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Author Topic: Resistor in series with MOSFET gate?  (Read 1270 times)
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Hey, this is just a quick question.

On another post, I was having trouble with a mosfet and a motor and I mentioned the fact that I just had the output line hooked directly to the mosfet. Someone recommended having a 100-ohm resistor in series with the gate. What is the purpose of that? Is it just to protect the arduino in case you connect the wire into the wrong hole on the breadboard (e.g. the drain of the mosfet instead of the gate?) I had one of those "inductive spike suppressing" diodes. I thought that the fact that the gate is isolated and controlled by voltage instead of current was the advantage of a mosfet over a regular transistor.
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Rapa Nui
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..what kind of mosfet do you use? the power mosfets might have between 100pF to 3nF input capacitance on the gate, so based on application the 100ohm in series may help or may not help.. smiley
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Hey, this is just a quick question.

On another post, I was having trouble with a mosfet and a motor and I mentioned the fact that I just had the output line hooked directly to the mosfet. Someone recommended having a 100-ohm resistor in series with the gate. What is the purpose of that? Is it just to protect the arduino in case you connect the wire into the wrong hole on the breadboard (e.g. the drain of the mosfet instead of the gate?) I had one of those "inductive spike suppressing" diodes. I thought that the fact that the gate is isolated and controlled by voltage instead of current was the advantage of a mosfet over a regular transistor.

The main purpose of a series gate resistor on a mosfet is to limit the current peaks the arduino output pin has to supply to the gate. A mosfet gate acts like a capacitor as seen from the arduino output pin, in that it only draws current to charge (to turn on) or dischage (to turn off) the gate capacitance. After charging/discharging to full on or off, there is no current flowing from or to the output pin as the gate is insulated from the drain and source of the mosfet, but there is real current flowing during the switching transitions of off to on and on to off.

That help?

Lefty
« Last Edit: August 19, 2012, 12:43:37 pm by retrolefty » Logged

Rapa Nui
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..therefore the power mosfets use drivers (ie special chips, transistors, etc) - as you need to supply a big current to charge/discharge their input capacitor fast.. when charging/discharging them slow (without having enough current) they may spend too much time in the "linear region" during the charging/discharging - thus producing a lot of heat (smoke) smiley
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Big power MOSFETs are quite large value capacitors (most of the capacitance is from gate to source, but some is from gate to drain which leads to issues at high-voltage operation.  The resistor protects the Arduino output pin from overload and perhaps also protects a little from spikes coming via the gate-drain capacitance from the drain. 

MOSFET specs always show a graph of total gate charge against gate-source voltage - its edifying to look at this as there is a plateau in the graph - this is all the charge needed to open up (widen) the source-drain channel as the drain current rises.  Typically charge might be something like 100nC.  To switch the MOSFET in 1us you'd need to provide 100mA - more than the Arduino can provide without risking the pin drivers from cooking. 

So with that resistor you'll be switching on the order of 10us perhaps - this allows PWM frequencies up to a kHz or so before switching losses become a definite issue.  Faster than that and a MOSFET driver (typically they provide 100mA to several amps to charge/discharge the gate capacitance in something like 50 to 150ns).  However such fast switching needs attention to circuit layout and stray inductance/capacitance.
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