Hello everyone, The following problem has probably several solutions, some of which I've seen implemented in power supplies.
PROBLEM: In many applications the metal part of the TO220 package (that's the square with metal backing and 3 legs in a row) is (or should be considered) under "live" voltage. At the same time, for thermal reasons, it is required that this metal be coupled to the best heat sink around = chassis. This obviously creates a problem of isolation, which seems to be solved by using a thermally conductive isolator sheet between the transistor and the chassis heatsink. However, if a screw, which passes through the hole in the TO 220 package is used (even if it is a plastic screw) then creepage requirements cannot be maintained (because the insulator is very thin). Solutions I saw to this problem are based on not using the hole and pressing the transistor to the heat sink by other means. I would be grateful for any suggestion on how to solve the problem: cool the TO 220 through the chassis while maintaining at least 1.5 mm (possibly more for reinforced insulation) creepage between TO 220 and chassis. Thanks Moshe Valdman
