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LM5160A: Minimum Input voltage

Kuramochi Tadahiko
Guru 28715 points
Part Number: LM5160A

Hi,

Following is described on the datasheet p.24;

"The primary-side output voltage of a Fly-Buck converter must be no more than one half of the minimum input voltage."

Why is this restriction existed?

How is the device behavior if the output voltage overs one half of the input voltage?

Best Regards,

Kuramochi

  • 0
    TI__Mastermind 23815 points

    Hi,

    This restriction is due to the regulation of the flybuck on the secondary stage. 

    It is recommended to keep the duty cycle on the primary less than 50% such that the design can have a better regulation at the secondary. 

    If the output voltage is over one half of the input voltage, the regulation on the secondary will not be better compared to where the duty cycle is less than 50 percent. 

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Guru 28715 points

    Hi Arief-san,

    Could you tell me why the regulation of the flybuck on the secondary stage is worse by the duty cycle is more than 50 percent on the primary side?

    I need to know the principle.

    Best Regards,

    Kuramochi 

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 23815 points

    Hi,

    Here is some reading on flybuck

    It is mainly because the energy is delivered during 1-D time, therefore at duty cycle greater than 50% you have less time to deliver the energy, therefore the peak current tends to be higher. Now with the peak current higher, you have more losses too (leakage inductance, etc) and hence once you increase current on the secondary, you demand more energy and you have more losses on the path. 

    I hope the short explanation helps to answer the question

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Guru 28715 points

    Arief-san,

    Thank you for your explanation.

    I understood what you explained.

    Does it mean the reason that why regulation of the flybuck on the secondary stage becomes worse is increasing losses?
    Best Regads,

    Kumramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 23815 points

    Hi,

    Yes your understanding is correct. 

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Guru 28715 points

    Arief-san,

    I have an additional question.

    Duty ratio overs 50% in the case of VIN(min) on the below schematic from the datasheet.

    Why is this allowed?

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 23815 points

    Hi Kuramochi-san,

    The 50% duty cycle is a recommendation and it is not a hard restriction on the flybuck design. 

    If you want to operate at more than 50% duty cycle, then you will have to watch out for what is current on the secondary and how much secondary voltage will drop at maximum current. 

    Therefore you have to just be aware of the load regulation on the secondary when you operate at duty cycle greater than 50%

    Thanks

    -Arief