To determine the rise and fall time dissipation it's generally easier to use V and I since the equivalent R is constantly changing during that time. Thus a simple linear approximation is to multiply the voltage times the ON current times the rise-time and then take 1/2 of that to get the average power. The switching dissipation is then this value times the ratio of the rise-time to the pulse period. Do the same calculation for the fall-time.When it is fully on then you have current squared times rdson. When the edge of a pulse is happening, the it goes through a range of resistances, instead of rdson, as the current is ramping from 0 to max or max to 0. You need to integrate one of the three power expressions, over the rise or fall time, i.e. V i or v squared over R or i squared times R. Then add the dissipation during the rise and fall times to the i squared rdson of the fully-on time.
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