Advances in Capacitors for Logic-Level to Medium-Power Applications

­Semiconductors aren’t the only components that made significant strides in recent years. Capacitors — including multilayer ceramic capacitors (MLCCs), tantalum polymer capacitors, and supercapacitors — have also made impressive and impactful improvements that can greatly affect power delivery and conversion aspects of modern circuitry.

Application examples for cutting-edge MLCCs include new and improved power distribution trees for industrial microcontroller units (MCUs) and high-power processors that require ultra-stable voltages. Application examples for state-of-the-art tantalum polymer capacitors include efficient power conversion applications that require low-loss capacitors capable of handling high ripple currents. And application examples for the newest, most advanced supercapacitors include recovering kinetic energy and converting it into stored energy to support regenerative braking in electric vehicles, eliminate batteries in devices like industrial IoT (IIoT) nodes, and help reduce electronic waste. They’re also ideal for industrial robotics that require substantial amounts of instant power to complete heavy lifting or mobility tasks, like jumping. 

MLCCs

MLCCs are far and away the most common type of capacitor used in today’s electronic designs.  Recent evolutionary trends in MLCCs include higher capacitance values in ever smaller case sizes, rugged environmental resistance, and enhanced frequency response packages.

It is common to encounter MLCCs with case sizes ranging from 01005 to 1812. Depending on the dielectrics used, surface-mount technology (SMT) MLCCs can easily achieve capacitance values extending from tight-tolerance sub-pF values to bulk capacitance values of 100µF or more, and voltages can range from 4V to many kilovolts (kV). While the higher capacitance/unit-volume trend is easily recognized, the benefits of enhanced frequency response MLCCs are less obvious. Newer SMT MLCCs are widely available in low-inductance models for high-speed decoupling, three-terminal models that serve as broadband EMI Filters for mixed logic, and ultra-broadband models for AC coupling in high-speed data I/O and processing applications. The underlying advancement behind these three models is one of reduced parasitic inductance or transformed inductance. 

A comparison of the impedance vs. frequency response can be made between a standard 1206 case size MLCC, a reduced-inductance, reverse-geometry 0612 case size MLCC, a three-terminal (3T) feedthrough 1206 MLCC, and a reduced-inductance interdigitated capacitor (IDC). The resulting graph (See Figure 2) shows that 3T MLCCs exhibit a significant frequency response advantage over standard MLCCs and reverse-geometry MLCCs. These advantages enable 3T MLCCs to significantly improve signal integrity through broadband filtering on power trees for industrial MCUs and the high-frequency decoupling of ICs. Additionally, one 3T MLCC can take the place of multiple values of standard-configuration MLCCs in order to conserve board space and reduce weight. Next-generation 3T MLCCs come in cases sizes as small as 0402 and values as large as 15µF.

Unlike the name suggests, three-terminal capacitors actually have four solderable connections. The two terminations at the end of the long dimension provide an in-and-out path or current to the load, while the two center terminations provide a center tap and are connected to ground. The resulting equivalent model of these connections is a small nonferrous inductance placed in series with the load and a reduced parallel inductance center-taping, the combination of which creates an SMT T-filter.

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Figure 2: A three-terminal 1206 SMT feedthrough MLCC (left) and its equivalent electrical model (right)

Tantalum Polymer Capacitors

Bulk capacitors have made very impressive gains in terms of expanded case size options, enhanced reliability, and optimized electrical performance. Tantalum polymer capacitors offer bulk level capacitance values similar to traditional tantalum capacitors with an MnO2 material system, as well as several other important advantages, including:

·       Significantly lower ESR

·       Higher ripple current ratings

·       Improved capacitance retention at high frequency

·       Higher energy density (Joules/cc)

·       Wider voltage range (2.5V to 125V)

·       Less voltage derating required

·       Benign failure mode if short-circuited

MnO2 tantalum capacitors continue to be widely used in designs and have recently found widespread use as start-up capacitors in energy harvesting power management integrated circuits (PMICs) due to their exceptionally low leakage. Start-up capacitors don’t need to be an exceptionally high-value capacitor since their primary goal is to minimize leakage currents. In fact, the ideal value is around 100µF.

KYOCERA AVX developed a superlative series of low-leakage MnO2 tantalum capacitors, the TMJ S1gma Series, that have DC leakage (DCL) limits of 0.001CV on selected codes and 0.005CV on the others. These capacitors exhibit the lowest DCL of any tantalum capacitor currently available on the market, which makes them ideal for a variety of long-life battery-operated circuit applications, like industrial tooling and remote wireless modules.

On the other end of the spectrum, we have tantalum polymer capacitors like the J-CAP Series, which delivers the highest energy per volume compared to traditional MnO2 capacitors and is widely used as energy storage capacitors in dying gasp circuit applications. Tantalum polymer capacitors exhibit the ideal combination of high voltage ratings, high capacitance values, and low equivalent series resistance (ESR) in low-profile case sizes. Dying gasp circuit applications supply sufficient energy to the PMIC so it can provide voltage to the solid-state drive (SSD) and allow data from the fast dynamic random-access memory (DRAM) buffer to be transferred into non-volatile flash memory when the power fails. The capacitor in these applications is essentially used as an energy reservoir, and during the process, its voltage drops as its energy drains. As such, dying gasp circuits tend to utilize high-capacitance-value, optimized-case-size devices with low ESR, and the PMICs in these applications typically have a first-stage buck-boost type DC/DC converter.

Tantalum polymer capacitors, like the J-CAP series are ideal for providing hold-up power to dying gasp circuits in applications ranging from process control systems to SSDs and can also be used to provide low-level power hold-up in industrial controllers and hold-up power in highly mobile devices while the batteries are quickly exchanged. These high-capacitance, solid tantalum electrolytic chip capacitors have an under-tab design that enables tight packing densities and conserves board space in applications that require multiple capacitors and are available with capacitance values extending from 4.7µF to 1,500µF and voltage ratings spanning 6.3V to 50V.

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Figure 3: A KYOCERA AVX J-CAPSeries conductive polymer solid electrolytic under-tab chip capacitor (left) and dimensions for its 4 and 8 case sizes (right), which are rated for 6.3–35V

Supercapacitors

Supercapacitors are another attractive, cost-effective solution for many types of higher power dying gasp circuits, ranging from robotics controllers to uninterruptable power supplies (UPS).  Supercapacitors’ massive and ever-increasing capacitance range helps them solve evolving high-power-demand energy harvesting challenges as well as provide energy recovery through dynamic braking in electric drives and industrial automation systems.

Recent evolutionary trends in supercapacitors, including higher reliability, an expanding range of case sizes, easily designed and assembled custom modules, and an attractive price-performance ratio, continue to drive end-use design engineers to deploy them in an increasingly wide range of emerging applications. IC companies are also driving supercapacitor growth trends with the introduction of powerful, low-cost power management integrated circuits (PMICs) that control everything from capacitor state of charge monitoring to charge control and energy transition functions.

Expanding further on case sizes, supercapacitors are commonly available in two form factors — small radial cans and prismatic cells — with a range of voltage and power levels. The most basic case is a single radial can used for lower-voltage designs. Although, you can connect multiple cans to obtain the correct voltage/energy for higher-power loads and balance those cans using passive or active balancing methods. Low-cost, lower efficiency designs typically use passive balancing methods, whereas active balancing is intended for higher-power and -efficiency designs.  

Small radial-can supercapacitors are also attractive because of their minimal footprint. Radial supercapacitors are available in sizes as small as 6.3mm in diameter and 12mm in height for a 1F capacitor. They’re also available with a variety of physical and electrical characteristics for broad application suitability. (See Table 1 comparisons.) The end device selected depends upon the desired application run time and supercapacitor package characteristics.

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Table 1: Typical physical and electrical characteristics for radial can, radial module, and custom module supercapacitors

Supercapacitors need to be properly derated to achieve long-term reliable operation, regardless of the specific package configuration. Supercapacitor reliability is a function of applied voltage and temperature and follows the Arrhenius equation, which suggests that expected life is more than doubled for every 10°C decrease in operating temperature, and life expectancy doubles again for every reduction of 0.1V in operating voltage.

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Figure 4: The mean time to failure (MTTF) in years at various voltages and temperatures for KYOCERA AVX acetonitrile (ACN) supercapacitors

Summary

Capacitor technology is rapidly progressing. Continued advancements in materials, processes, and designs are fueling significant improvements in the physical, electrical, and environmental performance of MLCCs, tantalum polymer capacitors, and supercapacitors, including the increasingly widespread availability of higher-CV devices capable of operating across broader frequency spectrums. End users should expect advanced capacitors to help enable new use cases ranging from small signal applications, like embedding passives into packages and substrates, to low-loss, self-healing power capacitors engineered to improve power drive reliability. 

Some of the biggest growth opportunities and use cases for these advanced capacitors will come from the industrial sector, which relies on ever-increasing processing and control capabilities to support evolving automated manufacturing processes. Typical growth cases include machine controllers and process automation applications used to produce higher-quality products with optimized lifespans, ranging from vision systems to robotics.

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