Testing VRMs

[bushytails]

Re: Testing VRMs

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

--Randy

[sulbert]

Re: Testing VRMs

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

[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.

[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?

[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]

thanks AKOR you are most welcome to write FAQs

[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.