Power Electronics

http://en.wikipedia.org/wiki/Pulse-width_modulation

basiclly its a one way of average value operate.

It works surprisingly well.

Think of your furnace (assuming you have the kind most of us do). It's either on or off. You get varying amounts of heat by how long and how often it's on - a kind of average.

Tom

I don't really understand why power mosfets are driven by PWM by going through much minipulation to get,(let's say) a "sine wave" output when it seems the mosfet could be driven directly by a sine wave without the spaces! Some advantages might accrue in addition to simplicity. ie. lower noise, fuller duty cycle, less C and L influences to deal with, etc., etc.

You're missing the point of PWM. You can drive a MOSFET or BJT with a sine wave input - the problem is that you are putting the transistor in it's linear mode. Power dissipation goes way way up. In PWM mode, the MOSFET is either off (zero power) or in saturation on (minimum power), so you can control large currents with a modest device, and generate less heat in the process.

Thanks for the great answer! Clearest explanation I've seen. Even I get it now.

The turn on resistance of a MOSFET is very low; less then an ohm, which dissipates less heat build up when on, which means less of a heat sink sq area needed, which means less of a foot print and/or PCB area needed for design.