‚ÄčThe fringing flux is an effect appearing in gapped magnetic, where the magnetic flux lines bulge outward due to mutual repulsion. This is translated into a modification of the inductance and a possible overheating in the wires surrounding the gap.

Several methods exist to avoid the extra loss from , either by playing with the gap or with the winding. The latest will be discussed in this document.


Estimated loss per meter due to fringing flux in the winding.

Inductor design: 

  • PQ50/50
  • Gap 2mm
  • 12 turns, 1 parallel
  • Litz 420 x 0.05mm
  • 300 kHz
  • 14 Arms


In order to avoid winding manually, a bobbin with a swelling in the vicinity of the gap was designed.

With this special coil former, unwanted loss can be avoided while having a larger window than by just avoiding the central zone


Two tests were done for the design:

The first one with a normal coil former. The wires near the gap got hotter than the rest due to the fringing flux generating loss on them. 

The second test was done using the coil former with the temperature is more homogeneus and lower than the first test, because the wires are far from the flinging flux.


While fringing flux loss is unavoidable from the moment there is a gap, it can be reduced by placing every wire in the winding beyond 3 times the gap length, which can be done easily with custom-made coil formers.

Other methods, like distributed gaps or the use of lower relative permeability materials, will be discussed in future documents.

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