Testing VRMs

[AK0R]

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.

[willawake]

thanks AKOR you are most welcome to write FAQs

[willawake]

Re: Testing VRMs

Here is an explanation of what VRMs are and do which was contributed by AKOR on another thread.

VRMs (voltage regulator modules) are a specific class of MOSFETs; MOSFETS are a specific type of FETs. FETs have several advantages over bipolar transistors. For a basic description on the workings of a voltage regulator, search on "7805 data" on Google or Yahoo (7805 is a basic 5-volt regulator).

VRMs take one voltage (such as +5 VDC) and produce a different, lower voltage that is needed by various components (microprocessor, memory, etc.). Each VRM can only produce one output at a time, so two different VRMs are needed if the CPU and memory operate at different voltages. The output of the VRM can be varied slightly, depending on how external components are connected and controlled, so some motherboards are able to have settable voltages for memory. CPU voltage is usually, but not always, taken care of automatically. True variable power supplies, where the voltage is variable over a large range, aren't used in PCs. The middle leg is usually cut off, because it is connected to the case, which is soldered to the board. The case is often, but not always, connected to ground. To test one, you need to know what the input and output should be, then measure them with respect to a known ground. If the VRM produces no or low output with proper input, either it's bad, or an external controlling component is bad. If the input is bad, look for upstream damage. Usually, it's best to test a VRM in-circuit (with CPU, memory, etc. removed!) so that the output can be checked at the immediate output of the VRM, as well as at the point of connection of the powered device.

[willawake]

Re: Testing VRMs

can someone please round up this faq with an explanation of which is the input and output of the vrm.

i think the input is the source but is the output the drain or gate?

[kc8adu]

Re: Testing VRMs

you forgot the vrm chip.
it generates drive pulses for the gate of the mosfet.
the mosfet drives the inductor and the resulting spikes are rectified and filtered either with a syncronous rectifier fet controlled by the vrm chip or a diode.the chip gets feedback from the output to vary the duty cycle thus the output voltage.
the technical name is buck converter.

[sulbert]

Re: Testing VRMs

Converter?
In most datasheets I have seen they call it a buck controller.

[bushytails]

Re: Testing VRMs

buck converter is the entire circuit, buck controller is the ic that controls the converter.

--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.

[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

[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.

[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.

[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

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

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

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.

[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

[badman86]

Re: Testing VRMs

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

[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

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

[bgavin]

Re: Testing VRMs

See the tutorial in the Power Supply section.