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# Reverse Engineering a Server CPU Voltage Regulator Module

## Power on

I rigged up my bench power supply to the board using crocodile clips to clamp to the power and ground connections. Wads of paper are used on the ‘other’ side of the crocodile clips to prevent unwanted short circuits.

The Intersil IC claims to operate at 5V and 12V, both of which are present on a computer motherboard so I dialled in the lower 5V value and powered up. I probed the power output to see what was there. Nothing. OK, time to try 12V. This time I got a much better result. 3.3V and change was present on the output pins.

That’s a really useful output voltage for hobby use. I had a quick probe around the pins of the Intersil IC with an oscilloscope to see if I could locate a waveform that would give away the switching frequency. There it was on pin 13, the PHASE pin.

The frequency of that PWM waveform is 296.3kHz with an amplitude of 12V.

What I need to do now is set up a test rig to see how stable the output is under load, and the best way to achieve that is to use a piece of test equipment called a DC Electronic Load.

These devices apply a load to a circuit under test. You’ll get some combination of constant current, voltage and power modes depending on how much you want to spend. The maximum wattage is also a price differentiator. I got this Itech IT8511+ on Ali Express. Itech are the OEM for BK precision loads so you know that this is a quality part.

I’ll test this circuit in constant current mode. That means I dial in a current on the load and the device will sink up to that current while burning it off as heat. My bench power supply is limited to 3A output but if you recall your basic physics you’ll remember that power is conserved throughout the system so if I put in 1A at 12V and get 12W then a perfect power supply would deliver 12W / 3.3V = 3.6A at the output.

In reality the power supply is far from perfect and some of those 12 watts will be lost. Typical efficiencies for switching power supplies range from 70 to over 90% depending on a number of variables including the load current.

I ran the test and applied a gradually increasing load up to 5A. I stopped there, not because it was getting hot – it wasn’t, the heatsink was just warm but because I know the MOSFET driver is rated at a maximum of 6A and there may be other, lower rated components that I don’t know about.

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