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Electronic Design

Proton Polymer: The Key To Double-Layer Supercaps For High-Voltage Applications

The turn of the century saw the debut of a double-layer supercapacitor technology, the proton polymer. Since its introduction, there has been much success in pulse supercapacitor applications due to the intrinsically high conductivity associated with the proton-conducting polymer separators and nanoparticle carbon powders used in these capacitors.

These components exhibit a capacitance range from 5 mF to 1 F, equivalent series resistance (ESR) values ranging from 20 to 500 mO, and leakage currents ranging from 3 to 20 mA for pulse-power and low-baseline-power applications. This technology enables the manufacture of capacitors with voltage ratings up to 15 V, resulting in pulse supercapacitors with the highest voltage rating available today.

In 1845, Hermann von Helmholtz introduced the concepts of the double-layer capacitor (DLC) for storing charge on the surface of an electrode, between the electrolyte and the electrode. But a quantum model developed that could describe its electrochemical processes wasn’t developed until the early part of the 20th century. The first products based on these concepts were developed only 30 years ago.

The first supercapacitors for commercial applications had capacitance values ranging from a fraction of a Farad to 100 F, but with a low working voltage range of 2.3 to 2.7 V. These components find routine employment in backup applications today. In most of these backup applications, the current requirements are in the micro-amp range and the ESR values, ranging from 5 to 100 O, are more than adequate.

The most significant factor driving passive component manufacturing trends today is the OEM’s desire to develop smaller end products that are handheld and/or rely on battery power. Many passive-component innovations are traceable back to the need for smaller and lighter devices. For capacitors, that has resulted in shrinking case sizes from something like a 1210 down to 0201 and even to some 01005s.

Yet the functionality of handheld devices is continually increasing, and they have had to evolve to accommodate many new applications. These range from high-power data logging to wide-area network (WAN) interconnection with increasing multimedia and imaging capabilities.

While the downsizing trend in both active and passive technologies continues to shrink the size of any given device, their increasing power-on-demand budget can only be supported by new component families. One such family that can provide the necessary energy density, enabled by the development of advanced materials, is the pulse supercapacitor.

In the late 1990s, passive component manufacturers introduced supercapacitors employing a new material technology. For example, AVX’s BestCap supercapacitors feature a polymer that provides performance enhancements such as an initial voltage range from 3.5 to 5.5 V and low ESR values of 30 to 200 mO. These supercapacitors particularly suit pulse-power applications, since they can maintain low ESR values and retain the high capacitance values at pulse widths ranging from 0.5 to 5 ms.

They are still viable for deployment in digital wireless applications because of their low profile, around 1.9 to 6.5 mm, and their long cycle lives and low leakage currents. These devices still find use in backup applications where low profiles are desirable, as replacements for aluminum electrolytic components where size and profile are critical, and in other applications where battery chatter is an issue, from annoyance level to mission-critical.

New applications are constantly emerging due to the adoption of rechargeable, lightweight low-profile batteries and because of the opportunity to use primary, alkaline cells in many electronic applications where cost is paramount. Of particular importance and advantage, these pulse supercapacitors can extend battery life by as much as 200% to 300%.

The initial ranges of supercapacitors were an ideal fit for wireless voltage holdup (typically 3.6 V) and 4.5-V across-the-battery applications. Yet for voltage holdup of higher voltage lines, it is better to have the holdup actually on the line. This is because electrical energy is proportional to 1/2CV2, and having the holdup at a higher voltage proves far more efficient. Therefore, the focus of development began moving toward aqueous-based technology utilizing proton-conducting membranes that can be stacked and serialized for voltage ratings of 7 to 15 V.

The capacitors use environmentally friendly, solvent-free aqueous materials with multiple cells. In Figure 1, each cell is between two current collectors (solid red). Each cell consists of two carbon electrodes separated by the proton-conducting solid polymer separator. The separator relies on a special design with a polymerbased composition that results in a very low leakage current and low ESR, which is essential for high-current pulse applications.

The number of cells is easily adjustable for a variety of voltage ratings in the same cell with ranges from 3 to 15 V, limited only by the height of this device. This flexibility in a stack of cells in the same package offers advantages over comparable technologies where each cell is by necessity enclosed in a package.

The first significant differentiator of the proton-polymer system is the level of capacitance retention achieved in increasing frequency, a.k.a., short pulse-width applications. Figure 2 shows that proton-polymer system response is more akin to a bank of aluminum electrolytic capacitors.

Depending on rating, these proton-polymer parts have typical ESRs of 40 to 200 mO. However, the impedance characteristics are also significant. Unlike electrostatic capacitors, the impedance curve remains flat down to frequencies below 100 Hz. Given these characteristics, together with low leakage current and a wide range of rated voltages, proton-polymer technology is highly suitable for deployment in a wide range of pulse applications.

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Looking at a common example, the pulse holdup in a handheld device GSM/GPRS transmission requires a current of approximately 2 A for the pulse duration. Due to battery and circuit impedance, the input voltage to the transmitter drops during the pulse (Fig. 3).

The battery voltage slowly drops as it is depleted of its charge and the transmitter can operate only above a certain input voltage. Therefore, to maximize the usage time before charging or replacing the battery, VM needs to be minimized. One solution is to connect a capacitor close to the transmitter, as this will provide the necessary pulse holdup characteristics

Apart from providing a technical solution, proton-polymer capacitors offer other advantages. For example, their wide voltage range lets them be used across the GSM chip (3.5 V), across the battery (4.5 V), or at the dc-dc converter (5.5 V). Also, their lowprofile, prismatic form factor enables them to fit into tight-quarter designs such as a PCMCIA, USB card, or LED camera flash.

Essentially, pulse holdup applications are defined as a system where an energy storage device, like a battery or fuel cell, supplies a low-power continuous load and also charges a capacitor that’s capable of short-term, high-power delivery on demand for peak load requirements. While many technologies can be used for the capacitor, as peak duty cycles increase such as in GPRS-8 to GPRS-10 topologies, then the applications suit the combined high-capacitance and low-ESR characteristics of proton-polymer pulse supercapacitors.

In fact, an application may have a mixedload requirement whereby a smaller bank of tantalums and a pulse supercapacitor can be used. Such pulse applications are everywhere, including combinations of electrical and electromechanical support. Examples range from digital cameras, ensuring capture is maintained while zoom and focus motors operate, to remote valve controls as in automated faucet to wireless remote valve activation.

The key is that proton-polymer pulse supercapacitors offer more battery options. With pulse support, standard non-rechargeable alkalines can be used in place of lithium- ion rechargeables, which can be useful for commodity electronics operated away from any convenient recharge facility.

Prior to pulse supercapacitors, the traditional approach was to use a high-power rechargeable battery, charged by a low-power primary cell. The pulse supercapacitor solution provides a number of benefits to the designer, including a substantially lower voltage drop for pulse durations up to 100 ms, a discharge current limited only by the ESR of the capacitor, a small form factor down to 1.8 mm, and a wider temperature range enabling a substantially lower voltage drop at cold temperatures in the ballpark of –20°C.

In essence, proton-polymer technology is based on an aqueous system where cells exhibit extremely repeatable voltage capabilities. This enables a wide range of voltage ratings to be manufactured from less than 2 V to support low-voltage power supplies up to 15 V.

The voltage repeatability means that these components are more than suitable to serialization, either as discrete capacitors in serialized banks or in volumetrically efficient modules. Also, no balancing resistors are needed for discrete parts and are optional with larger serial banks.

TAGS: Components
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