Need assistance selecting the right P-Channel MOSFET

Thread Starter

Metalhead_1544277733

Joined Dec 8, 2018
5
I have a little bit of a project underway and I need a MOSFET to handle the following. Could someone please help me out finding something suitable? Every one I've found falls short somewhere and I can't find the perfect fit. It will need to handle/fit the below.

8.4v
64A
500W
Single P-Channel (not N)
Holethrough heatsunk type
 

Thread Starter

Metalhead_1544277733

Joined Dec 8, 2018
5
I mean, that could work...
Am I making the right assumption that Power dissapation (W) is the max watts the MOSFET can handle before melting in to oblivion. Effectively its going in to a custom ecigarette so it'll only ever receive power for up to 8-10 seconds. If I don't have to find something with a power dissapation of 500w then finding something suitable, and the price just became a whole lot better.
 

OBW0549

Joined Mar 2, 2015
2,982
Am I making the right assumption that Power dissapation (W) is the max watts the MOSFET can handle before melting in to oblivion.
No. The MOSFET's maximum power spec relates to the maximum power the MOSFET can dissipate, not the maximum power it can deliver to a load. Don't confuse the two.

The power dissipated by a MOSFET equals its drain current (which is the same as the load current) times its drain-to-source voltage (which hopefully will be close to zero when the MOSFET is turned on). This has nothing to do with the power delivered to the load, which in your case is slightly more than 500 watts (8.4 volts X 64 amps).

Here is a p-channel MOSFET that should meet your needs; it's available from Digi-Key, here, and probably from other distributors as well. Going by the Rds(on) specs in the data sheet, at a drain current of 64A it will be dissipating about 15-20 watts so it will need a heatsink, though not a huge one.
 

Thread Starter

Metalhead_1544277733

Joined Dec 8, 2018
5
No. The MOSFET's maximum power spec relates to the maximum power the MOSFET can dissipate, not the maximum power it can deliver to a load. Don't confuse the two.

The power dissipated by a MOSFET equals its drain current (which is the same as the load current) times its drain-to-source voltage (which hopefully will be close to zero when the MOSFET is turned on). This has nothing to do with the power delivered to the load, which in your case is slightly more than 500 watts (8.4 volts X 64 amps).

Here is a p-channel MOSFET that should meet your needs; it's available from Digi-Key, here, and probably from other distributors as well. Going by the Rds(on) specs in the data sheet, at a drain current of 64A it will be dissipating about 15-20 watts so it will need a heatsink, though not a huge one.
Thank you, thank you, thank you.
Can I assume then, if I was to place this in a metal hammond box and thermal glued the mosfet to the casing (thus using the case itself as a heatsink) that would be theoretically sufficient?
 

OBW0549

Joined Mar 2, 2015
2,982
Can I assume then, if I was to place this in a metal hammond box and thermal glued the mosfet to the casing (thus using the case itself as a heatsink) that would be theoretically sufficient?
I should think that would suffice, provided the Hammond box has unimpeded access to the surrounding air and your thermal glue (or whatever you use) has adequate thermal conductivity.
 

OBW0549

Joined Mar 2, 2015
2,982
Also, for added kudos to you. I can pick one of these up from the very place I intend to get a ruck of other bits and bobs.
Excellent. Here's wishing you success in the project. Be sure to give that MOSFET plenty of gate-to-source voltage when turning it on; that will keep Rds(on), and therefore power dissipation, as low as possible. Avoid situations where it is only partially on, because then it WILL get hot.

Cheers.
 

wayneh

Joined Sep 9, 2010
16,102
I mean, that could work...
Am I making the right assumption that Power dissapation (W) is the max watts the MOSFET can handle before melting in to oblivion. Effectively its going in to a custom ecigarette so it'll only ever receive power for up to 8-10 seconds. If I don't have to find something with a power dissapation of 500w then finding something suitable, and the price just became a whole lot better.
It dawned on me after my first post that this was the actual source of your confusion, and @OBW0549 explained it.

Passing 64A still requires a "beefy" MOSFET, but nothing close to what you need to handle 500W of dissipation. I think you can find a MOSFET in a TO-220 package that meets your ratings, and has the screw hole you mentioned. It won't be nearly as expensive as the one I linked previously.

This looks close: https://www.mouser.com/ProductDetail/ON-Semiconductor/BMS3004-1E?qs=8%2bXDBfnGdByOov%2bJt7LTrA==
 
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