Smps Ferrites, Frequency and Loss

[D from BC]

Peak density at f gives core loss in mw/cm^3 to be picked off of the
mfr's core loss chart.

Temperature rise is 1 degree C per milliwatt per cm^2 surface area for
the shape of transformer chosen. (+/- 20%). Don't forget copper loss.

Ferrites with high resistivity, used almost without exception for
power conversion above 50KHz, also exhibit currie temperatures
significantly in excess of most commodity safety isolation systems.

If you see a currie temperature below 150degC, you've picked a signal
transformer or EMC (intentionally lossy) material.

RL

Neato...

1deg C
__~___
/1mW /
/__ __/ 1 cm^2

With 20% error..

Yup... Was looking at wrong mixes too.
I looked at Steward toroid cores (material 35 is available from
Digikey).
Curie temp 150
Resistivity 100 ohmcm
But I think the big clue was "Common Mode Material" is written at the
top of the datasheet

Not a choice material for power conversion


D from BC
British Columbia
Canada.

 

[Paul Mathews]

Planar transformers/inductors are actually *easier* to deal with than the
"normal" magnetics. If you can live with the restrictions of few turns and
each coil takes a layer e.t.c. planar would be my first choice - especially
if I was only making a few because you don't need anything else than the
core (and some hotmelt glue).

Because of short magnetic path lengths and small window areas
(restricting max number of turns), it can be difficult to come up with
viable planar designs for higher voltages such as those required here.
For me, the tradeoff has usually worked like this: If very low profile
is required, raise switching freq high enough so that number of turns
is low enough to allow planar approach.
Paul Mathews