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UCC24612: Inquire SCH and layout review, and have missing pulse isse

user4773172
Intellectual 1410 points
Part Number: UCC24612
Other Parts Discussed in Thread: UCC256303, , CSD18503Q5A, CSD18504Q5A, UCC24624

Hi Sir,

May I have your help to review the SCH and layout as below:

Now we facing missing pulse issue, please refer below waveform, Ch1: UCC24612 Vgs, Ch2:UCC256303 Vgs.

This issue only happened in 9A to 20A load, and if load below 8A, the SR is work properly=>MOSFET part number: CSD18503Q5A

We have tried to use other MOSFET: CSD18504Q5A, the SR work properly across all load condition. 

May I learn from you why there have missing pulse condition with lower Rdson MOSFET?

Thank you.

Best regards,

Edward Chen

  • 0
    TI__Expert 8210 points
    Edward

    UCC24612 may have issues when it drives a SR FET that has an Rdson below 4.5mΩ. The CSD18503Q5A (not working FET) has a Rdson of 3.4mΩ while CSD18504Q5A (working FET) has an Rdson of 5.3mΩ. UCC24612 relies on the voltage sensed across the FET for proper operation, so when the Rdson is too low it can cause misbehavior like what you are seeing.

    If you need to use the lower Rdson CSD18503Q5A , I recommend using UCC24624. UCC24624 is a dual SR controller that is designed to work with LLC power stage like UCC256303 so it will not have this Rdson limitation.

    Eric
  • 0 in reply to Eric Faraci
    TI__Intellectual 1410 points
    Hi Eric,

    Please refer to SCH in first post.

    In the SCH, the two kinds of MOSFET will be parallel three of them.

    So it means two kinds of conditions are both below 4.5mohm. (3.4//3.4//3.4 v.s. 5.3//5.3//5.3)

    Thank you.

    Best regards,

    Edward Chen
  • 0 in reply to user4773172
    TI__Expert 8210 points
    Edward

    Sorry about that, I missed that they are parallel. 4.5 mΩ is a general recommendation for when the behavior starts to degrade. 4.5 mΩ is not a hard limit, slightly under it the performance will start to degrade since there will be additional dead time generated. As the resistance further decreases the sensing will become even worse where it will have exacerbated issues like what you are seeing.

    Eric
  • 0 in reply to Eric Faraci
    TI__Intellectual 1410 points
    Hi Eric,

    So may I know is there any way that we can try to solve this issue?

    Any work around solution that we have?

    Thank you.

    Best regards,

    Edward Chen
  • 0 in reply to user4773172
    TI__Expert 8210 points
    Edward

    We recommend changing SR controller to UCC24624. UCC24624 is designed for LLC so the voltage sense thresholds are better suited for lower Rdson like this design.

    If you need to use UCC24612, we recommend using the higher Rdson like CSD18504Q5A.

    Eric