Edward/Doug/et.al.- Any sources for those special clips? I've seen metal spring clips also...
And how about sources for the grey insulating sleeving/tubing that slips over the entire TO-220 package, providing insulation and good thermal transfer, and allowing use of a metal clip? The sleeving would of course have to be 94V-2 or better... Dwight -- DWIGHT HUNNICUTT Sr. Compliance Engineer VINA Technologies,Inc. POWELL, DOUG wrote: > > Moshe, > > I also have used an number of TO-220 transistor packages. In the past to > get the isolation I used thin ceramic substrate. Several things were done > to provide the 1.5 MM required. 1) a shoulder washer was used on the screw, > 2) there was a counter-bore in the heat sink to provide a little extra > creepage as the substrate was only 1mm thick. On occasion we have tried > kapton film but European standards require either multiple layers or 0.4mm > thickness. Also the kapton is not puncture resistant to the edges of the > transistor. > > My favorite solution is the use of a substrate with no holes drilled, and a > plastic clip is shaped like a "U" bracket with one leg shorter and the other > with an angled foot. The clip legs were about 8 mm square. The leg with the > angled foot receives a screw and as the screw is tightened pressure is > transferred to the transistor body. The transistor side of the bracket is > shaped to hold the transistor captive and prevents a compromise in creepage > and clearance. I have also used this successfully with thermal switches > mounted to copper bus bars. > > I know the group does not like graphics to be embedded in email so if you > would like a sketch please let me know which graphics format you prefer. > > One last choice was more difficult to achieve. We used TO-220 style > resistors and by using a diamond saw were able to section several and show > that the 0.4MM of solid dielectric was provided to the metal tab. However > my inspector requires an impulse test on a component in every shipment we > receive. IEC 664-1 recommends > 2.5 kV for our application. Some parts had to be removed from our > authorized vendor list for the part as they did not withstand the test (i.e. > no breakdown, punch-thru, flash-over, etc.). > > Doug Powell > Compliance Engineer > Advanced Energy Industries, Inc. > Ft. Collins, Colorado USA > > ---------- > From: [email protected] > To: [email protected] > Subject: isolation of TO-220 package > Date: Monday, June 08, 1998 4:32AM > > 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
