I have this buck converter - BD9G201EFJ-M
Buck converter specifications:
- Input voltage - 18V to 32V
- Switching frequency - 300kHz
- Output voltage 9V
- Load current - 0mA to 200mA maximum.
I am performing a test where in, I disconnect the input voltage using this toggle switch and measure how long the output 47uF capacitor takes to discharge the output 9V. I performed this test with different input voltages (18V, 28V & 32V) and different load currents (no load, 50mA, 100mA & 200mA) at 25degrees celsius and the results were normal and as expected at 25degrees celsius..
Toggle Switch :
When I perform the same test at 85degrees celsius, for 18V input voltage and 28V input voltage, the output is as expected. When I keep the input voltage as 32V at +85degC with no load connected at the output, my IC burns out but (smoke came) and for a brief moment, the output was same as the input voltage of 32V.
I took out the IC and measured continuity between Vcc and LX pins of the IC, and the multimeter showed continuity. So, there is a short between Vcc and LX pins.
I think due to the high input dV/dt (dV - 32V to 0V & dt in the order of 100ms) at the input, I think the internal MOSFET between the Vcc and LX pins got damaged and that's why we are getting a short between those 2 pins.
My Questions :
Why is the test giving proper results at +25degrees celsius but failing at +85degrees celsius?
Is high dV/dt the problem? If so what actually happens when we give high dV/dt to the drain of the MOSFET while the Gate is turned ON (N-MOS)
Sample waveform @ +85degC with NO LOAD:
For an input voltage of 18V, I get the below waveform with no load connected at the output :