Transistor power dissipitation

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eng_boody

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hi all,

I want to know what is the relations between the power dissipation an and temperature of the transistor to enable me keeping it at constant temperature T without sink
please tell me how to get these parameters from the datasheet

note I'll use a TO-220 package

thanks
 

The datasheet for a TO-220 transistor shows thermal resistance from its chip to the ambient air (without a heatsink).
For example a TIP3055 from Texas Instruments has a maximum thermal resistance Rja of 35.7 degrees C per Watt. Then when it dissipates 1W its chip will be 35.7 degrees C above the ambient temperature. Its maximum allowed chip temperature is 150 degrees C but is not recommended.

If the transistor is enclosed then it cannot be cooled with ambient air.
 

The relation is gibe by a linear factor which is called thermal resistance, e.g. Rth,jc, thermal resistance junction to case for a transistor to be mounted on a heatsink.
 

Hi,

there is a relationship between power dissipation and temparature_rise.
But there ar additional parameters that influence it: air, air flow, humidity, mounting direction and so on.

for a TO-220 you may find in the datasheet R_PJA about 65°C/W (junction - air)

**********
If you want to keep it at constant temperature you could do this with a regulation loop.
With a BJT V_BE changes with about -2mV/°C. you could use this effect to measure the BJT´s die temperature.

For higher precision you should switch between "heating" and "measuring". While heating base current changes. But allways use the same (constant) base current during measurement (1 ms may be sufficient)

Klaus
 
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