and filter capacitor of high frequency transformer may produce large space radiation and form radiation interference. If the capacitance filter capacity is insufficient or the high frequency characteristics are not good, the high frequency impedance on the capacitor will cause the high frequency current to be conducted to the AC power supply in a differential mode to form conductive interference. It should be noted that in the electromagnetic interference generated by the diode rectifier circuit, the di/dt of the reverse recovery current of the rectifier diode is much larger than the di/dt of the reverse recovery current of the continuous diode. As a source of electromagnetic interference, the rectifier diode has high interference intensity and wide frequency band. However, the voltage jump generated by the rectifier diode is much smaller than the voltage jump generated when the power switch tube is turned on and off. Therefore, the rectifier circuit can be studied as a part of the electromagnetic interference coupling channel without considering the │dv/dt│ effect produced by the rectifier diode.
  ④ Interference caused by distributed capacitance
  The switching power supply works at high frequency, so its distributed capacitance cannot be ignored. On the one hand, the contact area of the insulation sheet between the heat sink and the collector of the switch tube is large, and the insulation sheet is thin, so the distributed capacitance between the two cannot be ignored at high frequencies. The high frequency current will flow to the heat sink through the distributed capacitor, and then flow to the chassis, resulting in common mode interference; On the other hand, there are distributed capacitors between the primary stages of the pulse transformer, which can directly couple the primary voltage to the secondary side, resulting in common mode interference on the two power lines with the secondary side as DC output.
  ⑤Stray parameters affect the characteristics of the coupling channel
  In the conducted interference band (<30MHz), most of the coupling channels of switching power supply interference can be described by circuit networks. However, any of the actual components in the switching power supply, such as resistors, capacitors, inductors and even switching tubes and diodes, contain stray parameters, and the wider the frequency band studied, the higher the order of the equivalent circuit. Therefore, the equivalent circuit of the switching power supply including the stray parameters of each component and the coupling between components will be much more complex. At high frequencies, stray parameters have a great influence on the characteristics of the coupled channel, and the presence of distributed capacitance becomes the channel of electromagnetic interference. In addition, when the power of the switching tube is large, the collector generally needs to add a heat sink, and the distributed capacitance between the heat sink and the switching tube can not be ignored at high frequencies, which can form space-oriented radiation interference and common mode interference conducted by the power line.

2, switching power supply electromagnetic interference control technology
  To solve the electromagnetic interference problem of switching power supply, you can start from three aspects: 1) reduce the interference signal generated by the interference source; 2) Cut off the transmission path of interference signals; 3) Enhance the anti-interference ability of the disturbed body. Therefore, the electromagnetic interference control technology of switching power supply mainly includes: circuit measures, EMI filtering, component selection, shielding and printed circuit board anti-interference design。
  ① Reduce the interference of the switching power supply itself
  Soft switching technology: Adding inductors and capacitors in the original hard switching circuit, using the resonance of inductors and capacitors 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 it is turned off, to eliminate the overlap of voltage and current.
    Switching frequency modulation technology: by modulating the switching frequency fc, the focus on fc and its harmonics 2fc, 3fc... The energy on the above is dispersed to the frequency bands around them to reduce the EMI amplitude at each frequency. This method does not reduce the total amount of interference, but the energy is dispersed over the baseband of the frequency points, so that each frequency point does not exceed the limit specified by EMI. In order to achieve the purpose of reducing the noise spectrum peak, there are usually two processing methods: random frequency method and modulated frequency method. 
    Selection of components: Select components that are not easy to produce noise, conduction and radiation noise. Usually 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 high frequency rectifier of switching power supply.
    Active suppression of common-mode interference: try to take a compensated EMI noise voltage from the main circuit which is completely inverting the main switching voltage waveform causing electromagnetic interference, and use it to balance the original switching voltage.
    Filter: One of the main purposes of the EMI filter is to obtain a high insertion loss in the frequency range of 150kHz ~ 30MHz, but the frequency of 50Hz power frequency signal does not produce attenuation, so that the rated voltage and current pass smoothly, and must also meet certain size requirements. The conducted interference signal on any power line can be represented by differential mode and common mode signals. In general, the difference mode interference amplitude is small, the frequency is low, and the interference caused is small. The common mode interference amplitude is large, the frequency is high, and the radiation can also be generated through the wire, which causes greater interference. Therefore, in order to weaken the conducted interference and control the EMI signal below the limit level specified in the relevant EMC standards, the most effective way is to install an electromagnetic interference filter in the input and output circuits of the switching power supply.
    PCB design: PCB anti-interference design mainly includes PCB layout, wiring and grounding, the purpose is to reduce the electromagnetic radiation of the PCB and the crosstalk between the circuit on the PCB. The best way to layout a switching power supply is similar to its electrical design. After determining the size and shape of the PCB, the position of special components (such as various generators, crystal oscillators, etc.) is determined. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.
    Buffer circuit to reduce electromagnetic interference: It consists of a linear impedance stable network, which is used to eliminate potential interference in the supply power line, including power line interference, electrical fast transients, surges, voltage high and low changes and power line harmonics. These disturbances are not very significant for the general regulated power supply, but they have a significant impact on the high-frequency switching power supply.
    ② Cut off the transmission path of interference signals - common mode, differential mode power line filter design
    Power line interference can be removed using a power line filter. A reasonable and effective switching power supply EMI filter should have strong suppression effect on both differential mode and common mode interference on the power line.
    ③ Enhance the anti-interference ability of sensitive circuits