[005] High Frequency Power Inductor Design

Proximity Loss in Magnetics Windings

Exploring the use of custom versus off-the-shelf magnetics design. Standard parts are designed to a price point, and must be tested properly if you want to use them. 

Power Supply Design Tips 

In this article, we continue the theme of custom versus off-the-shelf magnetics design. The relatively simple output inductor of a forward converter is used as an example to show the issues and pitfalls involved in trying to use off-the-shelf inductor designs for high-frequency power supply applications.

Power Inductor Design

When I first started my career in power electronics, over 25 years ago, all magnetic components were custom designed and manufactured. In our current power supply design workshops, we dedicate an entire day to the theory and practice of making custom inductors for switching power supplies. Attendees at the course design, analyze, and build their own inductors and transformers in the lab.[1]  

Figure 1: Building custom magnetics in our design workshop.

In the last few years, numerous companies have developed kits of standard parts aimed at the semiconductor companies to integrate into their designs. This proliferation of parts has recently led me to wonder if custom inductor design was becoming a skill belonging more to the past than the future.

Figure 2 shows two inductor designs, one developed in our workshop for a rugged 60-W converter, and the smaller low-cost, off-the-shelf part. You can see a significant size advantage of the standard component.

Figure 2: (a) 47 µH custom inductor and (b) off-the-shelf commodity component

The 47 µH custom inductor was designed to operate continuously at 5 A, with plenty of margin for overcurrent situations.

The off-the-shelf inductor, also 47 µH, had a single number assigned to it on the parts kit—a current rating of 5 A. Since the parts kit is aimed at the non-experienced power supply designer, it seems reasonable to assume that the inductor is suitable for a 5 A converter application

The parts are quite different in their design, as shown in Fig. 3. The custom-made inductor has a single-layer winding on an RM8 core, with a gapped center leg. The windings are a significant distance away from the center leg gap, essential to avoid proximity losses in the windings at high frequencies.