Imac A1312 - Blown Diode twice in LED inverter Board!!

[reessi]

This is my first post in this forum so if there is anything wrong, let me know and hello!
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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.

[reessi]

I have annotated the pictures please have a look.

[reessi]

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[reessi]

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!

[Twoflower]

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.

[reessi]

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/R

Would 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!

[fzabkar]

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

[Twoflower]

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.

[reessi]

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?

[reessi]

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...

[fzabkar]

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

[james_s]

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.

[reessi]

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

[james_s]

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.

[reessi]

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??

[reessi]

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.

[fzabkar]

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 the forward direction across each diode. Does this happen when the inverter is loaded by the LED strings?

[reessi]

Without the LEDS it does not power up, it checks for them. So the 120v is with the LEDS! What does AISI mean please?

[fzabkar]

AISI = as I see it

Sorry, I'm a veteran of Usenet.


I expect that your circuit is using this kind of topology:

https://learnabout-electronics.org/PSU/psu32.php

If the output voltage is 42V, then I would have expected that the drain voltage would switch between ground and 42.6V.

[reessi]

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!

[fzabkar]

I'm confused, too.

[reessi]

 That makes 2 of us, hopefully someone will be along to enlighten us!

[fzabkar]

Did you measure the voltage drop across the current sense resistors?

[aqibi2000]

Testing a Switching circuit with zero load is never a good idea.

[reessi]

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.