TECHNICAL FIELD

The disclosure relates to the field of power conversion between alternating current (AC) and direct current (DC) for power networks or power grids, in particular medium voltage AC (MVAC) power grids, e.g., for applications in data centers and EV charging stations. The disclosure particularly relates to an AC to DC converter, a method for controlling such AC to DC converter, and an AC to DC converter arrangement.

BACKGROUND

Most efficient semiconductor switches that are currently available, are low voltage, i.e., < 1500V, and higher voltage devices which are either very expensive or have very poor electrical performance. Due to this limitation, some power converters use series connected semiconductor devices to achieve high voltage blocking capability. However, for achieving a proper dynamic voltage sharing, such series connection of devices might require prior device matching and precise tuning of the snubber circuit, etc. Such measures might significantly increase the cost (technical effort), complexity and volume of series connected switches. Moreover, the snubber circuits in high voltage, high power applications tend to be bulky and may require dedicated cooling, which increases the volume of the system and reduces the efficiency.

Another approach is to use high voltage rated devices where available. These devices could have high switching loss performances. Furthermore, in this case due to high voltage switching, high switching transients in the form of high dV/dt can occur, possibly leading to high stress on the insulation requirements. Secondly, high dV/dt create common mode currents passing through capacitive coupling elements and can lead to EMI issues.

The key problem of the existing AC/DC converter structures for high voltage applications is unavailability for mature high voltage devices or rather poor efficiency of currently available high voltage semiconductor devices.

SUMMARY

The disclosure provides an AC/DC converter structure for high voltage applications for overcoming the above-described problems. In particular, the disclosure provides a solution for a power converter circuit topology using low voltage semiconductor devices and a modulation technique and method for controlling such power converter circuit to achieve high voltage rectification.

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

In order to describe the disclosure in detail, the following terms, abbreviations and notations will be used:

AC Alternating Current

DC Direct Current

SST Solid State Transformer

PWM Pulse Width Modulation

MV Medium Voltage, in this disclosure between 1500V and 30kV, for example

LV Low Voltage, in this disclosure below 1500V, for example

SST Solid-State Transformer

HVDC High Voltage DC

PFC Power Factor Corrector

BCS Bidirectional Control Switch

IGBT Insulated-Gate Bipolar Transistor

MOSFET Metal-Oxide Semiconductor Field Effect Transistor

EV Electric Vehicle

UPS Uninterruptable Power Supply

SST is a Power Electronics based transformer that is controllable by a control unit. As compared to traditional AC/AC copper and iron based passive transformers, SST can perform additional tasks apart from AC/AC voltage transformation. SST can have different voltage levels and DC-DC/AC-DC/DC-AC conversions. Several level of protections can be implemented.

HVDC is a DC voltage grid in a range of higher than typically >100 kV. Typically used for bulk transport of high power.

PFC is a power electronics unit that improves the power factor characteristics and efficiency of AC/DC converters. IGBT is a gate controlled semiconductor transistor switch which has up to very high current and high voltage characteristics, but the operation speed is comparatively slower than a MOSFET switch, switching power losses are higher.

MOSFET is a gate controlled semiconductor transistor switch which has up to medium current and medium voltage characteristics, but the operation speed is faster than IGBT. Switching power losses are typically lower than IGBT.

In this disclosure, converters, i.e., power electronic converters are described. Power converters are applied for converting electric energy from one form to another, such as converting between AC and DC or DC and AC or between DC and DC. Power converters can also change the voltage or frequency or some combination of these. Power electronics converters (including some type of power electronics, such as transistors, diodes, etc.) are based on power electronics switches that can be actively controlled by applying ON/OFF logic (i.e., PWM operation, usually commanded by a closed loop control algorithm).

In Power Electronics, circuits used to convert AC voltages into DC voltages are called Rectifiers. In its simple form, a 3-phase rectifier can be formed using 6 diodes, for example. In rectifiers based on passive diodes, the DC bus voltage cannot be controlled, and the input current will contain undesirable current harmonics. A simple circuit modification by adding bidirectional switches significantly improves the power quality of the rectifier. Due to the pulse- modulated behavior additional filtering by input inductors may be necessary. In a bi-directional power flow rectifier, the power can flow from AC/DC as well as DC/AC. This bi-directional power flow rectifier can be implemented by using a controlled switch at the positions of the passive diodes in the circuit.

The disclosure presents a novel rectifier topology and its method of operation to convert AC voltage from the utility grid to DC voltage to be used in high voltage applications such as SST, HVDC Transmission, UPS, etc.

According to a first aspect, the disclosure relates to an AC to DC converter, comprising: an AC terminal for receiving an AC voltage; a first DC terminal for providing a reference voltage; a second DC terminal for providing a positive DC voltage relative to the reference voltage; a third DC terminal for providing a negative DC voltage relative to the reference voltage; and at least two switching circuits connected in series, the at least two switching circuits being arranged to form a switch arrangement connecting the AC terminal with the three DC terminals, each switching circuit comprising three input terminals and three output terminals, wherein each output terminal of a switching circuit is connected with a respective input terminal of a subsequent switching circuit to form the switch arrangement; wherein the three input terminals of a first switching circuit of the switch arrangement are connected to the AC terminal; wherein a first output terminal of a last switching circuit of the switch arrangement is connected to the second DC terminal, a second output terminal of the last switching circuit is connected to the first DC terminal, and a third output terminal of the last switching circuit is connected to the third DC terminal, wherein each switching circuit comprises a controllable bidirectional switch configured to switch the second input terminal to the second output terminal, a first diode connected between the first input terminal and the first output terminal, a second diode connected between the third output terminal and the third input terminal, a first capacitor connected between the first output terminal and the second output terminal, and a second capacitor connected between the second output terminal and the third output terminal.

Such an AC to DC converter provides the technical advantage that due to the series connection of the switching circuits, low voltage devices can be applied. With the appropriate modulation scheme, dynamic and static voltage balancing of these low voltage devices can be achieved. Due to a novel modulation scheme, each low voltage building block (see switching circuits 110a, 110b, 110c in Figure 1) can be switched separately and therefore, overall dV/dt of the system is reduced. Therefore, simplified insulation and low EMI issues are advantages of this concept.

In an exemplary implementation of the AC to DC converter, each controllable bidirectional switch comprises a control input for receiving a control signal, the control signal being formed to consecutively switch each of the controllable bidirectional switches of the switch arrangement according to a modulation scheme.

This provides the advantage that the control signal can individually control each controllable bidirectional switch, thereby enabling dynamic and static voltage balancing and reduction of overall voltage peaks improving electromagnetic interference.

In an exemplary implementation of the AC to DC converter, for a positive current direction of the AC terminal, a first current path is formed by the first diode and the first capacitor of a respective switching circuit when the controllable bidirectional switch of the respective switching circuit is turned-off, and a second current path is formed by the controllable bidirectional switch of the respective switching circuit when the controllable bidirectional switch is turned-on. This provides the advantage that for the positive current direction, the switching circuits can be individually controlled one-by-one resulting in high accuracy of the voltage conversion and reduced distortion effects.

In an exemplary implementation of the AC to DC converter, for a negative current direction of the AC terminal, a third current path is formed by the second diode and the second capacitor of a respective switching circuit when the controllable bidirectional switch of the respective switching circuit is turned-off, and a fourth current path is formed by the controllable bidirectional switch of the respective switching circuit when the controllable bidirectional switch is turned-on.

The same advantage as for the positive current direction can also be achieved for the negative current direction, i.e. , the switching circuits can be individually controlled one-by-one resulting in high accuracy of the voltage conversion and reduced distortion effects.

According to a second aspect, the disclosure relates to a method for controlling an AC to DC converter according to the first aspect described above, the method comprising: providing a control signal to the control inputs of each controllable bidirectional switch of the AC to DC converter, the control signal being formed to consecutively switch each of the controllable bidirectional switches of the switch arrangement according to a modulation scheme.

Application of such method provides the advantage that the serially connected low voltage semiconductor devices (i.e., the above-described switching circuits) can be accurately controlled to achieve high voltage rectification. The method allows for a dynamic and static voltage balancing of these low voltage devices. Based on the modulation scheme, each low voltage building block can be switched separately and therefore, the overall voltage change (dV/dt) of the system can be reduced which results in simplified insulation and low EMI issues.

In an exemplary implementation of the method, the control signal is formed to: initiate a transfer of the AC to DC converter from a zero state in which the controllable bidirectional switches of each switching circuit are turned-off to an on state in which all controllable bidirectional switches are turned-on by consecutively turning-on the controllable bidirectional switches of each switching circuit one-by-one until the on state is reached.

This provides the advantage that the method can efficiently and precisely control the different states of the AC to DC converter resulting in high voltage conversion precision and reduced distortion effects. In an exemplary implementation of the method, the control signal is formed to apply a pause between the consecutively switching of each of the controllable bidirectional switches.

This provides the advantage that in each state there is enough time for the transient effects to subside.

In an exemplary implementation of the method, the control signal is formed to: initiate a transfer of the AC to DC converter from the on-state to the zero-state by consecutively turning-off the controllable bidirectional switches of each switching circuit one-by-one until the zero state is reached.

This provides the advantage that the DC voltage can be gradually increased.

In an exemplary implementation of the method, during the transfer of the AC to DC converter from the zero-state to the on-state, the first capacitors of each switching circuit are consecutively charged; and during the transfer of the AC to DC converter from the on state to the zero state, the first capacitors of each switching circuit are consecutively discharged.

This provides the advantage that the same voltage waveform can be obtained for the transition from zero-state to on-state as well as for the transition from on-state to zero-state.

In an exemplary implementation of the method, a charging sequence for charging the first capacitors of each switching circuit during the transfer of the AC to DC converter from the zerostate to the on-state is opposite to a discharging sequence for discharging the first capacitors of each switching circuit during the transfer of the AC to DC converter from the on-state to the zero-state.

This provides the advantage that the modulation scheme can be symmetrical, thereby reducing processing complexity.

According to a third aspect, the disclosure relates to an AC to DC converter arrangement for a multi-phase system, the AC to DC converter arrangement comprising: a plurality of phase terminals of an AC power network; and a corresponding plurality of AC to DC converters according to the first aspect described above, wherein each AC terminal of an AC to DC converter of the plurality of AC to DC converters is connected to a respective one of the phase terminals. Such AC to DC converter arrangement provides the advantage that high voltage high power AC/DC converters can be used for building the AC to DC converter arrangement. These high voltage high power AC/DC converters are highly efficient and lower in cost compared to existing solutions. Secondly the high power density for such power supplies can be achieved due to lower insulation requirements and the fact that in case of three phase, no power storage buffers are necessary as compared to existing SST technologies using input series structures or series connected devices with bulky snubbers.

In an exemplary implementation of the AC to DC converter arrangement, the second DC terminals of the plurality of AC to DC converters are interconnected with each other to provide a positive DC voltage terminal of the AC to DC converter arrangement; and the third DC terminals of the plurality of AC to DC converters are interconnected with each other to provide a negative DC voltage terminal of the AC to DC converter arrangement.

This provides the example, that the AC to DC converter arrangement is applicable to multiple input AC to DC conversion scenarios, including onboard charging of the EV, Input stage of the SST, PFC, UPS where high voltage grid interfacing is needed to cater the high power supply requirements to the load.

The controllable bidirectional switch of each switching circuit may comprise, for example, a combination of a controlled semiconductor switch with an uncontrolled semiconductor element.

The controlled semiconductor switch may comprise, for example a MOSFET or an IGBT. The uncontrolled semiconductor element may comprise, for example a diode or an intrinsic body diode of a semiconductor element.

The controlled semiconductor switch may comprise, for example, a SiC (silicon carbide) MOSFET or a SiC IGBT. The uncontrolled semiconductor element may be based on SiC semiconductor technology, for example.

According to a fourth aspect, the disclosure relates to a computer program product including computer executable code or computer executable instructions that, when executed, causes at least one computer to execute the method according to the second aspect described above.

The computer program product may run on a controller or a processor for controlling the abovedescribed AC to DC converter. According to a fifth aspect, the disclosure relates to a computer-readable medium, storing instructions that, when executed by a computer, cause the computer to execute the method according to the second aspect described above. Such a computer readable medium may be a non-transient readable storage medium. The instructions stored on the computer-readable medium may be executed by a controller or a processor for controlling the above-described AC to DC converter.

The disclosure introduces a new design for an AC to DC converter which enables building of high voltage high power AC/DC converters which are highly efficient and lower in cost compared to existing solutions. Secondly, the high power density for such power supplies can be achieved due to lower insulation requirements and the fact that in case of three phase, no power storage buffers are necessary as compared to existing SST technologies using input series structures or series connected devices with bulky snubbers.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the disclosure will be described with respect to the following figures, in which:

Figure 1 shows a block diagram of an AC to DC converter 100 according to the disclosure;

Figure 2 shows a system architecture of a 3-phase high voltage rectifier 200 using the AC to DC converters 100 shown in Figure 1 for each phase;

Figure 3 shows a block diagram of a 3-phase high voltage rectifier 200 using multiple building blocks 100a, 100b, 100c of the AC to DC converter 100 shown in Figure 1 for each phase;

Figure 4 shows a block diagram of an AC to DC converter arrangement 200 using the building blocks 100a, 100b, 100c of the AC to DC converter 100 shown in Figure 1 for each phase;

Figures 5a), b), c) and d) show four circuit diagrams representing the operation of the switching structure with only two low voltage branches, representing the case with positive current direction;

Figures 6a), b), c) and d) show four circuit diagrams representing the operation of the switching structure with only two low voltage branches, representing the case with negative current direction;

Figure 7 shows an exemplary switching sequence 700 in the form of a table for the bidirectional switches of an AC to DC converter 100 as shown in Figure 1 ;

Figure 8 shows a phase leg example for the 3-phase system architecture 200 shown in Figure 2; and

Figure 9a) to 9e) show five example implementations for a bidirectional controlled switch as shown in Figure 1. DETAILED DESCRIPTION OF EMBODIMENTS

In the following detailed description, reference is made to the accompanying drawings, which form a part thereof, and in which is shown by way of illustration specific aspects in which the disclosure may be practiced. It is understood that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the disclosure is defined by the appended claims.

It is understood that comments made in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless specifically noted otherwise.

Figure 1 shows a block diagram of an AC to DC converter 100 according to the disclosure.

The AC to DC converter 100 comprises: an AC terminal 101 for receiving an AC voltage (A); a first DC terminal 103 for providing a reference voltage (N); a second DC terminal 102 for providing a positive DC voltage (DC+) relative to the reference voltage (N); and a third DC terminal 104 for providing a negative DC voltage (DC-) relative to the reference voltage (N).

The AC to DC converter 100 comprises at least two switching circuits 110a, 110b, 110c connected in series. In this Figure 1 , an exemplary number of three switching circuits 110a, 110b, 110c are shown. The at least two switching circuits are arranged to form a switch arrangement 110 connecting the AC terminal 101 with the three DC terminals 102, 103, 104.

Each switching circuit 110a, 110b, 110c comprises three input terminals 111 , 112, 113 and three output terminals 114, 115, 116. Each output terminal of a switching circuit is connected with a respective input terminal of a subsequent switching circuit to form the switch arrangement 110.

The three input terminals 111 , 112, 113 of a first switching circuit 110a of the switch arrangement 110 are connected to the AC terminal 101. A first output terminal 114 of a last switching circuit 110c of the switch arrangement 110 is connected to the second DC terminal 102. A second output terminal 115 of the last switching circuit 110c is connected to the first DC terminal 103. A third output terminal 116 of the last switching circuit 110c is connected to the third DC terminal 104.

Each switching circuit 110b comprises a controllable bidirectional switch T _x configured to switch the second input terminal 112 to the second output terminal 115, a first diode DTX connected between the first input terminal 111 and the first output terminal 114, a second diode DBX connected between the third output terminal 116 and the third input terminal 113, a first capacitor CTX connected between the first output terminal 114 and the second output terminal 115, and a second capacitor CBX connected between the second output terminal 115 and the third output terminal 116.

Due to the series connection of the switching circuits, low voltage devices can be applied. With the appropriate modulation scheme, dynamic and static voltage balancing of these low voltage devices can be achieved. Due to a novel modulation scheme, each low voltage building block (switching circuit 110a, 110b, 110c) can be switched separately and therefore, overall dV/dt of the system is reduced. Therefore, simplified insulation and low EMI issues are advantages of this concept.

Each controllable bidirectional switch T _x may comprise a control input for receiving a control signal 121. The control signal 121 may be formed to consecutively switch each of the controllable bidirectional switches T _x of the switch arrangement 110 according to a modulation scheme. An example for such a modulation scheme 700 is shown in Figure 7.

Each of the controllable bidirectional switches T _x can be switched separately according to the modulation scheme.

For each switching circuit (e.g., 110b), the first diode DTX and the controllable bidirectional switch T _x can be connected through the first capacitor CTX. For each switching circuit (e.g., 110b), the second diode DBX and the controllable bidirectional switch T _x can be connected through the second capacitor CBX. For each switching circuit (e.g., 110b), the first diode DTX is connected in forward direction between the first input terminal 111 and the first output terminal 114 and for each switching circuit (e.g., 110b), the second diode DBX is connected in forward direction between the third output terminal 116 and the third input terminal 113. Note that connection in reverse direction would not work. The AC to DC converter 100 can be controlled by a method implementing a modulation scheme for switching the controllable bidirectional switches T _x , Ti, TN of the switching circuits 110a, 110b, 110c.

Such a method may comprise: Providing a control signal 121 to the control inputs of each controllable bidirectional switch T _x of the AC to DC converter 100. The control signal 121 can be formed to consecutively switch each of the controllable bidirectional switches T _x of the switch arrangement 110 according to a modulation scheme. An example for such a modulation scheme 700 is shown in Figure 7.

The AC to DC converter shown in Figure 1 introduces a new converter structure for high voltage applications. The new converter structure is based on a power converter circuit topology using low voltage semiconductor devices and the corresponding modulation technique or method to achieve high voltage rectification. In the basic structure shown in Figure 1 , D _TX and D _BX (X=1..N) are high power low voltage diodes. T _x (x=1..N) are the bi-directional control switches based on low voltage devices. CT _X and CB _X are auxiliary capacitors. With the correct modulation scheme (method), dynamic and static voltage balancing of these low voltage devices are achieved.

Due to its novel modulation scheme, each low voltage building block 110a, 110b, 11 Oc ean be switched separately and therefore, overall dV/dt of the system can be reduced. Therefore, simplified insulation and low EMI issues are one advantage of this new converter structure.

The rated voltage of the full structure 110 may be based on voltage blocking capability of a single structure (110a, 110b, 110c) times the number of such structures.

Figures 2, 3 and 4 represent exemplary configurations where a 3 phase AC/DC converter 200 is configured using three such structures 110a, 110b, 110c as presented in Figure 1.

Figure 2 shows a system architecture of a 3-phase high voltage rectifier 200 using the AC to DC converters 100 shown in Figure 1 for each phase.

The 3-phase high voltage rectifier 200 is a special AC to DC converter arrangement 200 for a 3-phase system. The 3-phase high voltage rectifier 200 comprises a plurality of three phase terminals 201 , 202, 203 of an AC power network 210; and a corresponding plurality of three AC to DC converters 100 as described above with respect to Figure 1. Each AC terminal 101 of an AC to DC converter 100 of the plurality of AC to DC converters 100 is connected to a respective one of the phase terminals 201 , 202, 203.

The second DC terminals 102 of the plurality of AC to DC converters 100 may be interconnected with each other to provide a positive DC voltage (DC+) terminal 222 of the AC to DC converter arrangement.

The third DC terminals 104 of the plurality of AC to DC converters 100 may be interconnected with each other to provide a negative DC voltage (DC-) terminal 224 of the AC to DC converter arrangement 200.

The first DC terminals 103 of the plurality of AC to DC converters 100 may be interconnected with each other to provide a reference voltage, e.g., ground, terminal 223 of the AC to DC converter arrangement 200.

The AC to DC converter 100 according to the disclosure is applicable to multiple input AC to DC conversion scenarios, including onboard charging of the EV, Input stage of the SST, PFC, UPS where high voltage grid interfacing is needed to cater the high-power supply requirements to the load. Figure 2 shows one example of such application with three phase to DC rectification stage.

Figure 3 shows a block diagram of a 3-phase high voltage rectifier 200 using multiple building blocks 100a, 100b, 100c of the AC to DC converter 100 shown in Figure 1 for each phase.

The 3-phase high voltage rectifier 200 comprises a plurality of phase terminals 201 , 202, 203 of an AC power network 210 and a corresponding plurality of AC to DC converters 100 as described above with respect to Figure 1.

The AC to DC converters 100 comprise at least two switching circuits 110a, 110b, 110c, as shown in Figure 1 which are connected in series. The switching circuits are arranged to form a switch arrangement 110 as shown in Figure 1 , connecting the AC terminal 101 with the three DC terminals 102, 103, 104.

As described above with respect to Figure 1 , each switching circuit 110a, 110b, 110c comprises three input terminals 111 , 112, 113 and three output terminals 114, 115, 116. Each output terminal of a switching circuit is connected with a respective input terminal of a subsequent switching circuit to form the switch arrangement 110. The dashed line in Figure 3 illustrates that more than two switching circuits are included in each of the AC to DC converters 100 of the respective phases.

Figure 4 shows a block diagram of an AC to DC converter arrangement 200 using the building blocks 100a, 100b, 100c of the AC to DC converter 100 shown in Figure 1 for each phase.

The AC to DC converter arrangement 200 can be used for any multi-phase system, while the example in Figure 4 is directed to a 3-phase system.

The AC to DC converter arrangement 200 comprises: a plurality of phase terminals 201 , 202, 203 of an AC power network 210; and a corresponding plurality of AC to DC converters 100 as described above with respect to Figure 1.

Each AC terminal 101 of an AC to DC converter 100 of the plurality of AC to DC converters 100 may be connected to a respective one of the phase terminals 201 , 202, 203.

According to Figure 4, the second DC terminals 102 of the plurality of AC to DC converters 100 may be interconnected with each other to provide a positive DC voltage (DC+) terminal 222 of the AC to DC converter arrangement 200.

Similarly, the third DC terminals 104 of the plurality of AC to DC converters 100 may be interconnected with each other to provide a negative DC voltage (DC-) terminal 224 of the AC to DC converter arrangement 200.

The embodiment shown in Figure 4 can be applied for the N-level configuration. Each phase (i.e. A, B, C, ... ) consists of at least N-top diodes, at least N-bottom diodes, N-top capacitors and N-bottom capacitors and N bidirectional switches. DTX and T _x are connected through capacitor CTX. T _X and DBX are connected through the capacitor CBX. The Tx switches can be operated according to the modulation scheme 700 shown in Figure 7.

Figures 5a), b), c) and d) show four circuit diagrams representing the operation of the switching structure with only two low voltage branches, representing the case with positive current direction.

As can be seen in the diagrams, for the positive current direction of the AC terminal 101 as shown in Figure 1 , a first current path 501 , 503 (see Figures 5a and 5c) is formed by the first diode DTI and the first capacitor CTI of a respective switching circuit 110b when the controllable bidirectional switch Ti of the respective switching circuit 110b is turned-off. A second current path 502 (see Figure 5b) is formed by the controllable bidirectional switch Ti of the respective switching circuit 110b when the controllable bidirectional switch Ti is turned-on.

In the example of Figure 5, the series connected switches Ti and T2 are turned on/off sequentially after a short pause (e.g., of several hundred nanoseconds). Operation of such a single phase converter structure with two low voltage building blocks can be visualized in Figure 5.

The current path from the AC-terminal to the DC-terminal is the capacitor and the diodes. As illustrated in Figure 5a, when the T2 is turned-on, the current flows through the capacitor CT2, slightly discharging it and decreasing the voltage by several volts, and then through the diode DTI into the DC-bus. In the next step (Figure 5b), following the short delay, the Ti is turned-on and the bidirectional unit is fully on.

For the turn-off process, it is important to charge the previously discharged capacitor to its original voltage. For this reason, T2 is turned-off first and the current flows through DT2 into the CT2 followed by the Ti, as shown in Figure 5c. After another short pause, Ti is also turned-off.

Figures 6a), b), c) and d) show four circuit diagrams representing the operation of the switching structure with only two low voltage branches, representing the case with negative current direction.

The behavior of the bidirectional switch is the same during the negative current direction of the AC-terminal. The difference is that the bottom capacitors and diodes participate now in the switching. In Figure 6a, turning on T2 at first initiates the current flow through DBI and CBI . CBI is slightly discharged. Turning-on Ti afterwards redirects the current through the switches, as shown in Figure 6b. Turning-off is shown in Figure 6c and 6c.

Thus, for a negative current direction of the AC terminal 101 , a third current path 601 , 603 (see Figures 6a and 6c) is formed by the second diode DBI and the second capacitor CB2 of a respective switching circuit 110b when the controllable bidirectional switch Ti of the respective switching circuit 110b is turned-off, and a fourth current path 602 is formed by the controllable bidirectional switch Ti of the respective switching circuit 110b when the controllable bidirectional switch Ti is turned-on. Figure 7 shows an exemplary switching sequence 700 in the form of a table for the bidirectional switches of an AC to DC converter 100 as shown in Figure 1 .

The table comprises for each controllable bidirectional switch T _x , Ti, TN different switching states 701 , 702, 703 and respective switching patterns 711 , 710.

The modulation scheme or method shown in Figure 7 can be used for controlling the AC to DC converter 100 as shown in Figure 1. The modulation scheme or method controls a switching of the controllable bidirectional switches T _x , Ti, TN of the switching circuits 110a, 110b, 110c.

Such a method may comprise: Providing a control signal 121 to the control inputs of each controllable bidirectional switch T _x of the AC to DC converter 100 as shown in Figure 1 , the control signal 121 being formed to consecutively switch each of the controllable bidirectional switches T _x of the switch arrangement 110 according to a modulation scheme. Figure 7 shown an example for such a modulation scheme 700.

The control signal 121 may be formed to initiate a transfer of the AC to DC converter 100 from a zero state 701 in which the controllable bidirectional switches T _x of each switching circuit 110a, 110b, 110c are turned-off to an on state 702 in which all controllable bidirectional switches T _x are turned-on by consecutively turning-on the controllable bidirectional switches T _x of each switching circuit 110a, 110b, 110c one-by-one 703 until the on state 702 is reached.

The control signal 121 may be formed to apply a pause between the consecutively switching of each of the controllable bidirectional switches T _x .

The control signal 121 may be formed to initiate a transfer of the AC to DC converter 100 from the on-state 702 to the zero-state 701 by consecutively turning-off the controllable bidirectional switches T _x of each switching circuit 110a, 110b, 110c one-by-one 703 until the zero state 701 is reached.

During the transfer of the AC to DC converter 100 from the zero-state 701 to the on-state 702, the first capacitors CT _X of each switching circuit 110a, 110b, 110c may be consecutively charged.

During the transfer of the AC to DC converter 100 from the on state 702 to the zero state 701 , the first capacitors CT _X of each switching circuit 110a, 110b, 110c may be consecutively discharged. The charging sequence 710 for charging the first capacitors CTX of each switching circuit 110a, 110b, 110c during the transfer of the AC to DC converter 100 from the zero-state 701 to the on-state 702 may be opposite to a discharging sequence 711 for discharging the first capacitors CTX of each switching circuit 110a, 110b, 110c during the transfer of the AC to DC converter 100 from the on-state 702 to the zero-state 701 .

Figure 8 shows a phase leg example for the 3-phase system architecture 200 shown in Figure 2.

The voltage 801 at AC input 101 of the third phase (Phase C) is shown together with a phase leg example with six bidirectional switching arrangements according to the design shown in Figure 1.

The voltage 801 has a step-profile with the positive steps 1V, 2V, 3V, 4V, 5V, 6V and the negative steps 6V, 5V, 4V, 3V, 2V, 1V. The voltage at the capacitors CTX and CBX of the switching circuits of the AC to DC converter 100 follows this step profile.

Figure 9a) to 9e) show five example implementations for a bidirectional controlled switch as shown in Figure 1.

Figure 9a shows the bi-directional control switch (BCS) symbol. Figures 9b to 9e show various BCS implementations which can be applied in the AC to DC converter 100 shown in Figure 1. These configurations use a combination of both controlled semiconductor switches (IGBT, MOSFET, etc...) and uncontrolled semiconductors (diodes or intrinsic body diode of a device).

The controllable bidirectional switch T _x of each switching circuit 110b shown in Figure 1 may, for example, comprise a combination of a controlled semiconductor switch with an uncontrolled semiconductor element.

The controlled semiconductor switch may, for example, comprise one of a MOSFET or IGBT.

The uncontrolled semiconductor element may, for example, comprise one of a diode or intrinsic body diode of a semiconductor element.

The controlled semiconductor switch may, for example, comprise one of an SiC MOSFET or an SiC IGBT. The uncontrolled semiconductor element may, for example, be based on SiC semiconductor technology.

While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features or aspects of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms "include", "have", "with", or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term "comprise". Also, the terms "exemplary", "for example" and "e.g." are merely meant as an example, rather than the best or optimal. The terms “coupled” and “connected”, along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other.

Although specific aspects have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific aspects shown and described without departing from the scope of the disclosure. This application is intended to cover any adaptations or variations of the specific aspects discussed herein.

Although the elements in the following claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the disclosure beyond those described herein. While the disclosure has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the disclosure. It is therefore to be understood that within the scope of the appended claims and their equivalents, the disclosure may be practiced otherwise than as specifically described herein.