Grant said:
Hi there,
I put up MOSFET SOA charts here:
http://grrr.id.au/soa/ for three
devices: FQP33N10, RFP50N06 and IRFP4321PbF.
The third device is odd, a nice big TO247 power package, but lousy
low frequency to DC SOA. Compared to other MOSFETs, this one has
an extra, steep line added to the trace set.
What's the explanation?
Worth trying to see if these really can do better than 1A at 20V?
Why limited to 20W DC when they claim it's a 310W max power device?
150V at 78A (limited to 75A by package, hmm, not as bad as some IR
datasheets).
The maximum power is the maximum power at "absolute zero" that the package
can dissipate. It is not the expected power dissipation from the package in
standard operating conditions as that depends on too many factors for a
simple number to describe.
It has to do with thermal resistanc calculations and is a sort of ideal
power dissipation(all additional thermal resistances are 0).
Conceptually if you think of heat radiating from the junction to the outside
package it has to move through the device packaging. That packaging has
resistance which "slows down" the transport of heat energy from the
junction. If the termal resistance is very large the junction temperature
will rise to a very large number and of course will burn up.
If you add additional thermal resistances(heat sink, air, etc...) then it
only makes matters worse as the heat energy has more barriers to cross. By
giving a number that is independent of all those extra cases we get a pure
number that depends only on the device package. That number is pretty
consistent across all devices manufactured the same because of quality
control. Now when we want to go calculate the total dissipation in our case
we can do so because the thermal resistances will add as there will not be
any co-dependence between the different thermal cases.
Another way to think about it is that in the ideal case, that is complete
power absorbtion and removal from the device package, then the device
package can dissipate it's maximum power dissipation withthe junction
temperature being it's maximum.
Best to show an example I suppose.
2N7000 -
Maximum power dissipation = 0.4W @ STP
Thermal resistance - 312.5 C/W
(Note there is no junction to case resistance because one generally uses
these without any other thermal resistances. This should make sense as they
realized no thermal calculations would probably be needed since the devices
would almost all be used effectly the same. If you put this is liquid
nitrogen then you might get more power out)
312.5*0.4 = 125C.
This tells us that if we run the device at 0.4W we will get a junction
temperature of 125C, possibly it's maximum which will probably cause it to
burn up. The data sheet suggests that the maximum junction temperature is
150C so it probably will be fine to run at 0.4W.
The total thermal resistance is R_JC + R_CA = R_JA = 312.5C/W.
Hence what they are telling us here is that the maximum power dissipation is
pretty much what it says it to be. This is why many electrical enginners get
confused about power dissipation numbers because they treat them all the
same. In this case it so happens it is the same because they intended it to
be the same. That is, the maximum power dissipation is the maximum operating
power dissipation. If the ambient temperature changed then one could
calculate the new maximum operating power dissipation.
Now for a high power device one generally adds in thermal resistances that
effect the *maximum operating power dissipation*.
Take the IRFZ40 TO-220
Maximum continuous power dissipation @ STP = 140W
Maximum junction temperature = 175C
R_JC = 1C/W
Hence R_JC*140W = 140C
BUT THIS IS THE IDEAL CASE. This assumes that the heat dissipating on the
surface of the case is completely and instantaneously absorbed into the
ambient. If they could get the device into an absolute zero atmosphere then
they could run the device at 140W and it would produce a junction
temperature of 140C.
IN REALITY, we have additional resistances involved. If we do not use a heat
sink then
R_JA = 62.5C/W
and if we ran this at 140W then the unction temperature would be (62.5 +
1)*140W = 8890C!!!!! To make sure the device survives we can only run it at
140C/(62.5 + 1) = 2.2W.
Why is it much less than the idea situation? Because the air cannot draw out
the heat from the case quick enough so the junction temperature will
increase faster. 2.2W will produce a junction temperature of 140C if the
package is in ambient. Probably ambient with no convetion. If you used water
then you would get a different thermal resistance and could run it at a
different power level.
Suppose you needed to run it at 5W, then
140C = 5*(1C/W + X) ==> X = 27
This means you'll need to find something with a thermal resistance of 27 to
get that 5W power dissipation.
As you hopefully realize now, the maximum power dissipation is that which
brings the junction temperature up to it's maximum. It depends on all the
thermal resistances inbetween. By taking the ideal case, that is R_JA = 0,
they can get R_JC. This means you can do computations by simply adding the
thermal resistances.
In the 2N7000 they didn't need to an ideal case since the device was meant
to be used only in ambient air. For power devices one is meant to use a heat
sink and that adds an extra dimension. This requires a calculation and
without R_JC one cannot do it.
Ask yourself, what is the "maximum power dissipation of a device"? Can you
answer that? The answer is no. To many factors involved. Unfortunately in
the devices that are ment to run ambient they generally mean "maximum
operating power at STP an in ambient" and in power devices they mean
"maximum power dissipation at absolute zero". Hence the confusion when they
are using the same term with two different definitions.
In fact I don't even know why they need to give the maximum power
dissipation for powe devices at all since you never will use it in thermal
calculations. Of course They must give you either the ideal maximum power
dissipation or the maximum junction temperature and they will be related by
T = R_JC*P.
What you can do is know that if two devices or two different packages have
two different power ratings that the largest one will let you run it
"hotter". The difference between a 100W BJT and a 200W BJT effectively means
that the 200W BJT's package has 1/2 the thermal resistance than the 100W
BJT. This doesn't mean too much as your total thermal resistance will be
much larger than that of the package. That is, in your calculations you'll
have stuff like "small number + larger number" ~= large number. The small
number is R_JC. If you start using very efficient heat sink's and other
advanced cooling methods then it might start mattering.
If you look at most power devices in TO-220 their R_JA is approximately
60C/W. This is because the thermal resistance of air is much greater than
that of the case. R_JA = R_JC + R_CA ~= R_CA != 60C/W. it depends on the
surface area of the package so it can change significantly with the package
shape. This means most power devices can dissipate around 2W in air without
additional heat sinking.
