DE102023200560A1 - Drive circuit for operating a microelectroacoustic transducer device and method for operating a microelectroacoustic transducer device by means of a drive circuit - Google Patents

Abstract

In a method for operating a microelectroacoustic transducer device (19), a coil (8) of a drive circuit (1) is first charged by storing electrical energy in a magnetic field of the coil (8). This energy is used to charge a piezoelectric capacitance (13) of the microelectroacoustic transducer device (19). A storage capacitor (7) is charged by discharging the piezoelectric capacitance (13). The storage capacitor (7) is discharged in order to recharge the piezoelectric capacitance (13). The transfer of electrical energy between the storage capacitor (7) and the piezoelectric capacitance (13) is mediated by the coil (8).

Description

The present invention relates to a drive circuit for operating a microelectroacoustic transducer device and a method for operating a microelectroacoustic transducer device by means of a drive circuit.

Piezoelectric micromachined ultrasonic transducers (PMUTs), also referred to below as microelectroacoustic transducer devices, are finding increasingly widespread use in the generation and detection of ultrasonic signals. They are used, among other things, in medical imaging procedures, in non-destructive material testing using acoustic microscopy, and in ultrasonic radar systems for applications in mobility and for intelligent industrial machines. From the publications EN 10 2007 052 887 A1 and CN109952767A Drive circuits for microelectroacoustic transducer devices are known.

PMUTs have the decisive advantage that they can be produced cost-effectively in large quantities as microelectromechanical systems (MEMS) using microsystem technology processes, and at the same time are smaller, easier to integrate and more efficient than solid piezoelectric transducers. While the functional principle of the latter is based on the expansion or contraction of a plate made of piezoelectric ceramic when an electrical voltage is applied, PMUTs use the bending movement of a membrane on which a thin piezoelectric film has been applied. An electrical voltage leads to a lateral expansion or contraction of the piezoelectric film, which leads to a deformation of the membrane connected to it.

By repeatedly increasing and decreasing the voltage on the piezo film, the membrane can be set into vibration and caused to emit ultrasonic waves. Conversely, the excitation of the membrane by another sound source leads to the generation of an alternating voltage on the piezoelectric film, which can be measured and thus used to detect acoustic signals. For example, it is possible to determine the distance to an object based on the travel time by emitting a sound signal and then detecting the wave reflected from this object.

The use of thin piezo films for the actuators of a membrane has many advantages (cost reduction, miniaturization, etc.), but the difficulty is that under certain circumstances they can only be operated sensibly with a unipolar voltage - i.e. with either a purely positive or purely negative voltage. The need for operation with a unipolar voltage can arise particularly with piezoelectric films made of ferroelectric materials. Compared to other piezoelectrics such as aluminum nitride (AIN) or zinc oxide (ZnO), ferroelectrics such as lead zirconate titanate (PZT) typically show a more pronounced piezoelectric response, which is advantageous at least with regard to transmission power and reception accuracy. At the same time, ferroelectrics often exhibit undesirable hysteresis behavior.

If a certain positive maximum voltage is applied to the ferroelectric piezo film, the expansion of the film is also at its maximum. If the voltage is gradually reduced, the expansion of the film also decreases. If the voltage disappears, the piezo film still shows a small expansion due to remanent polarization. With a small negative voltage, this polarization is canceled and the piezo film shows no deformation. If the voltage is now reduced further, the piezo film contracts until the majority of the dipoles of the ferroelectric have been repolarized. Further reduction of the voltage to a negative minimum voltage then leads to an expansion of the piezo film again. If the voltage is now increased again to its positive maximum value, a contraction initially occurs again until the dipoles are repolarized at a positive voltage, before finally the maximum expansion is reached again.

One way to avoid the problem of repolarization and the resulting hysteresis is to apply a unipolar alternating voltage to the piezo film. In order to still cause the membrane to deflect in different directions, a second piezo film can be applied to it, which is operated in antiphase to the first piezo film. The thicker the piezo film, the greater the amount of voltage at which repolarization occurs. Classic ultrasound transducers use sufficiently solid piezo films that can be operated with a bipolar alternating voltage without significant hysteresis behavior occurring.

As will be shown below, a direct drive of the piezo film with a unipolar voltage is affected by potentially very significant reactive currents and reactive power losses, which cannot be easily compensated. For example, a sinusoidal alternating voltage of the form U (t) = Û cos (ωt) can be applied to the piezo film. Electrically speaking, the piezo film behaves essentially like a capacitor with a capacitance C _P . The current through the piezo film is therefore $$I_P\left(t\right)=C_P\frac{dU\left(t\right)}{dt}=-\omega {\text{C}}_P\widehat{\text{U}}\text{sin}\left(\omega t\right).$$

If it is assumed that the ohmic losses are described by a resistor R connected in parallel to the piezo film, the current through this resistor is therefore $$I_R\left(t\right)=\frac{U\left(t\right)}{R}=\frac{\widehat{\text{U}}}{R}\text{cos}\left(\omega t\right).$$

und die elektrische Gesamtleistung wird zu $$P\left(t\right)=U\left(t\right)I\left(t\right)={\widehat{\text{U}}}^2/Rco{s}^2\left(\omega t\right)-\omega C_P{\widehat{\text{U}}}^2\text{ sin}\left(\omega t\right)\text{cos}\left(\omega t\right)=P_R\left(t\right)+P_P\left(t\right).$$

The total current I, which must be supplied by the voltage source, is therefore $$I\left(t\right)=I_R\left(t\right)+I_P\left(t\right)=\frac{\widehat{\text{U}}}{R}\text{cos}\left(\omega t\right)-\omega C_P\widehat{\text{U}}\text{sin}\left(\omega t\right),$$

and the total electrical power becomes $$P\left(t\right)=U\left(t\right)I\left(t\right)={\widehat{\text{U}}}^2/RcO{s}^2\left(\omega t\right)-\omega C_P{\widehat{\text{U}}}^2\text{sin}\left(\omega t\right)\text{cos}\left(\omega t\right)=P_R\left(t\right)+P_P\left(t\right).$$

While the ohmic active power P _R (t) = Û ² /Rcos ² (ωt) is purely positive, the capacitive reactive power P _P (t) = -ωC _P Û ² sin (ωt) cos (ωt) changes its sign over the course of an oscillation period. The piezo film therefore feeds its stored energy back into the voltage source within one period. However, if the voltage source is not designed to reuse this fed-back energy, it is dissipated as waste heat.

ergibt. Der Strom durch die Spule kompensiert also gerade den Strom durch den Piezofilm und der Gesamtstrom $$I\left(t\right)=I_R\left(t\right)+I_P\left(t\right)+I_L\left(t\right)=I_R\left(t\right)=\frac{\widehat{\text{U}}}{R}cos\left(\omega t\right)$$

reduziert sich somit auf den unvermeidbaren Wirkstrom durch den ohmschen Widerstand. Ebenso reduziert sich die Gesamtleistung $$P\left(t\right)=U\left(t\right)I\left(t\right)={\widehat{\text{U}}}^2/Rco{s}^2\left(\omega t\right)$$

auf die am Widerstand dissipierte Wirkleistung.In this case, it is therefore necessary to eliminate the capacitive reactive power in the overall balance for the most efficient operation. Typically, this reactive power compensation is implemented with a coil of inductance L=1/(ω ² C _P ) connected in parallel to the capacitance of the piezo film C _P . The current I _L (t) through the coil is U (t) =L dI _L (t)/dt, whereby with the initial condition I _L (0)=0, by integration $$I_L\left(t\right)=I_L\left(0\right)+\frac{1}{L}{\displaystyle \int {}_0^t}U\left(t\text{'}\right)dt\text{'}=\frac{\widehat{U}}{L}{\displaystyle \int {}_0^t}cOs\left(\omega t\text{'}\right)dt\text{'}=\frac{\widehat{U}}{\omega L}sin\left(\omega t\right)=\omega C_P\widehat{U}\text{sin}\left(\omega t\right)=-I_P\left(t\right)$$

The current through the coil just compensates the current through the piezo film and the total current $$I\left(t\right)=I_R\left(t\right)+I_P\left(t\right)+I_L\left(t\right)=I_R\left(t\right)=\frac{\widehat{\text{U}}}{R}cOs\left(\omega t\right)$$

is thus reduced to the unavoidable active current through the ohmic resistance. The total power is also reduced $$P\left(t\right)=U\left(t\right)I\left(t\right)={\widehat{\text{U}}}^2/RcO{s}^2\left(\omega t\right)$$

on the active power dissipated at the resistor.

Let the piezo film be such that it must be driven, for example, with a unipolar sinusoidal alternating voltage of the form U (t) =U ₀ + Û cos (ωt), where |Uo| ≥ |Û| ensures that its sign does not change. However, if we now consider the currents through the individual components, it turns out that the approach of a coil connected in parallel to the capacitance of the piezo film C _P with the inductance L=1/(ω ² C _P ) does not lead to success. The current through the resistor now amounts to $$I_R\left(t\right)=\frac{U\left(t\right)}{R}={\text{<figure-callout id="0" label="U" filenames="DE102023200560A1\_0018.png" state="{{state}}">U</figure-callout>}}_0/\text{R}+\widehat{\text{U}}\text{/R cos}\left(\omega t\right).$$

da der konstante Gleichspannungsanteil bei der Zeitableitung wegfällt. Für den Spulenstrom I_L(t) muss die Quellspannung jedoch wieder nach der Zeit integriert werden, und man findet mit der Anfangsbedingung I_L(t)=0, dass $$\begin{array}{l}{I}_L\left(t\right)=I_L\left(0\right)+\frac{1}{L}{\displaystyle \int {}_0^t}U\left(t\text{'}\right)dt\text{'}=\frac{1}{L}{\displaystyle \int {}_0^t}\left[U_0+\widehat{U}\text{ cos}\left(\omega t\text{'}\right)\right]dt\text{'}=\frac{U_0}{L}t+\frac{\widehat{U}}{\omega L}\text{sin}\left(\omega t\right)\\ ={\omega }^2{C}_P{U}_{0}t+\omega C_P\widehat{U}\text{ sin}\left(\omega t\right)={\omega }^2{C}_P{U}_{0}t-I_P\left(t\right).\end{array}$$

The current through the piezo film is as before $$I_P\left(t\right)=C_P\frac{dU\left(t\right)}{dt}=-\omega C_P\widehat{\text{U}}\text{sin}\left(\omega t\right),$$

because the constant DC voltage component is eliminated in the time derivative. For the coil current I _L (t), however, the source voltage must be integrated again with respect to time, and with the initial condition I _L (t)=0, one finds that $$\begin{array}{l}{I}_L\left(t\right)=I_L\left(0\right)+\frac{1}{L}{\displaystyle \int {}_0^t}U\left(t\text{'}\right)dt\text{'}=\frac{1}{L}{\displaystyle \int {}_0^t}\left[{\mathrm{<figure-callout\; id="0"\; label="U"\; filenames="DE102023200560A1\_0018.png"\; state="\{\{state\}\}">U</figure-callout>}}_0+\widehat{U}\text{cos}\left(\omega t\text{'}\right)\right]dt\text{'}=\frac{U_0}{L}t+\frac{\widehat{U}}{\omega L}\text{sin}\left(\omega t\right)\\ ={\omega }^2{C}_P{U}_{0}t+\omega C_P\widehat{U}\text{sin}\left(\omega t\right)={\omega }^2{C}_P{U}_{0}t-I_P\left(t\right).\end{array}$$

In the overall balance, one finds that the current through the piezo film is compensated by the coil, but the coil supplies an additional, linearly increasing current ω ² C _P U ₀ t due to the DC component: $$I\left(t\right)=I_R\left(t\right)+I_P\left(t\right)+I_L\left(t\right)={\text{<figure-callout id="0" label="U" filenames="DE102023200560A1\_0018.png" state="{{state}}">U</figure-callout>}}_0/\text{R}+\widehat{\text{U}}\text{/R cos}\left(\omega t\right)+{\omega }^2{C}_P{U}_{0}t.$$

Due to this increasing coil current, this type of reactive power compensation is completely unsuitable if the piezo film is to be driven with a unipolar alternating voltage. In fact, it even leads to an increase in current and power consumption. One object of the invention described here is to enable a piezo film or piezo element to be driven with a unipolar alternating voltage with as little loss as possible.

In addition to high reactive currents, other difficulties often arise in practice. For example, the drive voltage often has to be converted or increased by a voltage converter (e.g. a transformer) (e.g. from an 8V direct current to an 80V alternating current) using a voltage converter (e.g. in the automotive sector). In addition, the energy source (in the automobile a 12V battery) has to be stabilized by another voltage regulator to prevent performance fluctuations.

Due to the necessary design of the components (e.g. the transformer), the transmission frequency is limited to a narrow range of around 50 kHz (e.g. 40 to 60 kHz in the automotive sector). In addition, an extension to several independently oscillating ultrasonic transducers (arrays) is only possible with a significantly higher circuit complexity. For example, an additional transformer would have to be introduced for each ultrasonic transducer.

One object of the present invention is to provide an improved drive circuit for operating a microelectroacoustic transducer device and to specify an improved method for operating a microelectroacoustic transducer device by means of a drive circuit. This object is achieved by a drive circuit for operating a microelectroacoustic transducer device and a method for operating a microelectroacoustic transducer device by means of a drive circuit having the features of the respective independent claims. Advantageous further developments are specified in dependent claims.

A drive circuit for operating a microelectroacoustic transducer device has a first and a second supply connection for applying a supply voltage, a first and a second storage connection for connecting to a storage capacitor, a coil, a first, second, third, fourth, fifth and sixth switch, a first and a second diode and a first and a second transducer connection for connecting a piezoelectric capacitance of the microelectroacoustic transducer device to the drive circuit. The first supply connection is connected to the first storage connection and the second storage connection is connected to the first switch. The storage capacitor can be switched between the first supply connection and the first switch. The first switch is connected to the coil. The coil is connected to the second switch and the first diode. The second switch is connected to the second diode. The first diode is connected to the third switch. The second diode and the third switch are connected to the first converter terminal. The first diode and the second diode are connected in opposite directions. The second converter terminal is connected to the first storage terminal. The piezoelectric capacitance can be switched between the first supply terminal and the second diode as well as the third switch via the converter terminals. The fourth switch is connected to the second supply terminal and the coil. The fifth switch is connected to the coil and the first supply terminal. The sixth switch is connected to the second supply terminal and the first converter terminal.

The coil can be charged by closing the fourth and fifth switches using the supply voltage. The piezoelectric capacitance of the microelectroacoustic transducer device can be charged using the charged coil and by closing the second switch and opening the fifth switch. The piezoelectric capacitance can be discharged by opening the second and fourth switches and closing the first and third switches, and a storage capacitance of the storage capacitor can be charged via the coil. The storage capacitance can be discharged by opening the third switch and closing the second switch via the coil, and the piezoelectric capacitance can be recharged.

The drive circuit is based on the idea of transferring energy stored in the piezoelectric capacitance of the microelectroacoustic transducer device via the coil to the storage capacitor and back again with minimal loss. A voltage curve on the piezoelectric capacitance is therefore not specified by an external voltage source, but rather realized by means of an oscillating circuit, with the coil serving as a transfer element for the transfer of electrical energy between the storage capacitor and the piezoelectric capacitance. The drive circuit advantageously makes it possible to apply a unipolar alternating voltage to the piezoelectric capacitance, which means that thin piezo films and/or ferromagnetic piezo elements in particular can be used. At the same time, the microelectroacoustic transducer device can be driven with particularly little loss, since reactive power losses can be avoided. The coil does not function as an energy store and therefore does not have to be dimensioned accordingly. The sole purpose of the coil is to transfer electrical charge from the piezoelectric capacitance to the storage capacitor - and back again - with as little loss as possible, so it can be dimensioned comparatively very small. Essentially, the piezoelectric capacitance and the storage capacitor serve as energy storage, which can save component costs.

In a conventional oscillating circuit, the capacitance and inductance determine an oscillation frequency. In order to be able to vary the transmission frequency, the drive circuit has two parallel elements, each consisting of a diode and a switch. The forward directions of the two diodes are connected in opposite directions and the two switches are never opened at the same time. Closing a switch closes the oscillating circuit, but can only oscillate in one direction because the diode is connected in series with the switch. By timing the switches accordingly, the time interval between two charge transfers can be specified from the outside. The time required for a charge transfer corresponds to half the period of the oscillating circuit. This must be sufficiently short to match the maximum desired drive frequency of the piezo film. However, since the charge transfer time or oscillating circuit period is proportional to the square root of the inductance of the coil, the inductance is primarily limited upwards, but not downwards. This means that even small coils - both in terms of installation space and inductance - can be used, which can lead to considerable cost reduction.

The series connection of the coil with the storage capacitor and the piezoelectric capacitance offers the advantage over a parallel resonant circuit that the inductance of the coil does not have to be matched to the capacitance of the piezoelectric load and the desired drive frequency of the microelectroacoustic transducer device in order to generate the highest possible quality and thus also an alternating voltage with the highest possible amplitude at the piezoelectric capacitance. The series connection allows much greater freedom in the choice of the inductance of the coil. This only has to be small enough so that the recharging process takes place quickly enough to achieve the desired drive frequency. However, it can also be chosen to be significantly smaller than this upper limit without significant losses. This also gives a high degree of freedom with regard to the shape of the drive signal, which is no longer limited to sinusoidal signals.

Since the concept is not tailored to a specific transmission frequency, this can advantageously be selected from a wide range. Algorithms for processing ultrasound signals can use this fact in many ways. One example is better artifact detection in object detection by using previously inaccessible transmission frequencies. Transmission frequencies optimized for the far and near field can also be used, whereas previously it was necessary to decide on one frequency. The range in the far field could be increased in this way, for example.

Another advantage of the drive circuit is that it only requires a low-voltage power source, but still allows the piezoelectric capacitance to be boosted to a much higher voltage. For example, it is possible to use a DC supply voltage of 8V, while the piezoelectric capacitance can be charged to 80V. Essentially, this is done according to the principle of a boost converter. An additional inverter/transformer is therefore superfluous.

In addition, fluctuations in the voltage supply (such as deviation from the nominal value or load-dependent dynamic fluctuations) can be compensated using the drive circuit. This means that no voltage regulator is required to stabilize the voltage supply. Another major advantage is the flexible controllability of the voltage applied to the piezoelectric capacitance. This can be freely adjusted over a wide range, which offers new possibilities in terms of calibration. For example, ongoing calibration of the performance could be implemented to compensate for aging effects.

In one embodiment, a storage capacity of the storage capacitor is as large as the piezoelectric capacity. Advantageously, the electrical energy and the voltage level can thereby be (almost) completely transferred from the piezoelectric capacity to the storage capacity (and back again) during a recharging process, thus avoiding excessive charging of the storage capacity or the piezoelectric capacity.

In one embodiment, the storage capacitor and/or the piezoelectric capacitance and/or the microelectroacoustic transducer device are part of the drive circuit. The storage capacitor, the coil and the piezoelectric capacitance or the entire microelectroacoustic transducer device can be arranged on a circuit board, for example. However, the storage capacitor and/or the piezoelectric capacitance or the microelectroacoustic transducer device do not necessarily have to be part of the drive circuit; they can also be external components that are designed to be connectable to the drive circuit via the storage connections and the transducer connections.

In one embodiment, the storage capacitor is designed as a further piezoelectric capacitance of the microelectroacoustic transducer device or a further microelectroacoustic transducer device. In other words, in this case the piezoelectric capacitance and the further piezoelectric capacitance serve each other as a storage capacitance. Advantageously, the drive circuit is therefore particularly suitable for operating arrays of piezoelectric capacitances, since circuit complexity is significantly reduced. For example, only one coil is required for each two piezoelectric capacitances of an array that serve each other as a storage capacitor. In addition, in particular, antiphase operation of two unipolarly driven piezoelectric capacitances can advantageously be enabled.

A method for operating a microelectroacoustic transducer device by means of a drive circuit according to one of the embodiments comprises the following method steps. A supply voltage is applied to the supply terminals and the fourth and fifth switches are closed, thereby charging the coil. The second switch is closed and the fifth switch is opened, thereby charging the piezoelectric capacitance of the microelectroacoustic transducer device. The second and fourth switches are opened and the first and third switches are closed, thereby discharging the piezoelectric capacitance and charging a storage capacitance of the storage capacitor. The third switch is opened and the second switch is closed, thereby discharging the storage capacitance and recharging the piezoelectric capacitance.

Advantageously, the method involves a transfer of electrical energy between the storage capacitor and the piezoelectric capacitance. If the piezoelectric capacitance reduces its electrical voltage during the contraction phase, the energy released is not converted into a The energy is not fed back into the voltage source, but instead temporarily stored in the storage capacitor for further use. In the next cycle, this energy is then transferred back to the piezoelectric capacitance, stimulating it to expand. This principle of energy recuperation allows the piezoelectric capacitance to be driven extremely efficiently.

In one embodiment, charging of the coil and charging of the piezoelectric capacitance are repeated until the piezoelectric capacitance is charged to a maximum voltage before the storage capacitance is charged. Advantageously, the piezoelectric capacitance can thereby be subjected to a voltage significantly greater than the supply voltage.

In one embodiment, the piezoelectric capacitance is charged or discharged to a minimum voltage after discharging by opening the first and third switches and closing the sixth switch. When the piezoelectric capacitance is recharged, the first switch is closed and the sixth switch is opened while the third switch remains open. The minimum voltage is the applied supply voltage.

In one embodiment, the process steps are repeated cyclically. In one embodiment, the piezoelectric capacitance is subjected to a unipolar alternating voltage. This advantageously allows particularly thin and ferromagnetic piezo films to be driven with low losses.

In one embodiment, a further piezoelectric capacitance of the microelectroacoustic transducer device or of a further microelectroacoustic transducer device is used as storage capacitance. The piezoelectric capacitance and the further piezoelectric capacitance are operated in antiphase. Advantageously, for example, a membrane of the microelectroacoustic transducer device on which the piezoelectric capacitance and the further piezoelectric capacitance are arranged can be deflected in opposite directions. The membranes of different microelectroacoustic transducer devices can also be operated in antiphase.

• 1: eine Antriebsschaltung gemäß einer beispielhaften Ausführungsform in Form eines elektrischen Schaltplans;
• 2 bis 6: verschiedene Zustände der Antriebsschaltung der 1 im Rahmen eines Verfahrens zum Betreiben einer mikroelektroakustischen Wandlervorrichtung;
• 7: einen Spannungsverlauf an einer piezoelektrischen Kapazität einer mikroelektroakustischen Wandlervorrichtung, die gemäß dem Verfahren der 2 bis 6 betrieben wird; und
• 8: eine beispielhafte Umsetzung der Antriebsschaltung der 1 auf einer Leiterplatte.

The drive circuit for operating a microelectroacoustic transducer device and the method for operating a microelectroacoustic transducer device by means of a drive circuit are explained in more detail below in conjunction with schematic drawings. They show:

• 1 : a drive circuit according to an exemplary embodiment in the form of an electrical circuit diagram;
• 2 until 6 : different states of the drive circuit of the 1 in the context of a method for operating a microelectroacoustic transducer device;
• 7 : a voltage curve at a piezoelectric capacitance of a microelectroacoustic transducer device manufactured according to the method of 2 until 6 operated; and
• 8th : an exemplary implementation of the drive circuit of the 1 on a circuit board.

1 shows schematically a drive circuit 1 according to an exemplary embodiment in the form of an electrical circuit diagram.

The drive circuit 1 has a first and a second supply connection 2, 3 for applying a supply voltage 4 and a first and a second storage connection 5, 6 for connecting the drive circuit 1 to a storage capacitor 7. The storage capacitor 7 has a storage capacitance Cs. In the exemplary embodiment of the drive circuit 1 of the 1 the storage capacitor 7 is part of the drive device 1 and for this reason is already connected to the storage terminals 5, 6. However, the storage capacitor 7 can alternatively be an external component that is not part of the drive circuit 1.

The drive circuit 1 further comprises a coil 8 with inductance L, a first, second, third, fourth, fifth and sixth switch S1, S2, S3, S4, S5, S6, a first and second diode 9, 10 and a first and a second converter terminal 11, 12 for connecting a piezoelectric capacitance 13 with a capacitance C _P of a microelectroacoustic converter device to the drive circuit 1. The storage capacitance Cs of the storage capacitor 7 can ideally be as large as the piezoelectric capacitance 13, C _P . In the exemplary embodiment of the drive circuit 1 of the 1 The piezoelectric capacitance 13 is part of the drive device 1 and is therefore already connected to the converter terminals 11, 12. The piezoelectric capacitance 13 or the entire microelectroacoustic However, the converter device may alternatively be an external component that is not part of the drive circuit 1. In the exemplary embodiment of the drive circuit of the 1 However, the storage capacitor 7, the coil 8 and the piezoelectric capacitance 13 are components of the drive circuit 1 and are connected in series with one another. Since the drive circuit 1 does not necessarily have to have the storage capacitor 7 and the piezoelectric capacitance 13, it can alternatively only be designed to connect the coil 8 with the storage capacitor 7 and the piezoelectric capacitance 13 in series.

The storage capacitor 7 can also be a further piezoelectric capacitance of the microelectroacoustic transducer device or of a further microelectroacoustic transducer device. In this case, the further piezoelectric capacitance serves as a storage capacitance. For this reason, it can be regarded as a storage capacitor 7. The piezoelectric capacitance 13 and the further piezoelectric capacitance can, for example, be part of a common microelectroacoustic transducer device, for example as part of an array of piezoelectric capacitances. However, the piezoelectric capacitance 13 and the further piezoelectric capacitance can also be parts of different microelectroacoustic transducer devices.

The first supply connection 2 is grounded and connected to the first storage connection 5 and the second storage connection 6 is connected to the first switch S1, whereby the storage capacitor 7 can be switched between the first supply connection 2 and the first switch S1. The first switch S1 is connected to the coil 8. The coil 8 is connected to the second switch S2 and the first diode 9. The second switch S2 is connected to the second diode 10. The first diode 9 is connected to the third switch S3. The second diode 10 and the third switch S3 are connected to the first converter connection 11. The first diode 9 and the second diode 10 are switched in opposite directions. The second converter connection 12 is connected to the first storage connection 5. The piezoelectric capacitance 13 can be switched between the first supply connection 2 and the second diode 10 as well as the third switch S3 via the converter connections 11, 12. The fourth switch S4 is connected to the second supply terminal 3 and the coil 8. The fifth switch S5 is connected to the coil 8 and the second converter terminal 12. The sixth switch S6 is connected to the second supply terminal 3 and the first converter terminal 11.

In the exemplary embodiment of the 1 drive circuit 1 further comprises a first resistor 14 and a second resistor 15. The first resistor 14 is connected to the converter terminals 11, 12. The first resistor represents effective losses of the microelectroacoustic converter device. The second resistor 15 is connected to or interposed between the sixth switch S6 and the first converter terminal 11. The second resistor 15 serves as a protective resistor, but it can also be omitted.

2 until 6 show schematically different states of the drive circuit 1 of the 1 as part of a method for operating the microelectroacoustic transducer device.

2 shows the drive circuit 1 of the 1 after carrying out a first method step. During this method step, a supply voltage was applied to the supply connections 2, 3. The fourth and fifth switches S4, S5 were closed, while the first, third and sixth switches S1, S3, S6 remained open. As a result, a current flows from the voltage source U ₀ via the fourth switch S4, the coil 8 and the fifth switch S5 to the earthed first supply connection 2. The direct voltage U ₀ is therefore applied to the coil 8 and a coil current I _L (t) = I _L (0) + U ₀ t/L, with an initial current I _L (0), builds up over time t. This accumulates energy in the magnetic field of the coil, which charges the coil.

In the exemplary embodiment of the drive circuit 1 with first resistor 14, the second switch S2 was also closed. As a result, in the state of the drive circuit 1 according to 2 due to the much higher resistance, there is effectively no current via the second switch S2 and the piezoelectric capacitance 13. However, closing the second switch S2 is not absolutely necessary at this time.

3 shows the drive circuit 1 in one of the 2 subsequent state after the execution of a further process step. The second switch S2 was closed or is starting from 2 remained closed. The fifth switch S5 was opened. This charges the piezoelectric capacitance 13 of the microelectroacoustic transducer device. This process step can be carried out if there is enough energy in the magnetic field of the coil 8. This leads to the coil current that has been built up now flowing through the second switch S2 and charging the piezoelectric capacitance 13. The second diode 10 between the second switch S2 and the piezoelectric capacitance 13 prevents the current from flowing in the opposite direction after charging the piezoelectric capacitance 13, since the coil 8 and the piezoelectric capacitance 13 would form an oscillating circuit without the second diode 10. The process steps according to 2 and 3 can be repeated until a desired maximum voltage U _MAX > U ₀ is applied to the piezoelectric capacitance 13.

4 shows the drive circuit 1 of the 3 in a temporally subsequent state after carrying out a further method step. The second switch S2 was opened and the first and third switches S1, S3 were closed after a voltage U _P on the piezoelectric capacitance 13 was held at a desired maximum voltage U _MAX for a predeterminable period of time, as a result of which the piezoelectric capacitance 13 is discharged and the storage capacitance Cs of the storage capacitor 7 is charged. The storage capacitor 7 is therefore charged by means of the piezoelectric capacitance 13, the coil 8 being designed as a transfer element to transfer the electrical energy. The first diode 9 between the coil 8 and the third switch S3 also prevents the current from flowing back. At the end of the recharging process, the voltage U _MAX is almost present on the storage capacitor 7, while the voltage on the piezoelectric capacitance 13 has dropped almost to 0V.

5 shows the drive circuit 1 in one of the 4 temporally subsequent state after the execution of a further method step. However, this method step is only optional. The piezoelectric capacitance 13 is in the state of the drive circuit 1 of the 5 , ie after unloading according to 4 , charged or discharged to a minimum voltage by opening the third switch S3 and closing the sixth switch S6.

6 shows the drive circuit 1 in one of the 4 temporally subsequent state after the execution of a further method step. The third switch S3 was opened and the second switch S2 was closed, whereby the storage capacity Cs is discharged and the piezoelectric capacity 13 is recharged. If the piezoelectric capacity 13 was charged according to 5 charged to the minimum voltage U _MIN = U ₀ , the first switch S1 was also opened and when the piezoelectric capacitance 13 was recharged it was closed again and the sixth switch S6 opened, while the third switch S3 remained open. At the end of the recharging process, the storage capacitor 7 is almost at the minimum voltage U _MIN = U ₀ , while the voltage at the piezoelectric capacitance 13 has risen almost to the maximum voltage U _MAX .

The process steps can be repeated cyclically. The cycle starts again from the beginning, with the difference that now a slightly higher voltage Us than U _MIN is applied to the storage capacitance Cs and a slightly lower voltage U _P than U _MAX is applied to the piezoelectric capacitance C _P (instead of 0V as at the very beginning). When charging the coil 8 and the piezoelectric capacitance 13, only the difference between U _MAX and U _P > 0 needs to be recharged, while when discharging the piezoelectric capacitance 13, the voltage drops almost to U _MIN (instead of 0 V).

7 shows schematically a voltage curve 16 at the piezoelectric capacitance 13, which is determined according to the method of 2 until 6 was operated. In addition, a voltage curve 17 on the storage capacitor 7 is shown. The voltages are given in volts and plotted against time in seconds.

It can be seen that the piezoelectric capacitance 13 was first charged to a maximum voltage and then subjected to a unipolar alternating voltage. The exemplary available supply voltage 4 of Uo = 8V could be used to charge the piezoelectric capacitance 13 to a maximum voltage of approx. 80V. The recharging process, in which electrical energy is transferred between the piezoelectric capacitance 13 and the storage capacitor 7 via the coil 8, is shown in the diagram of the 7 This can be seen from the fact that the voltage curves 16, 17 at the storage capacitor 7 and the piezoelectric capacitance 13 oscillate and are 180° out of phase with each other.

The initial charging process of coil 8 takes place only once in the initial phase, as 7 example for times t<2.7×10 ^-4 s. Once the voltage level has been built up, the energy (and thus also the voltage) is transferred back and forth between the piezoelectric capacitance 13 and the storage capacitor 7 by means of the coil 8 during the drive. These recharging processes are much faster than the initial charging process of the coil 8 and lasts shorter the smaller the inductance of the coil 8. Since the storage capacitor 7 is decoupled from the DC voltage supply by two switches, it can store the energy directly at the same voltage level as the piezoelectric capacitance 13 (e.g. 80V). The principle of the boost converter is only used after the initial charging phase to compensate for any losses. However, since only small voltage drops need to be compensated, the boost converter only needs to be activated for a very short time.

8th shows schematically an exemplary implementation of the drive circuit 1 of the 1 on a printed circuit board (PCB) 18.

Merely by way of example, the storage capacitor 7 is part of the drive circuit 1, while the microelectroacoustic transducer device 19 is also designed as an external component by way of example. For this reason, the storage capacitor 7 is arranged together with the coil 8 on the circuit board 18, while the microelectroacoustic transducer device 19 is not arranged on the circuit board 18. The supply connections 2, 3, the storage connections 5, 6 and the transducer connections 11, 12 are provided on the circuit board 18. The switches S _J are implemented together with a controller 20 for controlling the switches S _J in an application-specific integrated circuit 21 (ASIC). The application-specific integrated circuit 21 also comprises a signal amplifier 22 and a signal processing device 23, which are designed to output a received signal 24 of the microelectroacoustic transducer device 19. The microelectroacoustic transducer device 19 can be operated as a transmitter and/or as a receiver.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• DE 102007052887 A1 [0002]
• CN109952767A [0002]

Claims (10)

Drive circuit (1) for operating a microelectroacoustic transducer device (19), with a first and a second supply connection (2, 3) for applying a supply voltage (4), a first and a second storage connection (5, 6) for connecting to a storage capacitor (7), a coil (8), a first, second, third, fourth, fifth and sixth switch (S1, S2, S3, S4, S5, S6), a first and a second diode (9, 10) and a first and a second converter connection (11, 12) for connecting a piezoelectric capacitance (13) of the microelectroacoustic transducer device (19) to the drive circuit (1), wherein the first supply connection (2) is connected to the first storage connection (5) and the second storage connection (6) is connected to the first switch (S1), wherein the storage capacitor (7) is connected via the storage connections (5, 6) between the first supply connection (2) and the first switch (S1), wherein the first switch (S1) is connected to the coil (8), wherein the coil (8) is connected to the second switch (S2) and the first diode (9), wherein the second switch (S2) is connected to the second diode (10), wherein the first diode (9) is connected to the third switch (S3), wherein the second diode (10) and the third switch (S3) are connected to the first converter connection (11), wherein the first diode (9) and the second diode (10) are switched in opposite directions, wherein the second converter connection (12) is connected to the first storage connection (5), wherein the piezoelectric capacitance (13) is switchable via the converter connections (11, 12) between the first supply connection (2) and the second diode (10) as well as the third switch (S3), wherein the fourth switch (S4) is connected to the second supply connection (3) and the coil (8), wherein the fifth switch (S5) is connected to the coil (8) and the first supply connection (2), wherein the sixth switch (S6) is connected to the second supply connection (3) and the first converter connection (11). Drive circuit (1) according to Claim 1 , wherein a storage capacity of the storage capacitor (7) is as large as the piezoelectric capacity (13). Drive circuit (1) according to one of the preceding claims, wherein the storage capacitor (7) and/or the piezoelectric capacitance (13) and/or the microelectroacoustic transducer device (19) are part of the drive circuit (1). Drive circuit (1) according to one of the preceding claims, wherein the storage capacitor (7) is designed as a further piezoelectric capacitance of the microelectroacoustic transducer device (19) or of a further microelectroacoustic transducer device. Method for operating a microelectroacoustic transducer device (19) by means of a drive circuit (1) according to one of the preceding claims, with the following method steps: - applying a supply voltage (4) to the supply connections (2, 3) and closing the fourth and fifth switches (S4, S5), whereby the coil (8) is charged, - closing the second switch (S2) and opening the fifth switch (S5), whereby the piezoelectric capacitance (13) of the microelectroacoustic transducer device (19) is charged, - opening the second and fourth switches (S2, S4) and closing the first and third switches (S1, S3), whereby the piezoelectric capacitance (13) is discharged and a storage capacitance of the storage capacitor (7) is charged, - opening the third switch (S3) and closing the second switch (S2), whereby the storage capacitance is discharged and the piezoelectric capacitance (13) is charged again. Procedure according to Claim 5 , wherein the charging of the coil (8) and the charging of the piezoelectric capacitance (13) are repeated until the piezoelectric capacitance (13) is charged to a maximum voltage before the storage capacitance is charged. Procedure according to Claim 5 or 6 , wherein the piezoelectric capacitance (13) is charged or discharged to a minimum voltage after discharging by opening the first and third switches (S1, S3) and closing the sixth switch (S6), wherein when the piezoelectric capacitance (13) is recharged, the first switch (S1) is closed and the sixth switch (S6) is opened, while the third switch (S3) remains open. Procedure according to one of the Claims 5 until 7 , whereby the process steps are repeated cyclically. Procedure according to one of the Claims 5 until 8th , wherein the piezoelectric capacitance (13) is subjected to a unipolar alternating voltage. Procedure according to one of the Claims 5 until 9 , wherein a further piezoelectric capacitance of the microelectroacoustic transducer device (19) or of a further microelectroacoustic transducer device is used as storage capacitance, wherein the piezoelectric capacitance (13) and the further piezoelectric capacitance are operated in antiphase.

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