Help with PNP Mosfet "Safe Operating Area"

Thread Starter

MechE74

Joined Jun 21, 2018
13
I hate to ask but I feel like I am going around in circles. This is my first time messing with a PNP mosfet.
Lets say I am looking to turn a resistive load on and off. Using PNP because like in many automotive applications I do not have the ability to control ground side.
For a hypothetical situation, lets say 12V, 15A. Driving with a gate voltage of 10V. Steady state, as demand could be to be on for 2 seconds or 30 minutes.
I randomly see this Vishay Si4497DY.
Vds=30V, continuous drain current rating of 19-36A depending on how you look at it. "Pulsed" drain current up to 70A.

upload_2019-2-8_15-29-20.png


Looking at Rds:
upload_2019-2-8_15-32-42.pngupload_2019-2-8_15-32-27.png

Rds@25C=~0.0027 ohm, then using the normalized curve when its hot its 0.0027*1.5=0.00405 ohm.
P=I^2*V, I get 15*15*0.0405=0.911W..

Looking at Rth:
upload_2019-2-8_15-36-31.png

If i use the Rth from the table, for <10s and Rth of 35, then the temp rise will be:
Trise=0.911*35=32 deg C, for a Thot=57 deg C.
If I use Rth from the note C, its "maximum under steady state" is 80C/W
Trise=0.911*80= 73C which is Thot of 98C.

So normally at this point I would think I am well in the good. For worst case condition of Rds and Rth it still does not go over 150 Deg C, or even near it...

BUT... Then I notice this pesky Safe Operating Area chart. Note, I added the red and blue lines..
upload_2019-2-8_15-52-21.png

If I go to the Vds of 10V, using the red line and go up to the DC operating line, then over to the vertical axis, I end up at like 0.16A?? Does this mean that if I have 12V on the source, my load on the drain, and the other end attached to ground, now this MOSFET can only supply 0.16A??? This does not seem right.
On the flip side, if I say OK what is the voltage this MOSFET can take at 15A, using the blue lines that puts me down at just over 0.1Vds??

I must be interpreting this incorrectly as the only place there is a CONTINUOUS drain current mentioned above of 20A is in the pulsed regions??

Anyone able to help set me straight here?

Many thanks in advance!
 

Attachments

crutschow

Joined Mar 14, 2008
34,285
Then I notice this pesky Safe Operating Area chart.
That's a P-channel MOSFET or P-MOSFET.
PNP refers to bipolar junction transistors (BJT).

You are confusing the supply voltage with the MOSFET drain-to-source voltage (Vds) which are not always the same since, when the MOSFET is ON as a switch, most of the supply voltage is dropped across the load, and when the MOSFET is OFF with a Vds of 12V, no current is flowing.

The safe-area curves refer to the MOSFET Vds and current that is being applied at the same time.
Thus for up to about 20A current, the Vds ON transistor voltage will be about 0.1V and the current can be applied continuously (to DC intercept of blue line).
Then, as long as the turn-on or turn-off time is <10ms, the transistor is below the safe-area limits (extending red line vertically from 12V to the 15A point).
Since the normal MOSFET switching time is a microsecond or so, you should be well within the safe-area limits.
 
Last edited:

Thread Starter

MechE74

Joined Jun 21, 2018
13
Thanks a lot! Its not clear, but it is clear-er haha!
So.. i was almost too embarrased to put this in my orginal post, but I am going to ask it anyhow.

So when the MOSFET is ON, is the voltage represented by the X axis in the safe area essentially the voltage drop across the MOSFET(ie, Rds*ILoad)?? Which is like 15*.004?? The other day I was thinking maybe thats what it was.... But then thought it was probably just wishful thinking. But that seems to be what you are saying.

PS thanks for squaring me on the terminology. I will quit using NPN and PNP in reference to Mosfets. Sorry Im an amatuer :)
 

crutschow

Joined Mar 14, 2008
34,285
So when the MOSFET is ON, is the voltage represented by the X axis in the safe area essentially the voltage drop across the MOSFET(ie, Rds*ILoad)?? Which is like 15*.004?
That is correct.
I will quit using NPN and PNP in reference to Mosfets.
No problem.

A BJT has two junctions in their construction with three layers of doped semiconductor, and the N and P refer to the primary charge carrier polarity (N is -electrons and P is +holes) of each layer (collector-base-emitter).

A MOSFET has no junctions, and is constructed of one type of doped semiconductor, either N or P.
So you can see that referring to a P-MOSFET as a PNP is a misnomer.
 

danadak

Joined Mar 10, 2018
4,057
Look at the thermal R as well. Its a small package, so it cant handle much
DC power. Its 80 C/W steady state, does not take much power to get to
150C Tj-max.

Regards, Dana.
 
Top