Benefits of Supercapacitors in Industrial Handheld Devices
Portable electronic devices for commercial applications must meet more stringent performance requirements and specifications than their consumer counterparts, and may have many power hungry features which need to operate simultaneously. A supercapacitor offers several key benefits when incorporated into the power architecture of an industrial handheld device.
Portable electronic devices for commercial applications must meet more stringent performance requirements and specifications than their consumer counterparts, and may have many power hungry features which need to operate simultaneously. A supercapacitor offers several key benefits when incorporated into the power architecture of an industrial handheld device.
Pulse power support: The supercapacitor provides the peak current required by all high power loads in the device, including GSM/GPRS and RFID communications, thermal printers, LED flash, bar code scanners, GPS chips, display refreshes, hard disk drives and audio amplifiers. This reduces stress on the battery, minimizing voltage droop, improving run-time (transactions per charge) and extending battery life.
“Drop test” and “hot swap” support: Battery contacts can disconnect when a device is dropped, giving rise to transient losses of power (“battery contact chatter”). This can result in data loss, interrupted communications, and even shut down the device. A supercapacitor solves this problem by providing short-term, uninterruptable power during any such voltage transients. A suitably specified supercapacitor can even provide enough power to support a battery hot swap – without losing any data or requiring a system restart.
Support for “last gasp” transmissions and “graceful shutdown”: In the event of more permanent loss of power, a supercapacitor will ensure that the device shuts down in an orderly fashion – undertaking critical house-keeping operations, writing volatile cache data to secure storage, and even sending a final transmission regarding the impending/actual power loss. The supercapacitor removes the need for a separate back-up battery to meet these requirements.
Support for low temperature operation: Handheld terminals are often used in low temperature environments such as refrigerated warehouses, outdoor restaurants and parcel delivery trucks. Battery impedance rises rapidly as the temperature falls, increasing the voltage drop during high current events. This could cause an under-voltage lockout, and shut down the function (or even the device), even though there is still plenty of energy left in the battery. The low ESR of the supercapacitor, even at low temperatures, shields the battery from the peak loads, and enables the device to function at temperatures as low as -40°C.
Extended battery run-time and operational life: For much the same reason, battery run-time and operational life will be significantly improved by a supercapacitor, simply because the battery voltage droop will be reduced for any given load. As the battery ages, and its impedance increases, the support given by the supercapacitor becomes even more important, and extends its operational life.
… or smaller, cheaper, lighter and more efficient batteries/power supplies: With a supercapacitor now meeting the peak loads, the power supply can be optimized for energy and sized to provide the average current to charge the supercapacitor, rather than the peak current required for the loads. This allows the size, weight and cost of the battery and related DC:DC components to be reduced, or a lower cost alkaline/primary cell to be used, while still supporting all high power functions. And if environmental concerns are significant, or maximum energy availability/flexibility is the objective, it even opens up the potential to use a renewable, low power source such as solar or vibration.
The following block diagram of Fig 1 shows a possible architecture to deliver all the benefits listed above:
Figure 1: Power Architecture for a Hand held Terminal (full size image)
In this architecture, the one supercapacitor provides peak power for all functions, and provides backup power for the unit. The battery only needs to deliver average power to the DC:DC and the supercapacitor delivers peak power in bursts as needed by the different functions, and is re-charged in between peaks. This also means the DC:DC converter only needs to be sized for average power rather than peak power. Typically the battery would be a Li-Ion cell at 4.2V – 3.3V, and the high power rail at 5V, so the DC:DC converter would be a boost. When the unit is first powered on, it must manage the inrush current to the supercapacitor, which will look like a short circuit. The TPS6120x or TPS6103x families, or the LT3625 manage this. If two battery cells in series are used, at 8.4V – 6V, then a suitable buck converter with programmable current limit is the LTC3663.
If high powered LED Flash is one of the unit’s functions, then there are LED flash drivers designed to charge a supercapacitor that can be used as the boost converter, e.g. TPS61325, STCF04, CATS3224. These flash drivers will deliver up to 4A to pair of LEDs to achieve ~15W of flash power, giving superior photos in low light conditions. To achieve the voltage rating on the high power rail, the supercapacitor will need to comprise two cells in series. Supercapacitors are low voltage devices with a maximum rating typically of 2.3V – 2.7V/cell. The cells need a balancing circuit to ensure the voltage is evenly distributed and so neither cell goes over-voltage. The LED flash drivers listed, and the LT3625 are designed to charge supercapacitors and include a balancing circuit.
A CAP-XX supercapacitor with an active balance circuit will only draw a few microamps, which is important to maximise battery run time. Many hand held terminals have traditionally used thermal printers that run at 12V or 9V. However, for the one supercapacitor to perform the peak power and power backup functions, the thermal printer should run at 5V so it can draw its peak power directly from the supercapacitor. A suitable thermal printer is the Citizen MLT4280. The power select switch should be two ideal diodes, such as the LTC4415, configured to select the battery if present, and the supercapacitor otherwise.
CAP-XX supercapacitors deliver these benefits due to their unique combination of very high power (low ESR), high capacitance in a thin prismatic form factor, suitable for a slim handheld terminal, ultra-low leakage current (~1µA), wide temperature range (-40°C to +85°C) for industrial grade performance or use in a refrigerated warehouse, long life and operational and disposal safety: UL810A certification, RoHS, REACH and WEEE.
