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PMP7760: Zener Calculation

Maor Matityahu
Intellectual 600 points

Part Number: PMP7760

Hello,

How do i calculate R117 and D102? 

I understand that the role of these two is to limit the output voltage at no load.

Thank you

  • 0
    TI__Guru* 78270 points

    Hi Maor,

    Thank you for considering our products.  D102 basically clamps the max output voltage when the load goes to zero ampere. At no load the output rail may rise up owing to the leakage inductance. Select a zener that is slightly higher than the targeted voltage so it will not dissipate during normal load range. 

    R117 is chosen to limit the current flowing into the zener.  It should be selected such that the max current flowing into  the zener would not exceed the zener current ratng.

    Hope this clarifies.

    Best Regards,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Intellectual 600 points

    Hello Youhao,

    Thank you for the answer

    i have some questions:

    1) Why the leakage inductunce makes the output voltage goes higher at no load?

    2) Without the zener diode, what will be the output voltage at no load at the PMP7760 design?

  • 0 in reply to Maor Matityahu
    TI__Guru* 78270 points

    It is rooted in the flyabck converter operating principles.  When the main switch is turned off, the transformer stored energy needs to be released, along with the leakage inductance, and part of the energy is released to the output rail. If there is no load, it will continue charging up the output capacitor and raise the output voltage.  You either need a bleeder resistor to dissipate the energy or use the zener to clamp it.  Bleeder will be a permanent loss.  The Zener just dissipate at light or no load condition, so it is an efficient way to prevent excessive voltage at the output rail.

    Thanks,

    Youhao