# Any Professional can write equations for IRF510 MOSFET for Id=30mA;Vo=5;Vdd=9?

1. Jan 12, 2012

### vco.asm

guys i had spend more times on how to determine the drain current exactly using
the prefer equations , but I failed i think the problem comes from extracting the correct
Kp (constant of mosfet)
any one can put here in details how to calculating + writing the procedures&equations
extremely; DATASHEET did not including Kp;Ids(on); only they mentioned Rd(on)
WHY;;;;;;;;;;;;;;;;;;;;;;;;;;???????????????????????????????????????????????????//

Last edited: Jan 12, 2012
2. Jan 12, 2012

### vco.asm

ok here my tries :=
IRF510 Datasheet including this INFO:
gfs = 1.3 @ 3.4 A and sometimes in other datasheet 1.3@4 A. & VgsOFF=4V
i do not know what if that gfs is same as Kp or gm ;
i assume it is gm ; thus:
as Id = Kn (Vgs - VgsOFF)^2 .............................. (1)
also
gm = 2 root (Id * Kn) then ---> Kn = (gm)^2 / (4 * Id)
apply this equation yiald: Kn = (1.3)^2 / (4* 3.4) = 124.5 mA/V^2

the required ( given ) parameters are : design MOSFET with Id=10mA with output = 5V
Vdd=9 V ; with using the voltage-divider ... here i want not to includes Rs ( resistor at
source ) .
as Vdd= Vds + Id*Rd
thus Rd= (Vdd - Vds)/ Id = (9-5)/10mA = 400 ohms
at this point all things correct right? theoretically YES yes yes
then as Vgg = Vgs
Vgg = Vdd * (R2/R1+R2) = Vgs
multiply both denominator and divisor by R1/R1 gives :

Vgs = Vdd * Rbb/ R1----------------------- (2)

for MOSFET :
for Id-= 10mA ; assume R1= 10K then
using Equation (1) yiald: Vgs = root(Id/Kn)+VgsOFF = Root(10/124)+4V=4.28 V
Assume R1 =10K
using Equations (2) and solve for Rbb yealds: Rbb = (4.28/9V) * 10K = 4759.9 ohms

and as 1/Rbb = 1/R1 + 1/R2 -----> thus R2 = 1 / ( 1/Rbb - 1/R1) = 1/(1/4759.9 - 1/10K)
thus R2 = 9083 =~ 9K ohms

THE RESULTs :
Id = 10mA
Vgs = 4.28 > VgsOFF (i,e MOSFET is ON )
Id = 10mA @ V0 =5v for Vdd = 9 V

The DISASTERs :
when apply the design on LT-Spice it is not success WHY WHY WHY ....