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thmc45.doc
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2000 Texas Instruments
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9
THMC45
SINGLE-PHASE 5VDC BRUSHLESS FAN DRIVER WITH SINGLE WIRE CONTROL INTERFACE
Product Preview
H-Bridge Outputs (OUTA, OUTB)
The THMC45 PWM controlled H-bridge topology driving a bipolar-wound single-phase brushless motor
provides several advantages for small, 5V brushless DC fans over the unipolar-wound motor driven by
two commutated low-side switches typically used. The H-bridge topology requires only two IC drive
pins and the bipolar-wound motor also needs only two drive connections, reducing the number of traces
on the fan PCB. A bipolar wound stator has a single-wire winding which is simpler to manufacture, and
thus increases reliability and reduces manufacturing time.
All factors combine to allow a smaller
diameter fan center hub, and thus higher blade area / airflow for a given fan frame size. Generally, an
H-bridge drive method with bipolar wound stator increases fan motor torque density over the typical
unipolar drive method. The H-bridge drive method also eliminates the need for snubbing inductive
energy at commutation transitions.
Figures 2a and 2b show the THMC45 H-Bridge motor drive states with stator winding current being
driven from OUTA to OUTB and from OUTB to OUTA, respectively. As shown, the motor winding is
pulse-width-modulated (PWM’d) from the low-side, with inductive current re-circulating on the high-side
during tOFF. Re-circulation of current through the high-side switches during tOFF, known as synchronous
rectification, improves power conversion efficiency by maintaining the inductive energy in the stator
winding.
Figure 2a: H-Bridge PWM Drive with Synchronous Rectification
Figure 2b: H-Bridge PWM Drive with Synchronous Rectification
A to B Current Direction
I
RECIRCULATE
Motor
OUTA
VPWR
GND
PWM
OFF
ON
I
DRIVE
PWM
OUTB
Motor
OUTA
VPWR
GND
PWM
OFF
ON
PWM
OUTB
PWM
t
ON
PWM
t
OFF
OUTA
H
H+
H
H-
L
OUTB
PWM
B to A Current Direction
I
RECIRCULATE
OUTA
VPWR
GND
I
DRIVE
OUTB
OUTA
VPWR
OUTB
PWM
t
ON
PWM
t
OFF
PWM
OFF
ON
PWM
Motor
GND
PWM
OFF
ON
PWM
Motor
OUTA
H
H+
L
H-
H
OUTB
PWM