[009] Is your Input Filter Causing Trouble?

You can analyze your power supply filter quickly and easily with free downloadable software. This allows you to check for filter impedance interactions, attenuation, and proper damping.

Introduction

This article comes with a free giveaway to help you with your power supply design process. Whether you are designing complete PWM converters from scratch, working with modular power supplies, or something in between, you will be faced with input filter design issues. Once you have read this article, you can download the free program InputFilter.xls to analyze your input filter design quickly and easily.

Is your Input Filter Causing Trouble?

A perpetual problem in designing switching power supplies lies with the input filter. Modern power electronics started with this issue when the very first switching power supplies were built. It was discovered that adding an input filter can make a previously stable system unstable. Much has been written about this in the past, and the reader is encouraged to look at the literature available, starting with reference [1].

With modern dc-dc converters available in either fully packaged form, or as integrated controllers, many new engineers are placing power supplies on a board. The input filter interaction issue continues to plague many designs, especially for engineers who are not familiar with proper design guidelines.

Power supply input filters are used to attenuate switching power supply noise, and to prevent corruption of the input line. If you are designing to meet stringent emissions standards, at least a two-stage filter is needed to attenuate the noise to an acceptable level. If you are designing for board-mount power, and switching substantial currents, at least an LC filter is advised to prevent noise problems on the board.

Figure 1 shows a typical two-stage filter configuration. Two different capacitors are shown on the output of the filter. One provides bulk energy storage, and one provides low impedance at high frequencies.

Figure 1: Input filter with five reactive elements.

Input Filter Attenuation

An input filter works in two ways. First, it attenuates noise from the input source to the output of the filter. In this direction, it can be viewed as a voltage filter, and the transfer function is expressed as the voltage ratio from V_in to V_out.

Then, to attenuate the power supply noise, the filter acts as a current filter. The input of the power supply is a switching current, which drives the filter. The transfer function is from the current at the right of the filter to the current at the left of the filter, assuming the input is short circuited (a voltage source.)

In either case, the attenuation is the same. Figure 2 shows the attenuation for the example filter values of Figure 1.

Figure 2: Input filter attenuation.

You will need to measure the noise spectrum before the filter is added to determine how much attenuation is needed to meet your system requirements.