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Maximum Duty cycle of a Flyback converter

Aneesh TS
Expert 1735 points

Hi Friends,


What is the maximum duty cycle of a Flyback converter?. Can i go for 0.9 Duty cycle in Flyback?. What is the impact of 0.9 Duty cycle in flyback?

Regards

Aneesh

  • 0
    Genius 5910 points

    The is no rule what the maximal duty of a flyback. It depends on input, output voltage and transformer ratio. The max duty: is that there is enough time left for the transfer phase else your transformer will saturate,you can see this real good with a scope. 0.9 is really high so I think you need to modify the transformer ratio.

  • 0 in reply to evs
    TI__Genius 9130 points
    Aneesh,

    The duty cycle in DCM (discontinuous conduction mode) will change with both input voltage and output load current.

    The duty cycle in CCM (continuous conduction mode) will be almost the same at a given input voltage, i.e. it will change with input voltage and be almost constant with load (until the load decreases to the point where operation does DCM again).

    At the boundary between DCM/CCM (sometimes called BCM point, or transition-mode (TM) or critical conduction mode (CrM)), the duty cycle will depend on the input voltage and the reflected output voltage - as evs said, volt-sec balance must be maintained:

    Dccm = Vrefl / ( Vin + Vrefl)

    Vrefl = Np/Ns * (Vo + Vrect)

    where Vrect is the secondary side rectifier drop.

    At 50% max duty cycle, the Vrefl will exactly equal Vin.

    If you need or want higher duty cycle like 90%, Vrefl has to be a lot higher than Vin to maintain volt-sec balance. This has downsides - the Np/Ns ratio becomes much higher, so the primary side max voltage stress on the primary switch is a lot higher. Also the peak secondary current is much higher, since only 10% of the cycle is allocated to the secondary conduction interval - it follows that it needs a much higher peak value to deliver the same average load current.

    For typical Flyback designs that run over a wide Vin range, high pri Dmax will penalise the secondary, since the secondary conduction duty cycle is always the same (in DCM).

    Typically, 50% Dmax at DCM/CCM boundary at the nominal operating point is a good target to aim for, as this maximises the conduction interval for both pri & sec and minimises rms currents.

    However, many Flyback controllers also use Quasi-Resonant (QR) or valley-switching techniques to reduce switching losses. These must alsways operate in DCM and will do so by design. The power stage must then be designed to operate at or near the DCM/CCM boundary at minimum Vni and max power output, otherwise the QR behaviour would not allows them to deliver full power at min Vin & max power if the operation needed to go CCM to do so, the controller would prevent it and Vout would be power limited.

    There are mnay Unitrode/TI Power Supply Design Seminar papers available here, and many that deal with the details of Flyback converters:
    www.ti.com/.../login.shtml



    I hope this is helpful to you, if so please click the "verify answer" button.

    Thanks,
    Bernard