[007] Capacitors for Switching Power Supplies

The changing characteristics of the capacitors used in power supplies are examined. They are not as simple as most engineers assume.

Introduction

In several past articles, we have examined some of  the complex characteristics of the power magnetics of a switching power supply. In this article, we examine another major passive component of the power supply—the capacitor. This is often a component that is viewed as a simple part that doesn’t require too much attention.

In this article, we examine another major passive component of the power supply—the capacitor. This is often a component that is viewed as a simple part that doesn’t require too much attention.

Power Supply Capacitors

Figure 1 shows the simple buck converter. Given modern integrated controllers, the design task of the engineer is apparently very simple—all we have to do is select and inductor and two capacitors, and the job is done. The choice of inductors can become very complex, and now we’ll see how capacitors can be troublesome, too.

Figure 1: Buck converter with critical capacitor components. The output capacitor impedance determines the mid- and high-frequency response of the converter using either voltage-mode, current-mode, or any other form of control, including digital.

I’m often asked to perform worst-case analysis of switching power supply designs like this for companies. Step one of this process usually consists of the company sending me schematics, parts lists, and component data sheets. Before proceeding further, I always have to ask for working physical samples of the power supplies to test on the bench. To the alarm of people not familiar with power supplies, much of the worst-case analysis depends on measurements of existing designs, combined with documented datasheet variations which will shift the design.

Component data sheets are rarely adequate for properly characterizing parts, and a comprehensive analysis requires information either not readily available from manufacturers. Additional measurements are needed for magnetics, and additional measurements are needed for capacitors, too.   

Low-Impedance Capacitor Measurement

A simple fixed RLC tester with a single measurement frequency is not adequate for characterizing capacitors for use in a modern switching power supply. The capacitors must be measured over a wide range of frequency to fully characterize their behavior.

Figure 2: Measurement setup for low-impedance capacitors  

Figure 2 shows how to make measurements of low-impedance capacitors with a frequency response analyzer [1]. Proper choice of sensing resistor, and proper RF layout of the test circuits will allow you to measure impedances as low as 1 mOhm with this test setup. While many component testers will only look at a single frequency, or a narrow range of frequencies, it is recommended that you sweep the frequency to see the impedance of the component under test from 10 Hz to at least 10 MHz.