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UCC28070: Inductor current not balanced

Aaqib Sheikh
Prodigy 165 points
Part Number: UCC28070


Dear Sir, 

I am working with your Controller UCC28070 for designing of PFC boost Converter. Last time your expert solve my several problems related to this controller. Currently, I am facing the problem in unbalanced inductor current waveforms. Inductor currents are in CCM but they dont have equal value as you can see in the picture. Both inductor currents displays different RMS values. Kindly tell me how can I solve this Problem. 

  • +1
    TI__Mastermind 40580 points

    Hello Aaqib, 

    Ideally, the current-loop gains of the two channels should be identical. 
    In reality, Ch2 current is higher than Ch1 current by the ratio of 4.817A/3.376A = 1.427.  This ratio is curiously close to the square-root of 2, but that may be just a coincidence.  But whether it really is the square-root of 2 or not is irrelevant.  Ch2 current is significantly higher than Ch1 and it is not the result of differing inductances.   The Ch1 waveform (yellow trace) partially obscures the ripple amplitude of the Ch2 (red trace), but I suspect that the peak-to-peak ripple of both channels are almost the same.   (Different inductances would have differing pk-pk ripple currents, but the same average and rms values.) 
    So the rms difference in current is due to some other difference in current-sense gain factor. 

    Both channels of the UCC28070 current loops try to regulate their respective average inductor currents to the same IMO current reference signal.  Each current loop has two gain factors which normally are the same but may be inadvertently different (in this case).  They are: the turns-ratio of the current sense transformers (CTs) and the burden resistors on the CT outputs.   Since Ch1 has a lower current than Ch2, either Nct1 > Nct2, or Rcs1 > Rcs2.   

    A higher turns-ratio for Nct1 would make the current-sense signal of a lower inductor current look higher in amplitude and satisfy the comparison to Vimo. 
    A higher resistance for Rcs1 would make the current-sense signal of a lower inductor current look higher in amplitude and satisfy the comparison to Vimo. 
    From the waveforms, we cannot determine whether one side Nct or Rcs values are too high or one side is too low (compared to ideal design calculations). 
    We can only determine that there is a difference in the gains, so some difference in Nctx or Rcsx must account for it.

    Please examine the CTs and burden resistances on your test board to see which one is different from your calculated value for that location. 
    Ensure that Nct1 = Nct2 and Rcs1 = Rcs2.  

    Regards,
    Ulrich