Pulse Power Rating of Semiconductors
The admissible dissipation of diodes, rectifiers and Zener
diodes which operate from sinusoidal supplies is based on
the arithmetic mean value of junction temperature and
power dissipation. Devices which handle pulses are capable
of passing short-term currents far in excess of the
maximum admissible static dissipation, and in this case it is
admissible to exceed the continuous dissipation curve for
the duration of each pulse. The magnitude of the admissible
current is then inversely proportional to the pulse duty
factor, because power is dissipated only intermittently, and
the thermal capacity of the system and heat conduction
prevent an undue rise in junction temperature. Some of the
data sheets contain diagrams which allow the rating of a
device operating under pulsed conditions to be determined.
In Figure 1, which applies to diodes and rectifiers, the
maximum admissible pulse current amplitute is plotted as a
function of pulse duration for an ambient (or case)
temperature of + 25°C. If the device is to operate at higher
ambient temperatures, then it is necessary to derate the
current values derived from this diagram in accordance with
the “admissible dissipation versus temperature” curve.