Effect of transient voltages on transistors

During transient conditions, the maximum junction temperature is dependent on the energy delivered to the transistor. For resistive and capacitive loads, transient energy is delivered during the turn-on period because the internal space-charge capacity induces a forward current in the base, which is amplified. For an inductive load, energy is delivered during the turn-off period because of the high induced voltage. Analytical expressions are derived to show the different transistor parameters and circuit constants which influence the magnitude of transient energy. By far the most important parameter is the sustain voltage. If this voltage is exceeded in the resistive or capacitive load condition, the initial base current is amplified by a greatly increased current gain causing excessive dissipation. If the voltage induced in the inductive load exceeds the sustain voltage, the negative resistance collector characteristic may cause highly dissipative oscillations which usually destroy the transistor. The transistor may be protected from destruction by preventing the base from biasing in the reverse direction so as to avoid negative resistance.