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Testing VRMs

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    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 to AKOR and Reference (link) to this site given, Thanks

    Akor's VRM Test Procedure PDF

    Hope this Helps

    Cheers All
    Attached Files
    You step into the Road, and if you don't keep your feet, there is no knowing where you may be swept off to." Bilbo Baggins ...


      Re: Testing VRMs

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


        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.
        Mann-Made Global Warming.
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          Re: Testing VRMs

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


            Re: Testing VRMs

            See the tutorial in the Power Supply section.


              Re: Testing VRMs

              Originally posted by badman86
              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


                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?


                  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.


                    Re: Testing VRMs

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


                      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


                        Re: Testing VRMs

                        Originally posted by coolday
                        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 ..


                          Re: Testing VRMs

                          yes! especially laptop motherboards


                            Re: Testing VRMs

                            Laptop motherboards ... hmm.... if I have the time ...

                            //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.//
                            Is repairable, by making a bridge with a wire. Usually from one capacitor to the next one. Never occurred to me unless it is a manufacturer design.

                            Basically, it is pretty simple. If mainboard does not POST, check voltages.
                            If PSU voltages are within range, check voltages on motherboard. Cpu has pretty much standard voltage. About 1.4V is fine for newer CPUs, 1.8V for athlons and P3's, 2V or greater for first celerons and P2 (Some early P3)
                            If there is no power (voltage) to the CPU, check if you placed capacitors the right way, observing the polarity. Same goes for other circuits, RAM, AGP etc. ..
                            If VRM IC is busted, it would be hard to find a new one. Sometimes (rarely) only one of the mosfets break, usually the upper one. When it does that, the lower one migh go 'poof' as well (protecting the CPU).
                            If one breaks, replace both. Check IC for damage, might be good to replace that one as well, sometimes it is the cause of this mess.

                            //VRMs (voltage regulator modules) are a specific class of MOSFETs; //
                            Far from being true. A VRM module consists of the controlling IC and MosFet's (if we focus only on semiconductors).
                            However, it is true that there are many type of FET's. Mosfet being the most common one. JFET is one of the other type.


                              Re: Testing VRMs

                              Source = Input
                              Drain = Ground
                              Gate = Output


                                Re: Testing VRMs

                                Source = Output
                                Drain = Input
                                Gate = Gate

                                Current flows from the Drain to the Source afaik. (When the gate is (ON))
                                Can also flow backwards once the gate is on, however, the body diode will conduct when the gate goes off, leading to rapid heating.