F5T Mosfet options

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bikerbub

bikerbub

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Joined 2013
#1
I've been reading up on the F5T for a few weeks now, hunting around the forums and finding most of the answers I've needed. The one thing that keeps tripping me up is the output Mosfets, because the original Fairchild components specced by Mr. Pass are only available in bulk from suppliers such as Rochester and Harry Krantz.

I saw the FQA36P15 and FQA28N15 mentioned on a thread from a few years back with very little follow-up. They are slightly different from the FQA16N25 and FQA12P20, with lower Vdss and higher Id. They also have a significantly higher typical Crss of 110pF, which Fairchild still considers to be "low."

I'm wondering if anyone has had the opportunity to compare these devices with a curve tracer to see if they exhibit similar characteristics, as they are much easier to come by than their Pass-approved equivalents. If not, what are the current alternatives being used? I've heard of Toshiba parts with lower Id being used, but I'm not familiar with their model numbers or characteristics in comparison.

Thanks,
Josh

(Originally posted this in the SS section by mistake)
 
#3
Yes I took a brief look at the datasheets, and the curves are not quite the same, but I'm just wondering if anyone's used them with any sort of success, or deemed them too different to work without major rework of the schematic.
 
#9
I remember NP mentioning that the IR version of the IRFP9240 had a glitch in its transfer function that was audible, but not so bad and the parts are usable. These are the parts that are currently available. Fairchild's version of the same part did not have that glitch. I am running out of my stock of the original Fairchild parts, but that means I am building amps. 😉
 
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#11
flg said:
To be clear, IRFP does not have any "Defect". The "P" in IRFP stands for plastic as in TO-3P. 😀
Click to expand...

To be clear, the _P-channel_ MOSFET has a defect, or as Charles Hansen said:
"Every single P-channel MOSFET IR has ever made has a serious defect.
The transconductance changes by a factor of ~2 after about a millisecond."


From the BA-2 article by Nelson Pass:

"This figure deserves some discussion, as it illustrates the
notorious “IR P channel problem” in which the transconductance
of the International Rectifier P channel Mosfets exhibit a shelving
in the midband. For complementary followers, this shows up
as increased second harmonic about about 200 Hz, leveling
off around 1 Khz. Is this really a problem? Perhaps it is if
you judge by specs, but I have not heard actual complaints
from listeners, probably because second harmonic is relatively
inoffensive.

There are three common cures to this:
- Use different P channel Mosfets – this problem is specific to IR
P channel parts.
- Place the output stage in a feedback loop to correct it.
- Use these circuits in balanced output stages, where the second
harmonic will cancel.

Correcting this issue with one or more of the above will reduce
the mid to high frequency distortion by about half or more,
depending on which cures you pick.
Or you can say, “My, that lovely second harmonic certainly
warms up the midrange on cold winter nights.” "


So some people may or may not like this added 2nd harmonic,
but this does not mean the problem is non existent.
 
#13
BobEllis said:
I remember NP mentioning that the IR version of the IRFP9240 had a glitch in its transfer function that was audible, but not so bad and the parts are usable. These are the parts that are currently available. Fairchild's version of the same part did not have that glitch. I am running out of my stock of the original Fairchild parts, but that means I am building amps. 😉
Click to expand...

While the P channel transfer hump is there, it doesn't seem to limit Nelson from using them in his big boy amps. It affects the overall distortion spectra as well as the phase of that distortion. All in all, I have not heard a major difference between the Fairchild and the IRF.
 
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