Electronic – arduino – What happens with Source and Drain when I use PWM on the Gate of a MOSFET

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I see some people using PWM to control the Gate of MOSFETs, what happens in the MOSFET when you use PWM on the Gate ? If I use Arduino to PWM the Gate of the MOSFET, will it control the voltage between the Drain and Source or it will only turn on/off very fast ?

Best Answer

If you pick the right N channel MOSFET for the load and you have the load in the drain up to V+ and the source connected to 0V then applying PWM inputs to the gate (with respect to source) will effectively cause the FET to act pretty much like a switch opening and closing. This is an approximation but for low switching frequencies it's not a bad one.

It's never as clean-cut as a switch of course but it can be reasonably approximated to one. When the "contact" closes it has "on-resistance" which can be as low as 1 milli-ohm on some FETs. When the "contact" opens there will be a little bit of leakage current but probably not much more than 10uA.

When it switches, it doesn't do so instantaneously and this is where there can be a significant amount of power loss. The "contact", over a few micro-seconds or in some cases a few tens of nano-seconds gradually changes from high impedance to low impedance. Parasitic capacitances make this worse generally and you need to "drive" the gate quite hard to achieve decent efficiency.

The space-mark ratio that you drive the gate with multiplied by the power voltage (V+) roughly tells you the average voltage applied to the load. Thus, if your supply is 12V and you drive 40:60, then the average voltage on the load will be approximately 7.2V i.e. the FET in "on" for 60% of the time.

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