Re: motor current sense question

>>Ok. You could bypass the high-side FET, and connect that side of the
>>motor directly to the power rail.

This is not possible because I need to maintain the full-bridge

>>Maybe you already have it, but remember that there should be a path
>>for the inductor current to continue existing right after the FET
>>turns off. The current through an inductor has a high "inertia." This
>>task is usually done by one diode in anti-parallel with the high-side
>>FET that you are not using.

All of the MOSFETs (IRFS4310) have reverse biased diodes built into
them. In addition to this I have also incorporated transient voltage
suppressors across both the low and high side FETs to further divert
back emf voltage transients from reverse biasing the MOSFET.

>I am measuring current through the load with a current probe. Also at
>the source.

>>And what are the numbers for each of them?

OK, here are the numbers:

V-motor(rms) ~= 9.36 [volts] (measured with dmm)
I-motor(rms) ~= 36 [amps] (measured with scope current probe)
I-motor(peak-peak) ~= 12.5 [amps} (measured with scope current probe)
R-motor ~= 0.27 [ohms] (measured with dmm)

V-source(rms) ~= 48.0 [volts] (measured with dmm)
I-source(rms) ~= 9.4 [amps] (measured with scope current probe)
I-source(peak-peak) ~= 4.0 [amps] (measured with scope current probe)

And just to make sure we're on the same page:

I have a full-bridge MOSFET, but I am only driving the motor in one
direction or using half of the bridge. The high-side FET (top left of
bridge) is always ON. The low-side FET is being PWM'd such that the
the V-motor(rms) is as mentioned above. A MOSFET driver is being used.

Thanks for your time, any suggestions / insight is appreciated.

Mike

.

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