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

July 22, 2019 //By Dennis Earley
planar switching
Escalation in the use of higher frequencies for today’s power-conversion applications has designers increasingly looking at planar magnetics devices over traditional 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

1. Here’s an example of a bobbin and core transformer
for a 100-kHz switch-mode power supply.

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.

2. This is a planar magnetics transformer that’s optimized
for switching power supplies operating at up to 700 kHz.

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.

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