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Step down converter protection (fuse)

m s2
Prodigy 50 points
Other Parts Discussed in Thread: TPS62132

Hi,

I'm a bit stuck with the selection of the right step down converter and a fuse circuitry. The step down converter has an input voltage of 12V and an output voltage of 3.3V. Of course, it is a good idea to fuse the input side of the converter in case it is damaged. But is it necessary to adjust the fuse size such that it blows when the output side of the converter is shortened?

For example, the TPS62132 has a short circuit protection where it limits the current to 1.6A whereas the maximum output current is 3A. Is it necessary to fit the fuse size on the input side of the converter to the output 1.6A? Or can I just leave the converter enabled even in case of a short on the output side?

Thanks!

  • 0
    TI__Guru**** 191445 points

    That 1.6 A current limit is only during start up or fault conditions when Vout is below about 0.5 V.  Normal current limit is 3.6 A min 4.2A typ 4.9 A max.  You don't typically have to use a fuse for output short circuit protection.  Under some fault conditions, the internal FETs may be damaged.  You may want to provide some fuse protection for that case.  It depends on the source current capability of your input supply.

  • 0 in reply to JohnTucker
    Prodigy 50 points

    Thanks for your response!


    The input supply offers 25A (hiccup short circuit protection).

    You say "Under some fault conditions, the internal FETs may be damaged". I plan to use TVS diodes on both sides of the converter to protect it. Is this what you mean?

  • 0 in reply to m s2
    TI__Guru**** 191445 points

    If for some reason the internal FETs were to short out (due to EOS damage for instance), the the IC would present a low impedance path from VIN to GND.  If your input supply can source 25 A, you may see the IC catch fire and you may also possibly burn copper traces on your PCB.  I would probably use a fuse on your input.

  • 0
    TI__Guru*** 147810 points

    A fuse is not required, unless your system requires it for some other reason.  The TPS62132 can operate reliably with its output voltage shorted.

    Only if the SW pin were shorted might the IC be damaged.  A short at Vout is ok.

  • 0 in reply to JohnTucker
    Prodigy 50 points

    Ah yes of course! So I can use e.g. a 5A fuse on the input side of the converter and I don't have to worry about short circuits on the outside of the converter? I mean, if a component on the 3.3V side is damaged it doesn't matter if the converter blows 1.6A through it, right? The component is already broken and the converter doesn't care. 

    Is this approach common practice?

  • 0 in reply to m s2
    TI__Guru*** 147810 points

    Yes, you can use a fuse if you want to but it isn't required.  Yes, the IC can output 1.6A into a short and not be damaged.

    In some applications a fuse is required by the application, so it's your decision.

  • 0 in reply to Chris Glaser
    Prodigy 50 points

    Thanks for your hints! What kind of applications do you have in mind? Microcontrollers etc. don't work with 0.5 V anymore...

  • 0 in reply to m s2
    TI__Guru**** 191445 points

    The 0.5 V is just an internal threshold for the TPS62132 for the higher or lower current limit.  The output voltage will ramp thru that part during start up and may fall to that level during overcurrent.  TPS62132 cannot actually regulate to that voltage, so there is no "application" for 0.5 V operation.  Hopefully this answers your question.