Whether it is half-wave rectification or full-wave rectification, through the waveform, you will find that the whole is a pulsating wave, which is different from the constant DC we require, does not meet our expectations, so there is a higher requirement for filtering.

We want to make the waveform smoother and closer to the constant DC by filtering.

I think filtering is a very good word, the whole process is to filter out the excess waveform, so that the waveform is smoother.

There are many ways of filtering, can be capacitor filtering, can also be inductance filtering, can also be a combination of them, but the most common, the most commonly used filtering is capacitor filtering, in the dc bus of the low-voltage converter and the capacitor is the most typical application.

Capacitive filtering is based on the principle that the voltage at both ends of the capacitor can not change, the reason is that the capacitor exists in the circuit charge and discharge.

Take single-phase full wave rectification as an example. On the rising edge of the power waveform, the capacitor is charged, and on the falling edge, the capacitor is discharged to the load, and so on and so on, but the capacitor in the discharge process, no longer change according to the sine law, but the exponential law decreases, the slower the discharge time, the smoother the waveform.

Discharge time is proportional to the product of load resistance R and capacitance C. If there is no load behind at this time, it is equivalent to the open power supply, and the load R equals to infinity, then the DC voltage at this time approaches the maximum value of U2, which is the square root of 2U2.

Here, we have to mention the dc bus voltage of the low-voltage converter. When the converter is in standby mode, the DC bus voltage of the converter approaches 537V, which is 380 multiplied by √2. Of course, it varies with the supply voltage.

It can be concluded that filtering makes the waveform smoother and, under load, improves the average value of the DC voltage.

Under load, the average size of dc bus is equal to 1.2U2, which is significantly higher than 0.9U2 in single-phase full-wave rectifier circuit.

As mentioned just now, the discharge time is proportional to the product of load resistance R and capacitance C. Therefore, when selecting capacitance, the larger the capacitance, the better, the larger the electrolytic capacitor will appear in the filter circuit. One is not enough, and one is combined until it satisfies filtering requirements.