Understanding Closed-Loop Fan Speed Control

UNDERSTANDING CLOSED-LOOP FAN SPEED CONTROL

By Ken W. Gay, SMSC

When implementing cooling solutions for electronic equipment, system designers are confronted with a complex set of variables. The cooling requirements are typically dynamic, and ambient temperatures can vary widely. Airflow can be affected by different system configurations or external restrictions. Additional variables are introduced when cooling fans are utilized, including a variation of fan speed from fan to fan. Aging also affects fan speed, resulting in a wide range of actual fan speeds produced by a particular fan drive setting.

Figure 1 illustrates the difference between open- and closed-loop speed control as different fans are driven to a target speed from the OFF condition. Closed-loop control can drive fans with a wide variety of response characteristics to the same target speed.

The acoustic noise produced by a fan increases with fan speed, and this variation makes it difficult to optimize cooling efficiency while minimizing acoustic noise. Eliminating fan speed variation helps meet product performance and acoustic requirements and provides a competitive advantage in the market.

To help reduce the challenges associated with cooling solutions, engineers can utilize closed-loop fan speed control ICs (FSC-ICs) over open-loop fan drive ICs. The small geometry IC fabrication processes available today make it possible to provide a rich feature set at an economical cost.

Benefits of Closed-Loop Fan Speed Control
Many types of consumer and industrial products rely on brushless DC fans for cooling, including desktop and notebook computers, projectors and projection TVs and communications networking equipment. The rpm rating for a DC fan is a typical value at full-rated voltage and under specific air-pressure conditions. The actual fan speed will vary due to a variety of factors including:
• Fan to fan variation (of the same model)
• Fan aging
• Vendor to vendor variation
• Fan speed control circuit
• System airflow

A fan's speed can vary 5% to 10% when new, unless tightly controlled by the fan specification, which adds cost. When aging and other conditions are factored in, fan speed can vary up to 20%.

Several fan speed controller options can control fan speed including open-loop and closed-loop fan speed control. Open-loop fan control sets a speed based on a percentage of maximum fan speed, which is typically a voltage or pulse-width modulation (PWM) duty cycle. The fan speed might be monitored to detect a fan stall, but the speed isn’t adjusted to converge on a target speed. In many cases the fan speed is not even monitored, which allows use of a slightly less expensive fan without a speed signal output (tachometer).

Closed-loop fan control provides an ideal way to control fan speed because it drives the fan to a target fan speed by measuring a tachometer signal from the fan. It then automatically adjusts the drive setting until the target speed is reached. The closed-loop fan speed method eliminates the variables that cause fan speed variation.

Designer Challenges
For a given cooling requirement, there are other acoustic issues to consider besides keeping the fan speed as low as possible. One such issue is abrupt speed changes. Consumers notice sudden fan speed changes much more than gradual changes, so features that control the rate of change to the fan speed are often utilized.

Another issue is enclosure resonance. Many product enclosures have mechanical or acoustic resonance points that need to be avoided, or an unpleasant amount of noise will be emitted at resonance. In this situation, a system that makes fan speed changes in well-controlled steps can provide superior results.

Simply starting a fan can be a challenge. DC fans require a certain level of drive to begin to spin, and this drive level varies from fan to fan and increases as the fan ages. An effective but noisy approach is to turn on the fan fully for a period of time to ensure it starts up. Then the design engineer can reduce the drive to the desired target speed. More sophisticated closed-loop controllers provide spin-up options that ensure the fan starts but without a noticeable surge at spin-up.

Systems with two or more fans present an additional challenge, especially if the fans are the same type and speed rating. Operating two fans at approximately the same speed can create acoustic beat frequencies. Closed-loop fan speed control allows the system designer to offset the fan speeds to avoid this phenomenon.

Many product designs include microcontrollers, which have the resources and programmability to control the fan rpm directly; however, there are numerous issues with this approach. Closed-loop rpm control requires frequent read/adjust iterations that place a heavy burden on the MCU and the communication bus. This may limit the speed of convergence and the ability to respond to changing conditions. Also, adding fan control to an MCU-based product requires program code and data storage space and dedication of engineering time to develop fan control expertise. Fan control issues such as spin-up, detection of fault conditions, tachometer implementation and loop response are already solved in FSC-ICs, which are available today.

The Selection Process
A typical closed-loop fan control system is illustrated in Figure 2. ICs in this category share many common features, including one or more temperature sensors, variable fan drive, tachometer input, communications bus and a fan speed control algorithm. However, there are significant differences in implementation. When selecting an FSC-IC for closed-loop fan control, it is important to consider these features:
Temperature sensors: On-chip temperature sensors are used to set the fan speed. The accuracy of these sensors can vary from 1°C to 3°C, and other features such as digital averaging and resistance error correction can greatly improve the quality of the system solution. Fan Drive Output: This can be either a variable DC voltage (Linear) or a PWM signal. A Linear output can save cost by integrating the drive circuit, while a PWM output provides a wider fan speed range.

Tachometer Input: This input converts a square wave from the fan to a fan speed, which is used as feedback for the fan speed control algorithm. The accuracy of this speed measurement can vary widely due to the IC's oscillator variation. When tight fan speed control is required, select an FSC-IC with good tachometer accuracy or a clock input, which will eliminate errors due to the FSC-IC's internal oscillator variation.

Communication Bus: Common buses are I²C, SMBus and SPI. A communications bus is typically required or used to configure and monitor the FSC-IC. Many MCUs have built-in I²C masters, which also support SMBus.
Configuration and Control: The FSC-IC may be set up and controlled by the host MCU or configured for stand-alone fan control. The system designer needs to consider the MCU capabilities and system architecture to determine the optimal interaction between the host and fan controller.
Fan Speed Control Algorithm: The control algorithm can be either linear or table-driven for discrete fan speed steps. A table-driven algorithm supports a non-linear temperature to fan speed relationship and can avoid operating at undesirable fan speeds that might cause acoustic resonance.
Supervisory Functions: A variety of features are available to allow the FSC-IC to operate independently of the host MCU and utilize an interrupt signal to request host intervention when required. These features can greatly reduce the processing load experienced by the MCU.

The following sections elaborate on some of these important features.

Temperature Measurement
FSC-ICs can perform accurate temperature measurement using "thermal diodes," which may be either diode-connected transistors or substrate PNP transistors built into an ASIC or CPU. An accurate temperature can be determined by performing precise voltage measurements while sourcing two different currents through these diodes.

The accuracy of the temperature measurement can be reduced by series resistance, non-ideal thermal diodes and noise induced from nearby switching data/clock signals or regulators. Advanced features such as resistance error correction and digital averaging can combat these issues to improve the accuracy and integrity of the temperature measurement, thereby improving system performance.

Tachometer
The tachometer signal (TACH) from the fan is a square wave proportional to the fan speed. A typical two-pole fan provides two pulses per revolution of the fan. The tachometer signal is typically used to gate a higher frequency within the FSC-IC, resulting in a "TACH count" representing the number of high-frequency pulses measured in a single fan revolution. TACH counts are inversely related to RPM and when a target fan speed is set, a TACH target count value is used.

A wide range of fan speeds needs to be supported by a general-purpose, closed-loop, fan controller, ranging from about 500 rpm to greater than 15,000 rpm. For this reason, there is typically a method of setting the operating range of the fan in the IC configuration, allowing the counters and registers to provide adequate resolution.

The designer should also consider how accurately the fan speed needs to be controlled because the accuracy of the TACH can vary greatly and will limit the effectiveness of the fan speed control. The TACH is usually based on an internal ring oscillator, which shifts with temperature and process, often 10% or more.

A ring oscillator that has been temperature compensated and trimmed can maintain accuracy of 2% to 3%. If very accurate fan speed control is needed, some FSC-ICs have external clock inputs, which also allow synchronizing multiple fan drivers.

Configuration and Control Options
The product designer has several options available to configure and control the FSC-IC. In one common approach, the MCU monitors one or more temperatures, determines what the target speed should be and sends the target speed value to the fan controller over the communications bus. The FSC-IC then manages changing and maintaining the fan speed. The FSC-IC may also be programmed to control fan speed autonomously after initial configuration by the MCU. A table relating fan speed to temperature is programmed or a linear algorithm is configured to vary the fan speed as the temperature changes. This approach off-loads the task of controlling fan speed completely from the MCU.

Systems that do not have an MCU can utilize an EEPROM to configure some fan controllers. At power-up, the contents of the EEPROM are loaded into the FSC-IC to program the register contents for completely stand-alone operation.

Selecting a Fan Control Algorithm
Two common methods to control fan speed with temperature are the linear control algorithm and table-driven (steps) approach. The linear control algorithm, illustrated in Figure 3a, increases the fan speed proportionately between minimum and maximum temperatures. When the temperature is at or below the minimum set point, the fan speed will be either OFF or set to a minimum value. A temperature half way between minimum and maximum will drive the fan to 50% of the maximum speed, and so forth.

Table-driven controllers relate a number of temperature set points to corresponding fan speeds. As temperature increases to reach the next highest set point, the fan speed is increased accordingly as illustrated in Figure 3b. This approach is useful for avoiding resonant frequencies and to implement a discrete set of RPM controlled fan speeds in a thermal design.

With both approaches, the designer can control the rate of change of the fan speed setting with parameters such as update time and maximum step size. This allows the fan speed to change at a barely perceptible rate.

While some FSC-ICs use a single temperature to determine the resulting fan speed, more complex devices can utilize a combination of temperatures. Individual temperature set point registers can be programmed for each temperature channel. The fan speed is determined by the temperature channel that specifies the highest fan speed.

Supervisory Functions
FSC-ICs can perform a variety of monitoring functions that further reduce the MCU processing load while ensuring reliable thermal management and system protection. If conditions such as a fan stall occur, an interrupt signal can be provided to the MCU for intervention. Some conditions, such as exceeding a critical temperature or a missing tachometer signal can justify turning the fan to full speed or triggering a system reset or shutdown.

Summary
Closed-loop fan speed control offers many advantages to improve cooling system design and to reduce acoustic noise emissions. This approach also reduces engineering characterization time by removing the speed variable. Less accurate and lower cost fan drive circuits may be used because errors in the level of fan drive are cancelled by the closed-loop control. Finally, and perhaps most importantly, by compensating for the variation of many fan characteristics, rpm control allows the use of lower cost fans and makes multiple sources easier to manage.

Ken W. Gay is Director of Systems Engineering at SMSC. He can be reached by e-mail at [email protected]