Building an efficient, intelligent, and flexible thermal management systemThe rapid growth of hardware-based systems has increased 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 analog 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. Analog sensors require signal conditioning (i.e., noise removal and amplification) and analog-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 a TMS using a Programmable System on Chip
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
The PSoC is fed with the data acquired from the various types of temperature sensors to which it is interfaced. Different resources present inside the PSoC are used to interpret this data. Input from the analog sensors is filtered and quantized to convert it into the appropriate digital format while the data from digital sensors requires I2C/SPI interfacing with the MCU. Temperature readings are stored in EEPROM for event logging. Other peripherals, such as an LCD or UART, can be used to display fan speeds, temperature, and other necessary information. Both open loop and closed loop implementations are possible with PSoCs. In the case of closed loop control, the tachometer signal from the fan and an internal timer are used to calculate the RPM-speed of the fan.
Firmware and Hardware Fan control: A PSoC-based TMS also supports both firmware and hardware fan control. With complete firmware control, the MCU is responsible for the entire process, from measuring the temperature to PWM generation. This method is unsuitable when multiple sensors and fans need to be interfaced with the TMS because of the limited resources of the MCU itself. Furthermore, closed loop control will slow down the responsiveness of the TMS.
An approach using partial firmware and hardware fan control can be implemented using an array of small programmable devices called UDBs present inside the PSoC architecture. A UDB contains two small PLDs, a datapath, and status and control logic as shown in Figure 3. The routing channel consists of an extensive signal routing system called the Digital System Interconnect (DSI) connecting UDB arrays to MCU, DMA controller, I/O pins, global clocks, etc. UDBs are smaller than a complete CPLD or FPGA but can efficiently perform calculations, comparisons, and other data management tasks at hardware-based rates.
Figure 3: Block diagram of a UDB
With partial firmware fan control, PWM generation and fan speed monitoring is offloaded to the UDBs. The MCU still fully manages speed control. As shown in Figure 4(a), the MCU repeatedly calculates the target speed and sets the PWM duty-cycle based on the fan’s current speed.
With complete hardware-based fan control, the duty-cycle is calculated within the UDBs with no intervention from the MCU. The MCU can set the desired speed but the UDBs are responsible for adjusting the PWM duty-cycle to maintain that speed as show in Figure 4(b). This frees up the MCU and enables it to perform other important tasks.
Figure 4(a): Partial Firmware control
Figure 4(b): Hardware control
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