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  1. #1
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    Fault mode of synchronous recrifier IC with BCM flyback

    Hello
    We are using the LT8309 synchronous rectifier for a PFC’d flyback on 80-135VAC input…26vdc output. We are using it like in the attached LTspice schematic (which needs to be run with the attached textfile “bcmpfcfile” in the same folder….the sim file is called “bcmpfc”)

    As explained on page 9 of the LT8309 datasheet, there is a fault mode of this synchronous rectifier….this occurs when in DCM as follows…when the secondary coil has discharged, the primary drain voltage can ring down and turn on the primary fet’s intrinsic diode………when the ringing primary current then reverses, the primary fet diode will turn back off again, but due to its reverse recovery it wont turn off straight away, and so some “Phantom” primary current will flow……when it stops flowing, the secondary will then take its turn and flow current…..this can drive the secondary VDS voltage such that the sync rect IC turns on again when it should not….and then the primary fet may turn on at the same time giving unwanted cross-conduction.

    The ways given to solve this are either to use a schottky parallel to the primary fet, or to set the turns ratio of the flyback such that the primary drain voltage cannot ring down to ground…neither of these options are open to us because our max primary drain voltage is too great for a schottky…and no matter what we set the turns ratio to…there is always going to be a time when the primary drain voltage can ring down to ground…(because the primary bus voltage falls very low due to PFC operation with little primary bus capacitance).

    Do you know any other flyback sync rect IC’s which offer mitigation of this problem?

    LT8309 datasheet:
    https://www.analog.com/media/en/tech...ets/8309fa.pdf

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  2. #2
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    Re: Fault mode of synchronous recrifier IC with BCM flyback

    hefty snubbing may cure it - along with sending the snubber energy to the o/p ( for efficiency ) - PDF please?

    - - - Updated - - -

    Actually it is very unlikely that on a HV pri side the energy will take the pri drain to gnd - this will only happen if you have large leakage and a fair bit of C in the Tx wdg.

    Pretty easy to use an RCD snub on the Tx pri wdg ( and on the sec across the synch rect mosfet - just RC ) and your problems will not manifest.

    Cheers, EP.


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  3. #3
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    Re: Fault mode of synchronous recrifier IC with BCM flyback

    I suppose this sort of behavior would be rare in a DC-DC flyback, since normally the flyback voltage on the primary side is roughly equal to the input DC voltage. If the flyback voltage is much greater than Vin (as is the case with a PFC flyback near zero crossings of the AC line), then I can see this becoming an issue.

    I ran your sim, and there are times where the secondary side diode ringing does take the rectifier close to forward bias (it crosses zero, but not enough to really forward bias it), look around 1.42ms. However the LT8309 doesn't react to it. Even if I decrease the snubbing to make it worse, I don't see an extra pulse ever coming from the LT8309. Never used the chip, so I'm not sure how easy it is to fool.

    If you intend to operate in BCM, then why are you worried about this?


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  4. #4
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    Re: Fault mode of synchronous recrifier IC with BCM flyback

    its in BCM but there is a minimum on time, so in certain situations, it will be in DCM.



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