Input Filter Design for Switching Power Supplies
Input Filter Design for Switching Power Supplies:
Written by Michele Sclocchi
Michele.Sclocchi@nsc.com
Application Engineer, National Semiconductor
The design of a switching power supply has always been considered a kind of magic and art, for all the engineers that design one for the first time.
Fortunately, today the market offers different tools that help the designers. National Semiconductor was the first company to offer the “Simple Switcher” software, and an on-line simulation tool that allows the design and simulation of a switching power supply. New ultra-fast MOSFETs and synchronous high switching frequency PWM controllers allow the realization of high efficient and smaller switching power supply.
All these advantages can be lost if the input filter is not properly designed. An oversized input filter can unnecessarily add cost, volume and compromise the final performance of the system.
This document explains how to choose and design the optimal input filter for a switching power supply application.
The input filter on a switching power supply has two primary functions. One is to prevent electromagnetic interference, generated by the switching source from reaching the power line and affecting other equipment.
The second purpose of the input filter is to prevent high frequency voltage on the power line from passing through the output of the power supply.
A passive L-C filter solution has the characteristic to achieve both filtering requirements. The goal for the input filter design should be to achieve the best compromise between total performance of the filter with size and cost.
UNDAMPED L-C FILTER:
The first simple passive filter solution is the undamped L-C passive filter shown in figure (1).
Ideally a second order filter provides 12dB per octave of attenuation after the cutoff frequency f0, it has no gain before f0, and presents a peaking at the resonant frequency f0.
Figure 1: Undamped LC filter
One of the critical factors
Written by Michele Sclocchi
Michele.Sclocchi@nsc.com
Application Engineer, National Semiconductor
The design of a switching power supply has always been considered a kind of magic and art, for all the engineers that design one for the first time.
Fortunately, today the market offers different tools that help the designers. National Semiconductor was the first company to offer the “Simple Switcher” software, and an on-line simulation tool that allows the design and simulation of a switching power supply. New ultra-fast MOSFETs and synchronous high switching frequency PWM controllers allow the realization of high efficient and smaller switching power supply.
All these advantages can be lost if the input filter is not properly designed. An oversized input filter can unnecessarily add cost, volume and compromise the final performance of the system.
This document explains how to choose and design the optimal input filter for a switching power supply application.
The input filter on a switching power supply has two primary functions. One is to prevent electromagnetic interference, generated by the switching source from reaching the power line and affecting other equipment.
The second purpose of the input filter is to prevent high frequency voltage on the power line from passing through the output of the power supply.
A passive L-C filter solution has the characteristic to achieve both filtering requirements. The goal for the input filter design should be to achieve the best compromise between total performance of the filter with size and cost.
UNDAMPED L-C FILTER:
The first simple passive filter solution is the undamped L-C passive filter shown in figure (1).
Ideally a second order filter provides 12dB per octave of attenuation after the cutoff frequency f0, it has no gain before f0, and presents a peaking at the resonant frequency f0.
Figure 1: Undamped LC filter
One of the critical factors