I'm helping someone debug a basic buck converter that explodes when plugged into a battery pack, and I'm wondering if anyone has seen the ringing/clipping shown in the attached picture before.
The buck converter board takes power from a 16 cell li-po battery pack at ~65V, has an output of 30V at 2A, and uses an TI LMR38025 converter IC. The board works fine when powered from a bench PSU at 60v, but the converter IC explodes and decaps itself instantly when the connector for the Li-po pack is plugged in. This has been reliably reproduced.
I suspect that the inductance of the battery pack wires and the buck input MLCCs (no bulk caps) are ringing up to double the supply voltage, around 130V, which exceeds the rating of the converter IC, so the electricity probably arcs over within the IC and causes the IC to fail.
I attempted to measure the ringing on the scope and found a really odd waveform (see attached). Instead of ringing, the voltage of the input caps goes up to about 110V and is clipped there for 400us, before falling down to slightly below the battery voltage, staying flat there briefly, then ringing and stabilizing to the battery voltage. My best guess for this is that the failed buck IC is clamping the voltage to 110V somehow. Although, when I tried applying 120V DC on the input of the buck, no current was drawn.
To fix this issue right now we are use an NTC inrush limiting thermistor, however this is bad for efficiency since it will burn 0.2 to 0.5W under normal usage. In simulation, a 75V TVS diode is unable to clamp the voltage to a safe level since the current is so high. I'm thinking either a P-FET soft start circuit before the caps or a hot-swap IC might be the only good solutions.
Does anyone know why might cause this weird waveform? Any other theories on what could cause this sort of instant failure? Any other suggested solutions?
Thanks!