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WEBENCH® Tools/LM5160A: LM5160ADNTT with fly-buck topology

ITZIK MIZRAHI
Prodigy 210 points
Part Number: LM5160A
Other Parts Discussed in Thread: LM5160

Tool/software: WEBENCH® Design Tools

Hi forum,

i used LM5160ADNTT in its isolated configuration with fly-buck topology.

when i used webench, it was recommend me to choose Cout2 (the capacitor that switch to source during time TON) with the value 270uF, with 5mohm ESR and 5.8A Irms.

the ESR i can understand, the Irms value look over-rated (those two parameters didnt get some attitude on datasheet) and when i used the equation on datasheet to calculate the capacitence i get a value much more lower then the webench recommend me (6uF).

the parameters i was filled on webench are:

  • Vin = 8.5-65
  • vout = 5 +/- 5%
  • Iout = 1.5A

the papameters i was filled in the equation in the datasheets are:

  • Iout2 = 1.5A
  • (delta)Vout2 = 0.25
  • Vout1 = 3.8
  • Vin(min) = 8.5
  • fsw = 231kHz

it is not make sense and also make me to choose an exotic capacitor (very big and very expensive) 

BR,

Itzik

  • 0
    TI__Mastermind 49297 points

    Hi Itzik,

    If Vout1 is 3.8V and Vout 2 is 5V, then the turns ratio is (5V + 0.5V)/3.8V or 1 : 1.5. Thus, the secondary-side current is reflected by to the primary by a factor of 1.5. A secondary load current of 1.5A may result in a reflected primary peak current that exceeds the LM5160's peak current limit. I recommend checking this with the quickstart calculator (download it from the LM5160 product folder under "Design and Development").

    In terms of the output cap on the Fly-Buck winding, the higher cap was likely recommended based on the high ripple current and also cost. However, a few 47uF/10V ceramic caps is also a viable option.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 210 points

    Hi Tim,

    i did the quickstart calculator and filled by my values, what make me understand that the maximum current i get on Iout(secondery) is 1A (that is OK), but i still didnt understand, why does the webench recommand me to choose cout(secondery) so large when the the quickstart calculator tell 8.1uF minimum?

    also, how did you said "a few 47uF/10V", what is your assumptions?

    BR,

    Itzik

  • 0 in reply to ITZIK MIZRAHI
    TI__Mastermind 49297 points

    HI Itzik,

    It depends on your output voltage ripple spec and switching frequency. In general, more output cap implies less ripple and better transient response. 8.1uF is probably a bit low - hence my recommendation for ~100uF total (note also that the effective capacitance of ceramics reduces when voltage is applied).

    Regards,

    Tim.

  • 0 in reply to Timothy Hegarty
    Prodigy 210 points

    Hi Tim,

    8.1uF capacitor will give me max ripple of 101mV on the secondary output (by quick start, good for me).

    what about the Irms? there any note aboye it, how do i took it into account?

    BR,

    Itzik

  • 0 in reply to ITZIK MIZRAHI
    TI__Mastermind 49297 points

    HI Itzik,

    Note that there is a derating of ceramic effective capacitance with applied voltage, so I recommend using a 22uF cap or higher. The rms current shouldn't be a problem for a ceramic. You can use the output cap rms current equation for a flyback here if needed.

    Regards,

    Tim