[085] Low-Profile Inductors with Helical Foil Windings

Inductors made with helical foil windings.

Introduction

The latest generation of off-the-shelf inductors for dc-dc converters features flat windings with a helical structure. While this reduces AC losses in many cases, the AC resistance is still considerably higher than the DC resistance and must be considered carefully when selecting your inductor. 

Off-The-Shelf Power Inductor Evolution

The first generation of off-the shelf inductors from magnetics manufacturers were cheap components with multiple layers of windings on a drum core, as shown in Figure 1. These inductors are fine if you have nothing but dc current flowing through them, but they exhibit extreme losses when an AC component of current is added. 

The high turns count meant many layers of windings are needed, and this leads to very high proximity losses. Furthermore, the windings are placed between the top and bottom pole pieces of the core and fringing losses are large. If you are planning on using inductors like these, be prepared to thermally test them properly, and derate them as needed – frequently more than 50% from the manufacturer’s data sheets. 

Figure 1: Drum-Core Inductors have Excessive Losses when AC Currents Are Flowing