Green energy applications such as hybrid vehicles, wind energy, and solar energy stand to reap big benefits from regulated electronic power conversion common to switch-mode power supplies (SMPS).
Insulated gate bipolar transistor (IGBT) advances are now facilitating the development of switched-mode power in the 5- to 50-kW range, putting switched-mode power solidly in the realm of these green energy markets.
At these high power levels, the demands on all passive components, including inductors, are increasing dramatically. This trend is going to create new requirements for the inductor designer.
Historically, SMPS inductors were designed using low-cost, relatively high-loss core materials and solid wire or foil for their windings. Designers have been able to get away with this for three reasons.
First, ripple currents are typically low in this class of inductor. Second, low-volume cores can support higher loss densities. And third, smaller gauge conductors can support high-frequency ac ripple current with lower losses than very large conductors.
As power levels increase along with frequency, these old rules of thumb no longer apply. The larger cores required at these high power levels simply cannot support high loss density, and even small ripple currents can lead to core overheating.
Also, high absolute values of current require very large copper cross sections, which in turn lead to high ac copper losses. As a result of these factors, the design becomes far more complex.
UNIQUE ASPECTS OF SMPS INDUCTOR DESIGN
Switch-mode inductor designs are demanding for a number of reasons. One reason is there are many core options to choose from. An SMPS transformer is almost always a core-loss-limited design, and the designer is limited to a soft ferrite.
But inductors can employ many different types of cores, including powdered metals, stripwound cores, ferrites, and even laminated cores. Within each of these classes of core, there are further distinctions between the base metals and methods of manufacturing that greatly affect core properties as well as the cost, size, and electrical performance of the inductor.
Another reason for the difficulty in designing switch-mode power inductors is that they are typically dc-biased components requiring energy storage, involving biasing the switched current and voltage to one side of the zero point.
As a result of this dc bias, inductors are operating all or part of their duty cycle in the saturable region, and it’s essential to understand their performance under saturating conditions.
In particular, it’s essential to understand how inductance drops with dc current (Idc) as the core enters saturation. This can be a make or break failure mode for some power-supply designs, and often there is no good way to predict inductor performance in this region.
A final cause for inductor-design hardships is the tricky task of predicting copper losses, which, in an inductor, are a combination of dc and ac losses. We can calculate dc losses quickly and easily based on the direct current resistance (DCR) of the inductor.
But ac losses resulting from ac ripple depend on a complex relationship between the core geometry, gap location or locations if a gapped core is used, the type of conductor (solid wire, litz, or foil), and the positioning of the wire in the core window.
DESIGN EXAMPLE
To highlight some of the variables entailed in an inductor design, our specific design example focuses on an SMPS inductor (Fig. 1) for deployment in a low-kilowatt power application. The Table shows the properties of this inductor. Inductor design typically starts with the choice of a core.
Pressed from nickel and iron powder, powdered cores exhibit low permeability and gaps effectively distributed throughout the core. Majority iron blends, i.e., Micrometals -52 and -66 materials, offer relatively low cost and high effective perm.