Let us understand from the example that how to choose MOSFET breakdown voltage from the datasheet, in our example we will be taking a MOSFET of part number STB30N65M2AG.
in the above picture you see breakdown voltage of 650V, so in general this could be use in application where the blocking voltage is maximum upto 650V.
Consider the above circuit in the first cycle the inductor is charged by turning on the switch, in the next cycle the inductor is discharged by turning of the switch, here comes the important part when the switch is turned off all the energy of the inductor is dumped into the switch, since the switch is closed the energy will not pass and will convert into voltage higher and higher depending on energy. so for choosing the MOSFET at this location maximum peak voltage should be known.
now you can say peak voltage in my circuit comes around 600V, so I can choose FET of 650V that should be fine voltage wise but the answer is no if your peak voltage is 600V than also 650V FET will not work why?
the answer before deciding the FET voltage rating you should know the maximum peak and the most important parameter is operating temperature at that peak.
usually the ratings mentioned in the datasheet is for 25degC, and if your circuit operating temperature goes down upto -40degC than you need take FET which has 650V at the -40degC.
for the example FET at -40degC gives voltage around 598V which is too low than the 600V.
How 598V ?
check out this characteristics at -40deg (x axis) points to around 0.92 factor (y axis), now multiply this factor with rated voltage which is at 25degC i.e. 650V*0.92 = 598V.
usually with drain current MOSFET voltage doesn't change very high but almost remain same.
if you see at 25degC factor is 1 which means 650V*1 = 650V at 25degC.
so next time for your worst case design choose the maximum voltage rating of your FET this way.
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