Renesas Powers Temperature Sensor With Wireless LAN Waves

May 8, 2013
Tetsuo Nozawa, Nikkei Electronics
The antenna and rectifier circuit
The antenna and rectifier circuit
[ If it clicks, the expanded picture will open ]
The power-supply chip equipped with a boost circuit and a power management function
The power-supply chip equipped with a boost circuit and a power management function
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The power-supply chip is used to boost the output of a thermoelectric transducer that is powered by body temperature.
The power-supply chip is used to boost the output of a thermoelectric transducer that is powered by body temperature.
[ If it clicks, the expanded picture will open ]

Renesas Electronics Corp developed an "energy-harvesting (EH) system" that receives electric waves output from a 2.4GHz-band wireless LAN, etc and uses their energy as a power source.

The new EH system has a more than four times higher energy use efficiency than Renesas' previous EH system. Also, the company added a boost circuit and a function to switch the power source to a button battery when the output of electric waves is weak. As a result, the new system is more suitable for practical use, the company said.

Renesas will exhibit the EH system at the 16th Embedded System Expo (ESEC), which runs from May 8 to 10, 2013, in Tokyo.

The new EH system consists of (1) a board equipped with an antenna that collects the energy of electric waves and a rectifier circuit and (2) a power management chip that boosts the output power and has a function to switch the power source to a button battery.

Renesas developed and announced a similar EH system in the past. But the system was used 30cm away from a 2.4GHz-band omnidirectional electric wave source with an output of 250mW. On the other hand, the new system is used 60cm away from the same electric wave source. In other words, the use efficiency of electric waves was improved by more than 300%.

Specifically, when an 8cm-long antenna and a rectifier circuit are placed 60cm away from the electric wave source, the new system outputs a direct current of 0.2V (10μW). At this place, the collectable energy of electric waves is 60μW, and the use efficiency of electric waves is 16%, Renesas said. The company did not disclose the details of the rectifier circuit.

Renesas boosted the 0.2V output from the rectifier circuit to 1.8V with a power-supply chip that it developed for energy-harvesting of electric waves and confirmed that a temperature sensor can be powered by the output. The start-up circuit of the power-supply chip operates with a voltage as low as 0.2V.

The power consumption of the chip is 4μW. And the operation efficiency of the boost circuit is about 40%, which is one of the world's highest, the company said.

In general, the output and frequency of electric waves in the air are unstable. Therefore, Renesas added the function to switch the power source to a button battery in case of weak electric waves.

"Though it depends on the capacity of a capacitor to be used, the power source is switched to a button battery when electric waves have stopped or become weak for several milliseconds to one second," the company said.

Renesas also prototyped a system that enables to use the power-supply chip for boosting the output of a thermoelectric transducer. It will be exhibited at the ESEC.