I've been watching this thread for a while now...great work!!!
I think I can finally contribute a little bit.
Over many years, I have gained a LOT of experience in dealing with MOSFET cooling with the custom electronic loads I design. With the higher power unit being discussed, and ways to add additional cooling, I had a few thoughts...
- As mentioned earlier, MOSFETs can only be cooled from their "rear" mounting plate. The front gets hot but you'll never move any meaningful amounts of heat through the epoxy case.
- Water-cooling can be quite effective at moving the heat from point-sources to another location, but that comes at a cost, literally and figuratively. It's much more expensive than other methods, is MUCH more prone to failure (especially when shock and vibration are involved), and requires lots of maintenance. And once you move the heat away with a water-cooling block, you still have to deal with the heat. This requires a much larger and more expensive setup than using an effective combination of MOSFET, MOSFET mounting, heat spreader (if needed), heat sink, and fan...all are important. My latest electronic load sheds over 400W (continuous) of heat from four MOSFETs and the heat sink and fan are only the size of your fist.
- I highly recommend using Type III (hardcoat) anodizing for the MOSFET mounting surface, whether it's the heat sink or just the spreader. It will insulate up to several hundred volts, is tough as nails, and has an incredibly low thermal resistance due to it being so thin. Typically 0.5mils penetrates the aluminum and another 0.5mils sits on the surface. This will allow you to mount the MOSFETs without the heat sink being electrically hot. Lots of benefits there.
- Doing a quick thermal analysis of the existing setup can give you a lot of info regarding what might have to be done when raising the power levels. It can also show how much headroom there might be in an existing design, or how close it's running to the MOSFET's thermal limits. I'd be happy to help with this.
Looking forward to seeing how the project evolves!