Coupled Versus Uncoupled Inductors Procurement: Which is Right for Your Switching Power Supply?

I recently took a deep dive into the various types of topologies surrounding switching power supplies. This article examined the benefits and downfalls of some of the most common topologies and aimed to assist you in the process of choosing how to power your design.

The SEPIC topology is a very common switching mode power supply that can operate as both a Buck and a Boost converter. In other words, the SEPIC can act as a step-down (Buck converter) as well as a step-up (Boost converter) voltage regulator. This is a great design if you’re running your PCB off of a battery since these variable voltage supplies are, well, variable and at times require both step-ups and step-downs.

The specific piece of the topology that left me awake at night was the fact that the suggested use of coupled or uncoupled inductors was vague and undefined. Folks seem to glaze over the fact that either type of inductor can be used to achieve similar results, but that wasn’t good enough for me. There are so many things to consider when figuring out what is best for me. What options do I have in regard to the transmission line? How about mutual inductance or leakage inductance?

Nowadays, compact HDI PCB designs are considered normal; however, compact designs often don’t account for variables like different inductor designs, which could, in fact, make or break a design. That being said, let’s dive even deeper into this design choice, break down exactly what you’re getting yourself into when determining the type of inductor to incorporate, and discuss coupled inductor vs transformer.

The Coupled Inductor

So what is a coupled inductor? As its name suggests, the coupled inductor essentially involves two inductors wound within a single core acting as two separate inductors. These two coupled windings will be required to have the same values (speaking in terms of their volt-microsecond balance).

This can be beneficial due to the physical space saved while using a single core to achieve the result of two separate cores. Again, since we’re striving for smaller and more efficiently spaced designs, this may entice many engineers.

However, pay heed to the fact that these coupled inductors will most likely require their own custom design (depending on your intended application). This will take more time and effort on your end, and will, in turn, require more time and effort to procure than an off-the-shelf design of the uncoupled counterpart. If long lead times will drastically affect your time-to-market goals, this needs to be considered.

Additionally, coupled inductors will offer slightly greater efficiency gain in their performance. Typical results lie around 0.5% efficiency gain. This gain is associated with lower winding losses from the smaller inductors used in the coupled design. Although not a large gain by any means, this is still a consideration worth noting.

Lastly, the key advantage of coupled inductors is that the regulator’s current ripple is divided between the coupled inductors, which allows the inductance of each to be halved. This benefits the design due to a smaller input capacitor and simpler EMI filtering.

Any lowered EMI challenges are almost reason enough to consider these bespoke components in your design!

Coupling your inductors can save you space but may cost more to procure.

The Uncoupled Inductor(s)

As the name suggests, the uncoupled design incorporates two separate inductors, which means two separate cores and therefore more space occupied by these two components.

Despite what some engineers may assume, these two inductors do not have to match volt-microsecond values. Although a great way to ‘keep it simple, stupid,’ you could save a bit of space by opting for a smaller inductor on the output side, which in turn could help you save some cash. The output side inductor’s value can be calculated by scaling the value of the primary inductor by a ratio equivalent to typical values of VOUT/VIN.

Comparing the design complexity of the potentially complicated coupled inductor, these two uncoupled components are, in theory, much easier to procure. This will allow you to be much more flexible in your design, giving you more agility if (and when) design changes roll through. This will also shave loads of time from your time-to-market goals, depending on how crucial this factor is.

In contrast, you will feel the aforementioned 0.5% loss in efficiency, but again, this number is relatively small, and if it’s negligible in your design, this shouldn’t matter to you.

Uncoupled inductors can cause heartbreak in your PCB design, and also your relationship.

In the specific case of the SEPIC switching mode power supply, using a coupled versus uncoupled inductor is often overlooked. However, while we strive for greater efficiency in both space reduction and performance, any small gain may help us achieve these goals while leaving us with the greatest possible educated design for our applications.

All in all, if we’re seeking a simple design that will leave us lightweight (metaphorically speaking) and agile, uncoupled inductors are a great choice. These designs give us quicker time-to-market, easier procurement, and simpler design requirements at the loss of additional components to mount. This, of course, steals from that valuable board space.

On the other hand, if we have a little more wiggle room in terms of procurement resources and are willing to take on the challenge of these bespoke designs, we can save board real estate by incorporating a coupled inductor into our PCB. This would help us achieve a lowered component count while slightly improving our efficiency and lowering our EMI challenges.

Which inductor option best fits your application, and why? If you want to discuss your board-specific challenges with a PCB guru, talk to an Altium Designer expert today.