Introduction: Demands on high-temperature electronics
High-temperature electronics is a growing market, as industries seek increasingly to enhance monitoring and control using electronic technologies. There are two major trends: electronic systems are being deployed closer to heat sources operating above the usual specified temperature limits for electronic components, such as automobile or aircraft engines. Secondly, maximum operating temperatures are increasing significantly beyond 200°C, particularly in applications such as automotive systems or in well-logging equipment as exploration companies seek to exploit natural resources held deeper underground. Industrial, medical and space applications are also presenting demands for electronic systems offering precision and stability at temperatures up to 175°C.
Today it is possible to find silicon-based ICs capable of withstanding operating temperatures at 175°C. However, merely surviving in a high-temperature environment is not good enough. To meet end-user demands for accuracy, as well as safe and repeatable operation, engineers also expect predictable performance, with stability and reliability.
Accuracy is vital, for example, in measurement and data-acquisition systems for use in the petrochemical exploration sector. Data taken from deep bores is used to analyze formations and thereby identify deposits such as oil or gas. Data collected is also used to determine the best locations to begin extraction. As today's deepest wells can be more than 5km deep, measurement and data acquisition systems can encounter temperatures as high as 250°C. Since significant investment decisions ride on the data collected, the systems must operate accurately at these extremely high temperatures. High-temperature electronics are also required in equipment used subsequently to complete the bore and extract resources.
In the aerospace industries, the trend to replace traditional hydraulic actuators with lighter and more economical electrical equivalents is extending to areas such as engine components and braking systems. Electric oil pumps, for example, are replacing mechanical pumps in the lubrication subsystems for turbine engines, bringing motor-drive electronics into close proximity with lubricants operating at temperatures over 200°C.
The advance of electronic systems in the automotive sector has been rapid and extensive. Legislation covering emissions and safety has driven adoption of electronic engine management as well as controls for traction and braking. At the same time there is constant pressure on the packaging of automobiles, forcing engine and powertrain into smaller compartments so that body designers can maximize selling points such as cabin space and carrying capacity. Hence to reduce size, as well as the cost of wiring, the ECUs are mounted as close as possible to the engine, as well as in locations such as inside the gearbox or other transmission components. This requires electronic boards and components to perform accurately at the operating temperature of the engine or gear box. Moreover, smaller engine compartments impose tighter restrictions on heatsink sizes, and changes to vehicle aerodynamics deliver less cooling airflow to the radiator; hence the temperature increases under the hood.
Finally, the use of power electronics for motor drives in hybrid and electric cars is also pushing temperature requirements for automotive-qualified electronic components, that are now moving up from 150°C maximum to 200°C.
The tendency to mount electronic control and processing modules close to sensors and actuators, to prevent noise entering the system through remote wiring, is also driving high-temperature electronics into applications such as automotive braking systems as well as more diverse sectors including industrial process control. Medical electronic systems also have an interest in high-temperature electronics, for example to take advantage of enhanced temperature monitoring for sterilization systems.
In space, the ability to operate at very high temperatures up to 300°C can be a requirement for some planetary missions. In satellite applications, on the other hand, the temperature of electronic assemblies may be regulated. However, the greater reliability and stable performance of high-temperature components can enable more robust systems, and may in some instances obviate the need for temperature regulation or cooling. At the other extreme of the temperature spectrum, space systems can expect to operate in extremely cold conditions.
Designing with high-temperature ICs
To address data-acquisition and drive-and-control applications at high temperatures, designers need access to key functions such as analogue-to-digital converters and precision amplifiers optimized for minimum parameter shift with temperature, as well as drivers and power MOSFETs capable of operating at extremes without significant derating. Other commonly used components include timing devices such as 555 timers and clock generators, voltage regulators (both linear and switched) and references, and a variety of general-purpose components such as standard 74-series logic functions. Cissoid's product portfolio, for example, also has a roadmap to more highly integrated functions such as telemetry and RFID ICs, as well as higher resolution and ratings for devices such as ADCs and power MOSFETs.
The fabrication technology of choice for high-temperature electronics is Silicon-on-Insulator (SOI). Its extremely low leakage current, compared to standard silicon technology, is a key attribute allowing SOI devices to deliver enhanced performance at elevated temperatures. SOI is proven for high-temperature, low-temperature and radiation-hard applications. Indeed, engineers at the NASA Glenn Research Center have performed a number of tests on devices such as Cissoid's 555 timer, the CHT-555, at temperatures ranging from "195°C to 375°C, recording stable performance with low parameter drift over the full temperature range.
Beyond simply surviving exposure to extremes of temperature, the skill in designing devices using SOI technology lies in optimizing for operation over a wide temperature range and ensuring long-term reliability throughout sustained high temperatures and repeated temperature cycling. Careful attention to circuit design, modeling, layout and assembly is necessary to meet these demands at a marketable price.
Effective techniques include awareness and compensation of temperature-related effects occurring at the silicon level. The NASA analysis of the CHT-555, for example, tested the device at 375°C for 750 hours. During the test the device was configured to produce a square wave, and the shift in frequency was monitored to determine the extent of any drift. The square-wave frequency shifted from 3.208kHz at 23°C to 3.361kHz at 375°C. After 750 hours of continuous operation the oscillation frequency increased slightly further to 3.385kHz. Upon returning to room temperature the oscillation frequency had increased to 3.261kHz, corresponding to an increase of only 0.87%.
The NASA tests set out to understand the performance and deratings of SOI devices at temperatures above 225°C. Testing at elevated temperatures is also useful to accelerate qualification to lower operating-temperature limits such as 150°C. Testing at 250°C reduces the time to failure by a factor of 40 compared to 150°C, delivering a corresponding reduction in qualification time.
Careful design can deliver high-temperature semiconductors displaying stable performance over wide temperature ranges from "200°C to well over +200°C. However, system-level design must also take into account the effects of temperature variation on surrounding components. Effects include changes in the capacitance and equivalent series resistance (ESR) of capacitors, and increasing DC resistance (DCR) in inductive components.
| Figure 1|
Figure 1 shows the circuit architecture of a linear voltage regulator, which has been optimized to accommodate large variation in the ESR of the output capacitors connected externally. This topology compensates for process-related variations as well as changes in external components, and has demonstrated high stability over a wide range of input-voltage and load-impedance conditions.