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 jc ( ° C/W): thermal resistance from transistor junction to transistor case

 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) T J = T A + (  jc +  cs +  sa ) P D , transistor junction temperature

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 jc ( ° C/W): thermal resistance from transistor junction to transistor case

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  1. 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

  2. : ambient (air) temperature PD : case temperature jc = 3.125 °C/W TJ(max) = 150 °C nominalroomtemperature

  3. 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

  4. 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 transistor’s derated PD(max) 3.9 W < 26 W 76

  5. 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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