Buck Converter Problem (temperature)

[FreshmanNewbie]

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 voltage (18V, 28V & 32V) and different load currents (No Load, 50mA, 100mA & 200mA) @ 25degC and the results were normal and as expected @ 25degC.

Toggle Switch :



Whereas, when I perform the same test @85degC, for 18V input voltage and 28V input voltage, the output is as expected. But when I keep the input voltage as 32V @ +85degC with no load connected at the Output, My IC got burnt (smoke came) and for the same brief moment, the output also had the same 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.

My hypothesis:

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 :

1. Why is the test giving proper results @ +25degC but failing @ +85degC?
2. 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 :

 

[Easy peasy]

there could be any number of internal design faults that have nobeen characterised or discovered at + 85 deg C - it may onlu be rated to + 70C - check the data sheet - the choke could be going funny at + 85, the list is endless ...

--- Updated Dec 3, 2020 ---

I see it is rated to 105 C - supposedly, at +85 - the pad on the bottom could easily be greater than +105 - depending on your heasinking - if there is any - so unless it has over temp shut down -> poof ...!