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LM5576 12v design - IC failure on plugging in power

Ahmad Qadomi
Prodigy 80 points
Other Parts Discussed in Thread: LM5576

Hi. I'm currently designing a 12v from 48v circuit. I bought "WEBENCH BUILD IT BOARD LM5576" and other components from digi-key. values and digi-key part numbers are on the attached excel sheet

6886.LM5576 12v design.xlsx

the input power is connected via a RJ45 connecter. and the load is a resistive test load of about 8.9Ohms (draw almost 1.33A). I assembled 6 boards and ran two of them very well for about 15 hours but then when i plug out and in the power they started to fail all of them. the failure is that the input becomes shorted and the input impedance become less than 10 Ohms. I tried to replace the IC for one board and the input impedance become several mega Ohms but when i plug power in the same problem occurred again.. I'm so confused. do you have a clue? is there an important notes I should be more careful about?

  • 0
    Prodigy 80 points

    I don't know if this help more to know the problem but i desoldered the IC and test the input pins 3,4 and i found out that they are shorted to pins 10,17,18 and 20 !!

  • 0 in reply to Ahmad Qadomi
    TI__Genius 10115 points

    With the part removed and still having all of these shorts, try check your soldering of the components and look for solder bridging between nets.

     

    Regards,

    Marc

  • 0 in reply to Marc Davis-Marsh
    Prodigy 80 points

    I checked everything and the circuit actually worked more than 1 time, but without a clear reason it fails on the next run or third or forth run. Why? I'm thinking that the RJ45 Connecter may cause voltage spikes or any disturbance that damage the IC. is that possible? and if it is, what could I do to protect the IC?

  • 0 in reply to Ahmad Qadomi
    TI__Genius 10115 points

    If you are hot plugging the 48V input with the RJ45, it is possible that the input voltage is ringing upon insertion.  The ringing can be as high as twice the DC value if this is the case.  One way to improve this is to add a large capacitor on the input of the device with enough esr to damp the resonant ringing.

    Monitor the input voltage and see what type of ringing you are getting.  Then look at the application note simple success with conducted emi, if you can measure the inductance of the cable you plan to use, you can use the equations in that document to pick a damping capacitor.

     

    Regards,

    Marc

  • 0 in reply to Marc Davis-Marsh
    Prodigy 80 points

    First of all, Thanks Marc for responding and for these useful information, I'll study it and apply it and then I'll tell you about the results.

    Secondly, I want to tell you a detailed description of the failure so i make it clearer to you. I'm using a 48v power supply that has an indication LED which is on as long as the power supply is on but it turns off if the output are shorted together, also the load is 8.9Ohm resistive load plus an indication LED, When i plug the power in to the LM5576 board the load LED, power supply LED both start to blink for some seconds and then the tantalum capacitor (16v Rated) blow up !!

    P.S. This problem isn't necessarily appears in first time plugging, and this problem never appears after running properly even after hours it just appears at plugging power in.

    Regards,
    Ahmad

  • 0 in reply to Ahmad Qadomi
    TI__Genius 10115 points

    Hi Ahmad,

     

    Can you monitor the output voltage during start-up to see if it is well behaved and not overshooting the desired 12V.  Tantalum capacitors are very sensitive to over-voltage and large in-rush currents.  Check the rise time of the output voltage and make sure that the current into the capacitor c dv/dt is less than the current rating of the capacitor.  If you don't know the current rating just increase the soft start time to something that is slower than the current setting.

     

    Regards,

    Marc