Electronic – How are electric under blankets fail safe

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!

Most of the losses in a well-designed boost inductor are going to be:

• Resistive losses in the windings
• Hysteretic losses in the core

I won't attempt to trump Wikipedia's explanation of hysteresis losses:

When the magnetic field through the core changes, the magnetization of the core material changes by expansion and contraction of the tiny magnetic domains it is composed of, due to movement of the domain walls. This process causes losses, because the domain walls get "snagged" on defects in the crystal structure and then "snap" past them, dissipating energy as heat. This is called hysteresis loss. It can be seen in the graph of the B field versus the H field for the material, which has the form of a closed loop. The amount of energy lost in the material in one cycle of the applied field is proportional to the area inside the hysteresis loop. Since the energy lost in each cycle is constant, hysteresis power losses increase proportionally with frequency.

Essentially, the more you slosh around in the B-H loop, the more heat you make because sloshing around in the B-H loop generates heat. Higher frequency = more sloshing per unit time = more power loss. Also, since it's both the magnetizing current and the load current contributing to the B-H sloshing, higher power = more sloshing per unit time = more core loss.

I said "well-designed" for a reason. In my opinion, a well-designed boost inductor is going to use ferrite core material, which is essentially non-conductive and therefore practically immune to eddy current losses (i.e. there may be some, but they're insignificant compared with the hysteretic loss).