Pcb or other method for 100A mosfet e-fuse  - Page 1
Forum section: electronics / projects

author: MadScientist
author_rank: Frequent Contributor
timestamp: October 03, 2021, 12:31:38 pm
content: I looking at a low voltage ( 12-16v) 100A mosfet based e-fuse application. Just trying to work out how to mount d2pak mosfets to handle this current. To me it seems a PCB based design is impractical as though short the trace widths are huge. I’ve seen suggestions to consider bus bars but can’t seem to work out how to marry that to d2pak packages.Any suggestions how I might tackle this.ThanksDave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2021, 12:37:43 pm
content: What's wrong with it? Need more continuous power?Tim

author: sandalcandal
author_rank: Unknown
timestamp: October 03, 2021, 01:03:05 pm
content: Like this?https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/msg1248658/#msg1248658

author: MadScientist
author_rank: Frequent Contributor
timestamp: October 03, 2021, 01:17:20 pm
content: Sort of.What I can’t work out is how to attach the d2pak leads to my busbarsFor example I can cnc mill 1mm copper busbars big enough to take m10 connector bolts etc. But soldering a d2 pak legs to that isn't easily done

author: sandalcandal
author_rank: Unknown
timestamp: October 03, 2021, 01:25:18 pm
content: Quote from: MadScientist on October 03, 2021, 01:17:20 pmIn the above example (the kweld) the d2pak MOFETs aren't directly attached to the bus bar. They are soldered to the PCB and the busbar is mounted in close proximity, contacting exposed PCB area to provide heatsinking and current carrying capacity.

author: PartialDischarge
author_rank: Super Contributor
timestamp: October 03, 2021, 01:25:36 pm
content: Quote from: MadScientist on October 03, 2021, 01:17:20 pmThe busbar should be bolted to a PCB (2 layers, 3 or 4oz copper, with vias, remove soldermask) and the the d2pak is soldered to the PCB close to the busbar

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2021, 01:29:59 pm
content: Don't worry about a few mm of copper foil. Heatsinking is the play.You can also get SMT jumper bits, solid metal, pick-and-place-able. Still can't place them too close to component leads and stuff, but helps spread out the heat and current.BTW don't worry about removing soldermask, unless you meant where it's bolted for contact of course.Tim

author: MadScientist
author_rank: Frequent Contributor
timestamp: October 03, 2021, 01:35:23 pm
content: So are people suggesting a short 1-2mm pcb trace from the d2pak pins to the busbars is ok for 100A? There’s also the issue of how to ensure the bus bars contact the copper on the pcb. I could place hold down bolts into the pcbI’m less worried about heat sinking as I have solutions for that

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 03, 2021, 01:43:37 pm
content: Quote from: MadScientist on October 03, 2021, 01:35:23 pmSure, why not, take a look at the RC/drone world and the speed controllers for the brushless motors, they are routinely rated at a hundred amps or more and are just MOSFETs soldered to a PCB.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: October 03, 2021, 09:08:35 pm
content: High current connector blocks for PCBs:https://www.we-online.com/catalog/en/em/redcube_terminals

author: ejeffrey
author_rank: Super Contributor
timestamp: October 04, 2021, 03:30:41 am
content: Quote from: MadScientist on October 03, 2021, 01:35:23 pmSure. For such a short wide trace you should ignore the standard PCB trace ampacity calculators. Those typically are for a long trace that is predominately cooled through the surface to air usually assuming no forced convection. For a short connection like this most of the heat will be conducted out the ends i.e., to the MOSFET or bus bar. What you should do is calculate the I^2 R losses from the short trace -- including contact resistance and and then add it to your system thermal budget.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 04, 2021, 02:40:10 pm
content: Maybe you can find MOSfets in D-PAK that can handle your 100A, but if you can put 2 of them in parallel then you also have half the current through the PCB, as each MOSfet has it's own connection to the bus bar.It may also allow for cheaper, or for easier to obtain MOS-fets. (Or replace if your source dries out).But you do have to give such things a bit of thought.Usually the current distribution is not perfect 50/50 and one of the MOS-fets may experience short current peaks if they do not switch at the exact same time (But MOS-fets usually can withstand short peak currents)

author: tszaboo
author_rank: Super Contributor
timestamp: October 04, 2021, 03:30:59 pm
content: I don't think you should use D2 for those currents, not unless you are talking about those 7 pin D2PACK packages. There are better packages than that. LFPAK,8HPSOF, DirectFET, maybe able to handle those currents.But a 1-2mm long pcb trace will handle those currents. Just make sure to have enough area, not only where the current would go, but all around it. Also vias, run it through more layers.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 04, 2021, 04:35:57 pm
content: Another design option:https://www.digikey.com/en/products/detail/ixys/IXTN170P10P/1995386

author: uer166
author_rank: Super Contributor
timestamp: October 04, 2021, 07:03:55 pm
content: There is nothing special about 100A on a PCB. I've seen ~300A be done with 8 or so paralleled SMT FETs, and some small SMT copper blocks soldered to board in automotive application. Personally done ~160A at 400V on a 8-layer 2oz PCB. You just have to calculate out the thermals and see what the heat load is, and make sure the mechanicals/heatsink can reject it. Heavier copper and more layers helps. Keeping it small helps even more.

author: MadScientist
author_rank: Frequent Contributor
timestamp: October 07, 2021, 08:46:00 am
content: Thanks guys

author: Siwastaja
author_rank: Super Contributor
timestamp: October 07, 2021, 10:44:29 am
content: Short distance is the key. PCB calculators that give you huge track widths assume infinitely long traces.Short track cools into its surroundings. You get ballpark estimates by calculating power dissipation by utilizing "ohms per square" (Google it if unfamiliar); count squares of known resistance in series or parallel. Then compare this copper dissipation to how much the MOSFETs with similar footprint area dissipate! Chances are copper is just a tiny part of that, and if the MOSFETs can cool through the PCB, then the copper of similar area definitely can, using the same heatsinking practices.