How to select the peripheral capacitance of the power module?

一、 Select from the rated withstand voltage value of the capacitor

1. Selection of AC-DC rectifier filter capacitor withstand voltage value: in AC-DC rectifier filter circuit, according to the formula

We can calculate the theoretical DC voltage after rectification.

For example, in the rectifier circuit with an input of 90-265vac, the voltage range after rectification can be calculated by formula to be 127-375vdc. According to experience, we generally choose capacitors with withstand voltage of 1.1-1.3 times the theoretical calculated voltage value. Considering the cost factor, some designers will also consider capacitors with withstand voltage of 1.0 times the theoretical calculated voltage value.

2. In the application of DC-DC power supply, we generally choose 1.3-1.4 times the withstand voltage value of the external input capacitor.

For example, if the input voltage is 48V, the withstand voltage value of the filter capacitor we choose is 63v. The reason is that the starting current of DC-DC power supply is large, which is easy to cause the secondary impulse voltage to be too high, thus damaging the device.

二、 Select from the capacitance value of input capacitance

In many literatures, empirical formulas are often used for the selection of filter capacitance C:

It is considered that the larger the filter capacitance C, the better; In the maintenance of some filter circuits, technicians often use capacitors with larger capacity than the original circuit to replace the broken capacitors.

Theoretically, increasing the filter capacitor C capacity in the circuit can indeed make the waveform of the output voltage smoother and less fluctuant, but at the moment of circuit connection, the impact current factor generated in the circuit cannot be ignored; In the maintenance of some filter circuits, the replacement of filter capacitors also has the problem of impulse current. On the premise of not replacing other components, simply increasing the capacity of filter capacitors will greatly shorten the actual service life of the whole circuit, and even burn the whole circuit. Besides,

三、 Selection of output capacitance

The output ripple noise of the power supply and the load capacity of the power supply (capacitive load) are generally considered in the selection of output capacitance. When the cost allows, many people will think that the larger the external output filter capacitor is, the less the ripple noise interference and the more stable the power supply to the system. This is a wrong idea.