[048] Forward Converter Design - Part XIV Magnetics Design for Five Outputs

Designing custom magnetics for three- and five-output forward converters.

 Introduction

This article continues the series in which Dr. Ridley documents the processes involved in taking a power supply from the initial design to the full-power prototype. The second layout of the PC board incorporates significant changes in the power stage, changing the number of outputs from three to five. Some of the issues involved in the coupled-inductor magnetics design are discussed.

Three-Output Coupled-Inductor Design

The original specification for the power supply design was as follows:

  1. Output 1 – 35 VDC @ 10A isolated
  2. Output 2 – 35 VDC @ 10 A isolated
  3. Output 3 – 15 VDC Bias power and regulated output, primary referenced
  4. Maximum power 350 W (only one output fully loaded at a time, application is for audio.)
  5. Input – 180 – 265 AC
  6. Power Topology: Two-switch forward

Figure 1 shows the schematic of the three-output forward converter. A single core inductor is used with three windings, one for each of the outputs. This coupled-inductor approach provides the best cross-regulation between each of the outputs. There are two major advantages of coupled inductors – firstly, all of the outputs are tied together through the transformer action of the inductor, preventing them from having individual resonant frequencies. Secondly, regardless of individual loading on each of the outputs, the coupled inductor forces all of the outputs to be in either continuous or discontinuous mode concurrently, greatly improving the regulation.

As shown in an earlier part of this series, excellent regulation was achieved with a wide variation of output loading.

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Figure 1: Forward Converter with Three Coupled-Inductor Outputs

Figure 2 shows the winding layout for the three-output transformer. The first layer of winding was the first secondary output, consisting of 15 turns wound with triple-insulated kapton wire. This winding used a single strand of wire, with a gauge selected to exactly fill one layer of the bobbin. One layer of magnetics tape was added on top of this secondary to provide a smooth winding surface for the subsequent windings.

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