[043] Forward Converter Design - Part X PCB Layout Procedures and Spacing

PCB layout procedures including voltage spacing.  

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. In Part X, Dr. Ridley presents the rules for good PCB layout.

PCB Layout

In part IX of this series, an example was given of a crucial area of PCB layout around the control chip. This illustrated the details that need to be taken care of when laying out a switching power supply and its control circuit. Further examples of how to take care of specific issues on individual power supplies could take an entire book, something that is well beyond the scope of this design series. However, it is worth taking some time to highlight the important issues to be concerned with when laying out a switching power supply.

There are nine fundamental rules that should be followed for a switching power supply board.

Rule Number 1: Use at Least a 2-Layer Board

The cheapest possible power supplies use single-sided boards and jumpers where necessary. While this can be done, it makes a designer’s life extremely difficult. The preference is to use at least two layers. This provides much more design flexibility and a tighter, better performance and more rugged layout.

If working with more than two layers, it is advisable to keep critical power paths on the top and bottom layers where they can be clearly seen, and sometimes repaired in the event of failures during the development program. If inner layers are used, they should be properly spaced from each other, as discussed in Rule number 5 below.