Chapter 6 - Printed Circuit Board Layout for AC-DC and DC-DC Converters

Abstract

Nothing is more important than printed circuit board (PCB) layout. It is the bridge, the only bridge, connecting your wonderful mathcad-verified, spice-validated, IEEE-published, patent pending paper design, and a trouble-free finished product. Every topology is quite different in terms of its recommended layout. In addition, the chapter explains buck–boost PCBs. This topology is the hardest to implement at the layout stage, simply because both the input and output sections see a pulsating current waveform. It is important to minimize almost all the current loops, both on the input side and the output. They all see an edge of current and will therefore complain in the form of voltage spikes. Again, the only exception, in principle, is the inductor. Along with this, it highlights forward converter PCBs that though a forward converter is a buck-derived topology, things change somewhat in terms of PCB layout, by the inclusion of the transformer. A transformer operates on the principle of AC transfer, so it requires the currents on its primary and secondary windings to have sharp edges, or there would be no coupling at all. During a crossover transition the current flow in certain trace sections has to suddenly come to a stop, and in certain others it has to start equally suddenly (within 100 ns or less typically, which is the switch transition time). These trace sections are identified as the “critical traces” in any switcher PCB layout.