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UCC39002: UCC39002

Waseem Akbar
Prodigy 60 points
Part Number: UCC39002


Hi, 

I have a question about one of the equations in the application note. Is the following equation computed correctly? This is on page 23 of the UCC39002 Advanced Load-Share Controller's User's Guide, HPA027A. 

|GMOD(fzero0|= 10^(65/20) computes to 1778.27 not 691.784. Not sure if the value is incorrect or I solved incorrectly. Any feedback would be greatly appreciated. 

  • +1
    TI__Mastermind 40580 points

    Hello Waseem, 

    Thank you for your question on the UCC39002 Load-Share EVM User's Guide calculations. 

    You are correct that |GMOD(fzero0| = 10^(65/20) = 1778.27.  This is the estimated DC gain of the PT4484 power module measured from its Bode plot shown in Figure 10 on page 17 of the User Guide.  The author of the guide also estimates several poles and zeros and the cross-over frequency from the plot in order to construct an equivalent gain equation shown at the top of page 18.   

    Having established the power module gain equation, the author uses it to estimate the gain of the power module at 256Hz.  At the top of page 23, the author chooses to set the zero-gain crossover of the load-sharing loop to 1/100th of the crossover frequency (25.6kHz) of the power module voltage loop. 
    At 256Hz, the power module gain calculates to about 56.8dB, so |GMOD(fZERO)| = 10^(56.8/20) = 691.784, where fZERO = 256Hz and is the sharing loop zero-crossing, not the V-loop crossing.  This is the amount of gain that the load-share loop compensation must attenuate so that the load-sharing response does not interfere with the normal voltage regulation response. 

    Admittedly, the author left out some steps, and I calculated backwards from 691.784 to determine the 56.8dB of gain at 256Hz, but this appears to match well enough with the gain curve of Figure 11.  In reality, the accuracy of this estimation does not need to be so precise, as long as the load-share cross-over is chosen at least 1 decade below the V-loop cross-over.  Personally, I would have chosen about 440Hz because the V-loop gain is clearly at 50dB with no guess work at values between gridlines.  On the other hand, the gain equation GMOD(f) can calculate the gain vs. frequency, but remember that GMOD(f) itself is an equation estimated from the Bode plot. 
    When fZERO is low enough, such precision is not necessary to get good     response from the load-sharing. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 60 points

    Hi Ulrich,

    Thanks, this helps a lot! Where should the bulk capacitance of the power supply be placed. Should it go before the current sense resistor or after the current sense resistor?

    Thanks, 

  • +1 in reply to Waseem Akbar
    TI__Mastermind 40580 points

    Hi Waseem, 

    I think that by "...bulk capacitance of the power supply..." you are referring to the output filter capacitance, right?  (If I'm wrong please let me know.)
    The power supply's output capacitance should go before the current-sense resistor. 

    Re-phrasing this: the current-sense resistor should be placed after the output filter capacitor of the power supply.  The purpose is to sense only the DC or low-frequency component of output load current from the supply.  You do not want to sense any of the high-frequency switching-ripple current which is filtered by the output capacitance.  

    Regards,
    Ulrich