At ISSCC 2013, Holst Centre and imec have presented an innovative method for the power-management of piezoelectric energy harvesters. The proposed interface IC provides energy-aware supply voltage regulation from cold start with a power consumption of less than 140nW. A power efficiency of 90 to 94% is achieved in a wide power range of 1µW to 1mW. Key building block of the IC is a new active diode bridge rectifier with zero bias property.

Holst Centre and imec have developed an integrated piezoelectric energy interface IC with zero bias rectifier circuit and energy-aware supply regulator. The IC is suitable for regulating the voltage in battery -less smart sensors that are typically composed of a piezoelectric energy harvester, sensors, an energy storage system and wireless transceivers. With a power consumption of less than 140nW and a power efficiency as high as 90 to 94% in a wide 1µW-to-1mW power range, the circuit outperforms existing implementations that typically suffer from high energy losses.

The IC regulates the AC power from the piezoelectric energy harvester and supplies an energy-aware DC voltage to the user. The rectified output voltage is connected to a capacitor which accumulates the harvested energy. When this output voltage reaches a target startup voltage, a power-OK signal is set and a timer oscillator with 8- bit counter is enabled. When the output voltage reaches the threshold voltage set by a shunt regulator, a USE signal is generated and the counter is stopped. The USE signal triggers the users to draw current from the capacitor until the USE signal is reset. The harvested power will charge the capacitor again and the cycle is repeated.

The proposed circuit has been fabricated in a commercial TSMC 0.25µm CMOS process with 0.25mm2 active area. The harvested power was generated by an aluminum nitride vacuum packaged MEMS piezoelectric harvester. Tests have demonstrated that the IC itself consumes only 40nA of quiescent current and that this new method of power management delivers optimal power matching for input AC-peak voltages up to 5V.

A comparison with state-of-the-art interfaces shows that this solution consumes less power than previous solutions (only 140nW) while at the same time providing full cold-start, high efficiency AC-DC conversion, output regulation and energy awareness.