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Original question:

TMS320F2800157-Q1: What is the most effective method for reading the output of three digital Hall sensors in a BLDC motor...

TMS320F28335: hall sensor signal from the field

rajesh beesetty
Expert 1145 points
Part Number: TMS320F28335
Other Parts Discussed in Thread: AMC3330

Dear Experts,

I am working on design of a customized TMS320F28335 based PCB, in which the voltages and currents of the inverter are to be measured, which are in the order of 250A rms, 220V. Hall sensors are placed on the lines, which are off the board. Since the hall sensors are inherently isolated, would you recommend to have another isolation on the board, say AMC3330, which isolates the signal. My worry is, if the isolation is not provided, the micro-controller ground would extend to the inverter stack physically, where 220V switching takes place. Furthermore, the currents are quite high, around 250A, So, the EMI might effect the micro-controller performance.

Please find the current isolation section of the complete schematic attached. I can share the complete schematic in the chat.

My question is, would you recommend to have a double isolation if the hall sensors are placed away from the micrcontroller PCB, or I am just overthinking about EMI-EMC?

Inductor_current_AMC3330.pdf

Regards,

Rajesh BN.

  • 0
    TI__Expert 5510 points

    Hi Rajesh, 

    As per my understanding, you have your sensors & MCU on the baseboard and gatedriver & switches on the daughter card (Plug in type). In this case, if your MCU circuit is behind the isolation with proper groudning then you should be good with single isolation. 

    Double isolation is not needed in my opinion, As the conductive EMI are taken care with single isolation. Where as the radiated EMI's will always be there due to GND planes arrangement on PCBs etc...

    Just to double confirm I will loop with AMC experts if they have any other feedback as well. Let me know if you have any other concerns related to MCU.

    Thanks & Regards,
    Uttam Reddy Pailla

  • 0 in reply to Uttam Reddy Pailla
    Expert 1145 points

    The Hall sensors(image attached) are not fixed on the micro-controller board, but away from the board. It will be fixed to chassis, and the conductors run throught the hall sensor. The +/-15V are extended to the hall sensor, so it gives +/-4V to the micro-controller board, where the signal conditioning takes place. My doubt is, is this +/-4V is to be isolated as these hall sensors are away from the board, so the micro-controller ground would extend to the hall sensors, where they are situated away from PCB,say 60cm approx.

    As per your explanation, I understood that the isolation from the micro-controller gnd is not required, as the hall sensor is inherently isolated.

    But, would this argument still valid if the hall sensor is placed in a noisy zone or too away from the micro-controller board?

  • 0 in reply to rajesh beesetty
    TI__Guru**** 174851 points

    Hi Rajesh,

    I see you have the +/-4V from the LEM sensors divided down to the +/-1V input range of the AMC3330.  Schematically I don't see any issues - you have the four AMC3330 HGND pins tied together which isn't really necessary, but in this case the four input signals are already isolated so that does not really matter.  Is your +/-15V supply to the LEM sensors referenced to the HGND side of the AMC3330?

  • 0 in reply to Tom Hendrick
    Expert 1145 points

    I have accidentally closed the thread by touching the 'resolved option'. Pleas note that my doubts are not yet cleared.

    Thank you for the reply! The HGND's are connected through 0R resistor, which will not be populated on the PCB. I'll send the complete schematic in the chat box.

    The +/- 15V is isolated as per the present status, but the basic question is whether to isolate it or not!

    Restating my doubt, is it required to isolate +/-4V LEM signal again on the PCB, or can we keep the LEM power supply- +/-15V ground to be connected to the micro-controller gnd? In other words, can we extend micro-controller to the hall sensors that are in the field. Kindly note that this field is near the inverter stack, where 220V voltage switching takes place. The hall sensor, however isolated, would u recommend to isolate it again on the PCB ?

    Regards,

    Rajesh BN.

  • 0 in reply to Tom Hendrick
    Expert 1145 points

    Is your +/-15V supply to the LEM sensors referenced to the HGND side of the AMC3330? : 

    If u mean, "is the +/-15V gnd is tied to the HGNDs": then, yes.. the HGNDs are connected to +/-15V gnd

  • +1 in reply to rajesh beesetty
    TI__Guru**** 174851 points

    OK, thank you for letting me know.  Personally I don't think you need the second layer of isolation, but I also don't know anything about your final system requirements are.  If the creepage/clearance needs are satisfied by the LEM sensor, you don't necessarily need the AMC devices.