Considerations on External Heat Transfer in Saturated Bipolar Junction Transistors

When the BJT is deeply saturated and a resistor (R) is connected between the base and collector, the overall bipole seen through the base and emitter exhibits rectifying properties. By appropriately sizing R, only a small fraction of the current flows into the base, while the majority passes through R, causing a voltage drop almost equal to that between the base and emitter. The same current flows through the collector and emitter, resulting in a voltage drop on the order of tens of millivolts. The novelty lies in the fact that almost all the power is dissipated in R rather than in the BJT. In the presence of large currents, this leads to significantly lower power dissipation in the BJT compared to a diode of the same size. The reduced thermal stress can extend the Mean Time Between Failures (MTBF) of the BJT. In this study, we conduct a model-based analysis of the saturated BJT rectifier, which is then validated through experimental setups. Furthermore, it is demonstrated that the power dissipated in a saturated BJT-based bridge rectifier is significantly lower than that measured in a classical bridge rectifier configuration under the same operating conditions.