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LTC3895 step down converter burns off
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Srijal97:
Okay so the new PCB was assembled yesterday and we did the testing today. We had a single oscilloscope of not-so-good quality and really bad probes, so we decided to connect it to just see the output voltage and not the switching node waveforms. The input voltage was given using 7*(12V, 30A) fixed output SMPS in series plus a 48V variable SMPS (set to 20V), also in series, giving a total of 104V. 3*(120Ohm, 10W) resistors were connected in parallel at the output as load, expected to draw a current of 0.8A. EXTVcc was used to power on the LTC3895 at 11.7V.

The first test was done as follows, can be seen in this video here: https://goo.gl/LvMbk6
1- EXTVcc turned ON directly to 11.7V
2- The Vin SMPSs turned on directly to 104V
3- Output of 32.7V Mean was seen on the output (at least we have something now, yay! ;D)
4- Vin turned OFF
5- EXTVcc turned OFF after a certain delay, at least 10-20 secs

Now after this test we started everything up again with the same steps; EXTVcc first and then Vin, but we had just a tiny little bump in the output as seen in this video: https://goo.gl/RzePpP, and after that nothing, 0V. Since then, in the further tests, we have absolutely nothing on the output.

What could have gone wrong? The main suspect right now are the input SMPSs. So we'll be using a 0-300V/2A supply and a thermal camera to observe the temperatures in some more tests tomorrow and try and measure voltages at specific points to know what went wrong. We'll also get a second board assembled and test that if this one does not work.

Edit: I have added the latest schematic and PCB images as well. C10 and C11 were not added/soldered.
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