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08-15-2012 11:12 AM

Reduce interference of switching power to electricity grid

How to reduce the interference of the switch power on the grid? Switch power on the grid interference voltage is measured on the grid and power supply between the LISN 50O resistor. German standard VDE and American Standard FCC conducted interference limits in switching power supply grid requirements. The standard is divided into Class A and Class B Class A standard is for those devices used in industrial, commercial and office environment, while the Class B standard is set up for those devices for residents, as it involves the civil, the standard is more stringent, requirements and more high.

To reduce interference from the power grid include following ways:

1) to reduce the voltage overshoot. Reduce the voltage overshoot can prevent the pipe to withstand high voltage, it can reduce high frequency noise on the power grid. Select reverse recovery current of diode (such as silicon carbide diode) is also a viable method to reduce the sources of interference strength. Adjust the trigger pulse hopping along and increase the gate resistance, etc. can reduce the dv / dt, but this will increase the switching losses and reduce the efficiency of the device as a whole, considering the trade-offs from the performance of switching power supply.

2) improving the modulation method. The same modulation frequency to the random modulation, frequency modulation, and the so-called S ? modulation and so on. Interfere with low frequency of the modulating pulse frequency is fixed modulation frequency of the harmonic interference, low-band interference is mainly concentrated in the harmonic point, low frequency interference generated by the random modulation methods such as are scattered at a certain frequency band Therefore, the above measures conducive to the switching power supply by the spectral characteristics of electromagnetic interference testing, in line with the power of electromagnetic standards.

3) Increase the input filter. (1) and (2) two ways to proceed from the source of strength to reduce interference, increase the filter from the start to change the characteristics of the coupled channel. Increase the common mode filter can reduce the interference of the switching power supply grid. If you do not enter the filter, the interference power on the grid will greatly exceed the corresponding standard. Join the input filter, the power grid interference will comply with relevant standards. Measurements taking into account the low frequency interference of switching power on the grid is an integer multiple of the switching frequency harmonic band resolution of the scanner is 200Hz, 150KHz-30MHz frequency band frequency band resolution 9KHz addition, shielding measures can also reduce the interference of the switching power supply grid.

Power to achieve the requirements of the standard EMC test identification and switching power supply will not cause impermissible interference in the course are two different things. Meet the standards of the power to use or improper handling, use can also cause serious interference. Moreover, the switching power supply is both a power supply unit, is also a noise generating device, through the coupling channel between it and disturbed body are connected together, apparently coupled to the channel characteristics and the characteristic, combined with the disturbed body is not good cause serious interference phenomenon is possible, switching power supplies in parallel power supply caused by the instability of the whole system.

Some by the time domain waveform of the disturbing body on the interference-sensitive, if any, digital circuits interfere with the pulse will result in the malfunction and the time domain waveform, that is not only related to the pulse amplitude, but also with the pulse width. Even switching power supply to the standard, but the time-domain waveform foreign interference caused by more serious interference is possible. Based on the above considerations, some users of the switching power supply, in addition to electromagnetic compatibility in accordance with standard testing of switching power supply, but also should be increased under specific conditions of use in switching power supply of some interference with the performance of detection.

In addition, some switching elements turn on and off transition is different, generated when the dv / dt, when opened, will be greater than the turn-off dv / dt, external interference is also the former than the latter. Moreover, the transition when the conduction load factors. If you use the power of the random modulation and fixed-frequency modulation of power between the two switching elements, such as LGBT, the parameters of the drive circuit and the driving pulse are the same, therefore the voltage of the switching between the transition of the same; external interference in the two coupled channels the same interfere with test results on the grid frequency domain characteristics of the former over the latter. However, the time domain waveform of the external interference is still the same, not necessarily random modulation power than the power of the fixed-frequency modulation.
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