Vincent Ko
System Engineer
Freescale Semiconductor
Austin, Tex.

The basic construction of a brushless-dc fan consists of a fan blade attached to a permanentmagnet rotor that surrounds the electromagnetic coils of the stator and associated control electronics.

The term "dc fan" today normally refers to the type of fan that uses a biphase, brushlessdc (BLDC) motor. These fans are widely found in chip cooling or chassis ventilation. Yet cooling requirements have become more sophisticated as highperformance electronics grow more complex — and power hungry. Just count the number of cooling fans installed in a single personal computer today.

In the past, only a simple motor driver optimized for a constant fan speed controlled these fans. Those designs are only acceptable today for general cooling. More sophisticated products, such as microprocessors and high-performance electronics, need a controlled airflow as temperatures change with shifting demands. When cool, the fan should push only a minimal amount of air to save power. As the temperature of the device rises, airflow should rise so performance is unaffected.

Surprisingly, slower airflows may actually cool better. High airflow rates can create a vacuum between the device and the fan. This "forced-air" vacuum then acts more like a thermal insulator than a cooling airflow. Device temperatures buildup faster and to a higher degree.

Until recently, expense prohibited creating "smart" fan controls that adjust fan operation to meet varying demands. Low-cost microcontrollers (MCU) broke the price barrier to create economical variable-speed dc fans. The use of MCUs also includes several fringe benefits over more traditional designs.

MCUs normally contain flash memory that lets designers easily change control algorithms to make the same drive electronics fit different motor types. That translates into lower manufacturing costs through inventory reduction and volume purchasing.

Programmable memory also lets the designer match fan speed against the temperature curve to tailor fan operation. Temperature feedback comes directly from the ambient environment or from other electronics in the system. For example, many PC motherboards already contain several thermal monitors for critical devices. The MCU can tap these existing sources rather than create new ones at added cost.

Should faults occur, such as a malfunction or blockage that stops motor rotation, the MCU can take corrective action like shutting down the system to prevent further damage. Likewise, it can alert the system controller to the breakdown and even initiate its own repair order.

A typical biphase brushless fan motor is made from a permanentmagnet rotor assembly that surrounds four electromagnetic coils. The coils work in pairs, with coils A and C forming one phase and coils B and D the other phase. A Halleffect sensor monitors rotor position, providing feedback to the embedded MCU for commutation, speed regulation, and fault detection.

Most BLDC fan-motor designs consist of a permanent-magnet rotor surrounded by coil windings mounted on the stator poles. As its name implies, BLDC motors have neither brushes nor a commutator. Commutation takes place electronically when the rotor hits specific positions in its rotation. In most instances, Halleffect devices sense rotor position. The Hall-effect signal triggers electronic commutation at the proper time.

The typical biphase BLDC motor has one pole-pair per phase for a total of four poles mounted at right angles to each other. Each commutation step rotates the rotor 90°. Four commutation steps produce one complete 360° revolution.

Current flowing through the winding of one pole pair induces a magnetic field from pole to pole. The permanent magnet in the rotor aligns with the magnetic field between the poles. Once aligned, the energized coils are turned off and the other pair turned on. The process forms a commutation step. The permanent magnet in the rotor is both pushed and pulled by the new magnetic field, so that the rotor begins to turn.