I designed a circuit using a SMPS voltage regulator based on the tps65261rhbr triple synchronous buck converter. The circuit is rated for up to 18V. It is connected to a 12V lawnmower battery, which also starts and powers a gasoline engine that returns charge to the battery with an alternator.
The circuit has worked perfectly over many hours of testing and switching power on and off before I connected and started the engine. Immediately upon starting the engine it failed: one of the 25V 47uF tantalum capacitors (TPSD476K025R0150) feeding the SMPS exploded.
My oscilloscope has a max of 10V, so I can't see what the input voltage waveform looks like when the engine starts and runs. I tried anyway, and I see that when the engine is started the voltage briefly dips under 10V, but outside of that brief blip it is clipped to 10V so I can't tell what's happening. I assume the voltage must have exceeded 25V for the capacitor to explode.
I'm considering switching to higher voltage (50V?) aluminum polymer bulk capacitors and a high input voltage rated 12V LDO before the SMPS to protect against input voltage spikes.
Does this seem like a good approach? Should I get a better scope or build a voltage divider to see what's really happening? Does anyone have any experience with powering circuits from an engine alternator and battery in parallel that can weigh in on this power supply design?