Quote:
Originally Posted by rich2202
I think you need more than 1.5A as your source. One output port alone on the VRM is rated for 12V/2A peak, and 12V/1.5A continuous.
Part of your power-up problem may be not having enough input current.
Given the minimum wiring size of 24 AWG, they are expecting peak draw of 3.5 amps.
Assuming maximum continuous power draw on all ports, that would be 0.5 and 1.5 amps at 12 volts, and 0.5 amps and 1.5 amps at 5 volts. That means a minimum draw of 2.83 amps at 12 volts continuous, and peak draw minimum of 3.55 amps at 12 volts. Assuming 80% efficiency, that would be 3.5 amps input to deliver the maximum rated continuous power draw.
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That's great and all, but I'm only testing the 5V / 2A supply rail. If we do a simple power calculation (P = V * I) we find that for this specific power rail, the maximum power consumption is only 10W. Now, if we calculate the maximum theoretical input power, the result (assuming a 12V input voltage and a maximum supply current of 1.5A) is 18W. This means that if I were to fully load the output of the 5V /2A rail on the VRM, it would consume (I = P / V) 0.83A on the input supply. Now, if we take into account 80% efficiency (which is terrible for a switching regulator), that puts the total current draw from the source supply at just about 1A. 1A < 1.5A, so we're good!
If I wanted to test maximum voltage regulation on all channels simultaneously, then yes, you are correct, I would require a much larger supply. In this case, I'm not testing that. I'm only testing the 5V / 2A output, and a 1.5A supply is more than adequate!
Quote:
Originally Posted by Alan Anderson
That would definitely be an issue with a switching power supply, but the Keithleys with which I am familiar are linear supplies.
It's certainly possible that you have a faulty VRM. Out of several thousand in the hands of robotics teams at the moment, I would not be surprised if one doesn't work properly.
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Agreed Alan, this is a lab-grade Keithley. I took some more data on another VRM and produced similar results. These measurements are starting to worry me. I'll post pictures of my setup tomorrow, but I can assure you that I've performed supply characterization tests like this before. 2 failures in ~3000 modules is terrible QC!