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Old 26th March 2002, 01:15 AM   #31
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@80deg C case temp, IRFP240 is only good for about 80w.

Find a bigger mosfet cause that one is gonna die real quick.

Also for the 1ohm resistors, use 2 x 0.5ohm resistors in series or 2 x 2 ohm resistors in parallel so that you can spread the heat evenly across the heatsinks.

As suggested above, it would be good if you put the resistors on a seperate heatsink WRT the mosfets but given your design criteria I doubt you will do this.
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Old 26th March 2002, 04:47 PM   #32
Lenin is offline Lenin  United Kingdom
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Default HELP! IRF 240 alternative required.

Ouch!

What am I gonna do??!!!

Surely someone has already identified an alternative mosfet for this application? I really, really don't want to scrap this project altogether.

HELP!
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Old 26th March 2002, 05:11 PM   #33
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Why don't you parallel mosfets. Here is a link:
http://members.ozemail.com.au/~dkfin...ed_circuit.gif
I beleve Nelson Pass sugested that for higher power SOZ.
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Old 26th March 2002, 08:49 PM   #34
Lenin is offline Lenin  United Kingdom
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Thanks - (for waving your wand) you may just have saved my SoZ project!

Any idea of the case and junction temperatures this gives me?

(Or is it as simple as just halving the figures I already have?)

Has no-one really ever identified a high temp alternative?
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Old 26th March 2002, 10:22 PM   #35
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Half of the difference between Chip and heat sink.

You can run the Mosfets really Damn Hot, but you'll
want to keep some spares around.
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Old 27th March 2002, 09:01 AM   #36
ralf is offline ralf  Germany
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Question paralleling MOSfets in SOZ

Is that right,

that one can paralleling the mosfets to get higher power???

Does that sound good?

Whats with the PSU, when dubbling the MOSfets, should it be dubbled, too?

Ralf
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Old 27th March 2002, 02:59 PM   #37
ralf is offline ralf  Germany
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Default Alternatives irfp240

i tried irfp250 and irfp3170, i didn`t hear any differences. the input capacitances are little higher then that of irfp240.

Ralf
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Old 27th March 2002, 07:36 PM   #38
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You certainly can scale the SOZ up arbitrarily by paralleling
Mosfets and making bigger power supplies, but there are
some really elegant alternatives, and I want to explore them
in the Zen Variations series. Part 2 of that, not addressing
this problem, is due next week.
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Old 28th March 2002, 08:12 AM   #39
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As the owner of the parallelled SoZ circuit above, I would just clarify a few things.
Quote:
Is that right, that one can paralleling the mosfets to get higher power???
You do not gain higher power by doing this, the only gain is to share the FET junction-to-heatsink temperature differential between multiple devices. So for a given total W dissipation, the device junction temperatures will be lower, hence the devices will last longer.

In the example at hand, if the heatsink is 80C and the FETj 130C, then if I use 2 devices the FETj will fall to half the difference, ie 105C as Nelson states. The total W dissipation will be the same, irrespective of the number of FETs, as determined by the rail voltage selected for your SoZ.

As my first SoZ attempt was at 50W (the limit of Nelsons design) I thought 2 devices would be more robust, so wrote to NP who guided me toward the listed schematic. You will only need to consider this if (1) you run the heatsinks near the thermal limit for the FETs, and/or (2) build the amp at higher power levels, say 40+ W

Like any multiple FET circuit, if you choose 2 devices it pays to match them so they share the load equally.

cheers, mark
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