Power MOSFET selection question

Thread Starter

Chipper

Joined Jul 19, 2018
21
Greetings, I'm wanting to replace an automotive type cube relay (Omron G8JN-1C7T-R-DC12) in a circuit I built last year for the DC power distribution system in my home built camper with a power MOSFET circuit in order to save some power while in discharge mode. The relay is used in line with a 13.3v LiFePO4 battery and controlled with a small signal relay and an OrionJr BMS to send current to a fuse panel. Maximum current demand might be as much as 25-30 amps with everything running. The relay is constantly energized as long as the BMS allows discharge. With a 64 ohm coil, it amounts to a waste of power. A MOSFET seems to me to be a better solution. I've begun learning about these devices and circuits, but I get lost when I look at the specs sheets provided by suppliers. I trying to find a simplified circuit to help me make this change. Thanks for reading, Chipper
 

crutschow

Joined Mar 14, 2008
23,494
You could use a P-MOSFET as a high-side switch.
It should be rated for at least 30V with an on-resistance in the low milliohm range (as low as possible).

The power dissipated is the current squared times the on-resistance so, the lower the on-resistance, the smaller the heatsink required for the transistor.
For example, a 10mΩ MOSFET on-resistance will dissipate 9W @ 30A.
You'd need no more than 1mΩ to avoid having to add a heat sink.

All the other parameters on the spec sheet can generally be ignored for your purposes.
 

Thread Starter

Chipper

Joined Jul 19, 2018
21
You could use a P-MOSFET as a high-side switch.
It should be rated for at least 30V with an on-resistance in the low milliohm range (as low as possible).

The power dissipated is the current squared times the on-resistance so, the lower the on-resistance, the smaller the heatsink required for the transistor.
For example, a 10mΩ MOSFET on-resistance will dissipate 9W @ 30A.
You'd need no more than 1mΩ to avoid having to add a heat sink.

All the other parameters on the spec sheet can generally be ignored for your purposes.
thanks for the quick reply, I especially like the comment "can be generally ignored" I'll do some looking and post my findings
 

Thread Starter

Chipper

Joined Jul 19, 2018
21
crutschow- please explain the preference for P-MOSFET over N-MOSFET in this application. I see many more N-MOSFET devices with low on resistance specs
 

crutschow

Joined Mar 14, 2008
23,494
please explain the preference for P-MOSFET over N-MOSFET in this application
For control of a +12V line, you need to use a P-MOSFET in order to properly bias the gate-source to turn it on.
To use an N-MOSFET you would have to put it in the ground (-12V) line for proper bias.

If it's acceptable to put a switch in the ground line in your application than you could use an N-MOSFET.
I find this item, but not sure of current capabilities
Tha device has about a 10 milliohm on-resistance, so would require a heat sink, but that package is not conducive to putting it on a heat-sink.
For that you need a device suitable for heat-sink mounting such as those in a TO-220 or similar case.
 
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