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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 ?

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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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The gate of a MOSFET is basically a capacitive load. The FET's drain current is controlled by the gate to bulk voltage. For discrete FETs the bulk is often connected to the source terminal internally.

This means the problem of driving a FET gate can be modeled as charging a capacitor. So you can archive both cases described in your question.

If the PWM output can drive large currents the FET gate can be charged quickly and it will turn the drain current on or off with a high slope. This is used for example in switch mode power supplies.

If the PWM output can only drive low currents, the gate capacity together with the line resistance will form a low pass filter. You can increase this effect together with a resistor from PWM output to gate, to implement a very cheap form of digital to analog converter. The duty cycle of the PWM controls the average gate voltage. For this mode you must consider there is a minimum gate voltage (threshold voltage) to archive an effect on the drain.

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Controlling the voltage between the drain and source (\$V_{DS}\$) and turning on/off is the same thing.

When \$V_{DS}\$ is at its minimum:

  • channel impedance is at minimum, \$R_{DS(on)}\$ in the datasheet
  • channel current is at maximum, usually limited by the load connected to the transistor
  • gate voltage is significantly higher than the threshold voltage \$V_{GS(th)}\$
  • the transistor is said to be on

When \$V_{DS}\$ is at its maximum:

  • channel impedance is at maximum
  • channel current is at minimum, some very small leakage current specified in datasheet
  • gate voltage is below the gate threshold \$<V_{G(th)}\$
  • the transistor is said to be off

In most PWM applications, it is desirable to have the MOSFET on or off, but not something in between. This is because power is the product of voltage and current:

$$ P = VI $$

When the MOSFET is on or off, there is high current, or high voltage, but not both. Thus, the power in the MOSFET is low, it doesn't get hot, and less energy is wasted. When the MOSFET is between states, there is significant current and voltage, so power in the MOSFET is high, it gets hot, and battery energy is wasted.

The switching speed is limited by the gate driver's ability to sink or source current to charge or discharge the capacitance of the gate. There are, of course, some applications where a constant voltage between minimum and maximum is desired (for example, linear amplifiers), but these must be prepared to deal with the power load and heat that results.

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