Using 48V telecommunication xfmr in off-line converter - Page 1
Forum section: electronics / projects

author: blueskull
author_rank: Unknown
timestamp: April 13, 2017, 12:47:48 am
content: Before anyone says, I know this is stupid and dangerous.So right now I'm working on a high density power adapter, from 90V~250V to 19.5V 180W, for my laptop.In order to get the power density I need, I need to use a planar transformer, which I cannot easily prototype.Therefore, I have to buy COTS ones from suppliers. In this case, I chose CoilCraft's PL160.The xfmr is designed for up to 72V half bridge operation at 200kHz (72V across entire primary winding, 0.36mVs), and I plan to run it at 400V half bridge at 400kHz (200V across primary winding, 0.5mVs).I'm pretty confident that it will work, given the Vs product is not too high.However, my real question is, can it withstand the high voltage across windings? Will it kill me?The xfmr is rated 1.5kv RMS isolation from primary to secondary, which translates to 2.1kV peak.According to UL60950-1, the xfmr should withstand at least 2.5kv peak in order to be used at 220V/230V/240V systems.I did a test with my newly arrived UniT UT202A at 1kV (actually, 1035V), it shows >22G resistance, and no sign of breaking down.I then did a test at 2.5kV (actually, 2667V), it shows >22G resistance, but this time with some very tiny and infrequent crackling noise.I'm pretty sure the noise comes from partial discharge, but since I don't have a coulomb meter, there is no way I can characterize it.Primary to ferrite core is also >22G at 2.5kV, but secondary to ferrite breaks down at 2.5kV. At 1kV, both are fine (>22G).PI and DAR are both tested across primary and secondary, without a valid result due to over the test period, leakage resistance has never fallen below 22G, which is equipment's detection limit.I then conducted test with my Fluke 289 in the loop, as current sensor, reading less than 20nA after RELed out error (~10nA). This means >133G for both primary to secondary, and primary to ferrite.So, given the test result, should I trust my life on this transformer (yes, I have GFCI installed, but I rather not testing it with my body in the loop)?

author: Monkeh
author_rank: Super Contributor
timestamp: April 13, 2017, 03:25:09 am
content: Or just buy an off-the-shelf 180W supply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2017, 03:55:48 am
content: There's a rule for that, depending on which standard you need of course, but some allow you to pass with an inspection step (hi-pot) of a certain level, if the material thickness fails to meet the tables. The hi-pot is some amount higher than necessary, for obvious(?) reasons.If the output doesn't need to be isolated, you only need basic, which is probably not too bad as-is.You could also add a transient protection device -- which the standards don't really make an exception for, but would at least reduce the risk of insulation failure until such time as the TPD fails. (The exception would be if the situation counts as a reduced transient voltage environment, but to pass on that ground you probably need a couple other considerations, maybe including redundant TPDs?)(Also, the TPD would be line to ground, which usually needs to be MOV + GDT, to eliminate ground leakage hazard.)Tim