[44] Forward Converter Design - Part XI PCB Layout Current Loops

PCB layout procedures including current loops. 

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. Part X of this series of articles presented the first five rules for good PC board layout. Part XI concentrates on the sixth and seventh rules, which are concerned with the routing of high-frequency currents.

PCB Layout Rule 6: Keep High-Frequency Current Loops Small

Switching power supplies generate high-frequency waveforms with both high-current and high-voltage pulses. These waveforms generate a broad spectrum of frequency components for square-wave converters. If the PC board is not laid out properly, the high-frequency currents can create numerous problems, including large stray inductances with associated energy storage, increased component stresses, and EMI issues that are very difficult to suppress.

These problems can be controlled by following the sixth rule of good PCB layout: Keep high-frequency current loops as small as possible. This requires careful thought regarding circuit functioning and board layout.

Figure 1 shows the basic schematic of the forward converter with a single output. When the power switches are turned on, a high-frequency pulse of current flows through components C1, T2, Q1, T1, and Q2. A corresponding pulse of current flows through the secondary of the transformer, and the forward diode.

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Figure 1: Forward Converter Schematic with High-Frequency Primary Current Path

It is important that the loop containing these components presents a low inductance. Figure 2 shows the placement of the primary components, and a preliminary layout of the board. In this example, all of the traces are placed on the upper layer and shown in red. This facilitates the testing of the power supply, and is useful for a single-sided board. Attention has been paid to ensure that the traces are well separated for voltage breakdown considerations.

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Figure 2: Forward Converter Primary Component Layout on a Single Layer.

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