SOLVED Unmarked Component Identification Help - Page 1
Forum section: electronics / repair

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2020, 12:51:10 pm
content: Hi All,Trying to fix an LED driver board with an unmarked component which I think has failed and is causing a short. I can't identify it and not sure what to Google.. my guess is a diode of some description?In the picture it's the component to the left, paired with a MBR10100A which is a diode that is working fine. There is a tiny marking in the circle with L then underneath 3M (there are 4 identical components with different last letters)Any help would be appreciated!Thanks

author: flolic
author_rank: Frequent Contributor
timestamp: July 29, 2020, 01:16:57 pm
content: Unknown mosfet.

author: Per Hansson
author_rank: Unknown
timestamp: July 29, 2020, 01:21:48 pm
content: There is a loose leg on the inductor, did you notice that?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2020, 01:32:14 pm
content: Per Hansson, I did not! Thanks for pointing that out.Ah thanks Flolic, it's definitely shorted in that case. Will any N Channel MOSFET be ok to replace it as long as it's rated over the voltage? I.ehttps://uk.rs-online.com/mobile/p/mosfets/6711024/

author: wraper
author_rank: Unknown
timestamp: July 29, 2020, 01:36:31 pm
content: Quote from: Per Hansson on July 29, 2020, 01:21:48 pmDoes not look loose to me. Soldering might be better, but still there is solder within the hole and on the other side.

author: wraper
author_rank: Unknown
timestamp: July 29, 2020, 01:45:03 pm
content: Quote from: ajp8868 on July 29, 2020, 01:32:14 pmDid you forget about current? What is input voltage/current and total number of channels, output/voltage current? Considering diode used, you probably should use MOSFET rated for 20A, not 5.8A.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2020, 01:54:43 pm
content: I did completely.. I'll remove one of the others later and see what I can measure to work it out.Thanks for the help all, will update when I get a replacement.

author: wraper
author_rank: Unknown
timestamp: July 29, 2020, 01:58:01 pm
content: Quote from: ajp8868 on July 29, 2020, 01:54:43 pmThe best thing you could measure is RDS(on). Out of that you could figure out how beefy replacement you need.

author: Syntax Error
author_rank: Frequent Contributor
timestamp: July 29, 2020, 01:59:36 pm
content: Your LED board component reminded me of a similar defective DPAK device, which was a constant current regulator for LED drivers, the NSI45090.Similiar part:https://uk.farnell.com/on-semiconductor/nsi45090jdt4g/led-driver-to-252/dp/2535065

author: wraper
author_rank: Unknown
timestamp: July 29, 2020, 02:04:40 pm
content: Quote from: Syntax Error on July 29, 2020, 01:59:36 pmCertainly not that. It's a MOSFET in buck converter.

author: Syntax Error
author_rank: Frequent Contributor
timestamp: July 29, 2020, 02:20:10 pm
content: Quote from: wraperYou're right. Ignore the above.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2020, 04:07:55 pm
content: Well I tried to measure resistance between source and drain while the led was operating but it didn't seem to measure anything at any point as if they're not connected.. is this normal?

author: wraper
author_rank: Unknown
timestamp: July 29, 2020, 04:31:50 pm
content: Quote from: ajp8868 on July 29, 2020, 04:07:55 pmHow do you expect to measure resistance in buck converter while MOSFET is constantly switching?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2020, 04:40:47 pm
content: I was doing it wrong, should have been measuring voltage drop.. but I've found a MOSFET with the same gate voltage threshold that is being supplied and a large current/voltage.It's this one, I would guess it should work?https://uk.farnell.com/on-semiconductor/fqb19n20ltm/mosfet-n-ch-200v-21a-to-263ab/dp/2453431

author: wraper
author_rank: Unknown
timestamp: July 29, 2020, 05:03:07 pm
content: Quote from: ajp8868 on July 29, 2020, 04:40:47 pmWhy would you use 200V mosfet with awful RDS(on)? If you go to high voltage, other specs suffer. You still did not say anything about the question about voltage and current (power) ratings I asked.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2020, 05:19:10 pm
content: Ah yeah sorry I missed the other questions, it looks like it's around 12v which goes to the led first, then back through to the coil then to the MOSFET tab. The LED is thishttps://uk.rs-online.com/mobile/p/leds/9154705/So the channel that has blown is the blue, which should only be around 4.5a according to Farnell's specs.Does that answer the questions?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2020, 05:32:52 pm
content: So in this case, is it a P Channel MOSFET?Edit: Realised I was being stupid... So N channel MOSFET that will handle 12v at least 5a and with a 2v threshold is required. Is that correct? I'm not sure of any other factors that should be considered

author: wraper
author_rank: Unknown
timestamp: July 29, 2020, 06:35:37 pm
content: Dunno where you got that 2V threshold figure. So input voltage is 12V? Is there single LED connected, or multiple in series? Does it have any sticker with specs?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 29, 2020, 06:49:32 pm
content: When the LED is on, 2.5v is being applied to the gate. But there seems to be a lot of MOSFETs that start at 2vIt's a single LED, max current draw is 4.5a as stated on the attached spec sheet (page 10)

author: fzabkar
author_rank: Super Contributor
timestamp: July 30, 2020, 03:13:00 am
content: What are the marking on U1B?I'm intrigued by the 3 dissimilar parallel current sense (?) resistors (R100, R150, R200). The equivalent resistance would be 46 mOhm. What's going on here??

author: ajp8868
author_rank: Regular Contributor
timestamp: July 30, 2020, 06:29:44 am
content: I was wondering myself why there's 3 rather than one higher value, that chip is marked SQ9910B and is an LED driver. I've attached the datasheet.The gate pin goes to the gate of the MOSFET, and the cs pin must be detecting if it's actually turned on as it's connected to the source pin?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 30, 2020, 08:53:28 am
content: So, I've been reading more and more into this and, if I understand correctly, the MOSFET is limiting the current to the LED based on the voltage applied to the gate.Therefore I will need to replace it with one that limits the same amount with the same gate voltage applied otherwise, I risk blowing the LED.. am I correct in thinking this way?If this IS how it's working, how do I work out how much current is being limited when the LED is on?

author: wraper
author_rank: Unknown
timestamp: July 30, 2020, 09:33:55 am
content: Quote from: ajp8868 on July 30, 2020, 08:53:28 amHeck no. It's basically a buck converter as I already said.https://en.wikipedia.org/wiki/Buck_converterThe difference from usual buck converters is that switch is located on the low side and there is no output cap. Have you ever wondered why there is a big inductor? Also you provided controller datasheet but apparently never bothered to read even a first page of it. Now i suspect you "measured" so called "Vgs(th)" by simply measuring voltage on the gate by multimeter in DC mode.Again you simply ignored my question about specifications (ratings) of this LED driver. At least simply write it's model, I will google myself.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 30, 2020, 10:07:50 am
content: Sorry I'm clearly mis-undertood the buck converter.I did read the datasheet of the controller to work out what pins etc are what, and I did not know what specs you were after.. I've linked both the LED datasheet and the driver sheet.I missed the fact it says 10v MOSFET drive...Yes I did measure the gate voltage being applied that way. I assumed that was how much MOSFET required to turn on.The driver board itself hasn't got a model number, it's a custom board out of an Equinox Shard DJ Light.What ratings of the driver do you mean?

author: wraper
author_rank: Unknown
timestamp: July 30, 2020, 10:27:45 am
content: I meant like input voltage, power rating. So to confirm, there is only a single led on the output, board is powered from 12V, and there are8 channels/LED in total4 channels / 1 led with 4 colors in total?

author: wraper
author_rank: Unknown
timestamp: July 30, 2020, 10:29:34 am
content: QuoteThere is no DC, gate is switched at very high frequency. You cannot measure that with multimeter.

author: ajp8868
author_rank: Regular Contributor
timestamp: July 30, 2020, 10:35:18 am
content: Ah ok, so input to the board is 12v from a separate smps, there is only one LED on the output of each channel and there are 4 LEDs on the single chip, so 4 channels. There's 4 identical coils/ICs in a row.

author: wraper
author_rank: Unknown
timestamp: July 30, 2020, 10:59:47 am
content: IMHO use 40-60V N-channel MOSFET with 8-15A current rating, low RDSon, say below 80 mOhm and you should be safe. 5A MOSFET will work too but there will be more heat dissipation. And I have no idea how good is heat transfer on that board. Do not go into too high current rating since gate charge will increase and will become harder to drive for controller IC.For example RSD080N06TL or STD46N6F7

author: ajp8868
author_rank: Regular Contributor
timestamp: July 30, 2020, 11:54:15 am
content: Brilliant, thank you for your advice. I'll get one ordered and update you once it's back together!

author: fzabkar
author_rank: Super Contributor
timestamp: July 30, 2020, 06:38:33 pm
content: The current for the LED is set at around 5A (= 250mV / 46 mOhm).Vcs = 250mVcurrent sense resistor = 46 mOhmAssuming that each LED drops around 3V, then the power rating for all 4 LEDs would be 60W (= 4 x 3V x 5A). That matches the specs in my searches.I'm wondering whether the dissimilar current sense resistors allow for slightly different currents for each colour, perhaps for balancing the intensity?

author: ajp8868
author_rank: Regular Contributor
timestamp: July 31, 2020, 07:01:24 am
content: That all sounds correct, the light is supposedly 60w and the max current tested on the datasheet is 4.5a.Just a bit of a noob question, but how does the current sensing drop to 250mv? I thought the drain would be connected to the source once the MOSFET was turned on, causing that area of the board to be around 10v? Or does the coil drop that?

author: fzabkar
author_rank: Super Contributor
timestamp: July 31, 2020, 07:27:17 pm
content: Look at the application circuit in the datasheet. The current in the sense resistors is sensed via the Vcs pin. The converter regulates the current by monitoring this pin and maintaining a voltage of 250mV (as per the spec).BTW, does each LED have a different combination of sense resistors?

author: ajp8868
author_rank: Regular Contributor
timestamp: August 06, 2020, 04:11:30 pm
content: Ahh that makes sense, thank you for explaining. They all have the same set of 3 resistors.

author: ajp8868
author_rank: Regular Contributor
timestamp: August 09, 2020, 01:03:09 pm
content: Thanks very much for the help guys, that MOSFET worked like a charm and I now understand the theory behind it much better!Cheers