LC Filter Design

Abstract

All DC/DC converters from Ericsson Power modules have a built-in output fi lter, which mainly consists of L and C elements. However, in some applications there is a requirement for better ripple performance than what the product is specifi ed for. This design note is intended to be a design guideline and describes the steps to design a simple LC fi lter aimed to reduce the fundamental output ripple of our DC/DC modules. Other parameters such as cost, component availability and size should also be taken in consideration when designing the fi lter. Capacitors There is no perfect capacitor. The equivalent circuit below of a capacitors is above. R1 (ESR) value will decide performance of attenuation and not only the capacitance value. L value will affect max operating frequency for the capacitor. The impedance of a capacitors is: X C = 1/ (2* π * f * C. Inductors There is no perfect inductor. You will have the parasitic elements R1 and C1. R1 will lower the effi ciency due to resistive losses. C1 will conduct high frequencies due to capacitive coupling between windings and lower the usable frequency range. The impedance for the inductor is: X L = 2 * π * frequency * L. The fi rst step is to choose an inductor. The current rating of the inductor has to be equal to or larger than the maximum output current of the converter. The induc-tor must not be saturated. If the inductor saturates the inductance will decrease and the output ripple will increase. Remember that the inductance has a negative in-fl uence on the dynamic load response. If the inductance is high the dynamic response will be slower. It often requires a small value to achieve the desired damping of the output ripple. Selecting a large inductor will require a large capacitor in order to counteract the large voltage drops caused by load transients. The next step is to choose a capacitor. The aim here is to select a capacitor with as high a value as possible with considerations taken to voltage ratings, size, cost and dynamic response. The added cost, for choosing a higher value of the capacitance than the minimum required , is usually compensated for by the enhanced performance , in terms of ripple and dynamic load response, of the fi lter and should be considered when selecting components. The following equation is used …