This article explores three essential types of filters used in power supply circuits. inductor filters (L-input), pi ($\pi$) filters, and RC filters. It explains how each filter type functions to reduce ripple and maintain a steady DC voltage, their construction, performance characteristics, and typical applications.
L-Input Filters
An inductor is a device that has the ability to store and release electrical energy. It does this by taking some of the applied current and changing it into a magnetic field. An increase in current through an inductor causes the magnetic field to expand. A decrease in current causes the field to collapse and release its stored energy.
The ability of an inductor to store and release energy can be used to achieve filtering. An increase in the current passing through an inductor causes a corresponding increase in its magnetic field. Voltage induced in the inductor due to a change in its field opposes a change in the current passing through it. A decrease in current flow causes a similar reaction. A drop in current causes its magnetic field to collapse. This action also induces voltage in the inductor. The induced voltage in this case causes a continuation of the current flow. As a result, the added current tends to raise its decreasing value. An inductor opposes any change in its current flow. Inductive filtering is well suited for power supplies with a large load current.
L-Input Filter Circuit Configuration
An L-input filter is shown in Figure 1. The inductor is simply placed in series with the rectifier and the load. All the current supplied to the load must pass through the inductor (L). The filtering action of L does not let a pronounced change in current to take place. This prevents the output voltage from reaching an extreme peak or valley. Inductive filtering, in general, does not produce as high an output voltage as capacitive filtering. Inductors tend to maintain the current at an average value. A larger load current can be drawn from an inductive filter without causing a decrease in its output voltage. When an inductor is used as the primary filtering element, it is commonly called a choke. The term choke refers to the ability of an inductor to reduce ripple voltage and current.
Figure 1. Inductive Filter Circuit Diagram and Output
Combination Filters
In electronic power supplies, choke filters are rarely used as a single filtering element. Instead, combination filters that combine different components, such as inductance, capacitance, and resistance, are more typically used.
LC Filters
A combination inductor-capacitor or LC filter is more widely used. This filter has a series inductor with a capacitor connected in parallel with the load. The inductor controls large changes in load current. The capacitor, which follows the inductor, is used to maintain the load voltage at a constant voltage value. The combined filtering action of the inductor and capacitor produces a rather pure value of load voltage. Figure 2 shows a representative LC filter and its output.
Figure 2. LC Filter Circuit Diagram and Output
Pi Filters
When a capacitor is placed in front of the inductor of an LC filter, it is called a pi filter. In effect, this circuit becomes a CLC filter. The two capacitors are in parallel with $R_{L}$, and the inductor is in series. Component placement of this circuit in a schematic resembles the capital Greek letter pi ($\pi$), which accounts for the name of the filter. See the schematic of a pi filter in Figure 3.
Figure 3. Pi Filter Circuit Diagram
The operation of a pi filter can best be understood by considering $L_{1}$ and $C_{2}$ as an LC filter. This part of the circuit acts on the output voltage developed by the capacitor-input filter $C_{1}$. $C_{1}$ charges to the peak value of the rectifier input. It has a ripple content that is very similar to that of the C-input filter of Figure 4. This voltage is then applied to $C_{2}$ through inductor $L_{1}$. $C_{1}$ charges $C_{2}$ through $L_{1}$. $C_{2}$ then holds its charge for a time interval determined by the time constant of $C_{2}$ and $R_{L}$. As a result, there is additional filtering by $L_{1}$ and $C_{2}$. The ripple content of this filter is much lower than that of a single C-input filter. There is, however, a slight reduction in the DC supply voltage to $R_{L}$ due to the voltage drop across $L_{1}$.
A pi filter has very low ripple content when used with a light load. An increase in load, however, tends to lower its output voltage. This condition tends to limit the number of applications of pi filters. Pi filters have been used to supply radio circuits, stereo amplifiers, and TV receivers. A full-wave rectifier nearly always supplies the input to this filter.
RC Filters
In applications where less filtering can be tolerated, an RC filter can be used in place of a pi filter. As shown in Figure 4, the inductor is replaced with a resistor. An inductor is rather expensive, quite large physically, and weighs a great deal more than a resistor. This filter is similar to a pi filter; however, it uses a resistor in place of an inductor and produces a lower DC output voltage and more ripple. The performance of the RC filter is not quite as good as that of a pi filter. There is usually a reduction in DC output voltage and increased ripple.
Figure 4. RC Filter Circuit Diagram
When operating, $C_{1}$ charges to the peak value of the rectifier input. A drop in rectifier input voltage causes $C_{1}$ to discharge through $R_{1}$ and $R_{L}$. The voltage drop across $R_{1}$ lowers the DC output to some extent. $C_{2}$ charges to the peak value of the $R_{L}$ voltage. The DC output of the filter is dependent on the load current. High values of load current cause more voltage drop across $R_{1}$. This, in turn, lowers the DC output. Low values of load current have less voltage drop across $R_{1}$. The output voltage, therefore, increases with a light load. In practice, RC filters can be used in power supplies that have 100 mA or less of load current. The primary advantage of RC filters is reduced cost.
Review Questions
- The purpose of a filter circuit is to increase _____ and decrease _____.
- Two types of inductive filter circuits are _____ and _____.
- When a capacitor is attached to the front of the inductor of an LC filter, it is called a(n) _____ filter.
- A(n) _____ filter places an inductor in series with the rectifier and the load.
- The purpose of a(n) _____ connected across the output of a rectifier circuit is to regulate voltage.
- The purpose of a(n) _____ placed in series with the load is to regulate current flow.
- The _____ filter is suited for power supplies in which the load requires a high level of current and a pure value of load voltage.
Answers
- DC voltage, ripple
- LC, pi
- pi
- LC
- voltage regulator
- current regulator
- LC
Key Takeaways
Inductor, pi, and RC filters are fundamental components in power supply design, each tailored for specific performance and application needs. Inductor (L-Input) filters are ideal for circuits with high and steady load currents, offering reliable current smoothing. Pi filters, with their low ripple output, serve well in sensitive analog systems like radios and audio amplifiers. RC filters, though less efficient, provide a cost-effective alternative where load currents are minimal and ripple tolerance is higher.