how to figure how much current the gate of a mosfet will draw

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Rscott9399

Joined Jan 13, 2017
51
As stated, i am trying to understand the loading effect of a mosfet gate.
Obviously the impedance is very high but if you wanted to calculate how much current the gate of a mosfet will draw how does one go about that?

Thanks
 

Papabravo

Joined Feb 24, 2006
14,410
Almost none, at least in the steady state. There is no DC path to anywhere inside the chip. It just charges up a capacitor in a very non-linear way. The more current you source the faster the capacitor charges. The closer you get to the maximum voltage, the less current the gate draws and the slower the voltage moves toward the asymptotic limit. The datasheet will give you a Gate Charge number, usually in nano-Coulombs that you can use to calculate the switching times. Also gate capacitance is very non-linear with gate to source voltage so those spec are almost useless.
 
Last edited:

crutschow

Joined Mar 14, 2008
25,417
So the amount of current required to turn the gate on and off depends mainly upon the Gate Charge value for the particular MOSFET you are using and how fast you want to switch the MOSFET.
For fast switching of large MOSFETS, the peak gate current can exceed 1A.
 

ErnieM

Joined Apr 24, 2011
8,055
While the steady state current is essentially zero the gate current during switching can be quite significant.

Consider a simple grounded source configuration used as a switcher. To turn the device on or off you must charge the gate to source capacitance to move from the active to inactive regions. That is the easy part, as you can directly use the value from the device spec sheet.

How about gate to drain? Yep, there is capacitance there too, but it doesn't sit so nicely at the driving port. This capacitance gets amplified by the "Miller Effect" of the drain voltage moving in opposite sign to the gate voltage such that the effective capacitance is much larger that the inter terminal capacitance itself.

Its not a simple calculation.
 

TeeKay6

Joined Apr 20, 2019
572

ronsimpson

Joined Oct 7, 2019
923
how does one design a gate drive for high current if you need high speed turn on?
What signal do you have? From a micro computer? What voltage levels? more information
"High Speed" frequency? more information
Do you want one 8-pin IC to drive the MOSFET or do you want to build it from transistors?
 

Papabravo

Joined Feb 24, 2006
14,410
interesting, so then how does one design a gate drive for high current if you need high speed turn on?
The simple answer is that you make a voltage source with as high a voltage as the gate can stand with a very low internal resistance. However it is more complicated because your load consists of multiple non-linear capacitors whose value changes with time. You should look at gate turn on and turn off waveforms and ask yourself why they look the way they do. The answer is in the movement of charges.
 

crutschow

Joined Mar 14, 2008
25,417
how does one design a gate drive for high current if you need high speed turn on?
The easiest is to buy a gate drive IC that's dedicated for that purpose.
Depending upon the MOSFET and the speed required, an NPN-PNP emitter-follower push-pull driver may also suffice.
 

dl324

Joined Mar 30, 2015
11,518
Obviously the impedance is very high but if you wanted to calculate how much current the gate of a mosfet will draw how does one go about that?
The gate of a MOSFET behaves like a capacitor. The voltage on the gate will rise faster when the gate capacitance is charged with a higher current. How quickly (or slowly) the gate-source voltage changes affects the turn on time.
 

Jony130

Joined Feb 17, 2009
5,182
As first approximation you can use this:
From the current definition:
I = Q/t

Ig = Qg/t = Qg*F where Qg is a total gate charge.
 
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