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LM2576 overvoltage on output

Kally
Prodigy 80 points
Other Parts Discussed in Thread: LM2576

Hello,

I'm wondering if the LM2576 gets damaged if a voltage higher than the set output voltage is applied to the output of the switcher? What is the allowed maximum voltage on the output of the switcher?

Thank you very much in advance.

  • 0
    TI__Genius 15810 points

    Hello,

    The output voltage is controlled by the feedback divider. Do you have an alternate way to power the load? Please send us your schematic. The problem arises when the voltage at the output goes higher than the input voltage. This can damage the device. The max voltage at the output is limited by the max duty cycle and the input voltage.

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Prodigy 80 points
    e.g.:
    Vin = 24V
    Vout = 12V

    Now I get a short circuit on the output to 36V.
  • 0 in reply to Kally
    TI__Genius 15810 points
    Please send us your schematic. Are you saying that while measuring Vout you see 36V?
  • 0 in reply to Akshay Mehta
    Prodigy 80 points
    e.g. this one in the upper left corner:
    www.ianjohnston.com/.../BenchPsuSchematic.pdf
    This is a PSU. Now if e.g. there is a short circuit to another power supply on connector X2-2 with a higher voltage than on this. Would this damage the LM2576?

    Thanks in advance!
  • 0 in reply to Kally
    TI__Genius 11085 points

    Hi Kally,

    When the output is short to a higher voltage than the input of the LM2576, the body diode of the internal MOSFET will turn on to conduct current from OUTPUT to VIN. That might damage the LM2576. The solution is to add a schottky diode from the OUTPUT pin to the VIN pin to redirect the current through the schottky, to protect the LM2576. You can also add a zener diode at the output to clamp the voltage. The zener voltage should be higher than Vout, say 16V zener with Vout = 12. Then the output voltage wouldn't be higher than the input voltage.

    -Yang 

  • 0 in reply to YangZhang
    Prodigy 80 points
    Thank you very much for your help! Also to Akshay Mehta!
  • 0 in reply to YangZhang
    Prodigy 80 points
    I have noticed that the schottky diodes have a big reverse current. Aren't normal silicon diodes more suitable for this application? I couldn't find a value for the body diode forward voltage in the datasheet of the LM2576, but I suppose that it's bigger than 1V. Am I right?

    Another question:
    Do you think the LM2576 is the right choice as a preregulator in the schematic? Are there better alternatives? I really like the way of lowering the output voltage dynamically by changing the feedback voltage.

    Thank you very much in advance.