Measuring average current accurately? (500KHz triangle waveform)

[grizedale]

Hello,

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


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, its 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:-
**broken link removed**

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

 

[emontllo]

Hello,

First of all, if you need precission, you should use a Kelvin current sense resistor instead (like Ohminte LVK25R002FER or Vishay Y14880R00200B9R). Note that the soldered unions may ad some resistance.

In the other hand, I really don't think the digital meter you are using will measure the signal precisely. Note that its AC bandwidth is 300 kHz... you may find aliasing or other AD conversion problems when measuring the triangular shaped current at a frequency higher than the meter bandwidth (Page 208 of the datasheet Crest Factor Errors (non-sinusoidal inputs)).

In addition, the signal size will be very small and noise immunity poor; even using twisted pair, the signal measured outside the chamber would have lots of noise... According to your site requirements, you may need some precision amplification as close as possible to the sense resistor. With this analog signal you could take it out of the chamber (as a differential signal) and measure it using a more specific device, maybe a properly calibrated scope? Another option would be analog filtering the signal with a low pass and assume that the filtered signal is equal to the average.

Ernest

 

[FvM]

I don't understand about problems with this measurement.

The average of the triangle current waveform does exactly represent the DC output current as long as your measuring system is linear up to the low pass filter. In case of doubt, an additional RC filter before the instrument seems reasonable.

You may want to calibrate the measurement by feeding a true DC current through the sense resistor. Temperature coefficients have to be observed of course.

 

[mtwieg]

It should work fine, even if the bandwidth of the meter is lower than the switching frequency. The only thing I would be concerned about it the sense resistor itself. I'd recommend putting it in series with the output inductor, on the output side. Also you should get a current sense resistor with built in kelvin sense terminals. Simply soldering to the top of the terminals of a normal SMD resistor will probably introduce some error (but you never stated what kind of accuracy you need).

 

[grizedale]

thanks, the accuracy needed is to the nearest 100mA