They don't even have an e-stop input...
Yep, and actually it's easier to not trust it and not rely on it.
You'll not find MOSFETs with those specs at that price... Otherwise I would be using them ?
Of course - I just remembered your project is 48v.They are both 40 V max... You need at least 80 V ones.
I’m out camping with limited reception. As I recall, the limiting factor for fet disconnects is the heat sink, not individual fets.For the total thermal power, yes, but the main limit is the individual thermal power and it'll be too high if you put only 1/4 of the MOSFETs because then you'll have double the current per MOSFETs so 4x the dissipated power per MOSFET.
He wants to do a separate port BMS with the discharge side MOSFETs switched and the charge side low current signal controlled.
I’m out camping with limited reception. As I recall, the limiting factor for fet disconnects is the heat sink, not individual fets.
I’m out camping with limited reception. As I recall, the limiting factor for fet disconnects is the heat sink, not individual fets.
You can redo the calculations if you want to be sure but I confirm that in my case the limiting factor is the individual thermal power, mainly because the PCB is in the thermal path. I have some margin of course but not that much, something like 30-50 % on top of using the worst case parameters, even if it's hard to have an exact number because there's not much data available (basically only one article) about using vias as a thermal path (almost all the articles are about using the PCB itself as the heatsink, not inbetween the components and an aluminium heatsink).
For the fun here's a capture for a single MOSFET:
View attachment 36069
Thermal vias are 0.32/0.6 mm, standard vias are 0.4/0.75 mm and the bigger vias are 0.8/1.4 mm.
This is for a heatsink on the opposite side?
Looks like it would be way to many vias for good path in X and Y direction through copper plane. But good for reaching heatsink opposite side.
I just evaluate vias with Excel, based on barrel length and thickness.
Fan-out to sheet of copper before transfer to heatsink matters. It would seem like fanning out on surface layer is best, but one project some other guys did, solder mask adhesion to copper was poor and pulled away (heatsink epoxied to it.). Turned out to be better fanning out on a buried layer. May not be such a problem with bolts clamping heatsink.
I've seen vendor (and "engineer") recommendations for connecting to a sheet of copper on every layer between topside and bottom side. If an island isolated from ground plane, that doesn't do anything to help cooling; I use interleaved ground planes to carry heat away.
Heatsink will be around 2 °C/W so 20 °C/W equivalent per MOSFET which is a 34 °C increase.
The total is then a 37°C increase for a 40 °C budget.
Easy enough to measure if you can set up a representative air flow/restriction environment. Ought to do that if considering cutting down. However, projecting performance to a different length may be far from trivial.
Did you implement heatsink over-temperature as a reason for disconnect?
Do you expect convection to always keep it cool enough? If not, adding a thermostatic fan would extend it's capability,
Correct.the 3000VA unit still only has one high-current connection that is both charge/discharge, right?
Are you talking about the remote control inputs?