Time to Retire the IRF7317

[Aaron]

Well after a good couple of years at the top of the FET Stacking performance pile it’s time for the IRF7317 to retire.

In the coming weeks ausmicro.com will be launching our new performance FET for the Mini-Z and 1st Generation XMods.

Whats so great about the new FETs?
For a start based on spec sheets at manufacturer benchmarks these new FETs are over 7% more efficient at 2.5v which is important becuase as we all know the Mini-Z and Xmods boards aren’t the best at driving the FETs at a full battery voltage. They generally drive the boards with approximately 2.9-3.0v at which point the news FETs are 2.3% more efficient and this advantage is held all the way to the 4.5v gate voltage.

2.3% isn’t much, we agree, but of course means there’s 2.3% less power being lost as heat through the body of the FET. It’s the heat that kills FETs because they become less efficient the hotter they get, which in turn makes them hotter leading up to “pop”.

So will I see the difference?
It’s unlikely you will see the difference, yes you could expect a little bit more “punch” and maybe a small improvement in top speeds, and a reduced risk of failure.

If I can’t see it why change it?
Several reasons:

• The wholesale costs of the IRF7317 have risen dramatically in the past 12 months
• The new FETs are better and based on the latest design technology (Released in March 2006)
• We know that to many people 2.3% isn’t worth it, but to some it is!

Is there something better coming?
Not for a while, there’s another FET we’re getting in for testing which may be up to 17% more efficient that the IRF7317 but there’s a lot testing to be done.

When will we be able to buy them?

Over the next few weeks we will be clearing our IRF7317 stock and then the new FETs will be available.

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Site Owner Guy.

[betty.k]

sounds pretty cool, i guess it had to happen one of these days :8ball:

[Aaron]

Yeah – a thing I didn’t mention is that the new ones do not contain lead. Important safety consideration right there!

A.

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Site Owner Guy.

[PandaBear]

What’s wrong with lead… tastes sweet!

Lead-free solder is such a pain too, sucks heat like anything and much
harder to work with… I’d better go stock up on the ‘old’ stuff
before the greenies get too carried away.

[Aaron]

Well I’ve been doing some more testing and the news is all good. The new FETs are definitely working at least as well as the IRF7317 and markedly better under some circumstances.

The test chassis I’ve been focussing on is my original MR01, now fitted with MR02 PCB and a PN mount holding a pretty normal old-skool custom motor (PN Ultra BB case, Plasma Dash Armature, PN Carbon brushes, PN Neo Magnets, PN Wires). The board had run a 3×2 stack of IRF7317 that had the typical performance we expect from them. The speed checker clocked 51km/h with a 13T and “old” Sanyo Twicells. The new FETs were then popped in (Well actually I had them mounted externally like a turbo) and run using the same batteries, top speed was again 51km/h.

So our difference in outright speed is bugger all as expected. However they do have a higher current rating on the P-Channel, and this is a good thing, and when it came to hooking the chassis up to a non-battery 4.8v supply the real gain became obvious will less current overall being pulled in to get our peak speed reading.

Next test will be to load up the dyno a little bit (simulate running on ground) and basically run for the equivalent of a couple of battery cycles and see if the heat gets to them…

Now that our stock is here you can expect to see them listed in the shop within a few days, and keep reading of the results of further tests here.

A.

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Site Owner Guy.

[PandaBear]

Internal resistance of AAAs probably higher than everything else in the circuit…?

[Aaron]

Once you’re FET’d that’s pretty much right. That’s why I’ve switched to running a digital controlled power supply for testing. It makes the whole testing process much more consistent.

So close to being able to do some real dyno work!

A.

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Site Owner Guy.

[Author]

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