Motor Technology Upgrades Cooling Fans
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Laptop/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.
Single-Phase Versus 3-Phase
The Singlesense concept developed and patented by Quadrant Systems reduces the cost of a 3-phase brushless 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.
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.
The Drive System
The phase A, B and C drive is unipolar, which sacrifices some 3-phase efficiency in favor of lower cost. The cost goal 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:
where ΔVBE is an accurate analog of temperature, T is the absolute temperature, K is the Boltzman constant, q is the 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 single-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 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.
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.