LT1683 (DC477A) Closing the loop

Hello,

I am currently in a design which involved LT1683.

I have checked the design of demo board DC477A which uses LT1683. 

I have implemented it but using my design parameters (48V - 24V@100W) but the behavior is not OK, even after simulation on LTSpice.

I don't understand the feedback loop (TL431, optoc., and VC Pin) in order to get a right compensation.

Could anybody give me some hints about how it works?

Why the FB pin is not used? How the Optocoupler works in this desing? Why you can compensate the loop in pin VC?

Thank you for your support. 

best regards,



Link to DC477A added.
[edited by: FCalvez at 8:55 PM (GMT -5) on 18 Dec 2020]
  • 0
    •  Analog Employees 
    on Jan 6, 2021 1:08 AM

    Hi, 

    The LT1431 in combination with an optocoupler on the DC477A provides an option for you to create an "opto-isolated feedback system". Take note that R18 is not populated in the demo board so it should not have an effect with LT1683 and its compensation. The LT1431 provides a stable reference on the output, while the optocoupler reflects the variation on the output to the error amplifier and provides isolation on the primary side at the same time. You can modify the demo board for it to work properly, but you should take into account its effect on the loop gain.

    Regards, 

    Fritz

  • 0
    •  Analog Employees 
    on Jan 6, 2021 1:22 AM in reply to FCGonzales

    Also refer to the table on the schematic of DC477A for the non-isolated and the isolated mode configuration. Thank you. 

    Regards, 

    Fritz