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LM3478: Boost Convertor Not working

K.V.Pavan Kumar
Prodigy 10 points
Part Number: LM3478

I have designed a boost DC-DC convertor circuit using LM3478. The input voltage is 12V,13A (via AC-DC Power Supply) and the output voltage 24V,5A. Circuit has been worked fine for few days and suddenly the convertor is not working and it is supplying the same input voltage at the output side (11.76V). I have checked the frequency and i am able to read 110kHz. I have also replaced the LM3478 and the Inductor but it is not working. Can you please guide us what could be the major issue in this,

Please find the schematic for your reference,

  • 0
    TI__Genius 9540 points

    Hi K.V. Pavan,

    Thanks for using the e2e forum.

    If the output voltage equals the input voltage, the device is probably not switching anymore.
    Right now, I cannot tell if the device got damaged (as you said the device worked fine before), or if the issue lies within the design.

    It would be really helpful to see some waveforms of the device behavior.
    Interesting signals to analyze would be Vout, DR, Isen, and FB.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 10 points

    Hi Niklas,

    Please find the attached signals acquired by using oscilloscopes. The out put voltage is not coming correctly in the oscilloscope, when I measured with multimeter it is 11.76V. 

    Please have a look at the graphs and let me know if you require any additional information to solve this issue.   

    Thanks 

    K.V.Pavan Kumar

    +91-9885277473

  • 0 in reply to K.V.Pavan Kumar
    TI__Genius 9540 points

    Hi K.V. Pavan,

    Thanks for the details.
    I will check through the waveforms and give you feedback earlier next week.
    Please note that there will be a holiday in Germany on Monday, so responses might get delayed.

    Sorry for this inconvenience.
    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Genius 9540 points

    Hi K.V. Pavan,

    Sorry for the delay.
    I took a look at the waveforms.
    It looks like the device is trying to switch, but on runs with minimum duty cycle.

    The voltage at Isense show very high peaks that exceed the overcurrent protection threshold.
    Therefore I assume there is too much current running through the FET to GND.

    In the spec you mentioned the application is for 5A Iout?
    Can you please check if the inductor falls into saturation?
    In such a case, the inductance will decrease and eventually becomes so low that the inductor behaves like a short-circuit, directly connecting Vin to GND when the FET is turned on. This would explain the large voltage spikes at Isense.

    Best regards,
    Niklas