Filtering Circuit Tutorial (Schematic Diagrams)

Schematic Diagrams

L First-order FilterC First-order Filter
CL Second-order FilterRC Second-order Filter
LC Second-order FilterRCR Model T: Third-order Filter
LCL Model T: Third-order FilterCRC TT-Third-order Filter
CLC TT-Third-order Filtersingle stage loe pass filter circuit
DLC Second order Filterbipolar series low pass filter circuit

CLC П-Filter

1. Working Principle

Direction of Filter Current

a. When the positive pulse is input, C1 is charged first, the charging current is ic1, and meanwhile, reaching the peak voltage of pulse. The inductor L also has a linearly increasing current, and the magnetic energy is stored in L. With the increase of the current, more and more magnetic energy is stored, and the capacitor C2 is charged with voltage through inductor L (the charge current is ic2), the voltage of C2 is basically equal to the voltage on C1, in addition, the current IRL in load RL is also supplied by input pulse.

CLC Direction of Filter Current

b. When the input positive pulse disappearing, the current of the load RL is supplied by two channels: one is -ic2 provided by the C2 discharge, the other is converted from the magnetic energy stored by the inductor L, in other words, L connects with C1 to provide current -ic1. The current in the loaded RL is equal to the sum of the discharge currents of the two capacitors, that is IL=-(ic2+ ic1).

c. For DC, C1 and C2 in CLC filter are equivalent to open circuit, and the inductance of inductor L to DC component is zero, which is equivalent to short circuit, so DC component can pass through inductor L1 smoothly.

d. For AC: capacitors have a large capacity equivalent to short circuit, while inductors are sensitive to various sinusoidal waves, so AC components can not be removed or fewer passed.

2. Advantages

High output DC voltage, sometimes the highest peak voltage can reach the rectangular wave. It is suitable for large load current, the output voltage pulsation is small.

3. Disadvantages

It is used in the power supply without voltage regulator, and its load capacity is poor.

4. Application

CLC filter is usually used in switching power supply of pulse-amplitude modulation. The larger the capacitance and inductance, the better the filtering.

DLC Filiter

1. Working Principle

DLC Direction of Filter Current

a. When the secondary winding of the transformer is up positive and down negative, because the output voltage of the secondary winding of the transformer is positive and negative alternating rectangular wave, so the negative half cycle is removed from the D1 rectifier, the positive half cycle passes through the D1 rectifier, the magnetic energy is stored when current flowing through the inductance L. This current is partly charged for C1, and another is for load RL, at the same time, D2 off.

direction of filter current

b. When the inputting positive pulse disappears, the self-inductance voltage generated by the secondary winding of the transformer is up negative and down positive, so the rectifier D1 cut off. The filter has no input voltage, and the current supply of the load RL consists of two parts. One is the magnetic energy stored in the inductor converted to electric energy, the current direction is the same as the original current, and forms the circuit current iL through the fly-wheel diode D2; the other is -ic1 provides by the provided by C1 discharge.

2. The output DC voltage of the DLC filter is the average of the input rectangular wave value.

3. DLC filter is usually used in the pulse-width switching power supply. The output of the DLC needs a parallel resistor to the ground, commonly called "release resistor", in general, 30-50mA current is enough.

CRC П-Filiter

1. Working Principle

CRCπ Filter Circuit

a. The output voltage of the rectifier is filtered by C1 capacitor at first, filtering out most of the AC components. And the voltage after C1 is added to the RC filter circuit composed of RL and C2, then the AC component is further filtered by capacitor C2.

b. There is almost no inductance in the small capacitor C1, also its capacitive reactance is very small, so the high frequency interference component is easily filtered to the ground, that is to say, the filter effect of high frequency AC interference is better.

c. The capacitance capacity is large (C2>C1), the low-frequency AC component flows through C2, so the effect of low-frequency AC interference filtering is better.

d. The resistor has voltage drop and power loss effect on AC and DC, so CRC is only used for low load current.

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