It is difficult to solve the interference problem of LED driving power supply. The main reason is that the influence caused by interference is often unpredictable and unpredictable.

First, the cause of interference caused by LED drive power supply.

The LED driving power supply first converts the power frequency AC into DC, then converts it into high frequency, and finally outputs it through rectifying and filtering circuit to get stable DC voltage, so it contains a lot of harmonic interference. At the same time, electromagnetic interference (EMI) is formed due to the spikes caused by the leakage inductance of the transformer and the reverse recovery current of the output diode. Disturbance sources in switching power supply mainly focus on the components with large voltage and current changes, especially on switching transistors, diodes and high-frequency transformers.

Electromagnetic interference produced by switching circuits

Switching circuit is one of the main interference sources of switching power supply. Switching circuit is the core of switching power supply (the same as LED street lamp power supply and LED tunnel lamp driving power supply), mainly composed of switching tube and high-frequency transformer. The du/dt generated by the system has a large pulse, wide frequency band and abundant harmonic. The main reason for this kind of pulse interference is that the switch tube load is the primary coil of the high frequency transformer and the inductive load. At the turn-on of the switch, the primary coil generates a large inrush current and a high surge peak voltage appears at both ends of the primary coil; at the turn-off of the switch, part of the energy is not transferred from the primary coil to the secondary coil due to the leakage flux of the primary coil, which will be stored in the inductor and collected. Capacitance and resistance in electrode circuit form attenuation oscillation with spike, which is superimposed on switching off voltage to form switching off voltage spike. Power supply voltage interruption will produce the same magnetization impulse current transient as the primary coil when it is switched on. This transient is a kind of conductive electromagnetic interference, which not only affects the primary transformer, but also makes the conductive interference return to the distribution system, resulting in harmonic electromagnetic interference in the power grid, thus affecting the safety and economic operation of other equipment.

Electromagnetic interference generated by rectifier circuit

In the rectifier circuit, there is a reverse current when the output rectifier diode is cut off. The time to return to zero is related to the junction capacitance and other factors. Among them, the diode which can quickly recover the reverse current to zero is called hard recovery diode. This diode will produce strong high frequency interference under the influence of transformer leakage inductance and other distributed 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 is on. When it is turned off by the reverse bias voltage, the reverse recovery current will decrease sharply due to the accumulation of more carriers in the PN junction. Small and great current changes.

High frequency transformer

High frequency switching current loop composed of primary coil, switching tube and filter capacitor of high frequency transformer may produce large space radiation and form radiation interference. If the capacitor 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 transmitted to the AC power supply in a differential mode to form conduction interference. It should be noted that the di/dt of the reverse recovery current of the rectifier diode is much larger than that of the reverse recovery current of the rectifier diode in the EMI generated by the diode rectifier circuit. As an electromagnetic interference source, the reverse recovery current of rectifier diode has a large interference intensity and bandwidth. However, the voltage jump produced by the rectifier diode is much smaller than that produced when the power switch is turned on and off. Therefore, the rectifier circuit can also be studied as part of the EMI coupling channel, regardless of the / dt6550

Interference caused by distributed capacitance

Switching power supply operates at high frequency, so its distributed capacitance can not be ignored. On the one hand, the contact area between the radiator and the collector of the switch tube is large, and the insulator is thin, so the distributed capacitance between the two can not be ignored at high frequency. The high-frequency current flows to the radiator through the distributed capacitor and then to the casing, resulting in common-mode interference. On the other hand, there is a distributed capacitor between the primary and secondary stages of the pulse transformer, which can directly couple the primary voltage to the secondary side and produce common-mode interference on the two power lines with DC output at the secondary side.

The influence of stray parameters on coupled channels

In the conducted interference frequency band (< 30MHz), most of the coupling channels of switching power supply interference can be described by circuit network. However, any practical components of switching power supply, such as resistors, capacitors, inductors and even switches and diodes, contain stray parameters. The wider the frequency band, the higher the order of equivalent circuit. Therefore, the equivalent circuit of the switching power supply, including the stray parameters of the components and the coupling between the components, will be much more complex. At high frequencies, the stray parameters have a great influence on the characteristics of the coupling channel, and the distributed capacitance becomes the channel of electromagnetic interference. In addition, when the power of the switch tube is large, the collector usually needs to add a heat sink. The distributed capacitance between the heat sink and the switch tube can not be ignored at high frequency. It can form space-oriented radiation interference and common mode interference of the power line conduction.

Two, electromagnetic interference control technology of switching power supply

To solve the problem of electromagnetic interference in switching power supply, we can start from three aspects: 1) reducing the interference signal generated by the interference source; 2) cutting off the propagation path of the interference signal; 3) enhancing the anti-interference ability of the interfered body. Therefore, the electromagnetic interference control technology of switching power supply mainly includes circuit measures, EMI filtering, component selection, shielding and anti-interference design of PCB.

(1) reduce the interference of switching power supply itself.

Soft-switching technology: Increasing inductance and capacitance components in the original hard-switching circuit, reducing du/dt and di/dt in the switching process by using the resonance of inductance and capacitance, making the voltage drop before the current rise when switching on, or the current drop before the voltage rise when switching off, eliminating the overlap between voltage and current .

Switching frequency modulation technology: by modulating switching frequency fc, it focuses on FC and its harmonic 2FC, 3fc... The energy above is spread to the frequency band around them to reduce the EMI amplitude at each frequency point. This method can not reduce the total interference, but the energy is dispersed to the baseband of the frequency point, so that each frequency point does not exceed the EMI limit. In order to reduce the peak value of noise spectrum, there are usually two methods: random frequency method and modulation frequency method.

Selection of components: components which are not easy to generate noise, difficult to transmit and radiate noise. What is particularly noteworthy is the selection of winding components such as diodes and transformers. Fast recovery diode with small reverse recovery current and short recovery time is an ideal device for high frequency rectification of switching power supply.

Common Mode Interference Active Suppression Technique: A compensated EMI noise voltage which is completely inverse to the main switching voltage waveform causing electromagnetic interference is extracted from the main circuit and used to balance the original switching voltage.

Filtering: One of the main purposes of EMI filters is to achieve high insertion loss in the frequency range of 150 kHz to 30 MHz, but it does not attenuate the 50 Hz power frequency signal, so that the rated voltage and current pass smoothly, and at the same time must meet certain size requirements. The conduction interference signal on any power line can be represented by differential mode and common mode signal. In general, the amplitude of differential mode interference is small, the frequency is low, resulting in less interference; Common mode interference amplitude is large, high frequency, but also can produce radiation through the wire, resulting in greater interference. Therefore, in order to weaken the conducted interference, the EMI signal is controlled below the limit level stipulated in the relevant EMC standards. The most effective method is to install electromagnetic interference filter in the input and output circuits of switching power supply.

PCB design: PCB anti-interference design mainly includes PCB layout, wiring and grounding, the purpose of which is to reduce the electromagnetic radiation of PCB and the crosstalk between PCB circuit. The best way of switching power layout is similar to its electrical design. After determining the size and shape of PCB, the position of special components (such as generators, crystal oscillators, etc.) is determined. Finally, according to the functional unit of the circuit, the layout of all the components of the circuit is carried out.

A buffer circuit for reducing electromagnetic interference consists of a linear impedance stabilization network, which eliminates potential interference in the power line, including power line interference, fast transient, surge, voltage variations and power line harmonics. These disturbances have little effect on the general regulated power supply, but have significant effect on the high-frequency switching power supply.

The way to cut off the interference signal -- the design of common mode and differential mode power line filters

Power line interference can be filtered by power line filter. A reasonable and effective switching power supply EMI filter should have a strong suppression of differential mode and common mode interference on the power line.

Third, enhance the anti-interference ability of sensitive circuits.