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Power components

2-A supercap charger balances and protects cells in portable applications

December 22, 2010 | Steve Knoth | 222901984
2-A supercap charger balances and protects cells in portable applications Steve Knoth Senior Product Marketing Engineer, Power Products Linear Technology Corporation, outlines how more compact, simpler designs can be created using supercapacitors.
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Supercapacitors (also known as ultra-capacitors or supercaps) continue to carve out a niche in the market space between conventional capacitors and batteries. They are replacing batteries in data storage applications requiring high current/short duration backup power, and they are also finding use in a variety of high peak power applications in need of high current bursts or supplementary battery backup.

Compared to batteries, supercaps provide better power density with higher peak power delivery capability, smaller form factors, higher charge cycle life over a wider operating temperature range, and have lower ESR. Compared to standard ceramic, tantalum or electrolytic capacitors, supercaps offer higher energy density in a similar form factor & weight. A supercaps lifetime is maximized by reducing the capacitors top-off voltage and avoiding high temperatures (>50C). See Table 1 below for a comparison of key features.



Table 1. Supercapacitors vs. Capacitors vs. Batteries

Summary - Supercaps vs. Batteries:

Batteries:

Good energy density
Moderate power density
High equivalent series resistance (ESR) at cold temperatures

Supercaps:
Moderate energy density
Good power density
Low ESR even at cold temperatures
( ~2x increase at -20C vs. 25C)

Supercap Limitations:
Limited to 2.5 V or 2.75 V maximum per cell
Must compensate for leakage differences in stacked applications
Lifetime degrades quicker at high charge voltages and high temperatures

Early generation 2-cell supercap chargers were designed for low current charging from 3.3 V, 3xAA, or a Li-Ion/Polymer battery since these ICs have a boost topology. However, supercap technology improvements have expanded the market, resulting in a slew of medium to higher current opportunities not necessarily confined to the consumer product space. Primary applications include solid state disk drives and mass storage backup systems, high current portable electronic devices such as industrial PDAs and handy terminals, data loggers, instruments, medical equipment, and miscellaneous dying gasp industrial applications such as security devices and alarm systems. Other consumer applications include those with high power bursts including LED flash in cameras, PCMCIA card and GPRS/GSM transceivers, and hard disk drives (HDDs) in portable devices.
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