How to reduce the interference of the switching power supply to the power grid? -Solutions - Huaqiang Electronic Network

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The interference voltage of the switching power supply to the grid is measured by adding a LISN 50Ω resistor between the grid and the switching power supply. Both the German standard VDE and the US standard FCC regulate the limits of conducted disturbances in the switching power supply grid. The standard is further divided into Class A and Class B. Among them, the Class A standard is for those used in industrial, commercial and office environments, while the Class B standard is for those devices used for residents. Because it is for civilian use, its standards are more stringent and the requirements are more demanding. high.

There are several ways to reduce interference to the grid:

(1) Reduce voltage overshoot. Reducing voltage overshoot avoids excessive voltage on the tube and reduces high frequency noise on the grid. Selecting a diode with a small reverse recovery current (such as a silicon carbide diode) is also a viable method to reduce the intensity of the interferer. Although adjusting the jump edge of the trigger pulse and increasing the resistance of the gate can reduce dv/dt, this will increase the switching loss and reduce the efficiency of the whole device. It is necessary to comprehensively consider the trade-offs of the performance of the switching power supply.

(2) Improve the modulation method. The frequency-invariant modulation is changed to random modulation, frequency conversion modulation, and so-called "∑△" modulation and the like. The low frequency band generated by the modulation pulse with constant frequency is mainly the harmonic interference of the modulation frequency. The interference of the low frequency band is mainly concentrated on each harmonic point, and the low frequency interference generated by the random modulation method is dispersed in a certain frequency band. Therefore, the above measures are beneficial to the test of the spectral characteristics of the switching power supply through electromagnetic interference, so that it meets the electromagnetic standard of the power supply.

(3) Increase the input filter. The two methods (1) and (2) are mainly to reduce the intensity of the interference source, and the addition of the filter starts from changing the characteristics of the coupling channel. The added common mode filter can reduce the interference of the switching power supply to the grid. If no input filter is added, the interference of the power supply to the grid will greatly exceed the corresponding standard. When the input filter is added, the interference of the power supply to the grid will meet the corresponding standards. In the measurement, the low-band interference of the switching power supply to the power grid is mainly the harmonics of the integer multiple of the switching frequency, so the frequency band resolution of the frequency sweeper is 200 Hz, and the frequency band resolution in the 150 kHz-30 MHz frequency band is 9 kHz. In addition, the use of shielding measures can also reduce the interference of the switching power supply to the grid. (Source: http://)

The power supply is certified by electromagnetic compatibility test and meets the requirements of the standard. It is not the same as whether the switching power supply will cause unacceptable interference during use. Improper use or handling of the standard power supply can cause serious interference during use. Moreover, the switching power supply is both a power supply device and a noise generating device, and is connected to the victim body through the coupling channel. It is obvious that the characteristics of the coupling channel are not well matched with the characteristics of the victim body, causing serious interference. It is possible that the phenomenon that the switching power supply is connected in parallel causes the entire system to be unstable.

Some disturbed bodies are sensitive to the time domain waveform of the interference. If some digital circuits are under the action of the interference pulse, the malfunction will be related to the time domain waveform, that is, not only related to the pulse amplitude, but also related to the pulse width. Even if the switching power supply meets the corresponding standard, it is possible that the time domain waveform that causes interference to the outside causes more serious interference. Based on the above considerations, some switching power supply users, in addition to the standard to detect the electromagnetic compatibility of the switching power supply, should also add some detection of interference performance under the specific use conditions of the switching power supply.

In addition, some switching elements have different transitions when turned on and off. The dv/dt generated when turned on is greater than the dv/dt generated when turned off, and the external interference is also greater than the latter. Moreover, the transition during conduction is basically irrelevant to factors such as load. If both a random modulation power supply and a fixed frequency modulated power supply are used as switching elements, such as LGBT, the parameters of the drive circuit and the drive pulses are the same, so the voltage transitions of the switches are the same; the external interference coupling channels in the two In the same way, the former is superior to the latter in terms of the frequency domain characteristics of the interference test results for the grid. However, the time domain waveform of the external interference is still the same, and the power supply that is not necessarily randomly modulated is superior to the fixed frequency modulation power supply.