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LM51561H: First power on

TecnoLAB
Intellectual 400 points
Part Number: LM51561H
Other Parts Discussed in Thread: LM5155

Hello,

I power on for first time new DC/DC step-up from 24V to 100V.

At the moment load is 50kohm resistor, it will be an RC load, approssimately 200kohm in parallel with 100nF.

It seems working, I have 100V on output.

I found this signal after inductor, is it correct? why there is a secondary and third wave?

To improve efficiency and avoid EMC/EMI problems, I have to change inductor value and switching frequency?

Thank you a lot

Ch1 Vgate, Ch2 Vinductor

Ch1 Output (100Vdc), Ch2 Vinductor

  • 0
    Intellectual 400 points

    I found a problem: I can see that every 20-30ms switching stops for some ms.

    It make also audible noise.

    What can I check? In what way can I solve this problem?

    Thank you

    Plot of Voltage out of inductor:

  • 0 in reply to TecnoLAB
    TI__Genius 11215 points

    Hello, 

    Thanks for reaching out. I see that you have a cap from the bottom feedback resistor to gnd (C37), is there a reason for this big cap there? Adding this cap will slow down your feedback response.

    Furthermore, I see that C41 ad C42 are quite big there, did you check if this design has enough gain and phase margin? The audible noise coming from the inductor might be caused by instability or might depend on the inductor type that you are using. 

    Kind regards, 

    EM 

  • 0 in reply to EM
    Intellectual 400 points

    Hello EM,

    I added C37 1nF thinking it can help to reduce noise on FB pin, I tryed without C37 and it's still working well.

    I will check also C41 and C42.

    Noise was generated in another part of circuit, now I solved it!

    Thank you

  • 0 in reply to TecnoLAB
    TI__Genius 11215 points

    Hello, 

    Thanks for sharing this update. About the noise on the FB: usually in the case of a noisy feedback I would expect something in the pF range, otherwise the loop response might be significantly slowed down.  Please let me know once you checked gain margin and phase margin.

    Kind regards,

    EM 

  • 0 in reply to EM
    Intellectual 400 points

    Hello,

    could you help me? In what way I can check gain and phase margins?

  • 0 in reply to TecnoLAB
    TI__Genius 11215 points

    Hello, 

    Thank you for your answer. You can use the Excel quick start calculator at this link https://www.ti.com/lit/zip/snvc224 (the LM5155/56 share the same calculator) and type your desired values. The audible noise might be also caused by the fact that the converter stops switching because the Vout is increasing and then starts again, likely because your load is so small. Since the ringing on the inductor is quite high, I would anyway double check the compensation. Let me know if I can help you furthermore. 

    Kind regards, 

    EM