IRFP4668PbF Suitability in application

[Vindhyachal.takniki]

1. I have a application in which mosfet is used as switch.
Max VDs = 84Vdc
Max Id = 100Adc continuous & 150Adc for 10sec
Load is motor
Used 12 IRFP4668PbF in parallel.
Vgs = 10V from system

2. I was calculating what max current I can pass in single IRFP4668PbF.
P(ja)max = [ T(j)max - T(a)max ] / Rja

According to datasheet
T(j)max  = 175C
T(a)max = 85C (in application max temperature can go to 85C)
Rja = 40C/W
Putting values in formula
P(ja)max = (175-85) / 40 = 2.25W 

Id = [ P(ja)max / Rdson at T(j)max ] ^ 0.5
Rdson at T(j)max = Rdson @175C = 3.1*9.7mohm = 0.03007ohm
where 3.1 is normalizeing factor for Rdson at T(j)max
Putting values in formula
Id = (2.25 / 0.03007) ^ 0.5 = 8.65A


3. Comes out to be 8.65A per mosfet,
I am using 12 in parallel so total current = 8.65*12
= 103.80A

4. Are these calculations correct?

5. How to select correct heat sink in this?

6. Also in datasheet Page 4 in SOA graph,
continuous DC graph looking at 10A current, Vds is only 8V, as marked red in pic attached.

Does that mean this mosfet can not work in my application as mentioned in point 1 like Vds=84Vdc & if Id/mosfet is 8.65A.

Infact it looks from graph that at higher vds like above 10V, mosfet cannot pass more current than 1A, as marked green in attached pic.

Or I am doing something wrong in interpreting SOA.

[danadak]

You have to ballast the design to minimize Rdson device to device variation
from taking too much current / power in a specific MOSFET


http://web.mscsoftware.com/support/library/conf/wuc96/08a_para.pdf


http://www.vishay.com/docs/71619/71619.pdf


If using them as a switch, but driven in a PWM application, or gates
being driving at significant rate then gate dissipation also has to be
accounted for.


http://ww1.microchip.com/downloads/en/AppNotes/00898a.pdf



Regards, Dana.