I have a transistor with Vsupply = 24VDC, VCEO = 45VDC, ICE = 20mA. How to calculate the delta T (rise in temperature) of this transistor. I would like to know what thermal resistance I may have at maximum.
In my opinion:
Pdissipation (worst case): 24VDC*20mA = 480mW (current loop, almost all current goes through this transistor)
If the thermal resistance is for example 120 K/W (or C/W) the delta T would be: 120*0.48 = 57.6 (rise). This means my transistor will become around 25 (ambient) + 57.6 = 82.6 degrees.
Is my calculation correct, or do I need to observe other parameters as well? I need a very small transistor package, but it must be able to dissipate ~500mA and work in an ambient environment of +55 degrees.