Power Parameter: Load, Heat, or Same Thing?

The answer will be different whether you refer to a resistor, transformer or transistor.

For transistors, I would expect to see maximal power dissipation in the table. I verified, that it's apparently the case for Digikey. Not sure if your question to Digikey clearly referred to transistors. If so, the answer is inappropriate.

You shouldn't however expect the comparison table to be more intelligent than the datasheet. There are often multiple specifications for different operation conditions, e.g. regarding different heatsinking. Power transistors sometimes specify a theoretical maximum power dissipation with fixed case temperatur (corresponds to infinite heatsink).

Thx for your reply.

FvM said:

The answer will be different whether you refer to a resistor, transformer or transistor.

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Indeed, it's for BJT transistors. Sorry for not mentioning that.

FvM said:

For transistors, I would expect to see maximal power dissipation in the table. I verified, that it's apparently the case for Digikey.

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In the Digikey table, i see "Power - Max", not "maximal power dissipation".

FvM said:

Not sure if your question to Digikey clearly referred to transistors. If so, the answer is inappropriate.

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Which answer? The support agent seemed to give two different, contradictory ones.

FvM said:

Power transistors sometimes specify a theoretical maximum power dissipation with fixed case temperatur (corresponds to infinite heatsink).

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By that, i assume "Power - Max" refers to watts the device can dissipate as heat, and not watts the device can deliver to the load. Correct?

That's surprising to me, since i see values in the "Power - Max" table as high as 300 watts. I'm surprised that an application would dissipate 300 watts as heat!

Generally the Max Power refers the absolute power the device can dissipate under some ideal condition, and has nothing to do with the power delivered to the load.
In practice, the maximum dissipation is often a lot less than that.

It is indeed confusing. We see something like this in Digikey:


So there are "Ta" numbers and "Tc" numbers. Some FETs have one, some have both.
The Tc number refers to the amount of power the device can dissipate when its case temperature (aka Tc) is held constant at 25C. Basically it assumes you have a perfect heatsink and the ambient temperature is 25C.
The Ta number refers to the amount of power the device can dissipate when the ambient temperature (aka Ta) is 25C, but it's mounted on some sort of standard fixture (depends on the package, but usually it provides very little heatsinking).

What you can actually dissipate is usually between Ta and Tc, but usually these numbers are different by an order of magnitude or more. But these numbers are useful for making comparisons between devices and packages.

By that, i assume "Power - Max" refers to watts the device can dissipate as heat, and not watts the device can deliver to the load.

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Device specification refers to what it can handle regardless of what is delivered to the load. Anyway, one should consider Digikey's parameter filter as a general guideline, not the spec itself. Note that for semiconductors most often datasheet give the SOA graph that depending on biasing values, will lie quite below specified 'maxim power'.

Thx for replies.

mtwieg said:

What you can actually dissipate is usually between Ta and Tc

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Unclear. Between what and what? Where Ta and Tc are both mentioned, only one wattage value is given.

andre_luis said:

SOA graph

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What does SOA stand for?

What does SOA stand for?

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Safe Operating Area, a visual specification available for most semiconductor device datasheets, particularly transistors.

mtwieg said:

So there are "Ta" numbers and "Tc" numbers. Some FETs have one, some have both.
The Tc number refers to the amount of power the device can dissipate when its case temperature (aka Tc) is held constant at 25C. Basically it assumes you have a perfect heatsink and the ambient temperature is 25C.
The Ta number refers to the amount of power the device can dissipate when the ambient temperature (aka Ta) is 25C, but it's mounted on some sort of standard fixture (depends on the package, but usually it provides very little heatsinking).

What you can actually dissipate is usually between Ta and Tc

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There is so much untruth here.

Tc is the case temperature at which the power dissipation is specified. It has nothing to do with holding it "constant at 25C". The manufacturer may specify 10 Watts at 25C or 2 Watts at 50C. It actually comes down to the Junction-to-case thermal resistance and the maximum junction temperature.

Knowing max Pd at Tc and your ambient temperature, you can determine the required thermal resistance of your heatsink.

"What you can actually dissipate is usually between Ta and Tc". This statement makes no sense.