I've read some answers about resistors used in MOSFET and found that 1kOhm to 100kOhm is used to take MOSFET off when MCU is powering and a resistor from 10Ohm to 200-500Ohm is placed between gate and signal to keep out oscillation.

I've a question about how much current there is on this two resistors and how to choose their power rate. And, resistor values can influence MOSFET speed?

My doubts are: Since resistor tell how much current go through it, pulldown resistors of 10kOhm and 100kOhm should consume 0.33mA and 0.033mA respectively. And 100kOhm should be best choice for low power applications where several MOSFET are used. For gate resistor between MCU pin and MOSFET gate I'll have a 100Ohm resistor with maximum current of 33mA. But MOSFET really drain current from my signal pin? and it's correct to calculate currents in this way on resistors(since there is voltage divider, I'll have very very low current on 10-100kOhm resistor? but MOSFET influence this current?). Don't knowing how to answer about this I don't know if I should use a 1/8W resistor or 1/25W resistor.

About switching speed, this resistors influence switching on load? For example, 400-500Khz and 1Mhz are influenzed? or 10Mhz?

I'm doing this considerations on a low voltage and current MOSFETS, like 3.3v gate and 5v Source-drain, up to 1A drain. Something like this as MOSFET www.ti.com/lit/ds/symlink/csd13202q2.pdf .

My main question is quoted in the upside, additive question is to explain what I'm on doubt. I'd like a more pratical example based on values on datasheet I've liked to better reuse this on other MOSFETS.

closed as too broad by Fizz, PeterJ, Andy aka, Daniel Grillo, nidhin Nov 30 '15 at 6:06

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  • I'm not surely exactly what you're asking, but there are a lot of intro appnotes on MOSFET driving... you should probably read at least on of those e.g. or for a much shorter one this and then ask a more focused question. The answer to your question is "yes" the driving circuitry, including resistors (gate and/or pull-up, pull-down) definitely affect switching speed. – Fizz Nov 28 '15 at 13:06

Think of the MOSFET gate as a capacitor. To turn the FET on, you need to charge this capacitor to about 2 V; higher voltages will further reduce the on-resistance of the FET.

The R is used to reduce the rate of charging of this capacitor. When you drive in steady state (i.e. after a 'long' time), the capacitor is charged to the driving voltage, and there is no current in the resistor.

Generally power dissipation in this R is not a critical issue -- it would only be an issue if you were driving FETs at 10's of MHz.

In some cases, especially when the wire between the driver and the FET is long, a smaller value (100 ohm) is used to prevent the long gate lead from picking up RF signals or feedback from the drain and triggering a high frequency oscillation.

I do not know what you are driving but Load and load type have a measurable impact on how a MOSFET operates. Remember the MOSFET is foremost a resistor.

When switching there is always a compromise, While the MOSFET is in transition it is in the linear region and can dissipate a lot of power depending on the load. You can use Ohm’s law to calculate this but Saber(software) works much better.

There is are implied capacitors gate source, gate drain, drain source. The thing you need to watch out for is (miller effect), primary the gate source capacitance. The faster you try to switch the gate the more current required to drive the gate. This current is required during transisition, I have seen amps flow into the gate on high speed circuits. Once the drive voltage is stable the gate will draw about nothing. The data sheet will give you the capacitance which you can then use to calculate gate current.

I normally do not use gate resistors but when I have to I try to stay between 10 and 50 ohms, the lower the better. As for the pull down (turns it off) the lower the better but I put that resistor on the driver side of the resistor, this keeps impedance lower to the gate. My preference is to use a MOSFET driver IC or a pair of NPN,PNP transistors connected base to base, emitter to emitter with the emitter connected to the MOSFET gate and the NPN collector to positive and the PNP collector to negative.

Depending on the MOSFET and load you may have to add a diode to protect the MOSFET from inductive kickback. MOSFETs can switch on or off in a few nanoseconds and will generate a tremendous voltage spike (opposite polarity when turning off and unlimited voltage until limited by external or internal circuitry). There is inductance in the connecting wire(s), the effect is dependent on speed and length. Look at the data sheet, it will be on at 3V but it will not reach its full capacity (enhancement) until the gate is typically a few volts more. There should be a chart showing the RDSon verses the gate voltage.

If you check the App notes you may find several I wrote.

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