isolated mosfets

Are there any SMD parts with isolated tab? That would be interesting. OTOH, a ceramic tab made of AlN shouldn't be that expensive and the thermal parameters would be great.

Best regards, Piotr

There are TO-220s with isolated tabs.

I'll be using two n-fets per channel, in a multichannel programmable load board. If they were isolated, a pair of fets could share heat sinking.

Isolated DPAKs with the solder-on straddling heat sinks would be ideal, all surface mount.

Isolated surface-mount parts that share a heatsink.

Surface mount heat sink - it does not compute.

RL

Easier to buy an isolated heatsink. More versatile.

RL

There are tons of heat sinks in this style:

They straddle a DPAK on the board. One pass through the reflow oven can solder the fet and the heat sink.

My programmable load circuit sinks AC or DC. It uses two mosfets. If the source is AC, they share the dissipation. If DC, only one gets hot. So it would be great to share the heat sinking.

Using two non-isolated fets and two heat sinks, the trick would be to transfer heat between the two but keep them insulated. That could work.

There are lots of isolated tab D-Squared paks (TO-263 ?) Most we use will have a TO-220 and D-Squared package with the same die.

We also use D-Pak N channel FETs and P channel D-Pak FETs.

Look at IRF part numbers for D-Pak as I remember that we use.

And you can heat sink D2Pak FETs. Just have to think about it. We do kilo-watts with D-Squared isolated. 7 lead and 3 lead with the middle lead missing of course.

I can't find any isolated dpak fets. Got any part numbers?

There are isolated TO-220s, mostly obsolete. But if I use TO-220s, I can bolt them to a big heat sink with insulators. Last resort.

Some PCB layout tricks might couple them thermally but provide insulation. Not perfect, but better than nothing.

Got part numbers?

I have only used those types of parts in situations where, in the past, a clip-on might be used for through-hole parts.Nothing over a watt or two. None are made for coupling heat sources of significance. They are expensive, difficult to reflow and largely ineffective without forced air.

I've only used isolated TO220 (not TO263) where emc prohibited capacitive coupling - and then only TO220F (plastic) - nothing with an internal ceramic wafer ($$$)- something you'd need to retain the SMD mounting methods of TO263 (D2).

AC applications will distribute heat because they are AC. If you mean reversible DC, then equal heat distribution is worth persuing.

You seem to have a lot of small heat sources in mind. These are the kind that are least affected by an electrical isolation barrier.

RL

Actually, it does. In one of my projects I had a very similar set or requirements. A ceramic substrate PCB, ideally made of AlN, would be a perfect solution -- it is a great insulator and a decent heat conductor. Unfortunately, even the price of alumina turned out to be a killer.

Best regards, Piotr

Right. I have an AC/DC dummy load circuit with two mosfets. With DC, one of them dissipates all the heat.

8 loads on a board roughy 3x12 inches, two fets per load, good air flow. But the more power we can spec it for, the more people might buy it.

There are no doubt better thermally conductive laminates than FR4, but I'd rather try cleverness first.

AlN is magical, but it would be insanely expensive to make a pcb with it.

Individual AlN insulators, TO-220 size or so, are cheap. AlN conducts heat about as well as the average aluminum alloy.

I could use two TO-220s with AlN insulators per channel, maybe all 16 fets on one giant heat sink, but that's labor intensive.

Uh OH ! My brain COMPLETELY farted !

Either that or I was drunk. But I don't even drink !

For some reason I got isolated and non-isolated reversed !

Ooops. Sorry.

I have used isolated TO-220s before but I don't think I have seen an isolated DPAK either.

Thinking about this again... If a D-Pak has the middle drain pin cut, same as a typical TO-263 D2PAK, how would you access the drain except by the tab ?

>

I might do this with ordinary dpak or d2 fets:

If the fets could be made isothermal somehow, it would double the heat sinking when one is doing all the work. Dream on.

They are being thermally shunted mostly by the layer 2 ground plane. I can make that 2 oz copper and make the 1/2 dielectric thin.

I can via down to pours on layer 6, which will help a little. Long vias don't conduct heat very well.

I think I can model this in LT Spice, with lots of resistors.

I wonder if I can make layer 1 from 2 oz copper... I will have a bga FPGA too, FTG256 package but not many balls used.

It would need a middle pin.

There are some isolated-tab power opamps around, so it's not impossible. But I'd better work with standard parts to be safe.

onsdag den 23. juni 2021 kl. 17.45.21 UTC+2 skrev snipped-for-privacy@highlandsniptechnology.com:

expensive but,

Maybe they could bridge the gap between the two fets. Heat would be concentrated in the small pad area, so it would be best to use several of these along the gap.

The 0612 version is 4 k/w, so two or three of those might be pretty good, with 2 oz copper.

That's 4.6 k/w and not crazy expensive. Pick and place.

onsdag den 23. juni 2021 kl. 18.42.46 UTC+2 skrev snipped-for-privacy@highlandsniptechnology.com:

how about two duals like this

parallel one from a each so heat split between packages