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.) The input of the power supply is a switching current, which drives the filter. Then, to attenuate the power supply noise, the filter acts as a current 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. First, it attenuates noise from the input source to the output of the filter. Input Filter AttenuationĪn input filter works in two ways. One provides bulk energy storage, and one provides low impedance at high frequencies.įigure 1: Input filter with five reactive elements. Two different capacitors are shown on the output of the filter. 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.įigure 1 shows a typical two-stage filter configuration. 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. Power supply input filters are used to attenuate switching power supply noise, and to prevent corruption of the input line. The input filter interaction issue continues to plague many designs, especially for engineers who are not familiar with proper design guidelines. 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. Much has been written about this in the past, and the reader is encouraged to look at the literature available, starting with reference. It was discovered that adding an input filter can make a previously stable system unstable. Modern power electronics started with this issue when the very first switching power supplies were built. Is your Input Filter Causing Trouble?Ī perpetual problem in designing switching power supplies lies with the input filter. Once you have read this article, you can download the free program InputFilter.xls to analyze your input filter design quickly and easily. 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. This article comes with a free giveaway to help you with your power supply design process.
![input pi filter design input pi filter design](https://i.stack.imgur.com/5s1SU.jpg)
![input pi filter design input pi filter design](https://i.stack.imgur.com/XYoOl.png)
This allows you to check for filter impedance interactions, attenuation, and proper damping. You can analyze your power supply filter quickly and easily with free downloadable software. Information on Downloading Technical Documentsĭownloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.Is your Input Filter Causing Trouble? The next article will discuss capacitors and noise in greater detail. The impedance of paths in which common mode currents flow is raised to shut out the common mode noise.
![input pi filter design input pi filter design](https://www.electroniclinic.com/wp-content/uploads/2020/07/FILTERS-AND-THEIR-CLASSSIFICATION-BASIC-FILTER-TYPES.jpg)
In general, common mode filters for power supply lines are used in the inputs to switching power supplies. Common mode filters can be broadly divided into filters for power supply lines and filters for signal lines. To address common mode noise, a common mode filter (common mode choke) is used. Resistors: Convert noise currents into heat.Ferrite beads: Inductive component reflects low-frequency noise currents,Īnd resistive component converts higher-frequency noise currents into heat.Capacitors: Bypass noise currents to GND.These components act on noise components as follows. The example shown is an π-type filter using an inductor and capacitors. Input Filters for Switching Power SuppliesĪs input filters for switching power supplies, filters with characteristics suited to dealing with common mode noise and with differential noise respectively are used.ĭifferential mode filters employ capacitors, inductors, ferrite beads, and resistors. This time, we briefly explain input filters for switching power supplies, and thereafter give more detailed explanations of each filter type. In the previous article, basic measures to address noise were summarized, differentiating between common mode noise and differential noise.