GB2547448A

GB2547448A - Converter apparatus

Info

Publication number: GB2547448A
Application number: GB1602846.6A
Authority: GB
Inventors: Whitehouse Robert; Taghizadeh Esfanjani Hassan
Original assignee: General Electric Technology GmbH
Current assignee: General Electric Technology GmbH
Priority date: 2016-02-18
Filing date: 2016-02-18
Publication date: 2017-08-23
Also published as: GB201602846D0

Abstract

A converter, particularly a DC-DC converter 200, comprises first 210a1, 210a2 and second 210b1, 210b2 pairs of energy storage devices (e.g. capacitors, batteries) and a plurality of switches 206. The converter has first and second commutation states. In the first state the first storage device of the second pair 210b1 is connected in series to a first terminal 202 and in parallel to the second storage device of the second pair 210b2; the first storage device of the first pair 210a1 is connected in parallel to the first terminal; and a second terminal 204 is connected to ground via the second storage device of the first pair 210a2 in series with the first terminal. In the second commutation state the first storage device of the first pair is connected in series to the first terminal, and in parallel to the second storage device of the first pair; the first storage device of the second pair is connected in parallel to the first terminals; and the second terminal is connected to ground via the second storage device of the second pair in series with the first terminal. A number of converters could form units of a multistage converter.

Description

CONVERTER APPARATUS

FIELD OF THE INVENTION

The invention relates to converter apparatus, and in particular but not exclusively to Direct Current to Direct Current (DC- DC) converter apparatus.

BACKGROUND OF THE INVENTION

There are various known designs of DC-DC converters. One class, known as Switched Capacitor (SC) DC-DC converters, may have one of a number of topologies, for example, ladder topology, or a “Dixon charge pump” topology. Multistage DC-DC converter topologies are known in which each subsequent stage in the converter design increases (usually doubles) the voltage. This also generally requires that the voltage rating for switches increases in each stage. In such designs, faulty components (or stages) cannot be easily bypassed to allow the converter to continue normal operation, and the conversion ratio is usually fixed.

Some designs, for example as described in F. H. Khan and L. M. Tolbert, "A Multilevel Modular Capacitor-Clamped DC-DC Converter," Industry Applications, IEEE Transactions on, vol. 43, pp. 1628-1638, 2007, have modular configurations and better scalability. Such designs may be termed Multilevel Modular Switched Capacitor Converters (MMSCCs).

In one example of an MMSCC, shown in Figure 1, a number of similar modules 102, each providing a stage or level of the MMSCC, are provided. Each module102 comprises (labelled in relation to one module 102 only) a capacitor 104, three switches 106a-cand three terminals: a high voltage terminal HV, a low voltage terminal LV and a ground terminal GRD. Each module 102 may be provided as an integrated, replaceable, component. Additional, or redundant, modules 102 can be provided so that faulty modules 102 can be bypassed to allow the converter to continue operation. In each module 102, switches 106a and 106b have the same voltage rating whilst switch 106c has double the voltage rating of switch 106a. However, each module 102 requires a capacitor 104 having a different voltage rating. The first module 102 (i.e. the module closest to the low voltage side) requires a capacitor 104 rated at the input voltage, the second module 102 requires a capacitor 104 with twice that rating, the third module 102 requires a capacitor 104 with three times that rating and so on.

Therefore, in particular, it may be the case that different devices or designs are required for each of the stages/modules 102, and for the switches 106 within a module 102, increasing the complexity of design. As will be familiar to the skilled person, in some examples, a switch may comprise a plurality of switching elements (usually semiconductor switching elements such as Integrated Gate Bipolar Transistors (IGBTs), metal-oxide-semiconductor field-effect transistor (MOSFETs), Thyristors or the like) which are connected in series to provide a switch with a higher voltage rating than the individual switching elements. While such a series connection allows reuse of a single switching element type within a design, the switching elements may be controlled to operate substantially simultaneously to ensure that the voltage supported is shared over the series connection, and not by the switching element, or subset of switching elements, which switches first and that voltages remain balanced. This can add to the complexity of controlling such circuitry and/or to the complexity of circuit design. In addition, providing a large number of switching elements in series can result in increased switching losses. In some examples, diodes may be connected antiparallel with the switches.

Furthermore the capacitor 104 in each module 102 of the MMSCC will be subjected to a different voltage and therefore may have a different voltage rating depending upon its position within the structure. This means that, to allow operation with faults within the converter, a redundant module 102 rated for the highest anticipated voltage may be employed regardless of where the failed module 102 is located.

It will also be noted that the MMSCC as shown in Figure 1, the output capacitor 104 is only charged during one half cycle which leads to a large ripple on the output voltage.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a direct-current to direct-current (DC-DC) converter unit for converting an input voltage to an output voltage, comprising a controller, a first and second pair of energy storage devices and a plurality of switches, wherein each pair of energy storage devices comprises a first and a second energy storage device; the switches being configured to selectively connect the energy storage devices with a low voltage terminal and a high voltage terminal; and the controller being arranged to control the switches such that: in a first commutation state of the converter unit, the first energy storage device of the second pair of energy storage devices is connected in series to the low voltage terminal and in parallel to the second energy storage device of the second pair of energy storage devices, the first energy storage device of the first pair of energy storage devices is connected in parallel to the low voltage terminal, and the high voltage terminal is connected to ground via the second energy storage device of the first pair of energy storage devices in series with the low voltage terminal; and in a second commutation state of the converter unit, the first energy storage device of the first pair of energy storage devices is connected in series to the low voltage terminal and in parallel to the second energy storage device of the first pair of energy storage devices, the first energy storage device of the second pair of energy storage devices is connected in parallel to the low voltage terminal, and the high voltage terminal is connected to ground via the second energy storage device of the second pair of energy storage devices in series with the low voltage terminal.

In such a converter unit, there are two charging paths between the high and low voltage terminals of the converter which may reduce the voltage ripple at either or both terminals. The energy storage devices may comprise one or more capacitors, batteries, or other energy storage devices.

As will be familiar to the skilled person, in relation to the terminals of a converter, the terms ‘high’ and ‘low’ have a relative, rather than absolute, meaning. The terminals could alternatively be referred to as ‘higher’ and ‘lower’ voltage terminals, for example, with the high voltage terminal operating at a higher voltage than the low voltage terminal.

The converter unit may be a step up converter unit, in which case, in one commutation state, a first energy storage device of a pair is arranged to receive charge from the low voltage terminal and to transfer charge to the second energy storage device of the pair, and in another commutation state the second energy storage device of the pair is arranged to receive charge from the low voltage terminal and the first energy storage device of the pair, and to output charge to the high voltage terminal.

In other examples, the converter unit may be a step down converter unit, in which case, in one commutation state, a second energy storage device may be arranged to receive charge from the high voltage terminal and to transfer charge to the low voltage terminal and the first energy storage device of the same pair, and, in the other commutation state, the first energy storage device of the pair is arranged to receive charge from the second energy storage device of the pair and to transfer charge to the low voltage terminal.

In some examples, each connection between at least one energy storage device and a terminal comprises a number of switches which is proportional to the number of energy storage devices connected in series (or, for example, proportional to the capacitance thereof if the energy storage devices comprise capacitors) with the terminal. In such examples, the switches may have a common voltage rating. This is advantageous as it reduces complexity in manufacturing and repair, as the same type of switch may be used throughout the converter unit. Indeed, in some examples, all the switches are of substantially similar, or the same, current and/or voltage ratings. In some examples, each switch may comprise more than one switching element, for example connected in series and/or in parallel, to provide the desired switching capabilities.

In some examples, the switches are arranged in submodules, each submodule comprising (i) four switches in an H-bridge configuration, the controller being arranged to control two of the switches to be closed in the first commutation state and open in the second commutation state, and to control the other two switches to be open in the first commutation state and closed in the second commutation state or (ii) a pair of switches, and the controller is arranged to control one of the switches of the pair to be closed in the first commutation state and open in the second commutation state, and to control the other switch of the pair to be open in the first commutation state and closed in the second commutation state.

In such a converter unit, each submodule has the same number of switches in a given commutation state at a given time (i.e. the number of switches in a submodule which are in a given switching state in a given commutation state may be the same for all submodules), and therefore the submodules may have the same voltage and current rating.

In one example, at least two parallel submodules interconnect the low voltage terminal and the energy storage devices, and at least two series connected submodules interconnect the high voltage terminal and the energy storage devices. This is a convenient layout which allows the voltage and current rating of the submodules to be the same.

In some examples, the submodules may be provided as an integrated (e.g. preformed, or prefabricated) component. As such, when a switch fails, the submodule may be replaced. In examples where the rating of each submodule is the same, any failed submodule may be replaced with a single spare submodule: there is no need to provide different spare parts for different failed submodules. Such an integrated component may exclude some or all passive components, such as capacitors for example providing energy storage devices. Passive components may generally have a lower failure rate than the power electronic components and their associated control equipment. Capacitors are examples of relatively high voltage passive components. As they are relatively less likely to fail than switches and the like, and are normally relatively large (and often relatively expensive), excluding capacitors from the integrated component means that the component may be smaller, less expensive and more readily transportable and the like. In addition, in a multistage converter such as is described in greater detail below, the capacitors of each stage may vary between stages, thus this component is not interchangeable in the same way as submodules as described above.

According to a second aspect of the invention, there is provided a multistage converter comprising at least two converter units according to the first aspect of the invention wherein the high voltage terminal of one converter unit is connected to the low voltage terminal of a second converter unit.

In such a multistage converter, the energy storage device of two adjacent stages (units) transfer charge from one to another and provide a DC voltage difference between the two stages.

In examples where the converter units comprise switches arranged in submodules, the submodules of each stage (i.e. converter unit) may have the same current and voltage rating and therefore complexity in manufacturing and repair is reduced.

According to a third aspect of the invention, there is provided a method of DC-DC conversion comprising: providing a first and second pair of energy storage devices, each pair comprising a first energy storage device and a second energy storage device; controlling a plurality of switches into a first commutation state in which: the first energy storage device of the second pair is connected in series to a first voltage terminal, and in parallel to the second energy storage device of the second pair; the first energy storage device of the first pair is connected in parallel to the first voltage terminal; and a second voltage terminal is connected to ground via the second energy storage device of the first pair in series with the first voltage terminal; reconfiguring the plurality of switches into a second commutation state in which: the first energy storage device of the first pair is connected in series to the first voltage terminal, and in parallel to the second energy storage device of the first pair; the first energy storage device of the second pair is connected in parallel to the first voltage terminals; and the second voltage terminal is connected to ground via the second energy storage device of the second pair in series with the first voltage terminal.

The method may be arranged to step up a voltage, or to step down a voltage.

The method may further comprise reconfiguring the plurality of switches repeatedly between the first and second commutation states to provide voltage conversion over a period of time.

Features described in relation to one aspect of the invention may be combined with those of another aspect of the invention. In particular, the converter unit or multistage converter may be arranged to carry out at least some steps of the method of the third aspect of the invention.

The method, converter unit or multistage converter may be arranged for operation in a relatively high voltage environment (for example, in the kilovolt range and above).

The invention further comprises methods of use of the converter unit or multistage converter described above.

Embodiments of the invention are now described, by way of example only, with reference to the following Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a prior art example of a Multilevel Modular Switched Capacitor Converter;

Figures 2-5 show an example of a switched capacitor converter unit according to an embodiment of the invention;

Figure 6 shows a multistage converter according to an embodiment of the invention; Figure 7 shows another example of a switched capacitor converter unit according to an embodiment of the invention; and Figure 8 and 9 show examples of bi-pole converters.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 2 shows a switched capacitor DC-DC converter unit 200 comprising a low voltage terminal 202 and a high voltage terminal 204.

The converter unit 200 further comprises a plurality of switches 206, which comprise switches 206a of a first switching group and switches 206b of a second switching group. The switches 206 are arranged in submodules 208, each submodule 208 comprising four switches 206, two from each group. Although for simplicity the switches 206 are illustrated as solid state switches, in an example, they may each comprise one or a plurality of transistors, such as Insulated Gate Bipolar Transistors (IGBTs) or any other semiconductor switch, or indeed any other suitable switching device.

The switches 206 of a submodule 208 are arranged in an H - bridge arrangement. In this example, each submodule 208 may comprise a preformed, i.e. integrated, component. In other examples, a pair of switches 206, comprising one of each group, could instead be provided as a preformed, i.e. integrated, component. In such examples, two such components may form a submodule.

The converter unit 200 further comprises four energy storage devices, which in this example comprise capacitors 210, comprising two capacitors 210a1 and 210a2 of a first pair and two capacitors 210b1,210b2 of a second pair. Each pair comprises a first capacitor 210a1,210b1, and a second capacitor 210a2, 210b2. It will be noted that the capacitors 210, which may be relatively high voltage components, are outside the submodules 208. Capacitors 210b1 and210a1 have the same voltage rating. Capacitors 210b2 and210a2 also have the same voltage rating. However the capacitors 210 within a pair (for example capacitors 210a2 and 210a1) do not have the same voltage rating. Capacitor 210a2 will have a voltage rating “one stage” greater than that of capacitor 210a1. This means that capacitors at the same position in the converter unit 200 and which may be connected to the same submodule 108 have the same voltage rating as each other. Each capacitor 210 may comprise one or more capacitive components.

Two submodules 208 are arranged in parallel at the low voltage terminal 202, and a further two submodules 208 are connected in series between the low and high voltage terminals of the converter unit 200. The series connected submodules 208 are connected to the parallel connected submodules 208 via the capacitors 210, which clamp the voltages between the submodules 208, removing the voltage stress between the submodules 208

An example is now described in which the converter unit 200 is transferring power from a low voltage to a high voltage terminal but the skilled person will understand that the power could flow in reverse. The arrangement is such that, in use of the converter unit 200, all switches 206 are subject to substantially the same voltage and current.

Figure 3 shows charging paths 302, 304 in a first commutation state in which the switches 206b of the second group are open, while the switches 206a of the first group are closed. This commutation state connects the second capacitor 210a2 of the first pair to the high voltage terminal 204, such that it can discharge thereto (see dashed line marking path 302). The first capacitor 210a1 of the first pair is connected to the low voltage terminal 202. Both of paths 302 and 304 comprise one capacitor 210 and two switches 206a, with each switch 206a being in a different submodule 208.

Figure 4 shows a charging path 402 in a second commutation state in which the two capacitors 210a1,210a2 of the first pair are connected with the low voltage terminal 202. The switches 206b of the second group are closed, while the switches 206a of the first group are open. This allows the capacitor 210a 1 to discharge, and 210a2 to charge via the path indicated with the dashed line 402. The polarity of the capacitors 210a1 and 201a2 will be opposite. In particular, the second capacitor 210a2 of the first pair charges from the series connection of the input voltage and from the connected first capacitor 210a1. It will be noted that path 402 is via two capacitors 210 and four switches 206b, each of the switches 206 being in a different submodule 208.

From a simple visual inspection of Figure 4, it may appear that the capacitors 210a1 and 210a2 and the low voltage supply are all connected in series. However, from an electrical standpoint, the summation of the voltages between the series connection of the low voltages supply and the capacitor 210a1 are opposed to (and hence in parallel with) the other capacitor 210a2. Similarly, the capacitor 210a1 is in parallel to the low voltage terminal 202 when the switches 206a of the first group are closed.

Although the charging paths are not shown on the Figures to avoid over complication thereof, in the second commutation state, the second pair of capacitors 210b is connected in a manner similar to the first pair of capacitors 210a in the first commutation state. In particular, the second capacitor 210b2 of the second pair is connected to the high voltage terminal 204, and can discharge thereto. The first capacitor 210b1 of the second pair is connected to the low voltage terminal 202 with a first polarity. Similarly, in the first commutation state, the second pair of capacitors 210b is connected in a manner similar to the first pair of capacitors 210a in the second commutation state, the second capacitor 210b2 of the second pair charging from the connection of the input voltage and from the connected first capacitor 210b1.

To consider power transfer from the low voltage terminal 202 to the high voltage terminal 404, the first capacitors (210a1, 210b1) cycle between charging from the low voltage terminal and discharging into the second capacitor (210a2, 210b2) of the same pair. The second capacitors (210a2, 210b2) cycle between being charged from a first capacitor (210a1,210b1) and discharging to the high voltage terminal (which therefore provides a load). The two pairs of capacitors operate in antiphase. Each of the first capacitors 210a1, 210b1 is connected to the low voltage terminal 202 with one polarity in the first commutation state and with a reversed polarity compared to the first commutation state in the second commutation state.

It will be noted that each group of switches 206 operates in anti-phase, with one group being set to open when the other group is set to close, and this cycle may continue repeatedly. Operation of all submodules 208 within the converter unit 200 is preferably well synchronised so that there is no short circuiting of the interconnecting capacitors 210.

It will also be noted that there are two charging paths between the low voltage and high voltage terminals, which may provide substantially continuous charge transfer, and may reduce the voltage ripple at either or both terminal. The two pairs of capacitors 210 and groups of switches 206 operate in the same manner but in different halves of a charge transfer cycle.

Each charging path 302, 304, 402 comprises a number of switches 206 which is proportional to the number of connected capacitors 210 (i.e. in this example, four switches 206 when there are two capacitors 210 and two switches 206 when there is a single capacitor 210). This allows the voltage rating of all the switches 206 to be the same. Moreover, the switches 206 in a given state are evenly distributed throughout the submodules 208, such that the same number of switches 206 is employed in both halves of the cycle, and half of the switches 206 in any one submodule 208 are closed and half are open.

This therefore allows the voltage and current rating of each submodule 208 to be the same. The current rating of each submodule 208 may be determined to be equivalent to the high voltage/low current terminal of the converter unit 200. The power in/out at the low voltage terminal is at a higher current via the parallel connected modules with charge transfer being, in this example, via the capacitors 210 which also provide the DC voltage difference between the submodules 208 and the terminals.

It will be noted that a common voltage and current rating would also be seen if a submodule 208 was defined as comprising just two switches 206, one from each group. For example, such a submodule could comprise the two capacitors on the left or right side of an H-bridge submodule, or the two switches 206 forming the top half or the bottom half of an H-bridge submodule. In such an example, eight submodules each comprising two switches 206 could be provided and the submodules the voltage and current rating of each submodule to be the same.

Figure 5a-c show an alternative representation of the converter unit 200 shown in Figures 2-4

In Figure 5b, switches 206b are closed and switches 206a are open. The high voltage terminal 204 is connected to ground via the series connection of the second capacitor 210b2 of the second pair and low voltage terminal 202. The low voltage terminal 202 is connected to the capacitors 210a1, 210a2 of the first pair, and the first capacitor 210a1 of the first pair (which is charged in the other portion of the cycle) discharges into the second capacitor 210a2 of the first pair. The first capacitor 201 b1 of the second pair is charged from low voltage terminal 202.

In Figure 5c, the switches 206a are closed and switches 206b are open. The high voltage terminal 204 is connected to ground via the series connection of the second capacitor 210a2 of the first pair in series with the low voltage terminal 202. The low voltage terminal 202 is connected to the capacitors 210b1, 210b2 of the second pair, and the first capacitor 210b1 of the second pair discharges into the second capacitor 210b2 of the second pair. The first capacitor 201a1 of the first pair is charged from low voltage terminal 202.

Note that, in both states, the low voltage terminal current is split between a first and a second capacitor of a group. As is shown in Figure 5a, two ‘sub-units’ 502a and 502b are formed within the converter unit 200, the first 502a comprising two submodules 208 and the first capacitors 210a 1,210b 1, and the second 502b comprising two submodules 208 and the second capacitors 210a2, 210b2.

As the skilled person will appreciate, each sub-unit 502 of Figure 5 could be a stage of a multistage converter 600 as shown in Figure 6. Each sub-unit 502 receives a voltage from a source such as a battery 602, or from a preceding sub-unit 502 of the converter 600 and outputs voltage to a further stage or to a load 604 as required. The voltages and current across each switch 206, and within each switching module will remain consistent throughout the stages.

In a multistage converter 600, capacitors 210 in each voltage stage are charged by previous capacitors 210 in series with the input source in one commutation state and may discharge to the next stage capacitor 210 in connection with input source in the other commutation state. The DC voltage difference between the stages of a multistage converter 600 is provided by the difference voltage between the two converter unit capacitors 210.

The provision of multiple stages allows for a high ratio between input and output voltages.

The converter unit 200 shown in Figures 2-5 has the voltage conversion ratio 1:3. A complementary circuit for the circuit shown in Figure 5a can be developed as shown in Figure 7 which has the similar operation principal and gives the same conversion ratio but in opposite direction -1:-3. The complementary converter units 100, 700 can be connected together to create a bi-pole converter 800 as shown in Figure 8. A second configuration of a bi-pole converter 900 is shown in Figure 9. In Figure 9, the parallel connected modules of two circuits are connected. This may reduce the number of stages required to construct a converter of a particular conversion ratio and may reduce the capacitance requirement of a converter with a given conversion ratio by a factor of about four. Furthermore, the two complementary circuits of the converter 900 shown in Figure 9 can be controlled with a mutual phase shift to reduce the voltage ripple in both input and the output terminals.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Features from one embodiment may be combined with features from another embodiment.

The invention has been described with respect to various embodiments. Unless expressly stated otherwise the various features described may be combined together and features from one embodiment may be employed in other embodiments.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference numerals or labels in the claims shall not be construed so as to limit their scope.

Claims

1. A direct-current to direct-current (DC-DC) converter unit for converting an input voltage to an output voltage, comprising a controller, a first and second pair of energy storage devices and a plurality of switches, wherein each pair of energy storage devices comprises a first and a second energy storage device; the switches are arranged to selectively connect the energy storage devices with a low voltage terminal and a high voltage terminal; and the controller is arranged to control the switches such that, in a first commutation state of the converter unit, the first energy storage device of the second pair of energy storage devices is connected in series to the low voltage terminal and in parallel to the second energy storage device of the second pair of energy storage devices, the first energy storage device of the first pair of energy storage devices is connected in parallel to the low voltage terminal, and the high voltage terminal is connected to ground via the second energy storage device of the first pair of energy storage devices in series with the low voltage terminal; and in a second commutation state of the converter unit, the first energy storage device of the first pair of energy storage devices is connected in series to the low voltage terminal and in parallel to the second energy storage device of the first pair of energy storage devices, the first energy storage device of the second pair of energy storage devices is connected in parallel to the low voltage terminal, and the high voltage terminal is connected to ground via the second energy storage device of the second pair of energy storage devices in series with the low voltage terminal.

2. A DC-DC converter unit according to claim 1 in which each connection between at least one energy storage device and a terminal comprises a number of switches which is proportional to the number of energy storage devices connected with the terminal.

3. A DC-DC converter unit according to any preceding claim in which all the switches are of substantially similar current and voltage ratings.

4. A DC-DC converter unit according to any preceding claim in which the switches are provided as submodules comprising four switches in an H-bridge configuration, the controller being arranged to control two of the switches of the pair to be closed in the first commutation state and open in the second commutation state, and to control the other two switches to be open in the first commutation state and closed in the second commutation state.

5. A DC-DC converter unit according to any of claims 1 to 3 in which the switches are provided as submodules comprising a pair of switches, in which the controller is arranged to control one of the switches of the pair to be closed in the first commutation state and open in the second commutation state, and to control the other switch of the pair to be open in the first commutation state and closed in the second commutation state.

6. A DC-DC converter unit according to claim 4 or claim 5 in which at least two parallel submodules interconnect the low voltage terminal and the energy storage devices, and at least two series connected submodules interconnect the high voltage terminal and the energy storage devices.

7. A DC-DC converter unit according to any of claims 4 to 6 in which at least one of the submodules is provided as an integrated component.

8. A DC-DC converter unit according to any of claims 4 to 7 in which, in use of the converter unit, the number of switches in a submodule which are in a given switching state in a given commutation state is the same for all submodules.

9. A DC-DC converter unit according to any preceding claim in which the switches comprise a first group of switches and a second group of switches, wherein, in use of the converter unit, the controller is arranged to control the switches of one group to be open when the switches of the other group are closed, and the first and second group comprise the same number of switches.

10. A DC-DC converter unit according to any preceding claim in which at least one energy storage device comprises one or more capacitors.

11. A multistage converter comprising at least two converter units according to any preceding claim, wherein the high voltage terminal of one converter unit is connected to the low voltage terminal of a second converter unit.

12. A multistage DC-DC converter according to claim 10 as it depends on any of claims 4 to 8 in which the submodules of each converter unit have the same current and voltage rating.

13. A method of DC-DC conversion comprising: providing a first and second pair of energy storage devices, each pair comprising a first energy storage device and a second energy storage device; controlling a plurality of switches into a first commutation state in which: the first energy storage device of the second pair is connected in series to a first voltage terminal, and in parallel to the second energy storage device of the second pair; the first energy storage device of the first pair is connected in parallel to the first voltage terminals; and a second voltage terminal is connected to ground via the second energy storage device of the first pair in series with the first voltage terminal; reconfiguring the plurality of switches into a second commutation state in which: the first energy storage device of the first pair is connected in series to the first voltage terminal, and in parallel to the second energy storage device of the first pair; the first energy storage device of the second pair is connected in parallel to the first voltage terminals; and the second voltage terminal is connected to ground via the second energy storage device of the second pair in series with the first voltage terminal.

14. A method of DC-DC conversion according to claim 13 comprising a method of stepping up a voltage, wherein the first energy storage device of a pair receives charge from a first voltage terminal in one commutation state and transfers charge to the second energy storage device of a pair in another commutation state, and the second energy storage device of a pair receives charge from a first voltage terminal and the first energy storage device of the pair in one commutation state, and outputs charge to a second voltage terminal in another commutation state.

15. A method of DC-DC conversion according to claim 13 comprising a method of stepping down a voltage, wherein the second energy storage device receives charge from a first voltage terminal in one commutation state and shares charge with the first energy storage device in another commutation state, and the first energy storage device of the pair receives from the second energy storage device in one commutation state, and outputs charge to a second voltage terminal in another commutation state.

16. A method of DC-DC conversion according to any of claims 13 to 16 further comprising substantially continuously transferring charge between the first and second terminals.

17. A method of DC-DC conversion according to any of claims 13 to 16 further comprising reconfiguring the plurality of switches repeatedly between the first and second commutation states.

18. A bi-pole DC-DC converter comprising a first and a second converter unit according to any of claims 1 to 10.

19. A bi-pole DC-DC converter according to claim 18 in which the first and second converter units comprise a common low voltage input.

20. A DC-DC converter unit substantially as described herein with reference to Figures 2 to 5.

21. A multistage converter substantially as described herein with reference to Figure 6.

22. A bi-pole DC-DC converter substantially as described herein with reference to Figure 7, 8 or 9.