After what seemed, for many of us, to be an interminable winter it’s finally June and that means Mother Nature is getting ready to turn up the burners. But before summer gets too hot to handle and we occupy ourselves with such important tasks as finding the SPF 50 block to prevent sunburn, I thought it a perfect time to take a look at how passives—and specifically capacitors—are meeting the challenge of very high temperature applications. Temperature-critical applications such as down-hole drilling, aerospace (in particular jet engines) and automotive use are generating the need for capacitors with very high operating temperatures, often approaching or exceeding 200 °C.
The need to drill deeper has significantly increased temperatures in today’s wells. Customary high-pressure/high temperature (HPHT) wells with pressures up to 69MPa (MegaPascals, 1 MPa = 145.037738 pound-force/square inch [PSI]) and temperatures of up to 177 °C, have now been expanded to what are being called “extreme high-pressure/high-temperature” (xHPHT) wells with pressures up to 138 MPa and temperatures of 200 °C and above. These applications also involve strong vibration and shock. In this application high temperature capacitors are needed for the DC/DC converters used in drilling heads that experience rising ambient temperatures the deeper you drill.
In automotive electronics the most temperature demanding locations are in the engine, transmission and brake systems (some of the wheel-mounted components can reach or exceed 250 °C). The ambient temperature of engine control systems, which are typically placed very close to the engine itself can range from – 55 °C to 200 °C. Power electronics for fuel pumps, motor controls and electric braking require high temperature capacitors that can withstand extensive and stressful thermal cycling over a long operating life.
In the capacitive discharge ignition (CDI) systems used to ignite jet engines the operating temperature can reach 200 °C or more. High temp capacitors also are used in conjunction with jet exhaust sensor systems, landing systems, fuel pumps and other aviation applications.
Several dielectrics including plastic (e.g., polytetrafluoroethylene [PTFE] or PEI polyester), mica, aluminum electrolytic, tantalum and ceramic are used for manufacturing high temperature capacitors. These technologies provide very high stability and good mechanical and electrical characteristics in temperatures ranging from of -55 °C to +200 °C, depending on the dielectric technology, and some do not require voltage de-rating.
In these high temperature apps the two dominant capacitor technologies are ceramic and tantalum capacitors. With operation temperatures exceeding +200 °C and even approaching +250 °C ceramic and tantalum capacitors are available that incorporate advanced designs and construction methods to operate reliably at these elevated temperatures.
High Temperature Ceramic Capacitors
High temperature ceramic capacitors can offer low ESR and excellent inrush current and ripple capabilities. One disadvantage is that they tend to be physically larger than other types of capacitors.
Ceramic capacitors are divided into two applications classes: EIA Class I and EIA Class II. Class I (sometimes called temperature stable ceramics) have little to no change in capacitance value over a wider temperature range (reducing the effects of capacitance drift with temperature) than X7R dielectrics while also having very high voltage stability. Class 1 ceramic capacitors commonly have names like “NP0” or “COG”. Modern C0G (NP0) formulations contain neodymium, samarium and other rare earth oxides. In recent years, high temperature (200°C-rated) surface mount base-metal electrode C0G MLCCs have been developed for extreme operating conditions.
Class I types often have a much smaller capacitance in comparison to Class II X7R’s. The K value for Class I dielectric materials is very low resulting in lower capacitance ratings so a much larger building block might be needed in order to achieve sufficient capacitance and the standard stacked MLC capacitors are generally quite large.
For some xHPHT down-hole environments, a leaded device is used in order to design against severe shock and vibration environments, providing strain relief especially in drilling tools and especially with larger case sizes.
Class II (X7R) ceramics offer the benefit of higher dielectric constants resulting in higher volumetric efficiency and thus higher capacitance, Nevertheless there is significant capacitance loss at high temperatures. The capacitance over the temperature range from – 55 °C to +125 °C is typically flat; but the capacitance drop off at 200 °C can be very significant, depending on the CV (capacitance/voltage) of the part.
With X7R (Class II) types an important factor to consider is the drop of capacitance with applied voltage. Aging also plays a role in capacitance loss but it is not considered significant compared to the effects of temperature coefficient of capacitance (TCC) and voltage coefficient of capacitance (VCC).
High Temperature Tantalum Capacitors
Tantalum capacitors do not exhibit capacitance drop-off at high temperatures and their value is stable over a wide temperature range. Based on the fact that this technology offers excellent volumetric efficiency, higher capacitance values can be achieved in much smaller packages. Wet high temperature tantalum capacitors offer lower leakage current values compared to solid tantalum technology. Both wet and solid tantalum capacitors require derating due to temperature.
High temperature wet tantalum products are primarily designed for use in down-hole drilling equipment and avionic engine control due to their capabilities for high ripple current. Wet tantalum capacitors packaged in a hermetically sealed case are available in high temperature designs suitable for use up to 200 °C with a category voltage of 60% the room temperature rated voltage (0.6Vr).
Development of solid tantalum capacitors in hermetically sealed ceramics packages has resulted in 230 °C rated SMD capacitors. Solid Ta capacitors with manganese dioxide (MnO2) cathodes can withstand harsh application conditions. In a 2013 CARTS paper (“High Temperature TA/MnO2 Capacitors” by Y. Freeman et al) KEMET authors showed that replacing silver on the top coating with electrochemically plated nickel provided what they described as a “radical improvement” in the high temperature stability of the Ta/MnO2 capacitors. Another paper, this one from AVX (“SMD Tantalum Capacitors Break Limit of 200 °C for Continuous Operation, by R.Faltus and, T.Zednicek of AVX Czech Republic) describes a novel 200 °C capacitor concept and technical development and processes of enhancing operating temperature of solid tantalum capacitors. It discusses new technological processes and materials that have been implemented in order to enhance the operating temperature range of AVX’s THJ family tantalum capacitors.
Film capacitors are known for their reliability, high current and voltage withstanding capability and their resistance to intense mechanical shock. But whether they could withstand temperatures above 170 °C was not well known. In a CARTS 2013 paper (“Film Capacitors for High Temperature, High Voltage and High Current”, by Luca Caliari et al.) Kemet aimed at showing designers that film capacitors can be a choice for extremely harsh environment applications with a typical working temperature that exceeded 200 °C. The paper concludes that PEN film capacitors, either in SMD or radial technology, can be a choice for working temperatures up to 230 °C. The only aspect that has to be considered, the authors noted, is the right voltage de-rating to apply, which depends on the final temperature reached by the capacitors, considering also the potential self-heating effect.