Pulse Power Rating of SemiconductorsThe admissible dissipation of diodes, rectifiers and Zenerdiodes which operate from sinusoidal supplies is based onthe arithmetic mean value of junction temperature andpower dissipation. Devices which handle pulses are capableof passing short-term currents far in excess of themaximum admissible static dissipation, and in this case it isadmissible to exceed the continuous dissipation curve forthe duration of each pulse. The magnitude of the admissiblecurrent is then inversely proportional to the pulse dutyfactor, because power is dissipated only intermittently, andthe thermal capacity of the system and heat conductionprevent an undue rise in junction temperature. Some of thedata sheets contain diagrams which allow the rating of adevice operating under pulsed conditions to be determined.In Figure 1, which applies to diodes and rectifiers, themaximum admissible pulse current amplitute is plotted as afunction of pulse duration for an ambient (or case)temperature of + 25°C. If the device is to operate at higherambient temperatures, then it is necessary to derate thecurrent values derived from this diagram in accordance withthe “admissible dissipation versus temperature” curve.
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