Power Electronics

Fan Control Systems Are More Than About Blowing Hot Air Away

Thermal management is becoming more challenging for design engineers. ICs like microprocessors and graphics engines are become smaller, more powerful, occupy smaller pc-board spaces, and dissipate greater heat levels that must be managed and controlled.

Modern processors, like those from Intel, are helping designers achieve high-performance levels from compact processors, previously unattainable, with reasonable power dissipation levels and easier to handle thermal management challenges. One need only look at the Intel Atom Z520PT processor used in Kontron’s micro-ETXexpress-XL sub-8-W module to see this. The module is designed for −40°C to +85°C industrial temperature ranges, suiting it for extreme environmental applications. Intel offers an enhanced Intel SpeedStep software technology that allows the system to dynamically adjust processor voltage and core frequency, which can result in decreased average power consumption and decreased average heat production.

Typically, fans, heat sinks, heat pipes, and thermal transfer materials are being enlisted to solve the thermal management challenge, using active and passive convection, radiation, and convection methods. Convection cooling using on-board and system fans is generally the most widely used thermal management mechanism employed. Usually, heat is exhausted out of the chassis or is forced through side walls into the ambient environment. Here, placement of the properly selected fan atop a CPU yields the best heat-removal results (Fig. 1).

Choosing the Right Fan

Modern fans are very efficient, thanks to newer blade and bearing designs that provide little or no noise or vibration. But like everything else, no two fans are alike with each offering different ambient-air temperature limits. So, check the fan’s mean-time-between failure (MTBF) rating. Most, however, are useful for a large number of CPU heat-removal and cooling applications as long as proper heat-sinking is employed and chassis air space design is sufficient for good heat removal and proper thermal air flow is considered. Check to make sure that there are no “dead” zones in the air-flow path where heat may remain stagnant.

In server applications where there are hundreds of processors to cool, the choice of the right cooling fan is critical. Some of the more modern fans feature active and passive cooling. They contain copper heat sinks or cooling plates to increase the cooling surface area and facilitate the cooling process of the heat sink or plate. And some may feature liquid cooling.

Generally, fan speeds ranging from about 1800 to 3000 rpm can provide sufficient server system CPU cooling. There’s another facet about cooling fans that is often not taken into consideration: dust which falls on the cooling mechanism, be it a plate or heat sink. Dust acts as a thermal insulator and impedes air flow, thus it is important for users to periodically clean the fan with gas dusters or other means for optimal performance.

Fans also draw power, which in turn creates heat that can be a significant source of energy waste. According to Don Folkes, business manager for Maxim Integrated Products Inc., fans draw anywhere from 7 to 15% of overall power in a data center environment, a strong reason why thermal management through local temperature and remote temperatures sensing as well as fan control is needed.

Fans are not perfect. A fan is generally one of the earliest mechanical components in a heat-removal system that fails, leading to a lower MTBF parameter. That’s why it is necessary to employ modern software controlled monitoring systems thanks to fan-controller ICs that help mitigate this problem. In multi-core systems where more than one fan is being employed, the use of fan controllers is critical and can help the development of cost-effective thermal-management systems.

There are many fan-control ICs available on the market. One is the EMC 2113 single-RPM-based fan controller from SMSC. It features multiple zones and hardware thermal shutdown capability.

The programmable unit is 4-wire fan compatible for both low- and high-frequency PWM, is accurate to within 2% for fan speeds of 500 to 16,000 rpm with an automatic tachometer feedback signal, includes a temperature lookup table for controlling fan speed or PWM drive setting, and features eight steps that incorporate up to four temperature zones, user selectable simultaneously. It also allows up to three external temperate channels (150 kHz to 40 kHz) and features error and beta correction to within 1°C accuracy and 0.125°C resolution.

Another interesting fan controller IC is the Melexis single-chip MLX90287 Hall effect high-performance fan controller IC for driving low-noise speed-controlled single-coil brushless fans in energy efficient designs (Fig. 2). Designed to control fan cooling speeds using PWM or analog signals, it features a wide operating voltage range of 4.5 to 16 V, making it compatible with widely used 12-V applications. It can handle cooling fans with up to 550 mA of continuous drive current and can operate within a temperature range of -40 to 150°C, making it suitable for harsh environments like automotive, industrial, military and aerospace applications.

Single-chip Fan Driver

Diodes Inc. has developed what it says is the smallest fully featured single-chip fan driver with high-output drive capability to provide a cost-effective solution for fan and motor drive applications. Utilizing Diodes’ low-profile DFN packaging technology, it can provide an average motor drive current up to 500 mA. It integrates a high-sensitivity Hall Effect sensor, amplifier and an internal H-bridge driver output stage, suitable for single-coil fan motor applications. A PWM speed control pin provides enhanced motor speed control by varying the duty ratio of the PWM signal, in addition to the supply voltage. The device’s operating voltage range of 1.8 to 6 V suits it for low operating voltage BLDC fans requiring low-voltage start-up in both Vdd and PWM speed control modes. Additionally, the AH5795’s bi-directional full-bridge driver uses soft switching to minimize audible switching noise and electromagnetic interference (EMI). The AH5795 has an ambient operating temperature range of -40°C to 105°C, and is available in space-saving, low-profile DFN2020-6 and TSOT23-6 packages.

To protect the coil from overheating, the new motor controller includes integrated locked rotor detection and automatic self-restart functions, which serve to shut-down the output driver in the event of a locked rotor and then restarts the motor when the rotor is freed. In addition, a tachometer output is provided by an open-drain frequency generator, which allows external speed monitoring. The all-in-one smart motor controller eliminates the need for most external components, including a timing capacitor.

An additional member of the smart fan motor controller family will be launched later this year, which includes all of the features and functionality of the AH5795 plus internal timing advance to improve tail-end current, thus enabling further reduction of audible and EMI noise.

System Control

Typically, the need to cool hundreds of processors in a data center is a system-level task that involves monitoring and controlling hundreds of sensors and fans simultaneously. It also involves making use of software algorithms. Fan controller ICs, like the MAX31785 from Maxim Integrated Products Inc., can handle up to 6 fans simultaneously. It is designed to work with other Maxim products that form system-level tasks.

Maxim also provides the MAX34440/34441 multi-channel power-supply manager that integrates thermal management with “black box” fault-logging capabilities (Fig. 3). The chip includes ample non-volatile flash memory enabling it to record failure data for rapid root cause determination.

When it is combined with the aforementioned MAX31785 fan controller, it can function as a complex system manager that includes extensive non-volatile power-supply sequencing and margining in either a “big box” configuration or a “pizza box” configuration (Figs. 4a and 4b, respectively). In the former configuration, it controls and monitors up to six power supplies. In the latter configuration, it can be used as a network switch that supports up to five power supplies plus a fan.

The complexity of the number of ICs often used in fan control applications can be daunting when you consider the number of temperature sensing and fan control functions involved. This is particularly true for cooling systems that contains a large number of processors like those typically found in a data center.

For example, a simple diode or transistor can usually serve as the lowest cost and smallest-size approach for analog temperature sensing. However, such sensors with microvolt level voltages must interface with an analog-to-digital converter (ADC) and are not suitable for long distances. On the other hand, integrated ADCs are available, albeit at a higher cost, with the converter, a diode temperature sensor, and an I2C interface and are more suitable for digital temperature sensing over long distances. There are also thermocouples, thermistors, PWM-based digital temperatures sensors, etc.

Using Programmable Logic

Often, microcontroller units (MCUs) can be used for PWM signal generation where there are a large number of fans. MCUs are sometimes also needed for temperature sensing and must work with the host CPU which itself may be a CPU, FPGA, or an ASIC, in either dedicated or shared modes. This is why Cypress Semiconductor is making available a programmable logic-based cost-effective fan-control solution using its PSoC3 IC working with Cypress’ PSoC Creator software IDE (Fig. 5).

Fan control for either a 3- or 4-wire implementation can be performed by the PSoC3 through a PWM interface where the user can adjust the duty cycle of the PWM to modify the actual fan speed. It allows the customization of functional blocks to meet requirements for thermal management and control systems (Fig. 6). It can handle both digital and analog temperature sensors, as well as provides fan tach feedback for an open- or closed-loop control range.

If the use of fans and fan control circuitry to cool a CPU is too much of a hassle, there is a new alternative. Noise Limit Inc. has created what it considers is the next generation CPU-cooling solution which it claims is reliable, flexible and low noise. The SilentFlux passive pro cooler removes the cooling fan completely with the aid of a high-efficiency condenser. It uses a bubble pump technology that is self-contained and relies on the heat generated by the CPU it is cooling. This heat creates a hot liquid and gas bubbles which move through a hermetically sealed loop tubing to cool the processor. And it is said to be 100% recyclable.

Wortmann AG, a leading systems integrator in Germany, is reportedly using SilentFlux for cooling in Wortmannís TERA line of workstations in combination with 95-W CPUs such as the Intel Core 2 Quad Series. “Noise Limit’s cooling system products are intrinsically engineered to achieve a new low noise threshold, but the design also offers greater flexibility in footprint layout for various computing-based applications, including media centers, all-in-one PCs, servers, and high-end gaming products,” says Bob Senior, executive vice president at Noise Limit Inc.

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