SO-8 high voltage isolation gap - Page 1
Forum section: electronics / general

author: radioing
author_rank: Newbie
timestamp: March 05, 2023, 12:11:12 pm
content: Hi All,I use SO-8 IRS2104 half-bridge driver to feed N MOSFETs powered from 400 V DC link. The problem is that the maximal clearance & creepage distance I can reach between Vs (source of high-side MOSFET) and LO (gate of low-side MOSFET) pins is approx. 0.7 mm - too small for 400 V DC. What is the best method to increase insulation between these two SO-8 package pins? Solution which enables driver desoldering in the future (repair) is preferred.Thanks.

author: Benta
author_rank: Super Contributor
timestamp: March 05, 2023, 12:15:58 pm
content: Milling a slot in the PCB is a usual method.

author: tom66
author_rank: Super Contributor
timestamp: March 05, 2023, 01:16:12 pm
content: A bit of conformal coating (with high dielectric resistance) may also help if selectively applied to that area.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2023, 02:30:15 pm
content: With the lack of creep distance already at the case, a slot alone would not help. It would be more like potting / conformal coating.

author: jonpaul
author_rank: Super Contributor
timestamp: March 05, 2023, 02:32:43 pm
content: Normal VL/TUV/VDE clearance , creep and strike is much greater than any SMD can provide.See applicable directives and compliance regs in the environment and location of the equipment's.We just Use TH not SMD in safety/isolation areas.Jon

author: wraper
author_rank: Unknown
timestamp: March 05, 2023, 02:48:26 pm
content: Quote from: jonpaul on March 05, 2023, 02:32:43 pmThis has nothing to do with safety isolation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 05, 2023, 04:04:53 pm
content: FYI, they make SO-14 versions of these, for just this reason.You can mill a slot between pins, though quite a small slot is needed to fit between SOIC pins (requiring a small end-mill, PCB will cost slightly more), and as mentioned, this has no effect on clearance, only creepage. If you can apply potting to the area (maybe conformal coating as mentioned, but it needs to be done carefully to get under the package and seal the pins), that will solve that issue, but it's a whole process to go through (in addition, agencies may require an inspection process for approval).Note that only functional type insulation is required here, and the creepage distance across the semiconductor itself is quite short but they use a compound with low CTI (comparative tracking index). I think the trick is it's loaded with silica, very low resin fraction -- so the plastic is quite hard, and also hardly expands/softens when heated to soldering temperature (obviously quite important!). The resin still turns to char under arcing, but something about it is acceptable, I forget why. Needless to say, any arcing has already destroyed the circuit; they're only concerned about fire hazard and electrical safety.Tim

author: radioing
author_rank: Newbie
timestamp: March 06, 2023, 10:23:27 pm
content: Thank you all for the answers.To be more verbose, I can add:At the beginning, we supposed to use a standard conformal coating, but with two or three layers locally to ensure full coverage.But then we realized that the most critical is the coating contact with the PCB and package, mainly the area underneath the package.Based on your answers, it seems that the robust solution is to use the slot, then, after perfect cleaning, coat the area with conformal coating and, finally, fill the slot and space around with silicone sealant (20 kV/0.5 mm). The flexibility of the sealant will hopefully ensure sufficient long-term stability and repairability. Definitely, it needs to be tested. Thanks.