Hi all,
I'm hoping someone can comment on my problem. I have a simple buck regulator based on LM5008 which steps down 60V to 5V to power an MCU circuit.
Load at 5V side is low, around 15mA. Input comes from battery pack, which doesn't exceed 60V max.
I'm seeing failures of ceramic cap C7, which is C3216X7S2A225K160AB made by TDK, surface mount 1206 size X7S type.
Failure rate is below 1%, but several boards already failed with same exact results, which means a systemic problem.
Failed caps exhibit low resistance, which causes R1 to fuse, shutting down the circuit.
Failure happens after several weeks in the field, never during initial QC.
Circuit is permanently attached to a large lithium battery, so there should be no transients.
70V TVS is present to quench any transients, just in case.
I have a suspicion that a batch of caps of lower voltage rating were mistakenly used in manufacturing, but I don't know how this can be proven or even checked for.
SMD caps have no markings and look alike. Is there any way to test voltage rating?
For example, if a 50V cap was installed instead of 100V and system is running around 53V nominal, would a small number of caps eventually fail due to slightly exceeded voltage rating? I imagine actual voltage breakage has a range and variability to it, so maybe some caps would fail, while others don't?
Also, the nature of the failure where capacitor develops DC continuity over time, does it point to a voltage based breakdown or some other failure mode?
Any thoughts and feedback is appreciated.