Blow your load up with a 48V PSU?...(LC ring of output filter) - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: June 02, 2022, 03:01:24 pm
content: Hi,Why do OffThe Shelf PSU's not give the schem of their output?Its clear from the attached LTspice simulation, that if an OffTheShelf PSU of 48Vout (say) gets a load suddenly connected to its output, then the PSU can put out 85V transient peaks....which may well blow up the load.Clearly this is due to the LC in its output (always present in offtheshelf PSU's). So why dont manufacturers tell us what they've put in the output so we can investigate and anticipate such overvoltages?LTspice and jpeg schem attached

author: dmills
author_rank: Super Contributor
timestamp: June 02, 2022, 05:51:49 pm
content: Now try it with reasonable parasitic values for ESR in the inductors and caps.The real gotcha is generally long leads and all ceramic INPUT caps on the load, but that got nothing to do with the power supply as such. Cure is a reasonably large conventional electrolytic at the load end, the ESR lowers the Q.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 02, 2022, 07:12:56 pm
content: Have you ever seen such problem in real life?I don't.As dmills said, large MLCC banks at deviceinputsare indeed a real problem.

author: Faringdon
author_rank: Super Contributor
timestamp: June 02, 2022, 10:17:24 pm
content: Thanks, though here is a more rationalised version...but still rings up to 70V......For a 48V output.I have actually known kit that got messed up by this.One was a PSU for a LED driver.....in the product, the PSU soft started into the led driver and all was well....but during production assembly and test, they often turned the PSU on before connecting it to the led driver.....and when they then "hot" connected it........blew up the led driver...in many cases, didnt blow it up but weakened it and it blew up in the field.Also seen with a UV lamp ink driver......going back and forth over the material.......led driver turned on and off no load to full load as it stopped at the gantry end to go back the other way.........LC ringing coupled with noload to fullload transient ( and VV) resulted in destructive overvoltage.I believe we need to see what the manufacturer has put in the output of their PSU.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 03, 2022, 05:37:24 am
content: Quote from: Faringdon on June 02, 2022, 10:17:24 pmUsual problem, but the responsibility is with the LED driver. It was clearly poorly designed. (Was that by you? Because you didn't want to use electrolytic capacitors?)Usually the problem is solved with electrolytic capacitors, but low ESR caps are fine if you add explicit series resistors. TVS is also a good idea.Why this way? Because no matter how well the output of a PSU is damped, if you have long or inductive wiring, the loadstillhas to damp it.

author: dl6lr
author_rank: Frequent Contributor
timestamp: June 03, 2022, 06:43:50 am
content: That is not a new problem, it has already been adressed by LT AN88 written by Goran Perica "Ceramic input capacitors can cause overvoltage transients"https://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdfAnd yes, this is not an issue of the PSU design (it happens with batteries too), but that of the supplied device.

author: f4eru
author_rank: Super Contributor
timestamp: June 03, 2022, 06:48:30 am
content: saturation of the inductor and other parasitics could lessen this problem.