| Electronics > Projects, Designs, and Technical Stuff |
| Insulating washers & creepage/clearance issue at holes |
| (1/3) > >> |
| eevcandies:
Have an electrically "hot" semiconductor, let's say 400V or 600V TO-220 mounted to a chassis/heatsink. Using the typical shoulder washer & thermal insulator pad with good cut-through prevention. Or this might be a small "hot" high voltage metal plate mounted using four corner shoulder washers & insulating pad material to a heatsink However, at the hole itself, the metal heatsink is only thousandths of an inch away from the "hot" metal (via the wall of the hole)...separated only by the thermal insulator pad (which is not insulating at the open hole). Adding the shoulder washer does not seem to change things, since an arc could travel from the hot metal's hole rim, alongside the shoulder washer, down a few thousandths to the heatsink's metal hole rim. Seems like the shoulder washer ONLY prevents the screw from causing an arc/short. A few thousandths gap is certainly not enough distance for 600 volts or hipot tests. Is this mounting issue discussed anywhere?--if so, I can't seem to find any mention. I usually see "simply add a pad & shoulder washer to insulate", in shoulder wash catalogs, app notes, etc..but that seems to ignore this path. Should we ignore? Do any of the standards address this potential arc path? |
| Someone:
To create more isolation there are many options, spring clamping with a flat pad or sleeve: http://www.tglobaltechnology.com/wp-content/uploads/2018/03/PTM-THIN-Series.pdf shaped clamps for use with flat thermal pads: https://www.essentracomponents.com/en-nl/p/transistor-insulators Many others... |
| eevcandies:
Thanks for the tips!! Have you ever seen this specific condition discussed or actually mentioned anywhere (as well as in any standards)? |
| T3sl4co1l:
You can get shoulder washers with a longer shank to stick down into the mounting hole; that doesn't account for the gap between the outside of the shank, the thermal pad and the hole edge, but it helps the screw anyway. I'm not sure offhand if regulations would accept a tight-fitting thermal pad as sufficient. It could possibly work as electrical insulation. I would guess it's not very reliable. The preferred solution is a solid pad and a spring clamp, which also improves clamping force -- normally the off-center screw hole causes the device to tilt up, increasing thermal resistance between the body (where heat is actually generated) and the heatsink. You can use I(2)PAKs and MAX247s in the same way. Tim |
| eevcandies:
Thanks for the tip. However, what I'm saying is for years and years you see thermal pads or mica washers & a shoulder washer touted as "the solution"....yet I've never seen this potential issue even mentioned anywhere...so is it an issue? If it is not, why not? If it is an issue is there a link anywhere where it has been discussed? Again, the issue is the distance between the metal surfaces,at the hole, is only the thickness of the sil-pad, or mica washer. A pic of a mounting: http://www.diyforums.org/MOSFET-MAX/MOSFET-MAXheatsink.php They mention the washer insulates the screw...but they neglect to mention the extremely small creepage between the heatsink and metal tab of the transistor through the hole |
| Navigation |
| Message Index |
| Next page |