VIPER26K device fails when we tried to measure VDD pin voltage with respect to power ground
We designed a buck converter with (300-800) input DC voltage and the output voltage is 24V. The max load current is 150mA. We noticed two issues here:
1. The device fails when we try to measure VDD pin voltage with reference to Power Ground (DC input ground). Though the VDD voltage is with reference to device ground (pins 1,2,3,4,6), but why it should fail when we use power ground as a reference with a Digital Multimeter. This has happened thrice and in all the cases, the device failed silently (no sounds, no physical damage etc.,). All this happened at 150V DC input voltage itself.
2. In the second case, the device failed at 800V DC input when we tried to connect 100mA resistive load. No probe is connected anywhere in the circuit. However, we used the same compensation network as given in ST AN 5380 (15V, 1.5W buck converter and could not arrive at the values for 24V output voltage. If the compensation network is a problem, can someone please help us in arriving at the correct values for compensation circuit.