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Original question:

LM51581-Q1: Flyback design works in simulation but not in real

LM51581-Q1: FLYBACK converter working with only one Diode(SS34)-dutycycle is going max to min and seems oscillating.

Naveen Gupta
Prodigy 130 points
Part Number: LM51581-Q1

Tool/software:

Hello Experts,
https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1468092/lm51581-q1-flyback-design-works-in-simulation-but-not-in-real
This above thread explains my previous issue that in simulation the circuit was working, but in reality, at no load I was getting the perfect voltage, but even at light load the voltage was dropping.

The circuit and calaculation seem correct (as checked by Moritz in the above-mentioned thread).

So the solution for that was swapping the diodes (SR160->SS34) and my circuit worked perfectly as intended, but it had this weird switching waveform in which the dutycycle is going max to min and seems to be oscillating.
You can see in below video of SW NODE



But due to voltage limitation of SS34, I cant use that for whole range of voltage so i bought 
SS120L
SS115L
S210
SS510B
SS220B
SS110F 
These diodes, but surprisingly, none of them worked. I was seeing the same voltage drop issue?
I am confused why this is happening Any help would be alot.

Thanks and Regards,
Naveen

  • 0
    TI__Expert 4090 points

    Hi Naveen,

    Thanks for using the E2E forum.

    The attached video is for the no load condition? If yes, can you please attached the scope plot with the load?

    Regarding the output voltage drop, it can be possible that your converter is entering into a peak current limit at slight loads. Can you please attached the scope plot for the inductor current?

    And kindly also attach the scope plot for the measured secondary and Aux side voltages before the diode. 

    Best Regards,

    Hassan 

  • 0 in reply to Hassan Jamal
    Prodigy 130 points

    Hello Hassan,
    Thanks for replying,
    My apologies I didn't make it clear, but this video is with the 50% and 75% load.
    Which means 100mA on AUX (10V) and 75mA on S1,S2 each (15V,15V).(measured values were 9.45V(AUX),15.01V,14.85V(S1,S2))
    In No load condition the output on AUX  is 9.42V(measured via scope) and 15.2V,14.95V(S1,S2)(measured on scope).

    About the scope shots before diodes I will get those very soon and post it here.

    Thanks and Regards,
    Naveen

  • 0 in reply to Naveen Gupta
    TI__Mastermind 30200 points

    Hi Naveen,

    Thanks for collecting the requested scope shots.
    Hassan it out today, but will be available next week.
    At this time Moritz should also return to the office, so he might have a look as well as he is already familiar with the original design.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 130 points

    Hello Niklas,
    No issues
    Thank you for the information; I really appreciate it.

    Thanks and Regards,
    Naveen

  • 0 in reply to Naveen Gupta
    TI__Genius 15925 points

    Hi Naveen,

    Do you still use the same circuit you showed in the other thread?

    Or do you have the PCB built in the meantime? If yes, can you please share layout and schematic?

    Thanks and best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 130 points

    Hey Moritz,
    These test results are from the same type of prototype as earlier, but instead of wiring, I made proper solder GND Tracks.
    Here is picture of it 


    These are the ONLY connections on all the board.
    I have yet to start the testing with the PCB, but I don't think results would be that different.
    Here is the schematic I am using.
    Also the PCB layout, which I made but the testing with this PCB is yet to start.

    FLYBACK_LM51581_NEW.pdfFLYBACK_FRONT.pdfFLYBACK_BACK.pdf
    Please let me know your thoughts,

    Thanks and Regards,
    Naveen

  • 0 in reply to Naveen Gupta
    TI__Genius 15925 points

    Hi Naveen,

    Can you please try to use a non-schottky diode? In flybacks it is often better to use an ultra fast recovery diode instead of schottkys.

    For more investigations i would wait for you to test the PCB. I think this can make a lot of difference.

    Best regards

    Moritz