![]() Alternatively, when a given heat sink is used, Eq. The smallest heat sink with a thermal resistance equal to or lower than the calculated value is then selected. θ SA is determined by the size and style of the heat sink, (see Table 8-2).Įquation 8-21 may be used to calculate θ SA when all other quantities are known. Table 8-1 shows typical θ CS values for three case styles and three contact conditions. ![]() The contact may be dry, have a layer of heat-conducting compound, or have an insulating gasket. θ CS is dependent on the transistor case and on the mechanical contact between the case and the Heat Sink in Transistor. The value of θ JC depends upon the transistor case style, and it is usually specified on the data sheet. ![]() in degrees Celsius (☌), and the thermal resistances are in degrees Celsius per watt (☌ /W), Q is the power dissipated in watts (W). Ohm’s Law may be applied to a thermal series circuit exactly as in the case of an electrical series circuit.įor an electrical series resistive circuit, I = E/R, or, Also, the temperature drop across each thermal resistance is analogous to the voltage drop across each electrical resistance. The power flow in the thermal circuit is similar to the current flow in the electrical circuit. The thermal resistance series circuit is analogous to an electrical series resistive circuit. The temperature difference between the transistor collector-base Junction and the air surrounding the heat sink (T J – T A) causes the dissipated power (Q) to flow through each of the thermal resistances in turn. The size of heat sink required for a transistor with a given power dissipation may be determined from the thermal equivalent circuit. This consists of the three thermal resistances connected in series. Θ JC – junction-to-case thermal resistanceįigure 8-29(b) shows the thermal equivalent circuit for the transistor and heat sink. Each portion of the path that the heat must pass through has a thermal resistance. In many cases, a mica gasket is inserted between the transistor case and the heat sink for electrical insulation. The heat generated at the collector-base junction must flow from the junction to the transistor case, then from the case to the heat sink, and finally from the heat sink to the surrounding air. ![]() Figure 8-29(a) shows the cross section of a transistor fastened to a Heat Sink in Transistor. ![]()
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