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Choosing op amps and shunts for three phase current measurement for InstaSPIN-FOC.

Kristoffer Stork
Prodigy 60 points

Hi,

In the past I've used hall effect current feedback but now I want to move to a solution with shunt + op-amp. I'm an EE, but not very experienced with these amplifiers.

I'm pressing the "buy now" button on the HC (60V40A kit) any time now, but at the same time I'm starting designing a powerstage for volume production. The HC DRV3201 kit uses three tlv2781 op-amps with three 2mOhm shunts. In addition precision resistors set the gain is needed; as referenced from page 139 of the InstaSPIN FOC/MOTION user guide.

I've decided to use 300uOhm shunts (something along the line of this: http://www.mouser.com/ds/2/427/wsl2726-88189.pdf) and want to find a IC that preferably has a internal reference and fixed gain, basically doing the job a normal Op-amps does - but without the external resistors. This page shows some current shunt amplifiers made for high common voltage ranges - not what I need; but a step in the right direction with fixed gain. Could anyone point me in the right direction?

The motor I'll be driving is a ebike hub motor with about 170uH phase-phase inductance and 100mOhm phase-phase resistance. I will run 20kHz PWM.

The first 50 boards are being hand assembled and the extra cost of a IC can be justified with less parts to populate to the board.

Sincerely, 

  • 0
    Guru 47900 points

    Acknowledge your desire to reduce parts count & assembly effort/cost.  That said - you do trade design flexibility & limit part selection - via that desire.

    Our group's focus is motor control, (BLDC & brushed), custom motors and/or flat-screen control design/integration.  And very often - we find the ability to adapt to an unanticipated design change or alternative motor (or spec) a great advantage.  Many times - clients appear with needs or devices - beyond our initial knowledge or expectations.  Thus - a design which can accommodate this variation saves the time/effort/cost of board, "respin."

    Suggest that this added design flexibility will most always serve you well - and prove superior to any early design which may be too restrictive...

  • 0 in reply to cb1
    TI__Guru** 118000 points

    for our new 24V/10A boards we are going to release soon, these will use

    http://www.ti.com/product/opa2374

    for the current sense amplification.

     

  • 0 in reply to ChrisClearman
    Guru 117850 points

    Indeed - looks like very nice, capable op amp.

    Especially like, " input range extends 200mV beyond the rails...output range is within 25mV of rails."  (such has past - bedeviled us.)

    Might you comment upon our sense that, "flexibility" in design may prove a safer choice than that based upon, "gain-resistor reduction/elimination."

  • 0 in reply to cb1_mobile
    TI__Guru** 118000 points

    certainly in your case flexibility is key, and more importantly is just the fact that you want to get as good of measurements into the processor as possible, especially if you want the solution to perform as well as possible across all operating conditions. 

  • 0 in reply to ChrisClearman
    Guru 117850 points

    Appears you/I are in close agreement.  My post (design flexibility) was simple response to op - (and who appears to have left the building...)

  • 0 in reply to cb1_mobile
    Prodigy 60 points

    Left the building, for a little while. I'm back and I'll design the current feedback as it is stated in the user guide.

    Thank you both for feedback on this topic.

  • 0 in reply to Kristoffer Stork
    Guru 117850 points

    You are aware that, "green tick" may be awarded to 2 responders...

    Present tick is out of proportion to effort expended in your behalf - and fails to reward those motivated only by your future success... 

  • 0 in reply to cb1_mobile
    Guru 117850 points

    Thank you Chris.  (appreciated)  We will be including the "rail to rail +" op-amp you referenced in our next design spin...