[020] Buck-Boost Converter with Current-Mode Control

Free downloadable software shows the characteristics of the current-mode buck-boost (flyback) converter.

Introduction

In this article, Dr. Ridley presents a summary of current-mode control for the buck-boost converter. A free piece of analysis software, the final one in a series of six, is provided to readers of this column to aid with the analysis of their current-mode buck-boost converters.

Modeling Power Supplies with Current-Mode Control

The buck-boost converter (or flyback converter in its isolated version) is the most popular converter for generating low power with multiple output voltage levels. The converter can be run in many different modes – discontinuous conduction mode (DCM), continuous conduction mode(CCM),  quasi-resonant mode (DCM with switching at the bottom of the DCM ring wave), and various options of fixed or variable frequency.  The choice of operation depends on the power level, the application, and the control chip used.

Regardless of the mode of operation of the power stage, current-mode control is almost always used, as shown in Figure 1. Many designers strive to always keep their converter in DCM in order to avoid some of the complexities of control. This is not necessary if current mode control is used, and keeping the converter DCM under every condition of line and load can create large peak stresses in the semiconductors.

Figure 1: Buck-boost converter with current-mode control. The green components show the current feedback; without these, the control is voltage-mode.

While it is an intuitive control scheme, the proper analysis of current mode control is complex.  The dynamic analysis of current mode involves advanced techniques, including discrete-time and sampled-data modeling. This is essential to arrive at a model which explains all of the phenomena seen with your converter, and which accurately predicts the measured control-to-output response and loop gain of the current-mode converter. The full analysis of current-mode control can be downloaded from www.ridleyengineering.com.

Figure 2 shows a plot of the control characteristics of the buck-boost converter in both DCM and CCM. Notice that these characteristics do not change very much at low frequencies, making control optimization straightforward for converters that must operate in both regions.