High H field shielded inductors?

Can you elaborate why a DC magnetic field is an issue?

Mark

Might saturate the cores of the inductors?

Hello,

A uniform field of 50kA/m causes about 63mT (Vs/m2) in air. Depending on the shape of the ferrite (for example le/D ratio ) and the orientation to the field, the flux density inside the material may increase significantly (factor 10 is not uncommon).

The AM ferrite antenna oriented parallel to the field is an example of where the flux in the core material is far higher then the undistorted flux density. Such a ferrite stick will definitely show saturation.

Even a sphere will "attract" field lines, so your 50kA/m field will bias your ferrite material certainly above 100mT. When you use "open" coils (bobbin style inductors), inductance variation because of the DC field will not be that high as most wanted field already goes through the air.

You may notice more distortion in your analog circuit because of asymmetric behavior of DC biased core material. You can compensate for that by using two coils in series or parallel with opposite polarity

You will probably have to measure it yourself.

Hopes this help you a bit,

Wim PA3DJS

How about mu metal shielding?

Leon

You might think about how thick (relative to its longest dimension) it would have to be to not saturate.

Something with a lower permeability but higher saturation flux would work better, since perfect shielding is not needed, but just a gross reduction in the field applied to the magnetic components.

Perhaps GIRON:

or silicon iron or low carbon steel. But it had better be anchored pretty well.

If it's just LF filtering, would switched capacitor filters do the trick? Magnetic flux at low frequency is hard to shield, and sensitivity to ambient field isn't usually spelled out in the inductor data sheet.

If it's bucking converters that are an issue, simply using overrated units will probably work. Power component saturation isn't likely to do anything but tickle the overcurrent shutdown in any case. Filters that change character according to uncontrolled B-field, though, are a nuisance. I remember a friend trying to match capacitors who said the bridge was giving him different values every time... and the problem turned out to be BX type ceramic capacitors being altered by his body heat as he held them to attach the bridge probes.

Nonlinear inductors aren't any more tractable than those nonlinear capacitors were...

Heyyy, if that DC field is in the same direction all the time, you could use some of those magnetized inductors. :-)

Tim