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Topic: Calculating power dissipation in a tranzorb for inductive flyback clamp (Read 1 time)previous topic - next topic

jtw11

Mar 15, 2013, 04:49 pmLast Edit: Mar 15, 2013, 04:52 pm by jtw11 Reason: 1
Hi guys,

I'm using a bidirectional tranzorb to protect the drain of a MOSFET being used to switch an inductive load - so when the inductive flyback exceeds the breakdown voltage of the tranzorb, the pulse is conducted back to battery positive.

The device is from Vishay's range - http://www.vishay.com/docs/88301/15ke.pdf

Calculating the energy stored in the inductor is no problem, I've got 5mJ stored. Now, to calculate the heat dissipation in the tranzorb, I seem to recall reading/hearing it would be the energy stored by the inductor multiplied by the switching frequency, that is - P = E * F.

Would this be correct, if so - I'm not sure what to type into Google to read more about this, things like 'power dissipation switching frequency' seem to come up short. If correct, in my worst case situation I'm dissipating 415mW. Of course, some of the energy is dissipated in the wires, traces etc - but predominantly the tranzorb I presume.

Any thoughts? Cheers!

shelleycat

Well the units seem correct.    Frequency = Counts per second.   Joules * count(unitless) /seconds = Joules/seconds, which is correct units for Watts.

jtw11

Ah, of course - that makes sense. Thanks!

dc42

Yes, that is correct - in the worst case, all the stored energy is dissipated in the tranzorb, and the dissipation in that case is the stored energy times the frequency.

BTW you can get power mosfets that are rated at greater than 5mJ repetitive avalanche energy, so your tranzorb is theoretically redundant if you use one of those. But including the tranzorb is kinder to the mosfet.
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jtw11

BTW you can get power mosfets that are rated at greater than 5mJ repetitive avalanche energy, so your tranzorb is theoretically redundant if you use one of those. But including the tranzorb is kinder to the mosfet.

I had looked at these yes, but decided to stick with a transient suppression diode. Thanks for your input!

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