Are two inductors better than one?


Electronics’ continuous advancement into smaller devices with fewer and fewer components has been propelling the industry for decades. Notebooks and tablets, for example, are only possible due to highly integrated semiconductors with very limited – and tiny – passive components. This creates a small-form-factor product at a very reasonable cost. In fact, the latest in highly integrated power supply integrated circuits (ICs) uses two inductors instead of one to continue this trend. You read that correctly: Two inductors enable a smaller and more cost-effective solution than one. How can this be?

A two-phase converter such as the TPS62180 has two power stages inside of it, feeding a common output voltage rail. Its single 6A output is created from two separate 3A phases, each with their own inductor. This is a different architecture than a traditional one-phase solution such as the TPS54623. In a one-phase converter, only one inductor is present and it carries the entire 6A output current. With inductor size increasing with current capability, clearly 6A requires a larger inductor than 3A. But shouldn’t a 6A inductor be about the same size as two 3A inductors? 

For several years now, the smallest power-supply solutions have used a newer inductor technology: the chip inductor. As its name implies, it’s as small and thin as a semiconductor chip or potato-chip crumb. Several vendors make these chip inductors in standard sizes, from smaller than a 0805 (2012) to as large as a 1210 (3225) and at heights of 1mm and less. This is perfect for small portable gadgets. For example, the new TPS63050 evaluation module (EVM) uses a chip inductor in a 1008 (2520) size.

The best news is that these super-small inductors can now carry 3-4A of current, so you can use one for each 3A phase. There is not yet one large enough to handle 6A all by itself. The 6A rail usually uses a traditional wire-wound inductor, which can carry the required current but comes in a much larger size: 10mm x 10mm x 4mm is not uncommon. An example of such an inductor is the MSS1048 series, which is used on the TPS54623 EVM.

Clearly, two 2520-sized (2.5mm x 2mm x 1.2mm) inductors are 90% smaller than one 10mm x 10mm x 4mm wire-wound inductor, and 75% shorter. But isn’t smaller more expensive? Actually, no. Chip inductors have been around for several years now, have shipped billions of units and are made by multiple vendors. With mature manufacturing, a high-volume market and good old competition, chip inductor prices are very reasonable.

It turns out that the physical size of the inductor can be a good indicator of relative pricing. An inductor with more wire and metal costs more than one with less. While a host of factors influence pricing, a quick search on Digikey showed 1µH chip inductors costing about $0.10 each at 3ku volumes, while 1µH wire-wound inductors started at around $0.40 at 1ku volumes. The compared volumes are different because not as many wire-wound inductors fit on a single reel, being so much larger than chip inductors. Referring back to the TPS54623’s EVM inductor, the MSS1048 family is priced at $0.56 at 1.2ku volumes on Coilcraft’s website.

Figure 1 shows the total solution of the two-phase, 6A TPS62180 using chip inductors. Isn’t it good to know that this saves cost as well as space?

The 6A TPS62180 occupies less than 80 mm2 total

Figure 1: The 6A TPS62180 occupies less than 80 mm2 total

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