[080] Sepic Converter Measurement - Part I Control Transfer Function

Control measurements for the Sepic converter.

Introduction

This article shows the measured characteristics of a Sepic converter. Depending on the conditions under which you measure the converter, it can appear either easy to control, or extremely difficult. Make sure you always make an extended range of measurements on your Sepic design if you wish to ensure avoiding the troublesome regions.  

Sepic Converter with Wide-Range Input

The Sepic converter is becoming increasingly popular for non-isolated applications. It is a very useful topology in that it can provide either voltage step-up or voltage step-down while preserving the polarity of the input voltage. 

The equation for the voltage gain of the converter is shown in Figure 1. At duty cycles less than 0.5, the converter steps down, and at duty cycles greater than 0.5, the converter steps up. (The buck-boost converter has the same gain equation, but it inverts the output voltage.) 

Some designers are reluctant to use this converter, however, since it has a reputation for difficult control. Other designers have found that in their application, they have had no difficulty with the control design. 

The disparity of opinions about the controllability of the Sepic are aggravated by the difficulty of the analysis for the converter. It is not hard to get Spice simulations of the control bode plots, but analytical expressions are hard to come by. The only known full analysis of the Sepic converter was performed by Dr. Vatché Vorpérian [1], but few engineers are aware of this work. 

In this article, we will take a look at the practical measurements on the Sepic converter rather than trying to dissect or simplify the analysis. 

Figure 1 shows the schematic of the Sepic converter with component values. The capacitors are multilayer ceramics with very low ESR, and the inductors used were off-the-shelf parts. This is typically how most of these converters are designed, and the available standard inductors are quite good for lower power applications (<50 W). Above 50 W, many designers will switch to custom-designed inductors. 

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Figure 1: Sepic Converter Circuit for Control-to-Output Measurements with the AP300 Analyzer

The AP300 analyzer [2,3] was used to inject a test signal into the converter, and measure the gain and phase response from input to output. 

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