Testing VRMs

[styxbound]

Re: Testing VRMs

Thanks, PyrOBEAST, I'll see if I can properly apply that to some boards.

[PCBONEZ]

Re: Testing VRMs

Sorry, I'm old and senile. Can anyone simplify for me how to find the inputs and outputs on vrms? There seem to be any number of fets around the cpu on a lot of boards, and I can't figure out where the in and out is on the circuit.

'In' will have +12v connected one one side of caps.
[Continuity check to power plug.]

'Out' will have Vcore on one side of caps.
[Continuity check to toroid(s) NOT connected to +12v.]

.

[styxbound]

Re: Testing VRMs

And thanks for that, PCBONEZ. Certainly should help me figure it out.

[alihomee]

Re: Testing VRMs

Procedure for testing Voltage Regulator Modules (VRMs):
(This assumes that any bad capacitor problems have already been fixed.)

1. Turn off power to the computer.

2. Remove CPU, memory, and all peripherals from motherboard. Disconnect power supply.

3. This step verifies that the board is not damaged, and the VRM is not shorted:
a. Measure resistance from power supply pin (+3.3, +5, +12, -5, -12) to input of VRM. Resistance should be zero (too low to measure).
b. Measure resistance from output of VRM to load supply pin. Resistance should be zero (too low to measure).
c. Measure resistance from input of VRM to ground (should be greater than 1 megohm, usually much greater).
d. Measure resistance from output of VRM to ground (should be greater than 1 megohm, usually much greater).

4. Install CPU, memory, and all peripherals to motherboard. Connect power supply.

5. Turn on power to the computer.

6. This step actually tests the VRM (all voltages taken with respect to circuit ground):
a. Measure voltage at the power supply side of the power connector (+3.3, +5, +12, -5, -12).
b. Measure voltage at the motherboard side of the power connector (+3.3, +5, +12, -5, -12). Anything less than the value obtained in step 6.a indicates a high resistance in the power supply connector.
c. Measure voltage at the input to the VRM (+3.3, +5, +12, -5, -12). Anything less than the value obtained in step 6.b indicates a high resistance in the PC board traces to the VRM, and is probably not repairable, even with a schematic.
d. Measure voltage at the output of the VRM. The value obtained depends on the functions being powered by the VRM, so knowledge of the expected value is required. Anything different than expected indicates a failed VRM (note that output may be higher or lower than expected if bad).
e. Measure voltage at the device being powered. This requires knowledge of the device (which pins are power pins). Pinouts are readily available on the Internet for 30-pin, 72/144 pin, and 168 pin memory, as well as ISA, PCI, AGP, USB, etc. Pinouts for CPUs are either on the Internet or may be obtained from the manufacturer. Anything less than the value obtained in step 6.d indicates a high resistance in the PC board traces from the VRM, and is probably not repairable, even with a schematic.

so where i found the input of vram

[ajsurkhi]

Re: Testing VRMs

i need complete startup process of desktop motherboard
please help

[ajsurkhi]

Re: Testing VRMs

I have ASUS M2N68 PLUS Motherboard. When i connect power supply cpu fan start for second and then stop. After pressing power switch nothing happen. Power switch have 0.4v only, vrm section has not getting any power. some one says SIO chip is faulty.
any body have an idea.?

[momaka]

Re: Testing VRMs

i need complete startup process of desktop motherboard
please help

Check all power rails first and make sure they are what they should be. Usually it's at least the following:
CPU V_core (a.k.a. CPU Vcc)
NB Vcc (typically less than 1.5V)
RAM Vdd/Vddq (2.5V-2.6V for DDR, 1.8V-2.1V for DDR2, 1.35V-1.5V for DDR3)
RAM Vtt (half of RAM Vdd/Vddq)
SB Vcc (1V-3.3V)
and possibly a few intermediate rails.

Boards with an AGP 2.0 or 3.0 slot will also have a separate rail that is typically either 1.5V or 0.8V respectively.

[bianchi77]

Re: Testing VRMs

c. Measure resistance from input of VRM to ground (should be greater than 1 megohm, usually much greater).
d. Measure resistance from output of VRM to ground (should be greater than 1 megohm, usually much greater).

I measured it very low at 9 ohm....is it because of filter capacitor in parallel ?
buck controller = NCP5322
Please have a look at the schematic, typical 5322...at the input and output ...there are filter capacitors..
What happen if NCP5322 is bad ? the output voltage is not right ?

[budm]

Re: Testing VRMs

9 Ohms at the input or at the output?
The Output you will have lots of circuit connected in parallel: loads to GND, filter caps to GND, the Power MOSFETs to GND, so it is the matter isolating the circuit to narrow down where the low Ohms reading is coming from.

[bianchi77]

Re: Testing VRMs

9 ohms at the output....how much resistance do I expect normally in average?
CPU in socket....

[budm]

Re: Testing VRMs

It should be very high in the K Ohms range because the devices (Loads) are not active.

[bianchi77]

Re: Testing VRMs

that's good news at least something wrong on the output....which one do you reckon ?
Could it be one of the output filter caps short ? but they look ok ( no bulge )...I tried replacing with new one last time...still the same...
Is it possible that one of the ceramic caps near the CPU is short ? ( on the bottom of the CPU, inside the socket )
without CPU in socket, 34.6 ohm...
I'll retry now and see what happen and post it....

[bianchi77]

Re: Testing VRMs

I have removed all the output capacitors now....I got only 8 ohms between Vcore and GND...CPU in socket...I also have removed all the FETs and the resistance is 7.5 ohm....what could it be the faulty now ?????
I have removed the inductors as well so the CPU now isolated by itself....
CPU on socket....resistance = 8 ohm....so it will be some somewhere on the ceramic CPU caps ?
Is it possible that those caps are faulty ??????

[bianchi77]

Re: Testing VRMs

When I tried to measure one of the chip capacitor with my LCR45....I got value >20000uF
which is not right, mostly those capacitor is about 100nF to 500nF....
Is there any possibilities that they are the faulty ones ??

[bianchi77]

Re: Testing VRMs

one of these ? Multilayer Chip Ceramic Capacitor
http://www.ecvv.com/product/2803276.html

It's 10uF...but I got >20000uF on my meter.......another bad capacitors ?

[bianchi77]

Re: Testing VRMs

Photos :

[bianchi77]

Re: Testing VRMs

I've found two missmatch value with the code written on it..05A = 110 ohm...measured in circuit 66 ohm...
09A = 121 ohm....measured in circuit 67 ohm....and they are not in parallel...

Could it be the problem ?
Or this one :

[budm]

Re: Testing VRMs

I have removed all the output capacitors now....I got only 8 ohms between Vcore and GND...CPU in socket...I also have removed all the FETs and the resistance is 7.5 ohm....what could it be the faulty now ?????
I have removed the inductors as well so the CPU now isolated by itself....
CPU on socket....resistance = 8 ohm....so it will be some somewhere on the ceramic CPU caps ?
Is it possible that those caps are faulty ??????

Something does not make sense here, you had 9 Ohms before the output caps and MOSEFTs are removed, and now you have 7.7 Ohms, and 8 ohms?
Is Vcore connected to that power supply as shown in the diagram that you are getting the 9 Ohms reading?
Very confusing.

[budm]

Re: Testing VRMs

one of these ? Multilayer Chip Ceramic Capacitor
http://www.ecvv.com/product/2803276.html

It's 10uF...but I got >20000uF on my meter.......another bad capacitors ?

What resistance do you get on that 10uF cap?

[bianchi77]

Re: Testing VRMs

I haven't measured the resistance of that 10uF...I'll let you know...some of them are 5uF
One test that I can do...is, unsolder all the capacitors about 16 small smd capacitors and test the resistance again....