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jc (°C/W): thermal resistance from transistor junction to transistor case
cs (°C/W): thermal resistance from transistor case to heat sink
sa (°C/W): thermal resistance from heat sink to ambient (air)
TJ = TA + (jc + cs + sa) PD , transistor junction temperature
TC = TA + (cs + sa) PD , case temperature
TS = TA + (sa) PD , heat-sink temperature
PD
jc = 3.125 °C/W
TJ(max) = 150 °C
: ambient (air) temperature
: case temperature
nominal
room
temperature
cs °C/W
i.e., without forced airflow (no fan)
T (°C) =  (°C/W)  PD (W)
Example: PD = 6 W  T = (13)(6) = 78 °C  for TA = 25 °C, TC = 103 °C
Example:
TIP31 node voltages: V1 = 18.6 V, V2 = 26 V, V3 = 18 V
TIP31 node currents: I1 = 15 mA, I2 = 485 mA, I3 = –500 mA
PD = (26 V)(485 mA) + (18 V)(–500 mA) + (18.6 V)(15 mA) = 3.9 W
PD = 3.9 W  T = (13)(3.9) = 51 °C  for TA = 25 °C, TC = 76 °C
TJ = TC + (jc) PD
= 76 °C + (3.125 °C/W) (3.9 W )
TJ = 88 °C < 150 °C TJ(max) 
~26
PD actual
3.9 W
76
<
transistor’s derated PD(max)
26 W 
Open question – What is the bias voltage of the mounting tab on the TIP31?
If it is floating, then no electrical insulators are necessary
If it is tied to VC, VB, or VE, then insulation between the tab and the heat sink
may be needed to prevent shorting these voltages to ground.
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