* Rth ( j-a ) varies depending on the circuit board, the heat-dissipation conditions involving soldering methods and the method of temperature measurement. If the air temperature is 35☌ and the transistor case is not to exceed 60☌, determine the amount of power this transistor can dissipate safely. The transistor is cooled by air flowing over it with an average heat transfer coefficient of 50 W/m2-☌. During the positive half-cycle of the signal, the input circuit is forward biased and hence collector current flows. * Data listed here came from the results of measuring a specific production lot. A transistor with a height of 0.4 cm and a diameter of 0.6 cm is mounted on a circuit board. In class B power amplifier operation, the transistor is so adjusted that zero signal collector current is zero i.e. Please treat these only as reference data.* The values in following data are not the guaranteed values nor maximum / minimum values. Thermal Resistance of Standard Packages (Reference Data) This provides a lower inductance path to RF ground, which in an RF amplifier is critical to maximize gain. (Collector land area / thickness / materials plus circuit board material, size circuit width will also bring different measurement results on Rth(j-c). Note that not all power transistors have the case or tab connected to the collector, some RF power transistors have the case connected to the emitter. For instance, the case temperature could be lower in comparison even though the applied current is the same, when the circuit board has good heat dissipation characteristics.Īn illustration below shows that Rth(j-c) becomes lower as the collector land pattern on the circuit board gets smaller. Since the Rth(j-c) value varies depending on the types of circuit board and also on the heat dissipation conditions (which include soldering state), the formula above potentially wouldn't apply to your calculation since the the measured values on RHOMs circuit board may not be the same as the measured values on your circuit board. ** The value is considered an approximation when the applied current is not constant, shifting from time to time by assigning the averaged consumption figure. * Please note that the case temperature differs considerably depending on the measurement method/point. * Case temperature is measured by the radiation thermometer, as the maximum temperature on the surface of the package, where the marking is put.
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