[045] Forward Converter Design - Part XII PCB Layout Voltage Planes

PCB layout rules for use of ground and other planes. 

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. Parts X and XI of this series of articles presented the first seven rules for good PC board layout. Part XII finishes up the design rules for good PCB layout design, focusing on the judicious use of planes.

PCB Layout Rule 8: Use Planes to Capture Noise

Switching power supplies generate high-frequency waveforms with both high-frequency and high-amplitude current and voltage pulses. In the last article of this series, it was shown how to keep current loops small in order to minimize leakage inductance and magnetic EMI generation. This is an essential step since shielding of magnetically-generated noise is difficult. It is better not to generate it in the first place, hence the need to close the high-frequency current loops.

Noise is also generated by electrostatic antennae created by the power supply components. Nodes which have high-frequency voltage waveforms, when coupled with large-area components or heatsinks, will generate considerable amounts of EMI.

Figure 1 shows the basic schematic of the forward converter with a single output. The worst electrostatic noise generators are shown in blue, red, and purple. These are the nodes of the circuit with the highest dv/dt waveforms. Typically, the waveform shown in red is the worst offender since it is tied to the large surface area of the drain of the lower power FET.

Figure 1: Forward Converter Schematic showing Electrostatic Noise Generators

Shielding planes can be used to greatly reduce the effect of these noise-generating nodes. Three such planes are shown in Figure 2. These can be implemented in the PC board, if space and available layers allows, and if the power components are surface mounted. The planes should be tied to local RF ground points, such as the return of the main power rail on the input, or the return of the output voltage.