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2 common questions on FlyBUCK

Patrick Zeng
Intellectual 1090 points

Hi team,

  My customer Midea is developing Isolated DC/DC soultion. I promoted LM5160 flybuck solution for them. It'll be appriciated if you can answer my below  questions regarding to flybuck:

  1. Why is the duty cycle has to be around 50%, best in 40%? For stability? Regulation? Or something else?

  2. What devices can be used in Flybuck topology. I know TI suggest LM5016/7, LM5160, etc, but from what I understand, as long as it's synchronous BUCK and also support low-side sink and source current limit, it can be used in Flybuck topolgy. Is my understanding right? 

  Thanks.

Regards,

Patrick

  • 0
    TI__Intellectual 1090 points
    Hi team,
    Any update?
    Thanks


    Regards,
    Patrick
  • 0
    TI__Intellectual 2320 points
    Hello Patrick,

    Apologies for the late reply.
    In the Fly-Buck topology, we generally suggest limiting the duty ratio to 50% in order to ensure you can maintain the regulation on the isolated side output. If you increase the duty ratio to more than 50%, you will just not have enough off-time (the only time available) to charge the secondary side output capacitors and the regulation will struggle.

    You can use any synchronous Buck to support the Fly-Buck topology as long as you have a peak current limiting capability in the part (followed by an current limit off-timer preferably). The valley current limit functionality will not work with the Fly-Buck because of of the impact of the leakage inductance of the Tx on the low side FET current during the SW node off-time. In most cases, the valley current limit will fail to trigger.

    Hope these answers help. Let me know if you have more questions.

    Regards,
    Sourav