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Will LLC converter transformer overheat?

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We are doing a 7.4kW Full bridge LLC converter. (vin=400VDC, vout = 300-410VDC).
Please advise if we are overheating our transformer?

We are using a PM87/70 ferrite core (Epcos) for the transformer (N87 ferrite).
We have a 0.4mm gap in the centre leg of the transformer, which we calculate gives an AL value for the gapped core of 2263nH/N^2.
Our peak magnetising current is 3.4 Amps, giving a peak B of 0.187. (pk-to-pk B = 0.373)
This peak B occurs at 45KHz switching frequency.
(Our total winding loss is 12W.)

Epcos ferrite databook:

Page 49 of the above ferrite databook tells us that the N87 material has a B.Hz constant of 13000, meaning that at 45khz, the maximum allowable delta_B is 13000/45000 = 0.29.

As such we are operating the core significantly above the recommended delta_B level. Do you think that given suffient cooling means, we will still be allright?

Ltspice simulation of LLC converter attached.


  • LLC_full bridge_360-340-400v_TXFMR.txt
    20.7 KB · Views: 65

0.29 Tesla is the peak B not the peak-peak, therefore you are running with a safety margin, you have not mentioned the wire, keep it at least 1.5mm away from the gap else the wire there will get very hot indeed.
Depending on your construction, the wires at 45kHz, will suffer from increased Rac, esp if simple pri over sec winding, so expect a bit more than 12W at full power...

thanks, are you sure its pk?, this was discussed in another thread. I have written to Epcos to verify this about "pk B" or "pk-to-pk B" but they have not yet responded, I will put it out when reply comes back as others have asked for the reply.

p.s. the peak B occurs at the peak o/p voltage and max load at this voltage as this is where the freq will be lower AND the maximum volt-seconds is applied to the core....

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