The LC Choke

The LC choke input filter is used primarily in power supplies where good voltage regulation is important and where the output current is relatively high and subject to varying load conditions. The LC choke input filter is used in high-power applications such as those found in radar and communication transmitter power supplies.

In the illustration below you can see that the input filter consists of an input inductor or filter-choke (L1) and an output filter capacitor (C1).

Full-wave rectifier LC choke input filter.

Inductor L1 is placed at the input to the LC choke input 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 LC choke input filter, as shown in the next illustration below.

Waveforms for a LC choke-input filter.

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.)

The shunt capacitor (C1) charges and discharges at the ripple frequency rate, but 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. In addition, the reactance of the shunt capacitor (XC) 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 (XC) 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 (transformer), the small resistance of the diode, and the counter emf developed by the coil. Therefore, the RC charge time constant (the next illustration) is short compared to its discharge time.

LC choke-input filter (circuit resistance).

As a result, when the pulsating voltage is first applied to the LC choke-input filter, the inductor or filter choke (L1) produces a counter emf that 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 (RL). C1 will attempt to discharge as indicated in the next illustration. Because of its relatively long discharge time constant, C1 can only partially discharge.

LC choke input filter (discharge path).

The larger the value of the filter capacitor, the better the filtering action. However, due to the physical size, there is a practical limitation to the maximum value of the capacitor.

The inductor or filter choke (L1) maintains the current flow to the filter output (capacitor C1 and load resistance RL) at a nearly constant level during the charge and discharge periods of the filter capacitor.

The series inductor (L1) and the capacitor (C1) form a voltage divider for the ac component (ripple) of the applied input voltage. This is shown in the next picture below. As far as the ripple component is concerned, the inductor offers a high impedance (Z) and the capacitor offers a low impedance. As a result, the ripple component (Er) appearing across the load resistance is greatly attenuated (reduced). Since the inductance of the filter choke opposes changes in the value of the current flowing through it, the average value of the voltage produced across the capacitor contains a much smaller value of ripple component (Er), as compared with the value of ripple produced across the coil.

LC choke input filter (as voltage divider).

Now look at the next picture below, which illustrates a complete cycle of operation where a full-wave rectifier circuit is used to supply the input voltage to the LC choke input filter. The rectifier voltage is developed across capacitor C1. The ripple voltage in the output of the LC choke filter is the alternating component of the input voltage reduced in amplitude by the filter section.

Filtering action of an LC choke-input filter.

Each time the plate 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 ripple frequency of 120 hertz. Although the diodes alternate (one conducts while the other is nonconducting), the filter input voltage is not steady. As the plate 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-plate) resistance, and the counter emf developed by the choke. During the nonconducting interval, (when the plate voltage drops below the capacitor charge voltage), C1 discharges through the load resistance RL. The components in the discharge path cause a long time constant; thus C1 discharges slower than it charges.

The choke (L1) is usually of a large value, on the order of 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 RL, 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, we will look at it using actual component values in the next tutorial.

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