SMT cap melted off a smps/inverter PCB. Best way to repair with not schematic? - Page 1
Forum section: electronics / repair

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: January 21, 2018, 09:58:33 pm
content: So this smps board stopped working, I took it out of the monitor a while a go and tested a bunch of stuff and couldn't find the problem. Recently looking at the board again it looks like the transformer gets so hot it discolored the board and most likely melted this cap off mounted directly under it on the back side of the PCB. There are no service manuals for the monitor, and I can not find a schematic of the power supply board, a DELTA DAC-24m009.What I do know is that this missing cap is connected to pin 5 of aNXP1530A SMPSchip, which is the sense pin, and looking at page eight and a few other diagrams I see this same resistor to parallel resistor/cap combo that is on the pcb, except it seems that the parallel resistor/cap terminates in to ground on my particular unit, instead of a fet like the diagrams. When I plug the the PSU/Inverter board in by itself (i.e not connected with anything in the monitor) and test the VCC of the NXP chip it gradually builds from about 1volt to 4ish volts over the course of 30 seconds or so and then when it hits 4v the board starts clicking.So how in the heck do I go about figuring out how to replace this cap?? I only know very fundamentally how something like this SMPS chip works and have never used anything like it in my own projects, and the data sheet seems to be pretty vague listing no specific values for anything unlike a nice TI data sheet.Here is a picture of the part of the PCB with damagehttps://www.dropbox.com/s/m6npiney9oc9wzi/smps.jpg?dl=0

author: coromonadalix
author_rank: Super Contributor
timestamp: January 21, 2018, 10:10:10 pm
content: i would start with an 0.1 uf capacitor if you read the 7.13 section it explain the circuit, it is a slow start timing look a like circuit used int the sense pin

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 10:28:11 pm
content: For me it seems like assembly defect as glue is present over one pad and another pad looks exactly like when some parts are not assembled. I would say the cap was holding on the glue but was never connected to the pads. Then it fell of during assembly or some later point of time. I don't think the defect has anything to do with this cap, as it's impact on circuit operation should be quiet minimal. And what I'm most certain, it's impossible for solder to melt there during operation. BTW it's quiet normal for SMPS transformer to get hot, and phenolic board becoming dark when run hot for long time. EDIT: probably this id not phenolic board as it looks like double sided. Nonetheless FR-4 becomes darker too, just less susceptible to this.Quote from: coromonadalix on January 21, 2018, 10:10:10 pmIMO too much, this cap slows down current sense. My guesstimate is it should be somewhere in single digit nF range.Quote from: rwgast_lowlevellogicdesin on January 21, 2018, 09:58:33 pmCheck secondary side for shorts and particularly shorted rectifier.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: January 21, 2018, 11:04:52 pm
content: Ok I will do some more poking around if the cap does not fix it. I also saw the .1uf thing in the data sheet and understand its value isnt to critical but what I mostly need to know is what kind of voltage rating should the cap have?

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 11:16:44 pm
content: Quote from: rwgast_lowlevellogicdesin on January 21, 2018, 11:04:52 pmDoes not matter. Max voltage there is very low, certainly below 2V. Cap in the datasheet makes response faster, this cap does exactly opposite.

author: poot36
author_rank: Frequent Contributor
timestamp: January 23, 2018, 06:49:19 am
content: If it is clicking then there is a short on the output of the power supply or the startup power supply filtering cap for the switching chip has lost capacity and when it tries to start up it can not provide enough power to keep the switching chip running so it cycles on and off.