transistor temperature

[badboy_6120]

Hello to every one

I always wonder how to obtain a transistor temperature at a certain power dissapation or more percisely how to know how much power you can get from a transistor without its case getting warm

For example I want to use a transistor in an amplifier that voltage across emitter-collector of this transistor be 60 volt and a 10mA current pass through it
How to choose a transistor that doesn't get warm at room temperature in this amplifier?

Thanks in advance

 

[BigBoss]

It's normally mentioned in the data-sheet of the transistor as SOA ( Safe Operating Area ).
Or Rth should be given to calculate package temperature.For instance Rth=500 K/W ( From junction to Free Air) for BC857.If power dissipation 150mW, if ambient temperature is 25C, junction temperature will be 75C so transistor works under
max. permissible temperature range.You should check data-sheet of your transistor..

 

[badboy_6120]

Thanks for your reply
How did you calculate the package temperature here?

 

[crutschow]

It boils down to thermal resistance between the transistor junction and the air.
There are several components to this: junction-to-case, and case-to-air (if free air mounted), or case-to-heatsink, and heat-sink to air (if heatsink mounted).
Each case has it's own thermal resistance to air as determined by the case size and type.
For example this states the thermal resistance of a small TO-18 metal case 2N2222A transistor is 325°C/W junction-to-air and 150°C/W junction-to-case (if a heatsink is added to the case in which case you would need to add the heatsink case-to-air thermal resistance to get the total).
Note that how warm a transistor gets has little to do with the internal transistor design or type but is determined by the case style and heatsink (if any) and the amount of power the transistor is dissipating.

For example if that transistor were carrying 10mA with 60V across it (not saturated as a switch) then the power dissipated is 0.6W. With no heatsink the TO-18 case temperature would rise 0.6W * 325°C/W = 195°C above the ambient temperature, which would obviously burn the transistor out at a 25°C room temperature (since the max junction temperature is 200°C). So you would need to add a heat sink of the appropriate size to keep the junction temperature well below this maximum or use a transistor with a case that can dissipate more power.

 

[dick_freebird]

The convection cooled (thetaJA) case is impractical to calculate
because you are unlikely to know the airflow where it counts.

Measuring it however can be pretty straightforward. You
first pull the Vbe vs temp at some reference current (or
reference setup, like power supply and resistor). Then you
light up the transistor as you'd run it, wait for temp to get
stable (few seconds for a low mass package, maybe a
minute or more with a big heat sink mass) and quickly
flip from forcing power, to force/measure Vbe. Refer Vbe
back to temnperature, and you know your power level
for the as-run condition, so there's your thermal impedance
and your junction temp rise. Junction to case you can then
get from the datasheet, if you care.

 

[badboy_6120]

thanks for your reply
i've got my answer