New supercapacitor shrinks in size but grows in features

Supercapacitors are becoming increasingly important for today’s electronic devices and energy systems. By some estimates, the supercapacitor market is predicted to reach $3.5 billion by 2025, with a forecasted median compound annual growth rate of 20% between 2020 and 2025.

The growing electric vehicle (EV) industry alone has significantly bolstered the supercapacitor market. Supercapacitor devices are currently used in EVs to transform energy from regenerative braking systems, since the high power density of supercapacitors allows them to charge and discharge more rapidly than batteries. Supercapacitors cannot yet replace lithium-ion batteries in terms of energy storage, although the technology is improving every year.

New market opportunities, like smart grids and alternate energy sources, are also key growth areas for supercapacitors. These markets benefit from the wide temperature ranges supported by the devices. Furthermore, supercapacitors are more environmentally friendly than batteries, as well as less subject to global trade restrictions.

Selection criteria

Designing with supercapacitors is relatively straightforward as long as engineers follow a few key guidelines. The critical parameters to understand before selecting a supercapacitor include:

• Operating temperature range
• Application areas, such as energy harvesting, pulse power, power hold-up or battery replacement
• Importance of equivalent series resistance (ESR) and leakage current to the design
• Operating voltage
• Expected lifetime
• Cost feasibility

Perhaps the most important parameter is the operating temperature range for the desired application. Aside from the proper functioning of the devices, temperature directly affects the operating voltage of the supercapacitor, especially those based on organic electrolytes. As temperature goes up, the maximum operating voltage tends to go down, since it takes less voltage to start breaking down the electrolyte system. The temperature range of the devices is determined by the freezing and boiling points of the electrolyte fluid within the supercapacitor.

The next criterion for the designer to consider is the electronic or electrical characteristics of the circuit within the specific application, particularly the ESR, leakage current and operating voltage. Compared to batteries, supercapacitors have low internal resistance or ESR, thus enabling high power density.

Leakage current occurs over time when a supercapacitor is held at a rated voltage. It results from naturally occurring imperfections in the supercapacitor’s electrode material. Leakage current is a function of voltage and temperature.

Depending upon the application, designers may need to add several supercapacitors in series and/or parallel combination to meet electrical and energy storage requirements. But a series configuration may result in imbalances between supercapacitors that, in turn, affect the voltage and ESR values. To resolve this issue, supercapacitors may need to be balanced.

“When supercapacitors are used as large energy storage devices, it is very important to pay attention to the ESR and leakage current specs,” noted Eric DeRose, global product manager for AVX. The company offers both balanced and unbalanced versions of the series configured supercapacitor modules.

Another important parameter to understand is the expected lifetime of the supercapacitor for a given application. If the device will operate on a board or within a module with a 20-year product warranty, then the supercapacitor must be designed to last at least that long. While all supercapacitors age with time, that aging rate is once again a function of the application voltage and operating temperature. Extreme voltages and temperatures will shorten the lifetime of any supercapacitor.

Cost may also be one of the ultimate parameters. Supercapacitors are slightly more expensive than batteries, although they perform additional functions.

Comparison to other caps

Figure 1. AVX’s new extremely flat form factor, wide-temperature range PrizmaCap supercapacitor. Source: AVX Corp.To address the growing need for supercapacitors with wide temperature ranges and long lifetimes, especially in the automotive and renewable energy market, AVX has created a flat form factor, propylene carbonate-based electrolyte supercapacitor known as the PrizmaCap (Figure 1). It is part of the company’s SCP series product line.

These new devices are prismatic electric double-layer capacitors (EDLCs). Whereas ordinary capacitors use a solid dielectric, EDLCs or supercapacitors use electrochemical double-layer capacitance, which increased the capacitance of the capacitor.

The PrizmaCap supercapacitor provides among the lowest profile and highest capacitance available (Table 1). Used by themselves or in conjunction with batteries, they provide extended backup time, longer battery life and instantaneous power pulses as needed.

Table 1. Key parameters for AVX’s new flat form factor PrizmaCap line of supercapacitors.

Since the PrizmaCap uses an organic-based (e.g., propylene carbonate) electrolyte, it can survive lower temperatures that would damage other supercapacitors. Although, it is important to consider how electrical parameters of the supercapacitor, mainly ESR and leakage current, are affected at extremely low temperatures, and oppositely at warmer temperatures. Electrical characteristics versus temperature plots are detailed in all AVX SuperCapacitor datasheets.

What lies ahead?

While supercapacitors are used in many configurations with a primary or secondary battery, they have yet to actually replace a battery since capacitors do not actually produce equivalent energy. However, even that difference may be changing.

The electronics industry has developed power management ICs that work in conjunction with low-voltage application supercapacitors. These ICs typically utilize an internal buck-boost converter to charge the capacitor. While such a combination may slightly reduce the operating voltage on the supercapacitor as the temperature goes up, it has the desired effect of simultaneously boosting the output voltage to a constant value, just like a battery. In any capacitor, the voltage decreases as the current is drawn out of the device. A power management IC will change that process to provide a constant output voltage, effectively making the module look like a battery.

“Adding a power management IC with PrizmaCap should also provide a price advantage rather than stacking multiple supercapacitors in series to attain a higher voltage,” said DeRose. “Plus, it will be a lot easier to program the input and output voltages as desired.”

Summary

Supercapacitors have many advantages over traditional capacitors, including higher energy storage capacities, handling of high-power pulses, wider operating temperature ranges, lower ESR and leakage currents and longer cycle lifetimes. This is why they are well-suited to applications requiring pulse power handling, energy storage, energy and power holdup and battery assistance.

Better yet, AVX has the largest selection of supercapacitors in the ultra-flat prismatic form factor with capacitance rated for one Farad or more.