Method and a Device for Determining a Switching Current of a Converter of a System for Inductive Power Transfer and a Method of Control

Abstract

A method for determining a switching current of at least one switching element of a converter of a system for inductive power transfer, including determining a phase current of at least one AC phase line of the converter; determining at least one switching time point of the at least one switching element and the phase current value at said switching time point; and determining switching current of the at least one switching element depending on the at least one phase current value.

Claims

1. A method for determining a switching current of at least one switching element of a converter of a system for inductive power transfer, wherein the method comprises the steps of: determining a phase current of at least one AC phase line of the converter; determining at least one switching time point of the at least one switching element and the phase current value at said switching time point; and determining switching current of the at least one switching element depending on the at least one phase current value.

2. The method of claim 1, wherein the phase current is measured by a current sensor.

3. The method of claim 2, wherein a bandwidth of the current sensor is higher than 150 kHz.

4. The method of claim 2, the phase current value is digitized by an A/D converter, and wherein a bandwidth of the A/D converter is smaller than the bandwidth of the current sensor.

5. The method of claim 2, wherein the current sensor comprises a Rogowski coil or a current transformer.

6. The method of claim 1, wherein a phase voltage change over time is determined, and wherein the phase current is determined depending on the phase voltage change.

7. The method of claim 1, wherein a set of at least two successive switching time points of the at least one switching element and the phase current values at said switching time points is determined, and wherein the at least one switching current is determined depending on the at least two phase current values.

8. The method of claim 7, wherein a first switching time point determined depending on a first time point of a switch signal generation, wherein at least one further switching time point is determined depending on a further time point of a switch signal generation, and wherein a time difference between the at least one further switching time point and the further time point of a switch signal generation is different from the time difference between the first switching time point and the first time point of a switch signal generation.

9. The method of claim 8, the time difference between a switching time point and the time point of a switch signal generation increases for successively determined switching time points.

10. The method of claim 9, wherein successive switching time points are determined by adding an increasing offset value to each of successively determined time points of a switch signal generation.

11. The method of claim 8, wherein the switching current is determined as the local maximum of the course of phase current values for the varying time differences.

12. The method of claim 7, wherein a predetermined number of successive switching time points are determined and/or all switching time points in a time interval of a predetermined length are determined.

13. The method of claim 1, wherein a first switching element voltage across a first switching element in one leg of the converter and a second switching element voltage across a second switching element in said leg of the converter are determined, wherein the first and the second switching clement voltages provide input values of a XOR operation, wherein the switching time point is determined as the time point at which the output value of the XOR operation changes to a logic 1 signal, and wherein the switching current is determined as the phase current value at said switching time point.

14. A device for determining switching current of at least one switching element of an converter of a system for inductive power transfer, wherein the device comprises: at least one means for determining a phase current of at least one AC phase line of the converter; and at least one evaluation means; wherein a phase current of at least one AC phase line of the converter is determinable; wherein at least one switching time point of the at least one switching element and the phase current value at said switching time point is determinable; and wherein the at least one switching current is determinable depending on the at least one phase current value.

15. The device according to claim 14, further comprising: a current sensor and/or at least one means for determining a phase voltage change over time.

16. The device according to claim 14, further comprising: a control unit for generating switching signals for the at least one switching element.

17. The device according to claim 14, further comprising: at least one means for determining a first switching element voltage across a first switching element in one leg of the converter and a second switching element voltage across a second switching element in said leg of the converter.

18. A method for controlling an operation of at least one switching element of a converter, in particular of a system for inductive power transfer, wherein a switching current of the at least one switching element of the converter is determined by a method according to claim 1, and wherein the switching element is controlled based on the determined switching current.

Description

[0068] The invention will be described with reference to the attached figures. The figures show:

[0069] FIG. 1: A schematic block diagram of a device for determining a switching current,

[0070] FIG. 2: An exemplary time course of phase currents in three AC phase lines of an inverter,

[0071] FIG. 3: An exemplary time course of switching signals and a phase current and

[0072] FIG. 4: A schematic flow diagram of a method for determining a switching current.

[0073] In the following the same reference numerous denote elements with the same or similar technical features.

[0074] FIG. 1 shows a schematic block diagram of a device 1 for determining a switching current of at least one switching element 6 of an inverter 5 of a system for inductive power transfer, in particular to a vehicle (not shown).

[0075] A primary unit (not shown) comprises the inverter 5 which is designed with a B6 bridge topology. The inverter 5 comprises switching elements 6, wherein a bypass diode 7 is connected antiparallel to each switching element 6. The inverter 5 has three legs, wherein each leg comprises a series connection of two switching elements 6. Further, a phase line U, V, W is connected to a connection section of the two switching elements 6.

[0076] In particular, a first switching element 6 is connected to a high potential phase line, wherein the second switching element 6 is connected to a low potential phase line.

[0077] The switching element 6 of the inverter can e.g. be provided by a MOSFED or an IBGT.

[0078] The inverter 5 generates or provides AC (alternating current) phase voltages for the phase lines U, V, W of the primary winding structure 3. AC output terminals of the inverter 5 are connected to the phase lines U, V, W, respectively. In the shown embodiment, these phase lines U, V, W of primary winding structure 3 are electrically connected to AC phase lines of the inverter 5. Thus, phase lines U, V, W also denote AC phase lines of the inverter 5.

[0079] The primary winding structure 3 is a three-phase winding structure. Schematically shown is an inductance L.sub.U, L.sub.V, L.sub.W provided by each phase line U, V, W. Further shown are compensating capacitances C.sub.U, C.sub.V, C.sub.W in each phase line U, V, W, wherein a capacitance value of said capacitances C.sub.U, C.sub.V, C.sub.W is chosen such that the resonant frequency of the resonant circuit provided by the inductance L.sub.U, L.sub.V, L.sub.W and the capacitance C.sub.U, C.sub.V, C.sub.W of each phase line U, V, W matches an operating frequency.

[0080] Further shown are AC phase currents I.sub.U, I.sub.V, I.sub.W in each phase line U, V, W which correspond to phase currents in AC phase lines of the inverter 5.

[0081] Further, the device 1 comprises current sensors 8 which measure the phase currents I.sub.U, I.sub.V, I.sub.W in each phase line U, V, W and thus the phase currents in the AC phase lines of the inverter 5. In particular, the phase current sensors 8 also measure the phase currents I.sub.U, I.sub.V, I.sub.W which flow through one switching element 6 of a leg of the inverter 5 into or out of the respective phase U, V, W.

[0082] In arrowhead of each phase current I.sub.U, I.sub.V, I.sub.W indicates a positive direction of the phase current I.sub.U, I.sub.V, I.sub.W. A positive value of the phase current I.sub.U, I.sub.V, I.sub.W indicates a current flow with the indicated direction.

[0083] Further, the system 1 comprises an evaluation unit 9 which is connected to the current sensors 8 by a signal link (shown by dash lines). The evaluation unit 9 is connected to said current sensors 8 by a low-pass filter unit 12 and an A/D converter unit 13, respectively. An A/D converter unit 13 can e. g. be a successive-approximation A/D converter unit or any other sample and hold A/D converter unit. The current sensors 8 provide samples of the measured phase current I.sub.U, I.sub.V, I.sub.W, wherein these values are low-pass filtered. Only some, but not all of the samples generated by the current sensors 8 are then digitized by the A/D converter units 13. A phase shift introduced by the current sensor 8, the low-pass filter 12 and the A/D converter unit 13 should be zero or as small as possible. Alternatively, the introduced phase shift of the phase current values should be considered in the method of determining a switching current.

[0084] Further, the system comprises a memory unit 10 which is connected to evaluation unit 9 by a signal or a data link. Further shown is a control unit 11 for controlling an operation of the inverter 5, e.g. for controlling an operation of the switching elements 6. The control unit 11 is connected to the evaluation unit 9 by a signal or a data link. The control unit 11 can generate switch signals for the switching elements 6

[0085] By means of the shown device 1, a phase current I.sub.U, I.sub.V, I.sub.W of phase line U, V, W of the inverter 5 can be measured by the current sensors 8. Further, at least one switching time point SP1, SP2, SP3 (see e.g. FIG. 3) of at least one switching element 6 and the phase current value I.sub.U, I.sub.V, I.sub.W at said switching time point SP1, SP2, SP3 can be determined, e.g. by the evaluation 9. It is e.g. possible that the evaluation 9 determines the switching time point SP1, SP2, SP3, wherein only the phase current value measured by the current sensor 8 at the switching time point SP1, SP2, SP3 is digitized by the A/D converter unit 13.

[0086] Further, a switching time point of the at least one switching element 6 is determined depending on the digitized phase current value.

[0087] A bandwidth of the current sensors 8 can be higher than 500 kHz. Further, a bandwidth of the A/D converter units 13 can be smaller than the bandwidth of the current sensors 8. The current sensor 8 can comprise a Rogowski coil or a current transformer.

[0088] FIG. 2 shows an exemplary time course of phase currents I.sub.U, I.sub.V, I.sub.W over time t. The time course of each phase current I.sub.U, I.sub.V, I.sub.W is quasi-periodic, wherein the fundamental frequency corresponds to the operating frequency of the inverter 5, wherein each time course also contains higher order or so-called harmonic frequencies. Switching time points SP and corresponding phase current values are indicated by circles for each phase line U, V, W.

[0089] It is shown in FIG. 2 that a determination of the phase current value slightly before or after the actual switching time point can led to rather high deviations from a correct switching current because of the high current gradient provided around a switching time point SP.

[0090] FIG. 3 shows a schematic flow diagram of one preferred embodiment of the invention. In a first step S1, an offset value d is set to zero or to another value, e.g. a value of 10 ns. In a second step S2, a first switching time point SP1 (see FIG. 4) is determine as a sum of a first time point SG1 of a switch signal generation for a switching element 6 (see FIG. 1) and the offset value d. Further, a first switching current SI1 is determined as the phase current value at this first switching time point SP1.

[0091] In a third step S3, it is checked if a predetermined number of switching cycles C1, C2, C3 (see FIG. 4) have been performed, e.g. five to ten switching cycles C1, C2, C3. If this is not the case, the offset valued is increased by e.g. 10 ns and the second step S2 is performed again. In particular, a second switching time point SP2 is determined as a sum of a second time point SG2 and the (increased) offset value d. Further determined is a second switching current SI2 at this second switching time point SP2.

[0092] If a predetermined number of switching cycles has been performed, a set of multiple switching time points SP1, SP2, SP3 (see FIG. 4) and corresponding phase current values SI1, SI2, SI3 have been determined. This set and in particular its values can be stored in the memory unit 10 (see FIG. 1). Further, the switching current is determined as the local maximum of the course of phase current values SI1, SI2, SI3 for varying differences offset values d. The offset value d corresponds to a time difference between the switching time point SP1, SP2, SP3 and the time point of the corresponding switch signal generation SG1, SG2, SG3.

[0093] This is performed in a fourth step S4. Not shown is a control step wherein an operation of the inverter 5, in particular of a switching element 6 of the inverter 5, is controlled depending on the determined switching current.

[0094] FIG. 4 exemplarily shows a time course of a phase current I.sub.U in the first phase line U (see FIG. 1) of the inverter 5 for three switching cycles C1, C2, C3. Further shown is a time course of a switch signal, wherein a switch signal representing a closed state of the controlled switching element 6 is indicated by a value 1 and a switch signal representing an opened state of the switching element 6 is indicated by a value 0.

[0095] Further shown are time points SG1, SG2, SG3 of a switch signal generation and increasing offset values d for the different switching cycles. It is possible that the control unit 11 generates the switch signals, wherein an information on the time points SG1, SG2, SG3 of a switch signal generation is transmitted to the evaluation unit 9. In the first switching cycle C1, the evaluation unit 9 adds a first offset value, e. g. a value of 10 ns to the first time point SG1 of a switch signal generation in order to determine the first switching time point SG1. Then, the A/D converter unit 13 is controlled such that the sample of the phase current value at this switching time point SG1 is digitized.

[0096] In the second switching cycle C2, the evaluation unit 9 adds an increased offset value d to the second time point SG2 of the switch signal generation in order to determine the second switching time point SP2. Then, the A/D converter unit 13 is controlled such that the sample of the phase current value at this switching time point SG2 is digitized. Third, in third switching cycle C3, the third switching time point SP3 is determine as the third time point SG3 of the switch signal generation and an further increased offset value d. Then, the A/D converter unit 13 is controlled such that the sample of the phase current value at this switching time point SG3 is digitized.

[0097] Based on the digitized phase current values, the switching current is determined.