Offline LED drivers are blowing up at Mains  Power ON test - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: June 17, 2017, 06:45:05 pm
content: 10% of our LED drivers are blowing up on Mains Power ON test. Do you know why?(Schematic and LTspice simulation attached.)Please bare with me, this situation is a simple one to understand, but lengthy to explain….We had very many 230VAC, 150W, Offline LED driver PCBs made to which we add (by hand wired Modification to PCB) two 100nF X2 capacitors to help them through EMC.Aswell as this , we also (as a ‘hand’ modification to PCB) cut through some PCB copper which was connected to the 0.5W Buck converter’s switching node. We did this so it could pass common mode emissions testing. We cut the PCB copper with a Dremel Drill type thing.Now, the problem is that since doing these two modifications, we now get about 10% or so of manufactured products blowing up at Mains Switch ON test.What do you think is the cause of these blow-ups? The blow-ups never happened before these two modifications were done.The blown-up units have the two 1206, 0R56 resistors blown open, the 1A mains input fuse is blown, and the 470uH inductor is badly blasted and goes open…and the downstream 220nF ceramic capacitor is often cracked.We suspect that the blow-ups happen when the production test staff inadvertently switch the finished LED lamps ON at mains peak. This we think results in a large inrush current.The thing is, the FET M15 should act somewhat as an inrush limiter. However, we believe that the act of cutting the PCB copper with the Dremel tool is actually ‘rubbing’ the PCB in such a way that the NFET M15 is getting induced ON (Vgs voltage getting induced to >4V). Then when we sometimes inadvertently apply mains at mains peak, there is no inrush limitation and the blow-up happens.Do you agree that this could be our problem?Schurter 3403.0166 Fuse datasheet:http://www.farnell.com/datasheets/1911207.pdf?_ga=2.242578496.333053046.1497723066-1679746183.1489787856470uH (Wuerth 744045471) inductor datasheet..https://katalog.we-online.com/pbs/datasheet/744045471.pdf0R56, 1206 resistor (ERJ8BQFR56V) datasheet:http://nl.farnell.com/panasonic-electronic-components/erj8bqfr56v/resistor-1206-0-33w-1-0r56/dp/1717876

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2017, 07:28:21 pm
content: Wild guess you really do need a NTC inrush limiting resistor on the input, and a VDR across the mains input as well. That will both aid in stopping this failure, and as well the clamping of mains transients will improve longevity of the devices. You have a typical transient failure, and with this even if you had no problems before, you were either lucky or your test mains lead was made from fake copper coated steel wire with appreciable resistance on each conductor.

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2017, 07:46:54 pm
content: Since this didn't happen before the PCB was modified your explanation seems plausible. You could try reducing the value of the gate resistors or adding a high value gate to source resistor.In any case, I'm surprised the MOSFET isn't blowing up because I doubt it's designed to be used like this. It's supposed used as a switch, not as an analogue amplifier.

author: bson
author_rank: Unknown
timestamp: June 17, 2017, 08:05:51 pm
content: Quote from: treez on June 17, 2017, 06:45:05 pmHmm. This would indicate overshoot from the inductor.Could this happen?Power comes onStray charge on the M1 gate has it in an ON stateThe inductor begins to charge; current sees a short and there's a wild inrush; voltage rises.As the 10V supply that now controls M1 begins to regulate, the gate is discharged and M1 turns OFFInductor cracks skulls and breaks bones.An easy fix if this is the case is to tie the M1 gate to source with a 100k-1M resistor to make sure it doesn't collect stray charge.

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2017, 09:41:48 pm
content: Thinking about this again, unless there's a proper form of over current protection such as a fuse before the capacitors, they should be X1 or X2 rated, meaning they won't fail short circuit or burst into flames, but the fact they cracked, suggests this isn't the case.

author: Niklas
author_rank: Frequent Contributor
timestamp: June 17, 2017, 11:11:09 pm
content: Are the ceramic capacitors located close to the area that was modified with the Dremel? Package size 1206 or larger?If so, then vibrations from the modification could have caused cracks that short circuit the capacitor.

author: ocset
author_rank: Super Contributor
timestamp: June 18, 2017, 11:27:49 am
content: QuoteYes very near, the caps are 1812 size, 630VDC rated 220nF ceramics.

author: ocset
author_rank: Super Contributor
timestamp: June 18, 2017, 01:23:13 pm
content: Actually, having thought further about it, the NPN, Q5 would soon clamp the gate of M15 and limit inrush current. I am actually now thinking that maybe M15 is getting killed by the electrostatic situation of the dremelling and is just short from drain to source, and so not limiting inrush current at all.Do you think that the attached surge current waveform would be likely to blow up the 1206, 0R56 resistor?The power waveform is also shown.Unfortunately the ERJ8BQFR56V datasheet doesn’t allure to its surge handling capability.ERJ8BQFR56V datasheet (0R56,1206 resistor)http://www.farnell.com/datasheets/21...183.1489787856New simulation (ltspice) posted to show case of M15 being short from drain to source.

author: Zero999
author_rank: Super Contributor
timestamp: June 18, 2017, 01:34:14 pm
content: Quote from: treez on June 18, 2017, 01:23:13 pmIt's possible a pulse like that could kill the resistor. I've experienced a similar thing before but with large thin film resistor and more power. In my case, replacing the resistor with a wire wound one did the job, even though it had a lower continuous power rating, the surge capacity was much higher.

author: ocset
author_rank: Super Contributor
timestamp: June 18, 2017, 01:51:02 pm
content: ...Also our 1206 resistors are not stated as being surge proof, (ERJ8BQ type)...........http://nl.farnell.com/panasonic-electronic-components/erj8bqfr56v/resistor-1206-0-33w-1-0r56/dp/1717876....maybe the instantaneous power of 400W over 5us is killing them...I notice that even the 1206 , SG73-RT resistor is surge rated but over any <10us interval the power goes to 1000W or more it will blow up, as its datasheet tells...SG73-RT surge resistor...http://www.koaspeer.com/catimages/Products/SG73-RT/SG73-RT.pdfWe are using the ERJ8BQ resistor which isnt surge rated and therefore i am wondering if this is why its blowing when (if) the M15 FET is going short from D-S?

author: jbb
author_rank: Super Contributor
timestamp: June 18, 2017, 08:07:01 pm
content: The NMOS + NPN current limiter should indeed limit the inrush.Might I suggest you:Measure the MOSFET with a continuity beeper before power-up to see if you're getting dead MOSFETsUse a knife rather than a Dremel to chop the tracks near L3Here's a thought: could you talk to the original manufacturer and get them to revise the board? I guess that you don't want to make your own from scratch, but that doesn't mean the originals can't be adjusted.

author: ocset
author_rank: Super Contributor
timestamp: June 19, 2017, 05:32:52 am
content: ThanksDo you think the wuerth 744045471 470uH inductor can take the following (attached) surge current without blowing up?This is the current surge it gets if the FET M15 has already failed short drain-source744045471 470uH inductor datasheet:https://katalog.we-online.com/pbs/datasheet/744045471.pdf