James C. Lewis, KEMET
There is no Moore’s Law for passive components like capacitors, but relentless development is delivering the kinds of devices engineers need to deliver cutting-edge new products for modern living. Capacitors have for many years enabled electronic designers to manage energy within circuits and fulfill basic functions like filtering noise or harmonics, correcting power factor, stabilizing feedback circuitry, coupling/decoupling, interfacing between voltage levels, and storing energy. But the demands placed on these components continue to increase, as electronic devices are expected to be smaller, longer lasting, more feature rich and more robust.
The next generation of devices has to be smaller, slimmer, with lower parasitic effects, longer lifetimes, higher temperature and voltage ratings, greater stability over time and temperature, and improved high-frequency performance.
Higher capacitance, smaller Spaces
Multi-Layer Ceramic Chip Capacitors (MLCCs) are used for a wide range of decoupling, filtering, bypassing and smoothing duties. Designers rely on these devices where high capacitance is required within small case sizes in order to satisfy tight constraints on pc-board and enclosure dimensions. Improvements leading to even smaller case sizes, as well as lower cost and better performance allow these devices to be used as an economical alternative to tantalum, aluminum or film capacitors.
KEMET has developed a proprietary lead-frame technology that allows two high-voltage X7R MLCCs to be stacked vertically as shown in Figure 1, resulting in a device that offers twice the capacitance within the same pc-board footprint as a single 2220 (0.22” x 0.20”) SMT chip capacitor. KEMET’s KPS series devices featuring this technology have rated voltages of 500 VDC and 630 VDC, and also have low ESR and ESL. This delivers the properties needed to fulfill applications such as smoothing circuits in switched-mode power supplies, snubbers in lighting ballasts, and high-voltage coupling and DC blocking in inverters.
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Figure 1. An advanced leadframe allows vertical stacking of MLCCs to boost capacitance
Polymer tantalum capacitors are known to offer very high capacitance, low ESR and small case sizes, which are often needed for applications such as Solid-State Drives (SSD), wireless cards and GPS systems. The latest families combine improvements in capacitance and ESR performance with low mounted height and all solid-state construction.
For applications where very high capacitance is required, and a slightly higher component height can be tolerated, advanced devices such as Tantalum Stack Polymer (TSP) capacitors featuring an organic polymer cathode that enables two, three, four, or six discrete components to be combined in a vertical stack as shown in Figure 2. The resulting device occupies the same pc-board footprint as a single capacitor to deliver the advantage of greatly increased capacitance with the known strengths of this type of device, which include not only very low ESR but also high ripple-current capability for power-supply applications.
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Figure 2. FT-CAP technology increases the capacitor’s ability to withstand flex cracking
Tantalum capacitors are also favored for use in applications that require high reliability. These include decoupling and filtering in computing, telecommunications and defense systems, and industrial applications such as filtering for point-of-load and switched-mode power supplies. In circuits where a high fault current can occur, and little or no series resistance may be present to protect the capacitor, a short-circuit failure of the capacitor can occur and may disrupt the overall operation of the system.
The arrival of true fail-open tantalum fused capacitors now enables designers to guard against such situations. An example is KEMET’s T496 MnO2 tantalum Commercial Off-The-Shelf (COTS) family, which includes an internal fuse that opens within one second when a fault current is detected, and hence ensures the capacitor is isolated from the circuit.
In applications where an array of capacitors is used to achieve a high capacitance value, such as radar pulse generators, or point-of-load DC/DC and switched-mode power supplies in the computer, industrial, defense and telecom sectors, the latest tantalum polymer capacitor technologies enable designers to achieve the desired capacitance using fewer individual capacitors. Polymer capacitors also have lower ESR and higher power dissipation than comparable MnO2 devices, resulting in higher ripple current capability. In addition, the devices have a benign failure mode and hence are not prone to ignition or smoking if excessive voltage or current is applied.
As well as incorporating the latest commercial polymer technologies, COTS capacitors such as the T543 series are up-screened by exposing to at least 24 hours of voltage conditioning and surge-current options at +25°C and -55°C/+85°C. By using these devices, designers have been able to reduce the number of capacitors needed in power-management circuits by up to 90%.
Ruggedness and reliability
Historically, MLCCs have been the most vulnerable to mechanical stresses resulting from flexing of the pc-board. A number of package technologies have been developed that now provide effective protection against this so-called flex cracking. KEMET’s KPS series MLCCs, for example, feature mechanical isolation technology that allows the devices to withstand deflections of up to 10mm as the board flexes.
Various other flex-mitigation technologies have been developed, such as Open-Mode and Floating-Electrode devices with Fail-open technology capable of withstanding 2mm of flexing. FT-CAP flexible termination technology, shown in Figure 3, is featured in the latest C0G and Ultra-Stable X8R capacitors, and directs any stresses caused by flexing of the board away from the ceramic body and into the termination area. This enables the capacitors to withstand up to 5mm of pc-board flex. Devices featuring a combination of Fail-Open and Flexible-termination (FO-CAP), or Floating-Electrode and Flexible Termination (FF-CAP), technologies are also available.
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Figure 3. Tantalum Stack Polymer technology delivers high capacitance with low-ESR and robust performance
For applications requiring operation at elevated temperatures, FT-CAP C0G capacitors have an enhanced high-temperature dielectric rated for operation up to 125°C. In addition, new devices with X8R dielectric are able to maintain stable capacitance values at temperatures up to 150°C.
The automotive sector has become a significant consumer of advanced electronic components, including capacitors, for various purposes such as power conversion, energy storage, and lighting ballasts.
Ballasts for xenon High-Intensity Discharge (HID) headlamps incorporate a boost capacitor to help generate the potential of several hundred volts needed to stimulate the lamp. In today’s advanced controllers, which integrate both the igniter and ballast circuits as a single unit, the boost capacitor can experience temperatures of up to 150°C. Automotive-qualified versions of X7R and C0G MLCCs have emerged to provide the high-temperature capability needed to satisfy this, and other automotive systems that must operate at sustained high temperatures - such as under-the-hood applications. Advanced film capacitors may also be used for automotive high-temperature applications.
High-voltage, high-power applications
Growing demand for electronic lighting control and alternative energy applications are creating demands for new generations of aluminum electrolytic capacitors for power conversion and conditioning applications. In particular, these types of applications are driving the emergence of compact capacitors having unusually high rated voltages such as 450V or 500V. These elevated voltages allow derating to ensure safe operation at the voltages typically applied to a lighting load such as an LED array containing a large number of emitters connected in series.
Increasingly widespread use of LED lighting in applications such as commercial interiors, high-bay lamps, street lamps and car-park lighting is also driving demand for typical capacitor lifetimes to be extended to match those of the LED emitters. To satisfy this requirement, KEMET has introduced surface-mount high-voltage electrolytic capacitors that have a specified lifetime of up to 10,000 hours.
Alternative-energy applications such as wind turbines demand rugged electrolytic capacitors for use in the DC/AC inverters needed to produce grid-quality power. In a wind application in particular, the extremely noisy raw AC output from the turbine is first rectified and transmitted to the inverter across a DC link. Large numbers of high-voltage electrolytic capacitors are used to smooth the DC link voltage and so provide a stable input that allows the inverter to generate the correct to and voltage for feed-in to the grid. An important requirement is for these smoothing capacitors must have high ripple-current capability in order to achieve a long lifetime and thereby minimize turbine maintenance overheads.
Smaller film capacitors are increasingly in demand for similar smoothing applications in micro-generators, such as domestic solar or wind installations. KEMET has developed its C4AE radial film DC-Link capacitors for these types of applications. They are designed with low Equivalent Series Resistance (ESR) using a combination of innovations such as high-conductivity electrolytes, optimized internal device geometry, and enhanced production processes such as vision-guided welding of terminations.
Reducing ESR using these techniques not only helps increase energy efficiency but also reduces internal heating leading to enhanced reliability and lower Bill Of Materials (BOM) costs. Enhanced film properties allow the devices to operate at temperatures up to 105°C. This makes them popular in a wide variety of applications besides solar generation, such as Electric-Vehicle (EV) charging stations and in-vehicle charge controllers, as well as industrial power supplies.
Advanced technology has taken a leading role in realizing goals such as sustainable energy, safer transportation, energy-efficient lighting, and convenient digital living to improve the way we live and work. But pressure to improve performance and reliability comes with the territory, as does the expectation to become progressively smaller, lighter, and more energy efficient. Today’s most advanced capacitors are highly evolved to meet these demands in applications used throughout residential, commercial, automotive, and industrial environments.