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LM5071 with aux input, isolate or not

Ivan Dodig33
Prodigy 80 points
Other Parts Discussed in Thread: LM5071

Hi,
I'm designing a device which can be powered either by PoE or auxiliary DC input. Device has no other connections and is housed in a plastic enclosure. 

Device will be used in buildings for access control. 

I was thinking of using non-isolated buck converter and guess this would be fine if the DC input always comes from an isolated AC adapter. 

Can you please tell me if you think that I should always use isolated flyback if aux input is used? 

Thanks. 

  • 0
    TI__Mastermind 40145 points
    Hi Ivan,

    I recommend first consulting with your safety engineer. If you have control over what adapter is used in the system, you can use a non isolated solution. However, if you expect the end user might use a different (non recommended) adapter you might consider the flyback. When in doubt, isolated solution is recommended.
  • 0 in reply to Darwin Fernandez
    Prodigy 80 points

    Hi Darwin,

    since I don't have control over the adapter or power source I will isolate.

    I have couple of more questions

    1) In all reference designs of the LM5071 the aux power input is not isolated from the PoE. I'm presuming that these designs rely on isolated ac adapter?

    2) I suspect that my device will sometimes be powered from non-isolated power source connected to aux input (with Ethernet connected). I was thinking of connecting the aux power to the "isolated side" of the flyback and use an opto-coupler to drive the AUX pin of the LM5071. Is this the way to go or can you recommend other solution?

    What I want to accomplish is over-ride the PoE power if available when the aux power is present. 

    Thanks.

  • 0 in reply to Ivan Dodig33
    Prodigy 80 points

    After investigating further I would drive the UVLO pin low with an opto-coupler and connect only a pull-down on AUX as recommended.

    In this way the other power source should disable the LM5071 altogether.