Rectifier circuit with AC side short-circuiting function and synchronized switch harvesting on inductor converter

Rectifier circuit including a pair of input terminals, a pair of output terminals, and a first circuit interconnecting the pair of input terminals. The first circuit includes an energy storing element and a rectifier bridge, wherein the rectifier bridge includes at least one controllable switching element per bridge branch. An output of the rectifier bridge supplies the pair of output terminals and wherein the at least one controllable switching element per bridge branch is configured to provide a temporary conducting path via the rectifier bridge which bypasses the pair of output terminals and which short-circuits a series connection of the energy storing element and an energy source connectable to the pair of input terminals. A converter for synchronized switch harvesting on inductor including such a rectifier circuit and a method for rectifying an electrical current generated by an energy source are also described.

Embodiments of the present invention relate to a rectifier circuit. Some embodiments relate to a converter for synchronized switch harvesting on inductor (SSHI). Some embodiment relate to a method for rectifying an electrical current. Some embodiments relate to a modified parallel SSHI converter.

BACKGROUND OF THE INVENTION

Energy harvesting (also known as power harvesting or energy scavenging) is a process by which energy is derived from external sources (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic energy), captured, converted into electrical energy and stored for low-power wireless autonomous devices, like those used in wearable electronics and wireless sensor networks. For example, piezoelectric transducers are employed for harvesting electrical power from vibrations. Different AC-DC converters are described in the literature in order to rectify the AC power and extract the maximum amount of power.

Possible applications of energy harvesters comprising such AC-DC converters for piezoelectric generators are, e.g., in applications like highway bridges (structural health monitoring) or railway trains (tracking and tracing). The frequency range of the vibrations associated to these applications is between 2 and 50 Hz, whereas mean accelerations are around 0.1 g.

The admittance locus of a piezoelectric transducer has intrinsic information about for which piezoelectric transducers the employment of a SSHI converter instead of a diode bridge will provide a significant increase in the harvested power. The internal impedance of a piezoelectric element is complex, as described by J. Brufau-Penella and M. Puig-Vidal in “Piezoelectric energy harvesting improvement with complex conjugate impedance matching,” Journal of Intelligent Material Systems and Structures, vol. 00-2008, 2008. Therefore, the maximum output power of the piezoelectric. transducer is obtained when the complex conjugate of the internal impedance is connected as output load. However, this solution is unrealistic since the inductance needed as complex conjugate load would be too large due to the dominant capacitive characteristic of piezoelectric elements. If a resistor is connected as a load to the piezoelectric element, the output power obtained depends on the mechanical frequency that excites the piezoelectric element and the resistance. The maximum output power in this case is typically obtained with a resistance that is equal to the modulus of the equivalent Thevenin impedance of the piezoelectric element at the frequency where the ratio of the real and imaginary parts of the admittance of the piezoelectric element is maximized. Thus, the maximum output power is obtained at the frequency where the admittance of the piezoelectric element has its most resistive behavior.

The peak value of the ratio between the output power when a resistor is the load and the maximum power, depends on the impedance circle of the piezoelectric element. There are piezoelectric elements were the maximum of this ratio is close to one and others were this ratio is much lower. For the piezoelectric elements with a ratio close to 1, the employment of non-linear techniques is not going to provide an improvement over the rectifier bridge. However, for piezoelectric elements where this ratio is far away from 1, the non-linear converters are a better solution than the rectifier bridge.

The Synchronized Switch Harvesting on Inductor (SSHI) is a non-linear switching technique that provides DC (direct current) power from an energy source, such as a mechanically excited piezoelectric element. A SSHI converter typically consists of a switch and an inductor plus a diode bridge. The mechanically excited piezoelectric element typically produces an alternating electric voltage and an alternating current (AC).

An efficient AC-DC converter for piezoelectric elements during vibration is of special importance for maximizing the harvested power.

The AC power delivered by, for example, piezoelectric transducers can be rectified with a diode bridge and a filtering capacitor (linear technique).

A more recent AC-DC converter which employs an inductor connected through a switch to the piezoelectric element (called SSHI) is also available. The switch is closed when the piezoelectric peak displacement is reached. The connection of the piezoelectric element with the inductor causes a resonant effect and a fast inversion of the piezoelectric voltage. After the piezoelectric voltage inversion, the switch is opened until a new peak is detected. However, the diodes of the diode bridge still induce voltage gaps which causes losses in the harvesting circuit and limits the harvested power.

In a further development of AC-DC converters for SSHI converters, the switch or switches for the inversion of the piezoelectric voltage are separated and two diodes of the diode bridge are replaced by these switches. Hence, the circuit includes less components, thus reducing the cost and the dimensions of the circuit. Another benefit is the removal of the diodes that induce voltage gaps. Therefore, losses introduced by such voltage gaps in the harvesting circuit are limited and the harvested power is thus greater. However, the amount of energy extracted from the piezoelectric element per cycle may still be comparatively low and only a fraction of theoretically extractable energy amount.

SUMMARY

According to an embodiment, a rectifier circuit may have: a pair of input terminals; a pair of output terminals; a first circuit interconnecting the pair of input terminals, the first circuit including an energy storing element and a rectifier bridge, wherein the rectifier bridge includes at least one controllable switching element per bridge branch, wherein an output of the rectifier bridge supplies the pair of output terminals and wherein the at least one controllable switching element per bridge branch is configured to provide a temporary conducting path via the rectifier bridge which bypasses the pair of output terminals and which short-circuits a second circuit including the energy storing element, the pair of input terminals, and an energy source connectable to the pair of input terminals.

According to another embodiment, a converter for synchronized switch harvesting on inductor may have a rectifier circuit according to claim1.

According to another embodiment, a method for rectifying an electrical current generated by an energy source may have the steps of: applying the electrical current to a first circuit including a rectifier bridge and an energy storing element so that the rectifier bridge conducts the electrical current to an output of the rectifier bridge along a first rectification path associated with a first current flow direction of the electrical current; detecting a first specific pattern in a sense signal associated with the electrical current; reconfiguring the rectifier bridge, in response to detecting the first specific pattern, by switching a controllable switching element of the rectifier bridge from a non-conducting state to a conducting state so that the electrical current is conducted within the rectifier bridge along a first inversion path that short-circuits a second circuit including the energy storing element and the energy source and bypasses an output of the rectifier bridge; and reconfiguring the rectifier bridge or allowing the rectifier bridge to perform a self-reconfiguration in response to a change of the current flow direction from the first current flow direction to a second current flow direction so that the rectifier bridge conducts the electrical current to the output of the rectifier bridge along a second rectification path associated with the second current flow direction.

According to another embodiment, a rectifier circuit may have: a pair of input terminals; a pair of output terminals; a first circuit interconnecting the pair of input terminals, the first circuit including an energy storing element and a rectifier bridge, wherein the rectifier bridge includes at least one controllable switching element per bridge branch, wherein an output of the rectifier bridge supplies the pair of output terminals and wherein the at least one controllable switching element per bridge branch is configured to provide a temporary conducting path via the rectifier bridge which bypasses the pair of output terminals and which short-circuits a series connection of the energy storing element and an energy source connectable to the pair of input terminals.

According to another embodiment, a method for rectifying an electrical current generated by an energy source may have the steps of: applying the electrical current to a first circuit including a rectifier bridge and an energy storing element so that the rectifier bridge conducts the electrical current to an output of the rectifier bridge along a first rectification path associated with a first current flow direction of the electrical current; detecting a first specific pattern in a sense signal determined on the basis of the electrical current; reconfiguring the rectifier bridge, in response to detecting the first specific pattern, by switching a controllable switching element of the rectifier bridge from a non-conducting state to a conducting state so that the electrical current is conducted within the rectifier bridge along a first inversion path that short-circuits a series connection of the energy storing element and the energy source, wherein the first inversion path bypasses an output of the rectifier bridge; and reconfiguring the rectifier bridge or allowing the rectifier bridge to perform a self-reconfiguration in response to a change of the current flow direction from the first current flow direction to a second current flow direction so that the rectifier bridge conducts the electrical current to the output of the rectifier bridge along a second rectification path associated with the second current flow direction.

According to another embodiment, a rectifier circuit may have: a pair of input terminals; a pair of output terminals; a first circuit interconnecting the pair of input terminals, wherein the first circuit includes an energy storing element and a rectifier bridge, wherein the rectifier bridge includes parallel bridge branches between a first AC side terminal and a second AC side terminal of the rectifier bridge and at least one controllable switching element per bridge branch, wherein an output of the rectifier bridge supplies the pair of output terminals; wherein the at least one controllable switching element per bridge branch is configured to receive a control signal and to assume, in response to a specific state of the control signal, a conducting state for providing a temporary conducting path via one of the parallel bridge branches of the rectifier bridge including said controllable switching element, the temporary conducting path bypassing the pair of output terminals and short-circuiting a series connection of the energy storing element and an energy source connectable to the pair of input terminals.

The switching elements connect the inductor in parallel to the energy source (e.g., piezoelectric element) during the time that the piezoelectric voltage may be inverted, thereby performing an inversion of the energy source voltage (piezoelectric voltage), i.e. the voltage generated by the energy source. Especially when the inversion of the energy source voltage is performed with a specific timing, for example corresponding to a resonant mode of a resonator formed by the energy source and the (inductive) energy storing element, the power transferred out of the energy source may possibly be significantly increased compared to other AC-DC conversion methods. The load current typically also flows through the inductor. While this fact may add power losses in the inductor it allows the operation of the modified parallel SSHI topology with just two diodes and two switching elements, e.g. MOSFETs (Metal Oxide Semiconductor Field Effect Transistors).

According to further embodiments of the present invention, the energy storing element (or energy storage element) and the rectifier bridge may be connected in series.

The rectifier bridge may be configured to function both as an energy transferring component from the pair of input terminals to the pair of output terminals and as a voltage inverter for the pair of input terminals.

Each bridge branch may comprise a diode component and the at least one controllable switching element.

According to at least some further embodiment of the teachings disclosed herein, the rectifier circuit may further comprise a controller configured to generate at least one control signal for the at least one controllable switching element per bridge branch on the basis of a sense signal indicating a state of the energy source.

The controller may be configured to cause a switching of the at least one controllable switching element from a conducting state to a non-conducting state, or vice versa, upon a detection of a peak in the sense signal.

The energy storing element may form a resonant circuit with the energy source while the second circuit is short-circuited via the rectifier bridge.

According to at least some further embodiments of the teachings disclosed herein the energy storing element is connectable in parallel to the energy source while the second circuit is short-circuited via the rectifier bridge.

According to some further embodiments, the energy source may be a piezoelectric element.

Further embodiments of the present invention provide a converter for synchronized switch harvesting on inductor. The converter comprises a rectifier circuit as described above.

Further embodiments of the present invention provide a method for rectifying an electrical current generated by an energy source. The method comprises applying the electrical current to a first circuit comprising a rectifier bridge and an energy storing element so that the rectifier bridge conducts the electrical current to an output of the rectifier bridge along a first rectification path associated with a first current flow direction of the electrical current. The method further comprises detecting a first specific pattern in a sense signal associated with the electrical current and reconfiguring the rectifier bridge, in response to detecting the first specific pattern, by switching a controllable switching element of the rectifier bridge from a non-conducting state to a conducting state. In this manner, the electrical current is conducted within the rectifier bridge along a first inversion path that substantially short-circuits a second circuit comprising the energy storing element and the energy source and bypasses an output of the rectifier bridge. The rectifier bridge is then again reconfigured or allowed to perform a self-reconfiguration in response to a change of the current flow direction from the first current flow direction to a second current flow direction so that the rectifier bridge conducts the electrical current to the output of the rectifier bridge along a second rectification path associated with the second current flow direction.

With the SSHI converter topology according to the teachings disclosed herein, the number of switching elements such as MOSFETs and diodes employed is reduced in relation to previous SSHI converters and the energy storing element (e.g., an inductor) is at a new location compared to the previous SSHI converters.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, a plurality of details are set forth to provide a more thorough explanation of embodiments of the present invention. However, it will be apparent to one skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form rather than in detail in order to avoid obscuring embodiments of the present invention. In addition, features of the different embodiments described hereinafter may be combined with each other, unless specifically noted otherwise.

A rectifier typically may be used when piezoelectric transducers are employed in an energy harvesting power supply for rectifying the AC power. One of the simplest rectifiers is a diode bridge with a filter capacitor. When the absolute value of the piezoelectric voltage is lower than the rectified voltage, no current flows from the piezoelectric element to the load. When the absolute value of the piezoelectric voltage is higher than the rectified voltage, the diodes conduct and the current flows from the piezoelectric element to the output load.

Non-linear converters may also be employed with piezoelectric transducers to increase the harvested energy by active discharge. In comparison, in linear converters, an important part of the charge generated by the piezoelectric element is lost charging and discharging the internal piezoelectric capacitor. In non-linear techniques, when the maximum voltage on the piezoelectric element is achieved, the connection of an inductor to the piezoelectric element causes a resonance, due to the internal capacitor of the piezoelectric element, which results in the inversion of the piezoelectric voltage polarity in a very short time (t=π√(LC)) compared to the mechanical excitation period. Thus, the inversion of the piezoelectric voltage pursues that piezoelectric current and voltage are in phase, and therefore the operation of the non-linear converter simulates the complex conjugate of the piezoelectric impedance.

There are two SSHI techniques, namely the parallel SSHI technique and the series SSHI technique. In the case of the parallel SSHI technique, shown inFIG. 1A, a switch32and an inductor34are connected in parallel to a piezoelectric element12. A diode bridge42, a filter capacitor52, and the output load62are connected afterwards. The parallel SSHI technique is, for example, described in an article by S. Ben-Yaakov and N. Krihely, “Resonant rectifier for piezoelectric sources”, Applied Power Electronics Conference and Exposition, 2005. APEC 2005, pp. 249-253 Vol. 1, 6-10 Mar. 2005.

The working principle of the parallel SSHI circuit can be explained usingFIG. 1Bwhich shows waveforms at steady state associated to the circuit shown inFIG. 1A. The upper diagram inFIG. 1Bshows a piezoelectric displacement u, a piezoelectric voltage v1, and a load voltage VL. When a piezoelectric material is mechanically excited, it transforms the mechanical energy into electrical energy. The AC power available between its electrical terminals has to be rectified to obtain DC power. When a piezoelectric element is in open circuit, the derivative of its voltage dv1/dt and the derivative of its displacement du/dt are proportional since no electrical current is flowing out of the piezoelectric element, i.e. I=0.

The piezoelectric voltage, the piezoelectric displacement and the electrical current at the piezoelectric element12are related as
I=αdu/dt−C0dv1/dt,
where I is the electrical current flowing out of the piezoelectric element, u is the piezoelectric displacement, v1is the piezoelectric voltage, α is the force factor, and C0is the capacitance of the piezoelectric element.

While the piezoelectric voltage v1is lower than the rectified voltage, there is no current iPflowing from the piezoelectric element12to the diode bridge42. Therefore, the derivative of the piezoelectric voltage dv1/dt and the derivative of the displacement du/dt are proportional. Once the piezoelectric voltage v1is higher than the rectified voltage plus two times the voltage drop of the diodes, there is current iPflowing from the piezoelectric element12to the diode bridge42. When the piezoelectric voltage v1reaches a peak (positive or negative), the switch32is closed and current iSflows from the internal piezoelectric capacitor to the inductor34. At this moment, a resonant LC circuit is created with the piezoelectric internal capacitor and the voltage on the piezoelectric element changes its polarity in a time π√(LC). The current generated by the piezoelectric element flows through the rectifier capacitor52and the load62when the switch32is open and the absolute value of the piezoelectric voltage is higher than the rectified voltage on the load. When the piezoelectric voltage reaches a maximum, the switch32is closed. The switch that connects the inductor in parallel with the piezoelectric element is composed by two diodes D5 and D6 and two MOSFETs T1 (NMOS) and T2 (PMOS) as shown inFIG. 3. The current flows through inductor L, D5 and T1 when the piezoelectric element reaches a positive peak voltage whereas the current flows through inductor L, D6 and T2 when the piezoelectric element reaches a negative peak voltage. Once the piezoelectric voltage is inverted, the switch32ofFIG. 1Ais opened again. Since the piezoelectric element12is connected most of the time by the diode bridge42to the output load62, it cannot be assumed that when the piezoelectric voltage v1reaches a peak, the displacement u of the piezoelectric element12is maximal. A corresponding parallel SSHI circuit is described by S. Priya and D. J. Inman in “Energy Harvesting Technologies”, Springer, 2009, pp. 211-213.

If the piezoelectric voltage and current have the same polarity, the power harvested by the piezoelectric element12, and therefore the power delivered to the output load62, is higher than if piezoelectric voltage and current have a phase shift. The fact that piezoelectric voltage and current are in-phase makes possible to emulate the complex conjugate of the piezoelectric impedance as output load.

The piezoelectric equivalent circuit at a given resonant frequency can be represented by an internal sinusoidal current source and a capacitor in parallel (in S. Ben-Yaakov and N. Krihely, “Resonant rectifier for piezoelectric sources”, Applied Power Electronics Conference and Exposition, 2005, APEC 2005, Twentieth Annual IEEE, vol. 1, pp. 249-253, 6-10 Mar. 2005). The current source of the model is proportional to the velocity of the piezoelectric element, and therefore proportional to the derivative of the piezoelectric displacement. Hence, when there is a zero crossing of the sinusoidal current source, the optimum point for maximizing the harvested piezoelectric power takes place. Moreover, when the piezoelectric element12is in open circuit, the piezoelectric current source is proportional to the derivative of the piezoelectric voltage. In other words, the multiplication of piezoelectric current and voltage is positive since the SSHI converters invert the piezoelectric voltage through inductor L and this maximizes the output power. Therefore, the piezoelectric voltage and displacement derivatives are proportional (the slopes of both curves have the same sign). However when the piezoelectric power is rectified employing the parallel SSHI circuit, most of the time the piezoelectric element is connected to the output load, and therefore the derivative of the piezoelectric voltage and the derivative of the displacement are not anymore proportional. Thus, it cannot be assured that the zero-crossing of the piezoelectric current source occurs at the piezoelectric voltage peak which maximizes the harvested power. Nevertheless, for the case of the series SSHI, the piezoelectric element is usually in open circuit and this fact causes the zero-crossing of the internal piezoelectric source to occur during the piezoelectric voltage inversion and the multiplication of piezoelectric voltage and current to be positive, increasing the harvested power.

In the series SSHI circuit, which is illustrated inFIG. 2A, the inductor234and the switch232are connected in series to the piezoelectric element12and the diode bridge42and filter capacitor52. As can be seen inFIG. 2B, in the case of the series SSHI circuit there is no current i flowing from the piezoelectric element12to the load62except when the switch232is closed, as stated in “Energy Harvesting from Ambient Vibrations and Heat”, by D. Guyomar, G. Sebald, S. Pruvost, M. Lallart, A. Khodayari and C. Richard, Journal of Intelligent Material Systems and Structures, vol. 20, no. 5, pp. 609-624, March 2009. While the switch232is open, the derivative of the piezoelectric voltage and displacement are proportional and the piezoelectric internal capacitor is charged. Once the piezoelectric displacement u reaches an extreme, the switch232is closed and the current i flows from the piezoelectric element12to the output load62through the inductor234and the diode bridge42. Since the series connection of the inductor with the piezoelectric element creates a resonating circuit, the piezoelectric voltage is inverted. Afterwards, the switch is opened again. Prior to opening the switch232the piezoelectric voltage v1has a magnitude of Vbeforeand subsequent to closing the switch232(and accordingly subsequent to the inversion of the piezoelectric voltage v1and a phase during which the current i flows) the piezoelectric voltage has a magnitude of Vafter, with |Vbefore|>|Vafter|. The output voltage VDCat the filter capacitor is also illustrated inFIG. 2B.

Both the parallel and series SSHI circuits need a diode bridge for rectifying the AC power harvested by the piezoelectric elements in addition to a switching circuit for the connection of the inductor to the piezoelectric element.FIGS. 3 and 4show the detailed circuit of the parallel and series SSHI converters, respectively.

InFIG. 3, the switching circuit332comprises two transistors335,337and two diodes336,338in two parallel branches, each branch comprising a series connection of a transistor and a diode. The transistor335(T1) is an N-channel metal-oxide-semiconductor (NMOS) field effect transistor (enhancement type) and the transistor337(T2) is a P-channel metal-oxide-semiconductor (PMOS) field effect transistor (enhancement type). The diode336(D5) is connected in series to the NMOS field effect transistor335and allows a current flow in a first direction, only. The diode338(D6) is connected in series to the PMOS field effect transistor337and allows a current flow in a second direction, only. The NMOS field effect transistor335comprises a gate and also the PMOS field effect transistor comprises a gate. Both gates of the transistors T1 and T2 receive the same control signal vc. In dependence on the control signal vceither one of the two transistors T1, T2 is in a conducting state while the other transistor is in the blocking state.

InFIG. 4which shows a detailed schematic circuit diagram of the series SSHI converter, the switching circuit432comprises two transistors435,437and two diodes436,438. As with the switching circuit332for the parallel SSHI converter the switching circuit432for the series SSHI converter comprises two parallel branches, each branch comprising a series connection of a transistor and a diode. The transistor435(T1) and the diode436(D5) are connected in series in a first branch. The transistor437and the diode438are connected in series in a second branch. The transistors435,437are controlled by a control signal provided by a control circuit431. The control circuit431causes one transistor to be in a conducting state while the other transistor is in a blocking state. The diodes436and438prevent a current flow in an undesired direction after the piezoelectric inversion has taken place while the corresponding transistor is in the conducting state.

In the configurations shown inFIGS. 1A, 2A, 3, and 4, the diode bridge42comprises a first bridge branch and a second bridge branch. Furthermore, each bridge branch comprises a first section and a second section with an output of the bridge being tapped between the first and second sections of the first bridge branch and the second bridge branch, respectively. With the configurations shown inFIGS. 1A, 2A, 3, and 4(and with commonly used diode bridge configurations in general), the first section of a particular bridge branch is typically conductive while the second section of the same bridge branch is typically non-conductive, or vice versa, so an electrical current flowing through a first section is conducted through the load connected to the output of the rectifier circuit and continues along the second section of the second bridge branch. In other words, with these configurations it is avoided that the first section and the second section of the same bridge branch are concurrently conductive as this would short-circuit the AC side of the bridge. With a diode bridge this condition is fulfilled substantially automatically because with a given input voltage only one of the diodes within the first and second sections of a particular branch may become conductive.

FIG. 5shows a new version of the series SSHI converter which is illustrated and described in M. Lallart and D. Guyomar, “An optimized self-powered switching circuit for non-linear energy harvesting with low voltage output”, Smart Materials and Structures, val. 17, pp. 035030, 2008. In comparison to the series SSHI converter shown inFIG. 4, the functionality of the switching circuit432inFIG. 4is provided by the bridge542in the configuration shown inFIG. 5. The bridge542comprises two switching elements545(S1) and547(S2). Furthermore, the bridge comprises two diodes546(D1) and548(D2). Compared to the series SSHI converter shown inFIG. 2, the circuit inFIG. 5consists in separating the maximum and minimum detection switches and replacing the diodes D3and D4by these switches. Thus, the circuit shown inFIG. 5includes less components, thus reducing the cost and the dimensions of the circuit. Another benefit is the removal of the diodes that induce voltage gaps. Therefore, losses introduced by such voltage gaps in the harvesting circuit are limited and the harvested power is greater. The energy transfer from the energy source to the rectifier output is done in a (very) short time at the positive peak or at the negative peak of the input voltage. The inversion of the voltage occurs concurrently during this short time. The piezoelectric element is left in open circuit (S1 and S2 open) for the rest of the time.

FIG. 6shows a schematic circuit diagram of a parallel SSHI converter or rectifier circuit600according to at least one embodiment of the teachings disclosed herein. According to the teachings disclosed herein a novel SSHI topology is presented where the number of MOSFETs (or switching elements in general) and/or diodes employed is reduced in relation to the previous SSHI converters and the inductor is at a new location.

The parallel SSHI converter600shown inFIG. 6comprises a pair of input terminals614,616, an energy storing element34, a rectifier bridge642, the filter capacitor52, and a pair of output terminals654,656. However, the filter capacitor52is not necessarily part of the parallel SSHI converter600but could be an additional component or integrated with the load62. The bridge642comprises two parallel bridge branches between a first AC side terminal and a second AC side terminal of the bridge. Each of the two bridge branches comprises a controllable switching element. Accordingly a first bridge branch comprises an NMOS transistor645(T1) as the controllable element. A second bridge branch comprises a PMOS transistor647(T2) as the controllable switching element. Both controllable switching elements645,647receive a control signal vcfrom a control circuit631. As the first controllable switching element645is, in the depicted embodiment, an NMOS transistor and the second controllable switching element647is a PMOS transistor, the two controllable switching elements react differently to the control signal vc. The first bridge branch further comprises a diode646which is connected in series to the first controllable switching element645. The second bridge branch further comprise a diode648which is connected in series to the second controllable switching element647. In alternative embodiments, the diodes646and648could be replaced with further controllable switching elements.

The energy storing element may be an inductor34and is connected to a first input terminal614of the pair of input terminals614,616and to the first AC side terminal of the bridge. The second AC side terminal of the bridge is connected to the second input terminal616of the pair of input terminals. Thus, a first circuit601interconnecting the pair of input terminals is formed.

Both bridge branches comprise output nodes that are connected to one of the pairs of output terminals654,656. The output nodes are between the controllable switching elements and the other switching elements (i.e., the diodes) of the bridge branches. Accordingly, the output node of the first bridge branch is connected to the output terminal654and the output node of the second bridge terminal is connected to the output terminal656of the parallel SSHI converter600according to the embodiment ofFIG. 6of the teachings disclosed herein. A parallel circuit comprising the filter capacitor52and the output load62extends between the first output terminal654and the second output terminal656.

The two controllable switching elements645,647of the rectifier bridge enable a low-ohmic connection (substantially a short-circuit) between the pair of input terminals614,616to be formed via the first bridge branch or the second bridge branch. In this manner, a second circuit602comprising the energy storing element34, the pair of input terminals614,616, and the energy source12(e.g., the piezoelectric element) may be substantially short-circuited and the energy storing element34is connected in parallel to the energy source12. Accordingly, the rectifier circuit600may be regarded as a parallel SSHI converter. Note that the energy storing element34is considered to be a part of the first circuit601and also of the second circuit602. The grouping of several elements within the first circuit601and the second circuit602is done for explanatory purposes so that is becomes more readily clear, which components are affected by, for example, a substantial short-circuit caused by the rectifier bridge.

The modified parallel SSHI topology according to the embodiment shown inFIG. 6is just composed by two diodes and two MOSFETs which reduce the voltage drops of the diodes and MOSFETs with respect to the Parallel and Series SSHI circuits depicted inFIGS. 1 to 4. Since two diodes of the rectifier bridge have been substituted by two active MOSFETs, the power harvested by the modified parallel SSHI has increased in comparison with the other SSHI topologies. Moreover, no further diode or transistors are required which increases the efficiency as well. The use of the modified parallel SSHI topology not only harvests more power from the piezoelectric element but also broadens the frequency bandwidth of the converter compared with the other SSHI converter circuits.

The topologies of the modified series SSHI converter shown inFIG. 5and of the rectifier circuit600ofFIG. 6are somewhat similar, but they are controlled in different manners and therefore their operations are different. In particular, the SSHI converter shown inFIG. 5is a series converter which basically still has the same shortcomings as the series SSHI converters shown inFIGS. 2A and 4. In contrast, the rectifier circuit600shown inFIG. 6acts as a parallel SSHI converter.

In the modified parallel SSHI converter600, the load current is also flowing through the inductor34. This fact adds power losses in the inductor34but allows the operation of the modified parallel SSHI topology with just two diodes and two MOSFETs.

A converter for synchronized switch harvesting on inductor (SSHI converter) may comprise a rectifier circuit600according to the above description. An energy harvester (power harvester or energy scavenger) converter may comprise the rectification circuit and an energy source such as a piezoelectric element.

In the rectifier circuit600, the inversion of the source voltage12is done in a short time. The energy transfer from the energy source12to the output of the rectifier bridge642is done in between these short inversion times. Accordingly, the rectifier circuit600is configured to perform the inversion of the source voltage at a different time (during a different phase of a rectification cycle) than the rectification of the source voltage. In this manner, the energy source12is usually connected in a closed circuit: be it in a rectification circuit comprising the load62, the filter capacitor52, and two diagonal elements of the rectification bridge642, or be it in an inversion circuit that bypasses the filter capacitor52and the load62. In other words, the energy source12is substantially never in open circuit during operation.

FIGS. 7A to 7Dillustrate the rectifier circuit600in four different phases of operation.FIGS. 7A and 7Cshow rectifying or rectification phases of the operation of the rectifier circuit andFIGS. 7B and 7Dshow inversion phases of the operation of the rectifier circuit.

InFIG. 7A, the piezoelectric element12currently generates a piezoelectric voltage v1that is positive and greater (regarding the magnitude) than a rectified voltage at the filter capacitor52plus the voltage gap of one diode and a possible voltage drop over a controllable switching element. The controllable switching element647(p-channel MOSFET) is controlled to be in a conducting state by the control circuit631and the control signal vcwhich has a negative polarity during this phase. This fact and the fact that the piezoelectric voltage v1is greater than the rectified voltage causes the diode646(D1) to become conductive so that a current path is formed, the current path starting from a first terminal of the piezoelectric element12via the first input terminal614, the energy storing element or inductor34, the diode646(D1), the filter capacitor52and the load62, the controllable switching element647, and the second input terminal616back to a second terminal of the piezoelectric element12. The electrical current thus charges the filter capacitor52.

When the piezoelectric voltage v1reaches its positive peak, diode646(D1) conducts and the control signal vcchanges its polarity from negative to positive. Thus, the p-channel MOSFET647stops conducting (i.e., turns off) and the n-channel MOSFET645starts conducting, thereby connecting the inductor34in parallel to the piezoelectric element12, as can be seen inFIG. 7B.FIG. 7Billustrates a second phase of the operation of the rectifier circuit600during which an inversion of the piezoelectric voltage v1is performed. The inversion is made possible by the formation of a resonant circuit due to the parallel connection of the internal capacitance of the piezoelectric element12and the inductor34. In other words, during the second phase of the operation of the rectifier circuit600a temporary conducting path via the rectifier bridge is formed along a first bridge branch comprising the diode646(D1) and the n-channel MOSFET645(T1). In particular, the temporary conducting path is formed along the first bridge branch comprising the n-channel MOSFET645and the diode646. The temporary conducting path substantially bypasses the pair of output terminals654,656and substantially short-circuits the second circuit602which comprises the energy storing element34(i.e., the inductor), the pair of input terminals614,616, and the energy source12(i.e., the piezoelectric element) that is connectable to the pair of input terminals614,616.

Once the inversion is completed and the piezoelectric voltage v1reaches a negative peak value, diode646(D1) does not conduct anymore and the diode648(D2) starts conducting (FIG. 7C). During the rectification phase of the negative semi-cycle, D2and T1conduct.

FIG. 7Dillustrates a further phase of the operation of the rectifying circuit, namely an inversion phase of the negative semi-cycle. When the piezoelectric voltage v1reaches its negative peak, the control circuit631causes the control signal vcto change its polarity from positive to negative. Thus, the n-channel MOSFET645stops conducting and the p-channel MOSFET647starts conducting again connecting the inductor34in parallel to the piezoelectric element12. During the inversion phase of the negative semi-cycle the second bridge branch comprising the p-channel MOSFET647(T2) and the diode648(D2) form a temporary conducting path via the rectifier bridge642which substantially bypasses the output terminals654,656and which substantially short-circuits the second circuit602(seeFIG. 6). Once the inversion is completed and the piezoelectric voltage v1reaches a positive value, diode646(D1) starts conducting and the cycle starts over again with the rectification phase of the positive semi-cycle as illustrated inFIG. 7A. During the rectification phase of the positive semi-cycle, the diode646(D1) and the p-channel MOSFET647(T2) conduct.

The control circuit is configured to sense the piezoelectric voltage v1and to generate the control signal vcon the basis of the sensed piezoelectric voltage v1. For example, the control circuit may be configured to detect local maxima and local minima of the piezoelectric voltage v1and the control signal vcmay be switched from a positive level to a negative level upon detection of a local maximum or minimum, or vice versa.

During the rectification of the AC power, the modified parallel SSHI converter600acts as a half-synchronous rectifier since the circuit has two diodes646,648and two MOSFETs645,647. Thus, the efficiency during this phase (these phases) is increased with respect to the other existing SSHI topologies. A synchronous rectification of the diodes is also possible employing the control circuit for synchronous rectification that appears in the European patent application 10192761.4 entitled “Diodenersatzschaltung, Aufwartswändlerschaltung, Abwärtswandlerschaltung and Brückengleichrichterschaltung” of the inventor M. Pollack. Moreover, if the two diodes646(D1),648(D2) are substituted by two active MOSFETs, the efficiency may increase, in particular if the control circuit631consumes relatively little power. A synchronous rectifier may also be selected for the design of an application specific integrated circuit (ASIC) since diodes designed with, for example, some CMOS technologies may have a voltage drop of 1.8V for a continuous reverse voltage of 20 V. Discrete MOSFETs that may offer better efficiency results in SSHI converters are designed for high frequency applications although the vibration frequencies employed for exciting the piezoelectric elements are relatively low, e.g., below 110 Hz. The cause is that when the piezoelectric element is connected in parallel with the inductor through the MOSFET and the diode, a resonant LC circuit is established with a resonant frequency of 2π√(LC) and the low capacitances associated to high frequency transistors provide better results.

FIG. 8illustrates three recorded signals of an implemented modified parallel SSHI circuit according toFIG. 6. The piezoelectric voltage v1is drawn in a dashed line, the control signal vcfor switching MOSFETs T1 and T2 ofFIG. 6is drawn in a dotted line and the rectified voltage VDCat the output of the rectifier circuit600is drawn in full stroke. On the voltage axis one division corresponds to 5V and on the time axis one division corresponds to 25 ms. The behavior of the piezoelectric voltage v1is the same that can be observed by the parallel SSHI converter as illustrated for example inFIGS. 2A and 2B.

When the piezoelectric voltage v1reaches its positive peak value, diode646(D1) conducts and the control signal vcchanges its value from negative to positive. Thus, p-channel MOSFET647(T2) does not conduct anymore and n-channel MOSFET645(T1) starts conducting. Since D1 and T1 are turned-on, the piezoelectric element12is connected in parallel to inductor34(L1). Once the inversion is completed and the piezoelectric voltage v1reaches a negative value, diode D1 does not conduct anymore and diode D2 starts conducting. In this rectification phase of the negative semi-cycle, D2 and T1 conduct. When the piezoelectric voltage v1reaches its negative peak value, diode D2 conducts and the control signal changes its value from positive to negative. Thus, n-channel MOSFET T1 does not conduct anymore and p-channel MOSFET T2 starts conducting. Since D2 and T2 are turned-on, the piezoelectric element12is connected in parallel to inductor L. Once the inversion is completed and the piezoelectric voltage v1reaches a positive value, diode D2 does not conduct anymore and D1 starts conducting. In this rectification phase of the positive semi-cycle, D1 and T2 conduct.

With respect to alternative embodiments, substantially the same explanation is valid if diodes D1 and D2 are substituted by MOSFETs and controlled as in a synchronous rectifier.

The modified parallel SSHI converter according to the teachings disclosed herein does not need a separate circuit for the connection of the inductor34to the piezoelectric element12besides the rectifier circuit642for the connection of the piezoelectric element12to the load62. Instead just one circuit is employed and performs both functionalities. The modified parallel SSHI converter may utilize just two diodes and two MOSFETs instead of the six diodes and two MOSFETs that may be used by the parallel and series SSHI topologies shown inFIGS. 1A, 2A, 3, and4. Moreover, the location of the two MOSFETs in the rectifier bridge allows a half synchronous rectification which increases the efficiency of the AC-DC converter. Moreover, the bandwidth of this rectifier is also broader than the bandwidth obtained with the other AC-DC rectifiers.

FIGS. 9 and 10display the harvested power versus output load for the piezoelectric elements DuraAct P-876.A12 and Midé QP20W, respectively. In particular,FIGS. 9 and 10show different measurements done while a piezoelectric element was excited with sinusoidal vibrations generated by an electromagnetic shaker in order to compare the harvested power of different AC-DC converters (full-wave rectifier, the parallel SSHI and the modified parallel SSHI) as a function of the resistive load connected to the piezoelectric element. The piezoelectric element DuraAct P-876.A12 was excited at 0.1 g while the piezoelectric element Midé QP20W was excited at 1 g. Each graph has four curves. The piezo-electric element12is connected directly to a resistive load62in the AC curve (black crosses), to the resistive load62through a diode bridge in the case of the standard curve (black squares with white crosses), to the resistive load62through the parallel SSHI circuit for the case of the parallel SSHI curve (triangles) and through the modified parallel SSHI circuit for the case of the modified parallel SSHI curve (diamonds). The direct connection of the resistive load62to the piezoelectric element12is provided as a reference because this direct connection does not generate diode-related or switching-element-related losses. Accordingly, a relatively high output power can be obtained using the direct connection, which is, however, an AC power. InFIG. 9the standard curve (white crosses) and the curve belonging to the conventional-technology parallel SSHI circuit (triangles) show an approximately identical behavior with the standard curve even slightly outperforming the conventional-technology parallel SSHI circuit, in particular for high loads. InFIG. 10the conventional-technology parallel SSHI circuit (triangles) performs better than the standard circuit (white crosses), at least for small load resistances. In bothFIGS. 9 and 10, i.e. for the piezoelectric elements DuraAct P-876.A 2 as well as Midé QP20W, the modified parallel SSHI converter according to the teachings disclosed herein shows superior performance compared to the standard curve and the conventional-technology parallel SSHI converter at least for relatively small load resistances. InFIG. 9, a maximum of the harvested power can be observed for an output load of approximately 120 kOhm for the modified parallel SSHI converter according to the teachings disclosed herein. InFIG. 10, the maximum of the harvested power can be observed for an output load of approximately 10 kOhm.

From the measurements, the results of which are shown inFIGS. 9 and 10, it is deduced that the modified parallel SSHI converter offers better results in terms of efficiency than the parallel SSHI converter employing the same diodes, switching elements and peak detector circuit.

Table 1 below shows the results obtained for another type of piezoelectric element12, namely the V21BL piezoelectric element with three different rectifier circuits, the standard, the parallel SSHI and the modified parallel SSHI circuit. The piezoelectric element12was loaded with a 1 g tip mass, provided with a parallel electrical connection and an acceleration of 1 g. For each rectifier, three different values of power and frequency are given. The second data pair corresponds to the maximum harvested power and the frequency at which it is obtained. The first and the third data points give information about the frequencies at which half of the maximum power is obtained.

Table 2 provides a simplification of the information that appears in Table 1 since the normalized power is the harvested power divided by the power obtained with the standard rectifier. The frequency bandwidth is calculated as the difference between the frequencies at which half of the maximum power is extracted.

Tables 3 and 4 show the normalized version of the measured results obtained for the Midé QP20W and the DuraAct P-876.A 12 piezoelectric elements. For all the cases measured, the modified parallel SSHI circuit is the AC-DC converter that harvests more power.

From the measurements shown in Tables 1 to 4, it is deduced that the modified parallel SSHI converter offers better results in terms of bandwidth operation than the parallel SSHI converter employing the same diodes, switching elements and peak detector circuit.

According to alternative embodiments of the teachings disclosed herein, a rectifier circuit may comprise a pair of input terminals, a pair of output terminals, an energy storing element, and a rectifier bridge configured to function as a switching circuit for the energy storing element and as a rectifying circuit connecting the energy storing element with the pair of output terminals.

The switching circuit for the energy storing element (e.g, an inductor) may close a resonant circuit formed by the energy storing element and an energy source that is connectable to the pair of input terminals. In this manner, the amount of energy extracted from the energy source may be increased. For example, the piezoelectric voltage of a piezoelectric element acting as the energy source may be inverted by the action of the energy storing element and the switching circuit. When the rectifier bridge functions as the rectifying circuit, the energy stored by the resonance circuit is transferred to the output terminals as a direct current.

The energy source (piezoelectric element), the energy storing element (inductor), and the switching circuit may be connected to each other so that the energy source and the energy storing element are connected in parallel. Accordingly, the rectifier circuit functions as a parallel SSHI converter during the inversion phases and this function classifies the rectifier circuit as a “parallel SSHI converter”. During the rectifying phase, that is while the rectifying bridge connects the input terminals (via the energy storing element) with the output terminals in a periodically alternating manner in order to rectify the alternating voltage provided by the energy source, the rectifier circuit may actually have the configuration of a series SSHI converter, because the energy storing element is connected in series between the energy source and the rectifier bridge.

The rectifier bridge may be configured to function in an alternating manner as the switching circuit and the rectifying circuit.

The rectifier bridge may be configured to bypass the pair of output terminals and to provide a short-circuit for the energy storing element and an energy source connected to the pair of input terminals when functioning as the switching circuit.

FIG. 11show a schematic flow diagram of a method for rectifying an electrical current generated by an energy source according to an embodiment of the teachings disclosed herein.

At a step102of the method an electrical current is applied to a first circuit601(seeFIG. 6) comprising a rectifier bridge and an energy storing element so that the rectifier bridge conducts the electrical current to an output of the rectifier bridge along a first rectification path associated with a first current flow direction of the electrical current. Thus, electrical energy is transferred from an input side (AC side) of the rectifying bridge to a an output side (secondary side or DC side) of the rectifying bridge during a first rectifying phase of one cycle of operation.

At a step104a first specific pattern is detected in a sense signal associated with the electrical current. For example, the sense signal may be piezoelectric voltage output by a piezoelectric element that also generates the electrical current. The sense signal may be based on a voltage between two input terminals of the rectifier circuit connected to the energy source.

The rectifier bridge may then be reconfigured, as indicated at a step106, in response to detecting the first specific pattern. The reconfiguration of the rectifier bridge may be achieved by switching a controllable switching element of the rectifier bridge from a non-conducting state to a conducting state so that the electrical current is conducted within the rectifier bridge along a first inversion path that substantially short-circuits a second circuit602(seeFIG. 6) comprising the energy storing element and the energy source and bypasses an output of the rectifier bridge. This phase of the cycle of operation is an inversion phase because a voltage generated by the energy source is inverted by the energy storing element and the short-circuit caused by the first inversion path.

At a step108the rectifier bridge is reconfigured. Alternatively, the rectifier bridge may be allowed to perform a self-reconfiguration in response to a change of the current flow direction from the first current flow direction to a second current flow direction so that the rectifier bridge conducts the electrical current to the output of the rectifier bridge along a second rectification path associated with the second current flow direction.

According to further embodiments of the method for rectifying an electrical current, a second specific pattern may be detected in the sense signal. A third reconfiguration of the rectifier bridge may then be performed, in response to detecting the second specific pattern, by switching the controllable switching element of the rectifier bridge from the conducting state to the non-conducting state so that the electrical current is conducted within the rectifier bridge along a second inversion path that substantially short circuits the second circuit and bypasses the output of the rectifier bridge.

In addition to the controllable switching element mentioned above, the rectifier bridge may comprises a second controllable switching element. The reconfiguration of the rectifier bridge during the step106may then further comprise a switching of the second controllable switching element form a conducting state to a non-conducting state. Likewise, the third reconfiguration of the rectifier bridge may comprise a switching of the second controllable switching element from the non-conducting state to the conducting state. Accordingly, the second inversion path employs the second controllable switching element.

The self-reconfiguration of the rectifier bridge may be caused by at least one diode element of the rectifier bridge changing from a non-conducting state to a conducting state, or vice versa. Besides the controllable switching elements, the rectifier bridge may comprises diode elements. The mentioned behavior of the diode element(s) is typically caused by an inversion of the polarity of the voltage at the input of the rectifier bridge so that a diode in a first bridge branch of the rectifier bridge become conducting and another diode in a second bridge branch becomes non-conducting.

According to further embodiments of the teachings disclosed herein, the rectifier bridge may comprise two bridge branches connected in parallel between a first bridge input terminal and a second bridge input terminal, wherein a first bridge branch of the two bridge branches substantially coincides with the inversion path and a second bridge branch of the two bridge branches substantially coincides with the second inversion path.

The rectifier bridge may comprise a second controllable switching element, a third controllable switching element, and a fourth controllable switching element. The method may further comprise controlling the first, second, third, and fourth controllable switching elements individually in response to specific patterns in the sense signal. By controlling the first to fourth controllable switching elements in an individual manner, the rectifying phase(s) and the inversion phase(s) may be timed in a specific manner so that an energy transfer from the energy source to the load is substantially maximized.