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LM5023: Ripple at output problem

Bruno Pescarolli
Prodigy 30 points
Part Number: LM5023
Other Parts Discussed in Thread: TL431

Tool/software:

Hi,

I have SMPS project , where I have 6.5V @ 1.5A output as shown in the figure below:

However, the output with full load (around 1.3A) has this behavior as shown in the figure:

Do I need to change the capacitors (C13 and C15), is there any calculation? Or will I need to change other chip parameters?
What is the best way to proceed in this situation?
  • 0
    Prodigy 30 points

    Other information about the project:

    Vin (65 to 285VAC)
    C_Bulk = 24uF 450V

  • 0 in reply to Bruno Pescarolli
    TI__Genius 12795 points

    You need to set up the feedback loop properly. Please see our EVM design as an example.

    There are PSpice models you can use to validate your circuit as well.

  • 0 in reply to Ning Tan
    Prodigy 30 points 
    Hmm, sorry, can you explain it to me better?
  • 0 in reply to Bruno Pescarolli
    Prodigy 30 points 
    I will provide more information.... about:

    1. This is Schematic:
     

    The output on 5,6 of transformer
    when it is without load and this behavior at the output occurs:
    But , When applying load, the behavior starts to happen, as shown in the two respective images:
    the bigger the load the worse it gets
    I increased the values ​​of Capacitors C13 and C15 and noticed that it decreased but I feel that this will not be resolved...
    I also noticed that on the VCC pin I measure the capacitor nearby, this has a similar behavior to the last image...
  • +1 in reply to Bruno Pescarolli
    TI__Mastermind 40580 points

    Hello Bruno, 

    The original responder Ning Tan is travelling and cannot respond easily. 
    I'll try to help you debug this issue. 

    I don't think C13 and C15 are involved with this issue.  (But I do think that C15 should be relocated in parallel with C13, and a low-value cap placed after the inductor L2.  

    Your output waveforms show symptoms of loop instability similar to feedback time delay.
    In your schematic diagram, I see two capacitors on the LM5023 COMP pin: C19 and C24.  The image resolution is low and I am not sure about the reference designator numbers or the values.  But C19 appears to be 47nF.  I can't see what value C24 is.  

    In any case, too much capacitance on the optocoupler output can delay the feedback signal, possibly leading to the symptoms seen.
    Please remove C19 from the circuit, and see if the output ripple is mitigated.  
    If C24 is > 1000pF, please remove it, too. 
    Loop compensation should be dominated by components at the TL431 regulator. 

    Also, I recommend to move R23 to across the optocoupler pins 1 & 2. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 30 points

    Hi , Ulrich 

    I remove the C19(47nF) and C24 (1000pF 50V) firstly ... At first the output continued with the behavior but moments later it stopped (there wasn't even time to measure the ripple) ....

    I observed ,that the VCC capacitor(C17) voltage dropped to 2.83V (very below the stop level), I did not change the resistor R23 (1K) and kept the same capacitors at the output (before and after the inductor L2)
  • 0 in reply to Bruno Pescarolli
    Prodigy 30 points 
    I managed to solve the ripple problems by changing the feedback capacitance, but the peaks still prevail... any tips?