Testing VRMs

[alexanderjohn]

Re: Testing VRMs

yes! especially laptop motherboards

[sam67]

Re: Testing VRMs

so anyone can provide pictures .... it would help us a lot

Would also like some pics if anyone has any or links ..This is really interesting and should be further explored ..

[closetonanak]

Re: Testing VRMs

First You Should Know Where Is Vrm Section On Board And How It Works, Which One Is Positive Line And Negative Line Then Mosfets Of P And N Lines, Check The Voltages, If No Voltages Means May B Mfets Are Open Thn Take Them Out From There Place And Check Also Check On Pads If Still There Shorting Means May Be Another Mosd\fet Has To Take Out From Board And Chk It Like That Go Ahead If U C That Capacitor Is Leak So Fst U Hav To Repalce, And In Ths Case Pwm Ic Of Both Line ( +, _ ) Have To Replace And Chk, Buck Control Ic( For Power Good Signal) , Io All Thing Should Be In Lone To Replace

[coolday]

Re: Testing VRMs

so anyone can provide pictures .... it would help us a lot

[coolday]

Re: Testing VRMs

Admins and moderators please try to start a section on motherboard repairing. it would be very helpful for newbies like us. Maybe this would be the first website to do so.

[badman86]

Re: Testing VRMs

Thank you for your answers, I think i got it now!

Now, what is not clear yet is this:

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).

Power supply pin means the ATX connector on the motherboard?

[andlcs]

Re: Testing VRMs

Hi!
It's an MSI K9A. I'd like to measure the resistance from the input and output of VRM to ground.

I think you can measure the input at the toroid coil near the ATX+12V conector.
And the output at the shielded inductor near the CPU socket. I'm not sure tho...

power off your computer first

[bgavin]

Re: Testing VRMs

See the tutorial in the Power Supply section.

[badman86]

Re: Testing VRMs

Hi!
It's an MSI K9A. I'd like to measure the resistance from the input and output of VRM to ground.

[PCBONEZ]

Re: Testing VRMs

What kind of board? - They aren't all the same.
VRM in may be 5v or 12v.
VRM out is determined by jumpers, BIOS settings, or an IC controller chip based on what the CPU needs for Vcore.

[badman86]

Re: Testing VRMs

Could someone tell me what is the input and the output of the VRM? Thanks!

[starfury1]

Re: Testing VRMs

Just to add to AKOR's post on the VRM testing
(and yeah bump it a bit)

There is a guy Sean Kelly that goes into to the design of a VRM circuit
he has his work up as doc files but here is the main link page
Sean Kelly

These below are some what old but still fundamentally applicable

here is a link to PDF on VRM chip

Analog AD3188 pdf link Here

This above goes through the functions so is worth the read
(Thanks to Bob P )

Anothe link on the AMD Hammer VRM Eval kit ISL 6559 here

not overly as useful as the first but adds a bit I suppose.

Here a page on Designing high-current, VRM-compliant CPU power supplies Here

not a whole lot there but does raise some of the design aspects.

There are pages and pages out there but not all that much on how they basically work for the beginners....

I would search out "buck converters"

Also note the use of Driver chips for the MOSFET'S so maybe do a bit of searching on those.
here is the ADP3418 link here


I would if you want to know more of how the VRM is constructed on the Motherboard you have works

Try to identify the controller chip and mosfet driver chips (mosfets if you like too)
Then see if you can find PDF's and application note on them.

You may wish to also get the Intel VRM Design data
Version 9 here but think its at 11 point something now.

Lastly I thought since it seems useful to have, I pdfed his test procedure and spread it out a bit to make it a little easier to follow so hope AKOR and TC dont mind...credit to AKOR and Reference (link) to this site given, Thanks

Akor's VRM Test Procedure PDF

Hope this Helps

Cheers All

[arneson]

Re: Testing VRMs

I have looked at these but I think I'm only using 10% of my brain cells.
You did see where I made these things fly, literally.
When the PSU fails I notice it's either, old, overtaxed, dirty,
or all of the above.
So its always better to replace with a clean, beafy, new one.

[PeteS in CA]

Re: Testing VRMs

Actually, O/P voltage overshoot can also be controlled or not by the compensation components in the error amplifier circuitry.

If you want to get a better picture of how pulse-width modulation is accomplished - better than my short paragraph - I'd suggest checking out the datasheet for an SG3524 or a TL494, which have block diagrams as well as verbiage.

[arneson]

Re: Testing VRMs

Man I wish I could say all that,
I did mean the soft start ramp to keep the overshooting down.

[PeteS in CA]

Re: Testing VRMs

With voltage mode control, e.g. 3524 or TL494, you only have voltage feedback. It doesn't respond as well to load transients, though the load transient spec for an ATX P/S isn't very challenging. With current mode control, e.g. UC3842 (3843, 3844, 3845) you have voltage feedback and a sample of the inductor current is also fedback. In voltage mode control, the output of its error amplifier (which compares the output voltage sample to an internal reference voltage) is compared to a sawtooth ramp. The start of the ramp is also the start of the switch (chopper) on-time; when the ramp voltage rises above the error amplifier output voltage, the switch is turned off, and remains off until the ramp voltage is reset to "zero". Current mode control is similar, except the sawtooth voltage is a sample (scaled by the output transformer in AC-DC P/Ss) of the inductor current. This lets the P/S resond much more quickly to load transients and O/P short circuits.

I'm not sure whether "ramping up" refers to the sawtooth ramp or to the soft-start function at turn-on. Soft-start prevents large currents through the switch during the turn-on time, while the I/P voltage is still increasing. Basically, soft-start limits the switch on-time and current stress, so that the O/P voltage comes up more slowly. Another feature that is implemented in the 3842 family PWM ICs is a lock-out that keeps the device from turning on until its Vcc is high enough to maintain control, during turn-on, turn off, or a I/P voltage drop-out. I don't think the 3524 has this feature, and I'm not sure about the TL494 (they're almost 30-year-old designs).

[arneson]

Re: Testing VRMs

I've learned that the Chopper (i like that word), has to ramp up, and the reference is actually two feedback loops.
The outer loop, for voltage, and the inner loop for current.

My problem with smaller powersupplies has always been finding bad Zeners, little ones.

[arneson]

Re: Testing VRMs

This is one of the most interesting topics for me.
I really need to learn more, right about the stage where the "chip gets feedback to vary the duty cycle".
I recently read where the new MOSfet's have thicker or more forgiving junction that is way less static sensetive.

[bushytails]

Re: Testing VRMs

Afraid I don't know nearly enough about buck converters to write a faq on them... most I can give is a general overview, of the mosfet switching current through an inductor...

--Randy

[willawake]

Re: Testing VRMs

well the information is slowly coming in small pieces. i think we will get to learn motherboard power circuitry eventually.

actually i am really interested in this subject and i know a few others on this forum are also. i have tried reading some stuff on the net but getting bored quickly.

cmon bushy, do one small faq.