Electrical energy converter with piezoelectric element(s) and switching assistance circuit(s), associated electrical energy conversion electronic system

This electrical energy converter comprises:

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. non-provisional application claiming the benefit of French application No. 21 12926, filed Dec. 3, 2021, which is incorporated herein by reference in its entirety.

DOMAIN

The present invention relates to an electrical energy converter comprising a first switching bridge comprising at least one first switching branch, each first switching branch being connected between two input voltage supply terminals and comprising at least two first switches connected in series and linked together at a first midpoint; a second switching bridge comprising at least one second switching branch, each second switching branch being connected between two output voltage supply terminals and comprising at least two second switches connected in series and linked together at a second midpoint; and at least piezoelectric assembly, each piezoelectric assembly comprising at least one piezoelectric element and being connected between a respective first midpoint and a respective second midpoint.

The invention also relates to an electronic system for electrical energy conversion comprising such an electrical energy converter and an electronic device for controlling said converter.

The invention relates to the field of electronic systems for converting electrical energy, in particular those with a piezoelectric element, especially systems for converting into direct electrical current, i.e. DC-DC conversion systems, and also AC-DC conversion systems.

BACKGROUND

An electrical energy converter of the aforementioned type is known from the documents FR 3 064 850 B1, FR 3 086 471 A1, and FR 3 086 472 A1, as well as from the thesis manuscript “Convertisseurs DC-DC piézoélectrique avec stockage provisoire d'énergie sous forme mécanique” by Benjamin Pollet, and can be seen in FIGS. 19 and 20 of the document FR 3 086 472 A1.

The switches of the first and second switching bridges are operated cyclically at the main frequency of oscillation of the piezoelectric element about its preselected resonance mode with, between each application of a voltage via the closing of at least one switch, a phase in which the piezoelectric element is open-circuited (substantially constant load) via the opening of at least one switch. The closing of each switch is advantageously carried out at a voltage of approximately zero at its terminals, and in all cases, the closing of a switch never generates a significant variation in voltage at the terminals of the piezoelectric assembly(-ies) (less than 20%, and advantageously less than 10%, of the input voltage Vinor of the output voltage Vout).

In the steady state, a command cycle typically comprises six distinct successive phases, namely three phases of substantially constant voltage across each piezoelectric element and three phases of substantially constant load across said piezoelectric element, alternating between phases of substantially constant voltage and phases of substantially constant load.

For controlling such an electrical energy converter, a control strategy typically consists of regulating the output voltage to track a desired setpoint, while ensuring zero voltage switching and synchronisation with the internal current of the piezoelectric element, as described in FR 3 064 850 B1, or in the article “Implementation of control strategy for step-down dc-dc converter based on piezoelectric resonator” by Mustapha Touhami et al (EPE′20 ECCE Europe, pp. 1-9).

However, the control of such an electrical energy converter is not always optimal.

SUMMARY

The purpose of the invention is therefore to provide an electrical energy converter with at least one piezoelectric element, with improved operation.

To this end, the invention has as its object an electrical energy converter comprising:a first switching bridge comprising at least one first switching branch, each first switching branch being connected between two input voltage application terminals and comprising at least two first switches connected in series and linked together at a first midpoint;a second switching bridge comprising at least one second switching branch, each second switching branch being connected between two input voltage application terminals and comprising at least two first switches connected in series and linked together at a first midpoint;at least one piezoelectric assembly, each piezoelectric assembly comprising at least one piezoelectric element and being connected between a respective first midpoint and a respective second midpoint; andat least one switching aid circuit, each switching aid circuit being connected to a respective one of the first and second midpoints, each switching aid circuit being configured to, via the flow of a previously received current, discharge at least one parasitic capacitance of a switch of the respective switching bridge to which it is connected, and respectively charge at least one parasitic capacitance of another switch of said switching bridge.

With the state-of-the-art electrical energy converter, the switching of the switches of the first and second switching bridges, which are cyclically commanded to obtain the aforementioned phases of the command cycle, induce a maximum swing of a total voltage of the piezoelectric assembly(-ies), denoted Vp, which swing ranges from −Vin−Voutto +Vin+Vout, where Vinis the value of the input voltage and Voutis the value of the output voltage.

Each piezoelectric element is modelled as a capacitor and a resonant branch connected in parallel to the capacitor, the capacitance of said capacitor being referred to as the parallel capacitance, or blocked capacitance, or reference capacitance, and denoted C0.

This voltage swing consumes part of an internal current ILof the piezoelectric element, flowing in the resonant branch and available to charge/discharge the parallel capacitance C0. On the one hand, this internal current ILis limited by the piezoelectric material itself, due to its heating and limited deformation amplitude. On the other hand, the higher the internal current IL, the higher the losses, namely the mechanical losses in the piezoelectric material proportional to the square IL2of the internal current ILand the conduction losses in the switches. Furthermore, the dielectric losses in the piezoelectric material increase with the voltage swing.

With the electrical energy converter according to the invention, the or each switching aid circuit makes it possible to limit this voltage swing, by facilitating certain switchings of the first and second switching bridges, as will be described in more detail later.

Preferably, the or each switching aid circuit then comprises an additional inductor or piezoelectric element, in addition to the piezoelectric assembly(-ies) connected between the respective first and second midpoints.

In other beneficial aspects of the invention, the electrical energy converter comprises one or more of the following features, taken in isolation or in any technically possible combination:the switching bridge to which a respective switching aid circuit is connected comprises two switching branches, and said switching aid circuit is then connected between the respective midpoints (38;48) of the two switching branches of said bridge;the first switching bridge comprises two first switching branches, and the second switching bridge has two second switching branches; and the converter comprises two piezoelectric assemblies, each connected between respective first and second midpoints, the midpoints between which the piezoelectric assemblies are connected being distinct from one piezoelectric assembly to another;the switching bridge to which a respective switching aid circuit is connected comprises a single switching branch, and said switching aid circuit is then connected between the midpoint and an end of said switching branch of said bridge;the converter comprises two switching aid circuits, a first switching aid circuit being connected to the first switching bridge and a second switching aid circuit being connected to the second switching bridge;each switching aid circuit comprises no controllable switch, in particular no transistor;each switching aid circuit comprises an element selected from the group consisting of: an inductor; a first assembly formed of an inductor and a diode connected in series; a second assembly formed of an inductor and a capacitor connected in series; and an additional piezoelectric element;

each switching aid circuit preferably consisting of an element selected from said group;the switching aid circuit comprises an inductor and a diode connected in series, and the diode is oriented according to a direction of flow of the previously received current, the diode being configured to block the flow of a current going from the positive polarity to the negative polarity of a possible direct voltage component of said current;the switching aid circuit comprises an additional piezoelectric element, each piezoelectric element has a reference capacitance, each piezoelectric element being modelled as a capacitor and a resonant branch connected in parallel to the capacitor, the reference capacitance being the capacitance of said capacitor, and

the reference capacitance of the additional piezoelectric element is at least three times less than the reference capacitance of the piezoelectric element(s) of each piezoelectric assembly connected between respective first and second midpoints;the switches of the first and second switching bridges can be commanded to alternate between phases of substantially constant voltage across each piezoelectric assembly and phases of substantially constant load across each piezoelectric assembly;the previously received current is obtained in at least one substantially constant voltage phase preceding the discharge of the at least one parasitic capacitance of a switch and the charging of the at least one parasitic capacitance of another switch, respectively, by the respective switching aid circuit;each piezoelectric assembly consists of one of the group consisting of: a single piezoelectric element; a plurality of piezoelectric elements connected in series; a plurality of piezoelectric elements connected in parallel; a piezoelectric element and an auxiliary capacitor connected in series; a piezoelectric element and an auxiliary capacitor connected in parallel; and an arrangement of a plurality of parallel branches, each branch comprising one or more piezoelectric elements connected in series or an auxiliary capacitor;

the auxiliary capacitor preferably having a capacitance greater than, preferably at least three times greater than, a reference capacitance of the piezoelectric element(s), each piezoelectric element being modelled as a capacitor and a resonant branch connected in parallel to the capacitor, the reference capacitance being the capacitance of said capacitor.

The invention also relates to an electronic system for electrical energy conversion comprising an electrical energy converter and an electronic device for operating the electrical energy converter, the electrical energy converter being as defined above.

DETAILED DESCRIPTION

The phrase “substantially equal to” means being equal within 10%, and preferably within 5%.

InFIG.1, an electronic system5for electrical energy conversion comprises an electrical energy converter10comprising at least one piezoelectric assembly12, each piezoelectric assembly12having at least one piezoelectric element15and several switches K1, K2, K3, K4, K5, K6, K7, K8capable of being commanded to alternate between phases with a substantially constant voltage across the terminals of the piezoelectric assembly(-ies)12and phases with a substantially constant charge across the terminals of said piezoelectric assembly(-ies)12; and an electronic device20for operating the electrical energy converter10.

The electronic system5for electrical energy conversion is typically a DC power conversion system, such as a DC-DC conversion system capable of converting a first inputted DC power or voltage into a second outputted DC power or voltage, or an AC-DC conversion system capable of converting an inputted AC power or voltage into an outputted DC power or voltage of the conversion system5.

When the electrical energy conversion system5is an AC-DC conversion system, the electrical energy conversion system5preferably further comprises a voltage rectifier, not shown, connected to the input of the electrical energy converter10and capable of rectifying the AC electrical voltage received at the input of the conversion system5to provide a rectified electrical voltage at the input of the converter10, the electrical energy converter10preferably being a DC-DC converter capable of converting DC electrical energy or voltage into another DC electrical energy or voltage. The voltage rectifier is for example a bridge rectifier, such as a diode bridge. Alternatively, the voltage rectifier is formed in part by switches of the converter10.

The skilled person will note that these different examples for the conversion system5, whether it is a DC-DC conversion system or an AC-DC conversion system, are also presented in the documents FR 3 086 471 A1 and FR 3 086 472 A1, in particular in relation to theirFIGS.1and2.

The electrical energy converter10is preferably a DC-DC converter, and is also called a DC-DC converter. The DC-DC converter is generally intended to regulate a supply voltage to a load22to a stable value, by being supplied by a power source24providing a substantially DC voltage. The power source24is for example a battery or a solar panel.

The electrical energy converter10is then configured to raise the value of the DC voltage between its input and its output, and is then also called a DC-DC step-up converter; or is configured to lower the value of the DC voltage between its input and its output, and is then called a DC-DC step-down converter.

The electrical energy converter10is configured to provide N separate output voltage(s) from E separate input voltage(s), where E and N are each an integer greater than or equal to 1.

In the example shown inFIG.1, the electrical energy converter10is configured to provide an output voltage, denoted Vout, from an input voltage, denoted Vin, whereby the number E of input voltages and the number N of output voltages are each equal to 1.

Alternatively, not shown, the electrical energy converter10is configured to provide a plurality of distinct output voltages from one or more distinct input voltages, where the number N of distinct output voltages is greater than 1. Also alternatively, the electrical energy converter10is configured to provide one or more distinct output voltages from a plurality of distinct input voltages, where the number E of distinct input voltages is greater than 1. Also alternatively, the electrical energy converter10is configured to provide a plurality of distinct output voltages from a plurality of distinct input voltages, where the numbers E and N are each greater than 1.

Where the electrical energy converter10is configured to provide a number of distinct output voltages, the converter10is typically connected to a number of loads22, as shown for example in FIG. 17 of FR 3 086 471 A1.

Similarly, when the electrical energy converter10is configured to provide one or more distinct output voltages from a plurality of distinct input voltages, then the converter10is powered by a plurality of energy sources24.

The electrical energy converter10comprises the piezoelectric assembly(-ies)12each formed of one or more piezoelectric elements15, and the control device20is configured to operate the piezoelectric material of the piezoelectric element(s)15at its/their resonance in order to exploit load transfer phases to dispense with the use of an inductive element, while regulating the output voltage by maintaining the resonance of the piezoelectric material, i.e. with repeated switching cycles at an operating frequency depending on the resonance frequency of the piezoelectric element(s)15, and by adjusting the durations of the respective switching phases within the resonance cycle.

In the steady state, the piezoelectric assembly(-ies)12exchange a load and substantially zero power over a resonant cycle, except for losses. In other words, each piezoelectric assembly12gives back substantially as much energy and load as it receives over a period. Two operating conditions then apply to the steady state/settled state, namely load balance and energy balance over a resonant period. Even if during transients (start-up, variation of voltage steps, change of output current) this balance is not respected, it must still be possible to achieve it in steady state. In particular, this requires a certain arrangement of the voltage steps during the resonance period. For example, for three voltage step operation, the two extreme voltage steps are controlled during one half-period of a given polarity of a current ILflowing through the piezoelectric elements15, and the intermediate voltage step is controlled during the other half-period of opposite polarity of the current ILflowing through the piezoelectric elements15.

As is known per se, the mechanical oscillation of the piezoelectric assembly12is approximately sinusoidal, as represented inFIGS.3,7,8and10by the curve26showing the total mechanical deformation of the piezoelectric element(s)12during a respective resonance cycle. When the electrical energy converter10comprises a plurality of piezoelectric assembly(-ies)12, as in the examples ofFIGS.1,5and6, the total mechanical deformation of the piezoelectric assemblies12is the sum of the elementary mechanical deformations of each of the piezoelectric assemblies12.

An increase or decrease in the energy stored over a period leads to an increase or decrease in the oscillation amplitude, respectively. Furthermore, during a phase with a substantially constant load at the terminals of the piezoelectric assembly(-ies)12, i.e. when the piezoelectric assembly(-ies)12is/are placed in a substantially open electrical circuit, with little exchange of electrical loads between the piezoelectric assembly(-ies)12and the outside, an increase in the amplitude of the oscillations causes an increase in the rate of change of the voltage Vpacross the piezoelectric assembly(-ies)12, and during a phase with substantially constant voltage across the piezoelectric assembly(-ies)12, this increase in the amplitude of the oscillations leads to an increase in a current Ipexchanged between the piezoelectric assembly(-ies)12and the voltage stages.

Substantially constant load means an exchange of charge with the exterior that is less than 10% of the load that would have been exchanged with the exterior if the voltage had been kept constant. In other words, a substantially constant load means a variation in load of less than 10% of the load that would have been exchanged with the exterior of the piezoelectric piezoelectric assembly(-ies)12if the voltage across the piezoelectric piezoelectric assembly(-ies)12had been held constant over the time period in question.

Substantially open electrical circuit means a circuit in which any leakage current leads to a variation in the load of the piezoelectric assembly(-ies)12of less than 10% of the load that would have been exchanged with the exterior of the piezoelectric assembly(-ies)12if the voltage across the piezoelectric assembly(-ies)12had been held constant over the time period in question.

Substantially constant voltage means a voltage variation of less than 20%, preferably less than 10%, of the input or output voltage of the converter10. For example, if the input voltage of the converter10is 100V, then the voltage variation during each phase at substantially constant voltage, is less than 20% of this voltage, i.e. less than 20V; preferably less than 10% of this voltage, i.e. less than 10V. Each phase with substantially constant voltage is also called a voltage step.

The converter10then comprises a plurality of switches K1, K2, K3, K4, K5, K6, K7, K8capable of being controlled to alternate between phases of substantially constant voltage and phases of substantially constant load across the piezoelectric assembly(-ies)12, within periods of substantially constant duration corresponding to the operating frequency of the converter10, depending on the resonant frequency, also known as natural frequency, of the piezoelectric assembly(-ies)12. The phases with a substantially constant load make it possible, in steady state or permanent operation, to switch from one constant voltage to another and to close the switches that must be closed when the voltage at their terminals is preferably zero, in order to have a so-called zero voltage switching mode, also called ZVS. At the very least, the closing of a switch must not result in a sudden variation of the voltage Vp(less than 20%, and preferably less than 5%, of the input voltage Vinor output voltage Vout), which would be a source of significant losses, the capacitance C0of the piezoelectric being significantly greater (typically at least 3 times greater) than the parasitic capacitance of the switches.

In particular, the converter10comprises a first switching bridge30comprising at least one first switching branch32, each first switching branch32being connected between two input voltage Vinapplication terminals34and comprising at least two first switches36connected in series and linked together at a first midpoint38. Of the two application terminals34, one has a lower potential, denoted Vinn, than the other, denoted Vinp.

In the examples ofFIGS.1,5and6, the first switching bridge30comprises two first switching branches32connected in parallel between the two application terminals34. In these examples, the first switching bridge30preferably consists of said two first switching branches32.

In the example shown inFIG.9, the first switching bridge30comprises a single first switching branch32connected between the two application terminals34. In this example, the first switching bridge30preferably consists of this single first switching branch32.

In the examples ofFIGS.1,5,6and9, each first switching branch32comprises two first switches36connected in series and joined at the first midpoint38. Each first switching branch32preferably consists of the first two switches36.

In the examples ofFIGS.1,5, and6, with two first switching branches32, the first two switches36are denoted K5, K6for one of the first two switching branches32, and K7, K8for the other of the first two switching branches32respectively.

In the example shown inFIG.9, with a single first switching branch32, the first two switches36are denoted K5, K6for said first switching branch32.

The converter10comprises a second switching bridge40comprising at least one second switching branch42, each second switching branch42being connected between two output voltage Voutsupply terminals44and comprising at least two second switches46connected in series and linked together at a second midpoint48. Of the two supply terminals44, one has a lower potential, denoted Voutn, than the other, denoted Voutp.

In the examples ofFIGS.1,5and6, the second switching bridge40comprises two second switching branches42connected in parallel between the two application terminals44. In these examples, the second switching bridge40preferably consists of said two second switching branches42.

In the example shown inFIG.9, the second switching bridge40comprises a single second switching branch42connected between the two application terminals44. In this example, the second switching bridge40preferably consists of this single second switching branch42.

In the examples ofFIGS.1,5,6and9, each second switching branch42comprises two second switches46connected in series and joined at the second midpoint48. Each second switching branch42preferably consists of the two second switches46.

In the examples ofFIGS.1,5, and6, with two second switching branches42, the two second switches46are denoted K1, K2for one of the two second switching branches42, and K3, K4for the other of the two second switching branches42respectively.

In the example shown inFIG.9, with a single second switching branch42, the two second switches46are denoted K1, K2for said second switching branch42.

In the examples ofFIGS.1,5and6, the converter10comprises two piezoelectric assembly(-ies)12, each connected between respective first38and second48midpoints, the midpoints38,48between which the piezoelectric assemblies12are connected being distinct from one piezoelectric assembly(-ies)12to the other, each switching bridge30,40then comprising two respective switching branches32,42.

In the example shown inFIG.9, the converter10comprises a piezoelectric assembly12connected between the first midpoint38of the single first branch32and the second midpoint48of the single second branch42.

According to the invention, the converter10further comprises at least one switching aid circuit50, each switching aid circuit50being connected to a respective one of the first38and second48midpoints, each switching aid circuit50being configured to, via the flow of a previously received current, discharge a parasitic capacitance of a switch36,46of the respective switching bridge30,40to which it is connected, and respectively charge a parasitic capacitance of another switch36,46of said switching bridge30,40.

In the examples ofFIGS.1,5,6and9, the converter10comprises a single switching aid circuit50connected to either the first switching bridge30or the second switching bridge40. In the examples shown inFIGS.1and5, the single switching aid circuit50is connected to the second switching bridge40. In the examples shown inFIGS.6and9, the single switching aid circuit50is connected to the first switching bridge30.

In the examples shown inFIGS.1,5, and6, the second switching bridge40has two second switching branches42, respectively the first switching bridge30has two first switching branches32; and the switching aid circuit50is connected between the respective midpoints38,48of the two switching branches32,42of said bridge30,40. In the examples shown inFIGS.1and5, the switching aid circuit50is connected between the second midpoints48of the two second switching branches42of the second switching bridge40. Likewise, in the example shown inFIG.6, the switching aid circuit50is connected between the first midpoints38of the two first switching branches32of the first switching bridge30.

In the example shown inFIG.9, the first switching bridge30comprises the single first switching branch32, and the switching aid circuit50is then connected between the first midpoint38of the first leg32of the first switching bridge30and one end of said first leg32, which end is itself connected to a respective application terminal34.

Alternatively, not shown, the converter10comprises two switching aid circuits50, a first switching aid circuit being connected to the first switching bridge30and a second switching aid circuit being connected to the second switching bridge40.

In this alternative, the skilled person will understand that each switching aid circuit50is capable of being connected between the respective midpoints38,48of the two switching branches32,42when the bridge30,40to which it is connected comprises two respective switching branches32,42; or is capable of being connected between the midpoint38,48and a respective end of the corresponding switching branch32,42when the bridge30,40to which it is connected comprises a single switching branch32,42.

In addition, when a respective switching aid circuit50discharges a parasitic capacitance of at least one switch36,46of the respective switching bridge30,40to which it is connected, and respectively charges at least one parasitic capacitance of another switch36,46of said switching bridge30,40, the control device20is preferably further configured to command the opening of at least one first switch36and/or at least one second switch46arranged in series in a loop including said switching aid circuit50, the piezoelectric assembly(-ies)12, some of the first switches36of the first bridge30and some of the second switches46of the second bridge40. The opening of the at least one first switch36and/or the at least one second switch46included in this loop then prevents the switching aid circuit50from significantly charging or alternatively discharging a reference capacitance C0, described below, of the piezoelectric element(s)15of the piezoelectric assembly(-ies)12during this phase which is at substantially constant load, and then better respects the constancy of the load during the substantially constant load phase during which the respective switching aid circuit50is activated, i.e. implemented.

According to this complement, when the respective switching aid circuit50that is activated is a switching aid circuit connected to the first switching bridge30, the control device20is preferentially configured to command the opening of at least one second switch46arranged in series in said loop, as defined in the preceding paragraph. As a corollary, when the respective switching aid circuit50that is activated is a switching aid circuit connected to the second switching bridge40, the control device20is preferentially configured to command the opening of at least one first switch36arranged in series in said loop.

Each switch in the converter10, i.e. each of the first36and second46switches, also denoted K1, K2, K3, K4, K5, K6, K7, K8, is preferably a unidirectional current and unidirectional voltage switch. The switch K1, K2, K3, K4, K5, K6, K7, K8, comprises for example a transistor, or a diode, or a transistor and a diode in antiparallel, not shown. The switch K1, K2, K3, K4, K5, K6, K7, K8, preferably consists of the transistor, or the diode, or the transistor and the diode in antiparallel. Alternatively, the switch K1, K2, K3, K4, K5, K6, K7, K8, comprises a combination of several transistors, and preferably consists of such a combination of several transistors. Alternatively, the switch K1, K2, K3, K4, K5, K6, K7, K8, comprises a mechanical switch, such as a MEMS (MicroElectroMechanical System) microswitch.

The transistor is, for example, an insulated gate field effect transistor, also known as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Alternatively, the transistor is a bipolar transistor; an insulated gate bipolar transistor, also known as an IGBT (Insulated Gate Bipolar Transistor); a silicon (Si) based transistor; a GaN (Gallium Nitride) based transistor; a silicon carbide (SiC) based transistor; or a diamond-based transistor; or a thyristor.

Each piezoelectric assembly12consists of one of the group consisting of: a single piezoelectric element15; a plurality of piezoelectric elements15connected in series; a plurality of piezoelectric elements15connected in parallel; a piezoelectric element15and an auxiliary capacitor, not shown, connected in series; a piezoelectric element15and an auxiliary capacitor connected in parallel; and an arrangement of a plurality of parallel branches, each branch comprising one or more piezoelectric elements15connected in series or an auxiliary capacitor.

The auxiliary capacitor is typically larger, preferably at least three times larger, than a reference capacitance C0of the piezoelectric element(s)15.

When the converter10comprises two piezoelectric assemblies12, according to an optional complement, the two piezoelectric assemblies12share a common piezoelectric material, while having the electrodes of one piezoelectric assembly12be distinct from those of the other piezoelectric assembly12. According to this optional addition, the electrode pairs of one piezoelectric assembly12, and respectively those of the second piezoelectric assembly12, cover distinct material surfaces. Furthermore, the electrodes of a first piezoelectric assembly12cannot in this case directly induce a significant electric field in the part of the piezoelectric material belonging to the other piezoelectric assembly12. According to this further optional complement, the capacitance between any one of the electrodes of one piezoelectric assembly12and any one of the electrodes of the other piezoelectric assembly12is negligible (at least 10 times smaller) compared to the reference capacitance C0of each of the piezoelectric assemblies12, e.g. by not directly facing each other across the material. This pooling of the same material makes it possible, for example, to facilitate the implementation of the piezoelectric assemblies12, (limitation of the number of part(s), pooling of the fastening means); and also to synchronise the vibration of the two piezoelectric assemblies, without there being a significant transfer of energy from one assembly to the other (< 1/10thof the output power).

The piezoelectric element15is known per se, and is typically modelled, close to the resonance mode operated, as a capacitor52and a resonant branch54connected in parallel to the capacitor52, the capacitor52and resonant branch54being connected between a first electrode56and a second electrode58of the piezoelectric element15, as illustrated in the modelling of the piezoelectric element15shown in a bubble60inFIG.1. The resonant branch54is typically an RLC branch formed by a capacitor62, a resistor64and a coil66connected in series. The capacitance of the capacitor52connected in parallel with the resonant branch54is called the parallel capacitance, or blocked capacitance, or reference capacitance, and is denoted C0. The voltage at the terminals of the piezoelectric element15then typically corresponds to the voltage at the terminals of the capacitor52.

In the present description, a so-called total piezoelectric voltage Vpis by convention the voltage across the piezoelectric assembly12if the converter10comprises a single piezoelectric assembly12; or the sum of each of the voltages across the piezoelectric assemblies12if the converter10comprises a plurality of piezoelectric assemblies12. In particular, when the converter10comprises two piezoelectric assemblies12, namely a first piezoelectric assembly and a second piezoelectric assembly, each connected between a respective pair of first38and second48midpoints, the voltage across the first piezoelectric assembly is denoted Vp1, and that across the second piezoelectric assembly is denoted Vp2. The total piezoelectric voltage Vpis then equal to the sum of these voltages Vp1and Vp2, i.e. Vp1+Vp2. The two piezoelectric assemblies12are preferably identical, and have substantially the same voltage at their terminals within a possible offset voltage Voffset, so that the voltages Vp1and Vp2are equal to Vp/2+/−Voffset, according to the following equations:

The voltage Voffsetis a substantially constant component across a resonant period and has little impact on the charge or energy balance over a period. This voltage Voffsetevolves slowly with respect to the control frequency, its ripple is typically at a frequency at least 10 times lower than the control frequency of the piezoelectric assembly(-ies)12. Moreover, when the voltages Vp1+Vp2are added together, this offset voltage Voffsetdisappears, and we obtain the total piezoelectric voltage Vp, as described in the different cycles. In practice, this voltage Voffsetdoes not affect the control law, and allows completely independent potentials Vinnand Voutnat low frequency.

Furthermore, in the present description and as shown inFIGS.1,5,6, and9, the voltage between the first midpoints38is denoted Vpa, and is by convention equal to the difference in potentials (Vpa1−Vpa2), where Vpa1is the potential of the first midpoint38connected to the first piezoelectric assembly, and Vpa2is the potential of the other first midpoint38connected to the second piezoelectric assembly. The voltage between the second midpoints48is denoted Vpb, and is by convention equal to the difference in potentials (Vpb1−Vpb2), where Vpb1is the potential of the second midpoint48connected to the first piezoelectric assembly, and Vpb2is the potential of the other second midpoint48connected to the second piezoelectric assembly when the latter is present.

By convention and as shown inFIGS.1,5and6, the voltage across the first piezoelectric assembly Vp1is equal to the difference in potentials (Vpa1−Vpb1), and that across the second piezoelectric assembly Vp2is equal to the difference in potentials (Vpb2-Vpa2).

The resonant frequency is the frequency at which the piezoelectric element15oscillates and therefore its current IL, visible inFIG.1. The conversion cycle is synchronised to a mechanical movement of the piezoelectric element15, and the control frequency is then set to the mechanical oscillation frequency. In practice, this oscillation frequency depends on the operating point of the converter10: Values of the three voltage steps and the output current. Depending on the operating point, this oscillation frequency typically fluctuates between the so-called series resonance frequency of the piezoelectric element15(ωs=1/√(Lr·Cr) where Lrand Crcorrespond to the inductance and capacitance of the resonant branch54and the so-called parallel resonance frequency of the piezoelectric element15(ωp=1/√(Lr·Cr·C0/(Cr+C0))), also respectively called the resonance frequency and the anti-resonance frequency of the piezoelectric element15. The operating frequency of the converter10is then between these two resonance and antiresonance frequencies of the piezoelectric element15. The operating point varies slowly with respect to the oscillation frequency of the piezoelectric element15. The operating point typically changes at less than 10 kHz, while the oscillation frequency of the piezoelectric element15is typically 100 kHz or more. As a result, the operating frequency of the converter10changes little from one period to the next.

Generally speaking, for the electrical energy converter10with the piezoelectric assembly12and controlled by the electronic control device20, the number of phases at a substantially constant voltage is typically at least 2, preferably equal to 3, while it may be greater than or equal to 4 with the implementation of the control described in the application FR 21 07345 filed on 7 Jul. 2021.

Each phase with a substantially constant voltage is obtainable from a combination of the input and output voltages, in positive or negative values, or is at zero voltage. The energy converter10then allows energy to be exchanged between phases with substantially constant voltage, and consequently with the voltages or voltage combinations used to achieve these phases at substantially constant voltage. In particular, it is possible to transfer energy from a low-voltage, substantially constant voltage phase to a higher-voltage, substantially constant voltage phase, and by the above combinations ultimately obtain a step-down converter, which may seem counter-intuitive. Conversely, it is also possible to transfer energy from a higher-voltage, substantially constant voltage phase to a lower-voltage, substantially constant voltage phase, and by the above combinations ultimately obtain a voltage step-up converter. The skilled person will then understand that it is possible to have a step-up cycle seen by the piezoelectric assembly12while the electrical energy converter10is a step-down converter, and conversely to have a step-down cycle seen by the piezoelectric assembly12while the electrical energy converter10is a step-up converter.

By convention, if power is supplied to the piezoelectric assembly12in phase II, IV, VI at a substantially constant voltage corresponding to the highest voltage during a resonant cycle, then the cycle is considered a step-down cycle for the piezoelectric assembly12. Conversely, if power is delivered, or drawn, from the piezoelectric assembly12during said substantially constant voltage phase at which the voltage is highest during the resonance cycle, then the cycle is considered to be a step-up cycle for the piezoelectric assembly12. As noted above, the conversion cycle seen by the piezoelectric assembly12is likely to be a step-up cycle while the electrical energy converter10is operating as a step-down converter, and conversely the conversion cycle seen by the piezoelectric assembly12is likely to be a step-down cycle while the electrical energy converter10is operating as a step-up converter.

The electronic control device20is configured to control the electrical energy converter10, in particular to control the switches K1, K2, K3, K4, K5, K6, K7, K8of the converter, in order to alternate phases with a substantially constant voltage across the piezoelectric assembly12and phases with a substantially constant charge, i.e. in a substantially open circuit, across said piezoelectric assembly12.

The electronic control device20is, for example, designed as an electronic circuit with one or more electronic components.

Alternatively, the electronic control device20is implemented as a programmable logic component, such as Field-Programmable Gate Arrays (FPGAs), or as a dedicated integrated circuit, such as Application-Specific Integrated Circuits (ASICs), or as a computer, such as a microcontroller or processor.

Each switching aid circuit50is configured, via the flow of a previously received current ICALC, to discharge at least one parasitic capacitance of a switch36,46, preferably a switch to be closed, of the respective switching bridge30,40to which it is connected; respectively to charge at least one parasitic capacitance of another switch36,46, preferably a switch to be opened or kept open, of said switching bridge30,40.

Each of the switches of said switching bridge30is opened when the previously received current flows through the switching aid circuit50.

As a result of this current flow, the switch(es)36,46whose parasitic capacitance has been discharged by the switching aid circuit50is/are closed. The other switch(es)36,46whose parasitic capacitance has been charged by the switching aid circuit50remain(s) open.

Each switching aid circuit50is free of a controllable switch, and in particular each switching aid circuit50is free of a transistor. In other words, each switching aid circuit50does not comprise a controllable switch, in particular each switching aid circuit50does not comprise a transistor.

Each switching aid circuit50comprises, for example, an inductor70; or a first assembly of the inductor70and a diode72connected in series; or a second assembly of the inductor70and a capacitor74connected in series; or an additional piezoelectric element76, as shown inFIG.2.

Each switching aid circuit50is for example an inductor70, the inductor70preferably consisting of a coil and a magnetic circuit. Alternatively, each switching aid circuit50is in the form of the first assembly of series-connected inductor70and diode72, and preferably consists of said first assembly of inductor70and diode72. In yet another alternative, each switching aid circuit50is in the form of the second assembly of series-connected inductor70and capacitor74, and preferably consists of said first assembly of inductor70and capacitor74. Alternatively, each switching aid circuit50is in the form of the additional piezoelectric element76, and preferably consists of the additional piezoelectric element76.

The previously received current ICALCis obtained in at least one substantially constant voltage phase, the at least one substantially constant voltage phase then preceding the discharging of the at least one parasitic capacitance of a switch36,46, and respectively the charging of the at least one parasitic capacitance of a further switch36,46, by the respective switching aid circuit50. The skilled person will then understand that the obtaining of the current ICALCsubsequently used for the switching aid is obtained during one or more substantially constant voltage phases which precede the switching aid, i.e. are prior to the switching aid. Indeed, as mentioned above, the current ICALCis received beforehand, i.e. received before the switching aid is implemented, in particular to facilitate the discharge of the at least one parasitic capacitance of a switch36,46, and the charging of the at least one parasitic capacitance of another switch36,46, respectively. Different examples of obtaining the current ICALCare described below.

In the embodiment where the switching aid circuit50is in the form of the inductor70alone, the inductor70has its current increase over a half-period, i.e. when the voltage across it is positive; and then its current decrease over the other half-period, i.e. when the voltage across it is negative. This embodiment of the switching aid circuit50preferably requires that the voltage across the terminals of the inductor70be substantially zero on average, otherwise there is a risk of current drift. If the switching aid circuit50is connected to the second bridge40, in particular between the second midpoints48, the voltage across the terminals of the inductor70is the voltage Vpb. By extension, the switching aid circuit50is connected to the first bridge30, in particular between the first midpoints38, the voltage across the terminals of the inductor70is the voltage Vpa.

The variant where the switching aid circuit50is in the form of the inductor70and the diode72connected in series, allows the inductor70to be charged only over half a period with the correct polarity, in particular for cycles where the current ICALCis received during a time period with only one polarity, for example between the times t2and t3for the step-down cycles A1 and A2 described below (positive polarity of the current ICALCfor the step-down cycle A1 between the times t2and t3, negative polarity for the step-down cycle A3 between these times t2and t3). In particular, the diode72then avoids charging the inductor70with a reverse current between the times t3 and T. This unidirectional current operation also reduces the effective current seen by the inductor70and therefore the losses. Furthermore, the switching aid circuit50according to this variant is not sensitive to the presence of a DC component as long as the DC component is in the direction of blocking the diode72.

The variant where the switching aid circuit50is in the form of the inductor70and the capacitor74connected in series, makes it possible—compared to the example of the inductor70alone—to reduce or even eliminate a possible DC component. Nevertheless, the74capacitor can be quite large. Indeed, the voltage at the terminals of the capacitor74must change little, i.e. in a small proportion, compared to the input voltage Vinor the output voltage Vout, for example have an amplitude of less than 50% of the input voltage Vinor output voltage Vout.

According to the variant where the switching aid circuit50is in the form of the additional piezoelectric element76, from the moment the converter10is controlled between the resonance and anti-resonance frequency of the additional piezoelectric element76, the latter starts to oscillate and to produce a current ICALCsubstantially in quadrature with the voltage at its terminals, such as the voltage Vpbif the additional piezoelectric element76is connected to the second bridge40between the second midpoints48, or the voltage Vpaif the additional piezoelectric element76is connected to the first bridge30between the first midpoints38. The current ICALCthen passes through an extrema around the time t3 for the step-down cycles A1 and A2, which ensures the voltage inversion function Vpbbetween the times t2and t3; or similarly around the time t0for the step-up cycles E1 and E2 described below, which ensures the voltage inversion function Vpabetween the times t0and t1.

The additional piezoelectric element76is typically at least 3 times smaller than the piezoelectric element(s)15of the converter10, the additional piezoelectric element76only having to charge/discharge the parasitic capacitances of the switches36,46. The parasitic capacitance of the switches36,46is indeed considered to be at least three times lower than the reference capacitance C0of the piezoelectric element(s)15of the converter10. This variant where the switching aid circuit50is in the form of the additional piezoelectric element76is insensitive to any DC component (regardless of its polarity), and the switching aid circuit50is suitable for connection to both the first bridge30(voltage Vpa) and the second bridge40(voltage Vpb).

In other words, the reference capacitance of the additional piezoelectric element76is at least three times less than the reference capacitance C0of each piezoelectric assembly12connected between respective first38and second48midpoints.

The operation of the converter10in the example ofFIG.1will now be explained according to two step-down configurations, namely a first step-down configuration A1 and a second step-down configuration A2 as shown inFIG.3. The difference resulting from the switching aid circuit50according to the invention relates to the changes in the voltages Vpaand Vpbbetween the times t2and t3in the case of these step-down configurations A1, A2, and more particularly to the areas shown as dotted lines inFIG.3to mark the difference.

The conversion cycle of the converter10according to the invention is described below for the first A1 and second A2 step-down configurations, focusing on the differences with respect to the conversion cycle of a converter10of the prior art for the same step-down configurations.

The skilled person will note that a very high step-up configuration typically means a configuration where the gain, i.e. the ratio of the output voltage Vtdivided by the input voltage Vin, is greater than 2, i.e. Vout/Vin>2. By extension, a very low step-down configuration typically means a configuration where the gain, i.e. the ratio of the output voltage Voutdivided by the input voltage Vin, is lower than ½, i.e. Vout/Vin<½.

Similarly, a step-up configuration is typically one where said gain is between 1 and 2, i.e. Vin<Vout<2Vin. By extension, a step-down configuration is typically one where said gain is between ½ and 1, i.e. Vin/2<Vout<Vin.

For the first step-down configuration A1, between the times t1and t2, according to the example of the switching aid circuit50, the inductor70or the additional piezoelectric element76sees its current ICALCincrease, under the voltage Vpbequal to +Vout. At time t2, the current ICALCis positive.

Just before time t2, the total piezoelectric voltage Vpis equal to −Vin+Vout, the voltage Vpabeing equal to −Vin, and the voltage Vpbbeing equal to +Vout; and the switches K5, K8, K1, K4are closed.

At time t2, all switches that were closed open. The current ICALCthen charges the parasitic capacitances of switches K1, K4, while discharging the parasitic capacitances of switches K2and K3. Similarly, through the slowly evolving piezoelectric assemblies12, the current ICALCpartially charges the parasitic capacitances of switches K5, K8, while partially discharging the parasitic capacitances of switches K6, K7. The voltage Vpbthus changes from +Voutto −Vout, while the voltage Vpachanges substantially from −Vinto −Vin+2Voutplus the change of the total piezoelectric voltage Vpsince time t2.

The voltage inversion Vpbis considered completed before the total piezoelectric voltage Vpreaches the next step Va. Indeed, even if the amplitude of the current ICALCin the switching aid circuit50is much smaller than the amplitude of the internal current ILof the piezoelectric elements15(for example at least 3 times smaller to limit its size), it nevertheless only has to charge/discharge the parasitic capacitances of the switches36,46considered much smaller than the reference capacitance C0of the piezoelectric elements15(at least a factor of 3). Furthermore, the current ICALCin the switching aid circuit50approaches its maximum value at time t3, while the internal current ILapproaches 0 at said time t3.

Once complete reversal of the voltage Vpbis achieved (from Voutto −Vout), then switches K2and K3are closed so that the voltage Vpbis fixed, while the voltage Vpacontinues to rise to Vindue to the natural increase in the total piezoelectric voltage Vp.

At time t3, switches K6and K7are closed. Switches K2and K3are also closed if this has not been done already, i.e. if the voltage Vpbhas not yet reached −Vout.

In addition, if the switches K2and K3have an intrinsic reverse diode or an additional diode in parallel, they can be switched on naturally according to the sign of the internal current ILafter time t3, or according to the residual current ICALCbefore time t3.

The remainder of the conversion cycle of the converter10according to the invention remains substantially unchanged from the conversion cycle of the prior art.

The voltage swing of the total piezoelectric voltage Vpis thus limited between the times t2and t4, ranging from −Vin+Voutto Vin−Vout, instead of from −Vin+Voutto +Vin+Voutwith the converter10of the prior art, i.e. a swing of 2Vin−2Voutinstead of 2Vin, while ensuring zero-voltage switching of switches36,46.

For the second step-down configuration A2, between the times t1and t2, according to the example of the switching aid circuit50, the inductor70or the additional piezoelectric element76sees its current ICALCdecrease, under the voltage Vpbequal to −Vout. At time t2, the current ICALCis negative.

Just before time t2, the total piezoelectric voltage Vpis equal to Vin−Vout, the voltage Vpabeing equal to Vin, and the voltage Vpbbeing equal to −Vout; and the switches K6, K7, K2, K3are closed.

At time t2, all switches that were closed open. The current ICALCthen charges the parasitic capacitances of switches K2, K3, while discharging the parasitic capacitances of switches K1and K4. Similarly, through the slowly evolving piezoelectric assemblies12, the current ICALCpartially charges the parasitic capacitances of switches K6, K7, while partially discharging the parasitic capacitances of switches K5, K8. The voltage Vpbthus changes from −Voutto +Vout, while the voltage Vpachanges substantially from +Vinto +Vin−2Voutplus the change of the total piezoelectric voltage Vpsince time t2.

The voltage inversion Vpbis considered completed before the total piezoelectric voltage Vpreaches the next step Va. Indeed, even if the amplitude of the current ICALCin the switching aid circuit50is much smaller than the amplitude of the internal current ILof the piezoelectric elements15(for example at least 3 times smaller to limit its size), it nevertheless only has to charge/discharge the parasitic capacitances of the switches36,46considered much smaller than the reference capacitance C0of the piezoelectric elements15(at least a factor of 3). Furthermore, the current ICALCin the switching aid circuit50approaches its maximum value (its negative extreme) at time t3, while the internal current ILapproaches 0 at said time t3.

Once complete reversal of the voltage Vpbis achieved (from −Voutto +Vout), then switches K1and K4are closed so that the voltage Vpbis fixed, while the voltage Vpacontinues to rise to −Vindue to the natural decrease in the total piezoelectric voltage Vp.

At time t3, switches K5and K8are closed. Switches K1and K4are also closed if this has not been done already, i.e. if the voltage Vpbhas not yet reached +Vout.

In addition, if the switches K1and K4have an intrinsic reverse diode or an additional diode in parallel, they can be switched on naturally according to the sign of the internal current ILafter time t3, or according to the residual current ICALCbefore time t3.

The remainder of the conversion cycle of the converter10according to the invention remains substantially unchanged from the conversion cycle of the prior art.

The voltage swing of the total piezoelectric voltage Vpis thus limited between the times t2and t4, ranging from Vin−Voutto Vout−Vin, instead of from Vin−Voutto −Vin−Voutwith the converter of the prior art, i.e. a swing of 2Vin-2Voutinstead of 2Vin, while ensuring zero-voltage switching of switches36,46.

FIG.4shows an example of the sizing of the inductor70of the switching aid circuit50.

During the operation of the switching aid circuit50, the parasitic capacitances of the switches36,46of a respective switching bridge30,40which were closed just before are charged, and the parasitic capacitances of the switches36,46of a respective switching bridge30,40which were open just before are discharged. When the switching bridge30,40comprises two switching branches32,42, the parasitic capacitances of one half of the switches36,46are charged and the parasitic capacitances of the other half of the switches36,46are discharged. Cpara_totis the total parasitic capacitance of the switches36,46to be charged, this being considered equal to the total capacitance of the switches36,46to be discharged, by symmetry of the switches36,46.

The voltage swing over these parasitic capacitances due to the action of the switching aid circuit50is equal to the minimum voltage between the input voltage Vinand the output voltage Voutwhen the switching bridge30,40has two switching branches32,42. For a step-down configuration, such as one of the first A1 and second A2 step-down configurations, the voltage swing across these parasitic capacitances is then equal to the output voltage Vout.

The total electrical load to be provided by the switching aid circuit50, denoted QCALC, then satisfies the following equation:
QCALC=Cpara_tot·Min(Vin,Vout)  [3]

For a respective step-down configuration, said total electrical load QCALCthen satisfies the following equation:
QCALC=Cpara_tot·Vout[4]

For a respective step-down configuration, said total electrical load QCALCthen satisfies the following equation:
QCALC=Cpara_tot·Vin[5]

This load QCALCis ideally exchanged between the times t2and t3for the step-down configurations A1 and A2, and an average exchange current, denoted ICALC_utile, then satisfies the following equation:

For a respective step-down configuration, said average exchange current ICALC_utilethen satisfies the following equation:

The expression of the current in a switching aid circuit50consisting of inductor70, denoted IL-CALCin this case, typically satisfies the following equation:

where VCALCrepresents the voltage across the switching aid circuit50, i.e. across the inductor70, and L represents the inductance of the inductor70.

In the example of the first step-down configuration A1 ofFIG.3, considering the switching aid circuit50placed on the side of the voltage Vpb, i.e. connected to the second switching bridge40, as in the example ofFIG.1, the voltage VCALCat the terminals of the switching aid circuit50, corresponding then to the voltage Vpb, is substantially equal to +Voutbetween substantially the times to and t3(considering the transition durations from t0to t1and from t2to t3to be short in comparison with the duration from t0to t3), and respectively substantially equal to −Voutbetween the times t3and t6.

When the voltage VCALCacross the switching aid circuit50is equal to Vout, the current IL-CALCin the switching aid circuit50increases by t*Vout/L, i.e. with a first slope P1 equal to Vout/L. When the voltage VCALCacross the switching aid circuit50is equal to −Vout, the current IL-CALCin the switching aid circuit50decreases by −t*Vout/L, i.e. with a second slope P2 equal to −Vout/L.

Due to symmetry, the current IL-CALCin the switching aid circuit50passes through zero at time T/4, and its value is maximum around time t3equal to T/2 and is then worth a maximum current IL-CALC-maxequal to T/4*Vout/L, i.e. equal to T·Vout/(4 L).

As the duration between the times t2and t3is relatively short compared to the period T of the resonance cycle, the current IL-CALCis considered to be relatively constant between the times t2and t3and close to the maximum current IL-CALC-max.

The value of the inductance L is then chosen so that the maximum current IL-CALC-maxapproaches the average exchange current ICALC-utiledescribed above.

The maximum IL-CALC-maxand average ICALC-utileexchange currents typically satisfy the following inequation:

The value of the inductance L then satisfies the following equation:

A maximum value Lmaxof the inductance L, also known as the maximum inductance Lmax, therefore satisfies the following equation:

For a respective step-down configuration, the maximum inductance Lmaxthen satisfies the following equation:

In this example, the maximum value Lmaxthat the inductance L of inductor70must not exceed is then deduced from the minimum time t3-t2calculated previously, while noting that the smaller the value of inductance L, the higher the value of the maximum current IL-CALC-max.

As an example of numerical values, with the total parasitic capacitance Cpara_totfor example equal to 100 pF, an operating frequency of the converter10of 1 MHz, i.e. the period T of the resonance cycle equal to 1 μs, and a minimum duration t3-t2of 50 ns, then the inductance L must be less than the maximum inductance Lmaxequal to 1 μs*50 ns/(4*100 pF), i.e. 125 μH, according to the previous equation (12).

In the variant where the switching aid circuit50is in the form of the inductor70and the diode72connected in series, the current ILis zero at the time t0, and not at the time T/2, and there is therefore approximately twice as much time t0charge the inductor70, namely the time T/2 instead of the time T/4, which leads, for the same value of the maximum current IL-CALC-max, to twice as large a value of the inductance L of the inductor70.

The skilled person will understand that these are orders of magnitude, that a lower value of the inductance L also works, but induces a higher current and therefore higher losses; and that a higher value of the inductance L leads to an incomplete inversion of the voltage Vpbor respectively Vpa, but is still preferable to a total absence of inversion of said voltage Vpbor Vpa.

FIGS.6and7illustrate a second embodiment of the converter10in which the switching aid circuit50is connected to the first switching bridge30, for example between the first midpoints38of the first two switching branches32.

According to this second embodiment of the converter10, the difference from the previously described embodiment is that the switching aid circuit50is then connected to the first switching bridge30instead of being connected to the second switching bridge40according to the first embodiment. The other elements which are unchanged between the first embodiment and the second embodiment are repeated with identical references.

The operation of the converter10in the example ofFIG.6will now be explained according to two step-up configurations, namely a first step-up configuration E1 and a second step-up configuration E2 as shown inFIG.7. The difference resulting from the switching aid circuit50according to the invention relates to the changes in the voltages Vpaand Vpbbetween the times t0and t1in the case of these step-up configurations E1, E2, and more particularly to the areas shown as dotted lines inFIG.7to mark the difference.

The conversion cycle of the converter10according to the invention is described below for the first E1 and second E2 step-up configurations, focusing on the differences with respect to the conversion cycle of a converter10of the prior art for the same step-up configurations.

For the previously described step-down configurations A1 and A2, it was the voltage Vpbthat had a homogeneous polarity on each of the two half-periods with a polarity inversion between the two half-periods, and it was therefore preferable to have the switching aid circuit50on the Vpbside, i.e. connected to the second switching bridge40, between the respective second midpoints48.

This time, for these step-up configurations E1 and E2, it is the voltage Vpathat has a homogeneous polarity on each of the two half-periods with a polarity inversion between the two half-periods (same polarity on the Vaand Vcsteps and opposite polarity on the step Vb). For these step-up configurations E1 and E2, it is therefore preferable to arrange the switching aid circuit50on the voltage side Vpa, i.e. connected to the first switching bridge30, between the respective first midpoints38. This arrangement of the switching aid circuit50on the voltage Vpaside is preferable except in the case where the switching aid circuit50is in the form of the inductor70and the diode72connected in series, where the arrangement on the voltage Vpbside (i.e. connected to the second switching bridge40, between the respective second midpoints48) is still preferable so that the DC component, if any, does not turn the diode72to the on state.

For the first step-up configuration E1, between the times t3and T (or t0), according to the embodiment of the switching aid circuit50, the inductor70or the additional piezoelectric element76sees its current ICALCincrease, under the voltage Vpaequal to +Vin. At time T (or t0), the current ICALCis positive.

Just before time t0, the total piezoelectric voltage Vpis equal to Vin−Vout, the voltage Vpabeing equal to Vin, and the voltage Vpbbeing equal to −Vout; and the switches K6, K7, K2, K3are closed.

At time t0, all switches that were closed open. The current ICALCthen charges the parasitic capacitances of switches K6, K7, while discharging the parasitic capacitances of switches K5and K8. Similarly, through the slowly evolving piezoelectric assemblies12, the current ICALCpartially charges the parasitic capacitances of switches K2, K3, while partially discharging the parasitic capacitances of switches K1, K4. The voltage Vpathus changes from +Vinto −Vin, while the voltage Vpbchanges substantially from −Voutto −Vout+2Vinplus the change of the total piezoelectric voltage Vpsince time t0.

The voltage inversion Vpais considered completed before the total piezoelectric voltage Vpreaches the next step Vb. Indeed, even if the amplitude of the current ICALCin the switching aid circuit50is much smaller than the amplitude of the internal current ILof the piezoelectric elements15(for example at least 3 times smaller to limit its size), it nevertheless only has to charge/discharge the parasitic capacitances of the switches36,46considered much smaller than the reference capacitance C0of the piezoelectric elements15(at least a factor of 3). Furthermore, the current ICALCin the switching aid circuit50approaches its maximum value at time t0, while the internal current ILapproaches 0 at said time t0.

Once complete reversal of the voltage Vpais reached (from Vinto −Vin), then switches K5and K8are closed so that the voltage Vpais fixed, while the voltage Vpbcontinues to rise to Voutdue to the natural increase in the total piezoelectric voltage Vp.

At time t1, switches K1and K4are closed. Switches K5and K8are also closed if this has not been done already, i.e. if the voltage Vpahas not yet reached −Vin.

In addition, if the switches K1and K4have an intrinsic reverse diode or an additional diode in parallel, they can be switched on naturally according to the sign of the internal current ILafter the time t0.

The remainder of the conversion cycle of the converter10according to the invention remains substantially unchanged from the conversion cycle of the prior art.

The voltage swing of the total piezoelectric voltage Vpis thus limited between the times t5and t1, ranging from Vin−Voutto Vout−Vin, instead of from −Vin−Voutto Vout−Vinwith the converter of the prior art, i.e. a swing of 2Vout-2Vininstead of 2Vout, while ensuring zero-voltage switching of switches36,46.

For the second step-up configuration E2, between the times t3and T (or t0), according to the embodiment of the switching aid circuit50, the inductor70or the additional piezoelectric element76sees its current ICALCdecrease, under the voltage Vpaequal to −Vin. At time T (or t0), the current ICALCis negative.

Just before time t0, the total piezoelectric voltage Vpis equal to −Vin+Vout, the voltage Vpabeing equal to −Vin, and the voltage Vpbbeing equal to +Vout; and the switches K5, K8, K1, K4are closed.

At time t0, all switches that were closed open. The current ICALCthen charges the parasitic capacitances of switches K5, K8, while discharging the parasitic capacitances of switches K6and K7. Similarly, through the piezoelectric assemblies12whose voltage is changing slowly, the current ICALCpartially charges the parasitic capacitances of switches K1, K4, while partially discharging the parasitic capacitances of switches K2, K3. The voltage Vpathus changes from −Vinto +Vin, while the voltage Vpbchanges substantially from Voutto Vout−2Vinplus the change of the total piezoelectric voltage Vpsince time t0.

The voltage inversion Vpais considered completed before the total piezoelectric voltage Vpreaches the next step Vb. Indeed, even if the amplitude of the current ICALCin the switching aid circuit50is much smaller than the amplitude of the internal current ILof the piezoelectric elements15(for example at least 3 times smaller to limit its size), it nevertheless only has to charge/discharge the parasitic capacitances of the switches36,46considered much smaller than the reference capacitance C0of the piezoelectric elements15(at least a factor of 3). Furthermore, the current ICALCin the switching aid circuit50approaches its maximum value (its negative extreme) at time t0, while the internal current ILapproaches 0 at said time t0.

Once complete reversal of the voltage Vpais reached (from −Vinto +Vin), then switches K6and K7are closed so that the voltage Vpais fixed, while the voltage Vpbcontinues to rise to Voutdue to the natural decrease in the total piezoelectric voltage Vp.

At time t1, switches K2and K3are closed. Switches K6and K7are also closed if this has not been done already, i.e. if the voltage Vpahas not yet reached +Vin.

In addition, if the switches K2and K3have an intrinsic reverse diode or an additional diode in parallel, they can be switched on naturally according to the sign of the internal current ILafter the time t0.

The remainder of the conversion cycle of the converter10according to the invention remains substantially unchanged from the conversion cycle of the prior art.

The voltage swing of the total piezoelectric voltage Vpis thus limited between the times t5and t1, ranging from Vout−Vinto Vin−Vout, instead of from +Vin+Voutto Vin−Voutwith the converter of the prior art, i.e. a swing of 2Vout−2Vininstead of 2Vout, while ensuring zero-voltage switching of switches36,46.

In the example shown inFIG.5, the switching aid circuit50has the inductor70and the diode72connected in series, and the diode72is oriented according to the direction in which the voltage is to be varied. For example, in the first step-down configuration A1, to change the voltage Vpbfrom +Voutto −Vout, the diode72as shown inFIG.5is in the wrong direction. For another example, in the second step-down configuration A2, to change the voltage Vpbfrom −Voutto +Vout, the diode72as shown inFIG.5is in the right direction. The anode of diode72must be on the positive terminal side of the voltage to be changed and its cathode on the negative terminal side of the voltage to be changed. Finally, once the orientation of the diode72has been determined, it is necessary to ensure that the diode72can block any DC component of the voltage, i.e. in this case the average value of the voltage Vpb, i.e. check that the anode is on the side of the negative terminal of this DC component and that the cathode is on the side of the positive terminal of this DC component. In the example shown inFIG.5, the orientation of diode72allows a positive DC component for the voltage Vpb. If the diode is ever in the wrong direction with respect to this DC component, then the same must be done on the other switching bridge, and the switching aid circuit50must be connected to the other switching bridge.

This example ofFIG.5with the switching aid circuit50in the form of the inductor70and the diode72connected in series, then corresponds to the case where the arrangement on the voltage side Vpb(i.e. connected to the second switching bridge40, between the respective second midpoints48) remains preferable so that the possible positive DC component does not make the diode72pass (case of the step-down A2 or step-up E1 configurations; or the case of the step-down A1 or step-up E2 configurations if the direction of the diode72inFIG.5is reversed).

For the first step-up configuration E1, between the times t0and t1, the voltage Vpbchanges from −Voutto +Vout. To contribute to this, the current ICALCmust be negative, and the diode72is then arranged in the opposite direction to the direction conventionally taken for the current ICALC. For the second step-up configuration E2, the voltage Vpbchanges from −Voutto +Vout, and the diode72is then arranged in the other direction.

For the first step-up configuration E1, between the times t5and T (or t0), according to the embodiment of the switching aid circuit50, the inductor70or the additional piezoelectric element76sees its current ICALCdecrease, under the voltage Vpbequal to −Vout. At time T (or t0), the current ICALCis negative.

Just before time t0, the total piezoelectric voltage Vpis equal to Vin−Vout, the voltage Vpabeing equal to Vin, and the voltage Vpbbeing equal to −Vout; and the switches K6, K7, K2, K3are closed.

At time t0, all switches that were closed open. The current ICALCthen partially charges the parasitic capacitances of switches K2, K3, while partially discharging the parasitic capacitances of switches K1and K4. Similarly, through the piezoelectric assemblies12whose voltage is changing slowly, the current ICALCcharges the parasitic capacitances of switches K6, K7, while discharging the parasitic capacitances of switches K5, K8. The voltage Vpathus changes from +Vinto −Vin, while the voltage Vpbchanges substantially from −Voutto −Vout+2Vinplus the change of the total piezoelectric voltage Vpsince time t0. The voltage Vpbcannot be reversed completely because as the voltage Vinis lower than the voltage Vout, the voltage Vpareaches −Vinbefore the voltage Vpbreaches +Vout.

The voltage inversion Vpais considered completed before the total piezoelectric voltage Vpreaches the next step Vb. Indeed, even if the amplitude of the current ICALCin the switching aid circuit50is much smaller than the amplitude of the internal current ILof the piezoelectric elements15(for example at least 3 times smaller to limit its size), it nevertheless only has to charge/discharge the parasitic capacitances of the switches36,46considered much smaller than the reference capacitance C0of the piezoelectric elements15(at least a factor of 3). Furthermore, the current ICALCin the switching aid circuit50approaches its maximum value (its negative extreme) at time t0, while the internal current ILapproaches 0 at said time t0.

Once complete reversal of the voltage Vpais reached (from Vinto −Vin), then switches K5and K8are closed so that the voltage Vpais fixed, while the voltage Vpbcontinues to rise to Voutdue to the natural increase in the total piezoelectric voltage Vp.

At time t1, switches K1and K4are closed. Switches K5and K8are also closed if this has not been done already, i.e. if the voltage Vpahas not yet reached −Vin.

In addition, if the switches K1and K4have an intrinsic reverse diode or an additional diode in parallel, they can be switched on naturally according to the sign of the internal current ILafter time t0, or according to the residual current ICALCbefore time t0.

The remainder of the conversion cycle of the converter10according to the invention remains substantially unchanged from the conversion cycle of the prior art. In practice, the energy remaining in the inductor70is released after time t1at voltage Voutuntil the current in the inductor70is cancelled out and diode72is blocked.

The voltage swing of the total piezoelectric voltage Vpis thus limited, ranging from Vin−Voutto Vin−Vout, instead of from −Vin−Voutto +Vin+Voutwith the converter of the prior art, i.e. a swing of 2Voutinstead of 2Vin+2Vout, while ensuring zero-voltage switching of switches36,46.

For the second step-up configuration E2, the operation of the converter10with the switching aid circuit50in the form of the inductor70and the diode72is similar to that described with respect toFIG.7for the second step-up configuration E2, but with the diode72facing in the other direction.

As an optional addition, when the converter10comprises two switching aid circuits50, with the first switching aid circuit connected to the first switching bridge30and the second switching aid circuit connected to the second switching bridge40, then the switching aid circuits50being arranged on both the voltage Vpaside and the voltage Vpbside, the effects of the two switching aid circuits50add up. The only thing to do is to choose the switching aid circuit50according to the presence or absence of a DC component, and possibly according to the sign of this DC component if there is one.

The example inFIG.8corresponds to the particular case where the output voltage Voutis substantially equal to the input voltage Vin, and the converter10is essentially used to isolate the output Voutfrom the input Vin, without changing its amplitude. In this case, there are only two voltage levels, namely Vin−Voutand Vout−Vin. The principle of the switching aid circuit50still applies, and the switching aid circuit(s)50are then suitable to be arranged on the Vpavoltage side and/or on the Vpbvoltage side, the two voltages Vpaand Vpbbeing substantially symmetrical. In other words, in this case, the first switching aid circuit is connected to the first switching bridge30and/or the second switching aid circuit is connected to the second switching bridge40.

In practice, to compensate for losses, the input voltage Vinis slightly higher than the output voltage Vout. To improve the readability ofFIG.8, this gap has been slightly exaggerated inFIG.8.

In the example shown inFIG.8, the topology is bidirectional, and electrical energy is also transferable from the output to the input. In the latter case, the difference between the input voltage Vinand the output voltage Voutwill be in the other direction. However, due to the perfect symmetry of the converter10in this case, operation in the other direction is simply a matter of reversing the input and output of the converter10and applying the above lessons.

In the case ofFIG.8, the step Vchas disappeared. The time t5is then merged with the time t6, and the duration between the times t4and t5allows the polarities of the two voltages Vpaand Vpbto be reversed. Similarly, the duration between the times t2and t3allows the polarities of the two voltages Vpaand Vpbto be reversed. The switching aid(s)50then allow the voltages Vpaand Vpbto be inverted without having the total piezoelectric voltage Vpswing to Vin+Vout, or −Vin−Vout.

The switching aid circuit50then allows the voltage swing to be drastically limited, especially as the output voltage Voutis close to the input voltage Vin.

FIGS.9and10illustrate a third embodiment of the converter10in which the first30and second40switching bridges each comprise a single switching branch32,42, whereas according to the first and second embodiments described above, the first30and second40switching bridges each comprise two switching branches32,42. According to this third embodiment, the converter10comprises a single piezoelectric assembly12connected between the first30and second40switching bridges, and the total piezoelectric voltage Vpis then the voltage across said piezoelectric assembly12.

In the example shown inFIG.9, the switching aid circuit50is connected to the first switching bridge30, typically between the first midpoint38and one end of the single first switching branch32. In this example, the switching aid circuit50is then placed on the side of the voltage Vpa.

In the example shown inFIG.9, the switching aid circuit50is particularly connected between the first midpoint38and the application terminal34with lower potential Vinn. It is further noted that the potentials Vpa2and Vpb2are directly connected to each other, and that the lower potentials Vinnand Voutnof the application34and supply44terminals are also connected to each other.

Alternatively, not shown, the switching aid circuit50is connected to the second switching bridge40, typically between the second midpoint48and one end of the single second switching branch42. In this alternative, the switching aid circuit50is then placed on the side of the voltage Vpb.

Other elements which are unchanged from the first and second embodiments described above are repeated with identical references.

As an optional complement, the converter10comprises an auxiliary capacitor, not shown, connected between the first30and second40switching bridges, typically between a respective first midpoint38and second midpoint48, preferably between the first midpoint38and the second midpoint48to which the piezoelectric assembly12is not directly connected. The auxiliary capacitor is typically larger in capacitance, preferably at least three times larger, than the reference capacitance C0of the piezoelectric element(s)15of the piezoelectric assembly12.

According to this third embodiment, the switching aid circuit50is preferably in the form of the additional piezoelectric element76or in the form of the inductor70and the capacitor74connected in series.

The operation of the converter10in the example ofFIG.9will now be explained for the second step-down configuration, here denoted A2bis, as shown inFIG.10. The difference resulting from the switching aid circuit50according to the invention relates to the changes in the voltages Vpaand Vpbbetween the times t2and t3, and more particularly to the areas shown as dotted lines inFIG.10to mark the difference.

The conversion cycle of the converter10according to the invention is described below for the second step-down configuration A2bis, focusing on the differences with respect to the conversion cycle of a converter of the prior art for the same step-down configuration.

According to this third embodiment, the aim is not to invert the voltage Vpaand the voltage Vpb, but simply to change it from Vinto 0 or vice versa for the voltage Vpa, and respectively from 0 to Voutor vice versa for the voltage Vpb.

For the second step-down configuration A2bis, between the times t1and t2, according to the example of the switching aid circuit50, the inductor70or the additional piezoelectric element76sees its current ICALCincrease, under the voltage Vpaequal to Vin. At time t2, the current ICALCis positive.

Just before time t2, the total piezoelectric voltage Vpis equal to Vin−Vout, the voltage Vpabeing equal to Vin, and the voltage Vpbbeing equal to −Vout; and the switches K6, K2are closed.

At time t2, all switches that were closed open. The current ICALCthen partially charges the parasitic capacitance of the switch K6, while partially discharging the parasitic capacitance of the switch K5. Similarly, through the piezoelectric assembly12whose voltage is changing slowly, the current ICALCcharges the parasitic capacitance of the switch K2, while discharging the parasitic capacitances of the switch K1. The voltage Vpbthus changes from −Voutto 0, while the voltage Vpachanges substantially from +Vinto +Vin−Voutplus the change of the total piezoelectric voltage Vpsince time t2.

The voltage inversion Vpbis considered completed before the total piezoelectric voltage Vpreaches the next step Va. Indeed, even if the amplitude of the current ICALCin the switching aid circuit50is much smaller than the amplitude of the internal current ILof the piezoelectric assembly12(for example at least 3 times smaller to limit its size), it nevertheless only has to charge/discharge the parasitic capacitance of the switch36,46considered much smaller than the reference capacitance C0of the piezoelectric assembly12(at least a factor of 3). Furthermore, the current ICALCin the switching aid circuit50approaches its maximum value at time t3, while the internal current ILapproaches 0 at said time t3.

Once complete reversal of the voltage Vpbis achieved (from −Voutto 0), then the switches K1is closed so that the voltage Vpbis fixed, while the voltage Vpacontinues to rise to 0 due to the natural decrease in the total piezoelectric voltage Vp.

At time t3, the switch K5is closed. The switch K1is also closed if this has not been done already, i.e. if the voltage Vpbhas not yet reached 0.

In addition, if the switch K1has an intrinsic reverse diode or an additional diode in parallel, it can be switched on naturally according to the sign of the internal current ILafter time t3, or according to the residual current ICALCbefore time t3.

The remainder of the conversion cycle of the converter10according to the invention remains substantially unchanged from the conversion cycle of the prior art.

The skilled person will observe that in the example ofFIGS.9and10, the switches K1and K2can be simple diodes, which then open and close naturally, i.e. without needing to be controlled by the control device20.

The skilled person will understand that the first step-down configuration, noted here as A1 bis, ofFIG.10, corresponds to the case where the first piezoelectric assembly has been removed, instead of the second piezoelectric assembly as in the example ofFIG.9, and that the switches K8, K4have been short-circuited instead of the switches K7, K3.

In other words, in this case, the switching aid circuit50is particularly connected between the first midpoint38and the application terminal34with greater potential Vinp. It is further noted that the potentials Vpa1and Vpb1are directly connected to each other, and that the greater potentials Vinpand Voutpof the application34and supply44terminals are also connected to each other.

For the first step-down configuration A1 bis, between the times t1and t2, according to the example of the switching aid circuit50, the inductor70or the additional piezoelectric element76sees its current ICALCdecrease, under the voltage Vpaequal to −Vin. At time t2, the current ICALCis negative.

Just before time t2, the total piezoelectric voltage Vpis equal to Vout−Vin, the voltage Vpabeing equal to −Vin, and the voltage Vpbbeing equal to Vout; and the switches K5, K1are closed.

At time t2, all switches that were closed open. The current ICALCthen partially charges the parasitic capacitance of the switch K5, while partially discharging the parasitic capacitance of the switch K6. Similarly, through the piezoelectric assembly12whose voltage is changing slowly, the current ICALCcharges the parasitic capacitance of the switch K1, while discharging the parasitic capacitances of the switch K2. The voltage Vpbthus changes from +Voutto 0, while the voltage Vpachanges substantially from −Vinto −Vin+Voutplus the change of the total piezoelectric voltage Vpsince time t2.

The change in voltage Vpbis considered completed before the total piezoelectric voltage Vpreaches the next step Va. Indeed, even if the amplitude of the current ICALCin the switching aid circuit50is much smaller than the amplitude of the internal current ILof the piezoelectric assembly12(for example at least 3 times smaller to limit its size), it nevertheless only has to charge/discharge the parasitic capacitance of the switch36,46considered much smaller than the reference capacitance C0of the piezoelectric assembly12(at least a factor of 3). Furthermore, the current ICALCin the switching aid circuit50approaches its maximum value (its negative extreme) at time t3, while the internal current ILapproaches 0 at said time t3.

Once the voltage Vpbis has completely changed (from +Voutto 0), then the switch K2is closed so that the voltage Vpbis fixed, while the voltage Vpacontinues to rise to 0 due to the natural increase in the total piezoelectric voltage Vp.

At time t3, the switch K6is closed. The switch K2is also closed if this has not been done already, i.e. if the voltage Vpbhas not yet reached 0.

In addition, if the switch K2has an intrinsic reverse diode or an additional diode in parallel, it can be switched on naturally according to the sign of the internal current ILafter time t3, or according to the residual current ICALCbefore time t3.

The remainder of the conversion cycle of the converter10according to the invention remains substantially unchanged from the conversion cycle of the prior art.

It is thus conceivable that the electrical energy converter10according to the invention offers improved control through the switching aid circuit50.

Indeed, the at least one piezoelectric assembly12has a capacitive behaviour which induces a slow variation of its voltage, i.e. of the total piezoelectric voltage Vp. The search for zero voltage switching, or ZVS, via the natural change in the total piezoelectric voltage Vptowards Vin+Voutand/or −Vin−Voutpotentials has a cost in terms of duration, with the time period during which there is no power exchange, and also a cost in terms of the over-amplitude necessary on the current ILto go and find these extreme points of the total piezoelectric voltage Vp.

The switching aid circuit50thus provides a significant improvement to these problems of the prior art electrical energy converter.