Book Suggestion - How to calculate the power dissipation of a BTJ

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luiseduardo

Joined Jun 5, 2017
5
I did a exam that asked for "power dissipation" of a given BTJ circuit. The question was simple, but my professor considered my calculation of dissipated power wrong only because my answer was P = Vce.Ic + Vbe.Ib, but he told me that the correct answer is only P = Vce.Ic, so can someone indicate a textbook or a scientific article confirming my answer so I could talk to him again ?

I found some links on the internet confirming my answer, but he would not consider a websites...
 

OBW0549

Joined Mar 2, 2015
3,566
Your answer is correct.

Your professor's answer, while not necessarily very wrong (because the Vbe * Ib contribution is usually small compared to Vce * Ic), is wrong nonetheless.

To me, this is so blatantly obvious that I doubt you're going to find it mentioned in most books-- it's just common sense.
 

WBahn

Joined Mar 31, 2012
26,398
Ditto.

The fact that you have a professor that thinks your answer is wrong is actually quite troubling. It likely implies that he is a formula monkey that doesn't understand basic concepts.

You might ask your professor how the base current, though dropping through a potential of about 0.7 V, manages not to dissipate any power.

You might also set up a simple circuit with a battery, a base resistor, a collector resistor, and a transistor and solve for the base and collector currents. Choose the values so that the collector-emitter voltage is about 0.5 V (i.e., close to saturation but still in the active region). Choose a power transistor so that you can use a fairly small beta of, say, 25. Choose a small battery voltage of, say, 1.5 V (so that the transistor is dissipating a significant fraction of the total power. Then show that the energy balance doesn't work if you use his equation but that it does work if you use yours.

You can also set up a circuit that places the transistor in deep saturation such that the base-emitter power dissipation is many times the collector-emitter dissipation to further emphasize this.

As OBW said, this is something that is so obvious that most texts aren't going to mention it -- they assume that if you have gotten through working with power in passive circuits that you know the basic principles involved. I didn't find anything explicit in either Sedra & Smith or Horenstein, but I did find the following generic statement in Horenstein:

"The electrical power is equal to the product of the port voltage and port current. For a multiport device, the total electrical power input is given by the sum of the input power taken over all ports." - Horenstein, Mark M, "Microelectronic Circuits and Devices", Prentice Hall, 1990, p210
 

WBahn

Joined Mar 31, 2012
26,398
To be a bit more explicit using that quote from Horenstein, the input power is positive if the port current is entering the port and negative if it is leaving.

For the typical assignments on an NPN transistor, you have base and collector currents entering the device and emitter currents leaving the device, hence the total power is

\(
p_d \; = \; i_b \cdot v_b \; + \; i_c \cdot v_c \; - \; i_e \cdot v_e
i_e \; = \; i_b \; + \; i_c
p_d \; = \; i_b \cdot v_b \; + \; i_c \cdot v_c \; - \; \(i_b \; + \; i_c\) v_e
p_d \; = \; i_b \(v_b \; - \; v_e\) \; + \; i_c \(v_b \; - \; v_e\)
p_d \; = \; i_b \cdot v_{be} \; + \; i_c \cdot v_{ce}
\)
 

dl324

Joined Mar 30, 2015
14,303
I found some links on the internet confirming my answer, but he would not consider a websites...
Some teachers don't really know the subject matter they teach and they just parrot what they were taught.

For mental calculations, or estimations, ignoring Vbe.Ib is okay. I can't think of a time where it mattered for anything I've designed.
 

MrAl

Joined Jun 17, 2014
8,863
Hello there,

The fact is that depending on the context of this question the answer could be either just the Vce*Ic contribution or the sum of the base emitter power plus that.

The reason Vce*Ic can be said to be correct could be because of the dogma of the course as it progressed so far. In this case the professor sets the rules and the student has to follow that like the Bible. There is sometimes a good reason for this, other times there may not be.
One good reason would be that the students were working with transistors that have a high Ic/Ib ratio thus making the base emitter power insignificant for all the experiments they did up to that point, so the point of the lesson would be to see that the student knows enough to ignore the base emitter power rather than include it. So the student has to follow the dogma like it or not. The goal is a savings in time.

Of course outside of the classroom we know that the two powers summed is the *better* answer, but it's not the only answer.

What i would do is try to talk to the professor and ask him if the lesson was all about knowing to ignore the base emitter power, and then suggest that sometimes it may be good to know.

As another example, many years ago it was considered proper to ignore the gate power in the power MOSFET. This changed once gate drivers became in widespread use because the gate current was much higher then and needed to be added in order to properly calculate the efficiency.
 

WBahn

Joined Mar 31, 2012
26,398
Hello there,

The fact is that depending on the context of this question the answer could be either just the Vce*Ic contribution or the sum of the base emitter power plus that.

The reason Vce*Ic can be said to be correct could be because of the dogma of the course as it progressed so far. In this case the professor sets the rules and the student has to follow that like the Bible. There is sometimes a good reason for this, other times there may not be.
One good reason would be that the students were working with transistors that have a high Ic/Ib ratio thus making the base emitter power insignificant for all the experiments they did up to that point, so the point of the lesson would be to see that the student knows enough to ignore the base emitter power rather than include it. So the student has to follow the dogma like it or not. The goal is a savings in time.
Even if that is the case, it is completely improper to tell the student that including the base-emitter power is wrong, because it isn't. If they want to tell the student that it is negligible and perhaps even ding them a bit for failing to recognize the useful simplification if that was one of the goals of the question and the context of the question made that clear. But to tell them that the answer is straight up wrong sends the wrong message -- it leaves the student thinking that the base current does not contribute to the power dissipation in a BJT.
 

OBW0549

Joined Mar 2, 2015
3,566
But to tell them that the answer is straight up wrong sends the wrong message -- it leaves the student thinking that the base current does not contribute to the power dissipation in a BJT.
Or, in the case of a student who knows his stuff, it could leave the student thinking that the professor is not terribly bright-- which is what I've concluded.
 

WBahn

Joined Mar 31, 2012
26,398
Or, in the case of a student who knows his stuff, it could leave the student thinking that the professor is not terribly bright-- which is what I've concluded.
Agreed. I've certainly run into times when a student that is thinking well beyond the scope of the material being taught asks questions that I'd rather not go into detail on because it might confuse a good portion of the other students. Sometimes it's enough to say as much by just pointing out that they are correct, but that it is a level of complexity that we can safely ignore at this stage. I usually try to snag the student after class to discuss it one-on-one -- those are some of the most enjoyable student-teacher interactions for both sides; at least I know I always got a lot out of those side discussions when I was a student. I always enjoyed it when a teach told me to see them in their office after class!
 

ErnieM

Joined Apr 24, 2011
8,271
Hello luiseduardo, welcome to the forums!
I find it very pleasing your very first post contains a question you have a better answer than your poof has!

While I doubt you will convince your prof of the correct answer (it's worth one try before you start to become "that" student) your plight is not very uncommon. I had one prof in particular who gave me zero points on a question because I used his method to solve it, and he admitted he knew I knew the material!

Do keep in mind that P = Vce.Ic + Vbe.Ib only applies for a static situation. When the transistor is being used in a switched mode (such as in a switching power supply) the switching losses can and usually are much greater than the base and collector dissipation. This is because the devices do not switch on and off cleanly, so there are times when significant current is flowing while there is also a significant voltage, thus peaks of power.

This is best measured in circuit as opposed to calculated. I say this because my calculation would be little more tan a guess, though I expect there are those who can do a much better job of this than I.
 

MrAl

Joined Jun 17, 2014
8,863
Hello again,

Yes i agreed there was a better answer available, but that's what dogma brings us where the person in authority makes all the rules, and i think it is a necessary part of education. Think of what it would be like to have a basic class with DC voltage source and a resistor and have the student have to account not only for I=V/R but also for stray fields.
It is of course possible that the teacher does not know the correct answer too, the other view is just a possibility. We'd have to ask him to find out for sure. I Like to mention alternate views when they look possible, even if not necessarily the way it happened.
 

WBahn

Joined Mar 31, 2012
26,398
Some teachers don't really know the subject matter they teach and they just parrot what they were taught.
Quite true -- this is particularly true in Math. In many schools the worst students in the Math department are the Math majors -- and it is quite common for the majority of Math majors to also be Education majors planning to teach math in K-12.

For mental calculations, or estimations, ignoring Vbe.Ib is okay. I can't think of a time where it mattered for anything I've designed.
Most of the time, it's fine to ignore. The only times when I've had to take it into account are with power transistors in a switching role -- then the power from the base current can be significantly greater than the power from the collector current.
 
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