Electrical – Measure current for Field Oriented Control

current measurement

I want to use an AD8418 (http://www.analog.com/media/en/technical-documentation/data-sheets/AD8418.pdf) together with the ADC of the uC to measure the current of a BLDC motor for two phases, that is used for field oriented control. As a driver I use a DRV8313 (http://www.ti.com/lit/ds/symlink/drv8313.pdf) which allows to connect the shunt resistor to the low-side mosfet. My question is now where I should put my shunt resistors to measure the currents:

1.) Directly to the phases of the BLDC motor. This is found in the application note of the AD8418.


2.) Put the shunt resistors to the low side mosfets.

When I put the shunts to the low side mosfets, I need to measure the current only when the low-side mosfet is active. For that, I need center-aligned PWM to trigger the ADC in the right moment. Is this also needed when I put the shunt resistors to the BLDC phases and if yes, when needs the current to be measured? Are there any advantages with method 1.)? The actual PWM signal will be around 25kHz, where the CMRR is about 80dB, that should be still enough.

Here is also a schematic of how I planned to do it with the method 1.)

enter image description here

Best Answer

Both methods have advantages and disadvantages. In method 1 you don't need any serious bandwidth, just about 100kHz is enough. Also you get isolated measurement, which is very good. Just remember you can also sample the third phase to detect short circuit. In method 2 you don't get good short circuit protection, your bandwidth should be at least around 2MHz, since current doesn't always appear on shunt resistors, you don't get isolation for free, but you can use cheap and simple components.

So in the end it's your call, depending on application. I used both methods and can't say any of them is always better than the other.