Suitability of mosfets!!

[sandyhooker]

I am in the throes of Mini-A planning and am in a quandry concerning the output devices. Grey Rollins originally used irf644 which is a 250volt 14A device with a Ciss of 1300pf. I dont have any of those but I do have some FQP28N15 which are rated at150V 14A with a Cissof 1600pf max .All ratings are at Vds =25Volts.

I also have some BUZ71A which are 60Volt devices rated at 12Amps and have a Ciss of 900pf as well as some RFP12N10L. Ratings on these are 100Volts at12amps and a Ciss of 760pf.

On the spec sheets some are called avalanche devices and some are called enhanced . One can be triggered (the RFP) by a logic level voltage.

Moving through the Pass threads over a substantial period of time ,I've noted that IR devices seem to be almost a universal choice . Can anyone tell me why? Has anyone used any of the above devices or any devices of a different manufacture. ?

What would be your choice????

Thanks for your Help

[Nelson Pass]

IR simply was the largest manufacturer of power Mosfets, and
IRF240 / 9240 parts were common and inexpensive and filled
the need as a general part.

There's generally no reason you can't adapt parts with similar
voltage, dissipation, current, transconductance, and capacitance.

Your parts might have a lower Vgs, so you might need to make an
allowance for that on the Drain resistance coming out of the
input devices.

[sandstorm33]

Originally posted by Nelson Pass


Your parts might have a lower Vgs, so you might need to make an
allowance for that on the Drain resistance coming out of the
input devices.

Thank you for your reply!! I overlooked the Vgs threshold and as luck would have it , all devices mentioned are virtually identical with respect to that.

[GRollins]

As I believe I said in the Mini-A writeup, I used the '644 because it's what I had on hand. In fact, the entire thing was an exercise in making do, right down to some of the changed resistor values. I had run low on money and wasn't able to buy a bunch of parts, so I rummaged around to see what was in my junk box. The project was part feasibility study, part use-'em-up for leftover parts, and all stepping stone towards the Aleph-X. My point being, it's something for people to experiment and have fun with...not a religion.
The voltage capability of the '644 is waaay overkill for the Mini-A. They were left over from building two pairs of Aleph 2s, which do need higher voltage parts. Substitution of other MOSFETs is just fine...both with me and with the circuit. Stick with vertical MOSFETs (meaning relatively high Vgs of 3 to 5V) and you'll be fine. To an extent the front end can compensate for variations in Vgs. The differential just moves current from one side to the other, which moves the DC offset across the front end load resistor up or down according to the needs of the output device.

Grey

[sandstorm33]

[QUOTE]Originally posted by GRollins
[B]As I believe I said in the Mini-A writeup, I used the '644 because it's what I had on hand. In fact, the entire thing was an exercise in making do, ......... My point being, it's something for people to experiment and


Well, I'm having a Ball!! Working with 15 volt rails is a wonder... Found a 24VCT 10 amp trans. for $18... I have 8 -22000uf 35volt caps...

Let's see what happens!!!

[GRollins]

That's the spirit!

Grey