fet lookup (help please)

[rroback]

I'm doing my first big diy project.. which was inspired by another. I'm attempting to take a jeti 77 esc, and change the caps, in order for it to run on higher voltage ( like 12s lipo.. say 50v) Right now, I'm trying to find max voltage's for the fet's which the man who told me about this said was 60v, but I'd like to confirm for myself. How do I find the max voltage of the fet? they say
4850
SAG (delta symbol, not sure how to type it)
•W46B

any help?

thanks,


Rhett

[TMetalMan]

According to the search I did on Mouser, the FET's show up as 60v ones.

http://www.mouser.com/?Ne=500&handler=data.listcategory&Ntt=*4850*&terms=4850&crc=true&N=521&Ns=MfgrPartNumber%7c%7cSField

[rroback]

Thanks very much. My question now is.. given I see the prices for 90 volt capable fet's.. why does a company not use any!!?? They are a tad larger, but cost basically the same.

Rhett

[jeffs555]

How many mosfets are there on the board? I doubt that the SI4850 is used as the output mosfet on a 77amp ESC. The SI4850 is only rated for an average current of 6 amps, so it would take around twenty of them to safely handle 77 amps. You need to identify the manufactures logo on the parts. Here is a site with good pictures of semi manufacturers logos. http://clivetec.superihost.com/SemiconductorLogos1.htm

Jeff

[Comatose]

rroback, 90 volt fets can't take as much current per device. If you replaced all your 60v fets with 90v fets you'd likely see the current rating drop to about 40A for the same controller (issues with the drive electronics aside.) Also, 90V is considered to be into the "less safe" area of voltage.

And because no one I've ever heard of at least runs 90V.

[AndyOne]

Three problems as I see with what you are trying to do...

1. There will be a voltage regulator for the onboard processor etc, this may not be capable of taking the higher input voltage and even if it is the increased voltage drop may exceed its power handling capability, in short it could overheat.

2. If you add enough MOSFETs in parallel to handle the current at the increased voltage you may find the drive is inadequate to handle all the extra gate capacitance.

3. In speed controllers rated at a maximum of 3s lithium cells the MOSFETS are rated at 30V minimum, which is at least 2 x greater than the battery voltage. You would be advised to follow this rule otherwise transient voltages may blow MOSFETs rated at "just enough".

Andy

[rroback]

this esc is rated to 10s, and the caps are rated at 50v 300uf right now. I'm switching them out for 63v, 330uf caps., I'm going to stick a big heat sink on it, and a fan I think also. the current will actually be lower, as I'm regearing. I'd expect the current to be much lower.. between 20-60 amps peak, and the esc can handle 77.. The guy who told me about this has one running on 12s, but hasn't tested it very hard yet,but he had one blow up on 9s.. ( weird). Theoretically, on the fets, I have a 10v clearence, but unless there is some sort of voltage ripple.. or spike... I can't seen they would ever get blown. any idea how I can check the voltage regulator? Heat will certainly be and issue, although the controller may find running high voltage.. lower current better? although certainly the motor ( an actro outrunner)... will be spining fast, so switches losses will be more.

r

[rroback]

I checked the fets... on one side of the board there are 32 of them.. but this esc is 3 boards stacked, so i can't see between the other boards, but it looks like at least another 32 of them..is this possible?

r

[AndyOne]

Theoretically, on the fets, I have a 10v clearence, but unless there is some sort of voltage ripple.. or spike... I can't seen they would ever get blown. any idea how I can check the voltage regulator? Heat will certainly be and issue, although the controller may find running high voltage.. r

Rhett,

Only 10V clearance is not enough IMO as you can almost guarantee there will be a transient of some sort.

The best way to check the voltage reg is to find out what type it is and look up the datsheet on the web. You may also need to know how much current the control part of the ESC takes.

Andy.

[jeffs555]

I didn't realize you were talking about a brushless controller. Ten volts is not much of a margin on a high powered brushless controller. It will generate very hefty switching spikes, which could well exceed 10 volts. Capacitance and voltage are not the only ratings on the capacitors you need to be concerned about. I hope you compared the ESR on the original capacitors to the new ones. These capacitors absorb a lot of the switching spikes, and usually need to have a very low ESR. If you switched them out for higher voltage caps with a higher ESR, you could very well decrease the battery voltage it would take to blow the mosfets.

[rroback]

what you guys are saying makes sense, but my lack of electronics knowledge is limiting me. what does a voltage regular look like? I bought low esr caps, although I don't know what values. 330uf.... that's kinda like the capacity of the cap, as in how much charge it can take?

r