Multiple Mosfets or Single one for Electronic load - Page 1
Forum section: electronics / beginners

author: nour
author_rank: Frequent Contributor
timestamp: May 30, 2017, 09:02:11 am
content: I want to ask which is better when designing Electronic DC Load?Is it better to use Only one High power rated MOSFET, like for example this one:IXFN 106 N 20 in miniBLOC, SOT-227 B package, 200V, 106A, 20m, 520W Pd@tc=25==> 26.46$ from Digikey!http://www.tme.eu/en/Document/dc715ca3e02f9710cc6abbaffaa0f755/92804.pdfOr use a smaller and cheaper(I mean by cheap compared to the first one) 5 MOSFET that in total can handle the same amount of power dissipation and power flow?I want to be able to do up to 100V@ 15A or around 50V 30A or more which it is up to 1500W and of course heat dissipation will be a problem but this is a problem for another day

author: bktemp
author_rank: Super Contributor
timestamp: May 30, 2017, 09:10:55 am
content: 5 MOSFETs won't be enough for handling 1500W unless you use MOSFETs designed specifically for linear operation.The Pd specification is useless for linear operation, you need to look at the DC curve of the FBSOA spec in the datasheet. If it is missing, or a straight line, the manufacturer didn't care about linear operation and you can either guess and derate heavily or test the limits by destroying a couple of mosfets.

author: nour
author_rank: Frequent Contributor
timestamp: May 30, 2017, 09:33:33 am
content: Quote from: bktemp on May 30, 2017, 09:10:55 amWell, I was going to ask about the linearity issue and power dissipation because I don't have much knowlage\experiance about using Mosfet in the linear region.anyway, thanks a lot for this piece of information it will help a lot.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 30, 2017, 09:46:57 am
content: IXTN30N100LDesigned for Linear Operation.It does 300 Watt. If you keep it cool. They are not cheap though.Look at their special category:N-CHANNEL: LINEAR POWER MOSFETS WITH EXTENDED FBSOAS.

author: sleemanj
author_rank: Super Contributor
timestamp: May 30, 2017, 10:20:43 am
content: Note that if you use mutliple mosfets, do not make the mistake of thinking you can just put them all directly in parallel.it is best if each has their own sense resistor and op-amp.Parallel Mosfets will not "automatically equalise" when they are operating in the linear region at least.

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 30, 2017, 11:16:09 am
content: Quote from: nour on May 30, 2017, 09:02:11 amFor some reason this is a Very Frequently Asked Question here. Please do a search for "electronic load" and limit it to this sub-forum (Beginners) as well as the Projects, Designs and Technical Stuff sub-forum.* You need to use MOSFETs rated for linear operation, which is not normally the case. IXYS makes some which can actually be purchased at the usual distributors.* Even then you will need to spread out the heat dissipation so that each TO-247 package only has to handle, say, a max of 40W with a fan cooled heatsink (20W might be the most you can manage with natural convection, whereas ~80W-100W might be possible with a liquid chill plate).* Each MOSFET needs its own feedback circuit (driven with a common reference to set the current level, of course). DO NOT TRY TO PARALLEL MOSFETS IN LINEAR MODE!* Expect to blow up sufficient value in parts that buying a used electronic load off eBay would have made a lot more sense.

author: Cliff Matthews
author_rank: Unknown
timestamp: May 30, 2017, 11:42:55 am
content: FWIW, Scullcom electronics covers-off this whole theme in his DC load project. He starts easy with a single device and through the series he adds more Mosfet's while discussing thermal runaway, software additions etc.. If you have the patience to watch the series, he is a very good teacher.https://www.youtube.com/playlist?list=PLUMG8JNssPPzbr4LydbTcBrhoPlemu5D

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2017, 03:14:31 pm
content: Even if you can find a single or fewer devices which can meet your power requirements, it may be advantageous to use more devices to spread the heat around and relax the heat sink requirements.