Hi.


I don't really know the maximum input voltage, and the operating temperature of your application.


Having said that I can tell you that in term of voltage (assuming we won't be above 440VAC) the mosfet should be fine. Also in terms of peak current it should be ok, because the max pulsed current is 1.2A and you are using a LNK304. Seeing that the current through the external mosfet and the LNK304 is the same, this should be fine. You may have a thermal issue though.


According to my experience, on a PSU of around 0.4W output power, and using a very similar schematic, I found the dissipation on the external mosfet to be between 0.4W and 0.5W at 440VAC. You can do something to improve this, by adding a 400V electrolytic capacitor between Q1 and U1. This will help because you will be reducing the RMS current through Q1 (instead of having a pulsed current you will tend to have a flat current). I would advise starting by testing at 2.2uF.


One thing you may need to check out is your inrush current during turn on of the PSU at very high input voltages. You may need to place some resistors to limit this.


I hope this helps.