Winding inductor coils too close to a centre leg gap in the ferrite?.....heating from fringing fields near the gap?

[opampsmoker]

Do you know why Power Integrations are making no recommendations to move the coils away from the 1.3mm gap of their 300W PFC Ferrite inductor?
The fringing fields from this gap will surely fry the coil?

DER484 (below) on page 18, shows this PFC inductor to comprise a TDK PQ3230 Ferrite core gapped to AL = 145. The MDT design software tool shows that this means a 1.3mm gap in the centre leg, giving it an inductance of 221uH.

Pages 19-21 of DER484 clearly show the winding of this inductor, with coils being layed down right over the centre leg gap! Are they trying to fry the coil?

Page 66 of DER484 shows the high ripple current in this inductor…at 115VAC the ripple is showing a peak-to-peak value of some 4A almost right across the 10ms half sine. This represents a delta B of 177mT. Surely with the 1.3mm centre leg gap, this would cook the coil?

The coil actually uses 60*#38AWG “Served Litz Wire” which appears to be made by mwswire.com. It makes you wonder if this 60 strand Litz wire reduces eddy currents such that the coil’s close proximity to the centre leg gap doesn’t matter?

DER484
https://ac-dc.power.com/sites/default/files/PDFFiles/der484.pdf

Interestingly, the PFS7528H (PFC controller) datasheet discusses PFC “inductor design” features on page 13, and mentions skin effect, core loss, proximity loss, saturation, but makes no mention whatsoever about coil heating due to fringing fields in the vicinity of a core gap. Do you know why not?

PFS7528H datasheet
https://ac-dc.power.com/sites/default/files/product-docs/hiperpfs-3_family_datasheet.pdf

 

[JimB]

will surely fry the coil

Are they trying to fry the coil

this would cook the coil

To quote the late Patrick Moore (Astronomer) "We just don't know".

However given the culinary nature of your post, I can only suggest that you ask someone like Jamie Oliver or Gordon Ramsay.

JimB

 

[unclejed613]

probably because the effect you are describing isn't an issue. it's common to use gaps in the center leg of a core and have the outer legs of the core in contact to minimize EMI radiation from the inductor. the magnetic field from the current in the wire is already maximum at the center of the winding, so it really makes no difference whether there's a gap there or not.

 

[Diver300]

There can be additional heating of windings near the core gap. That is where the windings are subject to the most intense magnetic field, and therefore the most heating due to eddy-current and skin effect heating.

There are many factors that will affect how severe that heating is, and whether it makes any difference in practice. Most inductors or transformers won't run the windings at a temperature that is anywhere near the temperature limit of the winding insulation, so if there is some heating and a few degrees more rise near the gap, it just doesn't matter. However there will be other situations where the increased magnetic field at the gap causes intense heating.

 

[ChrisP58]

The PFS7528H operates at a variable frequency up to 123KHz. The skin depth for 123KHz is 0.0073 Inches. The diameter of 38AWG is 0.004 inches, which makes it small enough that there will be very little eddy current.

Now, I haven't read through the details of the ap note, but it's quite probable that the fringing effects were factored into the choice of wire for this inductor.

While moving the windings away from the gap is one way of addressing the effect of fringing, it's more of a text book exercise. It's not really practical in production using high speed winding machines.

 

[ronsimpson]

Some what related:
On high frequency supplies it is common to put a copper strap around the entire transformer. I have used gaps on all three legs many times. (like center leg batter) On the outside legs the copper strap is pushed tight against the core and touches the gap. If there was to be a "hot spot" on the strap I should have seen it. The center leg of the transformer, the wire is not so near the gap because the bobbin separates copper from the core.

 

[unclejed613]

and therefore the most heating due to eddy-current and skin effect heating.

hence the use of litz wire, which minimizes both effects