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LM5122: CCM flyback synchronous rectification

venkata subash1
Prodigy 85 points
Part Number: LM5122
Other Parts Discussed in Thread: UCC24610, PMP10541

Hi I want to know the advantages and disadvantages of using CCM flyback synchronous rectification using LM5122 over using external synchronous controller like UCC24610.

I observed that there is no RCD clamp in reference design PMP10541 ,IF we use synchronus rectification then there is no need of RDC clamp to clamp  the primary side MOSFET  drain voltage ? is there any document to explain this? in light load condition what precautions we need to take care ?

  • 0
    TI__Expert 7450 points

    Hi venkata,

    Thank you for your interest in LM5122.

    UCC24610 is a secondary controller which means it needs another primary controller. LM5122 can be used to control both primary and secondary FETs but it will need an isolation if needed because it does not has internal isolation.

    LM5122 will still need the RC snubber or even RCD clamp depending on how much voltage overshoot it needs to suppress.

    Thanks,

    Yinsong

  • 0 in reply to Yinsong Cai1
    Prodigy 85 points

    Hi Yinsong,

    Thanks for the reply, I understood that UCC24610/12 is a secondary controller and LM5122 is primary controller. 

    Now my question is if we use UCC24610/12 to improve this SR-Flyback light load performance and allow it operating in DCM  the UCC24610 compares the SR-MOSFET’s voltage of Vd and Vs, only when Vd falls more than - 150mV below Vs, the GATE output goes high and the minimum TON timer is triggered. How can we set the same thing with LM5122?

    How can we use the diode emulation mode of the LM5122 in case of flyback application?

    any advantages of implementing synchronous rectification using primary converter like LM5122 than secondary converter UCC24610/12

  • 0 in reply to venkata subash1
    TI__Expert 7450 points

    Hi venkata,

    LM5122's drive control is based on the adaptive dead time so there is not much settings you can change. The diode emulation mode can be triggered easily by applying a <1.2V MODE voltage. I think a primary controller is still needed if the secondary controller like UCC24610 is used.

    Thanks,

    Yinsong