Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

[suzukiyota]

Due to a power surge my Asus board does not power on, when I connect 24 pin power connector to motherboard (without turning on the board), the LDO regulator (UZ2085G / PU300) gets hot immediately. Hot I mean insanely hot! which burns my finger.
I replaced the LDO regulator, but it didn't help. Probably something is using high ampere and causing the regulator become hot.
So which components should I check? What's the best way to diagnose the issue?

Thanks

[peste]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

Due to a power surge my Asus board does not power on, when I connect 24 pin power connector to motherboard (without turning on the board), the LDO regulator (UZ2085G / PU300) gets hot immediately. Hot I mean insanely hot! which burns my finger.
I replaced the LDO regulator, but it didn't help. Probably something is using high ampere and causing the regulator become hot.
So which components should I check? What's the best way to diagnose the issue?

Thanks

you probably have a short piece on the ground at the output of the regulator ...

[suzukiyota]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

I found one resistor, one pin goes to regulator output and the other goes to +3VSB ATX.
Boardview shows it should have 1.15K OHM. But multimeter reads 1.13K OHM. Is that a problem?

[dicky96]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

Nahh that's close enough. As Peste already advised you - check for a short circuit to ground form the output of the LDO.

[momaka]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

Was the computer connected to Ethernet when the surge occurred? If yes, I would suspect the LAN IC, as that's usually the one that gets toast from surges and lightning storms (and it also is the one that usually uses 3.3VSTB generated from the motherboard.)

Of course, do check if any other IC is getting hot before removing anything - that is, check the SuperIO/LPC, LAN IC, Audio IC, and any Firewire or extra SATA RAID controller IC.

[suzukiyota]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

I can see the whole 3VSB rail is short to ground. No, Ethernet was not connected when surge occurred. I checked LAN and audio chips, only 3VSB pin is short to ground. I guess the chip is ok?
How do I find which component is short? Should I use external 3V to burn the component?

[dicky96]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

Most common method would be to use a bench power supply to power the 3VSB.

You want a constant current power supply. Set the output to 3V, then short the leads of the power supply together and set the current for about 1 amp. Now connect it to 3VSB and wait a minute to see if anything gets warm. Don't worry if the PSU goes down to a very low voltage (like less that 0.5V)

If nothing is getting warm, increase the current limit (not the voltage ) on the bench PSU to about 2 amps and try again

Keep on increasing the current half an amp at a time until either something gets hot or you reach the maximum rating of your PSU.

If this method doesn't help (nothing seems to get hot) then we have to try another way - but for that we are going to need the boardview. I have attached it here. Are you familiar with using boardviewer?

[suzukiyota]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

Voltage injection makes PCH warm. So the PCH is shorted then.

[dicky96]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

Maybe, maybe not. PCH generally get warm anyway.

When you injected the 3VSB voltage, did you use a bench PSU? What voltage and current could you see was supplied by the PSU?

Do you have a good multimeter? You need something that can read 4.5 digits or better resolution, so it can read 0.01 ohms on the lowest resistance range. I have a Uni-T at the workshop I use for this sort of purpose, I can't remember which one, but it wasn't expensive. I'll check in the morning.

Take the heatsink off the PCH, turn the power off, remove the CMOS battery and check the resistance across all the little capacitors integrated on to the top of the BGA. One or more of these should be 3VSB. Do you see a short there? If so read the resistance, then read the resistance at other points on the 3VSB rail. If you see the lowest resistance at the PCH then either it, or a MLCC capacitor very close to it, are likely to be short.

If you can't see the difference with resistance range on your multimeter, then re-apply voltage from your bench PSU and try this other way.

Set your multimeter to voltage, again you need a decent multimeter. Now open the board view I posted and find a 3VSB LDO that gets hot. Now on the boardview click on the LDO 3VSB pin and it will go red.

Highlighted in yellow you will now see all the other points on the motherboard that are connected directly to 3VSB.

Now measure the voltage from 3VSB on the LDO that gets hot to a ground pin on the same LDO. I would assume this is where you are injecting voltage from your bench PSU. Make a note of the reading.

Now work your way around the board using the board view. For each component measure the voltage from 3VSB pin to ground pin on the same component. If the component does not have a ground pin, then obviously it can not short to ground, so skip that component, it isn't the problem.

Take your time and work methodically around the board. Whichever component reads the lowest voltage from 3VSB pin to ground pin on the same component, is the short circuit one.

[piernov]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

PCH die should not get warm when injecting on a 3.3V rail. No point in wasting time.

[dicky96]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

Then I stand corrected

[dicky96]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

Yes for sure you are correct Piernov

I didn't think it through properly, if the 3.3V rail is the standby, then it isn't going to draw very much current under normal conditions so the PCH should not get warm by powering that rail

[suzukiyota]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

When I inject 3V to the board, it is taking 4.7 amps from power supply. And power supply voltage drops to 2.3 volt.
The multimeter currently I have cannot read fine resolution. It can read 0.1 ohm.
I’m not injecting voltage to the LDO, because LDO is not in 3VSB power rail, and there’s no ground pin on LDO. From LDO 3VSB_ATX (3VSB_ATX is not shorted to ground, only 3VSB is) goes to MOSFET (PQ301 on board view) and that MOSFET is connected to 3VSB power rail, so I’m injecting voltage to 3VSB pin on the MOSFET.
So I guess my only option is to check voltage from each component. To detect the voltage difference, do I need a fine resolution multimeter?

Thanks

[dicky96]

Re: Asus Z87 Expert - No Power, LDO Regulator Gets Hot!

The more sensitive/higher resolution your meter the better, but using the voltage drop method this is less important- because with this method ohms law is working in your favour

Take the example of a cheap multimeter such as the AN870
https://www.aliexpress.com/item/1005...archweb201603_

The lowest resistance range is 199.99R and it can resolve to 0.01R
The lowest voltage range is 19.999mV and it can resolve to 00.001mV

On your PCB
The ‘short circuit’ resistance is 2.3V & 4.7A = 0.49R
Ohms per volt = 0.49R / 2.3V = 0.21R and meter reads to 0.01R
Resolution is 0.49R / 0.01R = 49


Voltage range can read to 0.000001V (00.001mV)
Resistance is 0.49R
‘short circuit’ Volts per ohm is 2.3V / 0.49R = 4.694 V
Resolution is 2.3V x 0.000001V = 2,300,000

So using the voltage drop method, your meter has about 50,000 times the resolution than it would have in the resistance method

Also three other factors are in your favour:

1. The more current you can get into the short circuit, the higher the voltage drop, therefore the higher resolution of your meter

2. Because your multimeter is drawing no current (or as near as damn it) the resistance of your meter probes is not relevant, and you don't need four wire probes as you would with a milli-ohm or better resistance meter

3. The only current path on which you will see decreasing voltages is the path towards the short circuit, circuit traces branching away from that path will show no voltage drop as there is no current passing through them. Using this knowledge it, you can often trace which direction the current is flowing by measuring the voltage at different points on a circuit trace. You can't easily detect the same using resistance mode as almost no current is flowing from your meter.