Power Module and Double Layer Capacitor Harvest Energy from Radio Signals

Jul 1, 2010 12:00 PM
Harry Ostaffe AND Charlie Greene Powercast Corporation, and Bharat Rawal, AVX Corporation

RSSI AND DATA OUTPUT

The P2110 has the capability for a microcontroller to sample the received signal strength (RSSI) and receive data from the broadcasted power. Driving DSET high directs the harvested DC power to an internal sense resistor, and the corresponding voltage is available at the DOUT pin. The voltage on the DOUT pin can be read after a 50µs settling time. DSET can also be used to receive short datagrams that are encoded in the power broadcast. Example data could be the ID of a power transmitter or a code to be used for the activation of specific devices. Powercast's current 915 MHz transmitter uses Amplitude Shift Keying (ASK) to broadcast an 8-bit ID with random delays between IDs to avoid collisions when multiple power transmitters are being used. With multiple transmitters, location-based applications can be enabled by transmitting back the ID of the transmitter which provided the energy. The harvested DC power is not being stored when using the RSSI or data functionality, but this energy loss will be negligible in most applications.

The INT (Interrupt) pin digitally indicates that voltage is present at the VOUT pin. This pin can be used in systems that contain other energy storage elements and can be used as an external interrupt to bring a device such as microcontroller out of a deep sleep mode. The digital high level of the INT pin will be enabled between VMIN and VMAX. The INT pin can provide a maximum of 0.1mA of current.

The regulated DC output voltage from the P2110 is preset to 3.3V and can be adjusted by adding an external resistor to increase or decrease the output voltage between the range of 1.8V to 5.25V. Decreasing the voltage is accomplished by placing a resistor between VSET and VOUT, and increasing the voltage is accomplished by placing a resistor between VSET and GND.

SYSTEM CONSIDERATIONS

As distance from an RF energy source increases, the amount of available power decreases with the inverse square of the distance. At close range some devices can be powered directly from the RF energy, but as distance increases, the energy will typically have to be accumulated before being used by the device. This is especially true for devices such as wireless sensors that wirelessly transmit data, and the P2110 was specifically designed to provide intermittent power for these types of devices. Multiple nodes using the P2110 can be powered simultaneously from the same transmitter. By using multiple transmitters a network of potentially thousands of battery-free wireless sensor nodes can be powered perpetually or on-demand.

Power consumption should be an important design criterion for all aspects of a micro-power system, especially when intermittent power is being used. Wireless sensors that are battery-powered typically consume a lot of energy (compared to a single transmission) when the system is initialized and attempts to join a network. After the network ID is received and a connection to an access point or other mesh node is established, the individual transmissions typically consume significantly less energy. When using intermittent power, minimizing start-up overhead and transmission can significantly impact energy consumption.

SUPERCAPACITORS PROVIDE ENERGY STORAGE

The capacitor used with the P2110 is a Double Layer Capacitor (DLC), also known as supercapacitor or electrochemical capacitor or Electrochemical Double Layer Capacitor (EDLC), with equivalent series resistance (ESR) values of up to several hundreds milli-ohms rated at > 4 volts. These Double Layer Capacitors (DLCs) are new and different from the standard super-capacitors or electrochemical capacitors, which traditionally have ESR values of up to hundreds of ohms, at either 2.5 volts or at 5 volts, and have been used for back-up applications for more than 30 years. DLC devices are an excellent compromise between batteries, which have to be constantly replaced, and electrostatic /electrolytic capacitors, which do not have enough capacitance in practical sized packages like “button” cells.

In the last 10 years, the newly developed low ESR DLCs can provide several amps at less than 5 V for high pulse power applications. The low ESR DLCs also have low profile, ESR values of 20 to 500mΩ, high capacitance of 6.8mF to 1 F and voltage ratings of 2.5 to 20 volts. These devices are being designed in new applications like micro-power energy harvesting because these components now have a combination of two unique characteristics: low leakage currents and low ESR, not possible even five years ago. They are now preferred over other capacitors or over other small thin batteries that have been tested for these and similar applications. These low ESR, low leakage current, high current pulse devices) products are particularly suited for ambient energy harvesting because of a unique combination of these characteristics wherein these devices offer low ESR values along with low leakage currents of less than a few micro-amps.

Fig. 4 is a diagram showing the cross-section of an AVX BestCap DLC. It shows two active nano-particle carbon layers surrounded by an electrolyte with a “separator” in between. These carbon layers are in contact with current collectors which carry the current to the outside world. The two carbon layers consist of two capacitors in series: hence the name Double Layer Capacitor or DLC, and since the charge carriers within the capacitor are ionic in nature, the term electrochemical DLC (or EDLC) is used. Here the primary concentration of charges is at the current collector-carbon interface. The capacitance (C) is directly proportional to the active area (A) and inversely proportional the separation distance (d) between these charges. The separation between opposing charges for a double layer capacitor is in the nanometer range, and this is why the capacitance in DLCs is so large (because this separation is several orders of magnitude smaller compared to a separation between charges in an electrostatic capacitor).

BestCap devices, based on an aqueous electrolyte, utilize protons, the smallest ionic species, as charge carriers. The result is a significantly lower ESR per unit of active area compared to other DLC technologies where larger ionic species may be used, and this is accompanied with lower leakage current due to its design, and these devices have enhanced reliability. This also offers the potential to build various capacitors within the same package, and the result is the flexibility to have a variety of voltage ratings for capacitors in one package size. No external balancing is required within this one package.

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