Topology and control method for a DC-DC converter

Abstract

This invention provides a hybrid stage DC-DC converter comprising a power input; a power output; an isolation stage connected to the power input and comprising an AC input port, first and second transformers, and first and second isolation DC output ports; and a voltage regulation stage comprising an input DC port and an output regulation DC port connected to the power output; wherein: the first transformer is configured to receive power from the voltage input via the AC input port and to output power via the first isolation DC output port to the input DC port of the voltage regulation stage; and the second transformer is configured to receive power from the voltage input and to output power via the second isolation DC output port to the power output; and wherein: the voltage regulation stage is configured to regulate incoming power from the isolation stage and to output regulated power to the power output via the output regulation DC port.

Description

[0001]SP3074 - 1 - TOPOLOGY AND CONTROL METHOD FOR A DC-DC CONVERTER Field of the Invention This invention relates to aspects of a power supply system, and particularly to a DC-DC converter suitable for use in DC power supply systems. This invention also 5 relates to methods for controlling the DC-DC converter. Background of the invention DC-DC converters are widely used in power supply systems. In certain implementation use cases, the power 10 supply systems need to be able to provide a wide voltage variation range. Examples of such implementation use cases include power supply for water electrolyzers, battery charging systems, and at energy storage system interfaces. In order to provide the desired wide voltage 15 variation range, the converters that are used for such implementations are typically designed with two stages: a first, isolation stage and a second, voltage reduction stage. However, the utilization of two distinct power conversion stages tends to lead to high power losses and 20 low efficiency, primarily resulting from the switching losses that occur. It is against this background that the present disclosure has been set. 25 Summary of the Invention According to an aspect of the invention, there is provided a hybrid stage DC-DC converter. The converter comprises a power input, a power output, an isolation stage, and a voltage regulation stage. The isolation stage 30 is connected to the power input and comprises an AC input port, first and second transformers, and first and second isolation DC output ports. The voltage regulation stage comprises an input DC port and an output regulation DC port connected to the power output. The voltage regulation stage is configured to regulate incoming power from the 5 isolation stage and to output regulated power to the power output via the output regulation DC port. The first transformer (of the isolation stage) is configured to receive power from the voltage input via the AC input port and to output power via the first isolation DC output port 10 to the input DC port of the voltage regulation stage. The second transformer (of the isolation stage) is configured to receive power from the voltage input and to output power via the second isolation DC output port to the power output. 15 The above-described configuration advantageously can transfer power through two power conversion stages within the isolation stage (corresponding to the first and second transformers). Thereafter, only some of the power output from the isolation stage will be provided to the voltage 20 reduction stage; the remaining power from the isolation stage can be directly output to the power output of the converter. This configuration simplifies the topology of the isolation stages (especially as compared with prior art converters). It also reduces the power losses of 25 switches on the input side (of the isolation stage). In some instances, the isolation stage comprises first and second diode rectifiers. The first diode rectifier has / corresponds to / is connected to the first isolation DC output port; and the second diode rectifier 30 has / corresponds to / is connected to the second isolation DC output port. Optionally, the output regulation DC port of the voltage regulation stage is connected to the second isolation DC output port. Optionally, each of the diode rectifiers has an AC port. In such instances, the first transformer has a primary winding connected to the AC input port, and a secondary 5 winding connected to the AC port of the first diode rectifier; and the second transformer has a secondary winding connected to the AC port of the second diode rectifier. In such instances, the first and second transformers are also connected in series with one 10 another. The above-described configurations correspond to a particular implementation, and specifically to a particular arrangement of diode rectifiers and transformers, that enables hybrid partial power transfer 15 to be achieved. Specifically, the above-described connections between the two transformers and the two diode rectifiers allows the power being output from the isolation stage subsequently (via the diode rectifiers) to be split into two current paths, one through each diode 20 rectifier. Thereafter, the power from one of the pair of diode rectifiers can be passed directly to the power output, whilst the power from the other diode rectifier can be passed to the voltage regulation stage. In some instances, the isolation stage further 25 comprises a first switching arrangement operated at a first switching frequency; and the voltage regulation stage comprises a second switching arrangement operated at a second switching frequency. In such cases, the first and second switching 30 frequencies may be different. More specifically, the second switching frequency may be lower than the first switching frequency. Advantageously, the above-described switching configuration is able to reduce power loss (in the form of switching losses, and particularly in relation to the switching losses associated with the voltage regulation stage) even further. Additionally or alternatively, the first switching 5 frequency is fixed whilst second switching frequency is variable. Advantageously, this means that the switching frequency of the voltage regulation stage can be varied to regulate the output voltage from the converter as 10 necessary. This variation can be implemented without altering the switching frequency utilised by the isolation stage. In some instances, each of the transformers used in the isolation stage corresponds to a three-phase 15 transformer; and each of the diode rectifiers used in the diode rectifiers corresponds to a three-phase diode rectifier. Advantageously, implementing a three-phase structure in the isolation stage (as compared with a typical ‘two 20 phase structure’ that would normally be used) means that the current associated with each phase is reduced. As a consequence, the peak current passing through the switches that are associated with the isolation stage (for example, the first switching arrangement) is lower. This means that 25 the power losses associated with switching can be reduced in the three-phase structure – since power losses are proportional to the square of the current, any reduction in current greatly reduces the corresponding power losses. A lower current rating can also be maintained, which 30 further increases the safety of the arrangement. Optionally, the isolation stage comprises first and second filter inductors, the first filter inductor being connected to the first diode rectifier and the second filter inductor being connected to the second diode rectifier. The above-described configuration is particularly advantageous when implemented in relation to the three- 5 phase structure, since it can suppress unbalanced three- phase current. Brief Description of the Drawings Figure 1 schematically illustrates a DC-DC converter 10 topology that may be used in some known power supply applications; Figure 2 schematically illustrates an alternative DC-DC converter topology that may be used in some known power supply applications; 15 Figure 3 schematically illustrates a hybrid DC-DC converter topology according to an aspect of the present invention; Figure 4 illustrates an example modulation strategy that may be used during implementation of the DC-DC 20 converter of Figure 3; Figure 5 illustrates example modulation signals that may be used during implementation of the modulation strategy of Figure 4; Figures 6a to 6c schematically illustrate conduction 25 paths through the DC-DC converter of Figure 3 when implementing the modulation strategy of Figure 4; Figure 7 illustrates a hybrid DC-DC converter topology according to another aspect of the present invention; and 30 Figure 8 illustrates simulation results for an example implementation of the DC-DC converter shown in Figure 3. These drawings depict one or more implementations in accordance with the present teachings, by way of example only, not by way of limitation. In the figures, like reference numerals refer to the same or similar elements. Detailed Description of the Drawings Figures 1 and 2 schematically illustrate example DC- DC converter configurations that may be utilised in some conventional prior art systems. Figure 1 shows an example two-stage DC-DC converter that is described in more detail in CN 103595259. This converter topology is referred to in that document as a ‘double-transformer series-parallel isolation soft- switching DC converter’. It comprises input and output sources Uinand Uout, and a pair of transformers T1 and T2 each having primary and secondary windings NP1 / 2 and NS1 / 2. The primary windings of the two transformers are connected in series, and the secondary windings are connected in parallel. On the ‘primary side’ of the converter (i.e., between the input source Uin and the primary windings of the transformers T1 and T2), there is provided a pair of voltage dividing capacitors Cin1 and Cin2. These are connected in series with one another and in parallel to the input source Uin. Four switches S1-S4 are provided in paired relationship on the primary side of the converter, whereby the midpoint between pairs of switches S1-S2and S3-S4is connected to the midpoint of the pair of capacitors Cin1 and Cin2. In addition, the midpoint of each pair of switches is connected to one end of the primary windings: in this case, the midpoint of a first pair of switches S1 and S2 is connected to the primary winding NP1of the first transformer (via a filter inductor Lf); and the midpoint of a second pair of switches S3 and S4 is connected to the primary winding NP2 of the second transformer (via a blocking capacitor Cb). On the ‘secondary side’ of the converter (i.e., between the output source Uoutand the secondary windings of the transformers T1 and T2), there is provided a further pair of switches S5 and S6; and a four- diode rectifier D1-D4 and an output filter capacitor Co connected in parallel with one another. The midpoint of the pair of switches S5and S6is connected to the secondary winding NS1of the first transformer; the midpoint of the transformers is connected to the midpoint of a first pair of diodes D1-D2; and the secondary winding NS2 of the second transformer is connected to the midpoint of second pair of diodes D3-D4. This converter topology effectively combines the isolation and voltage reduction stages together, as the two diodes D1and D2are shared between the two stages. However, as the shared diodes D1 and D2 carry both currents through the two transformers, there is little reduction in the conduction loss of the diodes overall. Moreover, the switches S5 and S6 on the output secondary side have to operate at the same (high) frequency as the switches S1-S4 on the input primary side, which leads to high switching losses. Figure 2 shows an example two-stage DC-DC converter that is described in more detail in WO 2020 / 037291. The converter in this case also comprises isolation stages (referred to as ’DCX modules’) and a voltage reduction stage (referred to as the ‘first converter’). The output terminals of the DCX modules are connected in series, and the input terminal of one DCX module is connected to the output of the first converter. The input port of the other DCX module is connected to the input port of the first converter, creating the input terminal. As the output ports of the two DCX modules are connected in series, the switches on the output side experience high conduction losses and low efficiency. This drawback limits the applicability of this converter to certain scenarios only – namely, those with low output voltage and large current. The present applicants have appreciated the limitations that are associated with the above solutions, and have developed an improved solution that alters the topology of the DC-DC converter arrangement. An example of such improved topologies according to aspects of the present invention are illustrated in Figures 3 and 7. Referring first to Figure 3, the DC-DC converter illustrated therein comprises an input terminal Vin and output terminal Vout. As shown in Figure 3, the DC-DC converter also has a topology that consists of an isolation stage and a voltage regulation stage. The isolation stage comprises an input capacitor CSM; a full-bridge circuit Q1-4; a resonant inductor Lr; a resonant capacitor Cr; first and second transformers Tr1 and Tr2; and first and second diode rectifiers D1-4 and D5-8. The isolation stage also comprises a pair of output capacitors which are connected respectively to one of the diode rectifiers – specifically, a first output capacitor Co1 is connected to the first diode rectifier D1-4 and a second output capacitor Co2 is connected to the second diode rectifier D5-8. The two transformers Tr1and Tr2each have respective primary and secondary windings, whereby the primary windings are provided at the ‘input’ side of the respective transformer and the secondary windings are provided at the ‘output’ side of the respective transformer. The primary windings of the two transformers are connected in series with the resonant inductor Lr, the resonant capacitor Cr, and AC-ports of the full-bridge circuit Q1-4. Specifically in relation to the full-bridge circuit, it is noted that this circuit comprises two pairs of switches Q1-2 and Q3-4 arranged in parallel; and the AC- ports correspond to the mid-points of each pair of switches. In other words, the primary windings of the first transformer Tr1 are connected to an AC-port corresponding to the mid-point between switches Q1 and Q2; and the primary windings of the second transformer Tr2 are connected to an AC-port corresponding to the mid-point between switches Q3and Q4. The primary windings of the two transformers are also connected to one another to complete this circuit. The secondary windings of each of the two transformers are connected separately to AC-ports of one of the diode rectifiers. The AC-ports of the diode rectifiers correspond to mid-points of pairs of diodes: namely, (i) between D1 and D2, (ii) between D3 and D4, (iii) between D5 and D6, and (iv) between D7 and D8. Therefore, more specifically, the secondary winding of the first transformer Tr1 is connected to the AC-ports of the first diode rectifier – AC-ports (i) and (ii) mentioned above; and the secondary winding of the second transformer Tr2 is connected to the AC-ports of the second diode rectifier – AC-ports (iii) and (iv) mentioned above. The voltage regulation stage comprises a switch S; a diode D and a filter inductor Lo which is used to regulate the output voltage. The voltage regulation stage and the isolation stage are connected to one another. Specifically, an input DC- port of the voltage regulation stage is connected to a (output) DC-port of the first diode rectifier in the isolation stage – this is indicated by the voltage Vo1 label that is associated with the first output capacitor Co1of the isolation stage. An output port of the voltage regulation stage is connected to a (output) DC-port of the second diode rectifier – this is indicated by the voltage Vo2 label that is associated with the second output capacitor Co2of the isolation stage. The output port of the voltage regulation stage also constitutes / corresponds to the DC output port of the overall converter. 5 The present applicants have appreciated that by configuring the topology of the DC-DC converter in the above manner, the converter is able to transfer some of the incoming power through two power conversion stages, while the remaining incoming power is transferred directly 10 by a single stage only, resulting in significantly improved efficiency. In other words, some of the power passes through both the isolation stage and the voltage regulation stage, whilst some of the power only passes through the isolation stage and then passes directly to 15 the output port of the converter (bypassing the voltage regulation). This configuration means that the output voltage can still be regulated (since some of the power is still passed from the isolation stage to the voltage regulation 20 stage) and hence the DC-DC converter is still able to handle wide voltage variation ranges. However, as only a portion of the output current is processed by the voltage regulation stage, switching and conductions losses are minimised / reduced as far as possible. 25 This ability to transfer only partial power through two power conversation stages is achieved as a result of (i) the ‘splitting’ of the incoming power, that is to be converted, by the first transformer and the second transformer (and their associated diode rectifiers); in 30 combination with (ii) the configuration whereby the output of only one of the transformers (i.e., the first transformer Tr1) is connected to the input of the voltage reduction stage. The benefits / advantages provided by the above- described converter topology can best be appreciated when considering implementation examples. Some of these will now be described with reference to Figures 4, 5 and 6. Figure 4 illustrates an example modulation strategy for the topology of Figure 3; and Figure 5 illustrates corresponding example modulation signals. Figure 6 illustrates schematically current flow paths that correspond to those example modulation signals and strategy. Considering Figure 4, a first carrier wave having a first driving / switching frequency fs1 (and a switching period TS) is defined for the four switches Q1-4in the full-bridge circuit. In other words, the switching frequency of the isolation stage corresponds to this driving frequency fs1. A corresponding duty cycle DQ1 is defined, and driving signals for each of the switches in the full-bridge circuit GQ1, GQ2, GQ3and GQ4are thereby derived. These driving signals are the same in frequency but are complementary to one another for pairs of switches (i.e., when one pair of switches GQ1 and GQ4 is turned on, the other pair GQ2 and GQ3 is not – they are in anti-phase). Typically, the duty cycle DQ1 would be set to 0.5, such that each of the switches is turned on for half of the switching period Ts1(i.e., each of GQ1 / Q4and GQ2 / Q3are output sequentially / in turn for 0.5Ts1). However, in some cases the duty cycle DQ1 can be set to less than 0.5; in such cases, GQ2 / Q3 can be obtained by shifting GQ1 / Q4 by 0.5Ts1 such that the driving signals are still offset from one another by the same amount. A second carrier wave having a second driving / switching frequency fs2 is defined for the switch S in the voltage regulation stage. In other words, the switching frequency of the voltage regulation stage corresponds to this driving frequency fs2. A corresponding duty cycle DSis defined, and a driving signal Gs is thereby derived. The first and second carrier waves are synchronised with one another, such that they start at the same time. In the isolation stage, all switches are open-loop controlled with a fixed duty cycle DQ1and fixed switching frequency. All switches and diodes in the isolation stage can realize zero-current-switching, leading to a reduction in power losses. In the voltage regulation stage, on the other hand, the switch S is controlled with a variable duty cycle Ds, to regulate the output voltage. More specifically, the duty cycle DS varies with the input or output voltages / currents. It will be appreciated that a beneficial aspect of this configuration is that the two stages can be controlled with different switching frequencies. This means that a lower frequency fs2 can be utilized for the voltage regulation stage, than the frequency fSthat is utilised for the isolation stage. This helps to reduce power losses (resulting from switching losses), and thereby improve efficiency, even further. Now referring to Figures 5 and 6, example operational waveforms during operation of the DC-DC converter are illustrated in Figure 5. There are three ‘modes’ of operation within a given operational period Tsand the conducting path associated with each of those ‘modes’ is illustrated in Figure 6. a) ‘Mode 1’(from t0 to t1): In the isolation stage, switches Q1 and Q4 are turned on, and the primary AC voltage Vpthat is passed by those switches to the first and second transformers Tr1and Tr2increases to the input voltage Vin. Additionally, half of the diodes in the first and second diode rectifiers – namely D1, D4, D5, and D8 – conduct, and as a result the output voltages Vs1and Vs2from the first and second diode rectifiers vary to Vo1 and Vo2, respectively. The resonance between the resonant inductor Lr and resonant capacitor Cr begins. Simultaneously, in the voltage regulation stage, switch S is also turned on, and the current ILothat passes through the filter inductor Loincreases until S is turned off at t1. The conduction path during this time period / mode is illustrated in Figure 6(a). b) ‘Mode 2’(from t1 to t2): At t1, the switch S in the voltage regulation stage is turned off and the current ILo decreases. However, in the isolation stage, the switches Q1and Q4are still turned on and conducting, and the resonance between the resonant inductor Lr and resonant capacitor Cr continues. The conduction path during this time period / mode is illustrated in Figure 6(b). c) ‘Mode 3’(from t2to t3): At t2, the resonance between the resonant inductor Lr and resonant capacitor Cr ceases. As a result, in the isolation stage, the switches Q1 and Q4, the diodes D1, D4, D5, and D8 can be turned off with zero current. At that point, switches Q2 and Q3 are then turned on instead, and the primary voltage Vpvaries to -Vin. The other half of the diodes D2, D3, D6, and D7in the first and second diode rectifiers conduct, and the output voltages vs1 and Vs2 decrease to -Vo1 and -Vo2, respectively. The resonance between the resonant inductor Lr and resonant capacitor Cr starts again, and continues until t3at which point the current Irpvaries to zero again. Once this has occurred, the switches Q2and Q3in the isolation stage are turned off with zero current. All during this mode, the switch S in the voltage regulation stage remains turned off. The conduction path during this time period / mode is illustrated in Figure 6(c). In this manner, the switching frequencies of the two stages can be isolated so as to reduce the resultant switching losses. However, by changing the duty cycle of S, the output voltage Vocan be regulated. Another hybrid-stage DC-DC converter topology that has been envisaged is illustrated in Figure 7. This topology is a variant of that illustrated in Figure 3. The DC-DC converter that is shown in Figure 7 is similar to that shown and described above in relation to Figure 3. The topology of Figure 7 also comprises an isolation stage and a voltage regulation stage, and these two stages are also connected with each other in such a way that only a portion of the power that is output from the isolation stage also passes through the voltage regulation stage. The benefits associated with reduced switching losses and increased efficiency are hence also achieved for the topology shown in Figure 7. The same main components are also utilised in each stage. The isolation stage of the DC-DC converter shown in Figure 7 also comprises an input capacitor CCM; a bridge circuit Q1-6; resonant inductors Lra-rc and resonant capacitors Cra-rc; two transformers Tra1-rc1and Tra2-rb2; two diode rectifiers D1-6and D7-12; filter inductors Lfo1-fo2and filter capacitors Cfo1-fo2. The components used in the voltage regulation stage are substantively identical to those that were used in the corresponding stage of the converter topology illustrated in Figure 3. Moreover, the voltage regulation stage and the isolation stage are connected to one another in substantially the same was in the topology of Figure 7 as was described above in relation to Figure 3. Specifically, the input DC-port of the voltage regulation stage is connected to the (output) DC-port of the first diode rectifier D1-6 in the isolation stage. The output port of the voltage regulation stage is connected to the (output) DC-port of the second diode rectifier D7-12. The output port of the voltage regulation stage also constitutes / corresponds to the DC output port of the overall converter. The main difference between the topologies of the converters in Figures 3 and 7 is that the Figure 7 converter has a ‘three-phase (interleaved) structure’. In other words, each of the bridge circuit, first and second transformers, and diode rectifiers are ‘three-phase’ components. As a result, an extra one of the other components that are connected in series with any of these three-phase components in the isolation stage need to be provided in the topology of Figure 7 (as compared with the topology in Figure 3). Specifically, three each of the resonant inductors and resonant capacitors are provided in the isolation stage. In the isolation stage, a corresponding configuration (to that described above in relation to the topology of Figure 3) of the bridge circuit Q1-6, first and second transformers Tra1-rc1 and Tra2-rb2, and diode rectifiers D1-6and D7-12is utilised in the topology of Figure 7: the primary windings of the first and second transformers are connected in series with the resonant inductors, resonant capacitors and the AC-ports (i.e., the mid-points between pairs of switches) of the bridge converter; and the primary windings of the two transformers are also connected in series with one another. Additionally, the secondary windings of the two transformers are connected separately to AC-ports (i.e., mid-points of pairs of diodes) of one of the diode rectifiers. Filter inductors Lfo1 and Lfo2 are connected to the first and second three- phase diode rectifiers D1-6and D7-12respectively to suppress the unbalanced three-phase current due to the inconsistency of resonant inductors and capacitors. Additional advantages are envisaged in relation to use of the three-phase structure described above, particularly when considering implementations / applications involving higher power requirements. The switches in this three-phase structure withstand lower current stress than in the topology shown in Figure 3. Considered another way, it will be appreciated that by having three interleaved phases in the topology, the current in each phase will be lower; the peak current through the switches will hence also be lower. As a result, power losses associated with the switches will be lower (since power loss during switching is proportional to the square of the current passing through). Moreover, the three-phase structure may be considered to be a particularly safe implementation as a result of the lower current rating that can be achieved. The components in the ‘three-phase (interleaved) structure’ converter of Figure 7 would be operated in a similar manner to that described above in relation to Figure 4 and 5, albeit taking into account the additional components that are present in the ‘three-phase interleaved structure’ converter. For the three-phase bridge, the switches could still be controlled with a 50% square wave signal – i.e., the same switching frequency and duty cycle as that described above in relation to Figure 4 – such that the resulting driving signals for Q1 and Q2, Q3and Q4, Q5and Q6are complementary. However, the driving signals for Q3and Q5would need to lag the driving signal of Q1 by 1 / 3Ts and 2 / 3Ts, respectively, where Ts is the switching period of the switches. As for the voltage regulation range, the switch S would still employ the same (pulse-width-modulation) strategy as was used in the topology of Figure 3. Any further modifications that may be required to implement the three-phase structure would be readily derivable by the skilled person based on the modulation strategies that are described above for the single-phase configuration The proposed isolated DC-DC converter described above in relation to Figure 3 has been simulated / tested with the software PLECS. The results of these tests / simulations are presented below and in Figure 8. The following parameters were input: Input voltage Vin = 2kV Output voltage Vo= 0.5~1kV Transmission power Pt = 5MW (@Vo=800V) Switching frequency fs1 = 1000Hz Switching frequency fs2 = 500Hz Transformer ratio of Tr1 - K1:1 = 1:1 Transformer ratio of Tr2- K2:1 = 1:1 Resonant capacitor Cr = 250μF Resonant inductor Lr = 80μH Output filter capacitor Co1 = 200mF Output filter capacitor Co2 = 200mF The results of the simulation are shown in Figure 8. The reference output voltage is set to 800V. According to Figure 8, the output voltage Vomaintains stable at 800V and the voltage VCo1 is 1200V with the duty cycle of S regulated to 0.667. As the output voltage remains stable over the simulation time period, the implementation feasibility of the proposed hybrid-phase topology is verified. In view of the benefits that are achieved by the above-described embodiments, it will be appreciated that the proposed DC-DC converters are able to achieve a high efficiency over a wide operation range of output voltage. They are able to realize the galvanic insulation between the input and output terminals, and actively regulate the output voltage / current / power; whilst still minimizing the amount of power that is lost via switching and conduction 5 losses. The proposed converters of the present invention can be utilised to realise voltage conversion and galvanic insulation between two DC terminals. In particular, the potential applications of the proposed converters include 10 the power supply for water electrolyzers, energy storage system interfaces or battery charging systems (e.g., for electric vehicles). While many possible variations of the hybrid-stage DC-DC converter topology and its implementation have been 15 described above, it will be clear to the skilled person that additional variations and modifications can be made without departing from the scope of the invention as claimed in the appended claims.

Claims

SP3074 - 19 - C L A I M S 1. A hybrid stage DC-DC converter comprising: a power input; a power output; an isolation stage connected to the power input and 5 comprising an AC input port, first and second transformers, and first and second isolation DC output ports; and a voltage regulation stage comprising an input DC port and an output regulation DC port connected to the 10 power output; wherein: the first transformer is configured to receive power from the voltage input via the AC input port and to output power via the first isolation DC output port to the 15 input DC port of the voltage regulation stage; and the second transformer is configured to receive power from the voltage input and to output power via the second isolation DC output port to the power output; and wherein: 20 the voltage regulation stage is configured to regulate incoming power from the isolation stage and to output regulated power to the power output via the output regulation DC port. 25 2. The converter of claim 1, wherein: the isolation stage comprises first and second diode rectifiers, the first diode rectifier is connected to the first isolation DC output port and the second diode rectifier is connected to the second isolation DC output 30 port.

3. The converter of claim 2, wherein the output regulation DC port of the voltage regulation stage is connected to the second isolation DC output port. 5 4. The converter of claim 2 or claim 3, wherein: the first and second diode rectifiers each have a respective AC port; the first transformer has a primary winding connected to the AC input port, and a secondary winding 10 connected to the AC port of the first diode rectifier; the second transformer has a secondary winding connected to the AC port of the second diode rectifier; and the first and second transformers are connected 15 in series with one another.

5. The converter of any preceding claim, wherein: the isolation stage comprises a first switching arrangement operated at a first switching frequency; 20 the voltage regulation stage comprises a second switching arrangement operated at a second switching frequency; and the first and second switching frequencies are different. 25 6. The converter of claim 5, wherein the second switching frequency is lower than first switching frequency 30 7. The converter of claim 5 or claim 6, wherein the first switching frequency is fixed whilst second switching frequency is variable.

8. The converter of any of claims 2 to 7, wherein each of the transformers is a three-phase transformer and each of the diode rectifiers is a three-phase diode rectifier. 5 9. The converter of claim 8, wherein the isolation stage comprises first and second filter inductors, the first filter inductor being connected to the first diode rectifier and the second filter inductor being connected to the second diode rectifier. 10

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