Converter and method for extracting maximum power from piezo vibration harvester

A piezo energy harvesting system coupled to a vibration source, comprising:

• (a) a piezo harvester for generating an output voltage and an output current representing energy harvested from the vibration source;
• (b) an active rectifier including first and second switches coupled in series between a harvester output conductor and a first reference voltage, third and fourth switches coupled in series between the harvester output conductor and the first reference voltage, a first comparator for controlling the third and fourth switches, and a second comparator for controlling the first and second switches, a first terminal of the piezo harvester being coupled to a junction between the first and second switches and a first input of the first comparator, and a second terminal of the piezo harvester being coupled to a junction between the third and fourth switches and a first input of the second comparator, wherein the first and second comparators control the rectifying of the harvester output current to charge a capacitance coupled between the harvester output conductor and the first reference voltage, and wherein the first and second comparators also generate output signals indicative of direction reversals of the output current of the piezo harvester;
• (c) a DC-DC converter having a first input coupled to the harvester output conductor, a second input coupled to the first reference voltage, and an output for supplying current to a battery, the DC-DC converter including an inductor coupled to the harvester output conductor, a fifth switch coupled to the inductor, and a rectifying device coupled to the inductor;
• (d) a third comparator for comparing a voltage on the harvester output conductor with a second reference voltage to determine when to stop discharge of the capacitance into the inductor;
• and
• (e) enable circuitry coupled to the outputs of the first and second comparators, respectively, for both starting the discharge of the capacitance into the inductor and causing switching operation of the fifth switch to steer current in the inductor into the battery in response to each direction reversal to substantially eliminate waste of power for recharging the capacitance of the piezo harvester, wherein the enable circuitry operates as a state machine having a first state in which an enable signal generated by the enable circuitry is at a logical “1” level to enable the DC-DC converter and a second state wherein the enable signal is at a logical “0” level to disable the DC-DC converter, wherein the enable logic circuit switches from the first state to the second state in response to an output of the third comparator going from a “1” level to a “0” level, and wherein the enable logic circuit switches from the second state to the first state in response to either the output of the first comparator going from a “1” level to a “0” level or the output of the second comparator going from a “1” level to a “0” level.