Low Rds MOSFETs?

codesuidae · Post by **codesuidae** » Mon Nov 22, 2004 5:55 pm

I've noticed that ph2t experimented with some low Rds FETs recently. How much of a difference does this actually make?

I went looking for low Rds FETs and found some with Rds of 0.007 and 0.004 at 2.5v for pFET/nFET respectively, but they are a bit pricy. (for a ZZMT with the 5v gate you'd get 6.5mOhm and 2.7mOhm).

Does very low Rds make enough difference to justify the increased cost?

hogjowlz · Post by **hogjowlz** » Mon Nov 22, 2004 5:57 pm

watch my signature. what ya think?

ph2t · Post by **ph2t** » Mon Nov 22, 2004 6:09 pm

code, you've just hit the nail on the proverbial head.

"On resistance" is where it's at, period.

It's the fundamental design point behind my nelly turbos as well as the Biff turbo for 1/64 shit.

You WILL find a difference! That I garuntee....

eg: dual cell off road 2 speed booster bit. run's fine unboosted with traditional irlml2502 mosfet mod. put Biff in there and unboosted it was even faster.....boosted I don't wanna even talk about, lol....

Biff, uses lower resistance mosfets as well, see the sticky in science, "Improved MOSFET........blah blah blah..."

May I ask what mosfets you got those figures from? Was it a set from fairchild semi per chance? I guess it's a SO-8 pkg as well? Approx 30 milliOhms seems to be the limit on the SOT-23 pkg size....

EDIT: What hog is trying to say is that he has a Biff turbo and he wants to try and have sex with it I think....

cheers,

ph2t.

hogjowlz · Post by **hogjowlz** » Mon Nov 22, 2004 6:17 pm

codesuidae · Post by **codesuidae** » Mon Nov 22, 2004 6:29 pm

ph2t wrote: May I ask what mosfets you got those figures from? Was it a set from fairchild semi per chance? I guess it's a SO-8 pkg as well? Approx 30 milliOhms seems to be the limit on the SOT-23 pkg size....

They are SO-8's (plenty of room in a ZZMT, so I'm not concerned about the size), but they are Vishay parts, Si4453DY and Si4378DY. They run about 5 bucks for a pair in singles quantities. The figures are:

Si4453DY : p-channel SO-8
-1.8v :: 10.25mΩ
-2.5v :: 7.75mΩ
-4.5v :: 6.5mΩ

Si4378DY : n-channel SO-8
1.8v :: not rated for 1.8v operation, single cell vehicles need not apply.
2.5v :: 4.2mΩ
4.5v :: 2.7mΩ

Ratings are around 8-12A continious, far more than these NiMH packs are capable of sourcing (I get no more than 1.5 to 2A into a dead short). How much current can those LiPoly's deliver?

I'm just surprised that 20mΩ makes much difference when the motor's DC resistance is a couple of orders of magnitude greater. I guess I'll have to do some playing around with the motors and see if I can convince myself that there is a measurable difference.

I suppose the Si4378DY would be interesting to try for both the high side and low side of the forward bridge. In theory a double stack would give a total series resistance of 2mΩ.

Have you tried FETs with ~2mΩ Rds before? Should I bother with this?

ph2t wrote:What hog is trying to say is that he has a Biff turbo and he wants to try and have sex with it I think....

yes, makes me think of Fight Club.

ph2t · Post by **ph2t** » Mon Nov 22, 2004 6:41 pm

The biggest issue with the low rds on mosfets was finding a set that drove well at 3V (typical Vgs for bcg/zz pcb's). This low Vgs of 3V is borderline drive for many mosfets untill the last 6 months since the 2.5V Vgs mosfets have hit the scene.....

If you don't have a Rds(on) vs. Vgs graph on the datasheet, look at the Gate charge vs. Vgs graph, you need to be in the area at or above the "flatline" for the mosfet to enter it's "enhanced" state and therefore charge up ok.

Now that Vgs is almost 5V you shouldn't have any issue driving them...

Oh yeah, where are you buying these mosfets from? Digikey doesn't seem to stock of lot of Vishay Siliconix discretes.....unless they are a new adittion to their product line.

thanks,

ph2t.

codesuidae · Post by **codesuidae** » Mon Nov 22, 2004 7:10 pm

ph2t wrote:where are you buying these mosfets from? Digikey doesn't seem to stock of lot of Vishay Siliconix discretes

Mouser carries a good selection of Vishay stuff, I've used some of their parts for SE FET mods before. However, they aren't carrying these yet, probably too early.

So far only Arrow electronics carries them, and they are out of stock on the nFET at the moment. I haven't emailed them to see when they will have more.

Finks · Post by **Finks** » Tue Nov 23, 2004 7:51 pm

ph2t wrote: May I ask what mosfets you got those figures from? Was it a set from fairchild semi per chance?

That place is like 2 min from my work. /derailing comments off