SOZ - Will DMOSFETs offer any improvement? Lower capacitance.

[Brian Guralnick]

I've been looking into Fairchild's Mosfet data sheets & I discovered something they called their Q-FET's, described as a DMOSFet process.

After closer examination, I've found the to have less capacitance than their IRFxxxx counterparts.

http://www.fairchildsemi.com/ds/FQ/FQP2N30.pdf
http://www.fairchildsemi.com/ds/FQ/FQP1N50.pdf
http://www.fairchildsemi.com/ds/FQ/FQP1N60.pdf

Would any of these offer any improvements over the IRF610s?
Which 1 would be best?
Do any other Mosfet manufacturers have anything similar?

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Driver stage.....

I've also discovered that the higher wattage Q-FET devices, up to around 250 watts, offer substantially less capacitance over their IRFxxxx counterparts, in some cases, half.
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[Nelson Pass]

Looking at the specs, I'm not seeing any advantage.

The higher voltage parts generally have even less trans-
conductance to along with the lower capacitance, not
probably an advantage.

By the way, a 610 is not the recommended part for a SOZ,
not that it can't be plugged into such a circuit.

[Brian Guralnick]

Originally posted by Nelson Pass
By the way, a 610 is not the recommended part for a SOZ,
not that it can't be plugged into such a circuit.

My fault,

That's what I get for going through so many different schematics, including user mods, on this great site. Ok, comparing the IRFP240/244 to FAQ16N25/19N20

dev# V A Ron Watt InCap OutCap RevXfrCap ton+rise toff+fall
IRFP240 200 20 0.18 150 1300 400 130 65 81
IRFP244 250 15 0.28 150 1400 320 73 63 66
FAQ19N20 200 23 0.15 190 1220 220 30 210 135
FAQ16N25 250 18.5 0.23 190 920 190 23 157 120

After careful study, it looks like Fairchild lowered some of their capacitance figures, but, also slowed down the mosfet itself as a side effect.

I do however like the really low reverse transfer capacitance, however, I would not expect to see signals in the audio range which would make a difference.

I wonder if really slow medium power mosfets exist where the capacitance is even less.


..... darn graph alignment...

[Nelson Pass]

They are certainly worth trying, and I don't doubt that they
would work fine. Better? Don't know.

[JFETglare]

One of the most vital specifications to watch is the reverse transfer capacitance. Regardless of how little those four listed values affect audio, it is bound to affect it on some level... So it can be argued that less is more here.

[JFETglare]

Oops, forgot to add--Nelson's point about transconductance is a good one... it is definitely something to watch for.

The beefy FQA70N15 MOSFETs that I have aren't half bad in that department. Very soon they will be residing in a circuit that's similar to SoZ, but with two stages so that local feedback on the output can be used without needing a turbodiesel powered pre-amp to drive the thing (input impedence)

[Brian Guralnick]

Originally posted by JFETglare
One of the most vital specifications to watch is the reverse transfer capacitance. Regardless of how little those four listed values affect audio, it is bound to affect it on some level... So it can be argued that less is more here.

That's the feeling I had deep in my gut. The Fairchild FAQ16N25 with it's 23pf reverse transfer capacitance really caught my attention.

[Brian Guralnick]

I'm steering towards a SOZ design using Q-Fets, but, instead of the pull-up resistors, I plan on changing them to an extra simple BJT CCS using MJL1302A.

[Nelson Pass]

Keep in mind that it's the non-linearity of the Ciss
that usually the biggest issue. The actual values of
these capacitances isn't as big a deal, IMHO.