EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: BravoV on November 06, 2016, 06:43:54 pm
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Context : Switching the mosfet on/off, not in linear mode.
Questions :
1. Why sometimes at circuits there is a external resistor series to the gate, while others never placed ? Why ?
2. At mosfet datasheets, sometimes the Rg is never mentioned, is it safe to assume there is none ? And some its clearly stated at the spec sheet. Why ?
TIA
Example :
(https://www.eevblog.com/forum/beginners/whats-the-rule-of-thumb-for-mosfet's-gate-resistor/?action=dlattach;attach=268209;image)
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MOSFET may have a quite large gate capacitance and if driven for example with an op amp, the op amp may oscillate without the gate resistor.
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BravoV and the group,
When Rg is included in a MOSFET datasheet, they are sometimes they are talking about a resistor that is internal to the MOSFET. It is not a requirement for an external resistor.
In the datasheet you quote, the section of the datasheet concerns the switching speed when the MOSFET is being used as a switch.
In some circuits, for example a Dynamic load, the gate currents can still be very low, because of the resistor that is in the source of the MOSFET, the voltage on the gate changes by small amount.
For example if a MOSFET has a transconductance of 10, you only need to change the gate source voltage by 100mV for a change in drain current of 1A.
Regards,
Jay_Diddy_B
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These should help -
http://www.infineon.com/dgdl/an-937.pdf?fileId=5546d462533600a40153559ea1481181 (http://www.infineon.com/dgdl/an-937.pdf?fileId=5546d462533600a40153559ea1481181)
http://irf.custhelp.com/app/answers/detail/a_id/215 (http://irf.custhelp.com/app/answers/detail/a_id/215)
Regards, Dana.
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To switch on a MOSFET, you need to charge the gate capacitance. Effectively, you are charging a capacitor and all of a sudden you have an RC circuit.
A small resistance can act as a snubber and get rid of any second order oscillating effects you can get with the parasitic inductance of the wire connected to the gate of the MOSFET. It will stop spikes.
The trade off is that the transient period will take longer. You now have to wait however many RC time constants for your MOSFET to turn on.
A rule of thumb might be an ohm per nF if you like. But check that RC time constant.
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If you are performing switching then the function of a gate resistor is to a) give a better match to the gate to limit the amount of ringing due to parasitics and b) to limit slew rate/rise time of the gate (if you do not have a slew-rate controlled driver). As for the rise time you actually want the slowest rise time you can accept in terms of switching losses (due to spending more time in the linear region) because this will result in smallest voltage transients over G-S and lowest EMI.
Edit: As for the rule of thumb, I quite often put 10 ohms as a nice round number... It of course depends on your mosfet gate charge etc.
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slup169 blueskull mentioned earlier -
http://www.radio-sensors.se/download/gate-driver2.pdf (http://www.radio-sensors.se/download/gate-driver2.pdf)
Regards, Dana.
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Remember if you're driving a mosfet gate with a microcontroller, the instant you flip the pin high, the MCU is trying to output a logic 1 into a discharged capacitor. Without a gate resistor, you're temporarily shorting out the pin on the MCU. Some MCUs are fine with that, some aren't. If yours isn't, you better have a gate resistor to keep it happy.
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Thank you guys, thats a lot to read. :-+
I understand on the limitation of the driver's part as its probably not strong enough (low impedance) to fill the gate capacitance fast enough.
But say if its driven by mosfet driver, which is really capable to fill the gate capacitance really fast, as few mentioned about to limit slew rate/rise time of the gate, in this context as in switching as in power switching supplies, isn't this situation will create loss as longer rise time will make the mosfet dissipates more energy as it spends more time in linear zone ?
Or this will be a compromises for the pro and con between fast vs slow switching ?
Again, I'm not very sure if I'm asking this correctly, CMIIW.
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Rule of thumb for mosfet gate resistor:
10 ohms.