Analysis of various power supply filter circuits

The voltage output by the rectifier circuit is a unidirectional pulsating voltage and cannot be directly used by electronic circuits. Therefore, the output voltage must be filtered to eliminate the AC component in the voltage and become DC for use in electronic circuits. In filter circuits, devices with special impedance characteristics for alternating current are mainly used, such as capacitors and inductors. This article analyzes its various forms of filter circuits.

1. Types of filter circuits

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.

2. Filtering principle

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.

3. π-type RC filter circuit diagram identification method

Figure 4 shows the π-type RC filter circuit. C1, C2 and C3 in the circuit are three filter capacitors, R1 and R2 are filter resistors, C1, R1 and C2 constitute the first section of π-type RC filter circuit, and C2, R2 and C3 constitute the second section of π-type RC filter circuit. circuit. Since the form of this filter circuit is like the Greek letter π and uses resistors and capacitors, it is called π type RC filter circuit.

The principle of π-type RC filter circuit is as follows:

(1) The filtering principle of this circuit is: the voltage output from the rectifier circuit is first filtered by C1 to filter out most of the AC components, and then added to the filter circuit composed of R1 and C2. The capacitive reactance of C2 and R1 form a voltage dividing circuit. Since the capacitive reactance of C2 is very small, the attenuation of the divided voltage of the AC component is large, achieving the purpose of filtering. For DC, since C2 has a DC-blocking effect, the R1 and C2 voltage dividing circuits have no voltage division attenuation effect on DC, so that the DC voltage is output through R1.

(2) When the size of R1 remains unchanged, increasing the capacity of C2 can improve the filtering effect. When the capacity of C2 remains unchanged, increasing the resistance of R1 can improve the filtering effect. However, the resistance of the filter resistor R1 cannot be too large, because the DC current flowing through the load must flow through R1, and a DC voltage drop will occur on R1, which will reduce the DC output voltage Uo2. The greater the resistance of R1, or the greater the current flowing through the load, the greater the voltage drop on R1, resulting in a lower DC output voltage.

(3) C1 is the first filter capacitor. Increasing the capacity can improve the filtering effect. However, when C1 is too large, the charging time for C1 will be very long when the device is turned on. This charging current flows through the rectifier diode. When the charging current is too large and the time is too long, the rectifier diode will be damaged. Therefore, using this π-type RC filter circuit can make the capacity of C1 smaller, and the filtering effect can be further improved by rationally designing the values ​​of R1 and C2.

(4) There are three DC voltage output terminals in this filter circuit, which output three groups of DC voltages Uo1, Uo2 and Uo3 respectively. Among them, Uo1 is only filtered by the capacitor C1; Uo2 is filtered by the C1, R1 and C2 circuits, so the filtering effect is better, and the AC component in Uo2 is smaller; Uo3 is filtered by the 2-section filter circuit, and the filtering effect is the best Okay, so the communication component in Uo3 is minimal.

(5) The magnitudes of the three DC output voltages are different. Uo1 has the highest voltage. Generally, this voltage is directly added to the power amplifier circuit, or to a circuit that requires the highest DC operating voltage and the largest operating current; Uo2 voltage is slightly lower, because resistor R1 has a voltage drop on the DC voltage; Uo3 voltage The lowest, this voltage is generally supplied to the front-end circuit as the DC working voltage, because the DC working voltage of the front-end circuit is relatively low, and the DC working voltage is required to have less AC components.

4. π-type LC filter circuit diagram identification method

Figure 5 shows the π-type LC filter circuit. The π-type LC filter circuit is basically the same as the π-type RC filter circuit. This circuit only replaces the filter resistor with a filter inductor, because the filter resistor has the same resistance to DC and AC, while the filter inductor has a large resistance to AC inductance and a small resistance to DC. This can improve the filtering effect without reducing the DC output voltage.

In the circuit of Figure 5, the unidirectional pulsating DC voltage output by the rectifier circuit is first filtered by capacitor C1 to remove most of the AC components, and then added to the L1 and C2 filter circuits.

As for the AC component, L1 has a large inductive reactance, so the AC voltage drop on L1 is large and the AC component added to the load is small.

For DC, since L1 does not present inductive reactance, it is equivalent to a path. At the same time, the filter inductor uses a thicker wire diameter and a small DC resistance. This means that there is basically no voltage drop to the DC voltage, so the DC output voltage is relatively high. This is The main advantages of using inductor filters.

5. Electronic filter image recognition method

1. Electronic filter

Figure 6 shows the electronic filter. VT1 in the circuit is a transistor, which functions as a filter tube. C1 is the base filter capacitor of VT1, R1 is the base bias resistor of VT1, RL is the load of this filter circuit, and C2 is the filter capacitor of the output voltage.

The working principle of the electronic filter circuit is as follows:

① VT1, R1 and C1 in the circuit form an electronic filter circuit. This circuit is equivalent to a capacitor with a capacity of C1×β1. β1 is the current amplification factor of VT1, and the current amplification factor of the transistor is relatively large, so the equivalent The capacitance is very large, which shows that the filtering performance of the electronic filter is very good. The equivalent circuit is shown in Figure 6(b). C in the figure is the equivalent capacitance.

② R1 and C1 in the circuit form an RC filter circuit. On the one hand, R1 provides base bias current for VT1 and is also a filter resistor. Since the current flowing through R1 is the base bias current of VT1, this current is very small, and the resistance of R1 can be made relatively large, so that the filtering effect of R1 and C1 is very good, so that the DC voltage on the base of VT1 There is very little communication component. Since the emitter voltage follows the base voltage, there is very little AC component in the VT1 emitter output voltage, achieving the purpose of filtering.

③In electronic filters, filtering is mainly achieved by R1 and C1. This is also an RC filter circuit, but it is different from the RC filter circuit introduced earlier. In this circuit, the DC current flowing through the load is the emitter current of VT1, and the current flowing through the filter resistor R1 is the base current of VT1. The base current is very small, so the resistance of the filter resistor R1 can be set to a large value ( The filtering effect is good), but it will not cause the DC output voltage to drop a lot.

④The resistance of R1 in the circuit determines the base current of VT1, which determines the tube voltage drop between the collector and emitter of VT1, and also determines the output DC voltage of the emitter of VT1, so changing the value of R1 size, you can adjust the size of the DC output voltage +V.

2. Electronic voltage stabilizing filter

Figure 7 shows another electronic voltage stabilizing filter. Compared with the previous circuit, a voltage stabilizing diode VD1 is connected between the base of VT1 and the ground terminal. The principle of electronic voltage stabilization is as follows:

After the Zener diode VD1 is connected between the base of VT1 and the ground, the input voltage passes through R1 to make the Zener diode VD1 in a reverse biased state. At this time, the voltage stabilization characteristics of VD1 stabilize the base voltage of the VT1 tube, so The DC voltage output by the VT1 emitter is also relatively stable. Note: The stability characteristics of this voltage are determined by the voltage stabilization characteristics of VD1 and have nothing to do with the electronic filter circuit itself.

R1 is also the current limiting protection resistor of VD1. After adding the Zener diode VD1, changing the size of R1 cannot change the emitter output voltage of VT1. Since there is a PN junction voltage drop in the emitter junction of VT1, the emitter output voltage is slightly smaller than the regulated value of VD1.

C1, R1 and VT1 also form an electronic filter circuit, which plays a filtering role.

In some cases, in order to further improve the filtering effect, a double-tube electronic filter circuit can be used. Two electronic filter tubes constitute a composite tube circuit. In this way, the total current amplification factor is the product of the current amplification factors of each tube, which can obviously improve the filtering effect.

6. Summary of power supply filter circuit diagram recognition

Regarding the analysis of the power supply filter circuit, the following points should be noted:

(1) When analyzing the working principle of the filter capacitor, the "DC blocking" characteristic of the capacitor, or the charging and discharging characteristics, is mainly used. That is, when the rectifier circuit outputs a unidirectional pulsating DC voltage, the filter capacitor is charged. When there is no unidirectional pulsating DC voltage, the filter capacitor is charged. When DC voltage is output, the filter capacitor discharges to the load.

(2) When analyzing the working principle of the filter inductor, it is mainly to understand that the resistance of the inductor to direct current is very small and has no inductive reactance, but there is inductive reactance to alternating current.

(3) When analyzing the electronic filter circuit, you must know that the capacitor on the base of the electronic filter tube is a key component of filtering. In addition, it is necessary to analyze the DC circuit. The electronic filter tube has base current, collector and emitter current. The current flowing through the load is the emitter current of the electronic filter tube. Changing the size of the base current can adjust the collector of the electronic filter tube. The voltage drop between the tube and the emitter changes the DC voltage output by the electronic filter.

(4) The electronic filter itself does not have a voltage stabilizing function, but adding a Zener diode can make the output DC voltage more stable.