Introduction

In this article, Dr. Ridley continues the topic of frequency response measurements for switching power supplies. This seventh article discusses the approach of trying to use step-load testing to estimate stability and design the control loop.

Step Load Testing

In the last six articles in this series, the importance of frequency response measurements has been shown. Yet it is still a fact that many power supply designers do not make proper loop gain measurements needed for fast and reliable designs. The reasons are varied—lack of time, knowledge, or budget to purchase the right kind of equipment.

Many old-timers in the industry claim that they can see all the characteristics necessary by just looking at the step-load transient response, and that there is no need to make loop measurements at all. This misconception can often lead to expensive errors in design, long and expensive time delays in product development, and instability in the field.

In these next two articles, we will examine the approach of using step-load responses to see how it does not guarantee proper design of the feedback compensation.