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1400v TO-247 creepage distances
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Zero999:

--- Quote from: jusanother on June 10, 2019, 09:14:34 pm ---OK, so lets limit this to functional insulation. How does the TO-247 package meet that @ 1400v ?

--- End quote ---
Easily, because minimum creepage distances depend on the comparative tracking index and the package material will be better, than most PCB materials.
https://en.wikipedia.org/wiki/Comparative_Tracking_Index

In any case, this is irrelevant since the device should be protected against over-current by a fuse. Arcing over should present no safety/fire hazard, as the fuse shall fail first, before any flames appear.
jusanother:

--- Quote from: MagicSmoker on June 10, 2019, 09:25:44 pm ---Shortest distance between pins is ~3mm, which is easily capable of holding off 1400V even for "Pollution Degree 2"

--- End quote ---

So assuming the case has a CTI >= 600v putting it in material class 1, and using pollution degree 2, even at 1000v you need 5mm creepage spacing according to this table that stipulates it is for functional insulation. http://download.wecogroup.com/docs/en/TechInfo_F4_Creepage_distance.pdf

How do I prove ~3mm  is easily capable of holding off 1400V even for "Pollution Degree 2" ?


--- Quote from: Zero999 on June 10, 2019, 09:31:13 pm ---In any case, this is irrelevant since the device should be protected against over-current by a fuse. Arcing over should present no safety/fire hazard.

--- End quote ---

I haven't got as far as safety concerns, I can't understand how it reaches functional requirements.
duak:
I wonder if a Conformal Coating would be required here.  I don't have any hands-on experience with that package at those voltages.  I did design an assembly that used a Silgard product to solve a creepage problem with wirebonds to a flex circuit operating at about 100 VAC.   The pitch was much tighter, so the voltage gradient was higher.  The device is proprietary and I can't really go into too much detail.
MagicSmoker:

--- Quote from: jusanother on June 10, 2019, 10:44:33 pm ---So assuming the case has a CTI >= 600v putting it in material class 1, and using pollution degree 2, even at 1000v you need 5mm creepage spacing according to this table that stipulates it is for functional insulation.
...

--- End quote ---

Ahh yes, I see your point. You will likely have to apply an approved conformal coating or silicone to the lead area to meet creepage requirements UNLESS the overmolding epoxy can be considered as the same as glass, mica, or other materials that do not experience surface breakdown/carbonization. Note that such materials are not put into material group 1 (CTI >= 600), rather, they effectively have "infinite" CTI so only have to meet clearance, not creepage, distance requirements.

But if you think the example you gave in the OP is bad, check out the IXYS IXBH12N300 3000V rated IGBT in a standard TO-247 package:

https://www.digikey.com/product-detail/en/ixys/IXBH12N300/IXBH12N300-ND/2183282

Talk about brazen!
T3sl4co1l:
Molding compound seems suspect, but it looks like high-CTI compounds are indeed available, up to 600 but maybe not infinity.  The dominant component is silica fume, and it appears a wide variety of epoxy and phenolic resins are also involved.  Basically if an arc forms, it can burn away the resin, leaving a huge amount of silica "ash" that hopefully tends to insulate things.  This does assume the resin doesn't carbonize under all that silica, which I expect is why it's not usually more than 600?

Tim
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