# tip122 maximum power dissipation

#### mah

Joined Mar 15, 2010
393
will TIP122 can hold current 4 A as in the attached circuit. it is written in its datasheet that its maximum current is 5A. do i need a large heat sink? and how to increase the collector current to 4A if the opamp supply voltage is 12v ?

#### Attachments

• 27.8 KB Views: 2

#### Hymie

Joined Mar 30, 2018
1,272
Based on your circuit application the transistor is dissipating 115W, whereas the data sheet lists a maximum power dissipation of 65W with the case temperature maintained at 25⁰C. Therefore you need a transistor capable of a higher power dissipation or a re-design of the circuit.

#### dl324

Joined Mar 30, 2015
16,665
do i need a large heat sink?
The transistor will be dissipating around 100W. You need a huge heat sink.

#### ericgibbs

Joined Jan 29, 2010
18,630
hi mah,
As stated you need a big heatsink. The Red plot

What is the purpose of this circuit.?
E
V2 is a 0v to 30v ramp input

#### Attachments

• 33.3 KB Views: 6

#### mah

Joined Mar 15, 2010
393
i want to design electronic load that control current in the power resistor, so what is your advice to handle the high dissipated power in the transistor?I also need to use small heat sink.
how did you calculate the dissipated heat?p=Ic*VcE

Last edited:

#### bertus

Joined Apr 5, 2008
22,264
Hello,

The datasheet states a maximum collector power of 65 Watts.
See the SOA (safe operation area):

Bertus

#### Attachments

• 342 KB Views: 1

#### bertus

Joined Apr 5, 2008
22,264
Hello,

YES, for that transistor you will also need a large heatsink.
Look at the temperature for an infinite heatsink:

The datasheet also shows the SOA:

Bertus

#### mah

Joined Mar 15, 2010
393
so how to solve this dilemma? 50 w and small heat sink

#### BobTPH

Joined Jun 5, 2013
8,655
If you want a max of 4A through 6R8 then that requires a voltage of 27.2V. When you input 50V, the transistor dissipates, as others have said, about 100W. (I get 91, (50-27.2) * 4, don't know where the 100 and 115 comes from.)

Now, change the collector voltage to 30V and we get (30-27.2) * 4 = 11W, which is manageable.

What are the parameters over which your current source must work?

Bob

#### Papabravo

Joined Feb 24, 2006
20,985
so how to solve this dilemma? 50 w and small heat sink
To your question. There are exactly three methods of removing heat from a system and they are:
2. Conduction
3. Convection
Since you want a small heatsink, that limits your options for conduction, and pretty much eliminates improving that method with liquid cooling. Which of the remaining two methods would you like spend BOATLOADS of money on?

#### mah

Joined Mar 15, 2010
393
Now, change the collector voltage to 30V and we get (30-27.2) * 4 = 11W, which is manageable.
i get 27.2 only when the opamp power is 32 v and this isn't available. when i reduce supply to 12 v which is available i get emitter voltage 9.5v. so (30-9.5)*4=80w which isn't manageable.
the current source changes from 0 to 30v, i have a load 50w

#### mah

Joined Mar 15, 2010
393
would you like spend BOATLOADS of money on?
I need something simple and easy at low cost. what is the maximum heat that can be dissipated with small heatsink. i can connect two parallel loads with transistors if we ran out of solutions

#### BobTPH

Joined Jun 5, 2013
8,655
So your 6R8 resistor and the 50V input is just there to mislead us, I guess.

Please explain EXACTLY what you are trying to do. What is the available power? Don't keep changing it in every post. And what are the ranges of current and voltage required for the output.

Bob

#### mah

Joined Mar 15, 2010
393
So your 6R8 resistor and the 50V input is just there to mislead us, I guess.
no it is not misleading. i want 4 A to get it i had to increase the opamp power supply to 30 v.
I am trying to do constant power load, the transistor switches the current source which ranges from 0-30v and the load is 6.8R50w,opamp power12v.

#### BobTPH

Joined Jun 5, 2013
8,655
Gee everyone calculated the power based in your 50V input and 6R8 load, and you never corrected them, but that is not misleading. We should have known better.

Bob

#### Papabravo

Joined Feb 24, 2006
20,985
I need something simple and easy at low cost. what is the maximum heat that can be dissipated with small heatsink. i can connect two parallel loads with transistors if we ran out of solutions
I guess you don't really understand the drift here. Your requirements are in severe conflict. It is doubtful that you can accomplish your goal, especially since we have no idea what done looks like.