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Finding Bpk for sendust torroid in PFC?

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Advanced Member level 5
Jun 13, 2021
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We are doing 165W PFC (200VAC in, 390Vout, 60kHz)
L = 270uH.

Inductor = MS sendust torroid with 60 turns. (MS 157060-2)

With the attached excel spreadsheet it is calculated that the peak B is 0.15T (at 3.5A). The very long excel calculation attached had to be used to get Bpk. There is nowhere in the below catalog where an equation is to be found for this. Would you agree?

The method involved splitting the ur vs H graph for 60u sendust into linear sections, and thus finding equations giving ur at any H. Then H had to be “stepped”…giving deltaB = uo.ur.deltaH…….so then Bpk was calculated by adding up all the deltaB’s.

This is an extraordinarily long winded way, but there is no other way, would you agree?

MS 157060-2

(LTspice of PFC, and excel calc of inductor Bpk attached)


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B = Ur.Uo Ampere-turns / mag-path-length

So if the effective Ur = 60, the mag path length = 10cm say, turns = 50, and the peak amps are 10

Then, B = 1.256E-6 x 60 x 10 x 50 / 0.1m = 0.3768 Tesla

if the Ur rolls over at higher ampere-turns, this needs to be taken into account ....

you're welcome ...
Thanks, the ur starts rolling off at the higher 3 of the "micrometals 2021 alloy catalog" shows it rolling peak current is so low that indeed i could have ignored the little bit of roll off that i get at 3.5A.

Just realised there is an equation for ur vs i could have used that also....would have saved me splitting the ur vs H curve into loads of straight line sections.....though i would still need the excel spreadsheet so that i could step H and then calculate and accumulate the delta B's........
(though i agree in this case i am in the part of the ur vs H curve where it isnt really starting to roll off.)

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