A three-phase bidirectional power converter and a method thereof

Abstract

Embodiments of the invention relates to a three-phase bidirectional power converter (100) using a soft switching cell (120). The three-phase bidirectional power converter (100) comprises a switching network (110), a soft switching cell (120), an input (130), and a DC bus (140). The three-phase bidirectional power converter (100) is configured to: align the switches of the switching network (110) into a switching pattern, and to discharge a current from the soft switching cell (120) into the switching network (110) for zero voltage switching of the aligned switches of the switching network (110) according to the switching pattern. Furthermore, the invention also relates to a corresponding method.

Description

[0001]A THREE-PHASE BIDIRECTIONAL POWER CONVERTER AND A METHOD THEREOFTECHNICAL FIELDEmbodiments of invention relate to a three-phase bidirectional power converter and a method for a three-phase bidirectional power converter. BACKGROUNDThe ongoing demand for electric vehicles (EV) rectifiers and photovoltaic (PV) inverters has createdthe need for universal bidirectional power converter systems. Half-bridge (H-bridge) three‐phase power converters are used for converting three‐phase alternating current (AC) to direct current (DC). The H-bridge three‐phase power converters are therefore widelyused in applications such as power factor correction (PFC) rectifiers, EV fast charging stations and alsofor PV inverter systems. Power converters should efficiently convert an input voltage / current to an output voltage / current. SUMMARYAn objective of embodiments of the invention is to provide a solution which mitigates or solves thedrawbacks and problems of conventional solutions.Another objective of embodiments of the invention is to provide a power converter with small size, highpower density and high power conversion efficiency. The above and further objectives are solved by the subject matter of the independent claims. Furtherembodiments of the invention can be found in the dependent claims.According to a first aspect of the invention, the above mentioned and other objectives are achieved witha three-phase bidirectional power converter, TPBPC, comprising:a switching network comprising a first, a second and a third H-bridge leg, each H-bridge leg comprising an upper switch, a lower switch and a midpoint node arranged between the upper switch and the lower switch; a soft switching cell comprising a transformer and a current charging / discharging circuit, the transformer comprising a primary winding connected to the current charging / discharging circuit, and a first, a second and a third secondary winding connected to the first, the second and the third H-bridge leg, respectively; an input comprising a first, a second and a third port connected to the first, the second and thethird secondary winding, respectively, and being configured to receive a first ^^, a second ^^ and a third^^phase voltage, respectively; and aDC bus connected to the first, the second and the third H-bridge leg; wherein the TPBPC isconfigured to: align the switches of the switching network into a switching pattern, and discharge a current from the soft switching cell into the switching network for zero voltage switching of the aligned switches of the switching network according to the switching pattern.An advantage of the TPBPC according to the first aspect is that zero voltage switching is achieved incurrent mode as opposed to in voltage mode which causes high derivate of the voltage (dv / dt). Lesselectrical components are therefore needed in the present TPBPC compared to conventional solutionssince the therein disclosed power converter topology employs a soft switching cell with less componentsneeded. Also, the root mean square (RMS) current through the soft switching cell can be held low bydesigning the number of turns of the primary winding to be higher than the number of turns of thesecondary windings of the soft switching cell transformer. Further, the herein disclosed TPBPC hashigher DC bus voltage utilization compared to conventional solutions which implies increase in themodulation index of the power converter thus leading to higher power delivery to a load.In an implementation form of a TPBPC according to the first aspect, the current charging / discharging circuit comprises a soft switching cell switch connected in series with a soft switching cell capacitor.An advantage with this implementation form is that the soft switching cell can be compact and smalldue to the small number of components needed in the soft switching cell. In an implementation form of a TPBPC according to the first aspect, a positive terminal of the primarywinding is connected to a drain of the soft switching cell switch and a negative terminal of the primarywinding is connected to the soft switching cell capacitor. In an implementation form of a TPBPC according to the first aspect, an inductance of the transformer, when discharging the switch current into the switching network, resonates with a capacitance of the soft switching cell switch of the current charging / discharging circuit and the capacitances of the aligned switches of the switching network according to the switching pattern.An advantage with this implementation form is that all the aligned switches can achieve soft switchingsimultaneously according to the switching pattern.In an implementation form of a TPBPC according to the first aspect, the transformer is a flybacktransformer. In an implementation form of a TPBPC according to the first aspect, discharging the switching current into the switching network comprises: set the soft switching cell switch into a non-conducting mode.An advantage with this implementation form is that the soft switching cell switch only need to switchonce every switching cycle. In an implementation form of a TPBPC according to the first aspect, the soft switching cell switch is in the non-conducting mode ≤15% of a duty cycle for the TPBPC.An advantage with this implementation form is that the switching loss of the soft switching cell switchis low and can be neglected since the duty cycle of the switching cell switch is short.In an implementation form of a TPBPC according to the first aspect, aligning the switches of the switching network is dependent on a polarity of a first ^^, a second ^^and a third ^^phase current associated with the first ^^, the second ^^and the third ^^phase voltage, respectively. In an implementation form of a TPBPC according to the first aspect, the switches of the switching network are aligned according to the following table: Phase current polarities Aligned switches−^^, +^^and +^^^^, ^^and ^^−^^, −^^and +^^^^, ^^and ^^+^^, −^^and +^^^^, ^^and ^^+^^, −^^and −^^^^, ^^and ^^+^^, +^^and −^^^^, ^^and ^^−^^, +^^and −^^^^, ^^and ^^where ^^, ^^and ^^are the upper switches of respective H-bridge leg and ^^, ^^and ^^are the lower switches of respective H-bridge leg. In an implementation form of a TPBPC according to the first aspect, the TPBPC is configured to: align the switches of the switching network and discharge the switch current simultaneously.An advantage with this implementation form is that soft switching is achieved in the aligned switchesonce in a switching cycle. In an implementation form of a TPBPC according to the first aspect, the TPBPC is configured to: operate with a fixed switching frequency and in a continuous conduction mode. An advantage with this implementation form is simplicity in the power converter design and incontrolling the TPBPC. Also, small electromagnetic interference filter size is possible.In an implementation form of a TPBPC according to the first aspect, a positive terminal of the first, the second and the third secondary winding is connected to a source of the upper switch, respectively; and a negative terminal of the first, the second and the third secondary winding is connected to the first, the second and the third midpoint node, respectively. In an implementation form of a TPBPC according to the first aspect, a positive terminal of the first, the second and the third secondary winding is connected to the first, the second and the third midpoint node,respectively; and a negative terminal of the first, the second and the third secondary winding isconnected to a drain of the lower switch, respectively.In an implementation form of a TPBPC according to the first aspect, the TPBPC comprises a filter circuitconnected between the input and the first, the second and the third secondary winding.An advantage with this implementation form is that since the zero voltage switching is achieved incurrent mode and therefore has less common mode noise compared to zero voltage switching achieved in voltage mode. This behaviour helps to reduce the electromagnetic interference filter size. In an implementation form of a TPBPC according to the first aspect, the filter circuit comprises a first filter inductor connected between the first port and a negative terminal of the first secondary winding, a second filter inductor connected between the second port and a negative terminal of the second secondary winding, and a third filter inductor connected between the third port and a negative terminal of the third secondary winding. In an implementation form of a TPBPC according to the first aspect, the DC bus comprises a DC bus capacitor. In an implementation form of a TPBPC according to the first aspect, the DC bus is configured as an output source in a rectifier mode or an input source in an inverter mode of the TPBPC. An advantage with this implementation form is that bidirectional power flow in the TPBPC is possible. In an implementation form of a TPBPC according to the first aspect, a positive terminal of the DC busis connected to the drains of the upper switches and a negative terminal of the DC bus is connected tothe sources of the lower switches. According to a second aspect of the invention, the TPBPC comprises a controller configured to control the TPBPC.According to a third aspect of the invention, the above mentioned and other objectives are achieved witha method for a TPBPC comprising: aswitching network comprising a first, a second and a third H-bridge leg, each H-bridge legcomprising an upper switch, a lower switch and a midpoint node arranged between the upper switch and the lower switch; asoft switching cell comprising a transformer and a current charging / discharging circuit, thetransformer comprising a primary winding connected to the current charging / discharging circuit, and a first, a second and a third secondary winding connected to the first, the second and the third H-bridge leg, respectively; an input comprising a first, a second and a third port connected to the first, the second and thethird secondary winding, respectively, and being configured to receive a first ^^, a second ^^and a third ^^phase voltage, respectively; and a DC bus connected to the first, the second and the third H-bridge leg; the method comprising: align the switches of the switching network into a switching pattern, and discharge a current from the soft switching cell into the switching network for zero voltage switching of the aligned switches of the switching network according to the switching pattern. Further applications and advantages of embodiments of the invention will be apparent from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGSThe appended drawings are intended to clarify and explain different embodiments of the invention, inwhich: ^Fig. 1 shows a TPBPC according to embodiments of the invention;^ Fig. 2 shows a flow chart for a method according to embodiments of the invention;^ Fig. 3 shows different switching patterns;^ Fig. 4 shows a TPBPC in a first connection pattern according to embodiments of the invention;^ Fig. 5 shows a TPBPC in a second connection pattern according to embodiments of theinvention; and ^Figs. 6a to 6k shows different operating modes of a TPBPC according to embodiments of theinvention. DETAILED DESCRIPTION Over the past years, various conventional solutions for H-bridge power converters have been proposed.Soft switch technology such as zero voltage switching (ZVS) is a promising technique in such powerconverters. For a power converter operating at a high switching frequency, its volume or weight may bereduced due to the size reduction of passive components such as inductors, capacitors, etc. with softswitching. Meanwhile, hard switching i.e., the absence of ZVS, limits the increase of the powerconverter switching frequency and thus achieving soft switching in a robust and low-cost way is a keyto increasing the switching frequency and power density of power converters. Therefore, a powerconverter configured for ZVS is described in the present disclosure.Fig. 1 shows a TPBPC 100 according to embodiments of the invention. The TPBPC 100 comprises aswitching network 110 comprising a first H1, a second H2 and a third H3 H-bridge leg. Each H-bridgeleg comprises an upper switch ^^, ^^, ^^, a lower switch ^^, ^^, ^^ and a midpoint node A, B, C arrangedbetween the upper switch ^^, ^^, ^^and the lower switch ^^, ^^, ^^. The TPBPC 100 further comprises a soft switching cell 120 comprising a transformer 122 and a current charging / discharging circuit 124.The transformer 122 comprises a primary winding W1 connected to the current charging / dischargingcircuit 124, and a first W21, a second W22 and a third W23 secondary winding connected to the first H1, the second H2 and the third H3 H-bridge leg, respectively. The TPBPC 100 further comprises an input 130 comprising a first 132, a second 134 and a third 136 port connected to the first W21, thesecond W22 and the third W23 secondary winding, respectively, and being configured to receive a first^^, a second ^^ and a third ^^ phase voltage, respectively. The TPBPC 100 further comprises a DC bus140 connected to the first H1, the second H2 and the third H3 H-bridge leg.According to embodiments of the invention, the TPBPC 100 is configured to: align the switches of theswitching network 110 into a switching pattern, and further configured to discharge a current from thesoft switching cell 120 into the switching network 110 for zero voltage switching of the aligned switchesof the switching network 110 according to the switching pattern.Fig. 2 shows a flow chart of a corresponding method 200 for a TPBPC 100 as shown in Fig. 1. Themethod 200 for the TPBPC 100 comprises: ^Aligning 202 the switches of the switching network 110 into a switching pattern, and^ Discharging 204 a current from the soft switching cell 120 into the switching network 110 forzero voltage switching of the aligned switches of the switching network 110 according to the switching pattern. Thus, it is herein disclosed a power converter solution for achieving ZVS without e.g., changing a power converter characteristic in terms of bidirectional power flow, converter operation under different loadtypes or shorting of the DC bus voltage during the ZVS process as found in conventional solutions.Embodiments of the invention is therefore based on a hardware and a method for injecting a currentneeded for realizing the ZVS turn-on into conductive state of aligned switches in a H-bridge powerconverter topology.The soft switching cell 120 may in embodiments use a flyback transformer for energy storage which is injected into the soft switching cell 120 as a switching current when ZVS is needed or transfers theenergy to a clamping capacitor. Thus, the current charging / discharging circuit 124 may comprise a softswitching cell switch ^^connected in series with a soft switching cell capacitor ^^. In more detail, a positive terminal of the primary winding W1 may be connected to a drain of the soft switching cell switch ^^and a negative terminal of the primary winding W1 is connected to one terminal of the soft switching cell capacitor ^^while the other terminal of the soft switching cell capacitor ^^is connected to a source of the soft switching cell switch ^^. The flyback mode soft switching cell switch ^^turnsoff, i.e., is in a non-conducting state, for a short duty cycle e.g., ≤15% which is advantageous in makingthe soft switching cell switch ^^loss low or negligible. Further, the energy stored in the magnetizing inductance ^^of the flyback transformer 122 is used to achieve ZVS turn-on of the aligned switches simultaneously by injection of the mentioned current, i.e.,by discharging a current from the soft switching cell 120 into the switching network 110. This meanse.g., that the magnetizing inductance ^^ resonates with the capacitors of the aligned switches, i.e., ^^^,^^^, ^^^ and ^^^, or with ^^^, ^^^, ^^^ and ^^^, during ZVS when the current in each of the phasecurrents has the phase +^^, −^^, −^^ or −^^, +^^ , +^^, respectively. In other words, an inductance ^^of the transformer 122, when discharging the switch current into the switching network 110, resonateswith the capacitance ^^^ of the soft switching cell switch ^^ of the current charging / discharging circuit124 and the capacitances of the aligned switches of the switching network 110 according to the switching pattern. Generally, the TPBPC 100 may be configured to align the switches of the switching network 110 anddischarge the switch current simultaneously. This may be understood that the switch current goes below zero current allowing the body diodes of the aligned switches to conduct before the switches can beturned on under ZVS condition.Further, the TPBPC 100 may be configured to operate with a fixed switching frequency and in acontinuous conduction mode. Thereby, the TPBPC 100 is easy to design due to less complexity fromswitching at fixed switching frequency and is therefore also easy to control. The switching frequencymay be any suitable frequency such as from order of KHz and above, e.g., 20KHz, 50KHz, 100KHz, 150KHz, 250KHz, etc.Fig. 3 shows H-bridge converter with flyback proposed soft switching cell waveform in a switchingperiod ^^ is one period of modulation. Voltage ^^^^ , ^^^^ , ^^^^, ^^^^ , ^^^^ , ^^^^ , ^^^^ is the gate tosource voltage of switch ^^, ^^ , ^^, ^^, ^^, ^^, ^^, respectively, which is needed to drive the switch intoconducting state. When a switch is in conducting state a current flow through the switch contrary to thecase when the switch is in non-conducting state. Current ^^^, ^^^, ^^^, ^^^, ^^^, ^^^, ^^^ is the currentflowing through switch ^^, ^^ , ^^, ^^, ^^, ^^, ^^, respectively, which indicates that the switch is inconducting state. Voltage ^^^ is the voltage across the transformer 122 of the soft switching cell 120,which is the magnetising inductance ^^ of the transformer 122. Current ^^^ , ^^^ is the current throughthe magnetising inductance ^^ and the current through the DC bus capacitor, respectively. Current ^^ ,^^ , ^^ is the current through the filter inductor ^^ , ^^, ^^ , respectively.It may be noted that the modulation principle used may be based on two types of carriers, i.e., a ramp-up ^^^^ and a ramp-down ^^^^ carrier as shown in Fig.3. When the filter current of any of the phases ispositive the ramp-up carrier ^^^^ is selected and the ramp-down carrier ^^^^ is selected when any of thefilter current of any of the phases is negative. This means in Fig.3 that the phase current is +^^,−^^,−^^then the carriers ^^^^, ^^^^, ^^^^ is selected, respectively, and these operating points are used fordescribing embodiments of the invention in the following disclosure.The aligning of the switches of the switching network 110 in hard turn-on instant according to the phasecurrent polarities is shown in Table 1 where the left column shows the phase current polarities while theright column shows the aligned switches. Thus, aligning of the switches of the switching network 110is dependent on a polarity of a first ^^, a second ^^and a third ^^phase current associated with the first^^, the second ^^ and the third ^^ phase voltage, respectively. Before the soft switching cell 120 injectsthe switching current the switches of the switching network 110 are aligned if they are turned on withoutthe soft switching cell 120 injecting the switching current which is so called hard turn on. Hard turn onmeans that the current and voltage is not zero before the switch is turned on. Hence, with hard turn onthere will be high switching losses. In contrast, with soft switching by turning on the switch when theswitch voltage and current is zero the switching losses can be low or negligible. In summary, the chosentype of modulation makes the alignment of the switches hard turn on instant possible. Table 1. Aligned switches according to phase current polarities. Phase Current Polarities Aligned Switches−^^, +^^and +^^^^, ^^and ^^−^^, −^^and +^^^^, ^^and ^^+^^, −^^and +^^^^, ^^and ^^+^^, −^^and −^^^^, ^^and ^^+^^, +^^and −^^^^, ^^and ^^−^^, +^^and −^^^^, ^^and ^^Moreover, two different connection patterns are herein disclosed and shown in Fig. 4 and 5 to describeembodiments of the invention in more detail for deeper understanding of the present TPBPC 100. Theseconnection patterns are denoted first and second connection patterns, respectively. However, the two connection patterns have some common features.The TPBPC 100 may comprise a filter circuit 150 connected between the input 130 and the first W21,the second W22 and the third W23 secondary winding. In embodiments, the filter circuit 150 maycomprise a first filter inductor ^^connected between the first port 132 and a negative terminal of thefirst secondary winding W21 a second filter inductor ^^ connected between the second port 134 and anegative terminal of the second secondary winding W22, and a third filter inductor ^^ connectedbetween the third port 136 and a negative terminal of the third secondary winding W23.As also shown in Fig. 4 and 5 the DC bus 140 may comprise a DC bus capacitor 142. Thus, a positiveterminal of the DC bus 140 may be connected to the drains of the upper switches ^^, ^^, ^^ and a negativeterminal of the DC bus 140 may be connected to the sources of the lower switches ^^, ^^, ^^. Further, aload 160 may be connected to the DC bus 140 when operated in the rectifier mode. The load 160 maybe any electrical device, unit or arrangement needing electrical power for its functioning. Thus, load 160 may be considered as an electrical power consumer. The switches of the switching network 110 are illustrated as Metal-Oxide-Semiconductor Field EffectTransistors (MOS-FETs). Thus, each switch is shown with a diode and a capacitor and comprises adrain, a source and a gate. However, other types of switches may be employed such as Insulated-GateBipolar Transistors (IGBT).Fig. 4 shows a TPBPC 100 according to the first connection pattern according to embodiments of theinvention. The TPBPC 100 has filter inductors ^^, ^^, ^^ which has of two terminals of which the firstterminal of each phase is connected to the positive terminals of each phase of the three phase voltages ^^, ^^, ^^and each second terminal are connected to the source of the lower switching devices ^^, ^^^^, respectively. This connection forms midpoint nodes A, B and C as seen in Fig. 4. Current ^^^ is thecurrent flowing through the DC bus 140, current ^^^is the current flowing in the primary winding W1 and voltage ^^^is the voltage across the primary winding W1.The flyback transformer mode soft switching cell 120 comprises of a flyback transformer 122, aclamping capacitor ^^ acting as a voltage source, a magnetizing inductor ^^ which is a part of thetransformer, and a soft switching cell switch ^^. The flyback transformer 122 comprises a single primary winding W1 and three secondary windings W21, W22, W23. The positive terminal of the primary winding W1 is connected to the drain of the soft switching cell switch ^^followed by a connection to the clamping capacitor ^^which in turn isconnected to the negative terminal of the primary winding W1. The first secondary winding W21 withvoltage ^^^^^, the second secondary winding W22 with voltage ^^^^^ and the third secondary windingW23 with voltage ^^^^^ has their positive terminals connected to the drains of upper switches ^^, ^^ ^^and the negative terminals to the source of the lower switches ^^, ^^, ^^ which is the same as themidpoint nodes A, B, C. It may be noted that first W21, the second W22, and the third W23 secondarywindings are in series with the upper switches ^^, ^^, ^^, respectively.Fig. 5 shows a TPBPC 100 according to the second connection pattern according to embodiments of theinvention. The TPBPC 100 has filter inductors ^^, ^^, ^^ which has of two terminals of which the firstterminal of each phase is connected to the positive terminals of each phase of the three phase grid ^^,^^, ^^ and each second terminal are connected to the drains of the upper switches ^^, ^^, ^^ respectively.This connection forms midpoint nodes A, B and C as seen in Fig. 5.The flyback transformer mode soft switching cell 120 also in this case comprises of a flybacktransformer 122, a clamping capacitor ^^ acting as a voltage source, a magnetizing inductor ^^ whichis part of the transformer, and a soft switching cell switch ^^.The flyback transformer 122 comprises of a single primary winding W1 and three secondary windingsW21, W22, W23 as previously mentioned. The positive terminal of the primary winding W1 is connected to the drain of the soft switching cell switch ^^followed by a connection to the clampingcapacitor ^^ which in turn is connected to the negative terminal of the primary winding W1. The first secondary winding W21 with voltage ^^^^^, the second secondary winding W22 with voltage ^^^^^and the third secondary winding W23 with voltage ^^^^^has their positive terminals connected to the drainsof the upper switches ^^, ^^, ^^ and the negative terminals to the source of lower switches ^^, ^^, ^^which is the same as the midpoint nodes A, B and C. It may be noted that ^^^^^, ^^^^^ and ^^^^^ are inseries with lower switches ^^, ^^, ^^, respectively.For the described first and second connection patterns in Fig.4 and 5 the following holds. In embodiments, the TPBPC 100 comprises a controller 170 which is described in the following disclosure.In embodiments, the number of turns of the primary and secondary windings of the transformer 122 maybe selected depending on the application. An option is 1:1 turns ratio i.e., the same number of turns inprimary winding as in the secondary windings. Higher number of turns can also be chosen for theprimary winding if a lower RMS current is needed in the soft switching cell switch e.g., 1.1:1 or 1.2:1 turns ratio. According to Ohms law higher voltage means lower current so higher number of turns in the primary side means lower RMS current in the soft switching cell switch. In embodiments, the DC bus capacitor 142 acts as an input source in the inverter mode or an output source in the rectifier mode of the TPBPC 100. In embodiments, the soft switching cell 120 works with pulse width modulation (PWM) alignment of hard turn on the instant of the main switches according to Table 1 before the ZVS turn-on instant of the listed switches in Table 1 can be achieved simultaneously.In embodiments, the soft switching cell switch ^^ is turned off, i.e., in non-conductive state, for a shortduty cycle such as ≤15% to make switching loss negligible and create a path for current injection during the ZVS.The energy stored in the magnetizing inductor ^^ of the transformer 122 is used to achieve the ZVSturn-on of the aligned switches of the switching network 110. This means that the magnetizing inductor^^resonates with the capacitors of the aligned switches in Table 1. For example, in the operating pointused in the previous analysis the magnetizing inductor ^^ resonates with ^^^, ^^^, ^^^, ^^^, or ^^^, ^^^,^^^, ^^^ .Furthermore, Figs. 6a to 6k illustrates different operating modes of a TPBPC 100 according toembodiments of the invention. The operating modes relates to different time periods defined by timeinstances. The key operating waveforms of the flyback mode soft switching cell 120 are depicted in Fig.3 and the characteristics of the mode are analytically explained in the following disclosure. It may benoted the different components of the TPBPC 100 in Figs. 6a to 6k have the same reference numeralsand denotations as previously given in the disclosure.Mode ^^-^^(Fig.6a)Through modulation strategies hard switch is identified according to Table 1 and the switches arealigned according to embodiments of the invention.At time instance < ^^ the upper switches ^^, ^^, ^^ are about to be aligned and are in non-conductivestate.At time ^^ due to the filter currents ^^ > 0, ^^ < 0, ^^ < 0, the antiparallel diodes ^^, ^^, ^^ of the upperswitches ^^, ^^, ^^ conducts.The soft switching cell switch ^^is conducting at this stage and the magnetizing inductance ^^of thetransformer 122 is clamped to the auxiliary capacitor voltage ^^^. The equations for this mode are givenas: ^^^ = ^^^ = −^^^ (2)If the number of turns in the transformer 122 is the same in the primary winding W1 and in the secondary windings W21, W22, W23 the following relations hold: ^^^ = ^^ + ^^^ (4)where ^^^, ^^^ , ^^, ^^^ are the voltages across the magnetizing inductor ^^ transformer primary sidevoltage, the DC bus voltage and the current flowing through the magnetizing inductor ^^ respectively.^^^^^, ^^^^^, ^^^^^ are the transformer secondary side voltages across the secondary windings W21,W22, W23, respectively. Mode ^^-^^(Fig.6b)At time instance ^^ the soft switching cell switch ^^ is turned off and the clamping capacitor ^^ isbehaving as a constant DC source.The magnetizing inductance ^^ thus resonates with capacitors ^^^, ^^^, ^^^ and ^^^, where ^^^, ^^^,^^^ are discharging while ^^^ is charging.Hence, we have the following relations: Let ^^^, ^^^, ^^^ = ^^^ which gives:−3^^^^^^ ^ ^^^^+ ^^^^(^ ^^ )^^= ^^^ (7)Mode ^^-^^(Fig.6c)The voltage across capacitors ^^^, ^^^, ^^^ are already discharged to zero and capacitor ^^^ is chargedequal to ^^ + ^^^.The diodes ^^, ^^, ^^, ^^, ^^, ^^ are conducting which provides ZVS turn-on of switches ^^, ^^, ^^ attime instance ^^.The magnetizing inductance ^^ suppresses the reverse recovery current of the antiparallel diodesaccording to the formulas: ^^^^ ≈ ^^ + ^^^ (9)Thus, the magnetizing inductance ^^ suppresses the reverse recovery of the body diode of switch ^^when switch ^^ is conducting and vice versa and same principle applies to the other H-bridge legs.Mode (Fig.6d)The current through diodes ^^, ^^, ^^ decreases to zero and their respective switches ^^, ^^, ^^ aretherefore conducting.When the current ^^^ in the magnetizing inductance ^^ decreases to zero, diodes ^^ and ^^ stopsconducting and the magnetizing inductance ^^ is clamped to the voltage ^^ and continuously increases.Mode ^^-^^(Fig.6e)When current ^^^ decreases to the value of the phase current ^^, diode ^^ stops conducting and thefollowing current relation holds: As shown in Eq. (10) the energy has been transferred to the DC bus 140 during the total duration of timeperiod ^^ − ^^ and the following relation holds:^ ^ = ^^^^^^^^^= ^^ (11)^ = ^ = ^ = ^^^^^^^ ^^^^ ^^^^ ^^^(12) Mode (Fig.6f)The magnetizing inductance ^^ resonates with capacitors ^^^, ^^^, ^^^ and ^^^ and the antiparalleldiode ^^ of the soft switching cell switch ^^ is ready to be turned on.During this stage the capacitors ^^^, ^^^, ^^^ are charged while capacitor ^^^ is discharged.At time instance ^^ the following equations hold:^ =^^^^^^ ^^^^= ^^ (13)Let ^^^, ^^^, ^^^ = ^^^ which means that: Mode (Fig.6g)Switches ^^, ^^, ^^ are in conductive state. The soft switching cell switch ^^ is turned on under ZVScondition.Capacitor ^^ is charged by current ^^^ governed by the relation: ^^^ = ^ = −^^^.Switch ^^ turns off at time instance ^^ with a hard turn-off.Mode ^^-^^(Fig.6h)There is a third resonance at the beginning of mode ^^-^^.The phase current ^^ charges capacitor ^^^ and discharges capacitor ^^^. The phase current ^^ dischargescapacitor ^^^ and charges capacitor ^^^. The following equations are valid for this mode: At time instance ^^, switches ^^, ^^ turns on with ZVS after diodes ^^, ^^ begins to conduct as seen inFig 3. Mode ^^-^^(Fig.6i)Switches ^^, ^^, ^^ and ^^ remains on and the power flows from the three-phase AC side to the DC bus140.Current ^^^ charges the capacitor equal to: ^^^ = −^^^.Switch ^^ turns off at time instance ^^ with a hard turn-off.Mode ^^-^^^(Fig.6j)The phase current ^^ in mode ^^-^^^ discharges capacitor ^^^ and charges capacitor ^^^ according to: Mode ^^^-^^^ (Fig. 6k)The energy in the magnetizing inductance ^^is transferred into the clamping capacitor ^^in mode ^^^-^^^ until the operating mode returns to mode ^ < ^^.Moreover, in embodiments of the invention, the TPBPC 100 comprises a controller 170 or a controller arrangement 170 configured to control the TPBPC 100. This may be understood such that the controller170 is configured to control the different components / parts of the TPBPC 100 such as the switchingnetwork 110, the soft switching cell 120, the input 130, the DC bus 140, and the filter circuit 150. Thus, the controller 170 may be configured to control the components / parts of the TPBPC 100 such that theTPBPC 100: aligns the switches of the switching network 110 into a switching pattern and discharges acurrent from the soft switching cell 120 into the switching network 110 for zero voltage switching of the aligned switches of the switching network 110 according to the switching pattern.The controller 170 may be a digital controller connected to the components / parts of the TPBPC 100 forcontrolling them. Thus, the TPBPC 100 may comprise control lines and / or control buses between thecontroller 170 and the components / parts of the TPBPC 100. The control lines and / or control buses may carry control signals and control commands according to standardised procedures and protocols.The controller 170 may comprise a processor and a memory. The processor may be referred to as oneor more general-purpose central processing units (CPUs), one or more digital signal processors (DSPs), one or more application-specific integrated circuits (ASICs), one or more field programmable gate arrays (FPGAs), one or more programmable logic devices, one or more discrete gates, one or more transistor logic devices, one or more discrete hardware components, or one or more chipsets. The memory may be a read-only memory, a random access memory (RAM), or a non-volatile RAM (NVRAM). A computer program may be executed by the processor for controlling the TPBPC 100.Finally, it should be understood that the invention is not limited to the embodiments described above,but also relates to and incorporates all embodiments within the scope of the appended independent claims.

Claims

CLAIMS 1. A three-phase bidirectional power converter (100), TPBPC, comprising: a switching network (110) comprising a first (H1), a second (H2) and a third (H3) H-bridge leg, each H-bridge leg comprising an upper switch (^^, ^^, ^^), a lower switch (^^, ^^, ^^) and a midpoint node (A, B, C) arranged between the upper switch (^^, ^^, ^^) and the lower switch (^^, ^^, ^^); a soft switching cell (120) comprising a transformer (122) and a current charging / discharging circuit (124), the transformer (122) comprising a primary winding (W1) connected to the current charging / discharging circuit (124), and a first (W21), a second (W22) and a third (W23) secondary winding connected to the first (H1), the second (^^) and the third (H3) H-bridge leg, respectively; an input (130) comprising a first (132), a second (134) and a third (136) port connected to the first (W21), the second (W22) and the third (W23) secondary winding, respectively, and being configured to receive a first ^^, a second ^^and a third ^^phase voltage, respectively; and a DC bus (140) connected to the first (H1), the second (H2) and the third (H3) H-bridge leg; wherein the TPBPC (100) is configured to: align the switches of the switching network (110) into a switching pattern, and discharge a current from the soft switching cell (120) into the switching network (110) for zero voltage switching of the aligned switches of the switching network (110) according to the switching pattern.

2. The TPBPC (100) according to claim 1, wherein the current charging / discharging circuit (124) comprises a soft switching cell switch (^^) connected in series with a soft switching cell capacitor (^^).

3. The TPBPC (100) according to claim 2, wherein a positive terminal of the primary winding (^^) is connected to a drain of the soft switching cell switch (^^) and a negative terminal of the primary winding (^^) is connected to the soft switching cell capacitor (^^).

4. The TPBPC (100) according to claim 2 or 3, wherein an inductance (^^) of the transformer (122), when discharging the switch current into the switching network (110), resonates with a capacitance (^^^) of the soft switching cell switch (^^) of the current charging / discharging circuit (124) and the capacitances of the aligned switches of the switching network (110) according to the switching pattern.

5. The TPBPC (100) according to any one of the preceding claims, wherein the transformer (122) is a flyback transformer.

6. The TPBPC (100) according to any one of the preceding claims, wherein discharging the switching current into the switching network (110) comprises:set the soft switching cell switch (^^) into a non-conducting mode.

7. The TPBPC (100) according to claim 6, wherein the soft switching cell switch (^^) is in the non- conducting mode ≤15% of a duty cycle for the TPBPC (100).

8. The TPBPC (100) according to any one of the preceding claims, wherein aligning the switches of the switching network (110) is dependent on a polarity of a first ^^, a second ^^and a third ^^phase current associated with the first ^^, the second ^^and the third ^^phase voltage, respectively.

9. The TPBPC (100) according to claim 8, wherein the switches of the switching network (110) arealigned according to the following table: Phase current polarities Aligned switches−^^, +^^and +^^^^, ^^and ^^−^^, −^^and +^^^^, ^^and ^^+^^, −^^and +^^^^, ^^and ^^+^^, −^^and −^^^^, ^^and ^^+^^, +^^and −^^^^, ^^and ^^−^^, +^^and −^^^^, ^^and ^^where ^^, ^^and ^^are the upper switches of respective H-bridge leg and ^^, ^^and ^^are the lower switches of respective H-bridge leg.

10. The TPBPC (100) according to any one of the preceding claims, wherein the TPBPC (100) is configured to: align the switches of the switching network (110) and discharge the switch current simultaneously.

11. The TPBPC (100) according to any one of the preceding claims, wherein the TPBPC (100) is configured to: operate with a fixed switching frequency and in a continuous conduction mode.

12. The TPBPC (100) according to any one of claims 1 to 11, wherein a positive terminal of the first (W21), the second (W22) and the third (W23) secondary winding is connected to a source of the upper switch (^^, ^^, ^^) respectively; and a negative terminal of the first (W21), the second (W22) and the third (W23) secondary winding is connected to the first (A), the second (B) and the third (C) midpoint node, respectively.

13. The TPBPC (100) according to any one of claims 1 to 11, wherein a positive terminal of the first (W21), the second (W22) and the third (W23) secondary winding is connected to the first (A), the second (B) and the third (C) midpoint node, respectively; and a negative terminal of the first (W21), the second (W22) and the third (W23) secondary winding is connected to a drain of the lower switch (^^, ^^, ^^), respectively.

14. The TPBPC (100) according to any one of the preceding claims, comprising a filter circuit (150) connected between the input (130) and the first (W21), the second (W22) and the third (W23) secondary winding.

15. The TPBPC (100) according to claim 14, wherein the filter circuit (150) comprises a first filter inductor (^^) connected between the first port (132) and a negative terminal of the first secondary winding (W21), a second filter inductor (^^) connected between the second port (134) and a negative terminal of the second secondary winding (W22), and a third filter inductor (^^) connected between the third port (136) and a negative terminal of the third secondary winding (W23).

16. The TPBPC (100) according to any one of the preceding claims, wherein the DC bus (140) comprises a DC bus capacitor (142).

17. The TPBPC (100) according to any one of the preceding claims, wherein the DC bus (140) is configured as an output source in a rectifier mode or an input source in an inverter mode of the TPBPC (100).

18. The TPBPC (100) according to any one of the preceding claims, wherein a positive terminal of theDC bus (140) is connected to the drains of the upper switches (^^, ^^, ^^) and a negative terminal of theDC bus (140) is connected to the sources of the lower switches (^^, ^^, ^^).

19. A method (200) for a TPBPC (100) comprising: a switching network (110) comprising a first (H1), a second (H2) and a third (H3) H-bridge leg, each H-bridge leg comprising an upper switch (^^, ^^, ^^), a lower switch (^^, ^^, ^^) and a midpoint node (A, B, C) arranged between the upper switch (^^, ^^, ^^) and the lower switch (^^, ^^, ^^); a soft switching cell (120) comprising a transformer (122) and a current charging / discharging circuit (124), the transformer (122) comprising a primary winding (^^) connected to the currentcharging / discharging circuit (124), and a first (W21), a second (W22) and a third (W23) secondarywinding connected to the first (H1), the second (H2) and the third (H3) H-bridge leg, respectively;an input (130) comprising a first (132), a second (134) and a third (136) port connected to the first (W21), the second (W22) and the third (W23) secondary winding, respectively, and being configured to receive a first ^^, a second ^^and a third ^^phase voltage, respectively; and aDC bus (140) connected to the first (H1), the second (H2) and the third (H3) H-bridge leg; themethod (200) comprising: aligning (202) the switches of the switching network (110) into a switching pattern, anddischarging (204) a current from the soft switching cell (120) into the switching network (110)for zero voltage switching of the aligned switches of the switching network (110) according to theswitching pattern.

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