RF ENERGY harvesting techniques for micro-power can be employed for a wide range of radio frequencies, and the license-free industrial, scientific and medical (ISM) radio bands, such as 915MHz and 2.4GHz, can be used to intentionally broadcast RF energy for wireless power systems. To be an effective solution that scales across multiple devices and environments, the RF harvesting component must work over a wide range of operating conditions, including variations of input power and output load resistance. An RF-to-DC power module combines with a Double Layer Capacitor (DLC) to meet these energy harvesting requirements.
The Powercast P2110 Powerharvester™ receiver converts RF to DC with an input sensitivity less than -11 dBm and maintains conversion efficiency over a 100X range of input power. Any standard or custom 50Ω antenna may be used with the P2110 receiver, which is optimized for the 902-928MHz band, but will operate outside this band, including at 868MHz and 950 MHz, but with reduced efficiency.
As shown in Fig. 1, the P2110 stores the harvested energy using a DLC (also sometimes called a super capacitor or an electrochemical double layer capacitor / EDLC) at the VCAP pin. The value of the DLC determines the amount of energy available from the VOUT pin for each cycle of operation. As the received power will be in the milliwatt and microwatt range, the DLC should have a leakage current as small as possible, with a recommended amount of less than 1µA at 1.2V and the ESR should be 200mΩ or less.
OPERATION AND TYPICAL APPLICATION
Fig. 2 shows the timing diagram associated with the P2110. After charging from a starting voltage of 0V, the voltage on the VCAP pin under normal operation will vary between approximately 1.02V and 1.25V. This voltage range is fixed in hardware and enables operation at an increased distance vs. charging a capacitor into the range of 3-5V. If the harvested energy becomes too large, the voltage on the VCAP pin will be internally clamped to protect low voltage DLCs. Clamping will begin at approximately 1.8V and will limit the voltage to less than 2.3V at the maximum rated input power.
A typical application for the P2110 is to provide repeating, intermittent power for low-power, battery-free wireless sensors (Fig. 3). Charge is stored in the external DLC and when the activation threshold is reached (VMAX=1.25V) VOUT is switched on to the configured voltage until the lower threshold (VMIN=1.02V) is reached or a RESET is applied by a microcontroller, at which point VOUT is turned off.
The typical circuit shown was tested with a common microcontroller and a 2.4GHz, 802.15.4 compliant radio module, and powered from a 4W EIRP, 915 MHz transmitter. The circuit included temperature and light level sensors. The microcontroller, when powered from the P2110, would read data from the sensors. This data was then transmitted along with a node ID, transmitter ID, and the RSSI (received signal strength indicator) value back to a computer. The battery-free wireless sensor used approximately 15mA of average current at 3.3V for 10ms. The device operated about every 90 seconds at a distance of 42 feet from the transmitter, using a receiving antenna with a linear gain of 4, or 6 dBi.
SIZING THE CAPACITOR
Since the operation of the system is driven by voltage thresholds, the DLC can be sized for specific applications. Smaller value DLCs will charge more quickly but will result in shorter operation cycles. Larger value DLCs will charge more slowly, but will provide for longer operation cycles. The required DLC value (in farads) can be calculated as follows:
C = 15 × VOUT × IOUT × tON
VOUT = User-configured output voltage of the P2110
IOUT = Average output current from the P2110
tON = On-time of the output voltage
The RESET function of the P2110 allows the voltage from VOUT to be turned off before the storage DLC reaches the lower threshold, VMIN, thereby saving energy and improving the recharge time back to the activation threshold, VMAX. In applications where the energy used is known and consistent the capacitor can be sized appropriately. The RESET function provides the flexibility to create a single module that can be used for a range of applications with varying power requirements, or which modifies its operation based on measured conditions. The RESET function can be triggered by a microcontroller or external timer. When the function of the microcontroller is completed, driving the RESET pin high will disable the voltage from VOUT. For devices that only use 10% of the stored energy, this provides a 10X improvement in the rate of operation by reducing the recharge time to the activation threshold.
The DLC value is less important when using the RESET function. A larger value can be used to facilitate intermittent functions that require more energy including changes in operation based input conditions or received data. The RESET controls the amount of energy removed from the DLC during operation, which will minimize the required recharge time. It should be noted that when RESET is used, a larger DLC will not affect recharge time during continued operation, but it will require more time to initially charge from a completely discharged state.
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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.
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.