PC Fan Driver Closes the Loop on Airflow Temperature

Apr 25, 2007 12:12 PM

Andigilog’s single-phase fan motor controller, the aMC8520, provides closed-loop speed control that monitors both rotor speed and temperature. According to Andigilog, the device, which is placed within the hub of the fan rotor, is the first component in the industry to implement this dual-parameter control mechanism as a single ASIC requiring no firmware or software. As such, it represents an integrated solution for producing quieter and more reliable fan operation.

Not only will these features benefit PC OEMs, but server farms will also gain in the form of reduced noise and improved thermal regulation. Other features of the device, such as the suppression of current spikes during rotor commutation and programmable 1-s to 5-s motor startup ramping (a feature called QuietStart) will also have particular advantages in such facilities.

The Figure illustrates the architecture of the motor-speed control loop. The RPM signal represents the rotor speed. This is calculated from information provided by a single Hall sensor that detects the rotor position. The temperature signal is derived from a thermistor.

Figure. The aMC8520 is a pre-driver controller ASIC for BLDC fan motors that features a fully integrated control loop with separate inputs for measured temperature and desired speed.

When the thermistor is mounted at the air intake of the fan, a control system can be developed that instantly adjusts the fan speed according to the anticipated cooling capacity of the airflow. This provides potential energy savings by driving the fan with just enough power to cool the load. It can also potentially improve the reliability of the fan itself, because the fan can operate at lower average power than might otherwise be possible in a system that immediately drives the fan to high speeds in response to a need for increased cooling requirements.

The aMC8520 specifically targets single-phase BLDC motors operating at 12 V or higher. Because it is a pre-driver, it interfaces to the fan motor through a full-bridge driver. The low-side devices of the driver are driven by 100-mA CMOS outputs. The aMC8520 feeds the driver’s high-side devices with up to 40 V through open-collector outputs. An internal shunt regulator is provided for driving high-voltage motors.

The device can accept either analog or digital signals for motor speed control. It is also compatible with analog or digital Hall effect sensors. In addition to these features, the device provides standard protection features, such as cycle-by-cycle current limiting, overtemperature protection, and undervoltage lockout. Other advanced functions enabled by digital signal processing are resistance cancellation and digital filtering.

Complimenting the aMC8520, the aMC8510 is an open-loop controller that retains the flexibility and several of the advanced features of the former device. Both parts are additions to Andigilog’s Thermal Edge family of acoustic and cooling management products.

General samples and an evaluation kit for each part, including a tuning board, are available now, with production planned for Q3 2007. The aMC8510 is priced at $1.50 each, and the aMC8520 at $2.00, both in 1000-unit quantities.