Illustration of filtering principle of power supply filter circuit

There are mainly the following types of filter circuits: capacitor filter circuit, which is the most basic filter circuit; π-type RC filter circuit; π-type LC filter circuit; electronic filter circuit.

1. Characteristics of unidirectional pulsating DC voltage

As shown in Figure 1(a). It is a unidirectional pulsating DC voltage waveform. It can be seen from the figure that the directionality of the voltage is consistent at all times, but the voltage amplitude fluctuates. That is, on the time axis, the voltage appears periodic. changes, so it is pulsating.

However, according to the principle of waveform decomposition, this voltage can be decomposed into a DC voltage and a set of AC voltages with different frequencies, as shown in Figure 1(b). In Figure 1(b), the dotted line part is the unidirectional pulsating DC voltage U. The DC component in , the solid line part is the AC component in UO.

2. Principle of capacitor filtering

According to the above analysis, due to the unidirectional pulsating DC voltage, it can be decomposed into AC and DC parts. In the filter circuit of the power circuit, the AC component in the voltage can be filtered out by using the "blocking DC" characteristics and energy storage characteristics of the capacitor, or using the "blocking DC" characteristics of the inductor. Figure 2 shows the schematic diagram of capacitor filtering.

Figure 2(a) shows the output circuit of the rectifier circuit. After the AC voltage is passed through the rectifier circuit, the output is unidirectional pulsating DC, which is the UO in the circuit.

Figure 2(b) shows the capacitor filter circuit. Since the capacitor C1 is equivalent to an open circuit for DC, the DC voltage output by the rectifier circuit cannot reach the ground through C1 and can only be added to the load RL as shown in the figure. For the AC component output by the rectifier circuit, because C1 has a large capacity and a small capacitive reactance, the AC component flows to the ground through C1 and cannot be added to the load RL. In this way, through the filtering of capacitor C1, the required DC voltage +U is taken out from the unidirectional pulsating DC power.

The larger the capacity of the filter capacitor C1, the smaller the capacitive reactance to the AC component, so that the smaller the AC component remaining on the load RL, the better the filtering effect.

3. Principle of inductor filtering

Figure 3 shows the schematic diagram of inductor filtering. Since the inductor L1 is equivalent to a path for DC current, the DC voltage output by the rectifier circuit is directly added to the load RL.

For the AC component output by the rectifier circuit, due to the large inductance and reactance of L1, it has a great hindrance to the AC component and prevents the AC current from flowing through C1 to the load RL. In this way, through the filtering of the inductor L1, the required DC voltage +U is taken out from the unidirectional pulsating DC power.

The greater the inductance of the filter inductor L1, the greater the inductive reactance to the AC component. The smaller the AC component remaining on the load RL, the better the filtering effect, but the DC resistance will also increase.