How to choose filter capacitor for switching power supply?

The filter capacitor plays a very important role in the switching power supply. How to choose the filter capacitor correctly, especially the choice of the output filter capacitor, is a problem that every engineering and technical personnel is very concerned about.

On the power filter circuit, we can see a variety of capacitors with different capacitances, such as: 100uF, 10uF, 100nF, 10nF, etc. 

How are these parameters determined?

In the 50Hz power frequency circuit, ordinary electrolytic capacitors are used, whose pulsating voltage frequency is only 100Hz, and the charging and discharging time is on the order of milliseconds.

In order to obtain a smaller pulsation coefficient, the required capacitance is as high as hundreds of thousands of μF. Therefore, the goal of ordinary low-frequency aluminum electrolytic capacitors is to increase the capacitance. The capacitance, loss tangent value and leakage current of the capacitor are main parameters used to identify quality of them.

In the switching power supply with a sawtooth voltage frequency up to tens of kHz, or even tens of MHz, for the output filter electrolytic capacitor used on it, the capacitance is not its main indicator at this time.

The standard to measure the quality of high-frequency aluminum electrolytic capacitors is the "impedance-frequency" characteristic, which requires a lower equivalent impedance within the operating frequency of the switching power supply, and at the same time, it has good filtering effect to the high-frequency spikes generated by semiconductor devices.

Ordinary low-frequency electrolytic capacitors begin to appear inductive at around 10kHz, which cannot meet the requirements of switching power supplies.

Switching power supply process

The special high-frequency aluminum electrolytic capacitor for switching power supply has four terminals.

Current flows in from one positive terminal of the four-terminal capacitor, passes through the capacitor, and then flows from the other positive terminal to the load; the current returning from the load also flows in from one negative terminal of the capacitor, and then flows from the other negative terminal to the negative terminal of the power supply.

Because the four-terminal capacitor has good high-frequency characteristics, it provides an extremely advantageous means for reducing the pulsating of the voltage and suppressing the switching spike noise.

High-frequency aluminum electrolytic capacitors also have a multi-core form, that is, the aluminum foil is divided into several shorter sections, which are connected in parallel with multiple lead to reduce the impedance in the capacitive reactance.

And the use of low-resistivity materials as the leads improves the capacity of the capacitor to withstand large currents.