Flux fringing can cause additional loss in copper windings.
The images show a 500 W PFC inductor made on ETD49 core. The air gap has 4 mm length and is located only in the central leg. The ferrite material was N87. The windings are made with a Litz wire, 28 strands of 0.38 mm, 50 turns, 275 μH. Switching frequency was around 50 kHz (varied through cycle, as common for PFCs), driving the inductor in transition mode.
If a section of the winding is placed in the direct vicinity of the air gap then the fringing flux is penetrating the wires. The estimated additional loss was only around 2 W, so the effect on the overall efficiency of the 500 W PFC converter was very small. But it made the section of the winding to be heated to over 105°C when operating at room temperature.
The high-temperature spots at the top of the thermal images are the diodes in the input full-bridge rectifier from the mains supply. Their operating temperature was around 75°C at full load.
When the section of the winding was moved away from the gap (additional non-magnetic, non-conductive spacer inserted, so that the wire was wound on a larger diameter around the the gap) then the local losses decreased significantly and the hot spot was reduced to only 53°C (so around 50°C reduction).
The tests were carried out at full rated power, in similar ambient conditions (room temperature).
The images on the left are for normal winding, the ones on the right are for the improved winding.
Top images were recorded with a thermal imaging camera. In these images the PFC inductor is mounted upside-down, with the PCB at visible at the top of the image.
The middle images are FEM simulations (in FEMM software), with the colour scale showing the concentration of the magnetic field strength in and around the air gap. Purple values are those which exceed 40 kA/m.
The bottom images are the photographs of the actual PFC inductors, with visible correction for the central winding section.
 The results were generated with the software Finite Element Method Magnetics created by Dr David Meeker. |
leakage_flux_causing_heating_-_magnetica.jpg
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