With my firm we're trying to get the CE certificate for our device and one of the tests is the protection against power surges. For this, the test company is using a device that's connected to our device's 12V power input and gives +500V surge for 1.2 microseconds. For this test, we included a TVS diode parallel to ground to our schematic, and the final schematic looks like this:
D18 is an 18V TVS diode, SMBJ18A-HT. U12 is the voltage regulator,
In its datasheet, it's stated that SMBJ18A has a surge current of 20.5 Amperes, and a clamping voltage of 29.2 Volts. This looked satisfactory to our circuit, as our voltage regulator works between 5-36 Volts. In the actual test however, the TVS diode failed and shorted out, causing the power wires to burn and melt.
Is there any error on the choice of TVS diode for this purpose or should we use a different sort of protection for the 500V surge?