Not using a high voltage rated Y capacitor across the isolation barrier? - Page 1
Forum section: electronics / general

author: zenerbjt
author_rank: Regular Contributor
timestamp: August 30, 2020, 11:58:00 pm
content: Dear Engineers,We are using the THN 20-4811WI DCDC module withVin = 48VDCVout = 5VDCPout = 5.5W.THN 20-4811WIhttps://www.tracopower.com/int/model/thn-20-4811wi-a1….In the application notes section (from above link) , there is a PDF called “EMI considerations”. This shows Y caps being used of voltage 2kV.However, on our schematic, we don’t have such high voltage rated Y caps. In fact, we have a 500VDC rated capacitor across the isolation barrier, the VJ1206Y472KXEAC capacitor (4n7, 1206, 500VDC, X7R). This capacitor is not actually Y1 or Y2 rated, its just a general capacitor. Also across the isolation barrier, we have a 500V rated 1206, 1MEG resistor (ERJP08F1004V).Incidentally, there are also three other DCDC modules across the same isolation barrier. (These are JTK3048S05 DCDC modules by XP power).Do you think we will be OK not using such high voltage rated Y capacitors?We are hoping that due to our 1MEG resistor across the isolation barrier....the voltage between primary and secondary will be pretty equal, so we wont need such high value rated Y capacitors?VJ1206Y472KXEAC datasheet:https://www.mouser.co.uk/datasheet/2/427/vjcommercialseries-1764145.pdfERJP08F1004V datasheethttps://docs.rs-online.com/ef78/0900766b812cb812.pdfJTK3048S05 DCDC modulehttps://www.xppower.com/portals/0/pdfs/SF_JTK30.pdfTHN 20-4811WI datasheethttps://www.mouser.co.uk/datasheet/2/687/thn20wi-519375.pdf

author: Phoenix
author_rank: Frequent Contributor
timestamp: August 31, 2020, 12:04:34 am
content: Quote from: zenerbjt on August 30, 2020, 11:58:00 pmThe most important question - is this a safety isolation barrier?

author: zenerbjt
author_rank: Regular Contributor
timestamp: August 31, 2020, 12:16:29 am
content: Yes, the isolation is officially for safety reasons.The Input voltage is a 14S LiPO battery of vin = 42v to 59v, so i am not sure why we need safety though.I thought those voltages were too low for danger?I have certainly touched across that voltage and it didnt do much

author: Phoenix
author_rank: Frequent Contributor
timestamp: August 31, 2020, 12:48:44 am
content: I'm not familiar with applying component bridging requirements at such low voltage (and unknown standard)."Officially for safety" against which standard?

author: floobydust
author_rank: Super Contributor
timestamp: August 31, 2020, 02:03:55 am
content: How are the batteries getting charged? There can be a second safety isolation requirement that appears due to charging from hazardous voltage such as mains.Obviously, the isolation requirement should be known in order to do professional product design, verses a treez situation.Galvanic isolation and static dissipative typically use a 1MEG and non Y-class capacitor. Under 50V is not a shock hazard unless water is involved.

author: zenerbjt
author_rank: Regular Contributor
timestamp: August 31, 2020, 08:55:02 am
content: QuoteThanks, the batteries are removed for charging elsewhere, so they'll never be charged in this circuitQuoteI am not aware of this exactly , but its just so that any tester or operator who's working on it doesnt get electrocuted if the transformer in the DCDC modules somehow breaks over and ends up applying the 48V battery voltage to the person.(sometimes it can be up to 101V with our 24S LiPO products)

author: Jeroen3
author_rank: Super Contributor
timestamp: August 31, 2020, 12:26:17 pm
content: Your x7r ceramic capacitor will fail the first or second instance it is challenged on it's rating, which is 500V and not 3 or 8 kV like with Y caps.And the failure mode is a short circuit.Y-rated caps can take more transients and fail open, no fire.Make sure your fuse can take it if there is a shor.However this means your absolute maximum rating across the isolation barrier is 500Vdc, not very functional, not very safety.If you insist in using these cheap parts, add a spark gap so an ESD strike doesn't take it out.Also, a resistor across an isolation barrier? Make sure to use a resistor capable of the transients, often MELF.It's safer to leave all those components out. But your emissions test house may disagree.

author: zenerbjt
author_rank: Regular Contributor
timestamp: August 31, 2020, 04:07:06 pm
content: Thanks, I was just wondering…..supposing the battery is supplying the circuit…and of course the battery ground is not earth grounded…its floating…so it could maybe float up to an arbitrary voltage with respect to earth……then suppose someone plugs in a USB device or whatever to the secondary side, which results in the secondary ground getting connected to earth ground…..then there would be a voltage between secondary and battery grounds…….but what is the maximum that this could be?I mean, there are >1kW electric drives and motors in this product...so a lot of stray magnetic fields all over the place..so presumably the battery ground could get induced up to a high voltage with respect to earth ground?As you know, boith E & M fileds can be very strong and induce voltages into other things nearby...i remember once testing a 13kV radar PSU.....and a loose screw one metre away from the 13kV terminal started flashing over to an earthed enclosure which it was right next to....ticking away and blue flashing every second. I wouldnt mind betting that battery ground could get induced up to very high voltage with respect to earth ground...and the 1MEG resistor wouldnt be able to equalize it.To be honest, we have a transformer based dcdc module....and even if we directly connected primary and secondary grounds i dont see how it could ever be an electrocution danger to anyone.....after all, if the dcdc module was so badly mangled that the 50V input voltage somehow got to a terminal where the user could touch then that would be a very weird damage scenario...most likely, if the transformer got breached, then the primary side 50V would simply get shorted down to ground due to the damage....which would have to be pretty major damage in that scenario......like i say, that much damage would likely just result in the pri side 50v getting shorted to ground...........and having pri and sec grounds directly connected wouldnt do any harm to anyone.

author: floobydust
author_rank: Super Contributor
timestamp: August 31, 2020, 08:10:05 pm
content: I think it's just galvanic isolation, so a vehicle ground fault (battery, wiring, motor etc.) to chassis does not cause problems.If the battery pack/primary was connected to chassis ground, and any cell shorted to the chassis- where would the fuse be? They generally are on the output or after the pack. Not in the middle. So a battery box failure would cause a short unless the primary side is isolated.Yes you would get an ESD event connecting the isolated vehicle to a mains charger. Like aircraft, which require a ground clip connected first to the chassis. Vehicle capacitance to ground is quite large, depending on the tires.