A novel motor design and driver IC
reduce Hall sensor requirements,
enabling use of high-efficiency
3-phase brushless dc fans in laptops
and other low-cost products.
By Brad Marshall, President, USMicro-III, Webster, Mass.;
Chris Petersen, Vice President, Engineering and Development,
Quadrant Systems, Bourne, Mass.; and Kiyoaki Hodohara,
Market Development Manager, Asahi Kasei Microsystems, Tokyo
L
aptop/notebook PC cooling fans demand low
noise and high efficiency. The latter is particularly
important as notebook designers continually seek
to extend battery life. The use of 3-phase brushless
dc motors offers a means of reducing the power
consumed by fans. But to date, cost pressures have prohibited
the move to 3-phase brushless fan motors. However, a new
technology is now being introduced that allows the use of
3-phase fans without a cost penalty. Low noise and reliability
are added benefits.
Today’s 3-phase fan motors represent the next stage in
the advancement of cooling fans, which have evolved from
brush motors to brushless single coil, often called one or
single phase. Because the single-phase brushless motors
have peaked in performance, a move to 3-phase brushless
fans is required to further boost efficiency. But to satisfy the
low-cost demands of the application, PC designers need a
means to offset the higher cost of the 3-phase fans.
fan design to parity versus the single-phase system, while
establishing the performance advantages of three phases.
The table summarizes the performance advantages of
Singlesense technology when used to drive a 3-phase laptop
cooling fan.
Singlesense is a combination of motor technology and
electronics, a systems solution. Although this technology is
applied here to drive a fan motor, the control circuit may be
applied to 3-phase brushless motors of any size or power level
to achieve considerable cost and size savings.
To commutate a conventional 3-phase motor, three
sensors are used. This requires three Hall sensors embedded in the motor with 9 to 12 wires exiting the motor and
decoding logic to process the sensor signals. In contrast,
Quadrant System’s Singlesense method facilitates 3-phase
commutation with one Hall sensor. This technique has
been applied in the controller chip and the associated
motor. The controller chip was developed by USM-III and
Asahi Kasei Microsystems (AKM), based on Singlesense
technology. In the motor, the rotor is modified to contain
magnetic signals that allow one Hall sensor to control
3-phase commutation, using simple logic.
Single-Phase Versus 3-Phase
The Singlesense concept developed and patented by
Quadrant Systems reduces the cost of a 3-phase brushless
Power Electronics Technology August 2006
14
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3-phase fan
1-phase fan
Input power
0.9 W
1.9 W
Power savings
Efficiency
50%
25%
Power savings
5x
1x
50%
90%
500 RPM
>1000 RPM
One component, small
driver IC chip
Five to ten components,
large driver IC chip
Start torque
Torque ripple
Minimum speed
Driver cost
Advantage
Reliability—most failures occur because of mechanical friction stopping the rotation
Less noise
Less noise, flat torque curve
The single-phase fan uses full-bridge drive requiring
2.7 times the transistor area; no discrete components
Table. Comparison of 3-phase versus 1-phase for a laptop cooling fan.
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Fig. 1. In the PECOS 3-phase brushless motor, the rotor magnet is modified by creating cutouts between the north and south poles, resulting
in zero fields, which are then used to control commutation.
Fig. 2. The Hall sensor embedded in the motor-control IC produces the
response shown here to the magnetic profile of the rotor.
A schematic drawing of the rotor magnet is shown in Fig.
1. The axes of the A, B and C stator phases are shown in gray.
The north and south rotor magnet poles are shown in white.
The zero field is created in the rotor magnet with cutouts
between the north and south poles. The cutout is actually
molded and does not create a special manufacturing step.
The magnetic profile created by the rotor at the interface
with the PECOS/Singlesense motor-control IC is shown in
Fig. 2. The PECOS control IC is located in close proximity
to the spinning rotor magnet. As the rotor spins, the on-chip
Hall sensor detects the north, south and zero magnetic fields.
The chip logic then converts the signals associated with the
north, zero and south poles to drive signals for the output
FETs, which are connected to phases A, B and C. It can be
seen from Fig. 2 that the logic needed is quite simple. Phase
A FET turns on when the Hall sensor sees a north pole, phase
B FET turns on when the Hall sensor sees a zero field and
phase C is on with a south field.
of the PECOS/Singlesense system is to equal 1-phase systems,
as users want better performance for the same or lower cost.
The drive system is shown in Fig. 3.
The purpose of tach (tachometer) or alarm is to warn the
CPU of a cooling fan problem that may cause CPU failure
if not detected. Alarm is a dc logic-level high that signals a
stopped fan. Tach, often called FG, is an output square wave
needed by the PC CPU and produces two full cycles per
revolution of the fan motor. This standard was defined by
Intel and is part of its CPU cooling system logic spec. The
speed control is also a part of the Intel spec as a closed loop.
The CPU chip contains a silicon transistor in a diode-connected configuration. An analog chip (such as ADM1029)
is used to measure the junction temperature using a band
gap measurement technique given by:
∆V
VBE =
The Drive System
KT
× ln((N)
q
where VBE is an accurate analog of temperature, T is the
absolute temperature, K is the Boltzman constant, q is the
The phase A, B and C drive is unipolar, which sacrifices
some 3-phase efficiency in favor of lower cost. The cost goal
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Power Electronics Technology August 2006
MOTOR TECHNOLOGY
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Fig. 3. The PECOS 3-phase drive system reduces the design of the drive
circuit to a single component.
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Fig. 5. A popular 1-phase drive system employs a single Hall sensor, an
analog fan controller and numerous passive components.
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Fig. 4. Intel’s CPU cooling specification defines a fan control system in
which fan speed is regulated according to CPU die temperature, which
is derived from VBE measurements taken across a transistor on the die.
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carrier charge, N is the ratio of switched currents, typically
10. Fig. 4 shows the system.
The PECOS chip was targeted at a 70-mm × 15-mm fan
used in a popular notebook PC. The key parameters are
VSUPPLY=5V and a running current of 200 mA. It has also
been adapted to higher powered fans as a predriver driving
power FETs.
The PECOS drive chip is of mixed-signal design, which
eliminates the many discrete timing components needed in a
more complex analog chip now used in a very large number
of cooling fans. The PECOS chip also includes an embedded
Hall element, eliminating the four-pin discrete Hall sensor.
A monolithic device, the PECOS chip is fabricated in a special
CMOS process that allows integration of the Hall sensors
with analog CMOS on the same die.
In the 1980s and 1990s, PC cooling fans used a 2-phase
motor because the area available for the motor in fan designs
was larger and the cost of the control circuits was minimal.
The first 1-phase fans were introduced in the late 1990s.
The 1-phase motor is actually a 2-phase motor driven by
a full bridge.
The driver power electronics for one phase is more expensive than for two phases, requiring nearly three times the
transistor area in a single-chip driver. Nevertheless, the move
from two phases to one phase eliminated one of two stator
coils or half the wire in a fan motor, making the motor hub
smaller for the new generation of reduced size, improved
airflow fans (Fig. 5). Thus, for the same input power a singlePower Electronics Technology August 2006
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Fig. 6. A conventional 3-phase drive system requires three Hall sensors
in addition to a complex analog chip.
phase fan now provides more airflow.
While 3-phase motors have always been preferred for
performance (efficiency, noise, starting torque), the cost
has been prohibitive because of the need for three discrete
Hall sensors or a complex sensorless system. Fig. 6 shows the
conventional 3-phase motor-drive components using three
Hall sensors for comparison with the PECOS/Singlesense
fan with no discrete Hall sensors.
With better motor design, 1-phase or 2-phase fans can
achieve a higher efficiency, but cost and package size would
increase accordingly. Typical offerings measure 20% to 30%
efficiency as they are designed for low-cost, maximum airflow. Such low-efficiency fans have been cost reduced and
made physically smaller by removing windings and/or iron,
which results in requiring more power input to achieve the
same fan speed/airflow.
The shortcomings of 1-phase/2-phase fan motors can
be seen from Fig. 7, which compares torque versus rotation
16
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MOTOR TECHNOLOGY
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Cost Competitive
The printed-circuit board and stator used in the PECOS
motor are of conventional construction. The stator is
laminated iron with six poles rather than four. The rotor
consists of a high-energy magnet material, adding a cost
of approximately 3 cents to 5 cents, while the printed-circuit-board assembly saves about 10 cents. Royalties for the
motor-control technology add about 5 cents to the cost of
the design.
There is a need to design fans with optimized efficiency
for battery-powered applications such as laptops and in severe
cooling situations such as servers. The PECOS/Singlesense fan
is designed to produce as close to the theoretical maximum
efficiency as possible. Generally, 3-phase fans can be produced
at a cost that is competitive with 1-phase fans.
PETech
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for the two systems. Higher 1-phase/2-phase torque ripple
creates more noise and provides poor starting torque, which
is a major failure mode. Clearly, higher torque ripple will
result in more vibration. An improved torque characteristic
also allows lower initial (idling) fan speed without the risk
of stalling. This also explains poor efficiency as the current
is actually maximum when torque is nearly zero, which
wastes power. The 3-phase motors are seen to have improved
torque ripple.
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Fig. 7. A comparison of torque and rotation for 1-phase/2-phase fan
motors versus 3-phase motors reveals that the former produce higher
torque
ripple, which
in more 1:09
noise and
starting
CKE-PET
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7/12/06
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