How much power IRFP460 can proccess?
The 280 watt figure is really optimistic .you would get this on an infinite heatsink that was held by some means at 25celcius.Now the fet would be running at Tj max of 150 celcius .This is all totally unreasonable .For good reliability 100 celcius is more acceptable  and its reasonable that your heatsink could be at say 60 celcius .If the heatsink were to be at 25 celcius it would have to be infinitely large.So we only have a differential of 40 between the junction and the heatsink now.If we use some insulater like a mica washer or silpad etc then this adds to the total thermal resistance between the junction and the heatsink.Well 1 degree celcius per watt seems a reasonable total figure.Therefore 40 watts of dissipation is acceptable and your SMPS if its worth its salt will waste less than this per device .In fact a good SMPS that has 2 switches should waste 10watts per switch.