Adjustable non-inverting Buck-Boost converter failing right after starting in boost-mode
I found the problem. I saw the huge 150MHz ringing on the SW2 node. Tried to add a snubber, but the ringing was still huge - up to 30Vpp. No wonder this burns the controller. Then I measured the signals on gates of the MOSFETs and the ringing was there. Since the drivers for the MOSFETs are pretty fast, the trace inductance and gate capacitance makes good resonator. To damp those oscillations I need to put the resistor in series:
The dead time between switching is 45 ns;
The max gate capacitance is 650 pf;
The driver voltage is +8V and I need only 6V to fully open the MOSFET;
So this gives me maximum resistance of ~50R, I put 33R and the ringing is gone.
The output reaches 51V and works perfectly.