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What is the best Synchronous Rectifier controller for Quasi-resonant control like LM5023? LM5023 switching frequency on EVM?

John Stevens
Expert 7395 points
Other Parts Discussed in Thread: UCC24630EVM-636, LM5023, UCC24630, UCC24636, UCC24610, UCC28740, UCC28630

We are trying to promote LM5023 + UCC24630 for the adapter. UCC24630EVM-636 is a good demo for us. Is this still the best SR solution for LM5023?

 

Could you also please help check what is the LM5023 flyback SW frequency on the UCC24630EVM-636 board? I couldn’t find it in user’s guide.

http://www.ti.com/lit/ug/sluub81a/sluub81a.pdf

  • 0
    TI__Expert 7395 points
    LM5023 is a quasi-resonant mode controller (transition mode/Boundary mode) so our new SR controller UCC24636 device will be the best fit. UCC24636 is new version of UCC24630 that is optimized for high performance in DCM (Discontinuous Conduction Mode) and Transition mode(QR). See preview datasheet here: www.ti.com/.../UCC24636.
    In general, UCC24630 is better for CCM applications. UCC24610 can be used for CCM or DCM/QR just fine because it uses Vds sensing instead of V/S balance method like UCC24630/UCC24636. However, UCC24610 is limited to 5Vout. Also, UCC24610 is limited in its use at higher power with lower Rdson. For higher power you will see better performance of V/S method with UCC24630/UCC24636.

    From figures 9 and 10 in the EVM users guide it looks like the switching frequency of the LM5023 on this EVM at full load is about 125kHz. You can also see in the ripple waveforms figures 5 and 6 that frequency as well with about a 8us period.
  • 0 in reply to John Stevens
    TI__Expert 7395 points

    Blake,

    I want to make a correction here. UCC24636 is in preview so to be confident in expected performance we did a few internal tests.  

    The transition mode controller like LM5023 operation results in the need for a shorter toff (SR inhibit) time than the UCC24636 setting of 4.35us, especially at lighter loads where the QR frequency increases before going into standby-type mode. We saw generally better efficiency with the LM5023EVM with the UCC24630 SR controller vs. using UCC24636. The 5023 EVM is based on the UCC24630 controller with 2.5us toff timer setting.

     

    As such, I want to correct the general recommendations:

    DCM based primary Flyback controller (Like UCC28740 for example)=> use UCC24636 SR controller

    Quasi-resonant(Transition mode) Primary Flyback controller (like LM5023 for example)= UCC24630 SR controller

    CCM Primary Flyback controller (UCC28630 can be in CCM or just step into CCM during Peak power) = UCC24610 is best with what we have as of today.

     

    Note that CCM operation with SR controllers is most complex and requires careful design practices to see the efficiency benefits and prevent shoot-through current. Let me know if you'd like to discuss that in detail.

     

    Regards,

    John