You have to be Subtle with Supercapacitors
Designers are using supercapacitors (also called electrochemical double-layer capacitors, or EDLCs) in a wide variety of applications. These include local backup power for embedded systems with flash memory, pulse power, bridging or hold-up power, camera flash systems, energy harvesting, pulse applications, uninterruptible power supplies, industrial systems, wireless alarms, remote metering, and toys. As useful as they are, designers should be aware of the complexities and subtleties of supercapacitors, relative to conventional board-level capacitors.
In this blog, we will begin by looking at three important considerations when using supercapacitors and close with a brief review of supercapacitors from AVX Corp., Eaton, and KEMET, each optimized for a range of applications.
What to consider when designing with supercapacitors
Some of the supercapacitor subtleties that can introduce challenges for designers include:
Equivalent series resistance (ESR): ESR is not that simple with supercapacitors. Specifications may not appear to vary a lot between manufacturers’ datasheets, but the actual ESR of shipped devices can vary quite a bit. Also, ESR increases as supercapacitors age. A designer should look for supercapacitors that start with a low ESR and which stays relatively low, across a wide temperature range throughout their operational life. Factors that can affect ESR over time include material purity, the cleanliness of the manufacturing process, and how long (if at all) the devices were subjected to burn-in prior to shipping.
Maximizing supercapacitor lifetime: In general, higher temperatures and higher operating voltages at the cell level cause the ESR to increase faster and reduce supercapacitor lifetimes. Therefore, lowering the operating voltage per cell is the main tool designers have to maximize lifetime. The typical strategy is to put more cells in series, but this increases the ESR of the system. This, however, can be overcome by adding capacitance to lower the ESR.
Power conversion: Many designers are used to working with batteries or another more constant voltage source. When using supercapacitors, it is important to understand how the voltage drops as it powers the load and the effect that the rate of current can have on voltage. With a sub-optimal power converter, supercapacitors can be more expensive than needed. If the power electronics are designed to use a wider voltage window (full rated voltage to ½ rated voltage), it enables the use of the full energy stored in the supercapacitor. This allows the use of smaller supercapacitors, which helps lower costs and can reduce system size.
Five Farad supercap for high energy density
The PHV-5R4H505-R 5 Farad (F) supercapacitor from Eaton is a 5 volt device optimized for the needs of high energy density applications such as pulse power systems, uninterruptible power supplies, and industrial systems (Figure 1). It features integrated cell management (built-in balancing). Its ESR is 70 milliohms (mΩ) at 100 Hertz (Hz) and 65 mΩ at 1 kilohertz (kHz), and it has an operating temperature range of -40°C to +65°C, and an extended temperature range up to +85°C with linear voltage derating to 4.0 volts at +85°C. The PHV-5R4H505-R has a lifetime of up to 20 years, assuming operation within the specified charge voltage and temperature ranges.
Figure 1: Eaton’s PHV-5R4H505-R 5 F supercapacitor comes in a rectangular package for high power density. (Image source: Eaton)
Next, we’ll look at two 470 millifarad (mF) supercapacitors optimized for different sets of application needs.
400 mΩ, 470 mF supercap for pulse power
The SCMQ14C474PRBA0 is a 5 volt, 470 mF, series-connected supercapacitor module from AVX with an ESR of 400 mΩ at 1 kHz (Figure 2). It is optimized for use in energy harvesting systems, pulse power applications, and for supplementing or replacing batteries in energy hold-up circuits. When used in combination with batteries, these supercapacitors can extend backup times, contribute to longer battery life, and support instantaneous pulse power requirements.
Figure 2: The SCMQ14C474PRBA0 is a 5 volt, 470 mF supercapacitor optimized for energy harvesting systems and pulse power applications. (Image source: AVX Corp.)
25 Ω, 470 mF supercap for long-term backup power
The FC0V474ZFTBR24 is a 470 mF, 3.5 volt supercapacitor from KEMET that is suited for use in low voltage, direct current (DC) hold-up applications such as embedded microprocessor systems with flash memory and clock ICs (Figure 3). With its ESR of 25 Ω at 1 kHz, this device is particularly useful for providing back-up currents of 500 microamperes (μA) and below for extended periods.
Figure 3: The FC0V474ZFTBR24 is a 470 mF, 3.5 volt supercapacitor from KEMET in a surface mount package that does not require a holder. (Image source: KEMET)
Supercapacitors are more complex than conventional board-level capacitors: with many variations available to suit applications ranging from local power backup in embedded systems, pulse power, energy harvesting, uninterruptible power supplies, industrial systems, and remote metering, among others. Designers need to pay particular attention to ESR, the subtleties involved with ensuring extended operating lifetimes, and power converter design.