Understanding power dissapation of MOSFET's

I'm currently trying to understand how MOSFET's work in total but come across confusing information when looking into power dissipation of the mosfet; mainly how to calculate it. Say I have a circuit that will consume 5A at 3.5V how would I go about calculating the power dissipation required for the MOSFET? Would I simply just do P=IA; and need a MOSFET capable of 17.5W of power dissipation? Or is more at play here?

• ...and what would be the voltage across (Vds) the MOSFET when a current of 5A is (Ids) goes through it? Feb 19 '15 at 16:32
• How are you using the MOSFET? As a switch for an external load? As a linear amplifier? As part of a switching regulator? Right across the power supply to see where it blows up? Feb 19 '15 at 16:33
• Using the pwm to control the MOSFET, as an external load switch. Feb 19 '15 at 16:40
• If you're using PWM then the main losses are the I^2R losses due to the RDSon of the FET, and switching losses during the transition through the linear region. The switching losses depend on the PWM frequency and how quickly the FET switches. The RDSon losses depend on your gate drive voltage and FET die temperature. Feb 19 '15 at 16:54
• i'll be switching at 490Hz which I've been told is not much for mosfet; and to clarify; R is RDSon is the I^2R? Feb 19 '15 at 16:56