AXBM2001 datasheet(13/14 Pages) ZETEX

13 / 14 page

Biplolar driver active switch off

The problems of base turn off when driving

bipolar d evices has b een mentioned

previously. Figure 21 is an active turn off

circuit that can be added to each phase drive

transistor of the circuits shown in Figures 7 or

8. The transistor Q3A and Q3B can of the

multi-pack type and forms a simple double

inverter function to actively discharge the

base charge of Q1.

Removal of Speed Variation with

Supply Voltage

It has already been discussed that when using

a 10k

NTC thermistor and an external

potential divider an increase in supply

voltage results in a slower speed. It is

therefore possible when using a 10k

NTC

thermistor to remove supply voltage

variation by feeding a percentage of the

supply voltage change into the SPD pin.

To do this the circuit in Figure 22 is used. In

this circuit a Zener diode is used to partly

remove supply voltage variation. A Zener

voltage half that of the supply voltage should

suffice. R1 and R2 should be small in relation

to R3 and R4 and provides a partly regulated

voltage that in turn is used for the potential

divider for the Thermistor network. A raising

of the supply voltage would normally put a

higher voltage across the windings and so

increase the speed. This is now countered by

the fact that a small percentage of the supply

increase appears at the R1/R2 junction. This

percentage increase also appears at the

R3/R4 junction - the SPD pin. An increase in

the control voltage on the SPD causes a

lowering of the PWM drive and thus a lower

speed. Careful manipulation of the R1/R2 and

R3/R4 ratios will enable constant speed to be

attained over a wide supply voltage range but

still

allowing

speed

variation

with

temperature.

AN 36 - 13

Applications Note AN36

Issue1 - JULY 2002

Figure 21

Active switch-off circuit for bipolar dirver devices

Figure 22

Constant Speed versus Supply Voltage

AXBM2001 Datasheet(PDF) 13 Page - Zetex Semiconductors

AXBM2001 Datasheet(HTML) 13 Page - Zetex Semiconductors

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Datasheet Download

Link URL