There are two kinds of filter circuits commonly used in high power DC regulated power supply: passive filter and active filter. The main forms of passive filtering are capacitive filtering, inductance filtering and complex filtering (including inverted L-type, LC filtering, LC-pi filtering and RC-pi filtering).

The main form of active filter is active RC filter, also known as electronic filter. The magnitude of the pulsation component in DC current is expressed by the pulsation coefficient S. The larger the value, the worse the filtering effect of the filter.

Pulse coefficient (S) = larger fundamental value of AC component of output voltage/DC component of output voltage

The principle of power filter is an impedance adapter network: the greater the impedance adapter between the input and output sides of power filter and the power and load sides, the more effective the attenuation of electromagnetic interference is.

The specific working principle is as follows: the direction of AC after diode rectification is single, but the size (current intensity) is still in constant change. This kind of pulsating DC is generally not directly used to power radio installations. In order to turn pulsating DC into waveform smoothing DC, we need to do some more work of "filling in and aligning", which is filtering. In other words, the task of filtering is to reduce the fluctuation component of the output voltage of the rectifier as much as possible and transform it into a nearly stable DC current.

According to the characteristics of electromagnetic disturbance at power port, EMI noise filter of power supply is a passive low-pass filter. It transmits AC to power source without attenuation, and greatly attenuates EMI noise coming with AC. At the same time, it can effectively suppress EMI noise produced by power supply equipment and prevent them from entering AC network to interfere with other electronic devices.

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