Solving Transistor KFY34 Switching Load Homework

[CWatters]

Actually I've just noticed that the saturation voltage specified for the transistor is UCES ≈ 1,5V rather than the 0.6V assumed. Perhaps worth amending the last graph of U_CE.

It also changes the ON collector current slightly but only from 400mA to 375mA. Edit: Perhaps leave it at 400mA because U_CES is a maximum figure. It could be lower for the actual transistor used.

 

[NascentOxygen]

Best not to assume V_CEsat is 0.6 (unless you are sure it will be). Sounds too much like one diode drop, and your examiner may start wondering did you really think it was a V_D? To be on the safe side, I suggest something like 1.1V if higher current, or 0.2V if low current, or 0.1V if no current, as reasonable general values for any non-specific circuit.

Otherwise, graphs are what is expected.

 

[-SJ-]

I've amended the last graph to 1,5V but is with this correction Ube remained or changed to 1,5V also?
d)It might look like something like this: P=Ic*Uce_sat=400*1,5=600mW? Pmax=800mW, so no additional cooling is necessary.

 

[NascentOxygen]

Base current is less than 20mA? So base voltage should be just under 1 volt. Probably close to that I'd say. It's just a p-n junction. Data sheets will show this as a graph between 0.8 and 0.9.

 

[-SJ-]

I've look into the data sheet and although I haven't found there graph Ube depending on Ib, I've saw there Ube_sat=1,1V and Ube=0,75V for Ic=400mA. Which of these values should be chosen? Why is the voltage over the junction current depending?

 

[berkeman]

Ube_sat=1,1V

That looks to be a typo. You mean Vce_sat (or Ube_sat), right? And even so, that is pretty high for Vce_sat.

Can you post a link to the datasheet you are reading?

 

[-SJ-]

Sure, I was reading from this one posted here: http://www.datasheetarchive.com/dl/Scans-048/DSAGER000349.pdf
Page 21 and 22.

 

[berkeman]

I've look into the data sheet and although I haven't found there graph Ube depending on Ib, I've saw there Ube_sat=1,1V and Ube=0,75V for Ic=400mA. Which of these values should be chosen? Why is the voltage over the junction current depending?

Sure, I was reading from this one posted here: http://www.datasheetarchive.com/dl/Scans-048/DSAGER000349.pdf
Page 21 and 22.

So those curves look quite normal. They show Ic as a function of Vce and Ib, and the last one shows Vbe as a function of Ic and temperature. The graph at the bottom of page 21 is the best one to use to estimate Vce_sat for your Ic...

 

[-SJ-]

So, for Uce_sat=1,1V should be the P=Ic*Uce=400*1,1=440mW, right?

 

[NascentOxygen]

So, for Uce_sat=1,1V should be the P=Ic*Uce=400*1,1=440mW, right?

Is that 1.1V maximum or typical?

Don't forget the power into the base, it's not always negligible.

A device's power rating is under specific heat-sinking & ambient conditions. You need to consider how close to that your conditions will be. Exceeding about 250mW for a 440mW device would be unwise, at the best of times. If there exists an engineering mantra, it is to ALWAYS err on the side of caution.

 

[-SJ-]

That Uce should be typical. The KFY34 transistor has Pmax=800mW, so 440mW should be fine and no additional cooling is necessary.
OK, that's it. I'll send it to my examiner to check it out and be with hopes he will accept it despite the terrible time spent at solution.

I would like to say THANK YOU REALLY VERY VERY MUCH for patient guidance in this long-suffering thread. Now, I am hopefully a bit wiser about calculations with transistor and looking forward to learn more.
Thank You.