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What parameters should I look on when comparing MOSFETs and JFETs?

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JimLo

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Dear Senior Engineers.

for FET, what are the following parameters?

thank you very much.


V(BR)DSS
(volts)
IDSS
(milliamps)
VGS(off)
(volts)
Transconductance
(kS)
rDS(on)
(ohms) PD
(milliwatts)
TJ
(C) Package Type Packing Method


if there are two MOSFET made by different manufacturer. how to compare these parameters? what else to consider?

every little help is appreciated.

thank you very much.....
 

vvvvvv

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MOSFET ? JFET ?

This is static parameters :

Rds responsible for power losses and heating MOSFET Ploss = Rds*Ids^2
Ids = maximum allowed current for your application.
Vds = maximum allowed Drain to Source Voltage
Vgs = voltage treshold for opening MOSFET.


When you compare two MOSFET :
Ids and Vds greater = better for you
Rds less = better for you

Dynamic parameters :
Qtotal = total gate charge
Cin = input capacitance
Cout = output capacitance
all less = good for you

Switch parameters :
Turn-On Time = time for open DS channel
Turn-Off Time = time for close DS channel
all less = good for you

When you select MOSFET :
see first that Ids and Vds of MOSFET
greater then required for application
second (for switching application)
times for on and off must be 5..10 times less then
maximum switching frequency
At last see for Maximum Power Dissipation (PD),
and thermal conditions of MOSFET. When MOSFET
open Ids*Vds must < then Maximum Power Dissipation,
but it's very roughly, i can't explain in two words how to be with power dissipation it's include number factors.

If you are beginner no matter who is manufacturer of
your MOSFET.
 

    JimLo

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mobile-it

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Re: MOSFET ? JFET ?

do you have a good tutorial on JFET's and MOSFET's ?

I am looking for this...

thanks
 

JimLo

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MOSFET ? JFET ?

thank you all for great helps

could i know that for a certain Vgs, will Rds remain constant? is it independent to the Vds?
 

vvvvvv

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MOSFET ? JFET ?

Rds strongly depend on Vgs.
If Vgs at level of treshold , Rds depend on Ids.
When Vgs over treshold about 2..4 V, Rds becomes independent from Ids (see datacheet on MOSFET
figure Rds vs Ids and Rds vs Vgs)

Rds independent from Vds because
ds=Rds*Ids ~ 0.01V..1V when MOSFET opened.
Such low level of Vds does not influence on Rds.

As example Vishay SI2335 MOSFET :
Vgs treshold =0.45V
When Vgs = 4.5V and MOSFET is full opened
Rds = 0.05 Ohm and constant when Ids = 0...15A
Vds max = 0.05*15 = 0.75V too small for influence on Rds
 

    JimLo

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