Limiting high inrush current for multiple AC/DC switched power supply - Page 1
Forum section: electronics / projects

author: Hartvik
author_rank: Newbie
timestamp: September 13, 2018, 06:36:52 am
content: Hi thereI am currently troubleshooting a huge LED-panel powering up arrays of RGB LED for showing promotional content.The LED-panel have screens on two sides, where the electronics are powered by 128 pcs (!) AC/DC 230v - 5v switched power supply (SPS) of type Deltahttp://www.sanmpower.com/display.php?id=1.See attached schematic for the distribution. Three 3 phase 400V + N circuits supply about 42 SPS each.The installation (Installed in 2013) has had multiple issues with smoked circuit breakers and burned contactors. Also, a lot of the AC/DC's have been burned and replaced...Today, the screen is always on as they do not dear to turn the screen on and off...Initially, I suspect high inrush currents are the main cause of the problem.My questions are:1: How much inrush current do you guess these SPS draw, on average?I understand that this inrush current comes from bulk caps after the rectifier bridge.The datasheet says Max Inrush Current 110Vac≤20A;220Vac≤40A. Have anyone actually measured this, as it seems like a worst case thing, and probably a bit lower.For dimensioning purposes, using this number will give a very expensive distribution system compared to the steady-state currents.2: I found this product which seems to be meant for the problem:https://powersolve.co.uk/wp-content/uploads/2017/07/ESB3Phase.pdfThe datasheet says: Allowed CapacitiveLoad (max.): 2000 µF(I guess per phase)How large are the caps normally installed in these SPS's? Would connect 14 SPS per phase be too much?3: I found another product that would limit the inrush, by replacing the contactors with SSR zero-cross relays.https://uk.rs-online.com/web/p/solid-state-relays/7204063/Any thoughts on how much the inrush would be reduced?4. I'm I wrong suspecting the neutral wire will draw a lot of current in this installation?5: Do you have any other ideas to limit the inrush currents?I was thinking of sequential startup, but the SPS does not have a remote control input.I know this is a lot of questions, I really appreciate any help

author: H.O
author_rank: Frequent Contributor
timestamp: September 13, 2018, 07:03:44 am
content: I've used ICL thermistors from Ametherm's MS series to limit the inrush current on transformers which would otherwise trip the 32A circuit breaker "every time". It's a cheap and simple solution with the drawback that the devices runs very hot when in operation. But if you can mount them somewhere where the disipated heat doesn't present any issues it's a simple and effective solution.You could have a small one on each SMPS, or a larger on each phase of each sub circuit or an even larger on each phase on the incomming power depending the actual current drawn.Another thing to keep in mind with NTC's is that if you power down the system you need to allow the NTC's to cool down before you power it up again.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 13, 2018, 07:10:19 am
content: As you noticed, the problem lies in the many paralleled supplies.Usually the inrush of standard SMPS modules is limited, but assuming you switch on only one of them at the same time. If you turn on many of them simultaneaously, the inrush just adds and is too large for your standard line supply components.Larger power supplies or other higher powerer equipment uses a more sophisticated scheme to pre-charge its internal capacitors, usually the capacitors get first connected through a resistor limiting the inrush current, then a relay shorts out the resistor.One cannot easily apply this scheme externally to a bunch of random SMPS for various reasons, one of them would be: The SMPS shall not start operating before the external pre-charge process is completed, otherwise strange issues may arise.So if you cannot modify the SMPS modules, there's another way (that I've done successfully in the past):Switch them on in a sequence (one or two at a time, next ones maybe 2 or 3 periods of line frequency later), this spreads the inrush current over time, and switch them on at voltage zero crossing, this reduces the peak inrush.

author: Hartvik
author_rank: Newbie
timestamp: September 13, 2018, 09:08:52 am
content: Thanks for the fast reply!H.O:I am now looking into mounting Ametherm NTC on the DIN rail in the distribution, as this is a large cabinet that should dissipate the heat well.https://www.ametherm.com/inrush-current/selecting-an-inrush-current-limiter.htmlI have contacted the manufacturer Mean Well, wich produces similar power supplies (RSP-320-5), to get an estimation on the capacitance of the bulk capacitors. They produce the replacement power supplies so I guess they are quite similar to the Deltas in spec.capt bullshot:Yes, a sequential startup will probably do a lot. Unfortunately, the transformers themselves does not seem to be remotely controlled on/off.I think we will install delay blocs to the contactors so they don't couple simultaneous.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 13, 2018, 09:26:16 am
content: Quote from: Hartvik on September 13, 2018, 09:08:52 amThat's what I meant. You'll have to turn on their power sequentially. A remote on/off as a part of the supply wouldn't solve the problems, as the internal capacitors get charged when you connect the SMPS input to the line, regardless of remote on/off. This usually only disables the power stage, not the capacitors.

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2018, 12:51:27 pm
content: Zero crossing solid state relays will help a lot. The specified inrush current is the worse case value from applying power at the high point of the power line cycle.Switching power supplies with active power factor correction lack large input capacitors so have much lower inrush current.

author: ogden
author_rank: Super Contributor
timestamp: September 13, 2018, 01:02:55 pm
content: Quote from: Hartvik on September 13, 2018, 09:08:52 amThis is exactly what comes into mind. Just put many DIN time delay relays that split incoming power bus into multiple and problem solved. By adding 24 delay relays you supposedly decrease inrush current 24 times. Hopefully that display thing do not mind power-up cycle lasting couple of seconds.

author: DeanA
author_rank: Regular Contributor
timestamp: September 14, 2018, 02:03:14 am
content: Hi,We have done this, very similar application, multiple meanwell PSU powering LED displays.The relay shorts out the resistors after a few seconds, the resistors limit the current out to the meanwell PSUs at initial switch on.We also use these for one of the series resistors in case for any reason the relay fails the unit won't catch fire :-)https://www.digikey.com.au/products/en?keywords=DFP10112Regards,Dean.

author: Hartvik
author_rank: Newbie
timestamp: September 14, 2018, 06:34:00 am
content: Quote from: DeanA on September 14, 2018, 02:03:14 amThat also looks like a good solution, thanks for the reply

author: Hartvik
author_rank: Newbie
timestamp: September 14, 2018, 06:42:11 am
content: I am currently waiting for the Mean Well to come back to me with the size of the bulk capacitors for NTC calculations.But I found a repair diagram online (Attached).From what I understand by watching Daves power supply teardowns, the bulk capacitors are given by C5 and C6, totaling 300 μF for each power supply.Not sure about C8 and C9 though, any thoughts if these are off any significans?