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Original question:

UCC28070: UCC28070 - PMP4311

UCC28070: Shunt Resistors

StnGgc
Prodigy 90 points
Part Number: UCC28070

Tool/software:

Hello,

I've started designing at 10KW version of PMP4311.  I prefer to use Shunt resistors over Current Sense Transformers and have been reviewing information on the e2e forum.  I've found a post (link copied below) and have a question regarding one comment made in the post, specifically, "3) You can use the transformer isolation to reduce the effects of common mode noise.  Resistive sensing makes it difficult to isolate the high current ground from the low signal ground."  

Will this be a major concern at higher power levels, going from 5KW to 10KW if I use Shunt resistors?  If I choose to go this route, should I use Fully Differential Isolation Amplifiers (for current sensing), Isolated Gate Drivers, and an Isolated Power Supplies to split the ground signal between the high current ground and the low signal ground? 

I'd appreciate any insight.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/601672/ucc28070-current-sensing-of-ucc28070

Regards,

  • 0
    TI__Expert 8650 points

    Hello,

    We are looking into your question and will get back to you soon.

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    Prodigy 90 points

    Anybody have any insight?

  • 0 in reply to StnGgc
    TI__Expert 8650 points

    Hello,

    Is there a reason why you are considering shunt resistors over current sense transformers?

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    Prodigy 90 points

    Jonathan,

    There is no reason other than I'm more comfortable and have more experience using shunt resistors over current sense transformers.  Should I stick with current sense transformers for a higher power rated version of PMP4311?

    Regards,

  • +1 in reply to StnGgc
    TI__Mastermind 40500 points

    Hello Steven, 

    The current-sense transformers (CT) are intended to allow the designer to obtain higher voltage signals and reduced noise for current-sensing of high-power PFC without also incurring high power dissipation through sense resistors.    

    If you use shunt resistors instead of CTs, their values will likely be very low to avoid excessive power loss, and wide band-width amplifiers will be needed to restore the CS signals to usable levels.  These amplifiers can also amplify noise which can increase THDi. 

    And they will also amplify the gate-drive current peaks that are present in the MOSFET source current (or IGBT emitter current) at the turn-on instant of the PWM cycle.  These gate-drive peaks may interfere with accurate current sensing at lighter load levels, so they will have to be suppressed.   
    A MOSFET with a Kelvin-connected-Source pin for gate-drive can help avoid this, but is not absolutely necessary. 

    The comment from a previous E2E post on this topic years ago indicated that it can be "...difficult...", but not impossible, to "...isolate the high current ground from the low signal ground."  
    In addition to the gate-drive current spike issue, stray inductance in the system GND paths can also add spike voltages to the current sense due to high di/dt if the power and signal paths cannot be adequately separated.  This is heavily layout dependent and may be difficult to achieve during the routing of high-current return paths.   

    You don't need special Diff Amps and isolated gate drives and split bias supplies to accomplish successful shunt resistor sensing if you are aware of the issues that can arise by doing so.  You do need low-offset and high bandwidth amps and high gain.  Lower gain improves BW but requires higher power loss. 

    Early consideration of these issues can allow you to arrange/re-arrange things to minimize the effects on control signals before actually starting the physical design.  Trying to correct a problem after a board layout is already finished and system constraints are solidified is much more difficult to achieve. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 90 points

    Ulrich,

    Thank you for the reply.  Based on your description, it seems a Current Sense Transformer is a better solution for a high powered PFC. 

    I have a rough design laid-out using a Current Sense Transformer and was wondering if it would make more sense to switch to a Shunt Resistor.   I'll stick with convention and use the current sense transformer.

    Thanks for the feedback!

    Regards,

    Steven