Device for obtaining electric energy and energy generator comprising such a device

Abstract

A device for harvesting electrical energy includes a rectifier and a control device. The rectifier includes a first charging circuit for harvesting energy from a positive voltage of an energy harvester, and a second charging circuit for harvesting energy from a negative voltage of the energy harvester. The charging circuits include a common coil and a common electronic switch. Furthermore, each of the charging circuits includes a capacitor and a blocking element. Because the charging circuits use the coil jointly, the device is designed in a simple and compact manner. In addition, the energy harvesting is efficient, due to the one-stage AC-DC conversion and due to a maximum power point tracking function of the control device.

Claims

1. A device for harvesting electrical energy, comprising a rectifier comprising a first input terminal and a second input terminal for connecting to an energy harvester, a first output terminal and a second output terminal for providing an output voltage, a first charging circuit for harvesting energy from a positive voltage of the energy harvester which is applied to the input terminals, comprising an electronic switch, wherein the electronic switch comprises a series circuit which is made up of a first electronic switching element and a second electronic switching element, a coil, a first capacitor, and a first blocking element, a second charging circuit for harvesting energy from a negative voltage of the energy harvester which is applied to the input terminals, comprising the electronic switch, the coil, a second capacitor, and a second blocking element, a control device for actuating the electronic switch, wherein the control device comprises at least one charge pump, wherein one of the at least one charge pump is connected to one of the output terminals and a node between the first and second series-connected electronic switching elements of the electronic switch.

2. The device as claimed in claim 1, wherein the coil and the electronic are connected in series.

3. The device as claimed in claim 1, wherein the electronic switch comprises at least one body diode which is connected in parallel with a respective electronic switching element.

4. The device as claimed in claim 1, wherein a first body diode which is connected in parallel with the first electronic switching element, and a second body diode which is connected in parallel with the second electronic switching element, have opposite blocking directions.

5. The device as claimed in claim 1, wherein the coil, the first capacitor, and the first blocking element comprise a first loop for transmitting energy from the coil to the first capacitor.

6. The device as claimed in claim 1, wherein the coil, the second capacitor, and the second blocking element comprise a second loop for transmitting energy from the coil to the second capacitor.

7. The device as claimed in claim 1, wherein the blocking elements have different forward directions with respect to the coil.

8. The device as claimed in claim 1, wherein the first capacitor is connected to the first output terminal, and the second capacitor is connected to the second output terminal.

9. The device as claimed in claim 1, wherein the coil is connected to the first input terminal and a charging circuit node, the electronic switch is connected to the second input terminal and the charging circuit node, the first capacitor is connected to the charging circuit node and the first output terminal, the second capacitor is connected to the charging circuit node and the second output terminal, the first blocking element is connected to the first input terminal and the first output terminal, and the second blocking element is connected to the first input terminal and the second output terminal.

10. The device as claimed in claim 1, wherein the rectifier comprises a galvanic energy store which is connected to the output terminals.

11. The device as claimed in claim 1, wherein a voltage controller is connected to the output terminals.

12. The device as claimed in claim 1, wherein the control device comprises a control circuit for generating control signals, and a supply circuit for providing a supply voltage for the control circuit.

13. The device as claimed in claim 1, wherein a primary charge pump comprises a series circuit which is made up of a first diode and a third capacitor.

14. The device as claimed in claim 1, wherein an auxiliary charge pump is connected to the input terminals.

15. The device as claimed in claim 1, wherein an auxiliary charge pump is connected to a primary charging pump.

16. The device as claimed in claim 14, wherein the auxiliary charge pump comprises: a fourth capacitor which is connected to the first input terminal and a second node, a second diode which is connected to the second input terminal and the second node, a fifth capacitor which is connected to the second input terminal and a third node, a third diode which is connected to the second node and the third node, and a fourth diode which is connected to the third node.

17. The device as claimed in claim 12, wherein the supply circuit comprises an ohmic resistor which is connected in parallel with the first capacitor.

18. The device as claimed in claim 1, wherein the control device comprises a first switching sequence if a positive voltage is applied to the input terminals, wherein the following is true for the first switching sequence: TABLE-US-00006 Z.sub.1p Z.sub.2p Q.sub.1 1 0 Q.sub.2 1 0 where Q.sub.1 and Q.sub.2 denote the two series-connected electronic switching elements of the electronic switch, Z.sub.1p and Z.sub.2p denote two consecutive switching states of the first switching sequence, and 1 means ON and 0 means OFF.

19. The device as claimed in claim 1, wherein the control device comprises a second switching sequence if a negative voltage is applied to the input terminals, wherein the following is true for the second switching sequence: TABLE-US-00007 Z.sub.1n Z.sub.2n Q.sub.1 1 0 Q.sub.2 1 0 where Q.sub.1 and Q.sub.2 denote two series-connected electronic switching elements of the electronic switch, Z.sub.1p and Z.sub.2n denote the two consecutive switching states of the second switching sequence, and 1 means ON and 0 means OFF.

20. The device as claimed in claim 12, wherein the control circuit comprises a first comparator for generating a sawtooth voltage and a second comparator for generating control signals for the electronic switch.

21. The device as claimed in claim 12, wherein the control circuit is configured in such a way that an input impedance at the input terminals is adjustable to the energy harvester by means of the generated control signals.

22. An energy generator comprising a device for harvesting electrical energy, comprising a rectifier comprising a first input terminal and a second input terminal for connecting to an energy harvester, a first output terminal and a second output terminal for providing an output voltage, a first charging circuit for harvesting energy from a positive voltage of the energy harvester which is applied to the input terminals, comprising an electronic switch, wherein the electronic switch comprises a series circuit which is made up of a first electronic switching element and a second electronic switching element, a coil, a first capacitor, and a first blocking element, a second charging circuit for harvesting energy from a negative voltage of the energy harvester which is applied to the input terminals, comprising the electronic switch, the coil, a second capacitor, and a second blocking element, a control device for actuating the electronic switch, wherein the control device comprises at least one charge pump and wherein one of the at least one charge pump is connected to one of the output terminals and a node between the two series-connected electronic switching elements of the electronic switch, and an energy harvester which is connected to the input terminals for providing an AC voltage.

23. The device as claimed in claim 15, wherein the auxiliary charge pump is connected to a first node.

24. A device for harvesting electrical energy, the device comprising: a rectifier comprising a first input terminal and a second input terminal for connecting to an energy harvester, a first output terminal and a second output terminal for providing an output voltage, a first charging circuit for harvesting energy from a positive voltage of the energy harvester which is applied to the input terminals, comprising an electronic switch, wherein the electronic switch comprises a series circuit which is made up of a first electronic switching element and a second electronic switching element, a coil, a first capacitor, and a first blocking element, a second charging circuit for harvesting energy from a negative voltage of the energy harvester which is applied to the input terminals, comprising the electronic switch, the coil, a second capacitor, and a second blocking element, a control device for actuating the electronic switch, the control device comprising a control circuit for generating control signals, and a supply circuit for providing a supply voltage for the control circuit, the supply circuit comprising an ohmic resistor connected in parallel with the first capacitor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a schematic representation of an energy generator according to a first exemplary embodiment comprising a galvanic energy store,

(2) FIG. 2 depicts a time profile of a voltage provided by the energy harvester, and the control signals generated by a control device for switching between first switching states and second switching states of the energy generator,

(3) FIG. 3 depicts a schematic representation of a control circuit for generating the controls signals for an electronic switch of the energy generator,

(4) FIG. 4 depicts an equivalent circuit diagram of the energy generator during the start of operation of a supply circuit for providing a supply voltage for the control circuit,

(5) FIG. 5 depicts an equivalent circuit diagram of the energy generator in a first switching state, if an energy harvester provides a positive voltage,

(6) FIG. 6 depicts an equivalent circuit diagram of the energy harvester in a second switching state which follows the first switching state according to FIG. 5,

(7) FIG. 7 depicts an equivalent circuit diagram of the energy generator in a first switching state if an energy harvester provides a negative voltage,

(8) FIG. 8 depicts an equivalent circuit diagram of the energy harvester in a second switching state which follows the first switching state according to FIG. 7,

(9) FIG. 9 depicts a schematic representation of an energy generator according to a second exemplary embodiment without a galvanic energy store,

(10) FIG. 10 depicts a circuit diagram of a supply circuit of the control device of the energy generator according to FIG. 9, in a first charging state,

(11) FIG. 11 depicts a circuit diagram of the supply circuit in a second charging state, and

(12) FIG. 12 depicts a circuit diagram of the supply circuit in a third charging state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(13) A first exemplary embodiment of the present invention is described below with the aid of FIGS. 1 to 8. An energy generator 1 comprises an energy harvester 2 and a device 3 for harvesting electrical energy. The energy harvester 2 is configured electromagnetically. The energy harvester 2 is known and configured in a typical manner. The energy harvester 2 is also referred to as an energy harvester. The device 3 is used for utilizing or harvesting the electrical energy which is provided by the energy harvester 2. For this purpose, the energy harvester 2 is connected to a first input terminal 4 and a second input terminal 5 of the device 3. The energy generator 1 is used to provide a load 6 with electrical energy. For this purpose, the load 6 is connected to a first output terminal 7 and a second output terminal 8 of the device 3. The energy harvester 2 is connected to the input terminal 5 having a reference potential which is referred to as EH-GND (energy harvester GND).

(14) The energy harvester 2 provides a voltage v.sub.h which has an alternating polarity (AC voltage). The voltage v.sub.h over time t is depicted by way of example in FIG. 2. The portion of the voltage v.sub.h having a positive polarity will be denoted below by v.sub.hp, and the portion of the voltage having a negative polarity will be denoted by v.sub.hn.

(15) The device 3 comprises a rectifier 9 and an associated control device 10. The rectifier 9 forms a first charging circuit 11 which is used for harvesting energy from the positive voltage v.sub.hp of the energy harvester 2 which is applied to the input terminals 4, 5. Furthermore, the rectifier 9 forms a second charging circuit 12 which is used for harvesting energy from the negative voltage v.sub.hn of the energy harvester 2 which is applied to the input terminals 4, 5.

(16) The first charging circuit 11 comprises an electronic switch 13, a coil L, a first capacitor C.sub.p, and a first blocking element D.sub.p. On the other hand, the second charging circuit 12 comprises the electronic switch 13, the coil L, a second capacitor C.sub.n, and a second blocking element D.sub.n.

(17) The coil L is connected to the first input terminal 4 and a charging circuit node K. The electronic switch 13 is connected to the charging circuit node K and the second input terminal 5, such that the coil L and the electronic switch 13 are connected in series between the input terminals 4, 5. The first capacitor C.sub.p is connected to the charging circuit node K and the first output terminal 7 Furthermore, the first blocking element D.sub.p is connected to the first input terminal 4 and the first output terminal 7 in such a way that the first blocking element D.sub.p enables a current flow from the first output terminal 7 to the first input terminal 4. The first blocking element D.sub.p is configured as a diode. The coil L, the first capacitor C.sub.p, and the first blocking element D.sub.p accordingly form a first loop M.sub.p2. The second capacitor C.sub.n is connected to the charging circuit node K and the second output terminal 8. Furthermore, the second blocking element D.sub.n is connected to the first input terminal 4 and the second output terminal 8 in such a way that the second blocking element D.sub.n enables a current flow from the first input terminal 4 to the second output terminal 8. The second blocking element D.sub.n is configcared as a diode. The coil L, the second blocking element D.sub.n, and the second capacitor C.sub.n thus form a second loop M.sub.n2. The blocking elements D.sub.p and D.sub.n thus have forward directions which are opposite with respect to the coil L, such that a current flowing through the coil L is opposite in the loops M.sub.p2 and M.sub.n2.

(18) The rectifier 9 furthermore comprises a rechargeable galvanic energy store 14. The galvanic energy store 14 is connected to the first output terminal 7 and to the second output terminal 8. For this purpose, a negative terminal of the galvanic energy store 14 is connected to the first output terminal 7, and a positive terminal is connected to the second output terminal 8. An output voltage E.sub.b is provided at the output terminals 7, 8.

(19) The electronic switch 13 is configured as a series circuit which is made up of a first electronic switching element Q.sub.1 and a second electronic switching element Q.sub.2. The electronic switching elements Q.sub.1 and Q.sub.2 are respectively configured as normally-off n-channel MOSFETs. A source terminal S.sub.1 of the first switching element Q.sub.1 is connected to a reference node K.sub.0. The reference node K.sub.0 defines a reference potential of the control device 10 (control circuit GND). A drain terminal D.sub.1 of the first switching element Q.sub.1 is connected to the second input terminal 5. Furthermore, a source terminal S.sub.2 of the second switching element Q.sub.2 is connected to the reference node K.sub.0. A drain terminal D.sub.2 of the second switching element Q.sub.2 is connected to the charging circuit node K. The reference potential of the reference node K.sub.0 ensures that positive control voltages or control signals g.sub.1 and g.sub.2 are applied to gate terminals G.sub.1, G.sub.2 of the switching elements Q.sub.1 and Q.sub.2 with reference to the associated source terminals S.sub.1 and S.sub.2, if the switching elements Q.sub.1 and Q.sub.2 are to be switched on, and negative control voltages or control signals g.sub.1 and g.sub.2 are applied if the switching elements Q.sub.1 and Q.sub.2 are to be switched off.

(20) A first parasitic body diode F.sub.1 is configured in parallel with the first switching element Q.sub.1. The first body diode F.sub.1 is configured with respect to the source terminal S.sub.1 and the drain terminal D.sub.1 in such a way that a current flow in the direction of the first source terminal S.sub.1 is blocked. Correspondingly, a second parasitic body diode F.sub.2 is configured in parallel with the second switching element Q.sub.2. The second body diode F.sub.2 is configured with respect to the second source terminal S.sub.2 and the drain terminal D.sub.2 in such a way that a current flow in the direction of the second source terminal S.sub.2 is blocked. The body diodes F.sub.1 and F.sub.2 thus have opposite blocking directions. By means of the described back-to-back arrangement of the switching elements Q.sub.1 and Q.sub.2, it is thus ensured that both in the case of a positive voltage and in the case of a negative voltage which is applied to the electronic switch 13, at least one of the body diodes F.sub.1 or F.sub.2 blocks. As a result, parasitic currents and resulting losses are avoided.

(21) The control device 10 is used for actuating the electronic switch 13. The control device 10 comprises a control circuit 15 for generating control signals g.sub.1 and g.sub.2, and a supply circuit 16 for providing a supply voltage v.sub.cc for the control circuit 15. The control signals g.sub.1, g.sub.2 are control voltages.

(22) The supply circuit 16 comprises a charge pump 17, which comprises a diode D.sub.cc and a capacitor C.sub.cc. The diode D.sub.cc is connected to the second output terminal 8 and a first node k.sub.1. The diode D.sub.cc enables a current flow from the second output terminal 8 to the first node k.sub.1. The capacitor C.sub.cc is connected to the first node k.sub.1 and the reference node K.sub.0, such that the diode D.sub.cc and the capacitor C.sub.cc are connected in series. The charge pump 17 is configured as one stage. The supply voltage v.sub.cc is applied across the capacitor C.sub.cc, i.e., between the node k.sub.1 and the reference node K.sub.0.

(23) The supply circuit 16 furthermore comprises an ohmic resistor R.sub.b which is connected in parallel with the first capacitor C.sub.p and which is connected to the charging circuit node K and the first output terminal 7.

(24) By way of example, the following values hold true for the rectifier 9 and the supply circuit 16:

(25) TABLE-US-00003 Inductance of the coil L  33 μH, Capacitance of the first capacitor C.sub.p  10 μF, Capacitance of the second capacitor C.sub.n  10 μF, Capacitance of the capacitor C.sub.cc 0.3 μF, Value of the ohmic resistor R.sub.b  45 kΩ.

(26) The control circuit 15 comprises a first comparator 18 for generating a sawtooth voltage v.sub.s and a second comparator 19 for generating the control signals g.sub.1, g.sub.2 for actuating the electronic switching elements Q.sub.1, Q.sub.2. By means of the comparators 18, 19, open-loop pulse width modulation is achieved.

(27) The first comparator 18 comprises an operational amplifier 20 which has the supply voltage v.sub.cc and the reference potential of the reference node K.sub.0 as operating potentials. A voltage divider made up of a first ohmic resistor R.sub.1 and a second ohmic resistor R.sub.2 is connected to the node k.sub.1. The first ohmic resistor R.sub.1 is connected to the reference node K.sub.0, whereas the second ohmic resistor R.sub.2 is connected to the node k.sub.1. A voltage v.sub.x is tapped at a node n.sub.1 between the ohmic resistors R.sub.1 and R.sub.2 and is fed to a non-inverting input (positive input) of the operational amplifier 20. The node n.sub.1 is thus connected to the non-inverting input. An output of the operational amplifier 20 is connected to an inverting input (negative input) of the operational amplifier 20 via a third ohmic resistor R.sub.3. Furthermore, the output is connected to the non-inverting input of the operational amplifier 20 via an ohmic resistor R.sub.4. A capacitor C.sub.1 is connected to the inverting input of the operational amplifier 20 and the reference node K.sub.0. The sawtooth voltage v.sub.s is applied across the capacitor C.sub.1. The connection between the ohmic resistor R.sub.3 and the capacitor C.sub.1 thus defines a node n.sub.2 to which the sawtooth voltage v.sub.s is provided.

(28) The second comparator 19 is used for comparing the sawtooth voltage v.sub.s to a comparison voltage v.sub.y. For this purpose, the second comparator 19 comprises an operational amplifier 21. The operational amplifier 21 has the supply voltage v.sub.cc and the reference potential at the reference node K.sub.0 as operating potentials. The second comparator 19 comprises a voltage divider made up of an ohmic resistor R.sub.5 and an ohmic resistor R.sub.6. The voltage divider is connected to the reference node K.sub.0 and the node k.sub.1. For this purpose, the ohmic resistor R.sub.5 is connected to the reference node K.sub.0, whereas the ohmic resistor R.sub.6 is connected to the node k.sub.1. The comparison voltage v.sub.y is tapped between the ohmic resistors R.sub.5 and R.sub.6. For this purpose, a node n.sub.3 between the ohmic resistors R.sub.5 and R.sub.6 is connected to a non-inverting input (positive input) of the operational amplifier 21. The comparison voltage v.sub.y is thus applied across the ohmic resistor R.sub.5, i.e., between the node n.sub.3 and the reference node K.sub.0. The node n.sub.2 is connected to an inverting input (negative input) of the operational amplifier 21. The control signals g.sub.1 and g.sub.2 are provided at an output of the operational amplifier 21. The control signals g.sub.1 and g.sub.2 are applied to the gate terminals G.sub.1, G.sub.2 of the switching elements Q.sub.1 and Q.sub.2.

(29) The control device 10 is configured in such a way that, if the positive voltage v.sub.hp is applied, a first switching sequence having a first switching state Z.sub.1p and a subsequent, second switching state Z.sub.2p are achieved. For the first switching sequence, the following is true:

(30) TABLE-US-00004 Z.sub.1p Z.sub.2p Q.sub.1 1 0 Q.sub.2 1 0

(31) where 1 means ON and 0 means OFF. The switching state Z.sub.1p is illustrated in FIG. 5, and the switching state Z.sub.2p is illustrated in FIG. 6.

(32) The control device 10 is furthermore configured in such a way that, if the negative voltage v.sub.hn is applied, a second switching sequence having a first switching state Z.sub.1n and a subsequent, second switching state Z.sub.2n are achieved. For the second switching sequence, the following is true:

(33) TABLE-US-00005 Z.sub.1n Z.sub.2n Q.sub.1 1 0 Q.sub.2 1 0

(34) where 1 means ON and 0 means OFF. The switching state Z.sub.1n is illustrated in FIG. 7, and the switching state Z.sub.2n is illustrated in FIG. 8.

(35) The first switching sequence and the second switching sequence respectively extend over a period T.sub.s. The following is true for the period T.sub.s:T.sub.s=1/f.sub.s, where f.sub.s denotes a switching frequency of the control device 10. The respective first switching state Z.sub.1p or Z.sub.1n has the duration D.Math.T.sub.s, and the respective associated second switching state Z.sub.2p or Z.sub.2n has the duration (1−D).Math.T.sub.s, where D denotes a duty cycle.

(36) The function of the energy generator 1 is as follows:

(37) First, the supply circuit 16 is put into operation in order to generate the supply voltage v.sub.cc and to provide it to the control circuit 15. This is illustrated in FIG. 4. The supply circuit 16 is based on the single-stage charge pump 17, which uses the energy harvester 2, the coil L, and the second capacitor C.sub.n to generate a sufficient supply voltage v.sub.cc. According to the loop M.sub.0 plotted in FIG. 4, the following is true for the supply voltage v.sub.cc:
v.sub.cc=v.sub.cn+v.sub.L+v.sub.hmax−2.Math.V.sub.D=E.sub.b+v.sub.hmax−2.Math.V.sub.D,  (1)

(38) where E.sub.b denotes the output voltage or the voltage of the galvanic energy store 14, v.sub.hmax denotes the maximum voltage of the energy harvester 2, V.sub.L denotes the voltage across the coil L, v.sub.cn denotes the voltage across the second capacitor C.sub.n, and V.sub.D denotes the voltage across the diodes D.sub.cc and F.sub.1. The capacitor C.sub.cc is charged by means of the voltage v.sub.h of the energy harwester 2, such that a sufficient supply voltage v.sub.cc is provided. The ohmic resistor R.sub.b ensures that the voltage E.sub.b is distributed across the capacitors C.sub.p and C.sub.n, and prevents an essentially full application of the voltage E.sub.b across the capacitor C.sub.p. As a result, a reliable start of operation of the supply circuit 16 is ensured, and overloading of the capacitor C.sub.p is avoided. The ohmic resistor R.sub.b is chosen to have high resistance, such that the losses caused by the ohmic resistor R.sub.b are low.

(39) When providing a sufficient supply voltage v.sub.cc, the control circuit 15 generates the control signals g.sub.1, g.sub.2 or the control voltages g.sub.1, g.sub.2 for actuating the electronic switching elements Q.sub.1 and Q.sub.2. The energy harvester 2 generates the voltage v.sub.h from which the current i.sub.h results. If the positive voltage v.sub.hp is applied, the control circuit 15 achieves the first switching sequence. First, in the first switching state Z.sub.1p, the switching elements Q.sub.1 and Q.sub.2 are switched on synchronously, such that the current i.sub.h in the loop M.sub.p1 flows through the coil L. This is illustrated in FIG. 5. In the first switching state Z.sub.1p, electrical energy is thus stored in the coil L.

(40) In the subsequent, second switching state Z.sub.2p, the electronic switching elements Q.sub.1 and Q.sub.2 are opened, such that the coil L drives a current in the loop M.sub.p2 and charges the capacitor C.sub.p, due to the stored energy. The current i.sub.p flows in the loop M.sub.p2 via the first blocking element D.sub.p, which is configured as a diode. The voltage v.sub.cp is applied to the first capacitor C.sub.p. This is illustrated in FIG. 6. Due to the voltage v.sub.cp, a current i.sub.b flows which charges the galvanic energy store 14.

(41) If a negative voltage v.sub.hn is applied, the control circuit 15 achieves the second switching sequence. In the first switching state Z.sub.1n, the switching elements Q.sub.1 and Q.sub.2 are switched on synchronously, such that the current in flows in a loop M.sub.n1 through the coil L, and electrical energy is stored in the coil L. This is illustrated in FIG. 7. In the first switching state Z.sub.1n, electrical energy is thus stored in the coil L.

(42) In the subsequent, second switching state Z.sub.2n, the electronic switching elements Q.sub.1 and Q.sub.2 are opened, such that the coil L drives a current in the loop M.sub.n2 and charges the capacitor C.sub.n, due to the stored energy. The current i.sub.n flows in the loop M.sub.n2 through the second blocking element D.sub.n, which is configured as a diode. The voltage v.sub.cn is applied to the second capacitor C.sub.n. This is illustrated in FIG. 8. Due to the voltage v.sub.cn, a current i.sub.b flows which charges the galvanic energy store 14.

(43) Because the single coil L is used in each case if the positive voltage v.sub.hp is applied and if the negative voltage v.sub.hn is applied, the rectifier 9 is designed in a simple and compact manner Because only a single coil L is required, costs are also reduced. The coil L which is used jointly by the charging circuits 11 and 12 is not only a part of the rectifier 9, but is also used for starting operation of the supply circuit 16. Because the voltage v.sub.h of the energy harvester 2 is rectified in one step, the harvesting of electrical energy is optimized by means of the rectifier 9. In addition, the rectifier 9 enables the harvesting of electrical energy from an extremely low voltage v.sub.h. The electromagnetically configured energy harvester 2 typically has a voltage from 1 mV to 1.2 V, in particular from 10 mV to 750 mV, and in particular from 50 mV to 500 mV.

(44) The control device 10 is configured in such a way that an input impedance Z.sub.in at the input terminals 4, 5 is adjusted to the energy harvester 2, and the harvested electrical energy is thus optimized or maximized. The energy generator 1 or the device 3 thus enables or achieves so-called maximum power point tracking (MPPT). The control device 10 accordingly has a maximum power point tracking function. The measurement of a voltage, a current, or a zero-crossing detection or polarity detection is not required for this purpose. The device 3 is operated in a discontinuous conduction mode (DCM) and is an adjustable and purely ohmic element from the point of view of the energy harvester 2. The input impedance Z.sub.in or the input resistance of the device 3 is adjusted to the inner resistance of the energy harvester 2, whereby the maximum power point tracking is achieved. For the input impedance Z.sub.in of the device 3, the following is true:

(45) $\begin{array}{cc}{Z}_{\mathrm{in}}=\frac{2.Math.f_S.Math.L_0}{D^2}& \left(2\right)\end{array}$

(46) where L.sub.0 denotes the inductance value of the coil L, f.sub.s denotes the switching frequency, and D denotes the duty cycle. The input impedance Z.sub.in is set via the duty cycle D.

(47) Due to the back-to-back arrangement, the electronic switching elements Q.sub.1 and Q.sub.2 are operated synchronously. In addition, the electronic switching elements Q.sub.1 and Q.sub.2 are operated in a corresponding manner in the first switching sequence, i.e., if the positive voltage v.sub.hp is applied, and in the second switching sequence, i.e., if the negative voltage v.sub.hn is applied. This means that the electronic switching elements Q.sub.1 and Q.sub.2 are closed in the respective first switching state Z.sub.1p or Z.sub.1n, whereas the electronic switching elements Q.sub.1 and Q.sub.2 are open in the associated second switching state Z.sub.2p or Z.sub.2n. Because the reference node K.sub.0 is connected to the source terminals S.sub.1 and S.sub.2, the electronic switching elements Q.sub.1 and Q.sub.2 can be actuated directly by the control circuit 15 in a simple manner

(48) The adjustment of the input impedance Z.sub.in takes place via the design of the control circuit 15. By means of the comparators 18, 19, open-loop pulse width modulation is achieved. The first comparator 18 charges the capacitor C.sub.1 via the ohmic resistor R.sub.3 and compares the voltage v.sub.x to the voltage across the capacitor C.sub.1 in such a way that the capacitor C.sub.1 is discharged if the voltage across the capacitor C.sub.1 is greater than the voltage v.sub.x. As a result, the sawtooth voltage v.sub.s arises, which is applied across the capacitor C.sub.1. The second comparator 19 compares the sawtooth voltage v.sub.s to the comparison voltage v.sub.y and generates the control signals g.sub.1 and g.sub.2. By means of the comparison of the sawtooth voltage v.sub.s to the comparison voltage v.sub.y, the input impedance Z.sub.in is automatically determined, and the control signals g.sub.1 and g.sub.2 are generated in such a way that the input impedance Z.sub.in is adjusted to the energy harvester 2 by means of the duty cycle D. Since the energy harvester 2 enables essentially maximum energy harvesting at an input impedance Z.sub.in between 7Ω and 13Ω, an exact adjustment of the input impedance Z.sub.in to the energy harvester 2 is not necessary to ensure efficient energy harvesting. Fluctuations in the supply voltage v.sub.cc thus do not disadvantageously affect the efficiency of energy harvesting. The duty cycle D and the switching frequency f.sub.s may, for example, be set via the ohmic resistor R.sub.6 and the capacitor C.sub.1.

(49) The control signals g.sub.1 and g.sub.2 are thus set with respect to time in such a way that the respective first switching state Z.sub.1p or Z.sub.1n is set for a duration of 0<t<D.Math.T.sub.s, with reference to a period T.sub.s, and the respective second switching state Z.sub.2p or Z.sub.2n is set for the duration D.Math.T.sub.s<t <(1−D).Math.T.sub.s.

(50) The galvanic energy store 14 which provides the voltage E.sub.b is charged by means of the voltages v.sub.cp and v.sub.cn. The load 6 is supplied with electrical energy by means of the voltage E.sub.b.

(51) A second exemplary embodiment of the present invention will be described below with the aid of FIGS. 9 to 12. Unlike the first exemplary embodiment, the rectifier 9 does not comprise a galvanic energy store. The supply circuit 16 comprises a primary charge pump 17 and an auxiliary charge pump 22 for starting operation of the supply circuit 16 and for providing the supply voltage v.sub.cc. An ohmic resistor R.sub.b corresponding to the first exemplary embodiment is not required.

(52) The auxiliary charge pump 22 is connected to the input terminals 4, 5. The charge pump 22 is configured as a passive one-stage Villard charge pump. A fourth capacitor C.sub.b1 is connected to the first input terminal 4 and a second node k.sub.2. A second diode D.sub.b1 is connected to the second input terminal 5 and the second node k.sub.2 in such a way that the diode D.sub.b1 enables a current flow from the second input terminal 5 to the node k.sub.2. A fifth capacitor C.sub.b2 is connected to the second input terminal 5 and a third node k.sub.3. A third diode D.sub.b2 is connected to the second node k.sub.2 and the third node k.sub.3 in such a way that the third diode D.sub.b2 allows a current flow from the second node k.sub.2 to the third node k.sub.3. A fourth diode D.sub.b3 is connected to the first node k.sub.1 of the primary charge pump 17 in such a way that the fourth diode D.sub.b3 allows a current flow from the third node k.sub.3 to the first node k.sub.1 and the reference node K.sub.0. The primary charge pump 17 is configured corresponding to the charge pump 17 of the first exemplary embodiment.

(53) The energy generator 1 enables starting operation of the control device 10 without a galvanic energy store. For this purpose, the auxiliary charge pump 22 is first put into operation, and the primary charge pump 17 is subsequently put into operation with it. In a few cycles after the excitement, the energy harvester 2 typically provides a comparatively high voltage v.sub.h which is subsequently reduced considerably due to damping. The auxiliary charge pump 22 is put into operation during these cycles. If the negative voltage v.sub.hn is applied, the capacitor C.sub.b1 is charged via the diode D.sub.b1. For this purpose, a charging current i.sub.1 flows through the diode D.sub.b1 to the capacitor C.sub.b1. The active loop M.sub.1 is illustrated in FIG. 10.

(54) If the positive voltage v.sub.hp is subsequently applied, the charged capacitor C.sub.b1 having the applied voltage v.sub.b1 and the energy harvester 2 having the voltage v.sub.hp charge the capacitor C.sub.b2 via the diode D.sub.b2. The active loop M.sub.2 and the charging current i.sub.2 flowing through the diode D.sub.b2 are illustrated in FIG. 11. For the voltage v.sub.b2, the following is true:
V.sub.b=2.Math.v.sub.hmax−2.Math.V.sub.D  (3)

(55) where v.sub.hmax is the maximum voltage of the energy harvester 2 and V.sub.D is the threshold voltage.

(56) Subsequently, the capacitor C.sub.cc is charged via the diode D.sub.b3, whereby the control circuit 15 is supplied with a sufficient supply voltage v.sub.cc, and generates control signals g.sub.1, g.sub.2 for actuating the electronic switching elements Q.sub.1, Q.sub.2. As a result, the rectifier 9 is put into operation, and the output voltage E.sub.b increases. If E.sub.b is greater than the voltage v.sub.b2, the auxiliary charge pump 22 is automatically deactivated. Only the primary charge pump 17 is then active. The diode D.sub.b3 disconnects the auxiliary charge pump 22 from the primary charge pump 17, such that only the primary charge pump 17 is active. The active loop M.sub.3 and the charging current i.sub.3 are illustrated in FIG. 12. For the voltage v.sub.cc, the following is true:
v.sub.cc==v.sub.0+v.sub.hmax−2.Math.V.sub.D  (4)

(57) where v.sub.0 is the voltage at the input terminal 8. All diodes are preferably configured as Schottky diodes having a threshold voltage V.sub.D between 0.1 to 0.2 V.

(58) A voltage controller 23 is connected to the output terminals 7, 8. The voltage controller 23 is used for stabilizing the voltage E.sub.b, and provides a controlled output voltage E′.sub.b to output terminals 7′, 8′. The load 6 is connected to the output terminals 7′, 8′. Due to the changing charging state of the capacitors C.sub.p and C.sub.n, the voltage E.sub.b at the output terminals 7, 8 fluctuates. The voltage controller 23 compares the voltage E.sub.b to a setpoint voltage V.sub.ref and provides the controlled voltage E′.sub.b on the output side. For this purpose, the voltage controller 23 comprises a DC-DC converter, of which the voltage E′.sub.b on the output side is controlled. For this purpose, the DC-DC converter is part of a voltage control circuit which compares the output-side voltage E′.sub.b to the setpoint voltage V.sub.ref, and feeds a voltage difference between the setpoint voltage V.sub.ref and the output-side voltage E′.sub.b to a controller, which actuates the DC-DC converter for correcting the voltage difference. The controller is configured as a PID controller.

(59) With respect to the further design and the further functionality of the energy generator 1, reference is made to the description of the first exemplary embodiment.