Transistor Ptot (Total Dissipation) Practical Max

RAMBO999

Joined Feb 26, 2018
256
Just a quick one really

Take something like a 2N3904 transistor. A small signal NPN transistor.

The Ptot is given as 625 mW at Tc 25C.

When considering Ptot does this include the power dissipation of the CE and the BE collectively or just the CE?

Adjustments for heat sinks and ambient temperature apart, is there a good practice when it comes to picking a transistor with an appropriate Ptot?

What I mean is if your circuit demand turns out to be nW do you double it? Treble it?

Thanks

crutschow

Joined Mar 14, 2008
26,032
When considering Ptot does this include the power dissipation of the CE and the BE collectively or just the CE?
It's the total dissipation from both the BE and CE inputs or base current times base-emitter voltage, and collector current times collector-emitter voltage.
Note that this is the voltage and current at the transistor occurring at the same time, not necessarily the supply voltage.
if your circuit demand turns out to be nW do you double it? Treble it?
Typically you would want to dissipate no more than about 50-75% of it's maximum rating.
Note that you also need to derate the maximum rating for ambient temperatures above 25°C.

dl324

Joined Mar 30, 2015
12,255
When considering Ptot does this include the power dissipation of the CE and the BE collectively or just the CE?
It's both, but CE is going to account for most of it.
Adjustments for heat sinks and ambient temperature apart, is there a good practice when it comes to picking a transistor with an appropriate Ptot?

What I mean is if your circuit demand turns out to be nW do you double it? Treble it?
If it's nW, it's a don't care.

RAMBO999

Joined Feb 26, 2018
256
It's the total dissipation from both the BE and CE inputs or base current times base-emitter voltage, and collector current times collector-emitter voltage. Note that this is the voltage and current at the transistor occurring at the same time, not necessarily the supply voltage.
The voltage drop across the transistor not the supply voltage. Right?

Typically you would want to dissipate no more than about 50-75% of it's maximum rating.
Got it!

Thanks. That's what I was looking for.

RAMBO999

Joined Feb 26, 2018
256

dl324

Joined Mar 30, 2015
12,255
What does that mean?
Max power dissipation for general purpose transistors are going to be at least a couple hundred mW. If you only need nW, power dissipation capability won't matter.

crutschow

Joined Mar 14, 2008
26,032
What does that mean?
It means any practical discrete device will be able to dissipate mWs of power, so nWs are negligible.

RAMBO999

Joined Feb 26, 2018
256
Ahh! I see.

You misunderstood my nomenclature.

My use of the term "n" in nW was not to denote "nano".

I use it to denote any number of Watts in the algebraic sense.

Perhaps xW might have been a better choice.

Cheers.

Audioguru again

Joined Oct 21, 2019
2,405
I usually feel the temperature of a part with my fingers. I don't like to get burned so I keep the heating below 60 degrees C.
Then my fingers and my parts last forever.

Ian0

Joined Aug 7, 2020
1,158
I usually feel the temperature of a part with my fingers. I don't like to get burned so I keep the heating below 60 degrees C.
Then my fingers and my parts last forever.
If you can keep hold of it, it's 50 degrees. If you have to let go it's either above 60 degrees or it's live.

Audioguru again

Joined Oct 21, 2019
2,405
My body temperature is lower than normal. My resting heart rate is also lower than normal. My doctor says that then I will live to a very old age.
There are many people in old age homes. Many of them cannot walk anymore but they are younger than me. Today I was out walking my dog and we ran all the way home.