• State TI Thinks Resolved
  • Locked Locked
  • Replies 10 replies
  • Answers 4 answers
  • Subscribers 64 subscribers
  • Views 607 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC256302: UCC256302_Question about output rise failure at restart

Akira Kurata
Intellectual 280 points
Part Number: UCC256302
Other Parts Discussed in Thread: UCC256402, UCC25630-1EVM-291

UCC256302_20191122.pdf

  • 0
    TI__Expert 8210 points

    Akira-san

    Thank you for reaching out.  First off I'd like to let you know about our latest LLC controller UCC256402.  This device is pin-to-pin compatible with the device you are currently using  but has improved performance and features.  Details of these improvements are in the following app note https://www.ti.com/lit/an/slua966/slua966.pdf. 

    We recommend that you migrate your design to UCC256402.  To help with this migration we have the following excel design calculator for UCC25640x https://www.ti.com/lit/zip/sluc675 that you can use to verify the design is correct.  Some functionality has changed slightly so component values around the IC are different.

    In order to help with your design I have a few questions

    • Did you use the UCC256302 excel design calculator http://www.ti.com/lit/zip/sluc634?  If not please use this tool to verify that there are no errors in your design.
    • Do you have a schematic for this design? 
    • When you say you are adjusting the output voltage, how are you doing this exactly?

    Best Regards,
    Eric

  • 0 in reply to Eric Faraci
    Intellectual 280 points

     

    Hello　Eric-san

     

    <Continuation of answers and questions>

     

    The requested circuit diagram will be posted at a later date.

     

     

    The questions I want to know are as follows.

     

    Why does the 5V output stop at 3-4V when restarting with no load?

     

    This phenomenon seems to occur when there is no problem in the restart after a sufficient time has elapsed after the 5V output stops, and a little output voltage remains.

     

    When restarting, if the HS pin fails to detect the slew rate, is it related to maintaining the SR pin voltage at 7V or 0V and then maintaining the low voltage of 5V output?

     

     

    I am evaluating the UCC256302 evaluation board (UCC25630-1EVM-291) by remodeling it into 2 outputs of 5V and 24V.


  • 0 in reply to Akira Kurata
    Intellectual 280 points

    Hello　Eric-san

     UCC256302_Application_20191125.pdfThe application circuit diagram of UCC256302 is attached.

  • 0 in reply to Akira Kurata
    TI__Expert 8210 points

    Akira-san

    UCC256302 minimum slew rate that it can detect is 1 V/ns.  The newer UCC256402 minimum slew rate is 0.1 V/ns.  I cannot tell from the waveform provided if this is an issue with your design, but moving over to the newer UCC256402 will help insure it is not.

    From your schematic it looks like you changed the transformer, resonant capacitor, FET and feedback circuitry.  I recommend that you use the excel calculator http://www.ti.com/lit/zip/sluc634 to verify that there is nothing in the design that is preventing operation.  Use the 5V output as the output for the calculator.

    When I went thru the tool I found that the VCR capacitors (C9, C10) need to be changed.  These capacitors impact the soft start timing, which by your description may be causing some of the behavior you are describing.  This app note has more details on soft start with UCC256302 http://www.ti.com/lit/an/slua836a/slua836a.pdf.  Other components may need to change as well, I recommend you go thru the excel tool and verify that all the component values in your design are correct.  If the undesired behavior does not go away after going thru this tool please upload the filled out calculator.

    Best Regards,

    Eric

  • 0 in reply to Eric Faraci
    Intellectual 280 points

    Hello Eric-san

     

    Excel calculation software (UCC25630X design calculation tool) is set for single output.

    This question is a 2-output application. Can this tool be used in this case as well?

    In that case, two outputs (total 98W) of 5V10A (feedback control output) and 24V2A (dependent open output)

    How should I incorporate it into calculation software?

    Is it okay to set only 5V10A?

          Please answer.

  • 0 in reply to Akira Kurata
    TI__Expert 8210 points

    Akira-san

    Yes, you can use excel tool where you just use the 5V output.  Some of the parameters that are dependent on power level, like RISNS, calculation will be incorrect if you leave it at 10A.  To compensate for this we recommend to set the output to the total max power of the output assuming all the power is on the 5V output.  You can do this by adding 9.6A (48W/5V) to the 5V/10A output, making it 5V/19.6A.

    Best Regards,

    Eric

  • 0 in reply to Eric Faraci
    Intellectual 280 points

    UCC256302_20191129.pdf

    UCC25630x Design Calculator.5v.24v.pdf

    Hello Eric-san 
    Attach the contents of the re-question as a PDF file.
    Please answer again.

  • 0 in reply to Akira Kurata
    TI__Expert 8210 points

    Akira-san

    In your file you mention that VCR voltage is increased due to slew rate detection failure.  I'd like to make sure you're aware that the newer, pin-to-pin compatible UCC256402 has a minimum slew rate detection of 0.1 V/ns, which is 10x better than the 1 V/ns UCC256302 has.

    Does this problem where the 5V rail stops at 2.8V only occur at no load?  We recommend increasing the pre-load resistor to see if it still occurs.  If this is the case, but you cannot accept the lower resistor value, we recommend you review this app note to determine how to adjust the power stage design to get proper operation over your entire operating range https://www.ti.com/seclit/ml/slup263/slup263.pdf 

    In your 

    Best Regards,

    Eric

  • 0 in reply to Eric Faraci
    Intellectual 280 points

    Following the previous advice, the UCC25630-1EVM-291 LLC device was replaced from UCC256302 to UCC256402, and the status of the restart failure was compared and verified.

    The results, waveforms and questions are listed in the attached PDF.

    Please confirm the contents and answer.

    UCC256302_UCC256402.pdf

  • 0 in reply to Akira Kurata
    TI__Mastermind 27105 points

    Hi Kurata-san,

    In the UCC256302, because the slew rate is missed, the dead time is extended and this is truncating the high side pulse width. Note it is almost exclusively happening when the switch node is going from 0V to Vin so the high side pulse is getting truncated but the low side is not. Because the high side pulse width is getting truncated, very low current is injected into the resonant tank. Less current is available to drive the switch node up and down and this makes it harder to detect the slew rate. It's having trouble recovering from the missing slew rate to achieve normal switching.

    In UCC256402, the dead time is purposefully made to match between H->L and L->H transitions. Because the controller is no longer truncating the high side pulse, more current is available and this is helping the converter recover from the missed slew rate detection. UCC256302 does not have the dead time copy feature.

    Best Regards,

    Ben Lough