Can anybody show me how to calculate the gate resistance(Rg) in a MOSFET ?

Thread Starter

mishra87

Joined Jan 17, 2016
1,034
Do you want to calculate the RB resistor value? And this resistor is in series with the gate terminal ?
I wanted to calculate the gate resistance value of MOSFET.

Means how to calculate gate resistance value of an MOSFET and what all basic parameters we need.

Still awaiting for comment .????
 

Thread Starter

mishra87

Joined Jan 17, 2016
1,034
The datasheet for FQ%2FFQAF5N90 mosfet shows 100 na leakage at 30 volts. That would mean the gate resistance IN the mosfet is 30V/100na = 300 million ohms.

I do not think this is the right calculation (30V/100na = 300 million ohms) for Mosfet gate resistance.

If Mosfet gate has very impedance so why do we need gate resistance.

I tried to find in goggle but i did not find please anybody guide me . ???

Thanks !!!
 

Thread Starter

mishra87

Joined Jan 17, 2016
1,034
If you are referring to the MOSFET internal gate resistance, I believe this application note gives the information you are seeking. See Section A4.
Thank you so much for your kind reply !!!

I still do not understand the importance of internal gate resistance .

I was not talking about Mosfet internal gate resistance .

I an externally connected series resistor "Rgate" connected to MOSFET Gate.
Thanks calculation is given in

upload_2017-1-12_19-56-53.png

Let me understand thoroughly .

Thanks once again !!!
 

OBW0549

Joined Mar 2, 2015
3,566
I still do not understand the importance of internal gate resistance .
It appears in series with (and thus adds to) any external resistance connected to the gate.

I was not talking about Mosfet internal gate resistance .

I an externally connected series resistor "Rgate" connected to MOSFET Gate.
You are making it extremely difficult to figure out just what you're asking: first, you want to know about the Rg parameter (the MOSFET intrinsic gate resistance); then you say you're not talking about that, but rather about resistance connected externally in series with the gate. Which is it-- internal, external, or both?

The importance of gate resistance, both internal and external, is that they interact with the MOSFET capacitances to form a low-pass filter which limits the rate at which charge can be delivered to (or extracted from) the gate, thus reducing its switching speed, with a consequent increase in power dissipation.

As far as I know, it's as simple as that. Perhaps one of our silicon specialists can elaborate.
 

crutschow

Joined Mar 14, 2008
34,280
Often a small external resistance is added to minimize gate oscillations due to the gate capacitance interacting with stray external wire inductance, but typically any series gate resistance is assumed to be so small that it is ignored.
 

Thread Starter

mishra87

Joined Jan 17, 2016
1,034
Thank you all for your support .

Actually I want to calculate that of gate resistance !!!

Please find the attachment ।।।

Thanks once again
 

ronv

Joined Nov 12, 2008
3,770
Attachment thread #11
I'm not sure if there is a calculation for it although it seems like it could be calculated.
I usually just use a value at least as large as what is usually posted in the data sheet. To try to actually calculate it you would need to know about all the parasitic inductance and capacitance associated with the pcba and lead lengths.
upload_2017-1-12_21-11-5.png
Edit: If you run at a high frequency you need to make sure you don't overheat your driver.
 

Thread Starter

mishra87

Joined Jan 17, 2016
1,034
I'm not sure if there is a calculation for it although it seems like it could be calculated.
I usually just use a value at least as large as what is usually posted in the data sheet. To try to actually calculate it you would need to know about all the parasitic inductance and capacitance associated with the pcba and lead lengths.
View attachment 118578
Edit: If you run at a high frequency you need to make sure you don't overheat your driver.

As per my understanding -

Input Capacitance Ciss of MOSFET which is given in datasheet and External resistance gate value Rg(to be Calculated).
T= RgC time constant,
Should be less than(10 times) to the rise time of MOSFET given in datasheet .

10*RgC<tr
That is how we can calculate Rg value .
 

ronv

Joined Nov 12, 2008
3,770

Motanache

Joined Mar 2, 2015
540
MOEFST has a considerable capacity between Gate and Drain. Order of nF
In DC does not matter.

At high frequency is felt the effects of this capacity.
This capacity represents an input impedance lower if the frequency is higher.

MOSFET must have now a large current large Gate current.
It is why MOSFET drivers provide Gate signal of 1A or 2A (eg IR2110)

However, to not force the MOSFET driver must put a resitor for limiting Gate transient current.

It depends on the Gate voltage growth rate (dv/dt) and gate-drain capacitance (Cgd).
 
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Motanache

Joined Mar 2, 2015
540
For a capacitor we have the following equation(definition):
C=Q/V
where V=voltage Q=charge and C= capacity

We have the following definition for the current intensity:
I=dQ/dt
where t=time

Ohm law: I=V/R (or I=U/R)
The voltage across a capacitor is first formula from here. Look at formula V= 1 / C * Integrate I(t)dt)



Now you have in post # 8:
dV/dt. So derivatives V as function of t and get:
(Intagete is the derivation reverse process):
dv/dt=I*1/C
but I=V1/R
so,

dV/dt=V1/R*(1/Cgd)
V1=Vdrv-Vgs
R=R1+R2+R3
And identify who are each: R1, R2, R3

imag1.gif
Sorry, to read CGD instead of CGS. Here:
http://electronics.stackexchange.co...e-miller-plateau-region-during-mosfet-turn-on

 
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