# PNP Transistor Power Dissipation Calculation - help needed

#### mishra87

Joined Jan 17, 2016
1,000
Hi Guys !

How can i calculate the power dissipation of PNP Transistor in below circuit ?

Regards,

#### ericgibbs

Joined Jan 29, 2010
17,980
hi mishra,
You are a LTSpice user, why not use the Power option in LTS.?
E
BTW: You need to add the parameters of the transistor [ or use an equivalent model] to get a true reading.
E

#### mishra87

Joined Jan 17, 2016
1,000
hi mishra,
You are a LTSpice user, why not use the Power option in LTS.?
E
BTW: You need to add the parameters of the transistor [ or use an equivalent model] to get a true reading.
E
Thanks !
Well you know i am lt spice user.
I have got simulation values but i wanted to math for thermal rise.
That is why i wanted to calculated PD.

Regards,

#### ericgibbs

Joined Jan 29, 2010
17,980
hi m,
How can you calculate PD of the transistor, in that circuit, if you do not state the specification of the actual transistor.
Also all the details of the square wave drive signal.
E

#### Deleted member 115935

Joined Dec 31, 1969
0
find the current through the transistor and voltage drop with input high and low,
that gives you two powers
Define the high / low time of your input , that gives you a ratio,
the ratio then allows you to calculate the power

so if when high, you dissipated 1 watt, and when low, 0.1 wat
and you were high for 10 % of the time,

then power is 10% * 1 watt + 90% * 0.1 watt = 0.19 watt

#### mishra87

Joined Jan 17, 2016
1,000
hi m,
How can you calculate PD of the transistor, in that circuit, if you do not state the specification of the actual transistor.
Also all the details of the square wave drive signal.
E
Hi,

I forgot to add transistor datasheet and now here here is the link.
https://www.mouser.in/datasheet/2/916/PEMD10_PUMD10-1599566.pdf
Regarding square wave , it is digital output signal of 0 to 4V from microcontroller.

Regards,

#### mishra87

Joined Jan 17, 2016
1,000
find the current through the transistor and voltage drop with input high and low,
that gives you two powers
Define the high / low time of your input , that gives you a ratio,
the ratio then allows you to calculate the power

so if when high, you dissipated 1 watt, and when low, 0.1 wat
and you were high for 10 % of the time,

then power is 10% * 1 watt + 90% * 0.1 watt = 0.19 watt
Thank you so much !

#### ericgibbs

Joined Jan 29, 2010
17,980
hi,
You have a 8.4V supply.

Q2 Base Emitter is say 0.7v

So across the 2k2 there is 8.4V - 0.7v = 7.7v

I via the 2k2 is ~3.5mA

The bias resistor 47k and 47k on Q1 means that it is always conducting, the 4V signal input will not turn Off Q1 as its Emitter is at 8.4V.

What are you trying to do with that circuit, it is ......

E

For your reference, this is the LTS with the LTS model and an equivalent real model.

Try it

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#### mishra87

Joined Jan 17, 2016
1,000
hi,
You have a 8.4V supply.

Q2 Base Emitter is say 0.7v

So across the 2k2 there is 8.4V - 0.7v = 7.7v

I via the 2k2 is ~3.5mA

The bias resistor 47k and 47k on Q1 means that it is always conducting, the 4V signal input will not turn Off Q1 as its Emitter is at 8.4V.

What are you trying to do with that circuit, it is ......

E

For your reference, this is the LTS with the LTS model and an equivalent real model.

Try it
Thanks i understand that why it is always ON.
I think Q1 base drive signal V2 should be 8.4V as same supply V1. So will not have any problem.
Power across Q1 is 3mW but still i wanted to do math for power across Q1.

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Last edited:

#### ericgibbs

Joined Jan 29, 2010
17,980
Power across Q1 is 3mV but still i wanted to do math for power across Q1.
hi,
I guess you mean 3mWatts, but is it...???
Consider the 50:50 duty cycle.
E

Update:
Place the finger cursor over the power plot label, hold down the Cntrl key and Left click, this show thw Avg Pwr etc.
E

Last edited:

#### mishra87

Joined Jan 17, 2016
1,000
hi,
I guess you mean 3mWatts, but is it...???
Consider the 50:50 duty cycle.
E

Update:
Place the finger cursor over the power plot label, hold down the Cntrl key and Left click, this show thw Avg Pwr etc.
E
View attachment 215694
Thanks you much for your all support !
Edited : 3mW

Here is my math for Q1 power dissipation and that is comming 3.46mW. Which is closer to simulated value.
Q1_PD = Vbe * ib + Vce * ic

Thanks once again !
Regards

#### ericgibbs

Joined Jan 29, 2010
17,980
hi m,
You are missing the point I raised, it is only 3.5mW on each High level, but its only at the High for 50% of the time.!
So your value needs to be 3.5mW/2

E

#### mishra87

Joined Jan 17, 2016
1,000
hi m,
You are missing the point I raised, it is only 3.5mW on each High level, but its only at the High for 50% of the time.!
So your value needs to be 3.5mW/2

E
Thanks,
I know what you are trying to say !
Sometimes you are not driving with 50% duty cycle. it is just digital output !

Thanks again for mentoring me !!

#### slackguy

Joined Feb 11, 2016
76
if there is one error (circuit locked base issue or microcontroller dead issue), then all your average is only informational not actual.

Quantity[0.750638, "Milliwatts"] sustained 100% duty is what you are susceptible to receiving, in situations (locked gate, 2.2V supply is blocked). But I'm sure your simulator warned you of that and suggested optimal routing and solder pad locations?