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LM5001: Power management forum

Karan
Prodigy 100 points
Part Number: LM5001

i want to boost 4V @ 0.8A to 74V and i need isolated output. so Iam Using LM5001 in isolated flyback topology by connecting FB to GND and COMP to optocoupler feedback circuitary. My transformer turns ratio is 1:8 and Lpri = 53uH. When i turned on the supply, iam able to boost it to only to10V and when i probed the SW to GND, i see that its operating in full duty cycle ( Rt = 12k is used ). May i know if i can use this IC in isolated flyback  topology for mentioned voltage range ? ( if no, what can be better replacement ? ) if yes, where iam going wrong ? is my transformer turns ratio and Lpri sufficient ?  

  • TI__Genius 11215 points

    Hello Karan,

    Thanks for reaching out. Could you please share your schematic? 

    Kind regards,

    Elisabetta 

  • in reply to EM
    TI__Expert 8545 points

    Hi Elisabetta ,

    please find the attached image.

    I help customer to insert it.

    Thanks.

  • in reply to EM
    Prodigy 100 points

    PFA of schematic, I haven't fixed the R,C values in schematic. But on PCB i have used proper values. ( Iam using RT = 12K to 15K )

  • in reply to FRANK1
    TI__Genius 11215 points

    Hello Frank, 

    Thanks for your help. 

    Kind regards,

    Elisabetta 

  • in reply to Karan
    TI__Genius 11215 points

    Hello Karan,

    There are a few considerations that I would like to share with you:

    • With RT = 12kOhm you are switching at a very high frequency (around 1MHz), is this intended to be so? 
    • With these values of Vin, Vout and this transformer, the duty cycle is high (around 70%), which is usually something to avoid because it makes the compensation more complicated. I would recommend therefore to double check the compensation, and possibly change the transformer
    • Could you please provide the final schematic? I need to know the values of resistors and capacitors that you are actually using to check your design.

    I'm looking forward to your answer.

    Kind regards,

    Elisabetta 

  • in reply to EM
    TI__Genius 11215 points

    Hello,

    I haven’t heard back from you, so I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if this one has been locked due to time-out).

    Thanks,

    Elisabetta