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> Current Measurement Is Accurate?, Average of 500KHz triangle wave?
treez
Posted: February 16, 2012 11:03 pm
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Hello,

Please can you tell if our method of measuring a 500KHz triangular current waveform is accurate?

We are just measuring the voltage across a series sense resistor.


This is the shape of current waveform we are trying to measure the average value of:-
http://i39.tinypic.com/2vwsht1.jpg

So we are trying to measure the average inductor current in each inductor of a dual buck converter.
(we need to do this to assess what actual maximum current the load (FPGA) is actually drawing)



We are measuring the current by using a 2mR series resistor. (its a 1% surface mount sense resistor)
-we measure the voltage across the sense resistor with a Hewlett Packard HP34401A digital meter.
(obviously wires (twisted pair) are taken from the sense resistor terminals to the meter)

HP34401A Digital meter User Guide:-
http://www.ee.buffalo.edu/courses/elab/hp34401.pdf


-the load current, we know is around 15 to 18A , but we need to know it exactly.
-anyway, each inductor's current is obviously a triangle current waveform with a few amps peak to peak and 7 to 9A average.


The buck is actually a dual-buck converter supplying an FPGA

Vin = 12V
Vout = 0.9V
Each FET switches at 500KHz (there’s interleaving)
Inductors are 470nH
Iout(max) = What we are measuring here!!!!

So please do you know if our sense resistor voltage being read by the HP34401A is going to give us an accurate way of measuring the current?


NOTE:
Before taking the measurements, we got the software engineer to code the FPGA with code that will make it draw the maximum current.
The dual buck and the FPGA are actually in the environmental chamber at 80degC when we measure the current.
(obviously we cannot put a current probe into the chamber at 80degrees C……even if we could, we’d still have to break the pcb track and bring a wire through the current probe’s “jaws” which would not be advisable at this high frequency)

HP34401A Digital meter datasheet:-
http://www.metrictest.com/catalog/pdfs/pro...s/hp_34401a.pdf

HP34401A Digital meter User Guide:-
http://www.ee.buffalo.edu/courses/elab/hp34401.pdf
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johansen
Posted: February 17, 2012 01:13 am
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you're going to have to add a resistor and capacitor across each inductor.

size each accordingly (ceramic capacitor, maybe 10uF? and a 100K ohm resistor.

your multimeter should have at least 10M ohm imput impedance, and you need to put the capacitor on the output (dc side) of the buck regulator.
the voltage across the capacitor will be the dc voltage drop across the inductor.
if the ripple is under 10% then you can just assume that the result should be within 2% of the actual value.

of course, you're going to have to know the resistance of the inductor.
however, you can determine that by pulling a known current from the buck regulator.

you could use one capacitor and two resistors, however you won't know if they are sharing current equally.


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treez
Posted: February 21, 2012 11:25 am
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Hi

Sorry but my rolling contract has been terminated due to the fact that I presented this method of current measurement as being accurate

http://cp.literature.agilent.com/litweb/pdf/5968-0162EN.pdf

…page 2 of the above clearly shows that HP34401A cannot be deemed accurate below 100mV.

-My readings were around 8mV so the accuracy would have been pitiful.

The product was for a very expensive robotic product and they said I nearly jeopardized the whole project by presenting an inaccurate current measurement.

When I asked them why my contract had not been renewed they pointed out that I had not been aware about how ridiculously inaccurate my current measuerement technique (first thread above) would have been.
-they virtually laughed me out of the door…..particularly since they found out I have a first class Bachelors degree.
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telomere
Posted: February 21, 2012 04:18 pm
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I am sorry to hear that. Take the opportunity for some self-analysis, learn where your weaknesses are, and redouble your efforts in those areas. ten years from now, you may find this to have been a pivotal point in improving your career.


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