Power Management
The core of any EH module is the harvesting circuit, which draws power from the transducer (e.g. solar panels, piezoelectric bimorphs, temperature differential, etc.), and manages energy storage. Figure below shows the simplified diagram of an energy harvesting wireless sensor. TMP’s power management (PM) system (patent pending) maximizes energy efficiency, enhances device reliability, and consequently, lengthens the life of the sensor. The harvested energy can come from different transducers such as solar cells, piezoelectric bimorphs, or temperature gradients. It is important to note that no matter what the source energy is, the PM circuit needs to maximize energy harvested from the source. For example, PZT bimorphs have high output impedance that results in considerable loading and therefore, minimal output power. If this impedance is matched adaptively, there will always be maximum power routed and stored in the energy storage device. Similarly, for solar energy harvesting, system operation must be at the maximum power point. TMP’s PM circuit adaptively calculates the output impedance of the transducer and consequently, adjusts key parameters to maximize charging of the energy storage device. The PM circuits are extremely low power resulting in high efficiency energy harvesting.
For vibrational EH, in a practical environment, vibrations may not be at a fixed frequency, and hence a circuit designed for a fixed frequency (or limited bandwidth) will not harvest the maximum possible energy when the frequency of the vibrating source changes. In order to harvest maximum energy, it is necessary to have an intelligent, variable impedance PM solution. Similarly, different piezoelectric transducers have different equivalent series capacitance resulting in different impedances. TMP’s PM solution uses highly efficient active circuits and a feedforward technique to detect source frequency and/or transducer impedance and accordingly adjust its impedance to facilitate high efficiency energy transfer to the storage device. Preliminary results obtained for EH from vibrations have shown greater than 70% efficiency for low excitation levels of less than 0.3 gpeak using discrete circuits. Higher efficiency is expected when the circuits are implemented in IC form using 0.18 µm process. For solar EH, the PM circuits allow operation close to the MPPT point so that maximum power is transferred to the storage device at all times.
Figure below shows the basic architecture of the intelligent power management solution. The emphasis is on using as minimum power as possible and therefore low power circuit architectures are used. These circuits will be fabricated on an integrated circuit (IC) with extremely low power consumption (of the order of a few µWs).
Texas MicroPower has now pre-released information on their solar, vibration, and combined solar and vibration power management evaluation modules. Please contact us via email info@texasmicropower.com for additional information.