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Old 01-06-2009, 03:16 PM   #129 (permalink)
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Originally Posted by MazdaMatt View Post
Question regarding mosfets... (sorry, i'm a computer engineer, that's like electrical without any analog electronics but more programming and logic...) Their output current is relative to the input voltage, nay? Why PWM in a high voltage at a variable duty instead of provide a variable voltage? Is it something to do with the efficiency of the mosfets? Aren't there switching losses or something in PWMing a mosfet?
Great question! There's 3 pins. Gate, Source, and Drain. Source is connected to ground. When gate is near 0 volts (compared to source, which is ground), the resistance from Drain to Source is around 1 million ohms or something ridiculously big, so basically no current flows from Drain to Source. Once gate gets over like 4 volts, the resistance from Drain to source suddenly plummets to like 0.004 Ohms. That's when huge current (they can handle like 100 amps or even more!) goes from drain to source.

So, there is almost no heat loss in the mosfet when the gate is 0v (no current flowing from drain to source), and almost no heat loss when the gate is over like 4 volts, but when the mosfet is transitioning from off to on, or on to off, there is a huge heat loss. So, you want the switch to happen as fast as possible, to minimize "switching losses".

Usually, to turn on the mosfet, people set the gate to at least 12v (but less than like 18v), because the gate behaves like a small capacitor. You want to fill it up as fast as possible so the voltage from gate to source gets above the turn on point as fast as possible.

So, a mosfet is an on/off switch that HATES to be in the transition state between off and on.
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