Their AC-powered power supplies application is typically immediately after the bridge rectifier circuit and before the switch-mode control circuit. A constantly changing output load or high current drift will result in poor voltage regulation. The Pi filter requires a stable output voltage to be effective. In addition, such components will be bulky and expensive, impacting board design. This limitation also has to be weighed up with the high input capacitance requirements and high voltage rating. This current will also flow through the inductor, meaning that an inductor with a high power-rating will be required in applications with a high output voltage. However, any current flow through the filter when a load is applied to the output will result in a voltage drop, and so the Pi filter cannot provide voltage regulation. Its other main advantage over different filter types is good ripple reduction. The Pi filter will produce a high output voltage with minimal current drain, producing only a very small voltage drop at the output. Finally, the output capacitor filters any AC component that has passed through the inductor. Next, the inductor performs the next filtering stage, effectively removing any ripple. The input capacitor performs the first and foremost stage of filtering out the AC component. The three components that form the Pi filter each act to block alternating current flow and pass direct current flow. ![]() This article will only be looking at the low-pass filter arrangement. The high-pass filter equivalent is formed by using a capacitor in series between the input and output with two inductors, one across the input and the other across the output. ![]() ![]() The main application of Pi filters in power supplies is to smooth a rectifier’s output by acting as a low pass filter. The low-pass filter used for power supply filtering is formed from an inductor in series between the input and output with two capacitors, one across the input and the other across the output. Pi filters can be designed as either low pass or high pass filters, depending on the components used. Pi Filters are a type of passive filter that gets its name from the arrangement of the three constituent components in the shape of the Greek letter Pi (π).
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