Coupled inductors for power supplies: advantages and compromises

Advanced semiconductor technology allows an increasing number of transistors to be integrated onto ever-smaller silicon chips. This increases the power density, however, and presents power-management challenges, especially in computing and telecommunications applications. First, the higher the integration, the higher the power required to operate the chip. In addition, with the transistors working at higher frequency, there is more dynamic load on the power supply. The output voltage can be decreased to avoid overheating the silicon, but the lower operating voltage tightens the voltage-regulation requirements.

In these applications, the most-widely adopted circuits are the multiphase buck converter shown in Figure 1, and its isolated versions, including full-bridge, half-bridge, and forward converters. To optimize the design, several factors, such as control and power devices, need to be considered. Another important factor is the output filter inductor, which influences several aspects of a power-supply's performance. With a larger inductance, there is smaller current ripple. This leads to higher efficiency, lower output voltage ripple, and lower EMI noise. Unfortunately, larger inductance also means that more energy is stored in the magnetic, which results in a larger footprint and slower transient response.

Figure 1. A two-phase buck converter

This article will discuss inductor design for switching converters, and a solution that uses coupled inductors. First, the fundamental limitations of the traditional inductor in low-voltage, high-current, fast-transient applications are described. Next, the operation and behavior of coupled inductors is explained. New problems are encountered when implementing coupled inductors, however, and several controller-based solutions are proposed. Lastly, a two-phase buck converter design is shown as an example.