LC Choke-Input Filter

The LC choke-input filter is used primarily in power supplies where voltage regulation
is important and 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.

Notice in figure 4-19 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, as
shown in the figure. The reactance of the inductor (X_{L}) 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.

The shunt capacitor (C1) charges and discharges at the ripple frequency rate, but the
amplitude of the ripple voltage (E_{r}) is relatively small because the inductor
(L1) tends to keep a constant current flowing from the rectifier circuit to the load. In
addition, the reactance of the shunt capacitor (X_{C}) presents a low impedance to
the ripple component existing at the output of the filter, and thus shunts the ripple
component around the load. The capacitor attempts to hold the output voltage relatively
constant at the average value of the voltage.

The value of the filter capacitor (C1) must be relatively large to present a low
opposition (X_{C}) to the pulsating current and to store a substantial charge. The
rate of the charge for the capacitor is limited by the low impedance of the ac source (the
transformer), by the small resistance of the diode, and by the counter electromotive force
(CEMF) developed by the coil. Therefore, the RC charge time constant is short compared to
its discharge time. (This comparison in RC charge and discharge paths is illustrated in
views A and B of figure 4-20.) Consequently, when the pulsating voltage is first applied
to the LC choke-input filter, the inductor (L1) produces a CEMF which opposes the
constantly increasing input voltage. The net result is to effectively prevent the rapid
charging of the filter capacitor (C1). Thus, instead of reaching the peak value of the
input voltage, C1 only charges to the average value of the input voltage. After the input
voltage reaches its peak and decreases sufficiently, the capacitor C1) attempts to
discharge through the load resistance R_{L}). C1 will only partially discharge, as
indicated in view B of the figure, because of its relatively long discharge time constant.
The larger the value of the filter capacitor, the better the filtering action. However,
because of physical size, there is a practical limitation to the maximum value of the
capacitor.

Figure 4-20A. - LC choke-input filter (charge and discharge paths).

CHARGE PATH

Figure 4-20B. - LC choke-input filter (charge and discharge paths).

DISCHARGE PATH

The inductor (also referred to as the filter choke or coil) serves to maintain the
current flow to the filter output (R_{L}) at a nearly constant level during the
charge and discharge periods of the filter capacitor. The inductor (L1) and the capacitor
(C1) form a voltage divider for the ac component (ripple) of the applied input voltage.
This is shown in views A and B of figure 4-21. As far as the ripple component is
concerned, the inductor offers a high impedance (Z) and the capacitor offers a low
impedance (view B). As a result, the ripple component (E_{r}) appearing across the
load resistance is greatly attenuated (reduced). The inductance of the filter choke
opposes changes in the value of the current flowing through it; therefore, the average
value of the voltage produced across the capacitor contains a much smaller value of ripple
component (E_{r}) than the value of ripple produced across the choke.

Figure 4-21. - LC choke-input filter.

Now look at figure 4-22 which illustrates a complete cycle of operation for a full-wave
rectifier circuit used to supply the input voltage to the filter. The rectifier voltage is
developed across the capacitor (C1). The ripple voltage at the output of the filter is the
alternating component of the input voltage reduced in amplitude by the filter section.
Each time the anode of a diode goes positive with respect to the cathode, the diode
conducts and C1 charges. Conduction occurs twice during each cycle for a full-wave
rectifier. For a 60-hertz supply, this produces a 120-hertz ripple voltage. Although the
diodes alternate (one conducts while the other is nonconducting), the filter input voltage
is not steady. As the anode voltage of the conducting diode increases (on the positive
half of the cycle), capacitor C1 charges-the charge being limited by the impedance of the
secondary transformer winding, the diode's forward (cathode-to-anode) resistance, and the
counter electromotive force developed by the choke. During the nonconducting interval
(when the anode voltage drops below the capacitor charge voltage), C1 discharges through
the load resistor (R_{L}). The components in the discharge path have a long time
constant; thus, C1 discharges more slowly than it charges.

Figure 4-22. - Filtering action of the LC choke-input filter.

The choke (L1) is usually a large value, from 1 to 20 henries, and offers a large
inductive reactance to the 120-hertz ripple component produced by the rectifier.
Therefore, the effect that L1 has on the charging of the capacitor (C1) must be
considered. Since L1 is connected in series with the parallel branch consisting of C1 and
R_{L}, a division of the ripple (ac) voltage and the output (dc) voltage occurs.
The greater the impedance of the choke, the less the ripple voltage that appears across C1
and the output. The dc output voltage is fixed mainly by the dc resistance of the choke.

Now that you have read how the LC choke-input filter functions, it will be discussed
with actual component values applied. For simplicity, the input frequency at the primary
of the transformer will be 117 volts 60 hertz. Both half-wave and full-wave rectifier
circuits will be used to provide the input to the filter.

Starting with the half-wave configuration shown in figure 4-23 , the basic parameters
are: With 117 volts ac rms applied to the T1 primary, 165 volts ac peak is available at
the secondary [(117 V) X (1.414) = 165 V]. You should recall that the ripple frequency of
this half-wave rectifier is 60 hertz. Therefore, the capacitive reactance of C1 is:

Figure 4-23. - Half-wave rectifier with an LC choke-input filter.

This means that the capacitor (C1) offers 265 ohms of opposition to the ripple current.
Note, however, that the capacitor offers an infinite impedance to direct current. The
inductive reactance of L1 is:

The above calculation shows that L1 offers a relatively high opposition (3.8
kilohms)
to the ripple in comparison to the opposition offered by C1 (265 ohms). Thus, more ripple
voltage will be dropped across L1 than across C1. In addition, the impedance of C1 (265
ohms) is relatively low with respect to the resistance of the load (10 kilohms).
Therefore, more ripple current flows through C1 than the load. In other words, C1 shunts
most of the ac component around the load.