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Home > Technology List > Capacitors for high-temp, high-rel apps

Capacitors for high-temp, high-rel apps

Update Time: 2019-12-18 00:00:13

Capacitors for high-temp, high-rel apps

As the electronic content in automotive, aerospace, and other heavy-duty applications increases, the demand for components with ever-higher temperature ratings poses a critical design challenge. Today’s OEMs are constrained by the traditional 125°C upper limit for components, and see this limit as a serious design constraint impeding the development of distributed control systems, smart sensors, and actuators.

Pulse-energy capacitors are especially suited for rugged applications in temperatures to 180°C.

To keep pace with this challenge, capacitor technology is being developed for a variety of new applications requiring high reliability and stability at high temperatures. Component vendors are developing a new breed of devices to perform at temperatures well beyond the capabilities of today’s ceramic capacitors.

Dielectric characteristics

Ceramic capacitors can be grouped in dielectric classifications based on their electrical performance and basic chemical composition. Class I dielectrics are stable with temperature, display a linear capacitance variation with temperature, present lower dielectric loss, experience no dielectric absorption, and experience no aging. Class II dielectrics offer significantly higher dielectric constants and facilitate higher capacitance values in a given package size but demonstrate significantly poorer temperature, voltage, aging, and loss performance.

Low-temperature coefficient of capacitance (LTCC) is important for ensuring circuit stability and function, both at elevated operational temperature conditions and to tolerate temperature swings. Increasing the inherent volumetric capacitance of a given dielectric is also a desirable result which, when coupled with stability over temperature and voltage ranges, equates to a higher-reliability component under extreme temperature and voltage conditions.

The challenge for capacitor manufacturers is to confront the fundamental principles of materials science, which force a tradeoff between high values of capacitance (that is, dielectric constant) and high-temperature stability. Multilayer capacitors (MLCs) are categorized by dielectric performance with temperature — or temperature coefficient — since these devices vary in behavior over temperature. The choice of component is therefore largely determined by the temperature stability required of the device, that is, the type of dielectric and the size necessary for a given capacitance voltage rating.

Determining dielectric class

Operational temperature and voltage conditions will help the designer determine which class of dielectric is best suited for the application. Class I systems offer the best stability in regard to temperature and voltage.

However, the volumetric capacitance limitation of these systems will increase board-space requirements. Extended-temperature-compensating dielectrics are a subgroup of Class I systems, which display near-linear and predictable temperature characteristics with dielectric constant values three times or more that of base Class I systems. Both categories offer predictable linear behavior with temperature within prescribed tolerances.

Class II dielectrics offer much higher dielectric constants, but have less stable properties when it comes to temperature, voltage, frequency, and time. However, even with capacitance loss at elevated temperature and voltage, Class II dielectrics typically have higher effective capacitance than Class I systems, allowing for smaller case sizes and improved board-spacing efficiency.

Although Class II dielectrics have variations several orders of magnitude greater than those for the linear dielectrics, offering dielectric constant values of 20 times or more over Class I dielectrics can result in significant advantages depending on circuit application requirements.

Multilayer capacitors are typically classified by temperature coefficient of dielectric materials.

New and emerging high-temperature, high-reliability MLCs address the very high operating temperature ranges of today’s applications. Selection considerations are based on the devices’ material chemistries and design requirements.

Class I and II capacitors

Among the latest offerings of Class I and II dielectric MLCs are devices available with operating temperature ranges from –55° to 150°C, 160°C, and 200°C in SMT case sizes from 0805 to 7565. These capacitors are specifically designed and tested to operate at rated high temperatures above the current military specification requirements of 125°C.

These high-temperature devices can be used in distributed power systems for engine-compartment circuitry and oil exploration, data logging, and geophysical probes. Design considerations include determining required temperature range and if high-reliability screening is necessary, although many capacitor manufacturers strongly encourage high-reliability screening to ensure any infantile failures are removed from a given lot of capacitors.

Encapsulated capacitors

Leaded, high-temperature encapsulated and coated radial capacitors can be used in very harsh environments where isolation and protection of the device is required for optimum reliability. Components can be supplied with 22-AWG tinned copper leads in sizes from 1515 to 7565, marked with capacitance and voltage ratings. Where spacing is critical, an epoxy coated format version is available, and custom shapes can be commissioned to fit specific system constraints.

Encapsulated and coated radial-leaded capacitors are often chosen for use in harsh environments.

Pulsed energy capacitors

Pulse-energy (“R” dielectric) capacitors can be used for especially rugged applications that reach temperatures up to 180°C, including harsh environments such as the control of high-temperature materials, oil-exploration sensing systems, down-hole equipment, and automotive/avionics engine-compartment circuitry. Circuit applications include detonation, power-supply filtering, energy storage, and coupling/decoupling.

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