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Original question:

UCC28070: How high speed op-amp is needed for resistive sensing

UCC28070: Resistive current sensing method

Satoshi Yoshida
Prodigy 660 points
Part Number: UCC28070

Hi John-san & team,
Our customer is now considering to design their product with similar schematic to 1401.Sch-1.pdf(TIDA-0130).
Please allow us to ask you some questions about a resistive current sensing method as below,

1.
They are struggling to select OP-Amp for sensing current.
Could you let us know how to determine the specifications(BW, SR ...) of amplifier?
John-san mentioned in the related Q&A as below,
        "For example if your switching frequency is 100kHz then you can expect to be required to detect pulse widths of 1uS or so."
but do you have any calculating formula to lead the spec of amplifier properly?

2.
They found the degradation of power factor in case of using lower-speed amplifier for sensing current.
Do you have any other concerns with using lower-speed amplifier except for power factor?

3.
Please give us the detailed explanation that why degraded power factor in above case.

4.
We infer that the downslope of inductor current during the switching period OFF-time is synthesized by the input of VINAC and VSENSE pin.
Are there any concerns to the quality of the synthesized waveform of OFF-time due to the degradations of the input waveform to CSA/CSB-pin during ON-time?

Best regards,
Satoshi

  • 0
    TI__Mastermind 34465 points
    Hi Satoshi-san,
    In order to achieve a high power factor you need to accurately measure the input current and this is true whether you use a current transformer or a resistor to sense the current. The main disadvantages of the current transformer are the false signals you get due to ringing around the zero crossing and the minimum off time that is required in order to reset the magnetic core.
    But the big advantage is there is an inherent signal amplification and no significant issue with switching frequency limitation.
    With resistive sensing you will need some sort of amplifier to generate a signal of significant amplitude on the CSA/CSB pins.
    You should expect to require the use of a high speed OPAMP if your switching frequency is fairly high.
    The scope of the reference design did not involve a detailed study of the detailed requirements of the OPAMP used for this method of line current sensing. I tested several high speed OPAMPs . I monitored the input and output signals and selected a device based on performance and availability.
    No doubt further study could be devoted to this testing in order to optimise cost and performance but this was beyond the scope of the reference design. I recommend that your customer continue with this testing and eventually they will find a compromise between performance and cost.
    The more accurately that CSA/CSB is generated the better the power factor will be.
    If the CSA/CSB waveform is not correct then the downslope will also be in error so if you look at the current amplifier output signals , CAOA/CAOB, they will also be wrong.
    If the CSA,CSB signals are generated accurately then you can expect to achieve very good performance for the pfc with resisor current sensing on the boost MOSFETs.

    Regards
    John
  • 0 in reply to John Griffin1
    TI__Prodigy 660 points

    John-san,
    Thank you for reply.
    We got an additional question from customer.
    Which part of the waveform of CSA/CSB inputs is significant to the performance?
    (Please refer attached ppt.)

    8688.waveform.pptx

    Best regards,
    Satoshi

  • 0 in reply to Satoshi Yoshida
    TI__Mastermind 34465 points
    Hi Satoshi-san
    The middle section (with the slow rising slope) is the important measurement.
    This represents the inductor current during the ON time.
    The current during the OFF time is synthesized from this waveform and the sections on either side of this waveform are not used by the UCC28070
    Regards
    John