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UCC256402: Question about IrespeakOCP1 and IsecpeakOCP1

Darren (FAE)
Expert 5483 points
Part Number: UCC256402

I understand IrespeakOCP1 is the OCP limit for the primary side, and IsecpeakOCP1 is the OCP limit for the secondary side.

From the DS (「8.2.2.16 ISNS Pin Differentiator」) I also understand the equation to calculate IrespeakOCP1;
and from this IsecpeakOCP1 is just IrespeakOCP1 times the transformer turns ratio.

In the current setup the IrespeakOCP1 = ~6[A].
The turns ratio can't be modified, and we are looking at some other way to reduce IrespeakOCP1 to around 20~30[A].

Is there any way to modify the other values in the eq. to reduce IrespeakOCP1 and thus reduce IsecpeakOCP1?
It looks like it can be done by adjusting CISNS and CR, right...?

Regards,
Darren

  • 0
    TI__Mastermind 40120 points

    Hello Darren, 

    In your third paragraph, I assume that you meant to "...reduce IsecpeakOCP1 to around 20~30[A]", correct? Please verify. 

    What you propose may seem possible, but changing the values of Cisns and/or Cr, etc. in equation (55) of the UCC25640x datasheet will have unintended consequences. For the example cited, you are proposing to reduce IrespeakOCP1 to ~1/4th of its existing value. 

    Cr, Cisns, and Risns have values already calculated previously from your system design requirements.  They cannot be changed in equation (55) (to reduce the result of equation (56)) without altering the performance of your design.  

    Specifically, Cr is chosen for a target resonant frequency, so it's value can't be changed by 1/4 without seriously affecting the entire resonance behavior of the LLC.  
    Examining equation (53), we can rearrange terms and see that Risns*Cisns = kisns*Cr.  kisns is function of system constraints and previous calculation, so the right-side product is a constant which makes the left-side product a constant.  Thus in equation (55) you can't change Cisns up without changing Risns down inversely proportionately.  Changing only one of them will alter the peak current sensing and severely limit your output power.    

    For this design example of 15A at 12V out, the secondary current peaks at the OCP1 level will necessarily be as high as ~99A (per equation (56)).
    Therefore the secondary-side components must be chosen to reliably accommodate this stress under the OCP1 condition.   

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Expert 5483 points

    Hi Ulrich,

    I understand in order to change IsecpeakOCP1 through modifying CISNS or CR mean other system parameters have to change.

    Is there any option here with minimal board changes that could be tested?
    Or, could you provide a couple component parameters that could be adjusted to change these current limits with minimal system impact...?

    Regards,
    Darren

  • 0 in reply to Darren (FAE)
    TI__Mastermind 40120 points

    Hi Darren, 

    Unfortunately, there are no minimal changes that can be done to reduce the secondary peak currents without also affecting the output power. 

    The only way to change the secondary peak currents is to change the resonant component values ( Cr, Lr, Lm) and other system paraments associated with and dependent upon these values.  

    Regards,
    Ulrich