The interference of LED driver is not a good solution, the main reason is the effects of interference is often impossible to guard against, and difficult to predict, in the face of all kinds of interference in the size of the how to do this, we need to find the root of the interference problem.

The cause of interference in a LED drive power supply

The LED driving power first rectifies the power frequency AC rectifier to DC, and then inverters it to high frequency. Finally, after rectification and filter circuit output, a stable DC voltage is generated, so it contains lots of harmonic interference. At the same time, due to the leakage of the transformer and the peak of the reverse recovery current of the output diode, electromagnetic interference is formed. The interference sources in the switching power supply mainly focus on the components of large voltage and current, which are shown on the switch tube, diode and high frequency transformer.

Electromagnetic interference (EMI) produced by switching circuits

Switching circuit is one of the main interference sources of switching power supply. The switch circuit is the core of the switching power supply (and the same LED street lamp power supply is the same as that of the LED tunnel lamp). It is mainly composed of the switch tube and the high frequency transformer. The du/dt produced by it has a large amplitude pulse, wide frequency band and rich harmonics. The main reason for this kind of pulse interference is that the load of the switch tube is the primary coil of the high frequency transformer, and it is the inductive load. In the turn-on instant, the primary coil produce large inrush current, and voltage surge peak high in both ends of the primary coil; when the switch is disconnected instantly, because the leakage flux of the primary coil, resulting in a part of the energy from a coil to the two coil transmission, storage in this part of the inductor energy and capacitance, the collector circuit resistance is formed with peak attenuation oscillation superimposed on the turn off voltage, turn off voltage spike formation. Interruption of supply voltage will produce impulse current and instantaneous magnetizing coil connected to the primary variable, this is a transient conduction type electromagnetic interference, not only affects the transformer primary, will make the conduction interference return distribution system harmonics caused by electromagnetic interference, thus affecting other equipment safety and economic operation.

The electromagnetic interference produced by the rectifier circuit

In the rectifier circuit, there is a reverse current at the end of the output rectifier diode, and the time it recovers to zero is related to the factors such as the junction capacitance. Among them, the diode that can restore the reverse current to zero quickly is called the hard recovery characteristic diode. The diode will generate strong high-frequency interference under the influence of transformer leakage and other distributed parameters, and its frequency can reach dozens of MHz. High frequency rectifier circuit of the rectifier diode turn-on when positive current flowing through the larger, in the reverse bias voltage and turn off, because there are more carriers in PN junction accumulation in carrier disappeared before a period of time, the current will flow, resulting in the disappearance of the reverse recovery current carrier decreased rapidly large current changes occur.

High frequency transformer

High frequency switching current loop composed of primary coil, switch tube and filter capacitor may generate larger space radiation and form radiation interference. If the capacitance filter capacity is insufficient or the high frequency characteristic is not good, the high frequency impedance on the capacitor will cause the high-frequency current to transmit to the AC power in differential mode, and form the conduction interference. It should be noted that in the electromagnetic interference produced by diode rectifier circuit, the di/dt of rectifier diode reverse recovery current is much larger than the di/dt of reverse current recovery of freewheeling diode. As a source of electromagnetic interference (EMI), the interference intensity of the rectifier diode reverse recovery current is large and the frequency band is wide. However, the voltage jump produced by the rectifier diode is far less than the voltage jump produced when the power switch tube is turned on and off. Therefore, dv/dt can also influence, regardless of the rectifier diode, the rectifier circuit as a part of the electromagnetic coupling interference channel to study.

Interference caused by distributed capacitance

The switching power supply operates at high frequency, so its distributed capacitance can not be ignored. On the one hand, the contact area between the fin and the collector of the switch tube is large, and the insulation piece is thin, so the distribution capacitance between them is not negligible at high frequency. High frequency current through the capacitor current distribution to the heat sink, and then flows into the shell, common mode interference; on the other hand, between the primary and secondary pulse transformer has distributed capacitance, the primary voltage can be directly coupled to the side, as the two power line in the DC output side on common mode interference.

The influence of the stray parameters on the characteristics of the coupling channel

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

Two, control technology of electromagnetic interference of switching power supply

To solve the electromagnetic interference problem of switching power supply, we can start from 3 aspects: 1) reduce the interference signal generated by the interference source; 2) cut off the transmission way of the interference signal; 3) enhance the anti-interference ability of the interfered body. Therefore, the control technology of electromagnetic interference of switching power supply mainly includes: circuit measures, EMI filtering, component selection, shielding and PCB anti-jamming design.

Reduce the interference of switch power itself

Soft switching technology: increase the inductor and capacitor in the hard switching circuit, the resonant inductance and capacitance, reduce the switching process of du/dt and di/dt, the switching device voltage drop current rise before the opening, closing or rising current voltage drop before the break, to eliminate the voltage and current overlap.

Switching frequency modulation technology: by modulating the switching frequency fc, the switch is focused on FC and its harmonic 2FC, 3fc... The above energy is scattered over the band around them to reduce the EMI amplitude at each frequency point. The method can not reduce the total amount of interference, but the energy is scattered to the base band of the frequency point, so that each frequency point does not exceed the limit of EMI. In order to reduce the spectral peak of noise, there are usually two methods: the random frequency method and the modulation frequency method.

Selection of components: selection of components that are not easy to produce noise, not easy to transmit and radiate noise. It is usually particularly noteworthy that 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 part of the switching power supply.

The active suppression technology of common mode interference: try to extract a compensation EMI noise voltage which is completely opposite to the main switch voltage waveform causing the electromagnetic interference from the main loop, and use it to balance the original switching voltage.

Filtering: one of the main purposes of EMI filter is to get higher insertion loss in the frequency range of 150kHz to 30MHz, but not to attenuate frequency 50Hz power frequency signal, so that the rated voltage and current pass smoothly, and at the same time, we must meet certain size requirements. Any conduction interference signal on any power line can be expressed by a differential mode and a common mode signal. In general, the interference of differential mode interference is small, the frequency is low, the interference is small, the common mode interference is large, the frequency is high, and it can also generate radiation through the wire, resulting in greater interference. Therefore, if we want to weaken the conduction interference and control the EMI signal below the limit level specified by the EMC standard, the most effective way is to install the electromagnetic interference filter in the input and output circuit of the switching power supply.

PCB design: the PCB anti-jamming design mainly includes the PCB layout, wiring and grounding, which aims to reduce the electromagnetic radiation of PCB and the crosstalk between the circuits on the PCB. The best method of switching power supply layout is similar to that of electrical design. After determining the size and shape of the PCB, the position of the special components (such as various generators, crystal oscillators, etc.) is determined. Finally, according to the function unit of the circuit, all the components of the circuit are laid out.

The buffer circuit to reduce electromagnetic interference is composed of linear impedance stabilization network, which eliminates potential interference in power supply line, including power line interference, electrical fast transient, surge, voltage variation and power line harmonics. These disturbances are not very important to the conventional power supply, but it has a significant impact on the high frequency switching power supply.

The transmission way of cutting off interference signals - Design of common mode and differential mode power line filter

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

Enhance the anti-interference ability of the sensitive circuit