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# A must-see for power engineers: a collection of various filter schemes! (very complete)

2023-10-10【Archive】

The output voltage of rectifier circuit is a unidirectional ripple voltage, which cannot be used directly for electronic circuits. Therefore, it is necessary to filter output voltage, eliminate variable component in voltage and use it for electronic circuits after it becomes constant.

Filter circuits mainly use devices with special impedance characteristics to AC, such as capacitors and inductors. This article will analyze various forms of filter circuits.

**1. Types of filter schemes**

Filter circuits mainly include following types: capacitor filter circuit, which is simplest filter circuit; π-type RC filter circuit; π-type LC filter circuit; electronic filter circuit.

**Second, filtering principle**

1. Characteristics of Unidirectional Ripple DC Voltage

**Figure 1(a)** shows waveform of a pulsating unidirectional DC voltage. It can be seen from figure that directivity of voltage is constant at all times, but in amplitude of voltage. fluctuating, that is, on time axis, voltage shows periodic changes, so it is pulsating.

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

Figure 1: Decomposition of a unidirectional ripple voltage

2. Capacitive filter principle

According to above analysis, unidirectional ripple DC voltage can be decomposed into two parts: AC and DC. In a power supply filter circuit, AC voltage component can be filtered out using "DC blocking" characteristic and energy storage characteristic of a capacitor or "DC blocking" characteristic of an inductor. **Figure 2** shows circuit diagram of a capacitive filter.

**Figure 2(a)** is output circuit of rectifier circuit. After passing an alternating voltage through rectifier circuit, output is a unidirectional pulsating direct current, that is, U in circuit.

**Figure 2(b)** is a schematic of a capacitor filter. Since capacitor C1 is equivalent to an open DC circuit, output DC voltage of rectifier circuit cannot pass through capacitor C1 to ground, but can only be added to load R. For AC component output by rectifier circuit, since C1 has a large capacitance and a small capacitance, AC component flows to ground through C1 and cannot be added to load R. Thus, by filtering capacitor C1, required DC voltage +U is extracted from unidirectional pulsating DC.

The larger capacitance of filter capacitor C1, smaller capacitive reactance with respect to AC component, so smaller AC component left on load R, better filtering effect.

Figure 2: Schematic diagram of a capacitive filter

3. The principle of an inductive filter

**Figure 3** shows circuit diagram of an inductor filter. Since inductor L1 is equivalent to a DC path, DC voltage output by rectifier circuit is directly added to load R.

For AC component output by rectifier circuit, since L1 has a large inductance and inductive reactance, it has a strong restraining effect on AC component, preventing AC current from flowing to load R through C1. Thus, by filtering inductor L1, required DC voltage +U is extracted from unidirectional pulsating DC current.

The larger inductance of filter inductor L1, larger inductive reactance of AC component, so smaller AC component remains on load R, better filtering effect, but DC resistance will also increase.

Figure 3: Schematic diagram of an inductive filter

**Third method for π-type RC filter circuit recognition**

**Figure 4** shows a π-type RC filter circuit. C1, C2 and C3 in circuit are three filter capacitors, R1 and R2 are filter resistors, C1, R1 and C2 make up first π-type RC filter circuit, and C2, R2 and C3 make up second π-type RC filter circuit. . Because this filter circuit is shaped like Greek letter π and uses resistors and capacitors, it is called a π-type RC filter circuit.

Figure 4: π-type RC filter circuit

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

The filtering principle of this circuit is that output voltage of rectifier circuit is first filtered by capacitor C1 to filter out most of AC components, and then added to filter circuit consisting of resistors R1 and C2. The capacitance C2 and R1 form a voltage divider circuit. Since capacitance C2 is very small, attenuation of divided voltage of AC component is large, and purpose of filtering is achieved. For DC, since C2 has a direct blocking action, voltage divider circuit of R1 and C2 has no effect of partial voltage reduction on DC, so DC voltage can be output through R1.

When size of R1 remains same, increasing capacitance of C2 can improve filtering effect. When capacitance C2 remains same, increasing resistance R1 can improve filtering effect. However, value of resistance of filter resistor R1 should not be too large, because DC current flowing through load will flow through R1, and a DC voltage drop will form on R1, which will reduce output DC voltage U. The greater resistance R1 or greater current flowing through load, greater voltage drop across R1, resulting in a lower DC output voltage.

C1 is first filter capacitor, increasing capacitance can improve filter effect. However, when C1 is too large, it will take a long time to charge C1 at startup. This charging current flows through rectifier diode. When charging current is too large and time is too long, rectifier diode will be damaged. . Therefore, using this π-type RC filter circuit can reduce capacitance C1, and filtering effect can be further improved by rationally calculating values of R1 and C2.

This filter circuit has three DC voltage output terminals that output three sets of DC voltages U, U, and U, respectively. Among them, U is only filtered by capacitor C1; U is filtered by C1, R1 and C2 circuits, so filtering effect is better and AC component in U is smaller; U is filtered by two filter circuits, and filtering effect is best Good, so AC component in U is minimal.

The three DC output voltages are different. The voltage U is highest, and usually this voltage is added directly to power amplifier circuit, or to circuit that requires highest DC operating voltage and largest operating current; voltage U is slightly lower because resistor R1 has a voltage drop to DC voltage; voltage U is lowest, this voltage is usually supplied to prestage circuit as a DC operating voltage, since operating voltage is constantth current in circuit of preliminary stage is relatively low, and alternating component in operating voltage of direct current should be less.

**The fourth method of π-type LC filter circuit recognition**

**Figure 5** shows a π-type LC filter circuit. The π-type LC filter circuit is basically same as π-type RC filter circuit. This circuit simply replaces filter resistor with a filter inductor, because filter resistor has same resistance for DC and AC, and filter inductor has a large inductance for AC and a small inductance for DC, which will reduce DC output voltage.

In diagram in **fig. 5** The unidirectional ripple DC voltage output by rectifier circuit is first filtered by capacitor C1 to remove most of AC components, and then added to filters L1 and C2. scheme.

Figure 5: Schematic of a π-type LC filter

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

For DC, since L1 has no inductive reactance, it is equivalent to a path. At same time, diameter of filter inductor is relatively large, and DC resistance is small. So there is basically no voltage drop for DC, so DC output voltage is relatively high, which is main advantage of using inductive filters.

**5. Image recognition method by electronic filter**1

1. Electronic filter.

**Figure 6** shows an electronic filter. VT1 in circuit is a triode that acts as a filter tube, C1 is VT1 base filter capacitor, R1 is VT1 base bias resistor, R is load of this filter circuit, C2 is output voltage filter capacitor.

The principle of operation of electronic filter circuit is as follows:

Figure 6: Schematic of electronic filter

VT1, R1 and C1 in circuit form an electronic filter circuit. This circuit is equivalent to a capacitor with capacity C1×β1. β1 is current gain of VT1, and current gain of transistor is relative. Therefore, equivalent capacitance is very large, which shows that filtering performance of electronic filter is very good. The equivalent circuit is shown in **fig. 6(b)**. C in figure is equivalent capacitance.

R1 and C1 in circuit form an RC filter circuit. On one hand, R1 provides base bias current for VT1 and is also filter resistor. Because current flowing through R1 is base bias current of VT1, this current is very small, and resistance value of R1 can be relatively large, so that filtering effect of R1 and C1 is very good, so that DC voltage on base of VT1 Communication component is minimal. Since emitter voltage tends to follow base voltage, variable component in emitter output voltage VT1 is also very small, which solves filtering problem.

In electronic filters, filtering is mainly implemented by resistors R1 and C1, which are also an RC filter circuit, but different from previously presented RC filter circuit. In this circuit, constant current flowing through load is emitter current VT1, and current flowing through filter resistor R1 is base current VT1, and base current is very small, so resistance value of filter resistor R1 can be set very large (good filtering effect) , but this will not cause a large drop in DC output voltage.

The value of resistance R1 in circuit determines base current VT1, thereby determining voltage drop on tube between collector and emitter VT1, and also determines output constant voltage of emitter VT1, so size of resistor R1 can be changed. adjust size of DC output voltage +V.

2. Electronic voltage stabilization filter

**Figure 7** shows another electronic voltage stabilization filter. Compared to previous circuit, voltage stabilization diode VD1 is connected between VT1 base and ground. The principle of electronic voltage regulation is as follows:

After Zener diode VD1 is connected between base of VT1 and ground, input voltage passes through R1 to put Zener diode VD1 into a reverse bias state. The emitter output of VT1 is also relatively stable. Note: The stability of this voltage is determined by voltage stabilization characteristic of VD1, and has nothing to do with electronic filter circuit itself.

R1 is also a current-limiting protective resistor VD1. After adding zener diode VD1, changing size of R1 cannot change output voltage of emitter VT1, since there is a voltage drop of PN junction at emitter junction of VT1, so emitter output voltage is somewhat less than adjustable value of VD1.

C1, R1 and VT1 can also form an electronic filter circuit that plays role of a filter.

In some cases, a two-pipe electronic filter circuit can be used to further improve filtering effect, and two electronic filter tubes make up a composite pipe circuit. Thus, total increase in current is product of increase in current of each tube, which obviously can improve filtering effect.

**6. Power Filter Circuit Summary**

When analyzing power filter circuit, mainly pay attention to following points:

(1) When analyzing working principle of filter capacitor, it mainly uses “direct blocking” characteristic of capacitor or charging and discharging characteristics, that is, when rectifier circuit outputs a unidirectional pulsating DC voltage, filter capacitor is charged. filter capacitor discharges load.

(2) When analyzing working principle of a filter inductor, it should be generally understood that inductor has little DC resistance and no inductive reactance, but has AC inductive reactance.

(3) When analyzing electronic filter circuit, it is necessary to know that capacitance at base of electronic filter tube is a key component of filter. In addition, to analyze DC circuit, electronic filter tube has a base current, a collector current and an emitter current, and current flowing through load is emitter current of electronic filter tube. and emitter, which leads to a change in output DC voltage of electronic filter.

(4) The electronic filter itself has no voltage stabilization function, but adding a zener diode can make DC output voltage relatively stable.

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