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TO-3 regulator failure modes

Jonald Cousins
Prodigy 20 points

Hi e2e Lin Reg Forum,

     I've read about and witnessed a typical failure mode of the TO-220 package voltage regulator in which it keeps signaling, but with an output that is elevated above its normal regulated specification.   This obviously can damage connected devices, if the high output of the failed regulator is above their abs. max. tolerance...  

     Is this a typical failure mode for all linear regulators - even those with the TO-3 package?    I'm considering retrofitting an old power supply with crowbar protection circuits and wanted to make sure that this was not going to be unnecessary, since I don't want to disturb the original circuits otherwise. 

Thank you...

  • 0
    TI__Guru 55940 points
    Hi Jonald,

    Do you have a particular device that you are working with?

    Unfortunately since device failure modes are dependent on the conditions that lead to the failure, it is difficult to answer a general question like the one you are asking. We simply need more information about a failure in order to effectively debug an application.

    That being said, the most frequent cause of an LDO outputting above the set voltage is due to an an unintentional external leakage path biasing the output voltage high. Due to the topology of most LDOs, it is only the load that will pull a biased high voltage back within regulation (LDOs do not generally sink current).

    Very Respectfully,
    Ryan
  • 0
    TI__Mastermind 22610 points

    "... I've read about and witnessed a typical failure mode of the TO-220 package voltage regulator in which it keeps signaling, but with an output that is elevated above its normal regulated specification ..."

    Without knowing more details it's impossible to give any concise reply.

    Ryan's answer is valid for the most common question/complaint that we get today.

    If this is a simple application, with a single 3-terminal voltage regulator, no secondary supply, no battery back-up, etc, then I'm guessing that this failure mode could be described as 'Vout= Vin'. This generally indicates damage to the pass element (Vout to Vin), and/or damage to the control loop feedback (Vout to GND), or internal loss of the Ground Reference, such that active voltage regulation is not possible.

    For any given linear voltage regulator, where both a TO-3 and a TO-220 package is offered, the internal silicon die is almost always the exact same.

    If the failure is due to electrical over-stress, violation of ABSMAX ratings, excessive reverse current, reverse voltage bias, ESD, etc, etc, there is no reason to believe that simply moving to another package type would solve the root electrical over-stress problem.

    If the TO-220 failure is due to a  mechanical mounting issue (i.e. over-torque of the TO-220 attachment screw to the heat-sink) where the lead-frame is distorted and the silicon die cracks after several thermal cycles, or bond-wires break, then moving to another package type might solve a mechanical mounting problem.

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Thanks for your replies, Mr. Jones and Mr. Eslinger.   I will go ahead and implement the crowbar circuits in the multiple-PSU.