Small potted mains transformer - Pri/Sec capacitance, Gate drive power supply - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 05, 2021, 07:33:20 pm
content: Hi folks,Just thinking about the power supply for a gate driveI'm wondering if I can replace DC/DC bricks with small potted classic transformers as they look suitableWindings are far away so capacitance should be no issueThey will be subject to fast common-mode transient way over 20V/nsWonder if someone uses them this way

author: sandalcandal
author_rank: Unknown
timestamp: September 05, 2021, 07:35:23 pm
content: As in you plan to use these transformers to roll your own isolated DC/DC converter or use them as a gate drive transformer?

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 05, 2021, 07:44:31 pm
content: Quote from: sandalcandal on September 05, 2021, 07:35:23 pmTo replace DC/DC converters

author: sandalcandal
author_rank: Unknown
timestamp: September 07, 2021, 06:36:28 pm
content: All the isolated gate driver power supplies I've seen operate using a HF switching design.Mains transformers are obviously designed to work at mains frequency and I suppose you could do that but the smoothing capacitor will need to be relatively large. Might also cause some conducted EMI issues.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2021, 07:53:00 pm
content: https://www.eevblog.com/forum/beginners/can-a-high-rate-of-voltage-rise-(dvdt)-affectcause-a-dielectric-breakdown/Apparently this is the same problem with motors driven directly from high frequency inverters so there are lots of search results:https://www.sciencedirect.com/topics/engineering/insulation-failure

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 07, 2021, 08:12:35 pm
content: I know thin insulations have a problem with itThat is why I start thinking about it as primary and secondary side is nicely separated in this type of transformers

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2021, 08:19:17 pm
content: Have seen iron core transformers used invery oldindustrial designs. Nothing wrong with it, just more expensive, bulky, etc.Indeed, you want the split-bobbin type for its low isolation capacitance. Mind the leakage inductance, the open-circuit voltage is quite a bit higher than the nominal-load voltage. You may want a linear regulator after the rectifier.Tim

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 07, 2021, 08:47:12 pm
content: Quote from: T3sl4co1l on September 07, 2021, 08:19:17 pmThat surprises me, they are not more expensive than hf onesA small potted transformer is 2-3$Whereas low capacitance switched ones are more expensive + need not cheap driver, making it significantly more costly

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2021, 08:51:02 pm
content: $2-3? That's quite expensive, I think you'll find things are much more competitive in 1k+ quantities.And therein lies the key, I'll bet. Is this just one-offs? Your design time costs a hell of a lot more than the cost of any part, just do it and be done, doesn't matter how. FYI, they make complete gate driver boards with everything you could want, no faffing around with isolation or comms!Tim

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 07, 2021, 09:16:25 pm
content: I'm comparing potted transformer (from like Breve Tufvassons) to something like Murata 78253J + MAX253It has a price ratio of 1:3-4 in single piece or volumeNow it is more like research for solutions for future projects

author: sandalcandal
author_rank: Unknown
timestamp: September 08, 2021, 09:19:22 am
content: Not sure what DC-DC converter modules you're looking at. Even these nice compact Murata ones are only like $3.55 one off and you don't need the separate cost and footprint of rectifiers and capacitorshttps://au.mouser.com/ProductDetail/Murata-Power-Solutions/NXJ1S1212MC-R13?qs=ROOYoSPRM2tx%252Bb9WJgCsRw==The more standard SIP DC-DC isolation modules can be had even cheaper.Would agree with the point Tim makes that the mains transformer is likely to end up more expensive if you're buying parts. If you just have some small mains transformers (and diodes+capacitors) on-hand lying around and just want play around to get something together that works then I think that'll be fine but I doubt it can be an overall cost saving.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 08, 2021, 10:00:35 am
content: I agree and I most likely end up with something like MGJ Series as it is rated for CMTI >200kV/uSAnd even when these NXJ are cheap and have low capacitance they in the datasheet for NXJ2 series say to use MGJ for gate drive purposes, maybe they want to just sell more expensive ones or they have some issues in this use

author: sandalcandal
author_rank: Unknown
timestamp: September 08, 2021, 11:10:43 am
content: Quote from: Miyuki on September 08, 2021, 10:00:35 amI haven't looked into the NJX2 but gate driving requires significant pulsed current compared to a regular "DC" load. I've seen many gate driver implementations including additional ceramic capacitors on the output of the DC-DC modules. Perhaps some performance issues there but I haven't any experience comparing the NJX2 and MGJ.