[LT4293] Inquries regarding aux power, ground and miscellaneous things

To whom may it concern, 

Hi i'm HS.

I'm posting this in order to ask several questions about aux power and ground and miscellaneous things.

Q1. LT4293 needs Isolated DCDC, such as Flyback, Forward as well as ideal bridge diode and its controller(LT4321)

I would appreciate if you provide me with how the grounds should be connected to each blocks.

I assume LT4293 should be connected to primary ground tho.. i would appreciate if you clarify  which point should be divided into three different grounds(Chassis, Primary, Secondary ground)

 

Q2. I will populate the DC Jack as well as PoE as power input like the diagram below.

Q2-1. Would you recommend me proper choke filter(coil) for LT4293?

I used LT4295 in my application before and the datasheet recommended a choke filter for LT4295.

But i couldn't find recommendation for choke filter for LT4293.

Q2-2. In my application, The priority of DC JACK should be higher than PoE, which means PoE should be shut down when the dc adapter is connected.

Also, i think output of DC adapter doesn't need to go through the isolated DCDC when the DC adapter is connected because it will cause the power loss for nothing since output voltage of both dc jack and isolated DCDC is 12V. 

I read the datasheet of LT4293 and found out that LT4293 can be shut down by the voltage of AUX pin of LT4293 with two voltage divider resistors.

As you see the simplified power diagram of my application, DC jack output should be connected to AUX pin of LT4293 in order to shut down the device, but the problem is that the ground of DC jack should be on primary side while end point DCDC/LDO is on secondary side.

Is there any way to shut down the LT4293 and bypass the isolated DCDC when the dc adapater is connected?

Or would you verify if there is no much of power loss going through isolated DCDC from 12V(primary) to 12V(secondary)?

Q3. Is there any layout guide for LT4293 with respect to Isolated DCDC and LT4321?

Thank you!

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  • 0
    •  Analog Employees 
    on Mar 2, 2021 11:58 PM

    Good Day HS,

    The LT4321, LT4293, and the input side of the isolated DC/DC converter will have their ground pins connected. The output of the DC/DC converter will have a separate ground.

    About the choke filter, are you talking about this filter (boxed in violet)?

    As for bypassing the isolated DC/DC converter, It is not possible since the 12V DC jack has a single ground. Connecting this to both final DC/DC stage and the LT4293 will connect the two grounds of the system and makes your system non-isolated. Losses due to the additional DC/DC converter stage cannot be avoided due to the controller's quiescent current and the non-ideal components in the circuit, but passing the 12V through this will maintain the isolation between the input and output of your system. These losses can be minimized through proper design of the DC/DC converter stage.

    You can refer to the LT4293's datasheet regarding the layout considerations. Relative to the other connecting stages, I suggest to keep the power traces short and thick to minimize resistive losses, since the LT4293 will be in between the bridge rectifier and the first DC/DC converter stage. The datasheet of the LT4293 also mentions additional considerations when using it with the LT4321.

    Best Regards,
    VGregori

Reply
  • 0
    •  Analog Employees 
    on Mar 2, 2021 11:58 PM

    Good Day HS,

    The LT4321, LT4293, and the input side of the isolated DC/DC converter will have their ground pins connected. The output of the DC/DC converter will have a separate ground.

    About the choke filter, are you talking about this filter (boxed in violet)?

    As for bypassing the isolated DC/DC converter, It is not possible since the 12V DC jack has a single ground. Connecting this to both final DC/DC stage and the LT4293 will connect the two grounds of the system and makes your system non-isolated. Losses due to the additional DC/DC converter stage cannot be avoided due to the controller's quiescent current and the non-ideal components in the circuit, but passing the 12V through this will maintain the isolation between the input and output of your system. These losses can be minimized through proper design of the DC/DC converter stage.

    You can refer to the LT4293's datasheet regarding the layout considerations. Relative to the other connecting stages, I suggest to keep the power traces short and thick to minimize resistive losses, since the LT4293 will be in between the bridge rectifier and the first DC/DC converter stage. The datasheet of the LT4293 also mentions additional considerations when using it with the LT4321.

    Best Regards,
    VGregori

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