ELMG Redux from 2010
This post was originally published in 2010. It is republished here because it looks like people are still doing the inductor-go-round.
So what is the inductor-go-round? It is like a merry-go-round (carousel) except with inductors. It can cost companies (or their startup backers) lots of time and money. And usually when you are on the inductor-go-round it feels much like a carousel at a fair. Round and round and sadly, not much progress.
Often the optimization of a power converter’s performance comes down to the optimization of the converter inductor. (It can also be optimization of the transformer but lets for the moment assume a non-isolated converter). For people with big and small inductors there is, it seems, an inductor go round.
What is a big inductor? At ELMG Digital Power we generally say that any inductor with mass more than 1 kg – and yes it says kilograms (1kg = 2.2 pounds) – is a big inductor. Some of you will say that until you have a 40kg inductor you don’t really have an inductor. Other inductors will be smaller. I guess it is all a matter of what you are familiar with.
For the inductors for grid connected inverters people start with laminated iron or some other laminated material. This is typically the inexpensive choice. Some others choose ferrite cores. Some start with another core material.
Air cored next
Then running first into core material problems they then move to air cored as the core is the cause of all the problems. They then either come up against either;
- the magnetic field interference problem (the enclosure is now the core),
- the fault rupture problems that occur with designing air cored mechanical restraints,
- or the size constraint.
From here most people head to a modern core material such a powder core. These materials are not necessarily homogeneous and so they then realize that to get long life times requires careful consideration of the wear out mechanisms and to meet this requirement means using a quality core material vendor (See www.micrometals.com/thermalaging_index.html for a good first discussion on thermal aging). The lesson here is that not all core materials are the same.
At this point some people go back to iron or ferrite and others go to an alternative core material like an amorphous alloy. If they then cannot meet the price point then they may well go back to laminated iron.
At this stage the realization is that there is no magic answer. The optimization of power inductors becomes the requirement and there is usually a realization that the inductor vendors are not as able as they seem. In the defense of the inductor manufacturers it must be said that most power electronics companies struggle to correctly specify inductors.
Getting off the inductor go round means designing the inductor completely.