Power Electronics

Stacked-Ceramic Caps Brave High Temperatures

A series of stacked-ceramic capacitors specially designed for SMPSs features the thermal performance of conventional ceramics without their mechanical drawbacks, enabling higher reliability and operating temperatures.

A growing number of applications require electronics that can survive and operate in extreme environmental conditions. Oil drilling, hybrid vehicles and aerospace provide application examples where electronic components must perform at very high temperatures. For example, electronic equipment used in the search for petroleum reserves may be exposed to temperatures in excess of 200°C, while also subjected to extreme shock and vibration.

Specific component types may have additional requirements. For example, capacitors in these applications must be able to supply high currents and also operate over a wide voltage range. A new stacked-ceramic-capacitor technology developed for use in switch-mode power supplies employs a specially developed dielectric and high-temperature solder to address extreme environmental and electrical requirements.

Previous Solutions

Prior to the adoption of 200°C ceramics, electronics systems were often placed in a relatively safe environment, such as the inside of an airplane fuselage or even in a remote temperature-controlled compartment. However, this increases the distance to the power source. The result is significant losses in the power cables, as well as the need for larger power supplies and additional filtering to overcome these issues. For example, it is not uncommon to see a jet engine with ceramic capacitors contained within the engine mount itself.

At 35,000 ft, air temperature is roughly -50°C, and a power supply in this environment would not be exposed to great amounts of heat. However, within 5 to 10 minutes, an aircraft can transition from this extremely low temperature at high-altitude to relatively high temperatures on the ground (caused by heat from the runway and the engine), resulting in a rapid temperature increase beyond 40°C. Similarly, extreme temperature transitions also exist in the down-hole logging environment, where specialized equipment probes for petroleum reserves.

To address these requirements, AVX developed a stacked-capacitor design based on high-temperature ceramics. The capacitors produced with this design are members of the SMX series. SMX parts were designed to meet varied conditions, such as voltage ratings from 25 Vdc to 500 Vdc, with a capacitance of 340 µF at 25 Vdc. Many design constraints had to be overcome to create this technology.

A stacked capacitor must be able to withstand the shock and vibration of everyday applications, and meet the 200°C operating temperatures. To do this, a special high-temperature solder was used to adhere the lead frame of the structure to the body of the individual capacitors in the stack.

This solder has a melting point well in excess of 250°C and approaching that of 300°C. Another drawback to using a traditional capacitor composed of X7R dielectric material is the loss of capacitance with increasing temperature. Some traditional capacitors exhibit a capacitance loss of over 80%. Using a specially developed dielectric, the stacked capacitors reduce this loss to 55% of the nominal rated capacitance value at 200°C (Fig. 1).

One of the key features of SMX capacitors is their intrinsic ability to retain capacitance over temperature (rated for -55°C to 200°C) and frequency. Additionally, these capacitors were designed to endure the severe shock and vibration associated with harsh environments and mission-critical applications. In general, ceramic capacitors are known to have cracks due to board flexure or other mechanical stresses applied to the board. AVX has developed a proprietary process to alleviate these stresses and allow the capacitors to operate in these stressful environments. Furthermore, these capacitors exhibit extremely low ESR and ESL, which makes them ideal for high-current, high-power circuits or absorbing extreme transients. These factors all combine to make this an ideal snubber, output/input filter, dc converter, decoupling or bypass capacitor.

Requirements for Extreme Applications

Several competing capacitor technologies have been used in the past for extreme applications, but the main one has been wet tantalum. The advantages of a stacked-ceramic capacitor stem from the dielectric materials of these devices. The lowest ESR capacitors traditionally have been ceramic capacitors, and by placing these into parallel stacks, one gets the benefit of lower ESR and the added value of higher current and surge-handling capability.

For example, as shown in Table 1, the ceramic can handle in excess of 3.5 times the ripple rating of a similar tantalum, while maintaining a minimal loss of capacitance. High-frequency performance is also enhanced. Table 1 also shows the other main advantages of using a ceramic capacitor in these high-temperature applications. While not shown in the table, most ceramic capacitors usually have the additional benefit over wet tantalums of lower, in some cases by a significant margin. Typically, tantalum capacitors have a much higher ESR than that of their equivalent ceramic capacitors and thus need more capacitance to overcome the ESR. The ceramic capacitors are also nonpolar, which allows an extra degree of freedom. Perhaps most important is that the ceramic capacitors are designed to handle a high current and voltage surge that would likely cause a catastrophic failure in the equivalent tantalum capacitor. Ceramic capacitors can be used over a wide frequency range, whereas the typical wet tantalum is characterized around 1 kHz.

One item that many engineers overlook is the capacitance loss under operating conditions. These conditions can be a combination of frequency, voltage and temperature. Table 1 shows a sample of data for these conditions.

For example, if a design calls for 10 µF of capacitance, the engineer will often either specify much higher capacitors in anticipation of this capacitance loss, or place multiple parts in parallel. Both of these options can significantly add to the cost of the design, as well as the size of the pc board.

X7R capacitors are designed to operate from -55°C to 125°C. X8R capacitors are designed to operate up to 150°C. The composition of the dielectric of the SMX series (X9U, with precious metal electrode system) allows users to operate the capacitor up to 200°C and still retain the superior capacitance versus temperature characteristics of the X8R dielectric. For several data points across the full operating-temperature range, the high-temperature X9U dielectrics actually have better performance than the typical equivalent X8R ceramics, as shown in Fig. 2.

Estimating Capacitor Reliability

One of the most critical characteristics of a capacitor is a prediction of how the capacitor will perform over an extended period. For example, the reliability of large electrolytic-type output capacitors is a major factor in the failure rate of an SMPS that uses them. When dealing with temperatures and potential thermally induced failures, an Arrhenius plot is often used to predict the outcome. Specifically, the Arrhenius plot is useful for finding the activation energy (EA) of a certain process. The relationship could involve any of a multitude of variables, each a function of temperature. For example, in high-temperature capacitors, an Arrhenius plot could project leakage current as a function of 1000/T, where T is the absolute temperature in Kelvins (Fig. 3). This gives the information about the exact conditions under which free carriers are liberated, a state that could cause the capacitor to become unstable.

This information is directly related to the life expectancy of the capacitor. As with any electronic component, the higher the voltage or temperature that is applied to the device, the shorter the life expectancy of the device.

This life expectancy is a key design parameter of the capacitor and dielectric. AVX has performed extended testing of the SMX high-temperature ceramic capacitors. The initial testing was to put the device under an accelerated test plan, which equates to more than 1.5 million hours of operating life. Initial laboratory test results indicated a failure rate of 0.43% per 1000 hours at 200°C and full rated voltage. Actual environmental and operating results have proven to be significantly better, with zero failures over 20,000 hours of actual operation.

High-temperature capacitors can be used in a variety of applications where operating conditions exceed 150°C. They also will be a valid design option in future systems, such as unmanned exploration platforms, which could experience temperatures exceeding 300°C.

Table 1. Properties of wet tantalum and ceramic capacitors.
Parameter Condition Unit Capacitor type
Wet tantalum SMX
ESR 10 kHz Ω 0.222 0.034
Capacitance reduction 10 kHz % 60 2
100 kHz % 84 2
Maximum capacitance change -55°C % -45 -15
+125°C % 25 -15
Maximum dc-leakage current +25°C µA 9 0.5
+125°C µA 36 5
Maximum rms ripple current 40 kHz A 1.85 7
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