MOSFET or Capacitor ? which one creates the short ???

[hgg]

Hi guys,

I am trying to repair a used laptop I've just bought with a motherboard that refuses to power on.
I've installed a new mSATA SSD, installed Windows normally and after a few reboots and a power off
the laptop never turned on again. 

No burnt fuses on the motherboard, power comes in normally and everything I've checked turned out
to be working fine except the following.  Two of the 5V MOSFETS driven from the peak current mode
power controller look like they have a short.

Below is the circuit that has the short:



MOSFETs Q89 & Q90 have a direct short between drain and source.   I have removed all power and CMOS
battery, have discharged the gates and the short is still there.

Capacitors C601 & RFC30 are shorted as well.




My question is which component do you think its more likely to create the short?
One of the capacitors or one of the MOSFETs?   
I don't want to start removing components randomly, especially these tiny capacitors.
Is there any other test I can perform to isolate the faulty component?

Thanks!
p.s. The full schematic of the power controller:  https://postimg.org/image/t3nv3wycj/

[capt bullshot]

One way to find the shorted component could be:

Inject a few amperes from a lab supply into the shorted node and watch for rising temperature, voltage drop on the PCB traces, ...
The easier way would be to desolder the components and check them one by one.

[danadak]

I would unsolder L to isolate upstream side of converter. With your MOSFETs removed,
their gates tied to source, is Rds very high ? If so they are probably OK, asasuming
they are enhancement mode devices.


Regards, Dana.

[hgg]

So there is no other way to identify MOSFET vs Capacitor without desoldering these 'microbes'..?

captbullshot's idea sounds promising but it seems a bit dangerous if you do not know
exactly what you're doing.  Would 5V / 1Amp be enough across Q90 for example ?

danadak do you mean tre L26 inductor?

[xavier60]

If you have a multimeter that has microvolt resolution you can trace the current path to ground by measuring voltage drops across very low resistance current paths like through the inductor or even the ground plane. I use about 200ma and a cheap AN8008 multimeter.

[hgg]

Hi, yes I have one.
Can you elaborate further on the setup I should use to measure the voltage drop you say?

[xavier60]

You could feed some current in to the right side of L26. For a start, measure the voltage drop across L26 to see if the current is flowing through it or not.

[capt bullshot]

Connect "-" output of your supply to GND (one side of the suspected short)
Set the lab supply to a low voltage (near 0), and a limited current (maybe 200mA as xavier supposes, maybe you'll need more)
Connect "+" output to the faulty node, somewhere on the PCB trace leading to the suspect components (other side of the suspected short)

Carefully increase the power supply output voltage setting (stay below 1...2V to avoid damage to your board) and watch for the current rising, while voltage stays low. If the voltage rises and current is low, you've done something wrong, start over.

Now, using the uV DMM, try to figure out where the current goes to. Measure various voltage differences along the PCB traces to the suspected components, a "good" component will have near to none voltage drop along the trace, the shorted one will have a noticable voltage drop across the PCB traces.

Your PCB layout (from the photo) doesn't look well suited for this method, since the components have very short and wide traces, it'll be difficult to figure out the path.

Suitable points to contact your power supply might be: upper pad of the 2R2 resistor (for "+") and lower pad of the small capacitor below the 2R2 resistor ("-"), where your C601 arrow points to. Then you can measure the voltage across the traces from this point to e.g. the pads of the MOSFETs.

[hgg]

Ok, for starters I applied 30mA and 3V (maybe a bit high voltage?) to the right side of L26 and ground.
I got 0.12mV across L26.

Shall I continue as captbullshot suggested ?
Thanks.

[xavier60]

I took some measurements on 2 laptop inductors at 200ma. A 3.3uh 5v rail inductor dropped 2mv. A 0.36uh VCORE inductor dropped 200uv.
The voltage drop across a few mm of ground plane can easily be measured. I take most measurements on the ground plane to find where the current is returning to it. It is best to connect the power supply negative and DMM negative to different points on the ground plane.

[xavier60]

Ok, for starters I applied 30mA and 3V (maybe a bit high voltage?) to the right side of L26 and ground.
I got 0.12mV across L26.

Shall I continue as captbullshot suggested ?
Thanks.

30ma at 3v isn't really a short.

[hgg]

What do you mean its not really a short?

I used current limiting on my power supply...
Should I increase it to 2V 200mA ?

For a start, measure the voltage drop across L26 to see if the current is flowing through it or not"

What does the voltage drop across L26 has to do with which MOSFET or Capacitor is burnt?

[hgg]

Suitable points to contact your power supply might be: upper pad of the 2R2 resistor (for "+") and lower pad of the small capacitor below the 2R2 resistor ("-"), where your C601 arrow points to. Then you can measure the voltage across the traces from this point to e.g. the pads of the MOSFETs.

Its very difficult to apply voltage to these points.  They are extremely tiny and hard to reach.
For example the MOSFET is 3x3mm  But if I manage to solder some magnet wires across
2R2 and C601 which point pairs should I measure next?  Top of 2R2 and drain of Q90 for example?

Sorry for my many questions but I have not tried this method before.

[xavier60]

What do you mean its not really a short?

I used current limiting on my power supply...
Should I increase it to 2V 200mA ?

For a start, measure the voltage drop across L26 to see if the current is flowing through it or not"

What does the voltage drop across L26 has to do with which MOSFET or Capacitor is burnt?

To get an idea of what's happening, we need to know the simultaneous current and voltage readings. 
The voltage drop across the inductor indicates that the current is flowing through the inductor to something on the left side.

[hgg]

Yes, but the problem is on the right side.
How does this info differentiate a burnt capacitor vs a burnt MOSFET?

[xavier60]

Yes, but the problem is on the right side.
How does this info differentiate a burnt capacitor vs a burnt MOSFET?

To be certain of that, you can feed the current to the left side of L26, then measure the voltage across L26 again.

[capt bullshot]

Suitable points to contact your power supply might be: upper pad of the 2R2 resistor (for "+") and lower pad of the small capacitor below the 2R2 resistor ("-"), where your C601 arrow points to. Then you can measure the voltage across the traces from this point to e.g. the pads of the MOSFETs.

Its very difficult to apply voltage to these points.  They are extremely tiny and hard to reach.
For example the MOSFET is 3x3mm  But if I manage to solder some magnet wires across
2R2 and C601 which point pairs should I measure next?  Top of 2R2 and drain of Q90 for example?

Sorry for my many questions but I have not tried this method before.

OK, you got it somehow wrong. The intention is: solder the two wires to suitable points at your PCB. There might be better accessible points available than the ones I said, you'll have to find them. Leave them there, apply the current through them.
Then take your DMM, use small and pointy tips to measure the voltage along the PCB traces, starting from one of these points. E.g. point one tip to the top pad of the 2R2 resistor (where you soldered the wire), the other at the pad of the nearby MOSFET that is connected by the PCB trace to the starting point. Read and note the voltage (should be a few microvolts). Next: leave the first tip in place, move the second to the other MOSFET, read and compare the voltage to the first reading. Now make your decision where the current goes to (there where's the larger voltage drop). Follow more traces if necessary.
As xavier says, this procedure can also be done across the GND plane of the PCB, it depends ...
The GND plane (if exists) is often easily accessible through many points on the PCB, you'll have to find them.

[hgg]

@xavier60:   0.0030mV

[xavier60]

@xavier60:   0.0030mV

That indicates that the short is on the left side of L26.
 I still need to know what the power supply voltage and current readings are while your power supply is connected to L26.
Saying 3v and 30ma indicates 100 ohms, which isn't really a short.

[hgg]

Ah ok, I see what you mean.
The current is maxed at 30mA and the voltage is almost zero.

[hgg]

@captbullshot:  I am measuring across top of 2R2 & Q90 drain and top of 2R2 @ Q89 drain the
same voltage drop:  0.0030mV   I've applied 2V @ 30mA.

[xavier60]

Ah ok, I see what you mean.
The current is maxed at 30mA and the voltage is almost zero.

That is a short for sure. Increase the current to 200ma. you need to find the place on the ground plane that has the highest voltage across it. This will be at the place where the current is flowing through the shorted component to ground. Hopefully it is a faulty capacitor and not a large BGA IC which this method doesn't work very well with. Read the previous posts. 

[capt bullshot]

20mA is to low a current. You'll need more voltage drop along the traces. Try to increase up to 200mA, maybe 2A, but watch the voltage at the output of your supply. It is supposed to stay low due to the short.

Also follow other traces, especially to and through the inductor, as the short may also be at the other side of the inductor.

Measure the voltage across the 2R2 resistor, should be near zero, otherwise the capacitor in series with it failed.

[hgg]

It looks like this is beyond me...
I started measuring capacitors all across the board and they all display a short.
I am not sure how can you go debugging this.
Anyway, guys thank you for your help!!

[xavier60]

Sounds like it has more that one shorted rail, I wonder what happened to it?