Applications Note AN36
Issue1 - JULY 2002
Another consideration of the output stage is
that of protecting the bipolar drivers from the
effects of the back emf when switching the
windings. This takes the form of a voltage
‘fly-back’ of several times the supply voltage
and so some form of clamping is required to
prevent this voltage from over-stressing the
driver devices.
Two forms of clamping can be
recommended, that in Figure 7 shows a
straightforward Zener diode placed across
the collector to emitter of Q1 and Q2. This will
absorb any voltage spikes appearing at the
collector. This arrangement also has the
effect of protecting Q1 and Q2 against any
negative going voltages appearing as a result
of the switching. The disadvantage is that the
size of the Zener needs to be sufficient to
absorb all the energy from the voltage spike.
Figure 7
Bipolar Output Drive Stage
Figure 8 provides and alternative method of
clamping whereby the Zener diodes are
connected between the collector and base of
Q1 and Q2 to form an active clamping
arrangement. Any voltages seen at the
collector will provide base current to the
drivers to turn them back on. This has the
effect of limiting the voltage excursion at the
collector and the power is thus absorbed in
the larger output devices.
The disadvantage of this active clamping
scheme is the need to protect for negative
voltage excursions of the power transistor
collector. The need for this is twofold, the first
being to limit the negative voltage induced
noise and the second to avoid exceeding the
emitter-base breakdown voltage of the power
transistors. To prevent this the diodes D1 and
D2 are included as protection to restrict the
negative excursion of the Q1 and Q2
collectors.
Figure 8
Bipolar Output Drive Stage Using Active Clamping
The ZXBM200x series have a Darlington
emitter follower output stage and therefore
rely upon an external method of turning the
drivers off. R3 and R4 provide that function. It
should be borne in mind that R3 and R4 will
detract from the base current drive provided
by R1 and R2 so the final choice of values will
be a balance of the two requirements, turn-off
and base drive.
AN 36 - 7
ZETEX [ ZETEX SEMICONDUCTORS ]
