Supercapacitor or battery? That's the question~

Many systems require backup power, and the question is: What are the options for storage media for this type of backup power? The traditional choices are capacitors and batteries.

It can be said that capacitor technology has played an important role in power transmission and distribution applications for decades. For example, traditional film and oil-based capacitors are designed to perform a variety of functions, including power factor correction and voltage balancing. However, a lot of research and development has been carried out in the past decade, resulting in significant advances in capacitor design and capacity. These advanced capacitors are called supercapacitors and are well suited for use in battery energy storage and backup power systems. Supercapacitors have a limited total energy storage capacity, but their energy density is very high. In addition, supercapacitors have the ability to quickly release high energy and recharge quickly.

Supercapacitors are not only compact, but also robust and reliable, meeting the requirements of backup power systems to cope with short-term power loss events. In addition, supercapacitors can be easily stacked in parallel or series, or even in series-parallel combinations to provide the necessary voltage and current for the end application. However, supercapacitors are more than just capacitors with very large capacitance. Compared to standard ceramic, tantalum or electrolytic capacitors, supercapacitors have higher energy density and greater capacitance of the same size and weight. Although supercapacitors require special maintenance, they can surpass or even replace batteries in data storage applications that require high current/short-term backup power.

In addition, supercapacitors can be used in a variety of high peak power and portable applications that require high burst current or short-term battery backup, such as UPS systems. Compared with batteries, supercapacitors provide higher burst peak power in a smaller size, have more charge cycles, and operate over a wider temperature range. The service life of supercapacitors can be maximized by reducing the upper cutoff voltage of the supercapacitor and avoiding high temperatures (> 50°C).

Batteries, on the other hand, can store large amounts of energy, but have limitations in power density and delivery. Because of the chemical reactions that occur inside a battery, the number of charge cycles it can sustain is limited. Therefore, batteries are most effective when delivering moderate amounts of power over a long period of time, and pushing large currents out of a battery very quickly can severely shorten its effective lifespan. Table 1 summarizes the advantages and disadvantages of supercapacitors, regular capacitors, and batteries.

Table 1. Comparison of the characteristics of supercapacitors, ordinary capacitors and batteries

Now that we know that supercapacitors, batteries, and/or a combination of the two can be used as a backup power source for almost any electronic system, what solutions are available?

First, any IC solution will need a complete lithium-ion battery backup power management system that must be able to keep the 3.5 V to 5 V rail powered when the main power fails. Batteries provide much more energy than supercapacitors, so they are more suitable for applications that require backup power for a long time. Accordingly, any IC solution will need an on-chip bidirectional synchronous converter to charge the backup battery with high efficiency and provide high-current backup power to downstream loads if the main power rail is interrupted. Therefore, when external power is available, the device will act as a step-down battery charger for a single-cell lithium-ion or LiFePO ₄ battery while giving priority to the system load. However, if the input power suddenly drops below the adjustable power-fail input (PFI) threshold, the IC will need to act as a boost regulator to provide several amps of current from the backup battery to the system output. Therefore, if a power failure occurs, the IC will need to perform power path control to provide reverse blocking and seamlessly switch between the input power and the backup power. Typical applications for this IC include fleet and asset tracking, automotive GPS data loggers, automotive telematics systems, toll collection systems, security systems, communications systems, industrial backup power supplies, and USB powered devices. Figure 1 shows a typical application schematic using the LTC4040 Li-Ion battery backup manager.

Figure 1. Backup power supply using the LTC4040 with user-set PFI thresholds.

The LTC4040 also has an optional overvoltage protection (OVP) function that protects the IC from input voltages greater than 60V through an external FET. Its adjustable input current limit function supports powering from a current-limited power supply while the system load current takes precedence over the battery charge current. An external disconnect switch isolates the main input power supply from the system during backup power supply. The LTC4040's 2.5 A battery charger offers 8 selectable charging voltages optimized for Li-Ion and LiFePO ₄ batteries. The device also features input current monitoring, input power loss indicator, and system power loss indicator.

Similar to batteries are supercapacitors. However, supercapacitors do not support situations where the main power source is lost for an extended period of time, but they are an excellent choice for systems that require high-power, short-term backup power. Therefore, any IC supporting such applications will typically need to be able to support a 2.9 V to 5.5 V rail during the main power interruption. It is well known that supercapacitors have a higher power density than batteries, which makes them an ideal choice for systems that require high peak power backup power for a short period of time.

For example, ADI's LTC4041 uses an on-chip bidirectional synchronous converter to provide high-efficiency step-down supercapacitor charging and high-current, high-efficiency step-up backup power. When external power is available, the device acts as a step-down battery charger for one or two supercapacitor cells while giving priority to the system load. When the input supply drops below the adjustable PFI threshold, the LTC4041 switches to boost mode of operation and can deliver up to 2.5 A of current from the supercapacitor to the system load. During a power failure, the device's PowerPath™ control function provides reverse blocking and seamless switching from input power to backup power. Typical applications for the LTC4041 include riding through "dying gasp" power supplies, high current ride-through 3 V to 5 V UPS, power meters, industrial alarms, servers, and solid-state drives. Figure 2 shows a typical LTC4041 application schematic.

Figure 2. LTC4041 supercapacitor backup power application schematic

The LTC4041 has an optional OVP function that uses an external FET to protect the IC from input voltages greater than 60 V. An internal supercapacitor balancing circuit keeps the voltage across each supercapacitor equal and limits the maximum voltage of each supercapacitor to a predetermined value. Its adjustable input current limit function supports operation from a current-limited source while the system load current takes precedence over the battery charge current. An external disconnect switch isolates the main input supply from the system during backup power. The device also features input current monitoring, an input power fail indicator, and a system power fail indicator.