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ICs tap alternative energy sources

Posted: 02 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:step-up converters alternative power sources? solar micro fuel cells? lithium batteries for portable consumer devices? power management for boost converters?

Imagine a cellphone that offers endless standby power or an MP3 player that never needs recharging. That is the promise of powering portable consumer electronics devices off renewable power sources. By harvesting power from external sources such as solar or micro fuel cells, systems can tap into what is, for all practical purposes, an infinite source of energy.

But designers building systems capable of drawing power from such sources face daunting obstacles. First, power circuits tapping the sources must support input voltages as low as 0.3V. "A single solar cell usually operates between 0.4V and 0.7V with no load. If you add a load to the output and pull out a couple of milliamps of current, your voltage drops down to between 0.5V and 0.4V or a little below," said Alex Friebe, product marketing engineer for DC/DC converters at Texas Instruments Inc.

That's far below the capabilities of the boost converters available from most power semiconductor suppliers today. Designed for rechargeable battery cells, most devices support input voltages in the 0.7V range and only support startup at 0.9V and above. And converters supporting alternative power sources must exhibit higher-than-normal efficiencies over a wide input range to maximize power transfer from a low-power source.

Furthermore, alternative energy sources present some unique performance considerations that designers usually don't see when their systems are drawing charge from the grid. A solar cell generates a current proportional to the amount of light falling on it. Non-uniform light exposure on a single cell or cells connected in series, for instance, can result in rising output impedance and reduced output power. Load conditions in micro fuel cells, on the other hand, can vary widely depending on their chemistry type.

In March, TI introduced a step-up converter designed to help designers cost-effectively use the non-standard voltages from alternative power sources to recharge the core Li-ion battery in a portable electronic device. The TPS61200 integrates a 1.5A switch and supports input voltages from 0.3V to 5.5V. It can manage power down to 0V if the undervoltage lockout pin is connected directly to the output voltage. It supports an extremely low, 0.5V startup in any load condition and still maintains efficiency of up to 90 percent.

Auxiliary output voltage is boosted to start the main circuit with less power.

"All of the power components we have were designed to operate with existing Li-ion or alkaline power cells," said Friebe. "So the challenge for our engineering team was how to do this when we had no existing control circuits or IP to operate an IC from this level."

TI engineers implemented an architecture that functions like a two-step inductive boost converter. An integrated startup boost stage initiates the device at 0.3V or 0.5V. Once the voltage reaches a predefined threshold, the converter seamlessly transitions to the main boost stage.

TI has added a boost converter that manages micro fuel cells and solar cells.

The step-up converter features a downconversion mode to protect the device when the input voltage exceeds the output. "Whenever input voltage exceeds output voltage, you get an unstable condition," said Friebe. "TI's downconversion mode decreases the input voltage when this condition occurs to meet the output voltage without compromising efficiency." Short-circuit protection and programmable undervoltage lockup are also provided.

Footprint, of course, is a consideration for any function supporting a portable device. "The question is: How big does the solar array have to be to meet the power needs required by the system?" noted Robert Castellano, president of The Information Network, a research firm that focuses on solar technology.

The TPS61200 was designed to operate from a single solar cell and eliminate the need for multiple cells in series, along with the protection circuitry that comes with a series connection. The approach lets designers create simple built-in solar-powered chargers for cellphones and other devices that, operating off indoor ambient lighting, can extend system standby time infinitely. "Sitting on your desk, a cellphone consumes 7-10mA and an MP3 player about 5mA," noted Friebe. "Even in bad conditions using indoor ambient light, a solar cell can generate 9-12mA, so if you connect it to your cellphone, you can theoretically extend standby time indefinitely."

The larger question may be whether a viable market exists for portable consumer electronics devices operating off alternative power sources. Tony Armstrong, marketing manager for power products at Linear Technology Corp., which offers step-up converters with guaranteed startup down to 0.7V, notes that in the industrialized world, users have almost constant access to a power source: through a wall socket, a USB port or an adapter in an automobile.

Moreover, improvements in battery and system design promise to extend lithium battery life to a point where it may no longer be a concern for users. "Interesting developments in Li-ion battery chemistries allow increased energy density and a better discharge profile in the same form factor," Armstrong said. "And new power devices are coming that consume less power, through lower quiescent current or better efficiency."

"Batteries have been increasing capacity at about 5 percent a year, while power demands on a battery have been significantly higher, so we've been trying to squeeze more efficiencies out of everything," said Mark Davidson, marketing director for National Semiconductor Corp.'s power management division. While the batteries currently in use discharge down to about 3V, batteries expected to sample later this year will operate to 2.5V and below. "We see that as a more likely answer to limited system power than a reliance on things like fuel cells or solar power," Davidson said.

- John Mayer

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