The role of capacitors in AC filter circuits

Updated: Sep 22

Most electronic devices are not designed to use voltage in ac form. A rectifier circuit is used to convert ac voltage to dc form. The voltage coming from a rectifier has unwanted ac components, and it is necessary to remove them. Removing these components from the rectified voltage helps to improve the overall performance of an electronic circuit. In order to achieve that, a smoothening component or circuit, usually known as a filter, is added after the rectifier. In this article, we will explore some of the most common filtering circuits and the important role that capacitors play in them.


Rectifier circuits


Before we dive into the operation of filter circuits, it is important that we understand how rectifiers work and the outputs they produce. The two most common types of rectifiers are shown in Figure 1 and Figure 2. The outputs produced by the two circuits are as shown in Figure 3 and Figure 4 respectively. The ac components present in the two outputs are undesirable, and filter circuits are used to remove them.



Figure 3

Figure 4

Implementation of filter circuits


So, how does a filter component or circuit eliminate unwanted ripples? In a power supply system involving ac to dc voltage conversion, a filter blocks ac components while allowing dc components to flow through the load. The presence of a filter helps to minimize the number of ac components that flow through the load. There are various types of filter circuits. In this article, we will focus on filters that are implemented using capacitors, resistors, and inductors.


Capacitors and inductors have unique electrical properties that make them popular for use in different types of filter circuits. To start with, a capacitor allows ac signals to pass while blocking dc components. On the other hand, an inductor allows dc components to pass while blocking ac signals. Filter circuits work by exploiting these inherent characteristics of capacitors and inductors. Filtering circuits differ depending on which component is used and the configuration of the components when more than one element is used.


The most common types of filter circuits include capacitor filters, inductor filters, LC filters, and RC filters.

Capacitor filters


Adding a shunt capacitor between the rectifier and the load helps to eliminate unwanted ripples. This shunt capacitor also acts like a smoothening device for the current that is fed to the load. When a capacitor of a suitable value is placed across the load, the undesired ripples from the rectifier are bypassed through the filter. This is because the shunt capacitor offers high impedance to dc components and very low impedance to high frequency ac components.


Figure 5 shows a half-wave rectifier circuit with a capacitor filter. When current is flowing from the rectifier, the capacitor is charged. The energy stored by the capacitor is then released to the load when the rectifier is not providing current. This cycle significantly helps to reduce undesired ac components and voltage fluctuations. Only a small amount of residual ac components manage to flow through the load. Figure 7 shows the output produced by a half-wave rectifier with a capacitor filter.


Figure 7

The value of the load resistance significantly determines the capacitor discharge time constant and the amount of ripples in the dc current. On the other hand, the capacitance of the filtering component determines the average voltage level as well as the amount of ripples in the output voltage. It is important to consider the output current of the rectifier when selecting a capacitor for filtering applications.

A small capacitance value results in a low average voltage. In addition, the amount of charge that such a component can store is low. In most cases, a component with a relatively high capacitance value is used. Although a component with a high capacitance value is required, it is important to consider that a component with a very high capacitance value has high current demands. A high current value can damage the diode and overheat the filtering component.


As compared to a half-wave rectifier, a full-wave rectifier yields a higher average voltage value and a lower number of ripples. This is because a full-wave rectifier provides current during both positive and negative cycles. Despite the good performance of this filter, its presence in a circuit causes an increase in the diode current. An LC filter is commonly used in many circuits to overcome this limitation. Figure 6 and Figure 8 show a full-wave rectifier circuit with a capacitor filter and the output produced by the circuit, respectively.


Figure 8

Inductor filters


One of the simplest ways of implementing a filter is by adding a series inductor of a suitable value after the rectifier circuit. This inductor restricts the flow of ac components to the load, thereby eliminating the unwanted ripples. The series inductor offers very low impedance to dc components thus allowing dc current to flow through the load.

Inductors store energy in form of magnetic fields, and the presence of a series inductor after a rectifier helps to smoothen current and eliminate undesired fluctuations. This component achieves that by storing energy when the current exceeds a certain limit and releasing it when it drops below the mean value. As a result, the load receives smoothened current with minimized fluctuations. Figure 9 shows a full-wave rectifier with a series inductor filter.


The presence of a series inductor also introduces a small resistance to the circuit. This resistance forms a voltage divider with the load resistance. In most cases, this resistance is ignored because it is very small as compared to the load resistance. In addition, this filter affects both output voltage and output current resulting in reduced effective and peak values. Using a combination of filtering components helps to overcome this limitation.


Inductor-capacitor (LC) filters


An LC filter is a combination of a capacitor and an inductor, and it delivers better performance than single-component filters. LC filters are commonly used in today’s circuits, and there are different ways of implementing them. In terms of performance, LC filters yield high efficiency, relatively low diode current and good ripple performance.

Some of the most common implementations of LC filters are L-section filters, capacitor input LC filters, and pi filters. In the case of an L-section filter, a series inductor is added after the rectifier followed by a shunt capacitor across the load as shown in Figure 10.


The inductor in the L-section filter circuit allows dc components to pass while restricting ac signals. It achieves this filtering by offering high impedance to ac components and low impedance to dc components. On the other hand, the shunt capacitor across the load bypasses residual ripples that manage to flow through the inductor.


In practical applications, several L-section filters are arranged in series to form a cascaded circuit. Cascaded L-section filters produce smoother outputs. Despite its impressive performance, this filter is not used in circuits where the size of the circuit is acritical factor. A capacitor input LC filter employs the same principle of operation, and the only difference is that the filtering process starts with the capacitor. A full-wave rectifier with a capacitor input LC filter is shown in Figure 11.


A pi filter is a special type of LC filters that is implemented by combining two capacitors and an inductor. Like the name suggests, the three components are combined to form a shape that resembles pi as shown in Figure 12. From the rectifier, the output is fed to a shunt capacitor that bypasses ac components while allowing dc components to flow through the inductor. The inductor allows dc components to flow through while restricting residual ac components that escape the first shunt capacitor. The second capacitor is connected across the load and bypasses the remaining ac components. Only a small amount of ripples manage to escape to the load.


Since the pi filter has more filtering components, it yields better filtering performance as compared to other inductor-capacitor configurations. Its average dc voltage is higher and the ripple factor is smaller. On the flip side, the peak inverse voltage and peak diode current produced by this configuration are usually high. In addition, the pi filter has poor voltage regulation.


Resistor-capacitor (RC) filters


An RC filter is similar to an LC pi filter except that a resistor is used instead of an inductor. The ac components that manage to escape the first filtering capacitor are dropped across the series resistor. The second capacitor is connected across the load and helps to bypass residual ac components. A full-wave rectifier with an RC filter is shown in Figure 13.


Although an RC filter overcomes some of the limitations of inductor-capacitor filters, its voltage regulation performance is poor. The presence of the series resistor causes a considerable voltage drop that lowers the overall efficiency of the circuit. Moreover, RC filters generate excessive heat and proper ventilation is required to ensure that the circuit does not overheat. These drawbacks limit applications of RC filters in today’s power supply systems.


Conclusion


Half-wave and full-wave rectification circuits produce outputs that contain unwanted ac components. A filter circuit is added after the rectifier to prevent the unwanted ripples from flowing through the load. Capacitors are widely used for filtering applications in both half-wave and full-wave rectifier circuits. Although a single component can be used as a filter, most filtering circuits employ a combination of a capacitor and an inductor (LC filter) or a capacitor and a resistor (RC filter). Some of the key factors to consider when selecting a suitable filter circuit for your application include voltage regulation performance, peak inverse voltage, peak diode current, and filtering efficiency.


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