Transistors–MOSFET
13MAY
A MOSFET is a type of transistor that is voltage-driven instead of current-driven like the BJT. This type of switch is also capable of extremely high PWMspeeds and typically has very low internal resistance. Mosfets usually include aninternal protection diode. It is generally acceptable to interface theArduino directly to a MOSFET switch; this is one less part that must be added into the circuit.
The MOSFET pins are labeled Gate (input), Drain (output), and Source (common).
Because MOSFET switches are voltage-driven and consume very little current, it is not necessary to use a current-limiting resistor in series with the gate pin of a MOSFET (as with a BJT), but it is good practice to use a resistor from the gate to source pin to fully turn the switch off when not in use.
A normal MOSFET requires around 10v applied to the Base pin to fully turn on. Because driving anything above 5v with an Arduino requires using a level-shifter or amplifier, we use what is called a logic-level MOSFET for direct integration.
As the PWM frequency that is applied to the MOSFET switch increases, MOSFET needs a lot of available current to switch quickly. Although the
40mA that the Arduino PWM output pin can supply is plenty of current to fully switch a MOSFET on or off slowly, it is not enough to fully charge and discharge the MOSFETs gate-capacitor at high PWM frequencies where the MOSFET capacitor needs to be fully charged and drained 10,000 to 32,000 times
per second!
Using a MOSFET driver IC (specialized signal-buffer) is the best way to drive a MOSFET switch because it can provide much more current during each switching cycle than the Arduino is capable of. You can also omit the pull-up or pull-down resistors from the gate pin when using a MOSFET driver to control a MOSFET—instead you should use a pull-down resistor at each input pin on the MOSFET driver IC, being driven from an Arduino PWM output pin.
40mA that the Arduino PWM output pin can supply is plenty of current to fully switch a MOSFET on or off slowly, it is not enough to fully charge and discharge the MOSFETs gate-capacitor at high PWM frequencies where the MOSFET capacitor needs to be fully charged and drained 10,000 to 32,000 times
per second!
Using a MOSFET driver IC (specialized signal-buffer) is the best way to drive a MOSFET switch because it can provide much more current during each switching cycle than the Arduino is capable of. You can also omit the pull-up or pull-down resistors from the gate pin when using a MOSFET driver to control a MOSFET—instead you should use a pull-down resistor at each input pin on the MOSFET driver IC, being driven from an Arduino PWM output pin.
Parallel Mosfets
The total current that can be transferred through a parallel set of MOSFETs is equal to the amount of current that can be passed by a single MOSFET, times the number of MOSFETs used in parallel. In addition, the total resistance of the parallel set of MOSFETs is equal to the Rds(On) rating divided by the number of MOSFETs in the parallel circuit. This means that by using two MOSFETs in parallel, you decrease the resistance by half—and when the resistance is decreased, so is the heat dissipation
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