IRF540 replacement

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iggy2k

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Joined 2013
#1
hello 🙂

an arcam alpha 7r went squeee whiff and ive traced it back to 2 of the irf540 mosfets, which seem to be no longer in manufacture, however, there are various other irf540(insert extra letters here).

IRF540PBF VISHAY - Transistor: unipolar, N-MOSFET | Transfer Multisort Elektronik looks to be about the closest replacement, does anyone know if this will be a 'close enough' part, its rated at 28A rather than 22A, and in general is specced a little higher on all max ratings. i have included found data pdfs for both units.

vanilla irf540 - HTTP 301 This page has been moved

irf540pbf - http://www.vishay.com/docs/91021/91021.pdf

cheers for your help 🙂
 
#2
Well, they seem to have been improved/upgraded along the years.
What's not to like about that? 🙂
Go ahead, just check it's the same size and type case (say, TO218 or 247) and not some "SMT version" or something, which is electrically compatible but perhaps not mechanically.
 
#3
The basic irf540 is available from many sources.
Most will stock a couple of variants.
Most will be current production.
The irf540 is not obsolete.

Is the output stage of each amplifier a single pair of irf540? This equals 4 devices in a stereo amplifier.
or
Is the output stage a 2pair of irf540? This equals 8 devices in a stereo amplifier.

If you have 2pair in the output stage, then the parallel pairs must be Vgs matched at the current they operate at. The upper pairs do not need to match the lower pairs. The opposite channel pairs do not need to match.
 
#4
there are 4 devices in total. i intended to replace all 4 anyway. i see youre a fellow scot, any tips on where to find stock ifr540? rs, farnell, tme and mouser all turn up later variants, and i assume it would be preferable to keep the circuit as close to the original as possible. looking at data sheets, the response curves of most of the later variants seem to be steeper, although in regards, at that point im just looking at pictures with a handful of crayons, if you catch my drift 🙂

thanks for the replies guys
 
#6
im not sure that parameter applies to mosfets? (like i even know what that means, i googled it, always learning 😀)

looks like the vishay pbf is the absolute closest im going to get. considering ive managed to get 10 years out of this amp, a couple of quid is not a bad bet to get her up and running again.
 
#7
For mosfets in linear power amplification, the Vth and gm and how these change with temperature are important. More modern devices unfortunately are more sensitive to temperature.
Have a good look for dry joints and failing capacitors. Something must have made the original devices blow.
 
#8
ive checked and double checked the board, the only visibly failing components are 2 resistors marked r102 and r2. these are directly inline between the relay and the mosfets. i have included some pictures for anyone that fancies a gander 🙂

also, nigel re-reading your comment i realise you were using yours as an example, you just had me a little confused there. an easily done thing these days.
 
#11
R2 and R102 were 4R7 resistors in series with 100nF capacitors at the outputs to give a resistive load to the amplifiers at high frequencies for stability. Once they go open, the amplifier outputs will self destruct. Its odd that both channels show damage, although they look like 0.5W, which is a bit low for the function. I would replace these with 1W rating, but don't use wirewounds.
 
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