Among the many new applications where integrated analog devices are showing up, we find energy harvesting applications in the forefront. For these applications, sensors are generally involved and power supply (or power supply support) circuitry is needed. So combining all onto an IC makes perfect — and probably elegant — sense.
One of the recent papers published as part of the 2013 IEEE International Solid-State Circuits Conference (ISSCC), is “A 1μW-to-1mW Energy-Aware Interface IC for Piezoelectric Harvesting with 40nA Quiescent Current and Zero-Bias Active Rectifiers.” The authors are Chris van Liempd, Stefano Stanzione, Younis Allasasmeh, and Chris van Hoof.
What these folks have suggested is to use an active bridge rectifier made up of small insulated gate N- and P-channel FETs. Since the end usage is for smart sensors, putting the sensor interface circuitry on the same IC as the FETs is the next logical step.
Why do you need active diodes? Because standard PN diodes will have an excessive voltage drop with even low forward current of several hundred millivolts. That ruins whatever possibility you had hoped for regarding high efficiency. Also, in some cases, your voltage source may be on the same order of magnitude as that diode-drop.
You would use the active bridge rectifier with a very low-power AC source such as a piezo-bender used to harvest energy. Here's a simplified model for piezo device and the active bridge power supply:
This and the following images are from the ISSCC paper cited above.
As the title of the ISSCC paper makes clear, this is for extremely low power levels — 1μW to 1mW. This is the order of magnitude for power levels you could expect from impact sensors used in freight shipments or in human-powered devices. In this case, “human-powered” means low levels of movement from your daily life or (for implanted devices) muscle movement. At these power levels, the power supply circuitry must be very efficient. That's why you need diodes with such a low forward voltage drop.
The authors claim voltage efficiency of 94 percent at a power draw of 1μW and 99 percent at 1mW. They further claim a forward voltage drop of around 20mV.
If you fabricate the “diodes” from MOSFETs on an IC, the next thing to put on that IC is the rest of the power supply circuitry, the microcontroller unit (MCU), the radio (e.g., Bluetooth), and any sensor conditioning circuitry for whatever the end product is. That would look something like this, without the sensor circuitry shown:
Sensor circuitry would typically consist of a few op-amps or instrumentation amplifiers connected to the ADC inputs of the MCU. Again, this can be included on the IC — more integrated analog functionality.
This gives you an idea of the chip size in which to squeeze the amount of circuitry shown. Have you worked on any energy harvesting projects? Do you have any suggestions for such projects or products?