Thoughts on other FET’s?

Are there any features that you would like to add to the VESC?
ekman
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Joined: 28 Jan 2016, 10:25
Location: Lund, Sweden

Thoughts on other FET’s?

Postby ekman » 09 Feb 2016, 17:25

Looking at the FET’s being used by the VESC today we find the IRFS7530TRL7PP from IRF.
I’ve seen discussions around using the IRF7749L2TR mainly because of cooling capabilities as I understand. What are the criteria that we could look at for FET’s for the VESC? I’ve been looking at Vishay’s new SUM70040E-GE3 FET maybe for higher voltage and/or lower amp applications? Anyone feels like chiming in on this?

IRF IRF7749L2TR
http://www.irf.com/product-info/datashe ... 9l2pbf.pdf

Vishay’s new SUM70040E-GE3
http://www.vishay.com/docs/62995/sum70040e.pdf

fet3.png
fet3.png (490.64 KiB) Viewed 4372 times

Andreas
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Joined: 04 Feb 2016, 19:02
Location: Switzerland

Re: Thoughts on other FET’s?

Postby Andreas » 09 Feb 2016, 21:04

Direct-FET packages do look nice on paper, but you really need to have a professional assembler soldering these to your board properly in order to benefit from the advantages. Without X-ray inspection after the solder process, you just can not tell how well the solder joints under that metal package turned out. Maybe it works well at first, but once you put the driver under full load a few times the half-bad solder-joint will give up (thermal stress with expansion/contraction) and a lot of your drivers allways fail after the first few rides. (I'm shure that would not make anyone happy :roll: ).
Since the project was more born out of a DIY setting, it makes perfect sense to have a FET package that can be soldered somehow at home with accessible tools and still have some way to control the quality of the soldering.
Of course, if you are aiming at a large series production from the beginning, and actually do what I would call "design for manufacturability" and not just "design for functionality and simplicity", direct-FET can be an option. But if you want to go down that route, you better make shure you know what you are doing and especially who (and how) assembles and inspects your boards. Once you are on that path, you will start to think about cost to manufacture. And difficult to solder packages associated with increased production failure rates, expensive/difficult rework options and all that nasty stuff no DIY project ever deals with can actually again drive you right back to the standard packages. Or you tolerate the increase in cost associated with that technology, if you have no other option, and can somehow push out that cost to your customer with higher product prices.

ekman
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Joined: 28 Jan 2016, 10:25
Location: Lund, Sweden

Re: Thoughts on other FET’s?

Postby ekman » 09 Feb 2016, 21:16

Andreas, thank you, perfect! Yeah that's pretty much the response I got when asking around as well. In my view the added cost/difficulties might not be worth it. But how about maybe assembling a list of alternative FET's that might also be used for the VESC? @Andreas, what's your view of the other FET on the list, the SUM70040E? Seems like there are more FET's that might be useful for the VESC in the TO-263-3 package range as well.

lizardmech
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Joined: 19 Jan 2016, 10:54
Location: Australia

Re: Thoughts on other FET’s?

Postby lizardmech » 10 Feb 2016, 07:50

I don't understand the directfets, the rating is for the package and nothing like mosfet inside on some models. I was looking at a 150v model listed as 375A, but in the datasheet the silicon was only rated to 60A or so.

Andreas
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Joined: 04 Feb 2016, 19:02
Location: Switzerland

Re: Thoughts on other FET’s?

Postby Andreas » 13 Feb 2016, 08:32

ekman wrote:@Andreas, what's your view of the other FET on the list, the SUM70040E? Seems like there are more FET's that might be useful for the VESC in the TO-263-3 package range as well.

Well, i would not really aim at a 100V rated FET if the driver is not built for that voltage anyway. You will usually get better specs if your voltage rating is lower, so going over 60V seems pointless unless you adapt the rest of the circuit to handle the 100V.

And about the Direct-FET ratings and differences between package and silicon current limits, thats the "problem" they have with direct-fets in general. The package is so good that the silicon is now the limiting factor, with all other technologies in plastic packages, it was always the package and connection of the silicon to the pads that limited the fets performance. Of course, some marketing guy must have opted to list the current the packages can handle, so it looks a bit more impressive. There is however no doubt about the fact that the same silicon will perform a LOT better in a direct-fet package than in any plastic package.
There is a somewhat old but still interesing read about fet packages from irf: http://www.irf.com/technical-info/white ... pn0204.pdf

ekman
Posts: 4
Joined: 28 Jan 2016, 10:25
Location: Lund, Sweden

Re: Thoughts on other FET’s?

Postby ekman » 02 Mar 2016, 00:57

For smaller solutions in the future maybe something like TI's CSD19533Q5A might be an option?
http://www.ti.com/lit/ds/symlink/csd19533q5a.pdf

rew
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Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Thoughts on other FET’s?

Postby rew » 27 Mar 2016, 09:28

In the smaller packages you can only dissipate about 1W before things get too hot. The bigger tolerate about 2W.

This means that the RDSon specification is VERY important.

So for the SO8, you roughly get P = I^2.R or I = SQRT (P/R) = sqrt(128) = 11A max.

My quick-and-dirty calculation would lead to a max current of 40A for the bigger D2PAK-7 fets that we now have. Increase the allowed dissipation to 3W and you get the 50A we now observe.

slapshot136
Posts: 27
Joined: 15 Sep 2016, 18:24

Re: Thoughts on other FET’s?

Postby slapshot136 » 04 Dec 2016, 22:43

What about something like this:

726-IPB010N06N

http://www.mouser.com/ds/2/196/Infineon-IPB010N06N-DS-v02_04-EN-226414.pdf

It actually has a lower RDSON than the IRFS7530TRL7PP (1mOhm vs 1.4), and the timings seem smaller (i.e. could we push the eRPM higher? it has a lower gate capacitance)

lizardmech
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Joined: 19 Jan 2016, 10:54
Location: Australia

Re: Thoughts on other FET’s?

Postby lizardmech » 05 Dec 2016, 13:53

These are a good alternative to direct fets, easier to solder at home. Direct fet package has lower thermal resistance on the top but less on the bottom. While the FDMT800 isn't as good on the top but retains low resistance to PCB.

http://mouser.com/ProductDetail/Fairchi ... MT80060DC/
You can buy them directly of the fairchild website for less if you want a batch of them.

slapshot136
Posts: 27
Joined: 15 Sep 2016, 18:24

Re: Thoughts on other FET’s?

Postby slapshot136 » 07 Dec 2016, 13:21

lizardmech wrote:These are a good alternative to direct fets, easier to solder at home. Direct fet package has lower thermal resistance on the top but less on the bottom. While the FDMT800 isn't as good on the top but retains low resistance to PCB.

http://mouser.com/ProductDetail/Fairchi ... MT80060DC/
You can buy them directly of the fairchild website for less if you want a batch of them.


Thermal issues aside, are they any better? I see the rdson is 1.1mOhm - was asking more as a direct replacement for the v4.12 hardware than for an entirely new design


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