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UCC24612: Synchronous Rectifier Controller for SEPIC

Darren DeBattista1
Intellectual 525 points
Part Number: UCC24612
Other Parts Discussed in Thread: UCC24630, UCC24610

Hi, 

I am designing a SEPIC SMPS with an output current of 10A. Given the output current, I'm considering the use of a synchronous rectifier controller to replace the output diode with a MOSFET, the same way as performed in FLYBACK converters. Are the synchronous rectifier controllers such as the UCC24612 suitable for use in SEPIC topologies?

Thanks

  • 0
    TI__Genius 11350 points

    Hi Darren,

    Thank you for your interest in the UCC24612 SR controller.

    Unfortunately, I don't have any experience using and SR controller in a SEPIC converter but I don't see a reason it cannot be used as you described. You can also consider the UCC24630 SR controller. In another post on this subject, Ulrich pointed out a limitation on the switching frequency. You didn't specify at what frequency your design would be operating, so please reference this post:

    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/692276/ucc24612-can-this-part-be-used-for-the-output-rectifier-in-a-sepic?tisearch=e2e-sitesearch&keymatch=ucc24612%2520AND%2520sepic#

    I wonder if you may be able to try out your concept with the SSR Controller Diode-Replacement Demo Board (note though that at higher currents you'll need to provide sufficient airflow) (https://www.ti.com/tool/UCC24612-1EVM)

    I hope this helped.

    Ray

  • 0 in reply to Ray DiCecco
    Intellectual 525 points

    Thanks for your reply. 

    My frequency of interest is around 500kHz, though it has not been decided yet. In the post you showed me, Frank was using the UCC24612-1 or UCC24612-2 which are respectively rated at 1MHz and 800kHz according to their datasheet. Can you please give more details on possible reasons why Frank was having issues already at 350kHz?

    Thanks 

  • 0 in reply to Darren DeBattista1
    TI__Genius 11350 points

    Hi Darren,

    In the post you are referencing, Frank did not specify what issue he was having and never closed the loop with us.

    At that time, Ulrich had some internal discussions and looked the potential pitfalls. I will assign this thread him and hopefully, he can recall the discussion from 4 years ago and can clarify for you. However, please allow a few days for Ulrich to respond.

    Thank you for your patience.

    Ray

  • 0 in reply to Ray DiCecco
    Intellectual 525 points

    Hi Ray,

    Thanks again. I will wait for Ultrich reply. 

  • +1 in reply to Darren DeBattista1
    TI__Mastermind 40580 points

    Hello Darren, 

    I cannot recall the internal discussion I had on this topic some 4 years ago, but I believe they centered around the potential for SR cross-conduction due to turn-off delay of the SR Fet.  As noted in the old post, the SEPIC topology can experience CCM current at high amplitudes (which we dubbed "deep CCM") and our concern was/is that the delay between detecting the reversal of CCM current and turning off the SR FET may be too long for safe and efficient operation at (or above) ~320kHz.  Your 10-A output at ~500kHz is subject to the same concern. 

    As for the 1MHz frequency "rating" in the features list for UCC24612-1, this range capability is not practicable under all possible operating conditions. Rather it is supportive of flyback applications where the peak current reduces as the frequency increases with diminishing load (such as QR and Active-Clamp Flyback), and where FET package parasitics are minimal.   As is often seen with all kinds of datasheets, the marketing department may tout a product's best case without necessarily listing all of the attendant limitations or restrictions for achieving that best case.  

    A controller's performance threshold and timing parameters that are guaranteed are listed in the Electrical Characteristics table.  The variability of these parameters limit their applicability to wave shapes tolerant of these variabilities.  The SEPIC current wave shapes at 500kHz may be intolerant of some of the UCC24612 timings, but the UCC24612 can adaptively adjust its proportional gate drive threshold to -150mV to mitigate this issue.  See the datasheet pages 15 and 16 on this topic.  If the SR MOSFET on resistance is chosen to be high enough to facilitate this behavior, then it may well work in your application after all.  Our concern is based on not having definitive experience with deep CCM at these frequencies. 

    A possible alternative controller may be the UCC24610 which features a SYNC input which can be triggered by the primary side to provide advanced warning to the SR that the primary side is turning on.  That device's direct drain sensing has a 50-V limit, however, the voltage can be extended higher using techniques discussed in this application note: https://www.ti.com/lit/pdf/slua860. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 525 points

    Hi Ulrich, 

    Thank you for the detailed explanation. I'll go through the datasheet of the UCC24612 in more detail and consider the UCC24610 for its sync input as suggested.