Need Help With High Current Switching using MOSFET - Page 1
Forum section: electronics / beginners

author: Cyril.Anthony
author_rank: Contributor
timestamp: June 23, 2014, 06:16:35 pm
content: Hey guys im trying Switch on and off a 600w load working on 12V using a MosfetI have few IRL8748. According to the datasheet Drain current is 78A @ 25C so can i use it?or will it heat up?Do you have any other mosfet which will work without heating?Thanks.

author: max_torque
author_rank: Super Contributor
timestamp: June 23, 2014, 06:27:24 pm
content: Will it heat up? Yes. Enough to fail? Ah, now we get to the meat of the question!Are we talking about just turning it on, and leaving it on, rather than rapidly turning it on / off (a-la PWM for example)Plenty of online calculations exist for working out heating in Mosfets, both their conduction losses and their switching losses (if appropriate)Once you have calculated the heat created in the device, then you can start looking at the thermal capability of the Mosfet, and more importantly, the way it is mounted and heat sinked, and what ambient environment it will be operated in etc!You also need to look at the package limited maximum current and then you'll find it's no mean feat to get busbars or a thick pcb copper layer to carry that sort of current continuously

author: rob77
author_rank: Super Contributor
timestamp: June 23, 2014, 06:54:39 pm
content: and make sure the mosfet can handle the 50A at 100celsius as well. if it's 78A @25celsius it doesn't ment it will handle it at 100celsius as well. btw.. it's the junction temperature, not the body/heatsink. (60C @ heatsink can easily be 100C junction).another point is the Rdson - it has to be very low for 50A continuous load. - even 10milliohm will cause 25W heat dissipation on the mosfet at 50Amps.

author: Cyril.Anthony
author_rank: Contributor
timestamp: June 23, 2014, 07:42:39 pm
content: Hey guys thanks for the reply and clearing things up for me can i use mosfets in parallel? im just gonna switch it on and off no pwm i just dont want it to heat up without any heatsink . I have 15 of irl8748 so can i hook them up in parallel or can i use irf8788?Thanks

author: rob77
author_rank: Super Contributor
timestamp: June 23, 2014, 08:02:06 pm
content: Quote from: Cyril.Anthony on June 23, 2014, 07:42:39 pmforget the "without heatsink" - that irl8748 have 5milliohm RDson (ok. 4.8mR - but that's 5) so you'll have 12,5W heat dissipation on the mosftet @ 50A - without heatsink it will last something like few secondsparalleling mosfets is ok, but you have to ensure that the current will be evenly distributed - you need mosfets with same Rdson (or compensate with series resistors) and make PCB traces or wires of equal length.but you'll be not able to run those without heatsink anyways... approx 1-2W is "safe" for TO220 package without heatsink... so no way.another point is the Rdson vs. Ugs - that mosfet is "logic level" , but to achieve the low Rdson you need to drive the gate with 10Vand don't forget the peak volatges during switch-off even a 30cm wire have enough inductance to generate nasty voltage peaks during switching off 50Amps - so be prepared for those peaksand last but not least ... reliable switching of 50A is kind of "electronics alchemy", definitely not something for young players

author: max_torque
author_rank: Super Contributor
timestamp: June 23, 2014, 09:17:13 pm
content: And to add: Are you switching the low or high side of the load? That makes a big difference for both the gate drive requirements, and the necessary inductive load protection at turn on/off.