LC Choke-Input Filter

TC 9-62

LC Choke-Input Filter

4-49. The LC choke-input filter is used primarily in power supplies where voltage

regulation is important. It is also used where the output current is relatively high and

subject to varying load conditions. This filter is used in high power applications such as

those found in radars and communication transmitters.

4-50. Figure 4-19 shows that this filter consists of an input inductor (L1), or filter choke,

and an output filter capacitor (C1). Inductor L1 is placed at the input to the filter and is in

series with the output of the rectifier circuit. Since the action of an inductor is to oppose

any change in current flow, the inductor tends to keep a constant current flowing to the

load throughout the complete cycle of the applied voltage. As a result, the output voltage

never reaches the peak value of the applied voltage. Instead, the output voltage

approximates the average value of the rectified input to the filter (see Figure 4-19). The

reactance of the inductor (XL) reduces the amplitude of ripple voltage without reducing the

DC output voltage by an appreciable amount (the DC resistance of the inductor is just a

few ohms).

Figure 4-19. LC Choke-input Filter

4-51. The shunt capacitor (C1) charges and discharges at the ripple frequency rate.

However, the amplitude of the ripple voltage (Er) is relatively small because the inductor

(L1) tends to keep a constant current flowing from the rectifier circuit to the load. The

reactance of the shunt capacitor (XC) also presents low impedance to the ripple component

existing at the output of the filter, and thereby shunts the ripple component around the

load. The capacitor attempts to hold the output voltage relatively constant at the average

value of the voltage.

4-52. The value of the filter capacitor (C1) must be relatively large to present a low

opposition (XC) to the pulsating current and to store a substantial charge. The rate of the

charge for the capacitor is limited by the following:

Low impedance of the AC source (the transformer).

By the small resistance of the diode.

By the cemf developed by the coil.

4-18

TC 9-62

23 June 2005