Designers of power delivery solutions for Electronic systems and subsystems, including ICs, application-specific ICs (ASICs), central processing units (CPUs), and field-programmable gate arrays (FPGAs), as well as USB power, are constantly looking for ways to improve efficiency while ensuring stable, noise-free power over wide temperature ranges in a compact form factor. They need to improve efficiency, stability, and reliability, lower cost, and shrink the solution form factor. At the same time, they must meet the ever-increasing power performance requirements of the application, including smoothing the input and output currents of power supply circuits, supporting peak power demands, and suppressing voltage fluctuations.
To meet these challenges, designers need capacitors that have low equivalent series resistance (ESR) and low impedance at high frequencies to support ripple absorption and ensure a smooth and fast transient response. In addition, both operational reliability and supply chain reliability are important.
Looking at the issues and options, polymer aluminum electrolytic capacitors emerge as a good solution as they feature high electrical performance, stability, low noise, reliability, a compact form factor, and low supply-chain risk as they do not use conflict materials. They combine low ESR (typically measured in milliohms (mΩ)) and low impedances at high frequencies (up to 500 kilohertz (kHz)), providing excellent noise suppression, ripple absorption, and decoupling performance on power lines. They also have capacitance stability at high operating frequencies and temperatures.
This article presents an overview of how polymer aluminum electrolytic capacitors work and how they are made. It compares the performance of these capacitors with alternative capacitor technologies, before looking at specific applications for polymer aluminum electrolytic capacitors. It closes with a review of representative devices from Murata and application considerations that designers need to be aware of when using these capacitors.
How are polymer aluminum capacitors made?
Polymer aluminum capacitors have an etched aluminum foil cathode, an aluminum oxidized film dielectric, and a conductive polymer cathode (Figure 1). Depending on the specific device, they are available with capacitances from 6.8 to 470 microfarads (µF) and cover a voltage range from 2 to 25 volts direct current (Vdc).
Figure 1: Polymer aluminum electrolytic capacitor model showing the relationship between the etched aluminum foil anode (left), the aluminum oxidized film dielectric (center), and the conductive polymer cathode (right). (Image source: Murata)
In Murata’s ECAS series devices, the etched aluminum foil is attached directly to the positive electrode, while the conductive polymer is covered with a carbon paste and connected to the negative electrode using a conductive silver paste (Figure 2). The entire structure is encased in a molded epoxy resin for mechanical strength and environmental protection. The resulting low profile, surface mount package is halogen-free and moisture sensitivity level (MSL) 3 rated. The multilayer (laminated) structure of the aluminum foil and oxidized film differentiates Murata’s ECAS series from typical aluminum electrolytic capacitors, such as can-type wound structures that can use either a polymer or an electrolyte as the cathode.
Figure 2: ECAS series polymer aluminum capacitor device structure showing the conductive polymer (pink), etched aluminum foil (white), aluminum (Al) oxidized film (blue), the carbon paste (brown) and silver paste (dark grey) that connect the conductive polymer to the negative electrode and the epoxy resin casing. (Image source: Murata)
The combination of the laminated structure and the materials selection enables ECAS capacitors to have the lowest ESR available for electrolytic capacitors. ECAS series polymer aluminum capacitors provide capacitances comparable to polymer tantalum (Ta) capacitors, Ta manganese dioxide (MnO2) capacitors, and multi-layer ceramic capacitors (MLCCs), with ESRs that are comparable with MLCCs and lower than polymer or MnO2 Ta capacitors (Figure 3).
Figure 3: Polymer aluminum capacitors (ECAS series) feature higher capacitance values and comparable ESRs compared with MLCCs, and lower ESRs with comparable capacitance to tantalum and can-type aluminum capacitors. (Image source: Murata)
For cost-sensitive applications, aluminum electrolytic capacitors and Ta (MnO2) capacitors may provide relatively inexpensive solutions. Conventional aluminum or tantalum electrolytic capacitors use an electrolyte or manganese dioxide (MnO2) as the cathode. The use of a conductive polymer cathode in ECAS capacitors results in lower ESR, more stable thermal characteristics, improved safety, and longer service life (Figure 4). MLCCs, while relatively inexpensive, suffer from DC bias characteristics not found in the other capacitor technologies.
Figure 4: Polymer aluminum capacitors provide the base combination of low ESR, DC bias characteristics, temperature characteristics, service life, and reliability. (Image source: Murata)
The DC bias characteristic refers to the capacitance change of an MLCC with an applied DC voltage. As the applied DC voltage increases, MLCC’s effective capacitance decreases. When the DC bias increases to a few volts, MLCCs can lose from 40% to 80% of their nominal capacitance value, making them unsuitable for many power management applications.
The performance characteristics of polymer aluminum electrolytic capacitors make them well suited for power management applications including power supplies for CPUs, ASICs, FPGAs and other large ICs, and for supporting peak power needs in USB power systems (Figure 5).
circuit” alt=”Why and How to Use Polymer Aluminum Capacitors to Effectively Power CPUs, ASICs, FPGAs, and USB”>Figure 5: In Ex. 1 (top): Polymer aluminum capacitors in a power management circuit used in target applications to eliminate ripple and smooth and stabilize voltage sources. Ex. 2 (bottom): Polymer aluminum capacitors can support peak power needs in USB power systems. (Image source: Murata)
Polymer aluminum capacitors have low ESR, low impedance, and stable capacitance, making them suitable for applications such as smoothing and eliminating ripple, especially on power lines subject to large fluctuations in the current load. In these applications, polymer aluminum capacitors can be used in combination with MLCCs.
Polymer aluminum capacitors provide power management functions, and MLCCs filter high-frequency noise on the power pin(s) of ICs. Polymer aluminum capacitors can also support peak power needs in USB power systems while maintaining a small pc board footprint.
Polymer aluminum capacitors
ECAS polymer aluminum capacitors are available in four EIA 7343 metric case sizes, depending on their ratings: D3: (7.3 millimeters (mm) x 4.3 mm x 1.4 mm high); D4 (7.3 mm x 4.3 mm x 1.9 mm high); D6 (7.3 mm x 4.3 mm x 2.8 mm high); and D9 (7.3 mm x 4.3 mm x 4.2 mm high). They are available in DigiReel, cut-tape, and tape and reel formats (Figure 6). Other specifications include:
- Capacitance range: 6.8 µF to 470 μF
- Capacitance tolerances: ±20% and +10%/-35%
- Rated voltages: 2 Vdc to 16 Vdc
- ESRs: 6 mΩ to 70 mΩ
- Operating temperature: -40°C to +105°C
Figure 6: ECAS polymer aluminum capacitors are offered in DigiReel, cut-tape, and tape and reel formats, and come in case sizes D3, D4, D6 and D9. (Image source: Murata)
Murata recently expanded the ECAS family to include 330 µF (±20%), 6.3-volt devices like the ECASD60J337M009KA0 with an ESR of 9 mΩ in a D4 case size. Higher capacitance values can contribute to improved ripple smoothing and a reduction in the number of capacitors required, reducing the overall solution size.
For example, when used to filter the output of a DC-DC converter switching at 300 kHz, the ECASD40D337M006KA0 330 µF (±20%), 2-volt polymer aluminum capacitor with an ESR of 6 mΩ will produce a ripple voltage of 13 millivolts peak-to-peak (mVp-p), compared with an aluminum polymer capacitor with an ESR of 15 mΩ, which produces a ripple voltage of 36 mVp-p, or an aluminum electrolytic capacitor with an ESR of 900 mΩ, which produces a ripple voltage of 950 mVp-p.
Other examples of ECAS capacitors include the ECASD40D157M009K00, rated at 150 µF (±20%) and 2 Vdc with an ESR of 9 mΩ in a D4 case, and the ECASD41C686M040KH0, rated at 68 µF (±20%) and 16 Vdc with an ESR of 40 mΩ, also in a D4 case. Features of ECAS polymer aluminum capacitors include:
- High capacitance combined with low ESR
- Stable capacitance with applied DC voltage/temperature/high frequencies
- Excellent ripple absorption, smoothing, transient response
- No voltage derating required
- Elimination of the acoustic noise created by ceramic capacitors (piezo effect)
- Polarity bar (positive) noted on product
- Surface mount construction
- RoHS compliant
- MSL 3 packaging
ECAS polymer aluminum electrolytic capacitors are optimized for use in power management applications; they are not recommended for use in time-constant circuits, coupling circuits, or circuits that are sensitive to leakage currents. ECAS capacitors are not designed to be connected in series. Other design considerations include:
- Polarity: Polymer aluminum electrolytic capacitors are polarized and must be connected in the correct polarity. Even a momentary application of a reverse voltage can damage the oxide film and impair the performance of the capacitor.
- Operating voltage: When these capacitors are used in AC or ripple current circuits, the peak-to-peak voltage (Vp-p), or the offset-to-peak voltage (Vo-p), which includes the DC bias, must be maintained within the rated voltage range. In switching circuits that may experience transient voltages, the rated voltage must be high enough to also include the transient peaks.
- Inrush current: If inrush current exceeding 20 amperes (A) is expected, additional inrush current limiting is required to maintain the peak inrush at 20 A.
- Ripple current: Each model of the ECAS series has specific ripple current ratings that must not be exceeded. Excessive ripple currents will generate heat that may damage the capacitor.
- Operating temperature:
- When determining the temperature rating of the capacitor, designers need to take into consideration the operating temperature of the application, including the temperature distribution within the equipment and any seasonal temperature factors.
- The surface temperature of the capacitor must remain within the operating temperature range, including any self-heating of the capacitor resulting from the specific application factors such as ripple currents.
It’s difficult for designers of power delivery systems to achieve the optimum balance of efficiency, performance, cost, stability, reliability, and form factor, particularly when supplying large ICs such as MCUs, ASICs and FPGAs, and when supporting peak power needs in USB applications. One of the main components of the power supply signal chain is the capacitor, and there are many characteristics of these devices that help meet designers’ requirements—if the right technology is used.
As shown, polymer aluminum capacitors help designers find the right balance. Their structure ensures low impedances at frequencies up to 500 kHz, low ESR, good ripple smoothing, as well as good noise suppression and decoupling on power lines. Also, they do not suffer from DC bias limitations, and they are self-healing, improving operational reliability. They also have a more reliable supply chain as they don’t use conflict materials. All told, polymer aluminum capacitors offer designers a higher performance option for addressing the requirements of a wide range of power management systems.
- Fundamentals: Understand the Characteristics of Capacitor Types to Use Them Appropriately and Safely