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Practical knowledge | Explain the working principle of filter circuit from the basics to the in-depth

Latest update time：2019-12-27

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The voltage output from the rectifier circuit is a unidirectional pulsating voltage, which cannot be directly used in electronic circuits. Therefore, the output voltage must be filtered to eliminate the AC component in the voltage and convert it into DC before it can be used in electronic circuits. In the filter circuit, devices with special impedance characteristics for AC are mainly used, such as capacitors and inductors. This article analyzes 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. From the figure, it can be seen that the direction of the voltage is consistent at any time, but the voltage amplitude fluctuates, that is, on the time axis, the voltage shows a periodic change, so it is pulsating.

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

2. Capacitor filtering principle

According to the above analysis, since the unidirectional pulsating DC voltage can be decomposed into AC and DC, in the filter circuit of the power supply circuit, the AC component in the voltage can be filtered out by using the characteristics of "isolating DC and passing AC" and energy storage of the capacitor, or by using the characteristics of "isolating AC and passing DC" of the inductor. Figure 2 shows the principle diagram of capacitor filtering.

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

Figure 2(b) is a capacitor filter circuit. Since capacitor C1 is equivalent to an open circuit for DC power, the DC voltage output by the rectifier circuit cannot pass through C1 to the ground, but can only be added to the load RL (Figure RL). For the AC component output by the rectifier circuit, since 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, the required DC voltage +U is taken out from the unidirectional pulsating DC power through the filtering of capacitor C1.

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

3. Inductor filtering principle

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

For the AC component output by the rectifier circuit, the large inductance and reactance of L1 greatly hinder the AC component, preventing the AC from flowing through C1 to the load RL. In this way, the required DC voltage +U is extracted from the unidirectional pulsating DC through the filtering of inductor L1.

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

3. π-type RC filter circuit diagram reading method

Figure 4 shows a 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 form the first section of the π-type RC filter circuit, and C2, R2 and C3 form the second section of the π-type RC filter circuit. Since the form of this filter circuit is like the Greek letter π and uses resistors and capacitors, it is called a 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 remove 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 divider circuit. Since the capacitive reactance of C2 is very small, the voltage divider attenuation of the AC component is very large, achieving the filtering purpose. For DC power, since C2 has a DC isolation effect, the R1 and C2 voltage divider circuit does not have a voltage divider attenuation effect on DC power, so 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 size 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, which will produce a DC voltage drop on R1, reducing the DC output voltage Uo2. The larger the resistance of R1, or the larger the current flowing through the load, the greater the voltage drop on R1, making the DC output voltage lower.

(3) C1 is the first filter capacitor. Increasing the capacity can improve the filtering effect. However, if C1 is too large, it will take a long time to charge C1 when the power is turned on. This charging current flows through the rectifier diode. When the charging current is too large and the charging time is too long, the rectifier diode will be damaged. Therefore, the use of this π-type RC filter circuit can make the capacity of C1 smaller, and the filtering effect can be further improved by reasonably designing the values of R1 and C2.

(4) This filter circuit has three DC voltage output terminals, which output three groups of DC voltages: Uo1, Uo2 and Uo3. Among them, Uo1 is filtered only by capacitor C1; Uo2 is filtered by C1, R1 and C2 circuits, so the filtering effect is better and the AC component in Uo2 is smaller; Uo3 is filtered by two filter circuits, and the filtering effect is the best, so the AC component in Uo3 is the least.

(5) The three DC output voltages are of different sizes. Uo1 has the highest voltage, which is usually directly added to the power amplifier circuit or to the circuit that requires the highest DC working voltage and the largest working current; Uo2 has a slightly lower voltage, which is because resistor R1 has a voltage drop to the DC voltage; Uo3 has the lowest voltage, which is usually supplied to the previous stage circuit as the DC working voltage, because the DC working voltage of the previous stage circuit is relatively low and requires less AC component in the DC working voltage.

4. Method of reading the π-type LC filter circuit diagram

Figure 5 shows a 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, and the filter inductor has a large inductance to AC and a small resistance to DC, which 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 filtering circuits.

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

For DC power, since L1 has no inductive reactance, it is equivalent to a path. At the same time, the wire diameter of the filter inductor is relatively thick and the DC resistance is very small. In this way, there is basically no voltage drop on the DC voltage, so the DC output voltage is relatively high. This is the main advantage of using an inductor filter.

5. Electronic filter image recognition method

1. Electronic filter

Figure 6 shows an electronic filter. VT1 in the circuit is a triode that acts 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, which is equivalent to a capacitor with a capacity of C1×β1. β1 is the current magnification of VT1, and the current magnification of the transistor is relatively large, so the equivalent 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). In the figure, C is the equivalent capacitor.

② R1 and C1 in the circuit form an RC filter circuit. 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, which is very small, the resistance value of R1 can be relatively large, so the filtering effect of R1 and C1 is very good, making the AC component of the DC voltage on the base of VT1 very small. Since the emitter voltage has the characteristic of following the base voltage, the AC component in the emitter output voltage of VT1 is also very small, achieving the purpose of filtering.

③ In electronic filters, filtering is mainly achieved by R1 and C1, which 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 value of the filter resistor R1 can be set very large (good filtering effect), but it will not cause the DC output voltage to drop a lot.

④ The resistance value of R1 in the circuit determines the base current of VT1, thereby determining the tube voltage drop between the collector and emitter of VT1, and also determining the output DC voltage of the emitter of VT1. Therefore, by changing the value of R1, the DC output voltage +V can be adjusted.

2. Electronic voltage stabilization filter

Figure 7 shows another type of electronic voltage stabilization filter. Compared with the previous circuit, a voltage stabilization diode VD1 is connected between the base of VT1 and the ground. The principle of electronic voltage stabilization is as follows: After the voltage stabilization diode VD1 is connected between the base of VT1 and the ground, the input voltage through R1 makes the voltage stabilization diode VD1 in a reverse bias state. At this time, the voltage stabilization characteristics of VD1 stabilize the base voltage of VT1 tube, so that the DC voltage output by the emitter of VT1 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 voltage zener diode VD1, changing the size of R1 cannot change the emitter output voltage of VT1. Since there is a PN junction voltage drop at the emitter junction of VT1, the emitter output voltage is slightly smaller than the regulated voltage value of VD1.

C1, R1 and VT1 also form an electronic filter circuit to play a filtering role.

In some cases, in order to further improve the filtering effect, a double-tube electronic filter circuit can be used, and two electronic filter tubes form 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 filter circuit diagram recognition

Regarding the analysis of power supply filter circuit, pay attention to the following points:

(1) When analyzing the working principle of the filter capacitor, the main use is made of the capacitor's "blocking DC and passing AC" characteristics, or charging and discharging characteristics, that is, when the rectifier circuit outputs a unidirectional pulsating DC voltage, the filter capacitor is charged, and when there is no unidirectional pulsating DC voltage output, the filter capacitor discharges to the load.

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

(3) When analyzing the electronic filter circuit, it is important to know that the capacitor on the base of the electronic filter tube is a key element of filtering. In addition, when analyzing the DC circuit, the electronic filter tube has base current and collector and emitter currents. The current flowing through the load is the emitter current of the electronic filter tube. Changing the base current can adjust the tube voltage drop between the collector and emitter of the electronic filter tube, thereby changing the DC voltage output by the electronic filter.

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

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