The voltage output from the rectifier circuit is a unidirectional pulsating voltage, which cannot be used directly in electronic circuits. Therefore, the output voltage needs to be filtered to eliminate the AC components in the voltage, converting it into DC for use in electronic circuits.
In the filtering circuit, devices with special impedance characteristics for AC are mainly used, such as capacitors and inductors. This article analyzes various forms of filtering circuits.
Types of Filtering Circuits
The main types of filtering circuits are as follows: capacitor filtering circuit, which is the most basic filtering circuit; π-type RC filtering circuit; π-type LC filtering circuit; electronic filter circuit.
1. Characteristics of Unidirectional Pulsating DC Voltage
As shown in Figure 1(a). This is the waveform of unidirectional pulsating DC voltage. From the figure, it can be seen that the direction of the voltage is consistent at all times, but the amplitude of the voltage fluctuates, showing periodic changes over time, hence 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 dashed part is the DC component in the unidirectional pulsating DC voltage U, while the solid line part is the AC component in UO.
2. Capacitor Filtering Principles
Based on the above analysis, since the unidirectional pulsating DC voltage can be decomposed into AC and DC components, in the filtering circuit of the power supply circuit, the characteristics of capacitors to "block DC and pass AC" and their energy storage characteristics, or the characteristics of inductors to "block AC and pass DC" can be used to filter out the AC components in the voltage. Figure 2 shows the schematic diagram of the capacitor filtering principle.
Figure 2(a) is the output circuit of the rectifier circuit. The AC voltage output from the rectifier circuit is unidirectional pulsating DC, which is UO in the circuit.
Figure 2(b) is the capacitor filtering circuit. Since capacitor C1 is equivalent to an open circuit for DC, the DC voltage output from the rectifier circuit cannot pass through C1 to ground, and can only be applied to the load RL as shown in the figure. For the AC component output from the rectifier circuit, since C1 has a large capacitance and small capacitive reactance, the AC component flows through C1 to the ground and cannot be applied to the load RL. Thus, through the filtering of capacitor C1, the required DC voltage +U is extracted from the unidirectional pulsating DC.
The larger the capacitance of the filtering capacitor C1, the smaller the capacitive reactance to the AC component, resulting in a smaller residual AC component on the load RL, thus improving the filtering effect.
3. Inductor Filtering Principles
Figure 3 shows the schematic diagram of the inductor filtering principle. Since inductor L1 is equivalent to a path for DC, the DC voltage output from the rectifier circuit is directly applied to the load RL.
For the AC component output from the rectifier circuit, since L1 has a large inductance and high inductive reactance, it poses a significant obstruction to the AC component, preventing the AC current from flowing through C1 to the load RL. Thus, through the filtering of inductor L1, the required DC voltage +U is extracted from the unidirectional pulsating DC.
The larger the inductance of the filtering inductor L1, the greater the inductive reactance to the AC component, resulting in a smaller residual AC component on the load RL, thus improving the filtering effect, but the DC resistance will also increase.
Identification Method for π-Type RC Filtering Circuit.
Figure 4 shows the π-type RC filtering circuit. In the circuit, C1, C2, and C3 are three filtering capacitors, and R1 and R2 are filtering resistors. C1, R1, and C2 form the first section of the π-type RC filtering circuit, while C2, R2, and C3 form the second section of the π-type RC filtering circuit. This filtering circuit is named π-type RC filtering circuit because its configuration resembles the Greek letter π and utilizes resistors and capacitors.
The principles of the π-type RC filtering circuit are as follows.
1) The filtering principle of this circuit is: the voltage output from the rectifier circuit first passes through the filtering of C1, which filters out most of the AC components, and then is applied to the filtering circuit composed of R1 and C2. The capacitive reactance of C2 and R1 forms a voltage divider circuit, and since the capacitive reactance of C2 is very small, the voltage drop for the AC component is very large, achieving the filtering purpose. For DC, since C2 has a blocking effect on DC, the voltage divider circuit of R1 and C2 does not have a voltage drop effect on DC, allowing the DC voltage to be output through R1.
2) When the size of R1 remains unchanged, increasing the capacitance of C2 can improve the filtering effect. When the capacitance of C2 remains unchanged, increasing the resistance value of R1 can also improve the filtering effect. However, the resistance value of the filtering resistor R1 cannot be too large, because the DC current flowing through the load must pass through R1, which will produce a DC voltage drop across R1, reducing the DC output voltage Uo2. The larger the resistance value of R1, or the larger the current flowing through the load, the greater the voltage drop across R1, resulting in a lower DC output voltage.
3) C1 is the first filtering capacitor, and increasing its capacitance can enhance the filtering effect. However, if C1 is too large, the charging time for C1 when powered on will be excessively long. This charging current flows through the rectifier diode, and if the charging current is too high and the duration is too long, it may damage the rectifier diode. Therefore, using this π-type RC filtering circuit allows for a smaller capacitance of C1, while further enhancing the filtering effect through the reasonable design of R1 and C2 values.
4) This filtering circuit has a total of three DC voltage output terminals, which output three sets of DC voltages: Uo1, Uo2, and Uo3. Among them, Uo1 is filtered only by capacitor C1; Uo2 is filtered through the C1, R1, and C2 circuit, so the filtering effect is better, and the AC component in Uo2 is smaller; Uo3 is filtered through two sections of filtering circuits, so the filtering effect is the best, and the AC component in Uo3 is the least.
5) The magnitudes of the three DC output voltages are different. Uo1 has the highest voltage, which is generally applied directly to the power amplifier circuit or to circuits that require the highest DC working voltage and the largest working current; Uo2 has a slightly lower voltage due to the voltage drop across resistor R1; Uo3 has the lowest voltage, which 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 requires less AC component in the DC working voltage.
Method for identifying π-type LC filter circuits.
Figure 5 shows the π-type LC filter circuit. The π-type LC filter circuit is fundamentally similar to the π-type RC filter circuit. This circuit simply replaces the filter resistor with a filter inductor, as the filter resistor exhibits the same resistance for both DC and AC, while the filter inductor has a high inductive reactance for AC and a low resistance for DC, thereby enhancing the filtering effect without diminishing the DC output voltage.
In the circuit of Figure 5, the unidirectional pulsating DC voltage output from the rectifier circuit is first filtered by capacitor C1 to remove most of the AC components, and then applied to the L1 and C2 filter circuit.
For AC components, L1 has a large inductive reactance, resulting in a large AC voltage drop across L1 and a small AC component applied to the load.
For DC, since L1 does not present inductive reactance, it is equivalent to a short circuit, and the wire diameter used for the filter inductor is relatively thick, resulting in a very small DC resistance. Therefore, there is basically no voltage drop for the DC voltage, so the DC output voltage is relatively high, which is the main advantage of using an inductor filter.
Electronic filter identification method
Figure 6 shows the electronic filter. In the circuit, VT1 is a transistor 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 output voltage filter capacitor.
The working principle of the electronic filter circuit is as follows:
The circuit consisting of VT1, R1, and C1 forms an electronic filter circuit, which is equivalent to a capacitor with a capacitance of C1×β1, where β1 is the current gain of VT1. Given that the current gain of the transistor is relatively high, the equivalent capacitance is also substantial, indicating that the filtering performance of the electronic filter is excellent. The equivalent circuit is illustrated in Figure 6 (b), where C denotes the equivalent capacitance.
② The R1 and C1 in the circuit form an RC filter circuit. R1 provides the base bias current for VT1 and also acts as 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 set relatively large, thus achieving good filtering effect with R1 and C1, making the AC component in the DC voltage at the base of VT1 very small. Due to the characteristic of the emitter voltage following the base voltage, the AC component in the output voltage of VT1's emitter is also very small, achieving the filtering purpose.
③ In the electronic filter, filtering is mainly achieved by R1 and C1, which is also an RC filter circuit, but different from the previously introduced RC filter circuit. 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 a significant drop in the DC output voltage.
④ The resistance value of R1 determines the size of the base current of VT1, thereby determining the voltage drop between the collector and emitter of VT1, which in turn determines the size of the DC output voltage at the emitter of VT1. Therefore, changing the size of R1 can adjust the size of the DC output voltage +V.
2. Electronic voltage regulator filter
Figure 7 shows another type of electronic voltage regulator filter. Compared with the previous circuit, a voltage regulator diode VD1 is connected between the base of VT1 and ground. The principle of electronic voltage regulation is as follows:
After connecting the voltage regulator diode VD1 between the base of VT1 and ground, the input voltage through R1 puts the voltage regulator diode VD1 in a reverse bias state. At this time, the voltage regulation characteristics of VD1 stabilize the base voltage of VT1, so the DC voltage output from the emitter of VT1 is also relatively stable. Note: The stability of this voltage is determined by the voltage regulation characteristics of VD1 and is not related to the electronic filter circuit itself.
R1 also serves as the current limiting protection resistor for VD1. After adding the voltage regulator diode VD1, changing the size of R1 cannot change the size of the output voltage from the emitter of VT1. Due to the PN junction voltage drop at the emitter junction of VT1, the output voltage at the emitter is slightly lower than the voltage regulation value of VD1.
C1, R1, and VT1 also form an electronic filter circuit, playing a filtering role.
In some cases, to further improve the filtering effect, a dual-tube electronic filter circuit can be used, where two electronic filter tubes form a composite tube circuit. Thus, the total current amplification factor is the product of the current amplification factors of each tube, which can obviously improve the filtering effect.
Summary of power supply filter circuit identification
When analyzing the power supply filter circuit, the following points should be noted:
1) When analyzing the working principle of the filter capacitor, mainly utilize the capacitor's "isolation of DC and passage of AC" characteristic, or the charging and discharging characteristics, that is, when the rectifier circuit outputs unidirectional pulsating DC voltage, the filter capacitor charges, 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 mainly to recognize that the resistance of the inductor to DC is very small, with no inductive reactance, while it has inductive reactance to AC.
3) When analyzing the electronic filter circuit, it is important to know that the capacitor at the base of the electronic filter tube is the key filtering component. Additionally, for DC circuit analysis, the electronic filter tube has base current and collector and emitter currents, with the current flowing through the load being the emitter current of the electronic filter tube. Changing the size of the base current can adjust the voltage drop between the collector and emitter of the electronic filter tube, thereby changing the size of the DC voltage output from the electronic filter.
4) The electronic filter itself does not have voltage regulation function, but after adding a voltage regulator diode, it can make the output DC voltage relatively stable.
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