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.
The image was 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.
etd49_heated_magnetica.jpg
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