Electronic – How to determine safe distance from a choke to prevent accidental coupling


In most of my power converter applications, I end up with a choke of some description, somewhere in a steel chassis. I have seen and heard tell of situations where the choke couples to the steel of the chassis. In some cases, the windings heat up more than desired, as does some part of the box. In other cases, the box itself has the powder coat baked off the metal. The general solution has been to keep the windings away from the steel, as well as any core gap.

However, "away" is a vague term. Is there any rule I can follow to determine the safe distances required to prevent coupling? Example situations include:

  • Distance between winding and steel box
  • Distance between choke gap and steel box
  • Distance between air core choke and steel box

I imagine that an exact solution is dependent on a detailed geometry of the system, so general rules are preferred.

Best Answer

It depends. And there are no hard and fast rules. The biggest factor is the design of the choke/inductor. There are some inductors that can be placed right up against some steel with no ill effects. Other inductors, in the same design, might have major problems. The type of core has a large effect (air cores are probably the worst, torroid is the best-ish), but there is enough variability that it is hard to give good rules of thumb that always work.

In the end, you need to try it and see what happens. Take the prototype, turn it on, and wave a piece of steel around and measure how it is affecting your device. You will learn more in 10 minutes of doing that then you every will reading answers like mine!

That being said, here is my experience: I design a lot of DC/DC converters in the 1 to 50 watt range. I use the typical off the shelf power inductors with powdered iron cores that are suitable for this sort of thing. Metal has to get within about 0.1 to 0.2 inches away from the top surface before there is a noticeable effect. But, as I said before, your mileage (kilometerage?) will certainly vary and don't take my word for it!