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Thermal Equivalent Circuit for a Transistor. Case Temperature. Heat Sink Temperature. Junction Temperature. Ambient Temperature. Thermal Resistance from Junction to Case. Thermal Resistance from Case to Heat Sink. Total Average Power Loss.
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Case Temperature Heat Sink Temperature Junction Temperature Ambient Temperature
Thermal Resistance from Junction to Case Thermal Resistance from Case to Heat Sink Total Average Power Loss Thermal Resistance from Heat Sink to Ambient
Example • TO-220 case style dissipating 5 watts • RJC = 3.0 C / Watt (from device spec) • TJmax = 150 C • TAmax = 50 C • Find the proper heat sink to keep the junction temperature from exceeding 150C in natural convection.
Need to determine a value for RCS – depends on the mounting technique. If the device is mounted without an insulator, but only with Thermalcote, (silicone grease), check page 57, Figure 9 for TO -220 case style.
Use Thermalcote RCS = 1 C/W No Insulator
Thermalloy P/N 6030 Temperature rise above ambient@ 5 Watts Power Dissipation = 66 C
Thermal Resistance RSA As long as RSA is less than 16 C/W, the requirement is met.