FETmodding Advancements

[Aaron]

Well it s been a bit of a slow day here so I ve been playing with some FETmod ideas I ve had sitting in the back of my mind for months.

Anyway I think I ve cracked it.

I was playing around with the “how to mount” problem for FET stacks and also the “Nelly sticks out like Dog nuts” problem.

Although I m quite happy with the results of FET stacking it s not for everyone and many of the newer chassis can t take more than a 2×2 stack. This ofcourse means that you re stacking is of less performance benefit. And suddenly a remotely mounted Nelly looks awesome.

But that got me thinking..

What would happen if I made a 2x stack using a dedicated N-Channel and P-Channel FET in SO-8 pacakging and did somethign tricky with mounting.

Ofcourse to make this work we need FETs that meet the following criteria.:
– Gate and Source must be on one side of the chip, and a common Drain across the other side.
– Both N & P-Channel must have same pinouts.
– On resistance at iWaver/Mini-Z drive voltages must be significantly lower than IRF7317 or other FET stacking solutions and pretty damn close to Nelly specs

So I started looking
It seems that the IRF7210 is pretty much the best combo of “cheap” “available” and low resistance for the P-Channel. It s also a SO-8 package which makes it ideal. IRF7210 is also used in Nelly, so we know we re on a good thing. At our drive voltages we can expect about 0.007 ohms resistance. Rated at 16A it ll also run a pretty tough motor.

Now for the N-Channel. I sifted through quite a few options until I struck the IRF7809AV, like the IRF7210 it s got the combo, and has the same pin-out as the IRF7210 which is vital for the mounting plan. The “On” resistance is between 0.007 and 0.0085 ohms (the later being at Mini-Z circuit voltages). With a 14.6A current rating it s a pretty good match for the IRF7210 and should drive some pretty tough motors.

Total ON resistance: 0.0155 ohms which is pretty respectable.

Now for the magic
The theory is to mount the fets in a pair (1xN and 1xP) in such a way that the pinouts match a stock Mini-Z/Iwaver FET (or IRF7389/IRF7317/SI4562DY).
This means the P-Channel gets mounted on the bottom with two of it s unused Source pins removed (ie the ones that line up with the N-Channel pads on the board)
The N-Channels get mounted on top offset two pins and linked down to their Source and Gate points on the board. It s a fair bit of a fiddle to mount up though. The other option is to remote mount the lot or use jumper wires…

Total cost would be the FETs which should retail about the AUD10.00 mark….

A.

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Site Owner Guy.

[mocky]

sounds like a plan. now lets await ph2t to voice his opinion. When this has been work and tested i wouldnt mind having it done, especially on the mr02 where u can only stack 2×2 without cutting and a 2×4 with cutting but then u would still have to stack them obliquely.
if u ask me sounds like a nelly compressed.

[Aaron]

Nelly is still the ‘best’ option for outright performance, what I’m trying to provide are ideas and alternatives that are a little cheaper (or a lot cheaper in this case) that deliver a noticable performance boost over stock FETs with good durability to handle most off-the-shelf warm motors not some of the customer wound hybrids some peopel are using 😉

Obviously it does come down to testing, and as I’ve just placed my FET order for the month it’ll be Jan before this moves forward. Probably to be tested in a MZM/iWav04

A.

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Site Owner Guy.

[jamiekulhanek]

Very scmick Aaron, i’ve often pondered things like this myself…

[micro_Amps]

Good stuff Aaron, such are the benefits of a slow day.
:)uA

[Author]

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