AXBM2001 datasheet(7/14 Pages) ZETEX

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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.

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

clamping

can

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 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.

AN 36 - 7

Applications Note AN36

Issue1 - JULY 2002

Figure 8

Bipolar Output Drive Stage Using Active Clamping

Figure 7

Bipolar Output Drive Stage

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