Resonant soft-switching voltage balancing converter for series-connected capacitors

Abstract

The present disclosure relates to a device for controlling a voltage balancing circuit for series-connected capacitors. The voltage balancing circuit comprises a first and second switching circuit being connected in series, and a resonant circuit. The first and second switching circuit may be connected in parallel to the series-connected capacitors. Each of the first and second switching circuit comprises a series connection of a first and second switching unit each comprising one or more semiconductor switches. A capacitor of the resonant circuit is connected between a node of the first switching circuit and a node of the second switching circuit. The device is configured to control, with a fixed duty cycle of 50% and a switching frequency computed using a resonant frequency of the resonant circuit, the first and second switching units. The device is configured to perform a resonant frequency correction method.

Description

[0001] A DEVICE FOR CONTROLLING A VOLTAGE BALANCING CIRCUIT FOR SERIES-CONNECTED CAPACITORS

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a device for controlling a voltage balancing circuit for series-connected capacitors, to a circuit comprising such a device and a voltage balancing circuit, and to a computer-implemented method for controlling a voltage balancing circuit for series-connected capacitors. The terms “voltage balancer” or “voltage balancer circuit” may be used as synonyms for the term “voltage balancing circuit”.

[0004] BACKGROUND

[0005] Converter circuits may be used for translating electrical power provided by a first electrical system to another form of electrical power used by a second electrical system. For example, a solar photovoltaic (PV) system translates solar energy into electrical energy in the form of direct current (DC) energy, wherein a converter circuit may be used to translated said DC energy into electrical energy in the form of alternating current (AC) energy for electrical supplying loads that are electrically powered by AC energy, such as home appliances, industry motors etc. A converter circuit may be also used to translate AC energy from an electrical energy source, such as mains, to DC energy to be provided to an uninterruptible power supply (UPS), a battery interface (e.g. Li-ion battery interface) etc. The term “power converter” may be used as a synonym for the term “converter”.

[0006] SUMMARY

[0007] Converter architectures with a split DC link have various advantages, such as lower device stress, improved efficiency and magnetic components size reduction. The split DC link means that a voltage is provided by a converter circuit across a series connection of two or more capacitors instead of providing the voltage across a single capacitor. The series connection of the at least two capacitors is / represents the split DC link. The series-connected capacitors may be referred to as “DC link capacitors”. The split DC link may be used for implementing a converter circuit as a multilevel converter. For example, a first converter stage may be electrically coupled to a second converter stage by the series connection of two or more capacitors being the split DC link. The voltage across the split DC link may be an output voltage of the first converter stage and an input voltage of the second converter stage when electrical energy is transferred from the first to the second converter stage and / or an input voltage of the first converter stage and an output voltage of the second voltage stage when electrical energy is transferred from the second to the first converter stage.

[0008] To avoid improper operation and possible damage of the components of a converter circuit comprising a split DC link, the voltage across the series-connected capacitors should be equalized. Asymmetric charging of the series-connected capacitors of the split DC link leads to unequal voltage sharing, which can affect the converter operation and damage the capacitors and other devices due to overvoltage. Therefore, voltage equalization circuitry methods may be used to ensure equal sharing between the series-connected capacitors.

[0009] For this, resonant balancer circuits, which are based on switching devices, i.e. semiconductor switches, with a resonant circuit, are an interesting option due to the promise of reduced switching losses with simple control. Herein, the term “semiconductor switch” may be abbreviated by the term “switch”. The capacitor of the resonant circuit transfers the charge between two series-connected capacitors (from the one with highest voltage to the one with lowest voltage) of the split DC link, resulting in an equalization of the voltage across both capacitors. An inductor may be added in series or in parallel with the capacitor to achieve resonance and reduce the switching losses.Resonant balancer circuits are attractive because they achieve voltage equalization with theoretically no switching losses, which enables to increase the switching frequency and thus reduce the size of components. A zero current switching (ZCS) of the semiconductor switches of the resonant balancer circuit may achieve no switching losses. Nevertheless, in practice, the resonant components value will vary (due to manufacture tolerances and changes in temperature and other parameters during operation). As a result, the resonant frequency will vary and the switching losses will increase.

[0010] Thus, when it comes to practical application, precise and expensive sensors may be required to avoid excessive increase in losses due to manufacture tolerances and changes in operating conditions. In the cases where the semiconductor switches of the voltage balancing circuit with a resonant circuit are switched at the resonant frequency to achieve ZCS, this low immunity leads to losing the ZCS operation, and increasing switching losses. This becomes more relevant at higher frequencies, which are typically preferred to reduce the size of the resonant components. One solution for countering this is to apply zero voltage switching (ZVS) instead. One approach is to reduce or increase the frequency in order to turn off the semiconductor switches while some current is still flowing to discharge the capacitances of the semiconductor switches before turn on. This approach requires a current sensor to switch at the adequate current level. Current sensors are bulkier and more expensive than voltage sensors. These current sensors need to provide fast and precise measurement, which also makes the implementation difficult at higher switching frequencies, since the delay of the current sensor can produce an increase in switching losses due to nonsufficient (or excessive) current at turn off.

[0011] Alternatively, precise sensors can be avoided at the expense of frequency reduction, with the consequent increase in the converter size. For achieving ZVS without the aforementioned current sensors (being high in costs and bulky) different modulation techniques may be used which do not allow operating the voltage balancing circuit with the resonant circuit at the resonant frequency of the resonant circuit. Instead, the switching frequency is higher. This becomes a problem when the balancing current is too low to fully discharge the devices capacitance before turn on, because switching losses cannot be avoided. In that case, the switching frequency is limited due to losses, and the size of the components will be larger for the same switching frequency, because they are designed for the resonant frequency, which is lower.

[0012] In view of the above, this disclosure aims to provide means that allow operating a voltage balancing circuit for series-connected capacitors with a reduced size and reduced costs. Another object may be to provide means that allow dealing with manufacture tolerances or changes in operating conditions of a resonant circuit of a voltage balancing circuit for series-connected capacitors without using expensive sensors to avoid excessive increase in losses due the manufacture tolerances or the changes in operating conditions.

[0013] These and other objectives are achieved by the solution of this disclosure as described in the independent claims. Advantageous implementations are further defined in the dependent claims.

[0014] Herein, the terms “electrically connect” and “connect” are used as synonyms. The term “controlled semiconductor switch” and “controllable semiconductor switch” may be used as a synonym for the semiconductor switches(s) being controllable by the device. The one or more semiconductor switches (controllable by the device) comprise a control terminal, such as a transistor, a thyristor, triac etc. The one or more semiconductor switches may be controlled via the control terminal to switch between the conducting state (i.e. on-state) and the non-conducting state (i.e. off-state). The terms “turn on” and “turn off’ may be used as synonym for “switching to the conducting state / on-state” and “switching to the non-conducting state / off-state”, respectively. Herein, semiconductor switches may be power semiconductor transistors, such as an insulated-gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs).Herein, the one or more semiconductor switches may be transistors. A semiconductor switch being a transistor may be an insulated-gate bipolar transistor (IGBT), a field-effect transistor (FET), a metal-oxide-semiconductor field-effect transistor (MOSFET), a bipolar junction transistors (BJT) or a junction gate field-effect transistor (JFET). Optionally, a semiconductor switch may be a semiconductor switch with a diode associated to it. For example, a semiconductor switch may be a transistor, such as a IGBT, with a diode connected in anti-parallel to the transistor. That is, in case a semiconductor switch is a transistor, optionally a diode is connected in anti-parallel to the transistor. For example, a semiconductor switch may be a transistor, such as a MOSFET, comprising an intrinsic body diode.

[0015] A first aspect of this disclosure provides a device for controlling a voltage balancing circuit for series-connected capacitors. The voltage balancing circuit comprises a first switching circuit and a second switching circuit being electrically connected in series to each other, and a resonant circuit comprising a resonant capacitor and a resonant inductor. The first switching circuit and the second switching circuit are configured to be electrically connected in parallel to a first capacitor and second capacitor, respectively, of the series-connected capacitors. Each of the first switching circuit and the second switching circuit comprises a series connection of a first switching unit and a second switching unit. Each of the first switching units and the second switching units comprises one or more semiconductor switches controllable by the device. The resonant capacitor of the resonant circuit is electrically connected between a node between the first switching unit and second switching unit of the first switching circuit and a node between the first switching unit and second switching unit of the second switching circuit. The device is configured to control, with a fixed duty cycle of 50% and using a switching frequency computed using a resonant frequency of the resonant circuit, the first switching units and second switching units to be alternately in a conducting state and non-conducting state such that the first switching units are never at the same time as the second switching units in the conducting state. The device is configured to perform a resonant frequency correction method by obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units or second switching units, the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the resonant circuit , and changing the used resonant frequency used for computing the switching frequency in case one of the obtained voltage sign measurements is negative.

[0016] In other words, the first aspect of this disclosure proposes performing a resonant frequency correction method for countering variations of the inductance value of the resonant inductor and / or the capacitance value of the resonant capacitor of the resonant circuit with regard to the values given in the data sheet of those components. These variations may be due to manufacturing tolerances and changes in temperature and other parameters during operation. For this, it is sufficient to use voltage sign measurements for detecting whether the actual values of the resonant capacitor and / or the resonant inductor deviate from the capacitance value and inductance value, respectively, of the data sheet such that the resonant frequency computed using the data sheet values deviates from (e.g. is greater or smaller than) the actual resonant frequency computed when using the actual values of the resonant capacitor and resonant inductor. Since for this merely voltage sign measurements) are done, low cost sensor(s) for measuring such voltage signs measurements can be used instead of using expensive sensors. Thus, the device of the first aspect allows achieving a voltage equalization of the series-connected capacitors with reduced switching losses, high immunity to components tolerances and variation (with temperature, DC bias, etc.), and overcurrent protection with the use of low-cost voltage sensors. The device according to the first aspect allows operating the voltage balancing circuit at a switching frequency close to the resonance (i.e. resonant frequency) of the resonant circuit with low cost voltage sign detectors and sensors. Performing the resonant frequency correction method allows operating the voltage balancing circuit with minimum switching losses regardless of components tolerance and variation, especially with regard to the components of the resonant circuit. Therefore, the device of the first aspect allows dealing with manufacture tolerances and changes in temperature and other parameters during operation of the resonant circuit of the voltage balancing circuit for the series-connected capacitorswithout using expensive sensors to avoid excessive increase in losses due to the manufacture tolerances and changes in operating conditions.

[0017] The term “constant duty cycle” may be used as a synonym for the term “fixed duty cycle”. The terms “resonant balancing circuit” and “resonant balancer circuit” may be used as synonyms for the term “voltage balancing circuit”. The resonant inductor may be electrically connected in series with the resonant capacitor between the node between the first switching unit and second switching unit of the first switching circuit and the node between the first switching unit and second switching unit of the second switching circuit. Alternatively, a terminal of the resonant inductor may be electrically connected with a node between the first switching circuit and the second switching circuit and a second terminal of the resonant inductor may be configured to be electrically connected with a node between the first capacitor and second capacitor of the series-connected capacitors. The first switching unit being in the conducting state means that all switches (e.g. being a single switch) of the first switching unit are in the conducting state. The first switching unit being in the non-conducting state means that all switches (e.g. being a single switch) of the first switching unit are in the non-conducting state. Accordingly, the second switching unit being in the conducting state means that all switches (e.g. being a single switch) of the second switching unit are in the conducting state, and the second switching unit being in the non-conducting state means that all switches (e.g. being a single switch) of the second switching unit are in the non-conducting state

[0018] The switching frequency computed using the resonant frequency may be computed to be close or equal to the resonant frequency of the resonant circuit. This allows achieving higher resonant frequencies and, thus, a minimum volume of the voltage balancing circuit.

[0019] The device is configured to perform the resonant frequency correction method. The resonant frequency correction method comprises the step of obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units or second switching units, the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the resonant circuit. The resonant frequency correction method comprises the step of changing the used resonant frequency used for computing the switching frequency in case one of the obtained voltage sign measurements is negative.

[0020] In an implementation form of the first aspect, the device is configured to control the first switching units and second switching units using a delay time such that after switching the first switching units or the second switching units from the conducting state to the non-conducting the delay time is waited before the second switching units or first switching units, respectively, are switched from the non-conducting state to the conducting state. This allows preventing a short circuit occurring via a respective series connection of the first switching unit and the second switching unit of the first or second switching circuit, which may lead to a damage of the voltage balancing circuit.

[0021] In an implementation form of the first aspect, the device is configured to control the switching of the first switching units and second switching units such that the switching frequency equals to the inverse of a sum of two times the delay time and the inverse of the used resonant frequency. This allows operating the voltage balancing circuit close to the actual resonant frequency of the resonant circuit, while preventing a short circuit across a respective series connection of the first switching unit and the second switching unit of the first or second switching circuit. Thus, this allows a small volume of the voltage balancing circuit and high switching frequencies.

[0022] In an implementation form of the first aspect, the device is configured to perform a resonant frequency correction method by increasing the used resonant frequency in case the voltage sign measurement in the first switching circuit is negative and thevoltage of the first capacitor is greater than the voltage of the second capacitor, or the voltage sign measurement in the second switching circuit is negative and the voltage of the first capacitor is smaller than the voltage of the second capacitor. This allows compensating a deviation of the actual values of the resonant capacitor and / or resonant inductor from the respective value of the data sheet. Thus, this allows dealing with manufacturing tolerances and changes in temperature and other parameters during operation of the components of the resonant circuit.

[0023] In an implementation form of the first aspect, the device is configured to perform a resonant frequency correction method by decreasing the used resonant frequency in case the voltage sign measurement in the first switching circuit is negative and the voltage of the first capacitor is smaller than the voltage of the second capacitor, or the voltage sign measurement in the second switching circuit is negative and the voltage of the first capacitor is greater than the voltage of the second capacitor. This allows compensating a deviation of the actual values of the resonant capacitor and / or resonant inductor from the respective value of the data sheet. Thus, this allows dealing with manufacturing tolerances and changes in temperature and other parameters during operation of the components of the resonant circuit.

[0024] In an implementation form of the first aspect, the device is configured to perform the resonant frequency correction method at a time when the device controls semiconductor switches, at which the voltage sign measurements are obtained, to be switched from the conducting state to the non-conducting state.

[0025] In an implementation form of the first aspect, the device is configured to periodically perform the resonant frequency correction method. This allows a more precise correction of a deviation of the actual values of the resonant capacitor and / or resonant inductor from the respective values of the data sheet.

[0026] In an implementation form of the first aspect, the device is configured to stop performing the resonant frequency correction method in case a set time elapses after start of performing the resonant frequency correction method, or the voltage sign measurement in one of the first switching circuit and the second switching circuit becomes negative after the voltage sign measurement in the other of the first switching circuit and the second switching circuit being negative.

[0027] In an implementation form of the first aspect, the device is configured to control the first switching units and the second switching units to be switched from the conducting state to the non-conducting state while a desired current flows through the resonant circuit by performing a second method. The second method comprises the steps of obtaining a voltage measurement of a voltage of the resonant capacitor or the resonant inductor, computing a peak current of the resonant circuit using the obtained voltage measurement, the used resonant frequency and additionally the capacitance of the resonant capacitor or inductance of the resonant inductor, and computing the switching frequency for switching the first switching units and second switching units using the computed peak current, the used resonant frequency and the desired current.

[0028] This allows applying an optimum off-current and thus achieving an optimal trade-off between turn off losses (caused by the circulating current at the moment of turn off of respective semiconductor switches) and turn on losses (caused by the discharging of a parasitic capacitance of the semiconductor switches). In other words, this allows providing an optimum level of current flowing through the resonant circuit at the moment of switching semiconductor switches to the off-state. The second method may be referred to as “off-current adjustment method”.

[0029] In an implementation form of the first aspect, the device is configured to compute the switching frequency by additionally using the delay time. This allows preventing a short circuit occurring via a respective series connection of the first switching unit and the second switching unit of the first or second switching circuit, which may lead to a damage of the voltage balancing circuit.In an implementation form of the first aspect, the device is configured to perform the second method after having performed the resonant frequency correction method.

[0030] In an implementation form of the first aspect, each of the first switching units and the second switching units is a semiconductor switch controllable by the device, the multiple semiconductor switches being electrically connected in series. The device is configured to perform the resonant frequency correction method by obtaining the voltage sign measurement of the voltage across the semiconductor switch of each of the first switching units or second switching units, and changing the used resonant frequency in case one of the obtained voltage sign measurements is negative.

[0031] In an implementation form of the first aspect, the voltage balancing circuit comprises a third switching circuit being electrically connected in series with the second switching circuit, and a second resonant circuit comprising a second resonant capacitor and a second resonant inductor. The third switching circuit is configured to be electrically connected in parallel to a third capacitor of the series-connected capacitors. The third switching circuit comprises a series connection of the first switching unit and the second switching unit. The second resonant capacitor of the second resonant circuit is electrically connected between the node between the first switching unit and second switching unit of the second switching circuit and a node between the first switching unit and second switching unit of the third switching circuit. The device is configured to control switching of the first switching unit and second switching unit of the third switching circuit using a second switching frequency computed using a resonant frequency of the second resonant circuit. The device may be configured to perform a second resonant frequency correction method with regard to the resonant frequency of the second resonant circuit by obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units or second switching units of the second and third switching circuits, the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the second resonant circuit, and changing the used resonant frequency of the second resonant circuit used for computing the second switching frequency in case one of the obtained voltage sign measurements in the second and third switching circuit is negative. This allows providing a voltage balancing circuit for a series connection of capacitors comprising more than two capacitors being electrically connected in series.

[0032] In an implementation form of the first aspect, the voltage balancing circuit comprises a third switching circuit and a fourth switching circuit being electrically connected in series to each other, and a second resonant circuit comprising a second resonant capacitor and a second resonant inductor. The third switching circuit and the fourth switching circuit are configured to be electrically connected in parallel to the second capacitor and a third capacitor of the series-connected capacitors, respectively. Each of the third switching circuit and the fourth switching circuit comprises a series connection of the first switching unit and the second switching unit. The second resonant capacitor of the second resonant circuit is electrically connected between a node between the first switching unit and second switching unit of the third switching circuit and a node between the first switching unit and second switching unit of the fourth switching circuit. The device is configured to control switching of the first switching units and second switching units of the third switching circuit and fourth switching circuit using a second switching frequency computed using a resonant frequency of the second resonant circuit. The device may be configured to perform a second resonant frequency correction method with regard to the resonant frequency of the second resonant circuit by obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units or second switching units of the third and fourth switching circuits , the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the second resonant circuit, and changing the used resonant frequency of the second resonant circuit used for computing the second switching frequency in case one of the obtained voltage sign measurements in the second and thirdswitching circuit is negative. This allows providing a voltage balancing circuit for a series connection of capacitors comprising more than two capacitors being electrically connected in series.

[0033] The device may be or may comprise at least one of processors), microprocessor(s), microcontroller(s), controller(s), application specific integrated circuit(s) (ASIC(s)) and field programmable gate array(s) (FPGA(s)).

[0034] In order to achieve the device according to the first aspect of this disclosure, some or all of the implementation forms and optional features of the first aspect, as described above, may be combined with each other.

[0035] A second aspect of this disclosure provides a circuit. The circuit comprises the device according to the first aspect, as described above, and a voltage balancing circuit. The voltage balancing circuit comprises a first switching circuit and a second switching circuit being electrically connected in series to each other, and a resonant circuit comprising a resonant capacitor and resonant inductor. The first switching circuit and the second switching circuit are configured to be electrically connected in parallel to a first capacitor and a second capacitor, respectively, of a series-connected capacitors. Each of the first switching circuit and the second switching circuit comprises a series connection of a first switching unit and a second switching unit. Each of the first switching units and the second switching units comprises one or more semiconductor switches controllable by the device. The resonant capacitor of the resonant circuit is electrically connected between a node between the first switching unit and second switching unit of the first switching circuit and a node between the first switching unit and second switching unit of the second switching circuit.

[0036] The circuit may be a converter circuit. The circuit may be part of a converter circuit, the converter circuit may comprise one or more converter stages. The converter circuit may be a converter circuit for a DC-to-DC conversion.

[0037] The above description of the device according to the first aspect of this disclosure is correspondingly valid for the circuit according to the second aspect of this disclosure.

[0038] The circuit of the second aspect and its implementation forms and optional features achieve the same advantages as the device of the first aspect and its respective implementation forms and respective optional features.

[0039] A third aspect of this disclosure provides a computer-implemented method for controlling a voltage balancing circuit for series-connected capacitors. The voltage balancing circuit comprises a first switching circuit and a second switching circuit being electrically connected in series to each other, and a resonant circuit comprising a resonant capacitor and a resonant inductor. The first switching circuit and the second switching circuit are configured to be electrically connected in parallel to a first capacitor and second capacitor, respectively, of the series-connected capacitors. Each of the first switching circuit and the second switching circuit comprises a series connection of a first switching unit and a second switching unit. Each of the first switching units and the second switching units comprises one or more semiconductor switches. The resonant capacitor of the resonant circuit is electrically connected between a node between the first switching unit and second switching unit of the first switching circuit and a node between the first switching unit and second switching unit of the second switching circuit. The method comprises controlling, with a fixed duty cycle of 50% and using a switching frequency computed using a resonant frequency of the resonant circuit, the first switching units and second switching units to be alternately in a conducting state and non-conducting state such that the first switching units are never at the same time as the second switching units in the conducting state. The method comprises performing a resonant frequency correction method by obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units or second switching units, the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to thenon-conducting state while a current flow is present via the resonant circuit, and changing the used resonant frequency used for computing the switching frequency in case one of the obtained voltage sign measurements is negative.

[0040] The above description of the device according to the first aspect of this disclosure is correspondingly valid for the computer implemented method according to the third aspect of this disclosure. The device of the first aspect may perform the method of the third aspect.

[0041] The method of the third aspect and its implementation forms and optional features achieve the same advantages as the device of the first aspect and its respective implementation forms and respective optional features.

[0042] The device according to the first aspect, the circuit according to the second aspect and the computer-implemented method according to the third aspect may be used in any application in which a voltage equalization of two or more series-connected capacitors by a voltage balancing circuit comprising a resonant circuit and semiconductor switches for controlling the voltage equalization is desired. Such applications include a solar photovoltaic (PV) system that translates solar energy into electrical energy in the form of direct current (DC) energy, wherein a converter circuit comprising a series connection of capacitors (as a split DC link) may be used to translated said DC energy into electrical energy in the form of alternating current (AC) energy for electrical supplying loads that are electrically powered by AC energy, such as home appliances, industry motors etc. Such applications include converter circuits comprising a series connection of capacitors (as a split DC link) for translating AC energy from an electrical energy source, such as mains, to DC energy to be provided to an uninterruptible power supply (UPS), a battery interface (e.g. Li-ion battery interface) etc.

[0043] All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

[0044] BRIEF DESCRIPTION OF DRAWINGS

[0045] The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which:

[0046] FIG. 1 shows an example of a device according to this disclosure for controlling a voltage balancing circuit for series- connected capacitors, and an example of a circuit according to this disclosure comprising the device and such a voltage balancing circuit.

[0047] FIG. 2 shows an example of an implementation form of the voltage balancing circuit shown in FIG. 1.

[0048] FIG. 3 shows an example of control signals providable by the device of FIG. 1 for controlling the voltage balancing circuit.

[0049] FIG. 4 shows the current through the resonant circuit of the voltage balancing circuit of FIG. 1 for an ideal case where the actual values of the components of the resonant circuit of the voltage balancing circuit equal to the valuesprovided in a data sheet for these components and a case where the actual values of the components of the resonant circuit deviate from the values provided in the data sheet.

[0050] FIG. 5 shows an example of a resonant frequency correction method performable by the device of FIG. 1.

[0051] FIG. 6 shows an example of an off-current adjustment method performable by the device of FIG. 1.

[0052] FIG. 7 shows an example of the current through the resonant circuit of the voltage balancing circuit of FIG. 1 over time with a desired value of said current at the time of turning-off switching units of the voltage balancing circuit.

[0053] FIG. 8 shows an example of a control scheme performable by the device of FIG. 1.

[0054] FIG. 9 shows an example of the current flowing through the resonant circuit of the voltage balancing circuit when the device of FIG. 1 performs no correction, performs the resonant frequency correction method and performs additionally the off-current adjustment method.

[0055] FIG. 10 shows an example of current flowing through the resonant circuit of the voltage balancing circuit when the device of FIG. 1 performs the off-current adjustment method without the resonant frequency correction method and when the device of FIG. 1 performs the off-current adjustment method and the resonant frequency correction method.

[0056] FIG. 11 shows an example of an implementation form of the voltage balancing circuit of FIG. 1.

[0057] FIG. 12 shows an example of an implementation form of the voltage balancing circuit of FIG. 1.

[0058] FIG. 13 shows an example of a computer-implemented method according to this disclosure for controlling a voltage balancing circuit for series-connected capacitors.

[0059] Same elements shown in the Figures are labeled with the same reference sign, and may be implemented likewise.

[0060] DETAILED DESCRIPTION OF EMBODIMENTS

[0061] FIG. 1 shows an example of a device according to this disclosure for controlling a voltage balancing circuit for series-connected capacitors, and an example of a circuit comprising the device and such a voltage balancing circuit. The device of FIG. 1 is an example of the device according to the first aspect of this disclosure. The description of the device according to the first aspect is correspondingly valid for the device of FIG. 1. The circuit of FIG. 1 is an example of the circuit according to the second aspect of this disclosure. The description of the circuit according to the second aspect is correspondingly valid for the circuit of FIG. 1.

[0062] The device 100 of FIG. 1 is a device for controlling a voltage balancing circuit 200 for series-connected capacitors 300. The device 100 may comprise a processor or processing circuitry (not shown) configured to perform, conduct or initiate the various operations of the device 100 described herein. The processing circuitry may comprise hardware and / or the processing circuitry may be controlled by software. The hardware may comprise analogue circuitry or digital circuitry, or both analogue and digital circuitry. The digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or multi-purpose processors. The device 100 may further comprise memory circuitry, which stores one or more instruction(s) that can be executed by the processor or by the processing circuitry, in particular under control of the software. For instance, the memory circuitry may comprise a non-transitory storage medium storing executable software code which, when executed by the processor or the processing circuitry, causes the various operations of the device to be performed. In one embodiment, the processing circuitry comprises one or more processors and a non-transitory memory connected to the one or more processors. The non-transitory memory may carry executable program code which, when executed by the one or more processors, causes the device 100 to perform, conduct or initiate the operations or methods described herein.

[0063] As shown in FIG. 1 , the voltage balancing circuit 200 comprises a first switching circuit 1 and a second switching circuit 2 being electrically connected in series to each other, and a resonant circuit RES 1. The resonant circuit RES 1 comprises a resonant capacitor Cr and a resonant inductor Lr. The first switching circuit 1 and the second switching circuit 2 are configured to be electrically connected in parallel to a first capacitor C 1 and second capacitor C2, respectively, of the series-connected capacitors 300. That is, the series connection of the first switching circuit 1 and the second switching circuit 2 is configured to be electrically connected in parallel to the series connection 300 of the capacitors Cl, C2 in order to equalize, i.e. balance, the voltages across each of said series-connected capacitors Cl, C2. As shown in FIG. 1, the node N3 (mid-point) between the first switching circuit 1 and second switching circuit 2 may be connected with the node N4 (mid-point) between the first capacitor Cl and second capacitor C2.

[0064] The first switching circuit 1 comprises a series connection of a first switching unit 11 and a second switching unit 12. The second switching circuit 2 comprises a series connection of a first switching unit 21 and a second switching unit 22. Each of the first switching units 11, 21 and the second switching units 12, 22 comprises one or more semiconductor switches controllable by the device 1. For example, according to FIG. 1, the first switching unit 11 of the first switching circuit 1 comprises a semiconductor switch SI 1, the second switching unit 12 of the first switching circuit 1 comprises a semiconductor switch SI 2, the first switching unit 21 of the second switching circuit 2 comprises a semiconductor switch S21, and the second switching unit 22 of the second switching circuit 2 comprises a semiconductor switch S22. The number of semiconductor switches of the first and second switching units shown in FIG. 1 is only by way of example and may be different. That is at least one, optionally each, of the first switching units 11, 21 and the second switching units 12, 22 may comprise more than one semiconductor switch, i.e. two or more semiconductor switches. Herein, the terms “two or more” and “multiple” may be used as synonyms. Optionally, diode(s) and / or capacitor(s) may be electrically connected between semiconductor switches of the multiple semiconductor switches of at least one switching unit, optionally of each switching unit 11, 12, 21, 22. According to the example of FIG. 1, the semiconductor switches Sil, S12, S21, S22 of the first and second switching units 11, 12, 21, 22 are IGBTs with a diode connected in anti-parallel to each IGBT. This is only by way of example and at least one, optionally each, of the semiconductor switches Sil, S12, S21, S22 of the first and second switching units 11, 12, 21, 22 may be implemented differently, e.g. by a different transistor type. The description of FIG. 1 is correspondingly valid irrespective of the number of semiconductor switches, optional diode(s) and capacitor(s) being present in the switching units and the type of the semiconductor switches. The capacitor Cp connected in parallel with each semiconductor switch, shown in FIG. 1, represents a parasitic capacitance of the respective semiconductor switch.

[0065] The resonant capacitor Cr of the resonant circuit RES1 is electrically connected between a node N1 between the first switching unit 11 and second switching unit 12 of the first switching circuit 1 and a node N2 between the first switching unit 21 and second switching unit 22 of the second switching circuit 2. As shown in FIG. 1 , the resonant inductor Lr of the resonant circuit RES1 may be electrically connected in series with the resonant capacitor Cr of the resonant circuit REST That is, the resonant inductor Lr may be electrically connected between the node N1 between the first switching unit 11 and second switching unit 12 of the first switching circuit 1 and the node N2 between the first switching unit 21 and second switching unit 22 of thesecond switching circuit 2. Alternatively, a terminal of the resonant inductor Lr may be electrically connected with the node N3 between the first switching circuit 1 and the second switching circuit 2 and a second terminal of the resonant inductor L2 may be configured to be electrically connected with the node N4 between the first capacitor C 1 and second capacitor C2 of the series-connected capacitors Cl, C2. This optional implementation form is shown in FIG. 2. Thus, FIG. 2 shows an example of an implementation form of the voltage balancing circuit shown in FIG. 1 , wherein the resonant inductor L2 is electrically connected in the voltage balancing circuit as previously outlined. The description of the device 100 and voltage balancing circuit 200 of FIG. 1 is correspondingly valid for the device 100 and voltage balancing circuit 200 of FIG. 2.

[0066] The device 1 of FIG. 1 is configured to control, with a fixed duty cycle of 50% and using a switching frequency computed using a resonant frequency of the resonant circuit RES1, the first switching units 11,21 and second switching units 12, 22 to be alternately in a conducting state and non-conducting state such that the first switching units 11, 12 are never at the same time as the second switching units 21 , 22 in the conducting state. The first switching unit 11 or 21 being in the conducting state means that all switches (e.g. being a single switch) of the first switching unit 11 or 21, respectively are in the conducting state. Accordingly, the second switching unit 21 or 22 being in the conducting state means that all switches (e.g. being a single switch) of the second switching unit 21 or 22, respectively, are in the conducting state.

[0067] For controlling the voltage balancing circuit 200 the device 100 is configured to provide control signals CS1 and CS2 to the control terminals of the semiconductor switches Sil, S12, S21, S22 of the first switching units 11, 21 and second switching units 12, 22. The device 1 is configured to control, with the fixed duty cycle of 50% and using the switching frequency being computed using the resonant frequency of the resonant circuit RES 1 , the first switching units 11,21 and second switching units 12, 22 to be alternately in a conducting state and non-conducting state by providing a first control signal CS1 to the control terminal of the semiconductor switches Sil, S21 of the first switching units 11, 21 and a second control signal CS2 to the control terminal of the semiconductor switches S21 , S22 of the second switching units 21 , 22. The first control signal CS 1 and the second control signals CS2 are complementary control signals, wherein the first control signal CS1 controls the first switching units 11,21 and, thus, the semiconductor switches Sil, S21 synchronously. The second control signal CS2 controls the second switching units 12, 22 and, thus, the semiconductor switches S12, S22 synchronously. The control signals CS1 and CS2 may have a fixed duty cycle with varying frequency.

[0068] FIG. 3 shows an example of control signals providable by the device 100 of FIG. 1 for controlling the voltage balancing circuit 200. The top graph of FIG. 3 shows the control signal CS1, the middle graph of FIG. 3 shows the control signal CS2 and the bottom graph of FIG. 3 shows the current Ir flowing through the resonant circuit RES1, e.g. through the capacitor Cr over time. Herein and in the FIGs, the current flowing through the resonant circuit RES1 may be also represented by “ires”. Thus, the horizontal axis of these graphs represents the time t, whereas the vertical axis of the top and middle graph represents the voltage level of the respective control signal CS1 or CS2 being either a low level or a high level, and the vertical axis of the bottom graph represents the current Ir through the resonant circuit RES1.

[0069] It may be assumed that when the voltage level of a respective control signal CS1 or CS2 is the high level then the semiconductor switch being controlled by the respective control signal is in the conducting state, and when the voltage level of the respective control signal is the low level then the semiconductor switch being controlled by the respective control signal is in the nonconducting state. This may be opposite. Whether the on-state is controlled by the high level or low level depends on the type of semiconductor switches used. As may be derived from FIG. 3, while the first control signal CS1 has the high level the second control signal CS2 has the low level, and while the second control signal CS2 has the high level the first control signal CS1 has the low level. Due to this control by the device 100, the first switching units 11, 21 are never at the same time as the second switching units 12, 22 in the conducting state. Without the optional delay times td, the time during which the first control signal CS1 or the second control signal CS2 has the high level (e.g. controlling the on-state of a respective semiconductor switch)equals the time during which the first control signal CS1 or the second control signal CS2, respectively, has the low level (e.g. controlling the off-state of the respective semiconductor switch). Thus, the device 100 controls the first switching units 11, 21 and second switching units 12, 22 with a fixed duty cycle of 50%.

[0070] As shown in FIG. 3, while doing so the device 100 may control the first switching units 11, 21 and second switching units 12, 22 using the optional delay time td such that after switching the first switching units 11, 21 or the second switching units 12, 22 from the conducting state to the non-conducting (e.g. respective control signal CS1 or CS2 goes from the high level to the low level) the delay time td is waited before the second switching units 12, 22 or first switching units 11, 21, respectively, are switched from the non-conducting state to the conducting state (e.g. respective control signal CS2 or CS1 goes from the low level to the high level). In this case the effective duty cycle of the switching (i.e. the ratio between the on-state to the sum of the on-state and off-state being one switching period) is not equal to 50%. Nevertheless, the device 100 controls the switching units 11, 12, 21, 22 with the 50% duty cycle while waiting the delay time td between switching the first switching units 11, 21 or the second switching units 12, 22 from the off-state to the on-state after switching the second switching units 12, 22 or the first switching units 11, 21, respectively from the on-state to the off-state. Namely, as soon as the delay time td is not implemented, the effective duty cycle equals to 50%. The delay time td may be referred as a time delay between off and on transitions.

[0071] The bottom graph shows the resulting current Ir flowing through the resonant circuit RES1 under ideal conditions as a result of the device 100 controlling switching of the first and second switching units 11,12, 21,22 using the first control signal CS1 and second control signal CS2 shown in FIG. 3. During the delay times td the current flow Ir through the resonant circuit RES1 is interrupted, as shown in FIG. 3. That is, under ideal conditions, if the switching period is equal to the resonant period plus the delay times td between turn off and turn on, the current Ir through the resonant circuit and, thus, voltage balancing circuit is a sinewave, and the turn on and turn off transitions occur at the zero-crossings of this sinusoidal current Ir, as illustrated in Fig. 3. That is, zero current switching (ZCS) is achieved. The resonant frequency fies.theo and switching frequency fsw,zcs,theo in this case may be calculated with the equations (1) and (2):

[0072]

[0073] In above equations (1 ) and (2), Lris the resonant inductance, Cris the resonant capacitance and td is the delay time between off and on transitions.

[0074] In practice, the inductance and capacitance value of the resonant inductor Lr and resonant capacitor Cr, respectively, will not be identical to theoretical values (e.g. the values provided by a data sheet), and the actual resonant frequency fies.prac of the resonant circuit RES1 will drift to equation (3).

[0075]

[0076]

[0077] Considering a maximum variation of the components of ±20%, the worst-case error of the resonant frequency is ±20%. As a consequence, a ZCS operation cannot be guaranteed and switching losses will increase. In order to counter this, the device 100 of FIG. 1 is configured to perform a resonant frequency correction method. For this, the device 100 is configured to obtain a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units 11, 21 or second switching units 12, 22, the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the resonant circuit RES1. In the example implementation of the voltage balancing circuit 200 shown in FIG. 1 , the voltage sign measurements of each of the second switching units 12, 22 are used. This is only by way of example and may be different. That is, alternatively, the voltage sign measurements of each of the first switching units 11, 21 may be used. In the example of FIG. 1, each of the semiconductor switch S12 and the semiconductor switch S22 are a semiconductor switch that is arranged in the respective second switching unit 12 or 22, respectively, such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the resonant circuit RES1.

[0078] For measuring the voltage sign of a voltage across the respective semiconductor switch, voltage sensors (e.g. voltage sign sensors) may be used, e.g. be electrically connected to the respective semiconductor switch. This is indicated in FIG. 1 by dashed circles, wherein the voltage sign measurement of the voltage across the semiconductor switch S12 of the second switching unit 12 of the first switching circuit 1 is referred to as “VSM1”, and the voltage sign measurement of the voltage across the semiconductor switch S22 of the second switching unit 22 of the second switching circuit 2 is referred to as “VSM2”. These two voltage sign measurements VSM1 and VSM2 are provided to the device 100. The voltage sensors (e.g. voltage sign sensors) may be implemented according to any known method, i.e. any known voltage sensor (e.g. voltage sign sensor) may be used. The voltage sign measurements VSM1 and VSM2 may be provided in any known way to the device 100. The description is correspondingly valid, when the voltage sign of the voltages across a semiconductor switch of each of the first switching units 11, 21 is used by the device 100 for the resonant frequency correction method. The device 100 may perform the resonant frequency correction method by changing the used resonant frequency used for computing the switching frequency in case one of the obtained voltage sign measurements VSM1 and VSM2 is negative.

[0079]

[0080] FIG. 4 shows the current through the resonant circuit of the voltage balancing circuit of FIG. 1 for an ideal case where the actual values of the components of the resonant circuit of the voltage balancing circuit equal to the values provided in a datasheet for these components and a case where the actual values of the components of the resonant circuit deviate from the values provided in the data sheet.

[0081] The graph (A) of FIG. 4 shows the current Ir through the resonant circuit RES1 under ideal conditions (i.e. the actual values of the resonant inductor Lr and the resonant capacitor Cr correspond to the values indicated in the data sheet, meaning there are no deviation between the actual values and the values provided by the data sheet). Thus, the graph (A) of FIG. 4 shows the same current Ir behavior over time as shown by the bottom graph of FIG. 3. The graphs (B) and (C) of FIG. 4 show the current Ir through the resonant circuit RES1 under real life conditions, where the actual values of the resonant inductor Lr and / or the resonant circuit Cr may deviate from the values indicated in the data sheet. Thus, the sine curve of the current Ir is changed, as indicated by the arrows in the graphs (B) and (C). The graph (B) shows the current behavior over time in case the value of the resonant inductor Lr and / or resonant capacitor Cr is smaller than the respective value of the data sheet, whereas the graph (C) shows the current behavior over time in case the value of the resonant inductor Lr and / or resonant capacitor Cr is greater than the respective value of the data sheet.

[0082] FIG. 5 shows an example of a resonant frequency correction method performable by the device of FIG. 1. As shown in FIG. 5, when the device 100 initially starts with controlling the voltage balancing circuit 200 (i.e. start up YES), the device 100 may use as the resonant frequency f for computing the switching frequency the resonant frequency fies.theo assumed under ideal conditions, i.e. when the values of the resonant inductor Lr and the resonant capacitor Cr do not deviate from the respective values provided by the data sheet. This resonant frequency fres.theo is computed using the above equation (1). In other words, a default value may be used as the resonant frequency. Thus, upon start up of the voltage balancing circuit 200 the device 100 may compute the switching frequency f according to above equation (2). Otherwise (i.e. start up NO), the device 100 may correct the resonant frequency f used for computing the switching frequency as follows.

[0083] As shown in FIG. 5, the device increases the used resonant frequency f in case the voltage sign measurement VSM1 in the first switching circuit 1 is negative (i.e. in the example of FIG. 1, the voltage Vsn across the semiconductor switch S12 of the second switching unit 12 of the first switching circuit 1 is negative (VS12 < 0)) and the voltage Vci of the first capacitor Cl is greater than the voltage Vc? of the second capacitor C2 (Vci > Vci), or in case the voltage sign measurement VSM2 in the second switching circuit 2 is negative (i.e. in the example of FIG. 1, the voltage Vs22 across the semiconductor switch S22 of the second switching unit 22 of the second switching circuit 2 is negative (VS22 < 0)) and the voltage Vci of the first capacitor Cl is smaller than the voltage Vc2 of the second capacitor C2 (Vci < Vci).

[0084] As shown in FIG. 5, the device decreases the used resonant frequency f in case the voltage sign measurement VSM1 in the first switching circuit 1 is negative (i.e. in the example of FIG. 1, the voltage Vsi2 across the semiconductor switch S12 of the second switching unit 12 of the first switching circuit 1 is negative (VS12 < 0)) and the voltage Vci of the first capacitor Cl is smaller than the voltage Vc2 of the second capacitor C2 (Vci < Vci), or in case the voltage sign measurement VSM2 in the second switching circuit 2 is negative (i.e. in the example of FIG. 1, the voltage VSM across the semiconductor switch S22 of the second switching unit 22 of the second switching circuit 2 is negative (VS22 < 0)) and the voltage Vci of the first capacitor Cl is greater than the voltage Vci of the second capacitor C2 (Vci > Vci).

[0085] In other words, as shown in Figure 5, considering an unbalance condition where the voltage Vci of the first capacitor Cl is greater than the voltage Vci of the second capacitor C2 (YES for Vci > Vci), there are three possible scenarios. In a first scenario, the voltage Vsn across the semiconductor switch S12 and the voltage Vsn across the semiconductor switch S22 are both positive. This means that both switches S12 and S22 have been turned off at zero current (i.e. ZCS operation is performed) so that the switching frequency corresponds to the switching frequency fsw,zcs,prac of above equation (4). Thus, no action is to be taken in order to achieve the ZCS operation. In a second scenario, the voltage Vsn across the semiconductor switch S22 isnegative (Vs22 < 0), e.g. current flows through the diode being connected in antiparallel to the semiconductor switch S22 being an IGBT. This means that the switches S12 and S22 have been turned off before the current Ir through the resonant circuit RES1 could reach zero Amperes and the switching frequency (used by the device 100 for controlling switching of the semiconductor switches of the first and second switching circuits 1 and 2) is greater than the switching frequency fsw,zcs,prac of above equation (4). To correct this, the device 100 may decrease the resonant frequency f used for computing the switching frequency and, thus, decrease the switching frequency. In a third scenario, the voltage Vsi2 across the semiconductor switch S12 is negative (Vsi2 < 0), e.g. current flows through the diode being connected in antiparallel to the semiconductor switch S12 being an IGBT. This means that the switches S12 and S22 have been turned off after the current Ir through the resonant circuit RES1 reached zero Amperes and the switching frequency (used by the device 100 for controlling switching of the semiconductor switches of the first and second switching circuits 1 and 2) is smaller than the switching frequency fsw,zcs,prac of above equation (4). To correct this, the device 100 may increase the resonant frequency f used for computing the switching frequency and, thus, increase the switching frequency.

[0086] The above described corrective actions will cause small variations of the used resonant frequency f (used for computing the switching frequency) and, thus, of the switching frequency. Thus, when the device 100 performs the resonant frequency correction method multiple times (e.g. after every switching period, i.e. each switching cycle), the switching frequency f used by the device 100 for controlling the switching of the semiconductor switches Sil, S12, S21, S22 converges to the switching frequency fsw,zcs,prac of above equation (4) and ZCS is achieved. Convergence may typically be achieved after few times of performing the resonant frequency correction method (e.g. after a few switching cycles), so the device 100 may perform the resonant frequency for a short duration.

[0087] The device 100 may be configured to perform the resonant frequency correction method at a time when the device 100 controls semiconductor switches (e.g. switch S12 and S22), at which the voltage sign measurements VSM1 and VSM2 are obtained, to be switched from the conducting state to the non-conducting state. For example, the falling edge of the respective control signal (i.e. the second control signal CS2 for the switches S12 and S22) may trigger the device 100 to perform the resonant frequency correction method. The device 100 may periodically perform the resonant frequency correction method, e.g. every switching cycle / after every switching period. The device 100 may stop performing the resonant frequency correction method in case a set time elapses after start of performing the resonant frequency correction method. The set time may be set such that the switching frequency converges to the switching frequency fsw,zcs,prac of above equation (4). Alternatively, device 100 may stop performing the resonant frequency correction method in case the voltage sign measurement VSM1 or VSM2 in one of the first switching circuit 1 and the second switching circuit 2 becomes negative after the voltage sign measurement VSM2 or VSM1, respectively, in the other of the first switching circuit 1 and the second switching circuit 2 being negative.

[0088] If the direction of the unbalance of the voltages across the capacitors Cl and C2 is opposite (i.e. the voltage Vci of the first capacitor Cl is smaller than the voltage Vc2 of the second capacitor C2), then the above described second and third scenarios are reversed, as shown in FIG. 5. The above description is accordingly valid. As outlined above, for the resonant frequency correction method merely the sign of the voltage across the respective semiconductor switches is relevant. Therefore, no precise voltage sensors are needed, but simple comparators can be used, which simplifies the implementation. Once the device 100 has computed the actual resonant frequency or a value close to the actual resonant frequency, a ZCS operation may be applied. The device 100 receives the voltage measurements of the voltages Vci, Vc2 of the capacitors Cl, C2 of the series-connection 300 of capacitors Cl, C2. These voltages may be measured according to any known method, i.e. any known voltage measurement means may be used.

[0089] In some scenarios ZCS operation of the voltage balancing circuit 200 is not the optimal solution for controlling the semiconductor switches in terms of losses. The device 100 may find the optimal point as a trade-off between turn-off losses (caused by the circulating current at the moment of turn-off, i.e. switching a semiconductor switch from the conducting to thenon-conducting state) and tum-on losses (caused by the discharging of the parasitic capacitance of the semiconductor switches at tum-on, i.e. switching a semiconductor switch from the non-conducting to the conducting state). This trade off can be obtained through online computation or with a look-up table and it provides the optimum level of current at the moment of switch off, i.e. a desired turn-off current. The device may obtain the desired value of the current iorr at the moment of switch off from a look-up table or by an online computation.

[0090] FIG. 6 shows an example of an off-current adjustment method performable by the device of FIG. 1. That is, the device 100 may be configured to control the first switching units 11, 21 and the second switching units 12, 22 to be switched from the conducting state to the non-conducting state while a desired current ioff flows through the resonant circuit RES 1 by performing a second method, which may be referred to as off-current adjustment method. The second method comprises the step of obtaining a voltage measurement of a voltage of the resonant capacitor Cr or the resonant inductor Lr. In the example of FIG.

[0091] 6, it is assumed that the voltage measurement VM3 of the voltage Ver of the resonant capacitor Cr is obtained, i.e. the voltage Ver of the resonant capacitor Cr is measured. Thus, in FIGs 1 and 2, the voltage measurement VM3 is provided to the device 100. For measuring the voltage Ver of the resonant capacitor any known method may be used. For example, a voltage divider may be used. This is only by way of example and the description is correspondingly valid in case of using a voltage measurement of the voltage of the resonant inductor Lr. Further, as shown in FIG. 6, the second method comprises the step of computing a peak current ires, peak of the resonant circuit RES1 using the obtained voltage measurement (e.g. the voltage Ver across the resonant capacitor Cr as shown in FIG. 6 or the voltage across the resonant inductor Lr), the used resonant frequency and additionally the capacitance of the resonant capacitor Cr (as shown in FIG. 6) or inductance of the resonant inductor Lr. The capacitance Cr of the resonant capacitor Cr or the inductance of the resonant inductor Lr may be used in case the voltage across the resonant capacitor Cr is measured. The capacitance Cr of the resonant capacitor Cr or the inductance of the resonant inductor Lr may be used in case the voltage across the resonant inductor Lr is measured and used. That is, either the capacitance or the inductance of the resonant circuit RES 1 may be used in the aforementioned two cases, because when the components are resonating, the AC voltage across each of them is the same but opposite (disregarding the resistance or other possible parasitics). The capacitance Cr of the resonant capacitor Cr may be obtained from the data sheet. The inductance of the resonant inductor Lr may be obtained from the data sheet. For example, as shown in FIG. 6, the device 100 may compute the peak current ires, peak of the resonant circuit RES1 by multiplying the voltage Ver of the resonant capacitor Cr, the resonant angular frequency corand the capacitance value Cr of the resonant capacitor Cr. The angular frequency coris computed using the resonant frequency. The second method comprises computing the switching frequency fswfor switching the first switching units 11, 21 and second switching units 12, 22 using the computed peak current ires, peak, the used resonant frequency fresand the desired current iorr.

[0092] Due to resonance, the current ires (also represented by “L” herein and the FIGs) through the resonant circuit RES 1 and, thus, the voltage balancing circuit 200 obeys the equation (6):

[0093]

[0094] In above equation (6), ires, peak is the peak current of the resonant circuit RES1. Hence, the resonant current will reach the desired level of the current iotr flowing through the resonant circuit RES1 when turning off the first switching untis 11,12 or second switching units 12, 22 at the time t’ioff, as shown in the following equation:

[0095]

[0096] The time t’ioff corresponds to the time from the beginning of the sinewave cycle to the first instant where it crosses ioff. FIG. 7 shows an example of the current iresthrough the resonant circuit RES1 of the voltage balancing circuit 200 of FIG. 1 over time with a desired value ioff of said current at the time of turning-off switching units of the voltage balancing circuit 200. As

[0097]

[0098] In reality, the capacitor voltage also comprises a voltage drop caused by the equivalent series resistance (ESR) of the resonant capacitor Cr:

[0099]

[0100] In above equation (12), AVer is the peak to peak amplitude of the capacitor voltage Ver. Due to resonance, the peak value of the sinewave will not vary abruptly, so the device 100 may use a filter to obtain the peak value of the signal, which can be sampled at lower frequency. The error of the calculated current peak with equation (12) if the resonant frequency had not been corrected by the resonant frequency correction method would be:

[0101]

[0102] In above equation (13), DFc is the dissipation factor or loss tangent of the resonant capacitor Cr. When the device 100 has performed the resonant frequency correction method and, thus, the resonant frequency has been adjusted, the effect of the inductance variation is eliminated:

[0103]

[0104] The error of the peak current calculation is then only dependent on the capacitor variation, which is typically lower than the inductor variation.

[0105] The above equations and explanation correspond to the case where the measured voltage is the voltage across the resonant capacitor Cr, and the capacitance of the resonant capacitor Cr is used to compute the peak current of the resonant circuit RES 1. As outlined already above, the voltage across the resonant inductor Lr may be measured and said inductor voltage measurement and an inductor AC voltage equation may be used for computing the peak current of the resonant circuit RES1. Optionally, the

[0106]

[0107] As shown in Figure 8, the device 100 may periodically perform the resonant frequency correction method (described above with regard to FIG. 5) to correct possible variations in the resonant frequency due to the actual value of the resonant capacitor Cr and / or resonant inductor Lr deviating from the values provided in the data sheet. As indicated in Figure 8, the device 100 may apply / perform the resonant frequency correction method for a very short duration, since convergence occurs after few switching cycles. During the rest of the time, the device 100 may apply / perform the second method (i.e. the off-current adjustment method) based on a lower frequency sampling of the resonant capacitor voltage Ver (or resonant inductor voltage). The second method is described above with regard to FIG. 6.

[0108]

[0109] FIG. 9 shows an example of the current flowing through the resonant circuit of the voltage balancing circuit when the device of FIG. 1 performs no correction, performs the resonant frequency correction method and performs additionally the off-currentadjustment method. The vertical axis represents the current flowing through the resonant circuit RES1 in Ampere, and the horizontal axis represents the time in seconds. As may be observed in FIG. 9 that convergence to ZCS by performing the resonant frequency correction method is reached fast, and once said function is deactivated, the frequency is changed to turn off at the desired current. For this, the device 100 may perform the second method described above, i.e. the off-current adjustment method.

[0110] FIG. 10 shows an example of current flow through the resonant circuit of the voltage balancing circuit when the device of FIG.

[0111] 1 performs the off-current adjustment method without the resonant frequency correction method and when the device of FIG.

[0112] 1 performs the off-current adjustment method and the resonant frequency correction method.

[0113] That is, Fig. 10 shows the effect of the off-current adjustment function of the device 100 with and without the resonant frequency correction method being performed by the device 100. The graph (B) of FIG. 10 shows the current flow through the resonant circuit RES 1 when the device 100 performs the off-current adjustment method without the resonant frequency correction method. Graph (A) of FIG. 10 shows the current flow through the resonant circuit RES 1 when the device 100 performs the off-current adjustment method and the resonant frequency correction method. The vertical axis of the two graphs of FIG. 10 represents the current in Amperes and the horizontal axis represents the time in seconds. It can be observed that the error is smaller when performing the resonant frequency correction method compared to the case when the resonant frequency correction method is not performed. In the examples of FIG. 10, it is assumed that the desired turn-off current i0£f is -6 A. As shown in the graph (A) of FIG. 10, the switching to the non-conducting state occurs at -4.8 A, and in the graph (B) of FIG. 10 the switching to the non-conducting state occurs at -21.2 A. Thus, for the graph (B) of FIG. 10 the error may be 253%, whereas for the graph (A) of FIG. 10 the error may be merely 20%.

[0114] The device 100 may use the calculation of the peak current of the resonant circuit RES1 for protection purposes. The peak value of the current Ir through the resonant circuit RES1 is not decided by the control algorithm, but rather depends on the unbalance between the capacitors Cl, C2 to be equalized. Under abnormal operation, the unbalance can increase excessively, causing the resonant current L to exceed the maximum rated value. The device 100 may be configured to detect said event with the peak current calculation and use it to trigger protective actions (e.g. shut the voltage balancing circuit down) to avoid damages.

[0115] As shown in FIG. 1, the device 100 and the voltage balancing circuit 200 may be part of a circuit 400. The circuit 400 is an example of the circuit according to the second aspect and, thus, the description of the circuit of the second aspect is correspondingly valid for the circuit 400 of FIG. 1.

[0116] As described above, the device 100 of this disclosure allows operating at a frequency close to resonance (i.e. the switching frequency being close or being equal to the resonant frequency of the resonant circuit RES1), ensuring minimum size of the resonant components compared to other solutions which use higher switching frequency than the resonant frequency. For using a switching frequency close to resonance (i.e. the resonant frequency), the device 100 of this disclosure does not require the use of current sensors. Instead, it is sufficient to use voltage sensors, which are smaller and more cost efficient. The device 100 of this disclosure allows operating the voltage balancing circuit 200 with the lowest possible switching losses at all operating conditions, regardless of the variation of the resonant capacitor Cr and resonant inductor Lr of the resonant circuit RES1. The error in the turn-off current iotr may be limited to the resonant capacitance variation, and can thus be minimized by using low tolerance capacitors. In consequence, the switching frequency can be maximized, achieving a smallest possible volume of the resonant components of the resonant circuit REST Since the two sensors across the semiconductor switches of the first and second switching circuits 1 , 2 provide information about the voltage sign, accurate sensors are not required, but rather simple comparator circuits can be used. The measured peak voltage across the resonant capacitor Cr or resonant inductor Lr, may befiltered and sampled at a lower frequency. The device 100 may provide a protection against overcurrent using only low-cost voltage sensors. The device 100 allows an implementation in industry due to a high immunity to the error caused by components tolerances and variation of the resonant capacitor Cr and resonant inductor Lr of the resonant circuit RES 1.

[0117] FIG. 11 shows an example of an implementation form of the voltage balancing circuit of FIG. 1. In the following mainly an optional feature of the implementation form of the circuit of FIG. 11 is described. The description of FIGs. 1 to 10 is correspondingly valid for the voltage balancing circuit of FIG. 11.

[0118] As shown in FIG. 11 , the voltage balancing circuit 200 comprises a third switching circuit 3 being electrically connected in series with the second switching circuit 2, and a second resonant circuit RES2 comprising a second resonant capacitor Cr2 and a second resonant inductor Lr2. The third switching circuit 3 is configured to be electrically connected in parallel to a third capacitor C3 of the series-connected capacitors 300. The third switching circuit 3 comprises a series connection of the first switching unit 31 and the second switching unit 32. The first switching unit 31 and the second switching unit 32 of the third switching circuit 3 may be implemented in the same way as the first switching units 11, 21 of the first and second switching circuits 1 , 2 and the second switching units 12, 22 of the first and second switching circuits 1,2. The second resonant capacitor Cr2 of the second resonant circuit RES2 is electrically connected between the node N2 between the first switching unit 21 and second switching unit 22 of the second switching circuit 2 and a node N6 between the first switching unit 31 and second switching unit 32 of the third switching circuit 3.

[0119] The device 100 (not shown in Figure 11) is configured to control switching of the first switching unit 31 and second switching unit 32 of the third switching circuit 3 using a second switching frequency computed using a resonant frequency of the second resonant circuit RES2. The device 100 may be configured to perform a second resonant frequency correction method with regard to the resonant frequency of the second resonant circuit RES2 by obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units 21, 31 or second switching units 22, 32 of the second and third switching circuits 2, 3, the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the second resonant circuit RES2, and changing the used resonant frequency of the second resonant circuit RES2 used for computing the second switching frequency in case one of the obtained voltage sign measurements in the second and third switching circuit is negative. The description of the control of the first and second switching units 11, 12, 21, 22 of the first and second switching circuits 1, 2 is correspondingly valid for a control of the first and second switching units 31, 32 of the third switching circuit 3, e.g. for a control of the first and second switching units 21, 22, 31, 32 of the second and third switching circuits 2, 3. The description of the device 100 performing the resonance frequency correction method is correspondingly valid for performing the second resonance frequency correction method. The device 100 may also perform the second method, i.e. the off-current adjustment method, in the implementation form of FIG. 11.

[0120] Additional switching circuits may be added to the voltage balancing circuit 200 as outlined with regard to the third switching circuit 3. The description of FIG. 11 is correspondingly valid for such one or more additional switching circuits and the control of such one or more additional switching circuits by the device 100. This allows equalizing the voltage across consecutive series-connected capacitors in case of more than three series-connected capacitors Cl, C2, C3. The voltage detection means are additionally added for each additional switching circuit and optionally each additional resonant circuit in line with the description of the implementation form of FIG. 1.FIG. 12 shows an example of an implementation form of the voltage balancing circuit of FIG. 1. In the following mainly an optional feature of the implementation form of the circuit of FIG. 12 is described. The description of FIGs 1 to 10 is correspondingly valid for the voltage balancing circuit of FIG. 12.

[0121] As shown in FIG. 12, the voltage balancing circuit 200 comprises a third switching circuit 3 and a fourth switching circuit 4 being electrically connected in series to each other, and a second resonant circuit RES2 comprising a second resonant capacitor Cr2 and a second resonant inductor Lr2. The third switching circuit 3 and the fourth switching circuit 4 are configured to be electrically connected in parallel to the second capacitor C2 and a third capacitor C3 of the series-connected capacitors Cl, C2, C3, respectively. Each of the third switching circuit 3 and the fourth switching circuit 4 comprises a series connection of the first switching unit 31, 41 and the second switching unit 32, 42. The first switching units 31, 41 and the second switching unit 32, 42 of the third and fourth switching circuits 3, 4 mays be implemented in the same way as the first switching units 11, 21 of the first and second switching circuits 1 , 2 and the second switching units 12, 22 of the first and second switching circuits 1 , 2. The second resonant capacitor Cr2 of the second resonant circuit RES2 is electrically connected between a node N8 between the first switching unit 31 and second switching unit 32 of the third switching circuit 3 and a node N9 between the first switching unit 41 and second switching unit 42 of the fourth switching circuit 4.

[0122] The device 100 (not shown in Figure 12) is configured to control switching of the first switching units 31, 41 and second switching units 32, 42 of the third switching circuit 3 and fourth switching circuit 4 using a second switching frequency computed using a resonant frequency of the second resonant circuit RES2. The device 100 may be configured to perform a second resonant frequency correction method with regard to the resonant frequency of the second resonant circuit RES2 by obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units 31, 41 or second switching units 32, 42 of the third and fourth switching circuits 3, 4, the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the second resonant circuit RES2, and changing the used resonant frequency of the second resonant circuit RES2 used for computing the second switching frequency in case one of the obtained voltage sign measurements in the second and third switching circuit is negative.

[0123] The description of the control of the first and second switching units 11, 12, 21 , 22 of the first and second switching circuits 1 , 2 is correspondingly valid for a control of the first and second switching units 31 , 32, 41 , 42 of the third switching circuit 3 and fourth switching circuit 4. The description of the device 100 performing the resonance frequency correction method is correspondingly valid for performing the second resonance frequency correction method. The device 100 may also perform the second method, i.e. the off-current adjustment method, in the implementation form of FIG. 12.

[0124] Additional pairs of switching circuits may be added to the voltage balancing circuit 200 as outlined with regard to the third and fourth switching circuits 3, 4. The description of FIG. 12 is correspondingly valid for such one or more additional pairs of switching circuits and the control of such one or more additional pairs of switching circuits by the device 100. This allows equalizing the voltage across consecutive series-connected capacitors in case of more than three series-connected capacitors Cl, C2, C3. The voltage detection means are additionally added for each additional switching circuit and optionally each additional resonant circuit in line with the description of the implementation form of FIG. 1.

[0125] FIG. 13 shows an example of a computer-implemented method according to this disclosure for controlling a voltage balancing circuit for series-connected capacitors. The method of FIG. 13 is an example of the computer-implemented method according to the third aspect of this disclosure. The description of the method according to the third aspect is correspondingly valid for the method of FIG. 13.The voltage balancing circuit may be as outlined above with regard to the FIGs 1 to 12. As shown in FIG. 13, the method comprises a step 1000 of controlling, with a fixed duty cycle of 50% and using a switching frequency computed using a resonant frequency of the resonant circuit, the first switching units and second switching units to be alternately in a conducting state and non-conducting state such that the first switching units are never at the same time as the second switching units in the conducting state. The method comprises performing a resonant frequency correction method by performing a step 2000 of obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units or second switching units, the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the resonant circuit. The resonant frequency correction method comprises the step 3000 of changing the used resonant frequency used for computing the switching frequency in case one of the obtained voltage sign measurements is negative.

[0126] The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

CLAIMS1. A device (100) for controlling a voltage balancing circuit (200) for series-connected capacitors (300), wherein the voltage balancing circuit (200) comprises:a first switching circuit (1) and a second switching circuit (2) being electrically connected in series to each other, anda resonant circuit (RES 1 ) comprising a resonant capacitor (Cr) and a resonant inductor (Lr), the first switching circuit (1) and the second switching circuit (2) are configured to be electrically connected in parallel to a first capacitor (Cl) and second capacitor (C2), respectively, of the series-connected capacitors (300), each of the first switching circuit (1) and the second switching circuit (2) comprises a series connection of a first switching unit (11; 21) and a second switching unit (12; 22),each of the first switching units (11, 21) and the second switching units (12, 22) comprises one or more semiconductor switches (Sil; S12; S21; S22) controllable by the device (100), andthe resonant capacitor (Cr) of the resonant circuit (RES 1) is electrically connected between a node (Nl) between the first switching unit (11) and second switching unit (12) of the first switching circuit (1) and a node (N2) between the first switching unit (21) and second switching unit (22) of the second switching circuit (2); whereinthe device (100) is configured to control, with a fixed duty cycle of 50% and using a switching frequency computed using a resonant frequency of the resonant circuit (RES 1 ), the first switching units (11, 21) and second switching units (12, 22) to be alternately in a conducting state and non-conducting state such that the first switching units (11, 21) are never at the same time as the second switching units (12, 22) in the conducting state, andthe device (100) is configured to perform a resonant frequency correction method by:obtaining a voltage sign measurement (VSM1 , VSM2) of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units (11 , 21 ) or second switching units ( 12, 22), the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow (Ir) is present via the resonant circuit (RES1), and changing the used resonant frequency used for computing the switching frequency in case one of the obtained voltage sign measurements is negative.

2. The device (100) according to claim 1, whereinthe device (100) is configured to control the first switching units (11, 21) and second switching units (12, 22) using a delay time (td) such that after switching the first switching units (11, 21) or the second switching units (12, 22) from the conducting state to the non-conducting the delay time (td) is waited before the second switching units (12, 22) or first switching units (11, 21), respectively, are switched from the non-conducting state to the conducting state.

3. The device (100) according to claim 2, whereinthe device (100) is configured to control the switching of the first switching units (11, 21) and second switching units (12, 22) such that the switching frequency equals to the inverse of a sum of two times the delay time (td) and the inverse of the used resonant frequency.

234. The device (100) according to any one of the previous claims, whereinthe device (100) is configured to perform a resonant frequency correction method by increasing the used resonant frequency in casethe voltage sign measurement in the first switching circuit ( 1 ) is negative and the voltage of the first capacitor (C 1 ) is greater than the voltage of the second capacitor (C2), orthe voltage sign measurement in the second switching circuit (2) is negative and the voltage of the first capacitor (Cl) is smaller than the voltage of the second capacitor (C2).

5. The device (100) according to any one of the previous claims, whereinthe device (100) is configured to perform a resonant frequency correction method by decreasing the used resonant frequency in casethe voltage sign measurement in the first switching circuit (1 ) is negative and the voltage of the first capacitor (C 1 ) is smaller than the voltage of the second capacitor (C2), orthe voltage sign measurement in the second switching circuit (2) is negative and the voltage of the first capacitor (Cl) is greater than the voltage of the second capacitor (C2).

6. The device (100) according to any one of the previous claims, whereinthe device (100) is configured to perform the resonant frequency correction method at a time when the device (100) controls semiconductor switches, at which the voltage sign measurements are obtained, to be switched from the conducting state to the non-conducting state.

7. The device (100) according to any one of the previous claims, whereinthe device (100) is configured to periodically perform the resonant frequency correction method.

8. The device (100) according to claim 7, whereinthe device (100) is configured to stop performing the resonant frequency correction method in casea set time elapses after start of performing the resonant frequency correction method, orthe voltage sign measurement in one of the first switching circuit (1) and the second switching circuit (2) becomes negative after the voltage sign measurement in the other of the first switching circuit (1) and the second switching circuit (2) being negative.

9. The device (100) according to any one the previous claims, whereinthe device (100) is configured to control the first switching units (11, 21) and the second switching units (12, 22) to be switched from the conducting state to the non-conducting state while a desired current (iorr) flows through the resonant circuit (RES 1) by performing a second method comprising the steps of:obtaining a voltage measurement of a voltage of the resonant capacitor (Cr) or the resonant inductor (Lr), computing a peak current (ires, peak) of the resonant circuit (RES 1 ) using the obtained voltage measurement, the used resonant frequency and additionally the capacitance of the resonant capacitor (Cr) or inductance of the resonant inductor (Lr), andcomputing the switching frequency for switching the first switching units (11, 21) and second switching units (12, 22) using the computed peak current (ires, peak), the used resonant frequency and the desired current (iopp).

10. The device (100) according to claim 9, when depending on claim 2, whereinthe device (100) is configured to compute the switching frequency by additionally using the delay time (td).

11. The device (100) according to claim 9 or 10, whereinthe device (100) is configured to perform the second method after having performed the resonant frequency correction method.

12. The device (100) according to any one of the previous claims, whereineach of the first switching units (11, 21) and the second switching units ( 12, 22) is a semiconductor switch (Sil; S21 ; S12; S22) controllable by the device (100), the multiple semiconductor switches (Sil, S12, S21, S22) being electrically connected in series, andthe device (100) is configured to perform the resonant frequency correction method by:obtaining the voltage sign measurement of the voltage across the semiconductor switch of each of the first switching units (11, 21) or second switching units (12, 22), andchanging the used resonant frequency in case one of the obtained voltage sign measurements is negative.

13. The device (100) according to any one ofthe previous claims, whereinthe voltage balancing circuit (200) comprises:a third switching circuit (3) being electrically connected in series with the second switching circuit (2), and a second resonant circuit (RES2) comprising a second resonant capacitor (Cr2) and a second resonant inductor (Lr2),the third switching circuit (3) is configured to be electrically connected in parallel to a third capacitor (C3) of the series- connected capacitors (300),the third switching circuit (3) comprises a series connection of the first switching unit (31) and the second switching unit (32), andthe second resonant capacitor (Cr2) of the second resonant circuit (RES2) is electrically connected between the node (N2) between the first switching unit (21 ) and second switching unit (22) of the second switching circuit (2) and a node (N6) between the first switching unit (31) and second switching unit (32) of the third switching circuit 1'3 ): wherein the device (100) is configured to control switching ofthe first switching unit (31) and second switching unit (32) ofthe third switching circuit (3) using a second switching frequency computed using a resonant frequency of the second resonant circuit (RES2), andthe device (100) is configured to perform a second resonant frequency correction method with regard to the resonant frequency of the second resonant circuit (RES2) by:obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units (21, 31) or second switching units (22, 32) of the second and third switching circuits (2, 3), the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow (Ir2) is present via the second resonant circuit (RES2), andchanging the used resonant frequency of the second resonant circuit (RES2) used for computing the second switching frequency in case one of the obtained voltage sign measurements in the second and third switching circuit (2, 3) is negative.

14. The device (100) according to any one of claims 1 to 11, whereinthe voltage balancing circuit (200) comprises:a third switching circuit (3) and a fourth switching circuit (4) being electrically connected in series to each other, anda second resonant circuit (RES2) comprising a second resonant capacitor (Cr2) and a second resonant inductor (Lr2),the third switching circuit (3) and the fourth switching circuit (4) are configured to be electrically connected in parallel to the second capacitor (C2) and a third capacitor (C3) of the series-connected capacitors (300), respectively, each of the third switching circuit (3) and the fourth switching circuit (4) comprises a series connection of the first switching unit (31; 41) and the second switching unit (32; 42), andthe second resonant capacitor (Cr2) of the second resonant circuit (RES2) is electrically connected between a node (N8) between the first switching unit (31) and second switching unit (32) of the third switching circuit (3) and a node (N9) between the first switching unit (41) and second switching unit (42) of the fourth switching circuit (4); wherein the device (100) is configured to control switching of the first switching units (31,41) and second switching unit (32, 42) of the third switching circuit (3) and fourth switching circuit (4) using a second switching frequency computed using a resonant frequency of the second resonant circuit (RES2), andthe device (100) is configured to perform a second resonant frequency correction method with regard to the resonant frequency of the second resonant circuit (RES2) by:obtaining a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units (31, 41) or second switching units (32, 42) of the third and fourth switching circuits (3, 4), the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow (Ir2) is present via the second resonant circuit (RES2), andchanging the used resonant frequency of the second resonant circuit (RES2) used for computing the second switching frequency in case one of the obtained voltage sign measurements in the second and third switching circuit (2, 3) is negative.

15. A circuit (400) comprising:the device (100) according to any one of the previous claims, anda voltage balancing circuit (200) comprising a first switching circuit (1) and a second switching circuit (2) being electrically connected in series to each other, and a resonant circuit (RES 1 ) comprising a resonant capacitor (Cr) and resonant inductor (Lr), whereinthe first switching circuit (1) and the second switching circuit (2) are configured to be electrically connected in parallel to a first capacitor (Cl) and a second capacitor (C2), respectively, of a series-connected capacitors (300),26each of the first switching circuit (1) and the second switching circuit (2) comprises a series connection of a first switching unit (11; 21) and a second switching unit (12; 22),each of the first switching units (11, 21) and the second switching units (12, 22) comprises one or more semiconductor switches controllable by the device (100), andthe resonant capacitor (Cr) of the resonant circuit (RES 1) is electrically connected between a node (Nl) between the first switching unit (11) and second switching unit (12) of the first switching circuit (1) and a node (N2) between the first switching unit (21) and second switching unit (22) of the second switching circuit (2).

16. A computer-implemented method for controlling a voltage balancing circuit for series-connected capacitors, wherein the voltage balancing circuit comprises:a first switching circuit and a second switching circuit being electrically connected in series to each other, and a resonant circuit comprising a resonant capacitor and a resonant inductor,the first switching circuit and the second switching circuit are configured to be electrically connected in parallel to a first capacitor and second capacitor, respectively, of the series-connected capacitors,each of the first switching circuit and the second switching circuit comprises a series connection of a first switching unit and a second switching unit,each of the first switching units and the second switching units comprises one or more semiconductor switches, and the resonant capacitor of the resonant circuit is electrically connected between a node between the first switching unit and second switching unit of the first switching circuit and a node between the first switching unit and second switching unit of the second switching circuit; wherein the method comprisescontrolling (1000), with a fixed duty cycle of 50% and using a switching frequency computed using a resonant frequency of the resonant circuit, the first switching units and second switching units to be alternately in a conducting state and non-conducting state such that the first switching units are never at the same time as the second switching units in the conducting state, andperforming a resonant frequency correction method by:obtaining (2000) a voltage sign measurement of a voltage across a semiconductor switch of the one or more semiconductor switches of each of the first switching units or second switching units, the semiconductor switch being arranged in the respective switching unit such that the voltage across the semiconductor switch is negative for a time after the semiconductor switch is switched from the conducting state to the non-conducting state while a current flow is present via the resonant circuit, andchanging (3000) the used resonant frequency used for computing the switching frequency in case one of the obtained voltage sign measurements is negative.

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