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

RF energy harvesting 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 to meet these energy harvesting requirements.

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 V_CAP pin. The value of the DLC determines the amount of energy available from the V_OUT 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 V_CAP 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 (V_MAX=1.25V) V_OUT is switched on to the configured voltage until the lower threshold (V_MIN=1.02V) is reached or a RESET is applied by a microcontroller, at which point V_OUT 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 × V_OUT × I_OUT × t_ON

Where:

V_OUT = User-configured output voltage of the P2110

I_OUT = Average output current from the P2110

t_ON = On-time of the output voltage

The RESET function of the P2110 allows the voltage from V_OUT to be turned off before the storage DLC reaches the lower threshold, V_MIN, thereby saving energy and improving the recharge time back to the activation threshold, V_MAX. 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 V_OUT. 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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