Applications Note AN36
Issue1 - JULY 2002
Biplolar driver active switch off
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.
The problems of base turn off when driving
bipolar devices has been 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.
Figure 21
Active switch-off circuit for bipolar dirver devices
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.
Figure 22
Constant Speed versus Supply Voltage
AN 36 - 13
ZETEX [ ZETEX SEMICONDUCTORS ]
