What’s the difference between conventional and planar switching power transformers?

The demand for higher efficiency and smaller packages has been the driving force behind advances in switch-mode power-conversion topologies including buck, boost, flyback, forward converters, and others. Requirements for smaller devices coupled with the demand for higher power densities are being achieved via innovative component packages.

Traditional MOSFET power-conversion topologies have responded to these demands through the development of devices designed to operate with lower switching losses at higher frequencies. Over the past several years, the emergence of wide-bandgap (WBG) devices capable of operating at yet even higher frequencies has accelerated the drive toward higher efficiency and smaller packaging. Planar magnetics devices are replacing traditional transformers and inductors in some of these higher-frequency power-conversion applications. 

This article offers a brief answer to two questions in this arena: What’s the difference between conventional and planar magnetics? How do you choose the right one for your application?

Conventional vs. planar transformer

A traditional switching power supply transformer (Fig. 1) consists of primary and secondary wire windings wound on a bobbin and ferrite core. Wire insulation and tape are used to separate the windings. The bobbin and core configuration are determined by the circuit topology.

A planar magnetics transformer (Fig. 2) replaces the wound wire and bobbin with thin copper sheets “wound” on a printed circuit board. The PCB is sandwiched between a ferrite core and fastened with rivets.