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.

 

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.


View attachment 127301

Starting with the SOA. What is shown is the on voltage across the FET; hence the comment about value in this area limited by Rds on. For example at 100 amps times an on resistance of .01 ohm there will be 1 volt across the FET.
There are a lot of ways to get to the result you want, but don't forget that ambient is probably about 25C so it must be factored in.
So just an exercise using round numbers:
10 in parallel = .001 ohms X 100 amps squared = 10 watts of power in the fets.
So if you want the maximum temperature to be 85C you can allow the temperature to rise 60C (85 - 25 ambient)
Then you would need a heat sink with a rating of 6C per watt.