I am designing a three phase inverter (SPWM) for Motor drive. As part of the protection circuit, I would like to use 3 shunt resistors (one in every phase), which will measure the AC current. The output voltage of the current sensor will go to a comparator, which in case of over-current will send a high output signal.
The voltage of my DC source is 48Volts, that means that each phase will take ±2*Vdc/3=±32Volts, but just to be sure lets assume ±48Volts! The maximum current, at which I want the comparator to send the high output signal is at 15Amps. The switching frequency of the Mosfets will be at 5Khz and the frequency of my current will be very low at 40Hz...
Note that accuracy is of high importance. I don’t want because of some noise to have a faulty output signal, because that signal will be used to disable the whole circuit. Adding to that, I would like the shunt resistor (or generally the resistance which will be added to the line) to be as small as possible (if possible to be less than 3mOhms). Finally the signal circuit is galvanically isolated from the power circuit. So if possible, it will be better to find an isolated sensing solution.
Thank you very much, in advance.
Sorry for getting back late. I've talked to several applications engineers and they recommend the AD7400 and AD7401 for your application. These products are isolated sigma delta converters designed for isolating current sense in motor control applications. They accept signals within +/-250 mV range with 16 bit accuracy. The output from both parts is a 1 bit data stream that needs to be reconstructed to create a 16 bit result. The typical method to do this is with a sinc3 digital filter but if the processing power is not available the AD7400/AD7400A can be converted to an analog signal with external amplifiers. You may also want to see this reference circuit: http://www.analog.com/en/circuits-from-the-lab/CN0185/vc.html
Also if you have any other concerns about the AD7400/AD7401, I recommend to post it on Precision ADC Community.