Application circuit description

AN2252

18/21

Figure 17. shows the principle of the emitter-switching operation of the ESBT. During the

turn-off time the BJT enters the storage time area which can be identified by a negative

base current. The amplitude of the base current during the storage time is equal to the

actual collector current just before gate turn-off. The base-emitter junction is not conducting

and all the collector current flows through the collector-base junction. This process is similar

to the reverse recovery of a standard diode. Since the recovery current is high, the storage

time is very short. The storage time can be easily read from Figure 17. and its value is of

about 800ns. Since the baseemitter junction is not conducting, current crowding and hot-

spot effects are greatly reduced, which gives rise to an excellent square Reverse Bias Safe

Operating Area (RBSOAR) similar to the Safe Operating Area (SOAR) of power MOSFETS.

Figure 17.

Detailed view of the primary switch base current

Figure 18. shows the part of the base current waveform that corresponds to the storage

time. Collector voltage is also represented. The Collector voltage rise indicates where

storage finishes and the current fall time begins.

Figure 18.

Detailed view of the primary switch base current

Note:

Channel 4 shows the base current (green) and channel 1 shows the collector voltage (blue).