[051] Forward Converter Design - Part XVII Loop Design and Measurement

Loop design and measurement for the five-output forward converter. 

Introduction

In this article, the loop gain of the five-output forward converter is measured and compared to predictions. The final loop design is compensated with a combination of theoretical design and empirical measurements, resulting in a Type I compensator for optimal results.

Measuring the Loop of a Five-Output Forward

Figure 1 shows a five-output forward converter with coupled inductors. In the last article of this series, the control-to-output transfer function was measured, and found to have significant differences when compared to the predictions. The discrepancies were due to the coupled-output structure of the converter, and parasitics of the magnetics. This leads to measurement results that can be difficult to predict.

After measuring the control-to-output transfer function, the compensation was designed, and the control loop was measured as shown in Figure 1. The loop was closed and measured on the 12 V output of the power supply. Normally, a Type II amplifier would be used for a current-mode control loop. However, the characteristics of the coupled-inductor design provide an area of increasing gain in the control-to-output transfer function with a corresponding phase lead. This allows us to use a simple integrator, or Type I compensation.

Figure 1: Forward Converter with Five Coupled-Inductor Outputs. Loop Gain Measured with the AP300 Analyzer on the 12 V Output.

Figure 2 shows the results obtained for the loop gain measurements. It can be seen that there are discrepancies in the measurement and prediction at all frequencies. This is a common result for multiple-output switching power supplies, which is one of the reasons that measurements are crucial for ensuring stability of hardware designs. Rarely are the models accurate enough for coupled-inductor power stages to properly predict stability of the system and ensure a reliable product.