Detailed analysis of the selection and calculation of power supply filter capacitor (I)

The impedance of the inductor is directly proportional to the frequency, and the impedance of the capacitor is inversely proportional to the frequency. Therefore, the inductor can choke the high frequency pass, and the capacitor can choke the low-frequency pass. If the two are properly combined, all kinds of frequency signals can be filtered. If the capacitor is connected to the load or the inductance on the load in the rectifier circuit, the AC ripple can be filtered. Capacitor filter is a voltage filter, which is the direct storage of pulse voltage to smooth the output voltage, the output voltage is high, close to the peak value of the AC voltage; it is suitable for small current, the smaller the current, the better the filtering effect.

Inductive filtering belongs to the current filter, which is by generating electromagnetic induction by current to smooth output current, low output voltage, lower than AC voltage effective value; suitable for large current, the greater the current, the better the filtering effect. Many characteristics of capacitors and inductors are quite the opposite.

In general, the role of electrolytic capacitors is to filter out the low-frequency signals in the current, but even the low frequency signals are divided into several orders of magnitude. Therefore, in order to be suitable for use at different frequencies, electrolytic capacitors are also divided into high-frequency capacitors and low-frequency capacitors (where the relative frequency is relatively high).

The low frequency filter capacitor is mainly used for the filtering of the city electric filter or the rectifier of the transformer. The frequency of the capacitor is 50Hz in accordance with the city electricity, while the high frequency filter capacitor mainly works after the rectifier of the switching power supply, and its frequency is from thousands of Hz to tens of tens of thousands of Hz. When the low frequency filter capacitor is used in the high frequency circuit, the high frequency characteristic of the low frequency filter capacitor is not good, and its internal resistance is larger and the equivalent inductance is higher when the high frequency charge and discharge is charged and discharge. Therefore, in the use of the electrolyte due to frequent polarization and produce a greater amount of heat. The higher temperature will make the electrolyte inside the capacitor vaporized, and the pressure inside the capacitor will increase. Eventually, the capacitor will burst and burst.