[035] Forward Converter Design - Part II Initial Testing

Initial testing on a forward converter design.  

Introduction

This article continues the series in which Dr. Ridley documents the processes involved in getting a power supply from the initial design to the full-power prototype. In part II, we focus on the interface between the power stage and controller, and getting the gate drive working properly.

Power Supply Development Testing

The results of initial testing are shown in this article for a two-switch forward converter. This was a project for which the initial prototype breadboard was complete, and it was time to apply control and power signals. As mentioned in the first article of this series, there were a total of 85 process steps involved in moving the power supply from the original non-working condition to a full power board. It would take too many articles to present every detail of each of these steps, so I will focus on the events which I have observed many times before in the industry.

Power Supply Requirements

The specification for the power supply was as follows:

Output 1 – 35 VDC @ 10A isolated

Output 2 – 35 VDC @ 10 A isolated

Maximum power 350 W (only one output fully loaded at a time, application is for audio.)

Input – 180 – 265 AC

Power Topology: Two-switch forward

Controller: Digital controller from TI

Power Stage and Control Interface Schematic

The full schematic of the power stage is shown in Figure 1, together with the signals for interfacing with the control circuit. The power stage was completely separated from the control so that each could be worked on individually, and layouts for each were independent. This is crucial for a digital power supply controller which needs fine-pitch layout versus the heavy copper suitable for the power stage.

It is quite common to find errors in either circuit that can require a new layout – keeping the power and control separate can greatly speed up the development process. For early testing, it is often a good idea to provide a connector between the two, with two separate boards.

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Figure 1: Power Stage and Control Interface Schematic

For high power work, using either digital or analog controllers, it is best to have only one ground connection between the control circuit and the power circuit. There should be no overlap of ground planes in order to prevent power currents from flowing near the control parts. A single point ground should be established between the two parts of the circuit.

The drive for the gate of an offline power supply should be galvanically isolated for both of the power FETs using a gate drive transformer as shown in Figure 1. This is much better than using an integrated high-side driver. A gate drive transformer has the advantage of providing negative gate voltage during the off-time which provides superior noise immunity from accidental turn on of the FET. A gate drive transformer is also much better during the development phase when power stage failures are likely to occur. If the high-voltage power FET is destroyed, the damage is usually limited to the secondary of the gate drive transformer, and the control circuit is left intact. This is crucial for finding the root cause of failures.

The current-sense network is also galvanically isolated. The current transformer network shown provides the cleanest signal for current sensing, and its wide bandwidth allows for protection of the power devices as we shall see later. 

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