Ultracapacitor Options (and Ways to Charge Them) Grow

Jun 1, 2007 12:00 PM
By David Morrison, Editor, Power Electronics Technology

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Ultracapacitors, also referred to as supercapacitors, continue to make inroads into a wide range of applications, where their high power density enables them to supplement or replace batteries. Today, they find use in numerous applications including consumer electronics devices, lighting systems, power tools, UPSs, windmills, hybrid electric vehicles, buses and trains, telecom systems and industrial equipment. In these applications, ultracapacitors perform a variety of energy-storage and power-delivery functions. For example, smaller value devices might be used to provide battery backup, while larger value ultracapacitors may help start a motor or store a vehicle's braking energy.

Because their voltage ratings are low, typically 2.5 V, ultra-capacitor cells are frequently combined into modules or packs where they are series connected to handle higher voltages. However, ultracapacitor manufacturers are working to develop ultracapacitor cells with higher voltage ratings to reduce the number of cells required for a given voltage rating. Some vendors offer 2.7-V rated parts, but work continues to push this specification higher.

Ultracapacitor development is also underway to reduce cell size and cost. These two issues, particularly the latter, are considered critical to the continued migration of ultracapacitors into new applications. As cost per Farad drops over time, ultracapacitor applications will tend to multiply. The same may be true of ultracapacitor vendors as new suppliers continue to enter the market. Although ultracapacitor specialists such as Maxwell Technologies and Ness have been the most visible producers of these components, mainline companies who offer electrolytics and other capacitor styles have begun to add ultracaps and supercaps to their portfolios.

A related development is the recent introduction by one semiconductor vendor of an IC specifically developed to manage supercapacitor charging. This part may be the first of many to address application needs for ultracapacitor power management.

New Devices

One of the mainline capacitor companies that now offers supercaps is Illinois Capacitor. This company recently entered the market by introducing its DLR series, which offers 14 capacitance values ranging from 4 F to 220 F with working voltage ratings from 2 Vdc to 2.5 Vdc. The company also offers multicapacitor packs with higher voltage ratings or capacitance values. Operating temperature ranges from -20°C to +70°C.

At 2.5 V, DLR life is rated at 500,000 cycles at <30% of initial value. The radial-leaded devices are offered in case diameters of 10 mm, 12.5 mm, 16 mm or 18 mm. The parts are fully RoHS compliant and each capacitor is 100% burn-in tested. Available from stock, DLR series supercapacitors are priced starting at $1 in lots of 1000 pieces. For more information, see www.illcap.com.

Meanwhile, Maxwell Technologies is currently developing a high-voltage ultracapacitor module targeting burst power and UPS application. It expects to introduce the new module in the coming months.

“The new module is highly robust and will find application in the pitch system market as part of wind energy machines,” says John M. Miller, vice president, Advanced Transportation Applications.

Supercapacitor Charger Chip

Semiconductor supplier Advanced Analogic Technologies (AnalogicTech) has introduced a chip it describes as the first supercapacitor charger IC. The AAT4620 is a P-channel current-limited MOSFET power switch for high-side load-switching applications in PC modem cards.

The device integrates all the circuitry required to limit current, protect the PC card connector, continuously charge the capacitor and notify the system when it is ready for use.

“Typically designers use a smart switch or MOSFET with a whole host of additional circuitry to create this function,” says Phil Dewsbury, product line director for AnalogicTech. “By combining a P-channel MOSFET current switch with two independent current limits, a power loop, reverse blocking and a system-ready pin in a tiny 12-pin TSOPJW package, the AAT4620 ensures the supercapacitor is charged as quickly as possible without violating the host power specification, while reducing component count and board space requirements.”

The AAT4620 offers two independent, programmable current limits to control current during host/card negotiation. As long as thermal dissipation is low, the integrated current limiting will allow the capacitor to charge until it reaches 98% of its final value. Set by two external resistors, the current limits support ±10% accuracy over the normal operating temperature range (see the figure).

An integrated digital power loop automatically monitors and reduces the charging current to a safe level when high levels of charge result in excessive temperature and threaten to send the chip into thermal shutdown. By regulating charge current and therefore die temperature, this circuit ensures the supercapacitor charges continuously and as quickly as possible.

Integrated reverse-blocking protection prevents discharge of the supercapacitor to the power supply. A system-ready output pin with externally programmable hysteresis alerts the system when the supercapacitor is fully charged and ready for use. The AAT4620 also features a quiescent current of 40 μA typical, which drops to 1 μA in shutdown mode.

The device is specified across the -40°C to +85°C temperature range. Pricing is $1.34 each in 1000-piece quantities. For more information, see www.analogictech.com.