Mosfet Rating Clarification

[dutty mech builder]

Hi all

I have, so far, been bulding mods using a mosfet and a resistor as this seems the safest way to use a non mechanical switch on a mechanical mod. I have been using a 3034 195amp 40v mosfet and a15kamp 1v resistor. I've had good results on a series 18650 box drawing around 20 amps.

I now want to build a parallel box where I may be drawing more like 30 or 40 amps, or even a triple box where I could be drawing even more (you do the math lol). Simple I thought. However somebody on another forum told me these mosfets are only safe upto 30 amps.

Doing more research (on the site I bought the mosfet) The mosfet I am using states a max continuous draw of 343 amps. This left me a little confused. I mean, a lot of products over-rate their components capabilities, batteries being a case in point, but this seems like quite a difference, and obviously potentially dangerous.

Anybody out there able to shed some light on this?

Thanks in advance.

DMB

 

[dc99]

Never heard of anything that could go that high. The 3034 is fine. You can parallel as many as you need and just use one resistor.

 

[dutty mech builder]

Thank you. I hadn't thought of parallel mosfets but that makes total sense. I have a few on order now.

 

[dc99]

I have a triple parallel with two in it that handles pretty much anything. I do fuse my boxes with these. One on each battery https://www.mouser.com/ProductDetail/Littelfuse/MHP-TA15-9-77/?qs=sGAEpiMZZMtarqWRZQNdzQKpdvOTG4qt

 

[david4500]

If you need a PCB to mount those fuses on, I have some on my Oshpark page: OSH Park ~ Profile for MosMaxElectronics

There are thermal limitations to a mosfet and any other electronic component. Those high current ratings, 100A+ plus... are only if the mosfet remains at a constant temperature of 25C (about room temperature). Have a look at the data sheet the crazy high current will have the temp listed next to it. If you are passing high current, guess what, it's going to heat up beyond 25C due to power dissipation... To keep the mosfet that cool, you'd need an enormous heat sink or some type of liquid cooling. The temp a mosfet can be used at will be listed on the data sheet as the operating junction/storage (TJ & TSTG) temperature range, for the commonly used 3034 mosfet it is listed at -55 to 175C.

Take a look at this video, it's full of all sorts of info but the final half or third goes into detail of power dissipation and thermal limitations: