Hi everyone,
I have some questions regarding transistor as I came across this in a book. Here it goes:
All transistors have maximum collector-current ratings (
I have some questions regarding transistor as I came across this in a book. Here it goes:
All transistors have maximum collector-current ratings (
IC,max), maximum collector-
to-base (BVCBO), collector-to-emitter (BVCEO), and emitter-to-base (VEBO) breakdown
voltages, and maximum collector power dissipation (PD) ratings. If these rating
are exceeded, the transistor may get zapped. One method to safeguard against BVEB
to-base (BVCBO), collector-to-emitter (BVCEO), and emitter-to-base (VEBO) breakdown
voltages, and maximum collector power dissipation (PD) ratings. If these rating
are exceeded, the transistor may get zapped. One method to safeguard against BVEB
is to place a diode from the emitter to the base, as shown in Fig. 4.38
a. The diode prevents
emitter-to-base conduction whenever the emitter becomes more positive than
the base (e.g., input at base swings negative while emitter is grounded). To avoid
exceeding BVCBO, a diode placed in series with the collector (Fig. 4.38b) can be used to
prevent collector-base conduction from occurring when the base voltage becomes
excessively larger than the collector voltage. To prevent exceeding BVCEO, which may
emitter-to-base conduction whenever the emitter becomes more positive than
the base (e.g., input at base swings negative while emitter is grounded). To avoid
exceeding BVCBO, a diode placed in series with the collector (Fig. 4.38b) can be used to
prevent collector-base conduction from occurring when the base voltage becomes
excessively larger than the collector voltage. To prevent exceeding BVCEO, which may
be an issue if the collector holds an inductive load, a diode placed in parallel with the
load (see Fig. 4.38
c) will go into conduction before a collector-voltage spike, created
by the inductive load, reaches the breakdown voltage.
My questions are:
1) In the figure 4.38a, how the diode would prevent the conduction from emitter to base whenever emitter becomes more positive? The diode would be in forward bias condition!
2) In Figure 4.38b, how would the diode protect BVCBo since it's in forward bias? (almost same as the case above)
3) How did the diode protect transistor against an induction load (for eg: relay) and when does the spike occur? Appreciate if anyone could explain this by drawing a diagram.
Thanks..
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