EPROM, IMO you're connecting your heatsink to a non effective location on this driver.
The negative input (-) connection ring you connected your heatsink to isn't directly thermally coupled (i.e. vs metal to metal heat conduction) to any of the main heat generating components (and the following first two are also failure prone) - the power Mosfet, the Schottky Rectifier and the big inductor, so that any heat transfer to the heatsink is done via the glass epoxy material which is a lousy heat conductor, although there are several vias in the PCB below the power Mosfet that go to a small track on the other side, but they aren't connected to the (-) ring.
On the other hand, all three ARE thermally coupled (as well as electrically) on the components side of the PCB in close proximity - at the power Mosfet's cooling tab. Thus the best place to connect a heat conductor would be to the power Mosfet's cooling tab and nearby Schottky Rectifier pin (which is that part's specified cooling path). Of course it would be necessary to carefully isolate this power carrying heat conductor so it doesn't short to anywhere, especially not to the (-) connection, which would result in a direct short between the (+) and (-) inputs through the coil.
I believe that this method would give a much better cooling effect. Too bad this good driver board doesn't offer easier cooling options.
Another possible method would be to fill the gap between the coil, Mosfet and rectifier with thermal glue and gluing them all (at the coil side) to a heatsink, but I doubt this would be effective because such glues have relatively poor heat conduction at those thickness levels compared to the metal to metal heat transfer mentioned above, as well as eliminating any possibility of electronic repair if needed.
Even inserting a relatively thin (1-2mm) heat tab between these parts and directly gluing to their plastic packages and the coil won't be very effective IMO because these parts' plastic packages aren't very heat conductive either.
I'll try playing with these ideas and come back with results. In any case, applause for all the good work that you, HANSV and others have done about this great driver.
Cheers!
