Hardware design engineers are always looking for opportunities to reduce component cost, size, and weight, while also meeting or exceeding component and system targets for efficiency and reliability. One of the most common and critical components for optimization are pc board mounted capacitors due to their bulk and wide usage. Reliability must also be a consideration when selecting a capacitor because of its susceptibility to leakage and capacity degradation over time when subjected to temperature extremes. This degradation can result in intermittent circuit malfunctions, compromising system efficiency and reliability.
While capacitor suppliers continue to improve on designs to increase energy density, reliability, and weight, the optimum part for an application may not be available due to long lead times caused by supply-chain issues.
This article discusses the role of filter and bulk storage capacitors. It shows how a single capacitor can replace other types of capacitors, such as an array of surface mount capacitors, resulting in fewer board components and circuit interconnects, improving the overall circuit reliability. Along the way, it introduces high-reliability aluminum electrolytic capacitors from Cornell Dubilier Electronics that have the dual advantages of a thin profile and very high energy density. As the capacitors are manufactured in the USA and are available for fast shipment to North American production facilities, the capacitors may also provide a path to shorter lead times.
Reliability of board-mounted capacitors
The lifetime of an electrolytic capacitor is determined by the rate of electrochemical degradation of its internal structure over time. Since this degradation is predictable under typical operating conditions, the functional lifetime of a capacitor can be easily calculated by the manufacturer. The reliability of a capacitor is a measure of how close a capacitor’s actual life matches its expected life in the face of variations of construction or exposure to extreme conditions.
While the wear-out lifetime of large and small capacitors are about the same, smaller capacitors are more reliable as there is less surface area between the anode and cathode surfaces. The larger the capacitor, the more reliability is a factor in its selection, as well as availability. As of the writing of this article, there are supply chain problems for Electronic components, including delays for many international shipments. For that reason, availability and lead-time have become critical criteria for electronic component selection.
Capacitors are not subject to the size optimizations common to many semiconductors, in that the size of a capacitor cannot be reduced by shrinking to a smaller process geometry. Due to the physics of capacitor design, the greater the capacitor rating in Farads (F), the greater the surface area between the anode and cathode, hence its larger physical size. Vertical mount capacitors, also called V-chips, are popular packaging options for saving board real estate, the tradeoff being a higher board profile and less clearance which can affect the packaging choices of nearby components.
The mounting position of a large aluminum electrolytic capacitor can also affect reliability. Large capacitors can get hot and require airflow or even heatsinking under some conditions. The applied DC voltage, ripple current, and ambient temperature extremes all shorten its operating lifetime due to parametric drift. Usually, the effective series resistance (ESR) of a capacitor is the first parameter to deviate from the datasheet specifications. As the ESR rises, the capacitor will get progressively hotter. It ultimately fails when it runs so hot that its internal structure breaks down and effectively shorts the anode and cathode. In very rare instances, the heat dries out the capacitor and it becomes an open-circuit.
Capacitor degradation in a system can first show up as random failures, which quickly turns into system failure when the capacitor shorts. This problem is amplified for banks of capacitors connected in series or parallel; if one capacitor fails, the entire bank fails. Capacitor banks reduce the reliability of the system because the failure rate of the bank is the failure rate of one capacitor multiplied by the number of capacitors in the bank. For this reason, capacitor banks are discouraged in high-reliability designs in favor of one large capacitor.
High-reliability, high-density capacitors
For space-constrained, high-reliability applications, Cornell Dubilier supplies the THA and THAS Thinpack aluminum electrolytic capacitors. Designed for very high energy density with a thin, low-profile package, the capacitors have a laser-welded case that encloses the electrolytic capacitor to prevent leaks. This laser weld eliminates the need for large end-seal gaskets that are widely used to seal the ends of most electrolytic capacitors. A valve in the case allows gas venting, relieving internal pressure which reduces swelling. The THA line is 8.2 millimeters (mm) thick and the THAS line is 9 millimeters (mm) thick. The THA and THAS capacitors’ design ensures 5,000 hours of operation at 85°C and 105°C, respectively. They have an energy density of 0.9 Joules per centimeter cubed (J/cm3).
For many filtering and motor control applications, designers can use the THAS131M450AD0C THAS series 130 microfarad (µF) capacitor (Figure 1). The capacitor is 66.5 mm long and only 25.4 mm wide. As mentioned, THAS series capacitors are only 9 mm thick, so when seated they enable a very low pc board profile. Rated at 450 volts, the series is suited to motor control applications and compact power supplies. Because the series is so slim, it is also appropriate for laptops or similar low-profile electronics where headroom for components is severely limited. The capacitor can also be mounted vertically on a pc board to save space, compared to similar capacitors.
Figure 1: The THAS131M450AD0C 130 µF capacitor is rated at 450 volts and is only 9 mm thick, making it suited to motor control and low-profile pc board applications. (Image source: Cornell Dubilier)
At 130 µF, the THAS131M450AD0C can be used to replace banks of smaller capacitors to improve reliability. The ESR of the THAS131M450AD0C at 25°C is 1.12 Ohms (Ω) at 120 hertz (Hz), which drops to 0.54 Ω at 20 kilohertz (kHz). Its low ESR makes it appropriate for switching power supplies where heat generation must be minimized. Ripple current at 85°C is rated at 1.36 amperes (A), also important for power supplies.
As part of the Cornell Dubilier THAS product family, the THAS131M450AD0C capacitor has a stainless-steel sleeve for improved durability. Its terminals are 20 AWG, appropriate for most through-hole pc board mounting applications.
For applications where voltage must be stored for a short period of time, designers can turn to the THAS322M050AD0C THAS series 3200 µF, 50-volt capacitor. It is also 66.5 mm in length with a thickness of 9 mm and has a stainless steel sleeve. The ESR at 120 Hz is 0.05 Ω, which drops slightly to 0.04 Ω at 20 kHz. It can handle a ripple current of 3.48 A at 20 kHz and 2.90 A at 20 Hz. With this low ESR and high current capability, it is appropriate for use as a 50-volt supercapacitor to temporarily supply power to a small circuit if the main power supply is unavailable.
Like all Cornell Dubilier THAS capacitors, the THAS322M050AD0C has a vent at the top, shown clearly in Figure 2. The vent allows gas to escape from the capacitor as part of normal operation, although gassing may increase under high temperatures. The vented gas is a mixture of hydrogen and waste gasses.
Figure 2: The vent seen on the top of the THAS322M050AD0C capacitor allows internal gasses that build up under normal operation to escape safely. (Image source: Cornell Dubilier)
The venting of internal gasses is important, especially in high-value capacitors. Hydrogen and other gasses can accumulate inside the steel casing, building pressure that can lead to failure. If a capacitor has insufficient venting, internal gases can build to a point where electrolyte can leak out onto the pc board and short out other electronics, or in some cases, the capacitor can even explode. Note, however, that it is important when laying out the pc board to make sure the capacitor vent has no obstructions.
For more charge capacity, Cornell Dubilier developed the THA series. THA Thinpack capacitors have aluminum sleeves, and at 8.2 mm in thickness are slightly slimmer than the THAS series. A THA series example is the 4400 µF 50 volt THA442M035AC0C. It is 53.8 mm in length and offers a very high energy density compared to similar capacitors. It has an ESR of 0.07 Ω at 120 Hz and 0.06 Ω at 20 kHz, making it appropriate for use as a source of temporary power for small electronics during brief power supply interruptions. A 4400 µF capacitor can also get very hot, so it’s important to provide adequate airflow to keep it well within its recommended operating range, which is -55°C to +85°C. For large-value capacitors, it is even more important to make sure the vent has no obstructions. It is also recommended that the vent doesn’t point towards anything flammable, as hydrogen gas is explosive.
Capacitors are critical components in electronic systems. By combining high reliability with high energy density and a low profile, designers can reduce the size and improve the operational life of electronic systems. A single, made-in-the-USA, energy-dense electrolytic capacitor can avoid long lead times, as well as replace banks of capacitors to conserve board space.
- THA and THAS Thinpack Aluminum Electrolytic Capacitors training module