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Design an efficient, smart thermal mgmt system

Posted: 29 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:thermal management systems? TMS? thermistors? RTD? MCU?

The rapid growth of hardware-based systems has intensified the need for efficient thermal management systems (TMS). A simple TMS can be an on/off thermostatic controller that switches a fan on when the temperature reaches a certain limit and turns it off when the temperature drops below the set threshold. However, sensitive electronic devices such as routers, switches, servers, and medical equipment require a more complex TMS approach to ensure that operation lies within the safe temperature range consistently. The fundamental processes involved in a TMS are illustrated in figure 1.

Figure 1: Basic block diagram of a TMS.

Temperature Sensors: A wide-range of both analogue and digital sensors can be used for measuring temperature. Commonly used sensors include thermistors, resistance temperature detectors (RTDs), thermocouples, and temperature sensor ICs. The choice of sensor depends upon the required range and precision of the temperature readings. Multiple sensors of various types can be used together for more accurate measurements.

Sensor Interface: The temperature sensor interface can be another limiting factor. Analogue sensors require signal conditioning (i.e., noise removal and amplification) and analogue-to-digital conversion before temperature values can be fed to the MCU for processing. On the other hand, digital sensors require the availability of I/O that supports communication protocols such as I2C, SPI, or 1-wire.

Temperature control algorithm: Running fans at their maximum speed can provide the best cooling effect but will consume a lot of power and produce acoustic noise. Furthermore, this reduces the lifetime of the fan. To increase the efficiency of the system, temperature control algorithms implemented on an MCU can run the fan at an optimal speed to balance cooling, noise produced, and operating life.

Fan Controller: The fan controller provides an interface to the MCU and, given the required speed by the MCU, drives the fan using a pulse width modulated (PWM) signal. The fan controller adjusts the PWM duty cycle to control the speed of the fan. Fan control can either be open loop or closed loop. In case of open loop fan control, the controller changes the duty cycle of the fan control signal without knowing whether fans are reaching the desired speeds. Closed loop control incorporates feedback from the fans to the controller to ensure the fans are running as expected. In addition, this feedback can be used to alert the MCU if there is a fault condition (i.e., fan stall or locked rotor).

Fans: Brushless DC (BLDC) fans are the most commonly used fans for cooling purposes. These fans come in different sizes, voltage ratings, and number of wires. The 2-wire fans are the simplest ones, with voltage connections only, while the 3-wire fans have an additional wire that provides the fan speed. The design of a TMS can significantly vary, based on specifications such as space constraints, maximum allowable noise, cooling requirements, and power consumption.

Building TMS using PSoC
Today programmable system on chip (PSoC) technology brings high levels of integration to microcontrollers to allow a single-chip approach to TMS design. For example, instead of separate fan controller circuitry, the fan controller can be implemented using programmable digital blocks present inside the SoC device running independent of the MCU sub-system. Moreover, PSoC devices provides the capability of interfacing with a number of digital sensors and other peripherals such as LCDs.

A PSoC device can serve as a more efficient alternative to the dedicated IC approach because of its mixed-signal functionality. It allows the digital peripherals to be configured by the user through an MCU sub-system. The configuration can be changed at run time as per the requirements thereby enhancing the flexibility. Moreover, several ICs, temperature sensors, and GPIOs can be integrated with the PSoC to build a complete TMS (figure 2).

Figure 2: Implementation of a TMS on Cypress' PSoC 3 device.

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