Imac A1312 - Blown Diode twice in LED inverter Board!! - Page 1
Forum section: electronics / repair

author: reessi
author_rank: Contributor
timestamp: December 15, 2020, 06:49:57 pm
content: This is my first post in this forum so if there is anything wrong, let me know and hello!-----------------------------------------------------------------------------------------------------------------I am trying to fix: A1312, iMac 27-Inch "Core i7" 3.4 (Mid-2011):First off I have the Imac Schematic but only for the mainboard, does anyone have the inverter one or a similiar one from another for better understanding?Searched all over for one!All voltages to the Inverter are correct:(12v in, 3.3v Enable and PWM Approx 3.0v).Output was 12v and moved a little but Inverter was obviously cutting off as D1 was SHORTED.After replacing this the IMAC works for 1 week exactly and dies again.When working, the output was 36v @ full brightness and 28v minimum. Before repairing the Inverter I tested the LEDs one by one in the display with a multimeter in diode mode and they light up and had same diode reading so I thought there was no fault there. I also tested the strips when out of the display on the Inverter and all lit as normal!I just confirmed the same Diode was Shorted. I had replaced with the exact type from RS electronics (V10P10), Datasheet attached.There are 2 others on the board which are still fine.There are 3 sets of Inductors that are sharing the load but why does this one always blow??I have removed all three sets of inductors and the 3 switching MOSFETS 23N06-31 (Tested working, switch on and off when charging the gate with multimeter), all inductors show conductance but can they fail another way? Like windings shorting?All the diodes have the same readings. All the capacitors tested ok I thought, the 2 input ones 420uF instead of 470uF, the output ones 41 instead of 47uF and on my ESR meter well within spec. I will replace them all now while waiting for replies.So any ideas as to why it has blown 2 so far!On the return of this IMAC I wired the board outside the unit and it gets very HOT!! Almost too hot to touch. It is hard to know exacltly what but appears the MOSFETS/INDUCTORS ! And the 0.24 ohm resistors which I'll label on the picture, I believe these are for current sensing.If I turn the brightness below acceptable level the board goes cold, indicating too much current draw I think! But can LEDS be faulty and do this, I thought they blow open circuit and surly the whole point is the Inverter is Current Limiting!!I have annotated the pictures please have a look.Many thanks.

author: reessi
author_rank: Contributor
timestamp: December 15, 2020, 06:52:45 pm
content: I have annotated the pictures please have a look.

author: reessi
author_rank: Contributor
timestamp: December 15, 2020, 07:00:28 pm
content: p

author: reessi
author_rank: Contributor
timestamp: December 15, 2020, 07:01:06 pm
content: The inductor casings had cracks in them but the windings looked ok under the microscope, I glued them back on.Something interesting I discovered, if you heat up the diodes with 200c hot air, they eventually go full short both ways and then return to normal!

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 15, 2020, 07:06:35 pm
content: The broken case of the inductor probably will have a dramatic effect on the inductance of the inductor. The case is part of the core.

author: reessi
author_rank: Contributor
timestamp: December 15, 2020, 07:32:23 pm
content: Ah I didn't realise the outside was part of it. The markings are:220,14CH11C.A google search is showing this:https://www.avnet.com/shop/emea/products/eaton/dra73-220-r-3074457345629315969/Saying -> Ind Power Shielded Wirewound 22uH 20% 100KHz 1.42A T/RWould these from RS be a correct replacement, I can get them and test:https://uk.rs-online.com/web/p/wire-wound-surface-mount-inductors/8135693/In the original specs it states 100khz but no mention in the RS specs, is that the maximum frequency it can work at?Is there a way to test inductors apart from Open Circuit?Cheers.Sorry the replacement does say 100khz, I realise this is the standard testing they do at that frequency!

author: fzabkar
author_rank: Super Contributor
timestamp: December 15, 2020, 07:57:06 pm
content: I would trace out a partial circuit diagram.BTW, these PCBs aren't that expensive, if you can find one (US$17 + shipping):https://www.ebay.com/itm/New-09-Apple-27-034-iMac-A1312-Backlight-Inverter-Board-612-0062-IV87169T-A11HF-5A-/263823324555

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 15, 2020, 08:09:50 pm
content: I'm not an expert there, but the inductor you selected is not identical to the defective one. DC-DC converter are sometimes very picky. There are more parameters than the inductance, resistance and current.

author: reessi
author_rank: Contributor
timestamp: December 15, 2020, 08:25:49 pm
content: I think this is the datasheet for original...I know I can buy one on ebay, only £12 but I am learning board repair!I already ordered those inductors and they were only £2.50, if anyone can explain why these are not a great match please explain as I do not know.In the meantime they come tomorrow so no harm in testing out!Twoflower which parameter are you saying is not identical?

author: reessi
author_rank: Contributor
timestamp: December 15, 2020, 08:26:24 pm
content: On my ESR meter all 6 inductors measure at 11 ohms. I'm not sure if an ESR meter is a good test.They measure short as expected on the multimeter.Datasheet for Ones from RS...

author: fzabkar
author_rank: Super Contributor
timestamp: December 15, 2020, 11:04:34 pm
content: Are there 3 separate LED strings?Is each pair of inductors in parallel or in series?What voltages do you measure across the current sense resistors? That should tell you the LED current.BTW, you forgot to attach the diode datasheet.V10P10, Vishay, High Current Density TMBS (Trench MOS Barrier Schottky) Rectifier, Vf = 0.453V at If = 5A, 10A, 100V, SMPC (TO-277A):https://www.vishay.com/docs/89006/v10p10.pdf

author: james_s
author_rank: Super Contributor
timestamp: December 15, 2020, 11:14:52 pm
content: Do you have an oscilloscope? Looking at the waveform at the diode should offer clues as to what is going on. I agree though that replacing the inductor is probably a good idea if it was cracked. Don't worry too much about the specs not being exact, so long as they are close there is a good chance it will work. This is not a particularly high power design and as someone pointed out, a replacement board is quite cheap if you screw it up.

author: reessi
author_rank: Contributor
timestamp: December 16, 2020, 12:05:46 am
content: Are there 3 separate LED strings?It is a bit confusing, all three outputs from the 3 sets of inductors are connected together after the diodes. Then that one output goes to a load of small inductors and SMD caps before going to a 12 pin connector!All the positives are tied together so 6 inputs and then 6 negatives not tied. I am presuming that means there a 6 seperate strings as the 6 negatives must be monitored for return current if I am correct??Now physically there are 2 LED strips, Left and Right along the bottom of the LCD. So each half has 3 positives and 3 negatives each receiving the 36v.Does that mean there are 6 strings of approx 12 LEDS assuming 3v voltage drop? I wish I had counted and checked all this before re-assembling the LCD Panel! Making 36 LEDS each half. It seams about right from memory!I will look for my photos.Each pair of Inductors is in series! I confirmed the connections by removing all the inductors and diodes and MOSFETS. It is very hard to follow the circuit board as it is painted black, thanks Apple!Yes thanks for the Datasheet, I did forget.In the morning I will solder everything back and annotate a picture of the board with all the voltages every where for you.I do have a 100Mhz oscilloscope which I would love to use! It doesn't get much use, only on a remote control boat I was fixing once.What frequencies will it be switching at? Should the ground be connected to the main ground on the board or the LED negative as that is not connected to ground? And same when measuring the voltages should the multimeter be on ground or LED negative?I found a photo, there 36 leds in each half . I tried to attach the photo but it says it fails security checks? Any idea why, came from my phone exactly as the other photos!This picture I showed earlier is the flow through the inductors and diodes if thats helpful?The red line is not connected to the diodes just the beginning of each set of inductors in series, in case that was mis-leading!Cheers

author: james_s
author_rank: Super Contributor
timestamp: December 16, 2020, 03:11:13 am
content: I would expect the switching frequency to be somewhere in the region of 20kHz to maybe 300kHz, you'll be able to find out with the scope. Connect the scope ground to the main ground of the PCB and see what you get. There is a reasonable chance that the LEDs will be modulated at the frame rate of the display so you may see bursts of the much higher switching frequency rather than continuous.

author: reessi
author_rank: Contributor
timestamp: December 17, 2020, 03:20:32 pm
content: Hello,So the Inductors I bought were totally wrong! They are tiny, I should have checked the physical size for a clue. They are 5mm and the originals are 12mm wide and much taller! But does that mean the specs I got up off google for the original are not correct? It stated 1.42a and these little ones are supposed to be that! Only one page comes up for the number on the original. The Inverter is 7.2A so to share the current for 3 x 2 in series would want at least 2.5 to 3 amp each? Since it is only 3 parralled!Anyway I put back together and took some measurements, see the pic. Usually around 105v to 120v at the MOSFETs (seams to change each time I turn it on but not by much once on) which is connected to the diodes, the diodes are rated for 100v!!What can be causing this!Many components are getting hot on the board!I have attached a picture of the oscilloscope connected to the same place. There is same waveform at each MOSFET. I do not understand how I work out the frequency on the oscilloscope, I messed around with the scale settings to it to show that waveform. Each cycle takes up 3 blocks on the picture, it has some timing info at the bottom but I'm not sure which segments it refers to?I am not sure which resistors are the current sensing ones but there are some tiny ones around the output connector. They change slowly while it is on anywhere from 1.8v to 6v, the brightness looks the same!The output is different everytime I turn it on from 36v to 42v! But stays the same once on!I'm not sure what else to check??

author: reessi
author_rank: Contributor
timestamp: December 17, 2020, 03:25:51 pm
content: Picture of the voltages at each MOSFET!The MOSFETS are rated at 60v yet we are getting over a 100v at the MOSFET Drain connected to the INductors!I do not understand the readings here though, why is there over 100v before the diodes but 42v after?By the way the input voltage to the board is 11.7v, is that within spec?Can the LEDS be drawing too much current or affecting the voltage drop if some were faulty? I thought not as the Inverter is limiting the current?Ok did some reading of my oscilloscope manual and got the frequency displayed, at full brightness it is 345khz approx.

author: fzabkar
author_rank: Super Contributor
timestamp: December 17, 2020, 07:31:40 pm
content: I would think that 11.7V would be OK. That's only 2.5% down on 12V.I can't understand the voltage readings on the MOSFETs, though. AISI you have 60V spikes in theforwarddirection across each diode. Does this happen when the inverter is loaded by the LED strings?

author: reessi
author_rank: Contributor
timestamp: December 17, 2020, 07:53:42 pm
content: Without the LEDS it does not power up, it checks for them. So the 120v is with the LEDS! What does AISI mean please?

author: fzabkar
author_rank: Super Contributor
timestamp: December 17, 2020, 08:16:45 pm
content: AISI = as I see itSorry, I'm a veteran of Usenet.I expect that your circuit is using this kind of topology:https://learnabout-electronics.org/PSU/psu32.phpIf the output voltage is 42V, then I would have expected that the drain voltage would switch between ground and 42.6V.

author: reessi
author_rank: Contributor
timestamp: December 17, 2020, 08:32:33 pm
content: Yes I was on that page earlier, I've been looking all day, there is a good video on Louis Rossmans youtube of how an Inductor works in the backlight circuit but I'm totally confused by the high voltages on the MOSFET drain before the diode!

author: fzabkar
author_rank: Super Contributor
timestamp: December 17, 2020, 08:35:00 pm
content: I'm confused, too.

author: reessi
author_rank: Contributor
timestamp: December 17, 2020, 08:42:20 pm
content: That makes 2 of us, hopefully someone will be along to enlighten us!

author: fzabkar
author_rank: Super Contributor
timestamp: December 17, 2020, 09:17:50 pm
content: Did you measure the voltage drop across the current sense resistors?

author: aqibi2000
author_rank: Regular Contributor
timestamp: December 17, 2020, 09:54:10 pm
content: Testing a Switching circuit with zero load is never a good idea.

author: reessi
author_rank: Contributor
timestamp: December 17, 2020, 09:54:38 pm
content: fzabkar -> I wasn't sure which ones they are? There are a load of resisters connected around the output pins, they measure from 1.8v to 6v, they change slowly for no good reason as far as I can tell since the brightness is not changing! Is that likly them, I see on other schematics they are connected on the return fromt the LEDS, these are.The board is hard to follow being black!aqibi2000 -> The LEDS are connected, why did you think there was no load? The inverters do not power up without LEDS, they check.

author: fzabkar
author_rank: Super Contributor
timestamp: December 17, 2020, 10:40:39 pm
content: I would think that the R240 resistors would sense the current.

author: james_s
author_rank: Super Contributor
timestamp: December 18, 2020, 04:39:39 am
content: BTW, you can edit an existing post to add more to it instead of posting 4 or 5 times in a row, it keeps things more consolidated and easier to read.

author: aqibi2000
author_rank: Regular Contributor
timestamp: December 18, 2020, 05:36:37 pm
content: Swap LED strip number 1 with another position (ensuring they have the same number of LEDs)Then see if D1 still gets hot to the touch....

author: reessi
author_rank: Contributor
timestamp: December 19, 2020, 09:26:09 pm
content: Hi there, I do not see what difference this can make since all three diodes have their outputs connected together and then that goes to the leds and all the positive inputs are connected together! The diodes do not drive seperate strings.Also 3 of the strings are all on one circuit board,There are 2 circuit boards of three on each, left and right half of the screen along the bottom only, you cannot seperate the individual stings and where they plug to the connecter they are all soldered together.I have ordered a new inverter board now as my friend needs his Imac back, if that one overheats then we know it the LEDS! But I do not see how they can as the inverter is current limiting, or supposed to be!!Cheers.

author: reessi
author_rank: Contributor
timestamp: December 23, 2020, 07:17:25 pm
content: Well the new Inverter gets hot as well but takes longer to get there! Can it be the LEDS but I do not understand how as the inverter is supposed to be current limiting, so it should not be possible to overload it just like on a bench power supply with current limiting??Does anyone one know if these inverters get hot anyway? I do not have it in the case at the moment so maybe should be cooled inside with the fans? Still the old one does the same but quicker, I do not want to give it back and the same diode fails on this new inverter!!Any ideas? Cheers.

author: james_s
author_rank: Super Contributor
timestamp: December 24, 2020, 04:51:09 am
content: How hot is it getting? I would expect the hotter parts to reach maybe 60C in still air. If it's running hotter than that then I would be concerned.

author: aqibi2000
author_rank: Regular Contributor
timestamp: December 25, 2020, 12:12:02 am
content: LEDs gets warm when operating at the specific current rating, testing with a DVM is not sufficient.The VF could have changed at the differing voltage level == over driving the SMPS?

author: fzabkar
author_rank: Super Contributor
timestamp: December 25, 2020, 06:54:09 am
content: This is the inverter for the same Imac at ifixit.com (you can see the pinouts):This Amazon listing identifies compatible inverters using the same Apple APP003SN chip:https://www.amazon.co.uk/TONELON-605-3035-661-5311-V267-601HF-Backlight/dp/B01AZDJFZMQuoteSearching for PDFs locates a potential circuit diagram, but the sites looked spammy, so I didn't try.Edit: Removed links to potential scam sites.

author: reessi
author_rank: Contributor
timestamp: December 25, 2020, 10:47:06 pm
content: I'll investigate those links.So I stuck a temperature sensor I have on the replacement board next to the inductors and it was up to 71c degrees after an hour on full brightness...So I'm guessing it has to be the LEDS?

author: fzabkar
author_rank: Super Contributor
timestamp: December 26, 2020, 12:00:33 am
content: Those spammy URLs are suspicious. I can't believe that "3.8M people liked" a Wordpress inverter schematic. Nor do I understand why there is only an invitation to "register", but nowhere for "registered" users to login. I'm wondering whether it's a credit card mining scam.Edit: I thought it best to remove the links.

author: reessi
author_rank: Contributor
timestamp: December 26, 2020, 12:48:06 am
content: I checked and also believe they are a scam, I recommend that you remove those links so nobody trys to sign up.So now I moved the temperature sensor from the LCD panel and stuck it to the back of the Inverter and it reads 95C Degrees!!! So the probe I was using wasn't accurate.No wonder they blow the diode. I am convinced now I need to replace the LEDs but can anyone explain what is happening?Surly the inverter is current limiting and it sets the voltage! I see that it changes the voltages for the brightness. It is my understanding the leds internal resistance changes with voltage hence you can control brightness with voltage. Can you also control brightness with current limiting?Under what fault condition are the LEDS to overload the inverter? I can only think that their internal resistance has lowered as they have got old but still this would not matter if the inverter was current limiting!By the way I spent extensive time looking for schematics before and datasheets for the chips on the board to no avail.I can buy only refurbished LED strips from Aliexpress! What about me replacing the LEDS, I'm sure can find them on RS electronics or ebay but how will I know the specs of them? I will have look tomorrow for the LCD panel datasheet!Cheers.

author: Rasz
author_rank: Super Contributor
timestamp: December 26, 2020, 12:56:40 am
content: Its Apple, 95C might be its intended operational temperature.

author: fzabkar
author_rank: Super Contributor
timestamp: December 26, 2020, 01:11:30 am
content: AFAICT the output power to the LED strips should be about 25W maximum. If anything, the inverter seems much too big and complex for the job.FWIW, here are the specs for an LG LM270WQ1-SLC2 panel that was used in an Imac A1312:https://www.avnet-integrated.eu/fileadmin/user_upload/Files/Displays/Colour_TFT/LM270WQ1-SLC2.pdfIt uses 4 LED strings with a maximum current of 125mA per string. The max power consumption of each string is 57V x 125mA = 7W."The LED Bar consists of 68 LED packages, 4 strings (parallel) x 17 packages (serial)."

author: james_s
author_rank: Super Contributor
timestamp: December 26, 2020, 08:06:29 am
content: I don't think 71C is all THAT unreasonable. It's warm enough that it would be nice to find someone with a similar system to find out how how theirs runs but it's certainly well within the range that semiconductors can tolerate. By the time it's been on for an hour I would expect the temperature to have stabilized.

author: reessi
author_rank: Contributor
timestamp: December 26, 2020, 01:32:24 pm
content: james_s-> actually it is running at 95 to 98c degrees as the first probe I have which is just a little temperature board and sensor off ebay was not accurrate. I moved the Imacs sensor from the LCD to the inverter and got over 95!Rasz-> Indeed ! But the inverter cannot take this abuse!!fzabkar-> actually in the Imacs for some reason they have a different version LM270WQ1- (SD) (E3): - Note the digits in brackets!It has 6 LED strings running @ average 36v each, the inverter is rated at input 12v * 7.2a Maximum = 86.4w.Maximum voltage output it is rated at is 42.3v.I have confirmed this from the wiring and I've counted the LEDS, there is LEFT and RIGHT bar. Each bar has 36 LEDS so TOTAL 72 LEDS total in the Panel.So there are 3 strings per BAR.Specs from Datasheet are:36v per string of 12 so 3v per LED.12.6w per string of 12 so 1.05w per LED.So 350mA per LED.So I am going to look for LEDS on RS electronics.What I would like to know is how can the LEDS be tested? I already checked each on on diode mode with the meter and they all lit dimmly, I'll check again if they show different readings. They all light up as normal when out of the panel!!DO I need to replace them all? But also if I only replace a few maybe they are different Lumen output and different colour temperature?I've attached the right datasheet for these panels.I wonder why there are panels with different backlight strings on the same model panel!!I still do not understand why they are causing the Inverters to overheat?I should point out that when the LEDS are on the Inverter is usually outputing 41 or 42v and the datasheet is saying typical 36v! Is this a clue that the LEDS are failing?It is like the inverter is running a MAXIMUM all the time!! Hence why it gets so hot and after as week fails!I just looked on RS and it is difficult to see an exact match. There are not any LEDS at 350mA and 3v and I've no idea what lumen output these are!

author: fzabkar
author_rank: Super Contributor
timestamp: December 26, 2020, 06:25:29 pm
content: I realise that the 4-string arrangement is different for the LM270WQ1 panel, but since it is for an Imac A1312, it stands to reason that the power specs for its backlight should be similar to the 6-string version.Here is the inverter for the 4-string setup:https://www.sergeraln.com/index.php?main_page=product_info&products_id=117050Notice that it is rated for 80W, even though the LEDs draw only 28W max."LCD Inverter Board Constant Current Source Booster80WFor LM270WQ1 12P Output"Your panel has 72 LEDs, whereas the other has 68. Therefore I expect that the LED current should be the same.In short, I believe that your backlight draws 28W, not 84W, and that the LED current is more like 120mA.

author: james_s
author_rank: Super Contributor
timestamp: December 27, 2020, 11:26:42 pm
content: Measure the voltage across each string of LEDs, if they are all the same within a percent or two then you can be pretty confident that the LEDs are fine. There are two failure modes I've seen with backlight LEDs, the most common is that one or more LEDs will short, causing the voltage of that segment to be reduced. The other is that an LED goes open causing that whole string to go dead. There is really nothing other than shorted LEDs that could cause the inverter to overheat, it will be a current source so up to a point it should deal with a higher or lower voltage across the string. Many will shut down if the detect a significant discrepancy though.95C is certainly toasty, not impossible that it was designed to run that hot but that's getting up there. Is the housing set up to benefit from the airflow of a fan, or thermal conduction to the case? It's possible that it runs hotter in open air than it does when it's all put together. Are you sure the inverter you got is genuine and not a low quality aftermarket part that might run hotter?

author: reessi
author_rank: Contributor
timestamp: December 28, 2020, 01:41:53 pm
content: In short, I believe that your backlight draws 28W, not 84W, and that the LED current is more like 120mA:In the datasheet for my panel it is saying 350mA, in the datasheet you posted it states 125ma Can it be that apple wanted a brighter panel?james_s:I'll do some more testing this evening. The inverter is a used inverter I bought, it is a later version but still overheats the same. The temperature measured is when it was inside as the LCD temperature sensor would not reach outside the unit! So the fans seamingly do not help. I have already cleaned all the fans in this IMAC. I also put a fan on it from an old graphics card and that did barely anything but there is not room for the airflow behind the inverter with a small fan....It cannot be normal as the back of the Imac is red hot where the inverter is. The only thing left is the LEDS if not the inverter!I have ordered some LEDS for this panel for £16, since I cannot find the correct replacements individually and need to try to resolve this soon.Both the inverters run at 95 to 98c!I thought it should shut down as well and I also thought like you that they're either short or open, but all of them light! So I am totally confused! It does shut down when there are no LEDS plugged in!

author: fzabkar
author_rank: Super Contributor
timestamp: December 28, 2020, 06:53:20 pm
content: Do you have something like this?https://ae01.alicdn.com/kf/HTB10ZxTau6sK1RjSsrbq6xbDXXaJ/HiDANCE-compteurs-de-courant-alternatif-220v-wattm-tre-num-rique-compteur-d-nergie-ue-watt-moniteur.jpgThey cost about AU$20.If so, then compare the power consumption of the Imac with inverter connected and disconnected.

author: james_s
author_rank: Super Contributor
timestamp: December 28, 2020, 07:17:04 pm
content: Apologies if you already mentioned this but is the input voltage correct? If the input voltage is wrong (or has a ton of ripple due to a bad capacitor somewhere) that could cause the inverter to overheat. The only other thing I can think of is a short from the LEDs to the chassis somewhere, or the inverter is being instructed to drive the LEDs harder than it should be. Does the display look unusually bright? Many LCD displays pulse the backlight in sync with the frame clock of the panel. I suppose it's possible that it's stuck on all the time when it's intended to be pulsed. Or maybe there is a common fault with the inverter and the used one you bought happens to have the same fault as the one you had. The thing that would really help here is if you were able to get your hands on another similar model computer to compare to and try your inverters in that known good machine.

author: aqibi2000
author_rank: Regular Contributor
timestamp: January 13, 2021, 11:45:21 pm
content: Fixed this now?

author: reessi
author_rank: Contributor
timestamp: February 06, 2021, 07:38:17 pm
content: No but I've got a new set of LEDS from china. Hopefully have time to test this week and then I'll update the results. I didn't see your message because this forum said I cannot use my normal hotmail email account.Cheers

author: reessi
author_rank: Contributor
timestamp: April 11, 2022, 03:00:06 pm
content: I forgot to update, this was fixed by replacing one of the led rows! After that the diode did not blow in the inverter!Cheers