Due to its miniaturization, light weight and high efficiency, the corresponding requirements of the switching power supply module are small and large capacity, ripple current resistance, high frequency and low resistance, long life at high temperature and more suitable high density. High frequency and low impedance means that the capacitor has strong ability to withstand ripple current, and its internal resistance is small. Relatively speaking, the product used in the circuit is not easy to heat itself and lead to premature failure.
The filter capacitor used in the output of switching power supply is different from that in power frequency circuit. The pulse voltage frequency of common electrolytic capacitor used as filter in power frequency circuit is only 100Hz, and the charging and discharging time is in the order of millisecond. In order to obtain a small ripple coefficient, the capacitance required is as high as hundreds of thousands of Micromethods. Therefore, the manufacturing goal of ordinary aluminum electrolytic capacitor for low frequency is to improve the quality of the capacitor High power capacity.
The electrolytic capacitor used as output filter in switching power supply contains abundant high-order harmonic voltage and current because most of the switching power supply work in the state of square wave or rectangular wave.
The frequency of sawtooth voltage is as high as tens of kilohertz or even tens of megahertz, which is different from that in low frequency application. Capacitance is not the main index, but its impedance frequency characteristic is the measure of its quality.
It is required that the impedance frequency characteristic of the electrolytic capacitor with rectifier output from switching power supply does not show an upward trend at 300kHz or even 500KHz. The ESR of electrolytic capacitor is low, which can effectively filter the high frequency ripple and peak voltage in switching power supply.
The common electrolytic capacitors begin to show an upward trend after 100kHz, and the filtering effect is relatively poor when applied to the output rectification and filtering of switching power supply.
The output capacitance of power module is generally high frequency and low resistance, because the rectifier output of switching power supply is high frequency DC pulse signal, it must be filtered by high frequency low resistance capacitor, so as to reduce the loss of capacitor itself, otherwise the capacitor will burst and leak after a long time of operation.