We have been working on a high voltage switching power supply design that needs to take 140V and step it down to 48V with infrequent bursts of current around 1A but normally drawing a few mA. We chose the LTC3779 as it has an example in its datasheet that's very close to our needs.
We have created a design that is similar to the datasheet's example but does not use the Boost side, so we removed those transistors. We've also chosen some different but similar components due to a high temperature requirement and availability.
We successfully simulated the design in LTSpice (the files are attached for anyone interested) and created a first prototype (schematic attached). However, testing the first prototype has not been successful. While it will run for hours at a time with a 60V input, we seem to randomly fry the LTC3779 IC. We've replaced it twice now and fried it four times on two separate boards (we need to order more devices). In addition to this, the no load input current is quite significantly higher (~55mA at 60V) than suggested by the simulation even at lower input voltages. We've had inconsistent results but when testing at higher voltages we seem to get run away heating in the inductor.
Any help would be greatly appreciated.