TECHNICAL FIELD

The present disclosure relates generally to electric power conversion, and more specifically to a switching cell for an electric power converter, to a method of operating the switching cell, as well as to electric power converters including the switching cell.

BACKGROUND ART

Electric power converters made up of modular switching cells or “legs” are widely used in diverse industries from photovoltaics to automotive. The two-level half-bridge (2L-HB) switching cells or “legs” found in a plethora of converters typically experience conduction and switching losses which are detrimental to the efficiency of the system. The reduction of turnon and turn-off losses with minimum effort is desirable to increase energy efficiency while not (or only minimally) penalizing a power density. Soft-switching (SS) strategies/topologies are a way to achieve this target.

Most SS strategies rely on the use of auxiliary components being active during commutation. These components increase the size of the converter and limit its power density.

A radical method to limit the size of this auxiliary/SS system is to limit its operation, e.g. to light load. This is because, for many applications, switching cells are operated at partial load most of the time and rarely at heavy loads.

Various exemplary solutions focus on the reduction of turn-on losses only. For example, a capacitor can be added in parallel to the main switches of the switching cell to reduce the turnoff losses. However, this results in higher turn-on losses in case of operation outside the SS region. This situation can be only avoided if the SS’s operating range covers the full load range, which in turn would result in large/costly SS circuits.

Other exemplary solutions achieve a reduction of turn-on and turn-off losses within a given operating range without affecting the rest of the range, at the expense of high current stress of inductors and auxiliary switches. This results in more chip area, bigger magnetics, and higher conduction losses.

Topologies and methods that can improve the energy efficiency at partial load without increasing the losses at heavy loads are desirable as they can lead to smaller and/or more cost- effective SS power conversion systems.

SUMMARY

It is an object to overcome these and other drawbacks. 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.

According to a first aspect, a switching cell for an electric power converter is provided. The switching cell comprises an input rail for a direct-current, DC, voltage source of the electric power converter; an output rail for a load of the electric power converter; a ground rail; a first main power switch being interposed between the input rail and the output rail; a second main power switch being interposed between the output rail and the ground rail; a series connection of a first auxiliary power switch and of a first coil of a coupling transformer being interposed between the input rail and the output rail; a series connection of a second coil of the coupling transformer and of a second auxiliary power switch being interposed between the output rail and the ground rail; a first discharging circuit, being configured to provide a lower electric potential than the ground rail; a second discharging circuit, being configured to provide a higher electric potential than the input rail; a first demagnetizing circuit, being connected to the first discharging circuit and to a common terminal of the first auxiliary power switch and the first coil of the coupling transformer; and a second demagnetizing circuit, being connected to a common terminal of the second auxiliary power switch and the second coil of the coupling transformer and to the second discharging circuit.

An electric power converter as used herein may refer to a class of electrical devices for processing and controlling a flow of electrical energy by supplying voltages and currents in a form which suits its electrical load. Electric power converters are usually classified based on the type of power conversion they achieve (e.g., DC/DC, DC/ AC, AC/DC, AC/AC). A switching cell or “leg” as used herein may refer to a class of circuit topologies which constitute modular building blocks of electric power converters. For example, one switching cell or more than one series-connected or parallel-connected switching cells may form part of an electric power converter.

A rail or electric rail as used herein may refer to a metallic conductor, such as a strip or bar, for electrical power feeder/transmission lines.

A power switch or electrical power switch as used herein may refer to an active electrical device for switching of high rated voltage s/currents. For example, common solid-state power switches may include power metal-oxide-semiconductor field-effect transistors (MOSFETs), power bipolar transistors or power insulated-gate bipolar transistors (IGBTs). Power MOSFETs may have a parasitic capacitance in parallel to their drain-source power path. Similarly, power IGBTs as well as power bipolar transistors may have a parasitic capacitance in parallel to their collector-emitter power path.

A coupling transformer as used herein may refer to a passive electrical device for inductive coupling of one electrical circuit including a primary winding (first coil) of the transformer and another electrical circuit including a secondary winding (second coil) of the transformer via a magnetic flux in a core of the transformer. Coupling transformers may have a leakage inductance in series with the mutually coupled transformer windings due to leakage flux traversing paths outside the primary and secondary windings.

A demagnetizing circuit as used herein may refer to an electrical circuit for demagnetizing an inductive circuit component, such as a winding of a transformer or an inductor.

A discharging circuit as used herein may refer to an electrical circuit for further discharging and thereby resetting the afore-mentioned inductive circuit component for a subsequent commutation.

A common terminal as used herein may refer to a common electrical potential of galvanically interconnected terminals of circuit components. The proposed switching cell reduces turn-on and turn-off losses in a given operating range (partial load) without detrimental effects on its energy efficiency at heavy load, e.g. when the soft-switching circuit is disabled.

More specifically, the magnetic coupling structure reduces current stress in semiconductors by 40%, reduces a semiconductor footprint by 52%, and reduces semiconductor turn-on and turnoff losses in the auxiliary circuit by 49%.

The coupling transformer with embedded leakage reduces a number and size of magnetic components by 50% and 18%, respectively.

Auxiliary DC voltages enable resetting the coupling transformer after every commutation.

Charging the auxiliary capacitor only when soft-switching is enabled avoids extra losses at heavy loads or when soft-switching is disabled. The soft-switching circuit can be optimized for a given operating range.

The proposed switching cell may be deployed as a 2L-HB leg in any DC/DC or DC/ AC converters that rely on 2L-HB legs as main switching units, e.g. in multiphase systems or in other architectures. The need for SS is particularly important in hard-switching topologies, such as PWM-driven DC/AC converters (inverters) used in photovoltaic and automotive systems.

In a possible implementation form, the main power switches may have respective parasitic parallel capacitances.

The parasitic parallel capacitances may be used for soft- switching and do not increase the size of the converter and limit its power density.

In a possible implementation form, the coils of the coupling transformer may have respective leakage series inductances.

The leakage series inductances may be used for soft-switching and do not increase the size of the converter and do not limit its power density. In a possible implementation form, the first demagnetizing circuit may comprise a series connection of first diodes being interposed between the lower electric potential of the first discharging circuit, and a common electric potential of the first auxiliary power switch and the first coil of the coupling transformer.

In a possible implementation form, the first demagnetizing circuit may further be connected to the output rail, and may further comprise a first soft-switching capacitor being interposed between an electric potential of the output rail and a common electric potential of the first diodes.

In a possible implementation form, the second demagnetizing circuit may comprise a series connection of second diodes being interposed between a common electric potential of the second auxiliary power switch and the second coil of the coupling transformer, and the higher electric potential of the second discharging circuit.

In a possible implementation form, the second demagnetizing circuit may further be connected to the output rail, and may further comprise a second soft-switching capacitor being interposed between the electric potential of the output rail and a common electric potential of the second diodes.

In a possible implementation form, the first discharging circuit may comprise a first DC voltage source being connected to the electric potential of the ground rail; and the second discharging circuit may comprise a second DC voltage source being connected to the electric potential of the input rail.

In a possible implementation form, the first and second discharging circuits may further comprise respective resistors being parallel-connected to the respective first and second DC voltage source.

In a possible implementation form, the first discharging circuit may further comprise a first electric power regeneration converter being interposed between the lower electric potential of the first discharging circuit and the electric potential of the ground rail; and the second discharging circuit may further comprise a second electric power regeneration converter being interposed between the higher electric potential of the second discharging circuit and the electric potential of the input rail. In a possible implementation form, the first and second electric power regeneration converters may respectively comprise a buck-boost converter circuit.

In a possible implementation form, the first and second electric power regeneration converters may respectively comprise a flyback converter circuit.

In a possible implementation form, the first and second DC voltage sources may form part of a power supply circuit for powering an electric circuit being connectable to the switching cell.

According to a second aspect, a multi-phase DC-to-AC electric power converter is provided, comprising a parallel connection of one or more switching cells according to the first aspect or any of its implementations.

According to a third aspect, a method of operating a switching cell according to the first aspect or any of its implementations is provided. The method comprises: during a commutation from the second main power switch to the first main power switch, switching the first auxiliary power switch conductive if a magnitude of a current output to the load is lower than a current threshold; switching the first main power switch conductive when a magnitude of a voltage across the same is lower than a voltage threshold; and switching the first auxiliary power switch non- conductive at the earliest when switching the first main power switch conductive.

In a possible implementation form, switching the first auxiliary power switch conductive may further comprise: switching the first auxiliary power switch conductive at the latest when switching the second main power switch non-conductive.

In a possible implementation form, the method may further comprise: during a commutation from the first main power switch to the second main power switch, switching the second auxiliary power switch conductive if a magnitude of a current input from the load is lower than the current threshold; switching the second main power switch conductive when a magnitude of a voltage across the same is lower than the voltage threshold; and switching the second auxiliary power switch non-conductive at the earliest when switching the second main power switch conductive. In a possible implementation form, switching the second auxiliary power switch conductive may further comprise: switching the second auxiliary power switch conductive at the latest when switching the first main power switch non-conductive.

According to a fourth aspect, a computer program is provided, comprising a program code for performing the method according to the third aspect or any of its implementations, when executed on a computer.

BRIEF DESCRIPTION OF DRAWINGS

The above-described aspects and implementations will now be explained with reference to the accompanying drawings, in which the same or similar reference numerals designate the same or similar elements.

The drawings are to be regarded as being schematic representations, and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to those skilled in the art.

FIGs. 1 - 3 respectively illustrate a circuit topology of a switching cell in accordance with the present disclosure in varying levels of detail;

FIGs. 4 - 7 respectively illustrate circuit topologies of discharging circuits in accordance with the present disclosure;

FIG. 8 illustrates a circuit topology of a multi-phase DC/ AC electric power converter in accordance with the present disclosure;

FIG. 9 illustrates a flow diagram of a method of operating the switching cell of FIG. 3; and FIGs. 10 - 11 illustrate operating modes of the switching cell of FIG. 3 during turn-on of the first main power switch.

DETAILED DESCRIPTIONS OF DRAWINGS

In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the present disclosure or specific aspects in which embodiments of the present disclosure may be used. It is understood that embodiments of the present disclosure may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.

For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding apparatus or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise.

FIGs. 1 - 3 respectively illustrate a circuit topology of a switching cell 1 in accordance with the present disclosure in varying levels of detail.

The switching cell 1 is suitable for an electric power converter 2, such as a multi-phase DC/AC electric power converter 2 explained in more detail in connection with FIG. 8 below.

In accordance with FIG. 1, the switching cell 1 comprises an input rail 101 for a direct-current, DC, voltage source UDC (DC-link voltage) of the electric power converter 2; an output rail 102 for a load (indicated by load current Iioad) of the electric power converter 2; a ground rail 103; a first main power switch 104, Si being interposed between the input rail 101 and the output rail 102; a second main power switch 105, S2 being interposed between the output rail 102 and the ground rail 103; a series connection of a first auxiliary power switch 106, S3 and of a first coil 107 of a coupling transformer 107, 108 being interposed between the input rail 101 and the output rail 102; a series connection of a second coil 108 of the coupling transformer 107, 108 and of a second auxiliary power switch 109, S4 being interposed between the output rail 102 and the ground rail 103; a first discharging circuit 110, being configured to provide a lower electric potential than the ground rail 103; a second discharging circuit 111, being configured to provide a higher electric potential than the input rail 101; a first demagnetizing circuit 112, being connected to the first discharging circuit 110 and to a common terminal of the first auxiliary power switch 106, S3 and the first coil 107 of the coupling transformer 107, 108; and a second demagnetizing circuit 113, being connected to a common terminal of the second auxiliary power switch 109, S4 and the second coil 108 of the coupling transformer 107, 108 and to the second discharging circuit 111.

The main power switches 104, Si _; 105, S2 may particularly include MOSFETs involving respective parasitic body diodes having parasitic parallel capacitances 114, C _0SS ,si; 115, C _oss ,S2.

The auxiliary power switches 106, S3; 109, S4 may have a bidirectional conduction capability (e.g. MOSFETs with/without an antiparallel diode or IGBTs with an antiparallel diode).

The coupling transformer 107, 108 is used to magnetically couple the first and second auxiliary power switches 106, S3; 109, S4 so that the current is shared between them during a commutation. For example, a turn ratio of the coupling transformer 107, 108 may comprise 1 :1.

The coils of the coupling transformer 107, 108 may have respective leakage series inductances 116, Ln; 117, L12. The leakage inductances may be used during commutations of the switching cell.

In accordance with FIG. 2, the first demagnetizing circuit 112 may comprise a series connection of first diodes 1121, Di _a ; 1122, Dib being interposed between the lower electric potential of the first discharging circuit 110, and a common electric potential of the first auxiliary power switch 106, S3 and the first coil 107 of the coupling transformer 107, 108.

Likewise, the second demagnetizing circuit 113 may comprise a series connection of second diodes 1131, D2 _a ; 1132, D2b being interposed between a common electric potential of the second auxiliary power switch 109, S4 and the second coil of the coupling transformer 107, 108, and the higher electric potential of the second discharging circuit 111.

In accordance with FIG. 3, the first demagnetizing circuit 112 may further be connected to the output rail 102, and may further comprise a first soft-switching capacitor 1123, Cxi being interposed between an electric potential of the output rail 102 and a common electric potential of the first diodes 1121, Di _a ; 1122, Dib.

Similarly, the second demagnetizing circuit 113 may further be connected to the output rail 102, and may further comprise a second soft-switching capacitor 1133, Cx2 being interposed between the electric potential of the output rail 102 and a common electric potential of the second diodes 1131, D _2a ; 1132, D _2b .

The first and second soft-switching capacitors 1123, C _x i; 1133, C _x2 are charged only when SS is enabled.

FIGs. 4 - 7 respectively illustrate circuit topologies of discharging circuits 110, 111 in accordance with the present disclosure.

In accordance with FIG. 4, the first discharging circuit 110 may comprise a first DC voltage source 1101, U _aux being connected to the electric potential of the ground rail 103 ; and the second discharging circuit 111 may comprise a second DC voltage source 1111, U _aux being connected to the electric potential of the input rail 101.

In accordance with FIG. 5, the first and second discharging circuits 110, 111 may further comprise respective resistors 1102, 1112, Rais being parallel-connected to the respective first and second DC voltage source 1101, 1111, U _aux .

In accordance with FIG. 6, the first discharging circuit 110 may further comprise a first electric power regeneration converter 1103 being interposed between the lower electric potential of the first discharging circuit 110 and the electric potential of the ground rail 103; and the second discharging circuit 111 may further comprise a second electric power regeneration converter 1113 being interposed between the higher electric potential of the second discharging circuit 111 and the electric potential of the input rail 101.

As a first example, the first and second electric power regeneration converters 1103, 1113 may respectively comprise a buck-boost converter circuit 1104, 1114, in accordance with FIG. 7. As a second example, the first and second electric power regeneration converters 1103, 1113 may respectively comprise a flyback converter circuit. The auxiliary power supplies, U _au x, work as sinks. That is, they facilitate a discharging/resetting of the coupling transformer 107, 108 after every use, thereby recovering energy during commutation. This energy can be dissipated through dedicated resistors (see FIG. 5), or it can be fed back into the DC-link or other auxiliary power supplies. For this, one or two power converters are needed (see FIG. 6). These converters can be either isolated or non-isolated. FIG. 7, for instance, illustrates the case wherein non-isolated buck-boost converters are used. Alternatively, fly-back converters (not shown) can be used to provide energy to a secondary power supply such as the one used to power the gate drivers, sensors, etc.

FIG. 8 illustrates a circuit topology of a multi-phase DC/ AC electric power converter 2 in accordance with the present disclosure.

The converter 2 comprises a parallel connection of more than one switching cell 1 according to the first aspect of the present disclosure or any of its implementations.

The particular implementation of FIG. 8 includes N=3 switching cells 1 and could be deployed in a three-phase inverter for driving a three-phase load (indicated by load currents Iioad A, Iioad B, and Iioad c). Similar arrangements can be extended to N number of phases without limitation. Although each phase A, B, C, ... would need a dedicated auxiliary/SS circuit, only two discharging circuits 110, 111 are needed regardless of the number of phases.

FIG. 9 illustrates a flow diagram of a method 3 of operating the switching cell 1 of FIG. 3.

The method 3 is for operating a switching cell 1 according to the first aspect of the present disclosure or any of its implementations.

The following steps 31 to 33 apply during turn-on of the first main power switch 105, S2.

The method 3 comprises, during a commutation from the second main power switch 105, S2 to the first main power switch 104, Si, a step of switching 31 the first auxiliary power switch 106, S3 conductive if a magnitude of a current output Iioad to the load is lower than a current threshold. This corresponds to operating mode “Mode 1 ” being explained in more detail below. This step may further comprise switching 311 the first auxiliary power switch 106, S3 conductive at the latest when switching the second main power switch 105, S2 non-conductive.

The method 3 further comprises a step of switching 32 the first main power switch 104, Si conductive when a magnitude of a voltage across the same is lower than a voltage threshold. This corresponds to operating mode “Mode 3 ” being discussed in more detail below.

The method 3 further comprises a step of switching 33 the first auxiliary power switch 106, S3 non-conductive at the earliest when switching 32 the first main power switch 104, Si conductive. This corresponds to operating mode “Mode 4” being described in more detail below.

Similar considerations/steps 34 - 36 apply during turn-on of the second main power switch 105, S ₂ :

The method 3 may further comprise, during a commutation from the first main power switch 104, Si to the second main power switch 105, S2, switching 34 the second auxiliary power switch 109, S4 conductive if a magnitude of a current input Iioad from the load is lower than the current threshold. This step may further comprise switching 341 the second auxiliary power switch 109, S4 conductive at the latest when switching 33 the first main power switch 104, Si non-conductive.

The method 3 may further comprise switching 35 the second main power switch 105, S2 conductive when a magnitude of a voltage across the same is lower than the voltage threshold.

The method 3 may further comprise switching 36 the second auxiliary power switch 109, S4 non-conductive at the earliest when switching 35 the second main power switch 105, S2 conductive.

FIGs. 10 - 11 illustrate operating modes of the proposed switching cell 1 during turn-on of the first main power switch 104, Si.

In a first operating mode (“Mode 1 ”) shown in FIGs. 10 and 11, the first auxiliary power switch 106, S3 is turned on with zero-current switching (ZCS). Zero-current switching as used herein may refer to a (soft) switching while a current on a power path of the switch is substantially zero (e.g. is lower than a given ZCS current threshold).

An antiparallel diode of the second auxiliary power switch 109, S4 is forward biased and the currents Ipri, I sec in the transformer windings 107, 108 increase at a rate given by 0.5I7 _DC /L _{ll 2} - The current I _mid flowing towards the main power switches 104, Si _; 105, S2 is the sum of these two currents Ipri, Isec-

In a second operating mode (“Mode 2 ”) shown in FIGs. 10 and 11, the leakage series inductances 116, L ₍₁ ; 117, L ₍₂ resonate with the parasitic parallel capacitances 114, C _0SS ,si; 115, C _oss ,S2 of the main power switches 104, Si _; 105, S2 when I _mid reaches a load current. More specifically, the parasitic parallel capacitance 115, C _oss ,S2 charges while the parasitic parallel capacitance 114, C _0SS ,si discharges.

In a third operating mode (“Mode 3 ”) shown in FIGs. 10 and 11, main power switch 104, Si is turned on with zero-voltage switching (ZVS) when the voltage across the parasitic parallel capacitance 114, C _0SS ,si is practically zero (e.g. is lower than a given voltage threshold).

Zero-voltage switching as used herein may refer to a (soft) switching while a voltage along a power path of the switch is substantially zero (e.g. is lower than a given ZVS voltage threshold).

As soon as the main power switch 104, Si is conductive, the currents I _pn , Lee in the transformer windings 107, 108 start to decay at a rate of 0.5[7 _DC /L _{(1 2} .

In a fourth operating mode (“Mode 4”) shown in FIGs. 10 and 11, the first auxiliary power switch 106, S3 is turned off with ZVS. The leakage series inductance 116, charges the first soft-switching capacitor 1123, C _X1 through first diode 1121, D _la .

If the energy in the leakage series inductance 116, L _tl is sufficient, the first soft-switching capacitor 1123, C _X1 will charge to a voltage U _cap = U _dc + U _aux equal to the DC-link voltage U _dc and the auxiliary supply voltage U _aux and Mode 5 will begin. Otherwise, the first soft- switching capacitor 1123, C _X1 will be charged to a lower value, and Mode 5 is bypassed since the current in the leakage series inductance 116, L _tl will have reached zero.

In a fifth operating mode (“Mode 5”) shown in FIGs. 10 and 11, the leakage series inductance 116, polarizes diode D _lb . The current in the transformer winding 107 ramps down a rate given by (U _dc + O.5t/ _aux )/L ₍₁ . At the end of Mode 5, the current through the leakage series inductance 117, L _i2 is zero. However, the current in the magnetizing inductance of the transformer 107, 108 remains. This gives rise to two more modes of operation.

In a sixth operating mode (“Mode 6”) shown in FIGs. 10 and 11, the current through the leakage series inductance 117, L ₍₂ becomes negative at a rate given by — 0.5(U _cap + U _aux )/L _i2 . This mode ends when the complete magnetizing current is circulating through the leakage series inductance 117, L _L2 .

In a seventh operating mode (“Mode 7”) shown in FIGs. 10 and 11, the current through the leakage series inductance 117, L _t2 increases at a rate of U _aux /L _mag and it feeds into the second discharging circuit 111, U _aux . At the end of this mode, the transformer is reset once the magnetization energy is recovered.

An eighth operating mode (“Mode 8”) shown in FIGs. 10 and 11 is activated during turn-off of the main power switches 104, Si _; 105, S2. For the main power switch 104, Si to turn off, the voltage across the parasitic parallel capacitance 114, C _0SS ,si needs to rise before the diode of main power switch 105, S2 conducts. For this purpose, the parasitic parallel capacitance 114, Coss, si is charged while the parasitic parallel capacitance 114, C _oss ,S2 and the first soft-switching capacitor 1123, C _X1 are discharged. This process reduces the current flowing through main power switch 104, Si during the commutation which leads to lower switching losses in the main power switch 104, Si.

Similar considerations apply for the second main power switch 105, S2.

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. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.