Power controller

Abstract

Power Controller The present invention relates to a method of controlling a DC-DC buck-boost converter, a method of controlling power transfer, and a method of transitioning between a step-up and step-down mode, as well as a control module, a converter system and a charging device. The DC-DC buck-boost converter comprises a first half-bridge having a high-side first switch controllable by a first PWM signal and a second half-bridge having a low-side second switch controllable by a second PWM signal. When the voltage demand requires the converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; when the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, and when the voltage demand requires the DC- DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%.

Description

[0001] Power Controller Related Applications The present case claims priority to, and the benefit of, GB 2317488.1 filed 15 November 2023 (15.11.2023), the contents of which are incorporated by reference in their entirety. Field of the Invention The present invention relates to a method of controlling a DC-DC buck-boost converter system, a method of controlling power transfer, and a method of transitioning a DC-DC buck- boost converter between a step-up and step-down mode, as well as a control module for a buck-boost converter, a buck-boost converter system including the control module and a charging device including the buck-boost converter system. Background Where electrical power flows from one DC source (e.g. a first battery) to another DC source (e.g., a second battery), there is a difference in voltage between the DC sources. Where the voltages of the DC sources change over time, as in the case of batteries, there is a need to modulate the relative voltage of the DC sources. This allows for the power transfer between the DC sources to be closely controlled. One common arrangement for such a power electronic regulator is a so-called “buck-boost” converter. A buck-boost converter typically comprises a first half bridge with a first high-side switch and a second half bridge with a second low-side switch. The first half bridge and second half- bridge are electrically connected by an electrical buffer, such as an inductor. The high-side and low-side switches are used to buck (step-down) or boost (step-up) the voltage across the buck-boost converter. An example of such a buck-boost converter circuit is shown in Figure 1A and 1B, showing the high-side switch A for stepping down voltage and the low-side switch B̅ for stepping up voltage across the converter. In addition, the first half bridge may include a synchronous low-side switch Ā, and the second half bridge may include a synchronous high-side switch B, which switch inversely to the corresponding low and high side switches A and B.̅ The relative on-time of the high-side switch (A) determines the degree to which the voltage is stepped down. The relative on-time of the low-side switch (B)̅ determines the degree to which the voltage is stepped up. In traditional buck-boost converter systems, transitioning between a buck-mode and a boost- mode can often be difficult to control. This transition region is particularly important for battery-to-battery charging. As the donor battery discharges, its voltage decreases, and as the receiver battery charges, its voltage increases. Therefore, during the charging process, the voltage of the donor battery first needs to be stepped-down and then needs to be 008698920 stepped-up. So, it is desirable for the buck-boost converter to smoothly transition between the buck mode and the boost mode during battery-to-battery charging. Traditional systems typically control both the high-side switch (A) and the low-side switch (B)̅ simultaneously, to control the voltage during the transition. For example, during a transition from a buck-mode to a boost-mode, the on-time of the high-side switch (A) is gradually increased while the on-time of the low-side switch (B)̅ is gradually decreased simultaneously. When the converter is neither stepping up nor stepping down the voltage, the high-side switch (A) is on (i.e., conducting) half of the time and the low-side switch (B)̅ is on half of the time – so the step down provided by side A and the step up provided by side B cancel each other out to provide a net 0 voltage over the electrical buffer. However, this dual switching is complex to control. The dual switching also results in a loss in efficiency in the circuit. There is an energetic cost to operate each of the switches in the buck-boost converter, and so operating all switches simultaneously requires more energy. Moreover, the switching itself has an energetic cost to the power transferred through the buck-boost converter. The energetic cost to the power transfer is greater at high current flows required in fast-charging applications. For example, high power MOSFETs needed for fast-charging applications typically require more power to switch, and the loss in efficiency of the power transferred is more pronounced at higher currents. Accordingly, operating the traditional buck-boost converters in this transition region is inefficient. There is a need for buck-boost converter control methods and systems which address these issues. Summary of the Invention At its most general, the present invention provides a method of controlling a DC-DC buck- boost converter wherein only one of a high-side first switch and a low-side second switch of the buck-boost converter are switched at a time. In particular, the high-side first switch is only switched when the low-side second switch is off, and the low-side second switch is only switched when the high-side first switch is on. In other words, the high-side first switch and the low-side second switch are not switched simultaneously. Accordingly, when the DC-DC buck-boost converter steps-down voltage, the high-side first switch is switching and the low-side second switch is off; when the DC-DC buck-boost converter is maintaining voltage, the high-side first switch is on and the low-side second switch is off; and when the DC-DC buck-boost converter steps-up voltage, the high-side first switch is on and the low-side second switch is switching. In the context of the present invention, a switch being “on” may be considered to be a conducting, on-state of the switch wherein the switch is configured to conduct current between a first side of the switch and a second side of the switch. For example, when the 008698920 switch is a p-type Metal Oxide Field Effect Transistor (MOSFET) the switch may be considered as “on” when a gate-source voltage of the MOSFET is higher than a gate threshold causing the MOSEFT to operate in its saturation region and enable current to flow between the drain and source of the MOSFET. A switch being “on” may be defined by a duty cycle of the switch - wherein when the switch is “on” the duty cycle is 100%. In contrast, a switch being “off” may be considered as a non-conducting, off-state of the switch wherein current cannot flow from one side of the switch to the other. A switch being “off” may be defined by a duty cycle of the switch - wherein when the switch is “off” the duty cycle is 0%. When a switch is “switching” this may be considered as a switching state of the switch wherein the switch is controlled to regularly transition between its off state and its on state. A percentage “on” time of the switch and percentage “off” time of the switch may be defined by a duty cycle of the switch, wherein when the switch is “switching” the duty cycle is between 0% and 100% (and not equal to 0% or 100%). The rate at which the switch transitions between the on state and the off state, when the duty cycle is between 0% and 100%, may be determined by a switching frequency. In general, the present invention provides a method of controlling a DC-DC buck-boost converter system, the DC-DC buck-boost converter system comprising: a DC-DC buck-boost converter comprising a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges, wherein when the DC-DC buck-boost converter steps-down voltage, the high-side switch is switching and the low side switch is off; when the DC-DC buck-boost converter is maintaining voltage, the high-side switch is on and the low side switch is off, and when the DC-DC buck-boost converter steps-up voltage, the high-side switch is on and the low side switch is switching. Specifically, in the transition region between stepping up and stepping down the voltage of the first source relative to the second source, the high-side first switch and a low-side second switch of the buck-boost converter are only switched one at a time. In the transition region, the high-side first switch is only switched when the low-side second switch is off, and the low- side second switch is only switched when the high-side first switch is on. This invention eliminates overlaps in switching of the first half-bridge and second half-bridge. The reduction in switching results in a more efficient buck-boost converter. The energetic 008698920 cost of switching the switches is reduced, as the rate of switching in the whole buck-boost converter system is reduced. The reduction in efficiency in the power transfer through the buck-boost converter is also minimised, as the power transfer is now interrupted by a single switching point, rather than two switching points. In other words, the voltage is only stepped-down or stepped-up once (by switching of one of the first high-side switch or the second low-side switch) in the converter to achieve the desired voltage difference – rather than being both stepped-down and stepped-up (by switching of both the first high-side switch and the second low-side switch) to achieve the desired voltage difference. Operation of the buck-boost converter system in accordance with methods of the invention is more efficient than known systems, in particular in the transition region between a buck-mode and boost-mode. The invention is excellent for transferring power at high currents and high- rates, such as in fast-charging applications, where the efficiency gains from reductions in switching rates are larger than low-power systems. The invention is particularly suited for battery-to-battery charging, such as in power transfers between electrical vehicle (EV) batteries. As a donor battery (e.g., donor EV battery) discharges, its voltage decreases, and as the receiver battery (e.g., receiver EV battery) charges its voltage increases. So, the buck-boost converter needs to smoothly transition between a buck mode and a boost mode during battery-to-battery charging. The switching in the invention is preferably achieved by controlling the duty cycle of each switch. The duty cycle of each switch is determined by a pulse-width modulation (PWM) signal. Where the PWM signal has a duty cycle of 0% the switch is always off. Where the PWM signal has a duty cycle of 100% the switch is always on. Where the PWM signal has a duty cycle of between 0% and 100% (e.g., 50%) the switch is on for a percentage of the time equivalent to the duty cycle (e.g., 50% of the time). In a first aspect of the invention there is provided a method of controlling a DC-DC buck-boost converter system, the DC-DC buck-boost converter system comprising: a DC-DC buck-boost converter comprising a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first pulse-width modulation (PWM) signal, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges; and a control module for controlling the DC-DC buck-boost converter, the control module comprising a PWM signal generator; the method comprising: generating the first PWM signal and the second PWM signal with the signal generator 008698920 based on a voltage demand, wherein the voltage demand determines if the DC-DC buck- boost converter steps-up, steps-down, or maintains the voltage of the first DC source relative to the second DC source, wherein: when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; when the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, and when the voltage demand requires the DC-DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. A voltage demand signal is used to determine if the buck-boost converter is required to step- up, step-down, or maintain the voltage of the first DC source relative to the second DC source. A first PWM signal and a second PWM signal is then generated based on the voltage demand. In this way, the operation of the buck-boost converter is controlled by modulating the duty cycle of the first PWM signal and the second PWM signal. In a second aspect of the invention there is provided a method of controlling power transfer between a first DC source and a second DC source using a DC-DC buck-boost converter system as described in the first aspect; the method comprising: transferring power between the first DC source and the second DC source using the DC-DC buck-boost converter, and controlling the rate of power transfer using the control module, by controlling the relative voltage of the first DC-source compared to the second DC-source, using a method according to the first aspect. In a third aspect of the invention there is provided a method of transitioning a DC-DC buck- boost converter between a step-up mode and a step-down mode using a DC-DC buck-boost converter system as described the first aspect; the method comprising: controlling the DC-DC buck-boost converter according to the method of the first aspect, and transitioning from a step-down mode to a step-up mode, wherein the first PWM signal duty cycle is increased from less than 100% to 100% while the second PWM signal maintains a duty cycle of 0%, and subsequently the second PWM signal duty cycle is increased from 0% to greater than 0% while the first PWM signal maintains a duty cycle of 100%; or transitioning from a step-up mode to a step-down mode, wherein the second PWM signal duty cycle is decreased from greater than 0% to 0% while the first PWM signal maintains a duty cycle of 100%, and subsequently the first PWM signal duty cycle is decreased from 100% to less than 100% while the second PWM signal maintains a duty cycle of 0%. 008698920 In a fourth aspect of the invention there is provided a control module for controlling a DC-DC buck-boost converter, the DC-DC buck-boost converter comprising: a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first pulse-width modulation (PWM) signal, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges, the control module comprising: a signal generator, where the signal generator is configured to generate the first PWM signal and the second PWM signal based on a voltage demand, wherein the voltage demand determines if the DC-DC buck-boost converter steps-up, steps-down, or maintains the voltage of the first DC source relative to the second DC source, wherein: when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; when the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, when the voltage demand requires the DC-DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. In a fifth aspect of the invention there is provided a DC-DC buck-boost converter system comprising: the control module of the fourth aspect, and a DC-DC buck-boost converter. In some embodiments the buck-boost converter is a four-switch buck-boost converter wherein: the first half-bridge comprises the high-side first switch and a low-side first synchronous switch configured to switch inversely to the high-side first switch, and the second half-bridge comprises the low-side second switch and a high-side second synchronous switch configured to switch inversely to the low-side second switch. Such embodiments are particularly advantageous, as the power transfer can be more easily controlled to be bidirectional and therefore provides a compact bidirectional system that can, if necessary, be provided as a single component. In a sixth aspect of the invention there is provided a charging device comprising: the DC-DC buck-boost converter system of the fifth aspect, 008698920 a first DC source, wherein the first DC source is in electrical connection with the first half-bridge, and a power connector for releasably connecting the DC-DC buck-boost converter system to a second DC source, wherein the power connector is in electrical connection with the second half-bridge of the DC-DC buck-boost converter system. In some embodiments, the first DC source comprises an electrochemical cell, and preferably the second DC source comprises an electrochemical cell, such as the electrochemical cell of an electric vehicle. Summary of the Figures The present invention is described with reference to the figures listed below. Figure 1A shows an example circuit diagram of a four-switch buck-boost converter. Figure 1B shows an example circuit diagram of a two-switch buck-boost converter. Figure 2 shows an overview block diagram of a source device, a sink device, and a charging device that contains the system of the invention. Figure 3 shows an overview block diagram of the system of the invention. Figure 4 shows a block diagram of the control and input systems of the invention. Figure 5 shows the process of the invention. Figure 6 shows oscilloscope plots of the switching behaviour of a conventional buck-boost converter at different input voltages and a constant output voltage. Figure 7 shows an oscilloscope plot of the switching behaviour of a buck-boost converter controlled according to the invention. Detailed Description of the Invention At its most general, the present invention provides a method of controlling a buck-boost converter, wherein only one of the buck-side and a boost-side of the buck-boost converter are switched at a time. In particular, the buck-side is only switching when the boost-side is not switching, and the boost-side is only switching when buck-side is not switching. This is the case irrespective of the step-up or step-down in voltage. Even when the step-up or step- down in voltage is relatively small (e.g., less than 20%, such as less than 10%, 5%, 3%, 2% or 1% change) only the high-side first switch and a low-side second switch are switched at a time. 008698920 In general, a DC-DC buck-boost converter comprises a buck-side and a boost side. The buck-side and boost-side are electrically connected by a device suitable for buffering electrical energy, such as an inductor. The buck side typically comprises a controllable high- side switch. The boost-side typically comprises a controllable low-side switch. The buck-side and boost-side are preferably interchangeable (e.g., in a bi-directional bust-boost converter), and so the high-side and low-side switches which are controlled may also be interchangeable, depending on the direction of current flow. The buck-side and boost-side can be independently controlled using a PWM signal. The duty cycle of the PWM signal is used to determine the ratio of the on / off time of a switch. A duty cycle of 100% corresponds to a switch being on 100% of the time. A duty cycle of 0% corresponds to a switch being on 0% of the time (i.e., off 100% of the time). A duty cycle of 50% corresponds to a switch being on 50% of the time and off 50% of the time. A duty cycle of less than 100% corresponds to a switch being off at least some of the time (that is, it excludes a duty cycle of 100%). A duty cycle of more than 0% corresponds to a switch being on at least some of the time (that is, it excludes a duty cycle of 0%). In the present invention, the buck-boost converter is only operated in a buck mode or a boost mode across all voltage ranges. When the buck-boost converter steps-down voltage then only the buck-side is switching, when it maintains voltage neither buck-side or boost-sides are switching, and when it steps-up voltage then only the boost-side is switching. More specifically, when the buck-boost converter steps-down voltage then only the buck-side is switching and the boost-side switch is off, when it maintains voltage the buck-side is on and the boost-side is off, and when it steps-up voltage then the boost-side is switching and the buck-side is on. This limitation may be provided using the duty cycle of the PWM signal used to switch the buck-side and the boost-side switches. When the buck-boost converter is to: step-down voltage, then the buck-side has a duty cycle less than 100%, and the boost- side has a duty cycle of 0%; maintain voltage, then the buck-side has a duty cycle of 100% and the boost-side has a duty cycle of 0%, and step-up voltage, then the buck-side has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. Switching of the buck-side acts to step-down the voltage across the buck-boost converter. The buck-side typically comprises a high-side switch. The buck-side may comprise a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first 008698920 switch. The buck-side may further comprise a first half-bridge comprising a low-side first switch. The low-side first switch is synchronous with the high-side first switch, and is to switch inversely to the high-side first switch. In other words, when the low-side first switch is on, the high-side first switch is off. Switching of the boost-side acts to step-up the voltage across the buck-boost converter. The boost-side typically comprises a low-side switch. The boost-side may comprise a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch. The boost-side may further comprise a second half-bridge comprising a high- side second switch. The high-side second switch is synchronous with the low-side second switch, and is configured to switch inversely to the low-side second switch. In other words, when the high-side second switch is on, the low-side second switch is off. In the present invention, only the high-side first switch or the low-side second switch are being switched across all voltage ranges. That is, if the high-side first switch is switching then the low-side second switch is not switching. Alternatively, if the low-side second switch is switching then the high-side first switch is not switching. When the buck-boost converter steps-down voltage then only the high-side first switch is switching, when it maintains voltage neither high-side first switch or low-side second switch are switching, and when it steps-up voltage then only the low-side second switch is switching. Even when the step-up or step- down in voltage is relatively small (e.g., less than 20%, such as less than 10%, 5%, 3%, 2% or 1% change) only one of the high-side first switch and the low-side second switch are switched at a time. More specifically, when the buck-boost converter steps-down voltage then only the high-side first switch is switching and the low-side second switch is off, when it maintains voltage the high-side first switch is on and the low-side second switch is off, and when it steps-up voltage then the low-side second switch is switching and the high-side first switch is on. This limitation may be provided using the duty cycle of the PWM signal used to switch the buck-side and the boost-side switches. The high-side first switch is controllable by a first pulse-width modulation (PWM) signal. The low-side second switch is controllable by a second pulse-width modulation (PWM) signal. When the buck-boost converter is to: step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, and step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. 008698920 Accordingly, in a first aspect of the invention there is provided a method of controlling a DC- DC buck-boost converter system, the DC-DC buck-boost converter system comprising: a DC-DC buck-boost converter comprising a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first pulse-width modulation (PWM) signal, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges; and a control module for controlling the DC-DC buck-boost converter, the control module comprising a PWM signal generator; the method comprising: generating the first PWM signal and the second PWM signal with the signal generator based on a voltage demand, wherein the voltage demand determines if the DC-DC buck- boost converter steps-up, steps-down, or maintains the voltage of the first DC source relative to the second DC source, wherein: when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; when the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, and when the voltage demand requires the DC-DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. Some methods of controlling buck-boost converters are known. For example, Texas Instruments® LM5118 Wide Voltage Range Buck-Boost Controller is an example of a known buck-boost controller which uses pulse width modulation (PWM) to control the duty cycle of a high-side first switch and a low-side second switch. In this controller, at extreme voltage steps (a large step-up or a large step-down) only the high-side first switch (buck side) or the low-side second switch (boost side) are switched. However, when the step-up or step-down are small (i.e., during the transition between buck and boost modes) both the high-side first switch (buck side) and the low-side second switch (boost side) are switched. US 2023 / 0144906 describes a buck-boost converter under PWM control. Figs.6A and 6B of US 2023 / 0144906 show waveforms for the buck-boost converter in buck and boost modes where only one of the phase-legs (the high-side first switch or the low-side second switch) is being switched at a time. In each case the high-side first switch, or the low-side second switch is being switched at a duty cycle of about 50%. The document does not discuss how the duty cycle is adjusted in a transition region between the buck and boost modes, e.g., 008698920 where the duty cycle of the high-side first switch approaches 100%. There is no suggestion that during the transition from a buck mode to a boost mode the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0% (e.g., where neither the high-side first switch or the low-side second switch are being switched). US 2023 / 0144906 also does not describe a control module including a signal generator configured to generate a first PWM signal having a duty cycle of 100% and a second PWM signal having a duty cycle of 0%, when the voltage demand requires the DC-DC buck-boost converter to maintain the voltage. In these known controllers it is necessary to overlap activation of the high-side first switch and the low-side second switch in order to provide a smooth transition between a buck-mode and boost-mode. For example, at the transition point (where the voltage is maintained, and no step-up or step-down occurs) the buck-side has a duty cycle of 50% and the boost-side has a duty cycle of 50%. Either side of this transition point, the duty cycle of the boost-side and buck-side are increased / decreased to just above / below 50% to provide a small step-up or step-down. Thus, in the known controller both the buck and boost-sides are switching simultaneously. However, simultaneous switching of both the sides simultaneously is inefficient. There is an energetic cost to operate each of the switches in the buck-boost converter, and so operating all switches simultaneously requires more energy. Moreover, the switching itself has an energetic cost to the power transferred through the buck-boost converter. The energetic cost to the power transfer is greater at high current flows required in fast-charging applications. For example, high power MOSFETs needed for fast-charging applications typically require more power to switch, and the loss in efficiency of the power transferred is more pronounced at higher currents. In the present invention, at the transition point (where the voltage is maintained, and no step- up or step-down occurs) the buck-side has a duty cycle of 100% and the boost-side has a duty cycle of 0%. Either side of this transition point, the duty cycle of only one of the boost- side or the buck-side is changed to provide a small step-up or step-down. Even at a step-up or step-down in voltage which is relatively small, such as less than 20%,10%, 5%, 3%, 2% or 1% change in voltage, only one of the sides are switched at a time. For a step-up, the boost-side has a duty cycle just above 0% while the buck-side retains a duty cycle of 100%. For a step-down, the buck side has a duty cycle just below 100% while the boost side retains a duty cycle of 0%. In this way, the buck-side and boost-side are never switching simultaneously. 008698920 Method of controlling a buck-boost converter In a first aspect of the invention there is provided a method of controlling a DC-DC buck-boost converter system, the DC-DC buck-boost converter system comprising: a DC-DC buck-boost converter comprising a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first pulse-width modulation (PWM) signal, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges; and a control module for controlling the DC-DC buck-boost converter, the control module comprising a PWM signal generator; the method comprising: generating the first PWM signal and the second PWM signal with the signal generator based on a voltage demand, wherein the voltage demand determines if the DC-DC buck- boost converter steps-up, steps-down, or maintains the voltage of the first DC source relative to the second DC source, wherein: when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; when the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, and when the voltage demand requires the DC-DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. The DC-DC buck-boost converter is described in further detail herein. The buck-boost converter may be a two-switch or a four-switch buck-boost converter. Preferably, the buck- boost converter is a four-switch buck-boost converter. The method comprises a step of generating the first PWM signal and the second PWM signal based on a voltage demand. The PWM signals are generated using any suitable means, such as signal generator. The signal generator is described in further detail below. The PWM signals are based on a voltage demand. The voltage demand determines if the DC-DC buck-boost converter steps-up, steps-down, or maintains the voltage of the first DC source relative to the second DC source. The voltage demand is typically determined by the control module, such as a controller of the control module. The voltage demand may be based on feedback or feedforward data. Determination of the voltage demand is described in more detail below. 008698920 The voltage demand modulates the duty cycle of the first PWM signal and the second PWM signal. This modulation results in the step-up, step-down, or maintenance of the voltage across the buck-boost converter. The duty cycle of the first PWM signal and the second PWM signal satisfy specific conditions such that only the switches on the first half-bridge or the second half-bridge are switching at any one time. Step-up refers to where the voltage is increased across the buck-boost converter. In other words, the voltage of the first DC source is increased relative to the second DC source. Step-down refers to where the voltage is decreased across the buck-boost converter. In other words, the voltage of the first DC source is decreased relative to the second DC source. Maintenance of the voltage refer to where the voltage is not stepped up or stepped down. In other words, the voltage of the first DC source is not changed relative to the second DC source. When the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%. In this way, the first PWM signal causes the high-side first switch to be switching, and the second PWM signal causes the low-side second switch to be always off. In some embodiments, the first PWM signal has a duty cycle which is not 100%, and the second PWM signal has a duty cycle of 0%. In some embodiments, the first PWM signal has a duty cycle which is not 100% or 0%, and the second PWM signal has a duty cycle of 0%. In some embodiments, when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle of more than 0% and less than 100%, such as more than 20% and less than 100%, more than 30% and less than 100%, more than 50% and less than 100%, more than 70% and less than 100%, more than 80% and less than 100%, more than 90% and less than 100%, while the second PWM signal has a duty cycle of 0%. That is, in the transition region the high-side first switch is switching (e.g., at a high duty cycle), and the low-side second switch is always off. When the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%. In this way, the first PWM signal causes the high-side first switch to be always on, and the second PWM signal causes the low-side second switch to be always off. In other words, the high-side first switch is not switching and the low-side second switch is not switching. When the voltage demand requires the DC-DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. In this way, the first PWM signal causes the high-side first switch to be always on, and the second PWM signal causes the low-side second switch to be switching. 008698920 In some embodiments, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle which is not 0%. In some embodiments, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle which is not 0% or 100%. In some embodiments, when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle of 100%, while the second PWM signal has a duty cycle of more than 0% and less than 100%, such as more than 0% and less than 80%, more than 0% and less than 70%, more than 0% and less than 50%, more than 0% and less than 30%, more than 0% and less than 20%, more than 0% and less than 10%. That is, in the transition region the high-side first switch is always on, and the low-side second switch is switching (e.g., at a low duty cycle). The method comprises a step of generating the first PWM signal and the second PWM signal with the signal generator based on a voltage demand. The generation of the first PWM signal and the second PWM signal from the voltage demand may include a calculation step and a first and second generation step. In some embodiments, the step of generating the first PWM signal and the second PWM signal from the voltage demand comprises: calculating a first demand signal and a second demand signal, wherein the first and the second demand signals differ by at least a threshold value equal to the difference between a minimum threshold value and a maximum threshold value, wherein the minimum threshold value corresponds to a duty cycle of 0% and the maximum threshold value corresponding to a duty cycle of 100%; generating a first PWM signal from the first demand signal; and generating a second PWM signal from the second demand signal. The first demand signal and second demand signal are calculated from the voltage demand. The first and the second demand signals differ by at least a threshold value equal to the difference between a minimum threshold value and a maximum threshold value, wherein the minimum threshold value corresponds to a duty cycle of 0% and the maximum threshold value corresponding to a duty cycle of 100%. For example, if a demand signal value of 0 corresponds to a duty cycle of 0% and a demand signal value of 1 corresponds to a duty cycle of 100%, then the difference between the first and second demand signal values must be 1 or more. Alternatively, if a demand signal value of 21 corresponds to a duty cycle of 0% and a demand signal value of 34 corresponds to a duty cycle of 100%, then the difference between the first and second demand signals must be 13 or more. 008698920 In this way, only one of the first or second demand signals can have a value corresponding to a duty cycle of between 0% and 100%. The other demand signal value is thus greater than 100% or less than 0%, and so results in a PWM signal of 100% or 0% (as explained below). For example, if the first demand signal value corresponds to a duty cycle of 75%, then the second demand signal value corresponds to a duty cycle of -25% (which gives a PWM signal of 0%). Alternatively, if the first demand signal value corresponds to a duty cycle of 125% (which gives a PWM signal of 100%) then the second demand signal value corresponds to a duty cycle of 25%. In some embodiments, the calculating step further comprises: comparing the first demand signal and the second demand signal to both the minimum threshold value and the maximum threshold value; and if the first and / or second demand signal is greater than a maximum threshold value, setting the demand signal to the maximum threshold value, if the first and / or second demand signal is less than a minimum threshold value, setting the demand signal to the minimum threshold value, and if the first and / or second demand signal is between the minimum threshold value and the maximum threshold value, leaving the demand signal unchanged. Where the demand signal is between a minimum threshold value and a maximum threshold value, wherein the minimum threshold value corresponds to a duty cycle of 0% and the maximum threshold value corresponding to a duty cycle of 100%, the demand signal is proportional to the demand signal relative to the minimum and maximum threshold values. Where the first and / or second demand signal is greater than a maximum threshold value, the demand signal is set to the maximum threshold value. Where the first and / or second demand signal is less than a minimum threshold value, the demand signal is set to the minimum threshold value. The step also includes generating a first PWM signal from the first demand signal; and generating a second PWM signal from the second demand signal. Where the demand signal is between a minimum threshold value and a maximum threshold value, wherein the minimum threshold value corresponds to a duty cycle of 0% and the maximum threshold value corresponding to a duty cycle of 100%, the PWM signal generated is proportional to the demand signal relative to the minimum and maximum threshold values. For example, if a demand signal value of 0 corresponds to a duty cycle of 0% and a demand signal value of 1 corresponds to a duty cycle of 100%, then a demand signal of 0.5 would 008698920 generate a PWM signal having a duty cycle of 50%. Similarly, a demand signal of 0.7 would generate a PWM signal a duty cycle of 70%. Alternatively, if a demand signal value of 21 corresponds to a duty cycle of 0% and a demand signal value of 34 corresponds to a duty cycle of 100%, then a demand signal of 27.5 would generate a PWM signal having a duty cycle of 50%. Similarly, a demand signal of 30.1 would generate a PWM signal a duty cycle of 70%. Where the demand signal is below a minimum threshold value, the demand signal is set to the maximum threshold value, and the PWM signal generated is equal to a duty cycle of 0%. For example, if a demand signal value of 0 corresponds to a duty cycle of 0% (the minimum threshold value is 0), then a demand signal value of 0 or less (e.g, -0.2, -23) would generate a PWM signal having a duty cycle of 0%. Where the demand signal is above a maximum threshold value, the demand signal is set to the minimum threshold value, and the PWM signal generated is equal to a duty cycle of 100%. For example, if a demand signal value of 1 corresponds to a duty cycle of 100% (the maximum threshold value is 1), then a demand signal value of 1 or more (e.g, 1.2, 23) would generate a PWM signal having a duty cycle of 100%. In some embodiments, in the calculating step, the first or second demand signal corresponds to the voltage demand and the other of the first or second demand signal corresponds to the difference between the voltage demand and the threshold value. In this context, the difference between the voltage demand and the threshold value can be applied as a subtraction (minus) or an addition (plus). Ins some embodiments, the first demand signal corresponds to the voltage demand and the second demand signal corresponds voltage demand minus the threshold value, or the second demand signal corresponds to the voltage demand and the first demand signal corresponds voltage demand plus the threshold value. In some such embodiments, the first demand signal corresponds to the voltage demand and the second demand signal corresponds voltage demand minus the threshold value. For example, if a demand signal value of 0 corresponds to a duty cycle of 0% and a demand signal value of 1 corresponds to a duty cycle of 100%, and the voltage demand is 0.5 then the first demand signal has a value of 0.5 and the second demand signal has a value of -0.5 (0.5 minus 1). This results in a first PWM signal having a duty cycle of 50% and a second PWM signal having a duty cycle of 0%. 008698920 Similarly, if the voltage demand is 1.7 then the first demand signal has a value of 1.7 and the second demand signal has a value of 0.7 (1.7 minus 1). This results in a first PWM signal having a duty cycle of 100% and a second PWM signal having a duty cycle of 70%. In other such embodiments, the second demand signal corresponds to the voltage demand and the first demand signal corresponds voltage demand plus the threshold value. For example, if a demand signal value of 0 corresponds to a duty cycle of 0% and a demand signal value of 1 corresponds to a duty cycle of 100%, and the voltage demand is 0.5 then the second demand signal has a value of -0.5 and the first demand signal has a value of 0.5 (-0.5 plus 1). This results in a first PWM signal having a duty cycle of 50% and a second PWM signal having a duty cycle of 0%. Similarly, if the voltage demand is 0.7 then the second demand signal has a value of 0.7 and the first demand signal has a value of 1.7 (0.7 plus 1). This results in a first PWM signal having a duty cycle of 100% and a second PWM signal having a duty cycle of 70%. Preferably, the step of generating the first PWM signal and the second PWM signal from the voltage demand comprises: calculating a first demand signal and a second demand signal, wherein the first and the second demand signals differ by at least a threshold value equal to the difference between a minimum threshold value and a maximum threshold value, wherein the minimum threshold value corresponds to a duty cycle of 0% and the maximum threshold value corresponding to a duty cycle of 100%, and the first demand signal corresponds to the voltage demand and the second demand signal corresponds voltage demand minus the threshold value, or the second demand signal corresponds to the voltage demand and the first demand signal corresponds voltage demand plus the threshold value; comparing the first demand signal and the second demand signal to both the minimum threshold value and the maximum threshold value, and if the first and / or second demand signal is greater than a maximum threshold value, setting the demand signal to the maximum threshold value, if the first and / or second demand signal is less than a minimum threshold value, setting the demand signal to the minimum threshold value, and if the first and / or second demand signal is between the minimum threshold value and the maximum threshold value, leaving the demand signal unchanged; generating a first PWM signal from the first demand signal; and generating a second PWM signal from the second demand signal. In some embodiments, the step-down in voltage is proportional to the duty cycle of the first PWM signal at all duty cycles, and the step-up in voltage is proportional to the duty cycle of the second PWM signal at all duty cycles. Preferably, the proportionality constant relating the duty cycle and the step-up in voltage is constant at all duty cycles. Preferably, the 008698920 proportionality constant relating the duty cycle and the step-down in voltage is constant at all duty cycles. This differs from known systems where dual switching is used, especially in the transition region, so this proportionality is not present across all duty cycles. Method of controlling power transfer In a second aspect of the invention there is provided a method of controlling power transfer between a first DC source and a second DC source using a DC-DC buck-boost converter system as described in the first aspect; the method comprising: transferring power between the first DC source and the second DC source using the DC-DC buck-boost converter, and controlling the rate of power transfer using the control module, by controlling the relative voltage of the first DC-source compared to the second DC-source using a method according to the first aspect. Accordingly, there is provided a method of controlling power transfer between a first DC source and a second DC source using a DC-DC buck-boost converter system, the DC-DC buck-boost converter system comprising: a DC-DC buck-boost converter comprising a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first pulse-width modulation (PWM) signal, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges; and a control module for controlling the DC-DC buck-boost converter, the control module comprising a signal generator; the method comprising: transferring power between the first DC source and the second DC source using the DC-DC buck-boost converter, and controlling the rate of power transfer using the control module, by controlling the relative voltage of the first DC-source compared to the second DC-source, using a method according to the first aspect. The DC-DC buck-boost converter is described in further detail herein. The buck-boost converter may be a two switch or a four-switch buck-boost converter Preferably, the buck- boost converter is a four-switch buck-boost converter. The method comprises a step of transferring power between the first DC source and the second DC source using the DC-DC buck-boost converter. This may be known as the “power transfer” step. 008698920 The power transfer step may be initiated by connection of the first and second DC source via the buck-boost converter. Alternatively, the power transfer step may be initiated by a start transfer signal from the first DC-source and / or the second DC-source. The power transfer step may be terminated by disconnection of the first and / or the second DC source to the buck-boost converter. Alternatively, the power transfer step may be initiated by a stop transfer signal from the first DC-source and / or the second DC-source. The method comprises a step controlling the rate of power transfer. This may be known as the “control step”. The control step typically comprises controlling the relative voltage of the first DC-source compared to the second DC-source. The magnitude in the difference in voltage between the first DC-source compared to the second DC-source effects the current flow, and thus the rate of power transfer, between the first DC-source and the second DC-source. A larger voltage difference results in a higher current flow, and a larger power transfer. Therefore, controlling the voltage difference allows for the power transfer to be controlled. The relative voltage of the first DC-source compared to the second DC-source is controlled using a method of controlling the DC-DC buck-boost converter according to the first aspect. In this method, the relative voltage is controlled based on a voltage demand. The description of the method of controlling the DC-DC buck-boost converter provided above for the first aspect also applies to this aspect. In some embodiments, the method further comprises the steps of: sending status data on the power transfer between the first DC source and the second DC source to the control module; and adjusting the voltage demand based on the status data. The step of sending status data on the power transfer between the first DC source and the second DC source maybe known as the “data sending” step. In some embodiments, the status data comprises status data on the rate of the power transfer between the first DC source and second DC source, such as current supplied to or delivered from the first DC source or second DC source and / or the voltage. The current and / or voltage may be measured using any suitable component, such as an ammeter and / or a voltmeter. Preferably the current and voltage are measured, to determine the power transfer. Alternatively, only the current is measured. The status data may be measured by generating a sense signal. The current and / or voltage may be measured by generating a current sense signal and / or a voltage sense signal. The status data may be in the form of an analogue or a digital signal. 008698920 The control module may comprise a receiver for receiving and optionally storing the status data. Any suitable means of measuring the status data may be used, such as sensors (e.g., voltmeter, ammeter, ohmmeter, thermistor, thermocouple, high input impedance ohmmeter). The control module may comprise a controller for processing and optionally storing the status data. For example, the power controller may comprise a chip for -post processing status data. The power controller may comprise a means of sending and / or receiving status data (e.g., for sending and / or receiving status data to / from a remote system (such as the cloud)). The status data may be obtained directly from the first DC-source, second DC-source, or the buck-boost converter. Where the first DC-source and / or the second DC-source are a battery, the status data may be obtained from the battery management system (BMS) of the battery. The step of adjusting the voltage demand based on the status data may be known as the “adjustment step”. Typically, the voltage demand increases if the status data is below a target range; the voltage demand decrease if the status data is above a target range; and the voltage demand is not changed if the status data is within the target range. In this way, the voltage demand can increase or decrease the power transfer to meet the target range. In some embodiments, the step of adjusting the voltage demand comprises: comparing the status data to a target range; and adjusting the voltage demand value in response to the comparison of the status data to the target range, such that subsequent status data is maintained within the target range or is moved into the target range. In some embodiments, the target range is a pre-set range. The pre-set range may be set by the first DC source and / or the second DC source, such as by a battery management system of an electric vehicle. The pre-set range may be set by the buck-boost converter. The pre- set range may be set by a user input. In some embodiment, the target range is 20 to 200A, preferably 40 to 150A, more preferably 50 to 100 A, yet more preferably 70 to 90A. The target range may be about 75A. In addition or alternatively, the target range may be 50 to 800 kW, preferably 100 to 400 kW, more preferably 150 to 200 kW. In some embodiments, the target range is a variable target range. The variable target range may be provided by the first and / or the second DC source, such as by a battery management system of an electric vehicle. 008698920 In some embodiments, a first variable target range is provided by the first DC source and a second variable target range is provided by the second DC source. The target range may be calculated from the first variable target range and second variable target range. For example, the target range may be calculated as the average of the first variable target range and second variable target range. Alternatively, the target range may be calculated as the overlapping region of the first and second variable target range. That is, the target range used may be equal to the region of overlap between the first and second variable target range. In some embodiments, the target range changes depending on the first DC source or the second DC source. For example, where the first DC source or the second DC source is a battery, the target range may change depending on the discharge or charge rate of the battery. If the battery is cyclable (e.g., chargeable or dischargeable) at a first C-rate range the target range may be a first target range, and if the battery is cyclable at second C-rate range the target range may be a second target range. For example, if the first C-rate range is from 0C to 3C, the first target range may be 0 to 150A, and if the second C-rate range is from 3C to 20C, the second target range may be 150A to 1000A. In addition or alternatively, if the first C-rate range is from 0C to 3C, the first target range may be 0 to 60 kW, and if the second C-rate range is from 3C to 20C, the second target range may be 60 to 400 kW. The control module may compare two or more types of status data. Preferably the control module compares status data on the power transfer and a piece of additional status data. Preferably the control module compares status data on the power transfer and an additional one or more types of status data, such as two or more, three or more, four or more, or five or more types of additional status data. The additional status data may include battery temperature, battery internal resistance, battery voltage, battery current, battery capacity, battery state of charge, or a combination thereof. Preferably the status data includes battery temperature, battery voltage and battery current. In some embodiments, the step of adjusting the voltage demand based on the status data uses a control loop. Any suitable control loop may be used, such as an open control loop, feedback control loop or a feedforward control loop. The feedback control loop may be a positive feedback loop or a negative feedback loop. The feedforward control loop typically 008698920 detects the change in the status data and then adjusts the voltage demand to minimize the effect of the change. The control loop may be a PID loop. In some embodiments, the adjustment step is carried out on a control module programmed with a suitable algorithm. The control module may include a semi-conductor chip, an integrated circuit, a logic circuit or a microprocessor circuit. The components of the control module may be disposed on a printed circuit board. In some embodiments, the adjustment step is repeated at a rate of 20 kHz or more, preferably from 20 to 100 kHz, more preferably from 40 to 60 kHz, yet more preferably about 50 kHz. In some embodiments of the method, the first DC-source is a first battery and the second DC- source is a second battery. Where the voltage of the first battery is greater than the voltage of the second battery, the DC-DC buck-boost converter is typically operated in a step-down mode to step-down the voltage of the first battery relative to the voltage of the second battery. Accordingly, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%. Where the voltage of the first battery is similar to the voltage of the second battery, the DC- DC buck-boost converter is typically operated to maintain the voltage, to maintain the voltage of the first battery relative to the voltage of the second battery. Accordingly, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%. Where the voltage of the first battery is less than the voltage of the second battery, the DC- DC buck-boost converter is typically operated in a step-up mode to step-up the voltage of the first battery relative to the voltage of the second battery. Accordingly, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. The second aspect refers to a power transfer between a first DC-source and a second DC- source. In some embodiments, the power transfer is in a first direction from a first DC-source to a second DC-source. In other embodiments, the power transfer is in a second direction from a second DC-source to a first DC-source. The power transfer may be bi-directional. Where the power transfer is bi-directional, the buck-boost converter is preferably a four-switch buck-boost converter. This allows the buck side and boost side of the buck-boost converter to be interchangeable. When power is transfer in a first direction, the first half bridge is the buck side and the first high-side switch is controllable by the first PWM signal; and the second half bridge is the boost side and the second low-side switch is controllable by the second PWM signal. 008698920 When power is transfer in a second direction, the first half bridge is the boost side and the first low-side switch is controllable by the second PWM signal; and the second half bridge is the buck side and the second high-side switch is controllable by the first PWM signal. In such embodiments, the control module determines if the first PWM signal goes to the first high-side switch or the second high-side switch, based on the direction of power transfer. In such embodiments, the control module determines if the second PWM signal goes to the first low-side switch or the second low-side switch, based on the direction of power transfer. The control module may determine the direction of power transfer based on the status data on the power transfer between the first DC source and the second DC source. Method of transitioning between a buck-mode and a boost-mode In a third aspect of the invention there is provided a method of transitioning a DC-DC buck- boost converter between a step-up mode and step-down mode using a DC-DC buck-boost converter system as described the first aspect; the method comprising: controlling the DC-DC buck-boost converter according to the method of the first aspect, and transitioning from a step-down mode to a step-up mode, wherein the first PWM signal duty cycle is increased from less than 100% to 100% while the second PWM signal maintains a duty cycle of 0%, and subsequently the second PWM signal duty cycle is increased from 0% to greater than 0% while the first PWM signal maintains a duty cycle of 100%; or transitioning from a step-up mode to a step-down mode, wherein the second PWM signal duty cycle is decreased from greater than 0% to 0% while the first PWM signal maintains a duty cycle of 100%, and subsequently the first PWM signal duty cycle is decreased from 100% to less than 100% while the second PWM signal maintains a duty cycle of 0%. Accordingly, there is provided a method of transitioning a DC-DC buck-boost converter between a step-up mode and a step-down mode, the DC-DC buck-boost converter comprising a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first pulse-width modulation (PWM) signal, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal, an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges, the method comprising a step of: controlling the DC-DC buck-boost converter according to the method of the first aspect, and 008698920 transitioning from a step-down mode to a step-up mode, wherein the first PWM signal duty cycle is increased from less than 100% to 100% while the second PWM signal maintains a duty cycle of 0%, and subsequently the second PWM signal duty cycle is increased from 0% to greater than 0% while the first PWM signal maintains a duty cycle of 100%; or transitioning from a step-up mode to a step-down mode, wherein the second PWM signal duty cycle is decreased from greater than 0% to 0% while the first PWM signal maintains a duty cycle of 100%, and subsequently the first PWM signal duty cycle is decreased from 100% to less than 100% while the second PWM signal maintains a duty cycle of 0%. The DC-DC buck-boost converter is described in further detail herein. The method comprises a step of controlling the DC-DC buck-boost converter according to the first aspect. The description of the method of controlling the DC-DC buck-boost converter provided above for the first aspect also applies to this aspect. When transitioning between the step-up and step-down mode the buck-boost converter transitions through a maintaining mode. That is, the buck-boost converter transitions through a mode where the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%. In this way, dual switching of the high-side first switch and the low- side second switch does not occur. The method comprises a step of transitioning a DC-DC buck-boost converter from a step-up mode to a steps-down mode. In this step, the first PWM signal duty cycle is increased from less than 100% to 100% while the second PWM signal maintains a duty cycle of 0%. That is, the converter transitions from a step-up to a maintaining mode. In addition, or alternatively, the second PWM signal duty cycle is increased from 0% to greater than 0% while the first PWM signal maintains a duty cycle of 100%. That is, the converter transitions from a maintaining mode to a step-down mode. The method comprises a step of transitioning a DC-DC buck-boost converter from a step- down mode to a step-up mode. In this step, the second PWM signal duty cycle is decreased from greater than 0% to 0% while the first PWM signal maintains a duty cycle of 100%. That is, the converter transition from a step-down to a maintaining mode. In addition, or alternatively, the first PWM signal duty cycle is decreased from 100% to less than 100% while the second PWM signal maintains a duty cycle of 0%. That is, the converter transitions from a maintaining mode to a step-up mode. In some embodiments, during the transition when the first PWM signal duty cycle is a duty cycle of less than 100%, the duty cycle is greater than 80 % and less than 100%, preferably greater than 90% and less than 100%, more preferably greater than 95% and less than 100%, yet more preferably greater than 99% and less than 100%. 008698920 In some embodiments, during the transition when the first PWM signal duty cycle is a duty cycle of greater than 0%, the duty cycle is greater than 0 % and less than 20%, preferably greater than 0% and less than 10%, more preferably greater than 0% and less than 5%, yet more preferably greater than 0% and less than 1%. Preferably the method comprises a step of transitioning a DC-DC buck-boost converter from a step-up mode to a step-down mode. In this step, the first PWM signal duty cycle is increased from greater than 80% and less than 100% to 100% while the second PWM signal maintains a duty cycle of 0%. That is, the converter transitions from a step-up to a maintaining mode. In addition, or alternatively, the second PWM signal duty cycle is increased from 0% to greater than 0% and less than 20% while the first PWM signal maintains a duty cycle of 100%. That is, the converter transitions from a maintaining mode to a step-down mode. Preferably the method comprises a step of transitioning a DC-DC buck-boost converter from a step-down mode to a step-up mode. In this step, the second PWM signal duty cycle is decreased from greater than 0% and less than 20% to 0% while the first PWM signal maintains a duty cycle of 100%. That is, the converter transition from a step-down to a maintaining mode. In addition, or alternatively, the first PWM signal duty cycle is decreased from 100% to greater than 80% and less than 100%, while the second PWM signal maintains a duty cycle of 0%. That is, the converter transitions from a maintaining mode to a step-up mode. More preferably the method comprises a step of transitioning a DC-DC buck-boost converter from a step-up mode to a step-down mode. In this step, the first PWM signal duty cycle is increased from greater than 95 % and less than 100% to 100% while the second PWM signal maintains a duty cycle of 0%. That is, the converter transitions from a step-up to a maintaining mode. In addition, or alternatively, the second PWM signal duty cycle is increased from 0% to greater than 0 % and less than 5% while the first PWM signal maintains a duty cycle of 100%. That is, the converter transitions from a maintaining mode to a step- down mode. More preferably the method comprises a step of transitioning a DC-DC buck-boost converter from a step-down mode to a step-up mode. In this step, the second PWM signal duty cycle is decreased from greater than 0 % and less than 5% to 0% while the first PWM signal maintains a duty cycle of 100%. That is, the converter transition from a step-down to a maintaining mode. In addition, or alternatively, the first PWM signal duty cycle is decreased from 100% to greater than 95% and less than 100%, while the second PWM signal maintains a duty cycle of 0%. That is, the converter transitions from a maintaining mode to a step-up mode. In an alternative embodiment, when transitioning between the step-up and step-down mode the buck-boost converter transitions without a maintaining mode. That is, the buck-boost 008698920 converter transitions straight from a step-down mode to a step-up mode, or straight from a step-up mode to a step-down mode. In such embodiments, in the step down mode the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; and in the step-up mode the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. In this way, dual switching of the high-side first switch and the low-side second switch does not occur. A transition may be required during battery-to-battery charging due to changes in battery voltage. As a donor battery decrease in state of charge (SOC) the battery voltage decrease and as a receiver battery increases in SOC the battery voltage increases. Accordingly, in some embodiments the first DC-source is a first battery and the second DC-source is a second battery. In some such embodiments, as the voltage of a donor battery decreases below that of the receiver battery, the method comprises a step of transitioning a DC-DC buck-boost converter from a step-down mode to a step-up mode, wherein the second PWM signal duty cycle is decreased from greater than 0% to 0% while the first PWM signal maintains a duty cycle of 100%. That is, the converter transition from a step-down to a maintaining mode. In addition, as the voltage of a donor battery decrease further the first PWM signal duty cycle is decreased from 100% to less than 100% while the second PWM signal maintains a duty cycle of 0%. That is, the converter transitions from a maintaining mode to a step-up mode. Control Module for a Buck-Boost converter In a fourth aspect of the invention there is provided a control module for controlling a DC-DC buck-boost converter, the DC-DC buck-boost converter comprising: a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first pulse-width modulation (PWM) signal, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges, the control module comprising: a signal generator, where the signal generator is configured to generate the first PWM signal and the second PWM signal based on a voltage demand, wherein the voltage demand determines if the DC-DC buck-boost converter steps-up, steps-down, or maintains the voltage of the first DC source relative to the second DC source, wherein: when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; when the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, 008698920 when the voltage demand requires the DC-DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. The control module is for carrying out the method of the first aspect. The description of the first aspect, above, also applies to the control module itself. In some embodiments, the control module further comprises: a receiver configured to receive status data on the power transfer between the first DC source and second DC source, and a controller configured to adjust the voltage demand in response to the status data. The control module may comprise a receiver. Any suitable receiver may be used, which is capable of receive status data on the power transfer. The status data may be in the form of an analogue or a digital signal. The status data may be a sense signal. The control module may comprise a receiver for receiving and optionally storing the status data. The control module may comprise a controller. Any suitable controller may be used, which is capable of adjusting the voltage demand in response to the status data. The controller may include a semi-conductor chip, an integrated circuit, a logic circuit or a microprocessor circuit. In some embodiments, the adjustment step is carried out on a controller programmed with a suitable algorithm. In some embodiments, the controller comprises a microcontroller programmed with an algorithm to carry out the adjustment to the voltage demand. In some embodiments, the controller comprises a circuit configured to carry out the adjustment to the voltage demand. The control module may comprise a controller for processing and / or storing the status data. For example, the power controller may comprise a chip for post processing status data. The power controller may comprise a means of sending and / or receiving status data (e.g., for sending and / or receiving status data to / from a remote system (such as the cloud)). Buck-Boost converter system In a fifth aspect of the invention there is provided a DC-DC buck-boost converter system comprising: the control module of the fourth aspect, and a DC-DC buck-boost converter. The Buck-Boost converter system is for carrying out the method of the first aspect. The description of the methods and control module, above, also applies to the Buck-Boost converter system. 008698920 The DC-DC buck-boost converter may be any suitable buck-boost converter known in the art. The buck-boost converter typically electrically connects a first DC source to a second DC source. The buck-boost converter is a converter able to step-up and step-down the voltage of the first DC source relative to the second DC source. A DC-DC buck-boost converter typically comprises: a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges. Typically, the buck-boost converter comprises a ‘boost’ side and a ‘buck’ side. The boost and buck side may be interchangeable in a bidirectional buck-boost converter. The boost-side is operable to step-up the voltage and the buck-side is operable to step-down the voltage. The buck-boost converter comprises an electrical connection between the buck-side and the boost-side. The electrical connection may be provided by any suitable means. The electrical connection is adapted to buffer electrical energy. The buffering of electrical energy refers to the smoothing of peaks and troughs in the electrical energy supplied, which occur as a result of the switching in the buck-side and boost-side. Any suitable buffer may be used, such as an inductor or a charge pump. Preferably, the buffer is an inductor. When operating in a first direction, current flows from the first DC source to the second DC source. Here, the second DC source is functioning as a sink or load. When operating in a second direction (opposite to the first direction), current flows from the second DC source to the first DC source. Here, the first DC source is functioning as a sink or load. The buck-side typically comprises a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first PWM signal. The buck- side is operable to step-down the voltage of the first DC source relative to the second DC source. The buck-side may further comprise a first half-bridge comprising a low-side first switch. The low-side first switch is synchronous with the high-side first switch, and configured to switch inversely to the high-side first switch. In other words, when the low-side first switch is on, the high-side first switch is off. The boost-side typically comprises a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal. The boost-side is operable to step-up the voltage of the first DC source relative to the second DC source. 008698920 The boost-side may further comprise a second half-bridge comprising a high-side second switch. The high-side second switch is synchronous with the low-side second switch and configured to switch inversely to the low-side second switch. In other words, when the high- side second switch is on, the low-side second switch is off. In some embodiments the buck-boost converter is a four-switch buck-boost converter wherein: the first half-bridge comprises the high-side first switch and a low-side first synchronous switch configured to switch inversely to the high-side first switch, and the second half-bridge comprises the low-side second switch and a high-side second synchronous switch configured to switch inversely to the low-side second switch. When operating in the first direction, the first half-bridge is the buck-side and the second half- bridge is the boost-side. Here, the high-side first switch is controlled by the first PWM signal to step-down the voltage of the first DC source relative to the second DC source, and the low- side second switch is controlled by the second PWM signal to step-up the voltage of the first DC source relative to the second DC source. The high-side first switch and low-side second switch operate synchronously and inversely to the high-side first switch and low-side second switch respectively. When operating in the second direction, the second half-bridge is the buck-side and the first half-bridge is the boost-side. Here, the high-side second switch is controlled by the first PWM signal to step-down the voltage of the second DC source relative to the first DC source, and the low-side first switch is controlled by the second PWM signal to step-up the voltage of the second DC source relative to the first DC source. The low-side second switch and high-side first switch operate synchronously and inversely to the high-side second switch and low-side first switch respectively. In other words, in a four-switch buck-boost converter the role of the first half-bridge and second half-bridge can be interchanged, depending on the direction of current flow through the converter. Charging Device In a sixth aspect of the invention there is provided a charging device comprising: the DC-DC buck-boost converter system of the fifth aspect, a first DC source, wherein the first DC source is in electrical connection with the first half-bridge, and a power connector for releasably connecting the DC-DC buck-boost converter system to a second DC source, wherein the power connector is in electrical connection with the second half-bridge of the DC-DC buck-boost converter system. 008698920 In some embodiments, the first DC source comprises an electrochemical cell. Preferably the second DC source also comprises an electrochemical cell. In some embodiments, the first DC source comprises an electrochemical cell having a maximum operable charge and / or discharge rate of 1C or more, preferably 3C or more, more preferably 5C or more, yet more preferably 10C or more, even more preferably 20C or more. In some embodiments, the first DC source comprises a battery comprising a plurality of electrochemical cells. Preferably the second DC source also comprises a battery comprising a plurality of electrochemical cells. Preferably, the battery is a battery provided in a road vehicle, such as an automobile, bus, moped or truck. Alternatively, a battery is provided in a rail vehicle, such as a train or a tram. The battery may also be provided in an electric bicycle (e-bike), a drone, an electric aircraft, and an electric or hybrid boat. Similarly, batteries may be provided in power tools such as powered drills or saws, garden tools such as lawnmowers, hedge trimmers, or grass trimmers, or home appliances such as toothbrushes, vacuum cleaners or hair dryers. Batteries of the invention may be used in cameras. The battery may be used in robots, such as delivery robots or warehouse automation robots. The battery may be provided in a regenerative braking system. The battery may be provided in a portable electronic device, such as a mobile phone, laptop or tablet. The battery may be provided in a power grid management system. In some embodiments of the method, the first DC-source is a first battery and the second DC- source is a second battery. In some embodiments, the first DC source comprises a battery having a maximum operable charge and / or discharge rate of 1C or more, preferably 3C or more, more preferably 5C or more, yet more preferably 10C or more, even more preferably 20C or more. Preferably, the first battery has a higher operable charge and / or discharge rate than the second battery. In some embodiments, the maximum operable discharge rate is 3C or more, where the capacity retention is at least 70% over 1,000 cycles, preferably 5 C or more, where the capacity retention is at least 70% over 1,000 cycles, more preferably 10 C or more where the capacity retention is at least 70% over 1,000 cycles. In some embodiments, the maximum operable discharge rate is 3C or more, where the capacity retention is at least 80% over 1,000 cycles, preferably 5 C or more, where the capacity retention is at least 80% over 1,000 cycles, more preferably 10 C or more where the capacity retention is at least 80% over 1,000 cycles. In some embodiments, the maximum operable discharge rate is 3C or more, where the capacity retention is at least 90% over 1,000 cycles, preferably 5 C or more, where the 008698920 capacity retention is at least 90% over 1,000 cycles, more preferably 10 C or more where the capacity retention is at least 90% over 1,000 cycles. The battery may be a battery as described in WO 2021 / 074406 or WO 2019 / 234248, the contents of which are incorporated herein by reference in their entirety. The battery typically comprises a working electrode, electrolyte, counter electrode, and optionally a separator. Preferably, the battery is a lithium-ion battery. The working electrode may be an anode or cathode during a discharge step, for example in a lithium ion battery. Preferably, the working electrode is the anode during a discharge step. In some embodiments, the working electrode comprises a niobium oxide or a niobium metal oxide. The niobium oxide may be selected from Nb2O5 polymorphs, NbO2, Nb2O3 or combinations thereof. The niobium metal oxide may be a compound (for example, having a crystalline structure) of a niobium oxide and an additional metal oxide. Suitable niobium metal oxides include niobium tungsten oxide (for example Nb16W5O55or Nb18W16O93), a titanium niobium oxide (for example TiNb2O7), a niobium molybdenum oxide (for example Nb2Mo3O14), or combinations thereof. Suitable niobium tungsten oxides include Nb12WO33, Nb26W4O77, Nb14W3O44, Nb16W5O55, Nb18W8O69, Nb2WO8, Nb18W16O93, Nb22W20O115, Nb8W9O47, Nb54W82O381, Nb20W31O143, Nb4W7O31, or Nb2W15O50or combinations thereof. In some embodiments the working electrode active material comprises Nb16W5O55, Nb18W8O69, Nb2WO8, Nb18W16O93, or Nb22W20O115, or combinations thereof.Preferably the working electrode comprises Nb16W5O55 or Nb18W16O93, or combinations thereof. In some embodiments, the working electrode comprises graphite, Si, SiOx (where x is typically from 0 to 2), LTO, or a mixture thereof. In some embodiments, the working electrode consists essentially of graphite Si, SiOx (where x is typically from 0 to 2), or lithium titanate (LTO). Preferably, the working electrode comprises these compounds (e.g. graphite) in particulate form. The working electrode may comprise a mixture of niobium tungsten oxide and an additional active material. The additional active material may be an additional metal oxide. For example, the working electrode may comprise a mixture of niobium tungsten oxide and an additional active material selected from lithium titanate (LTO; Li4Ti5O12), titanium niobium oxides (for example TiNb2O7), titanium tantalum oxides (for example TiTa2O7), tantalum molybdenum oxides (for example Ta8W9O47) and niobium molybdenum oxides (for example Nb2Mo3O14). The working electrode may comprise a mixture of niobium tungsten oxide and LTO. The additional active material may be graphite. The working electrode may comprise a mixture of niobium tungsten oxide and graphite. 008698920 Preferably, the working electrode consists essentially of niobium tungsten oxide and an additional active material. For example, the working electrode consists essentially of a mixture of niobium tungsten oxide and LTO. Alternatively, the working electrode may consist essentially of a mixture of niobium tungsten oxide and graphite. The working electrode may additionally comprise reduced graphene oxide, Ketjen black or Super P carbon. Preferably, the working electrode comprise a niobium tungsten oxide in particulate form. To improve conductivity at the working electrode, a conductive carbon material (e.g., carbon black, graphite, nanoparticulate carbon powder, carbon fiber and / or carbon nanotubes) is typically admixed with the working electrode material. Alternatively, the conductive carbon material may be coated onto the working electrode material. In one embodiment, the working electrode comprises porous carbon, such as porous reduced graphene oxide, which may wrap the larger niobium oxides particles. Typically, the working electrode contains 1-5% by weight of binders. The working electrode may be admixed with a binder. Some examples of binders include PVDF, PTFE, CMC, PAA, PMMA, PEO, SBR and co-polymers thereof. The working electrode is typically fixed to a current collector, such as a copper or aluminum collector, which may be in the form of a plate. In a specific embodiment the working electrode comprising a particulate niobium tungsten oxide using a standard electrode configuration of 8:1:1 active material / carbon / binder with a 2-3 mg·cm2loading of active material and a 1.27 cm2electrode area against a lithium counter electrode and using 1.0 M LiPF6in ethylene carbonate / dimethyl carbonate as electrolyte. The electrochemical cell typically comprises a counter electrode. The counter electrode may be an anode or cathode during a discharge step, for example in a lithium ion battery. Preferably the counter electrode is the cathode during a discharge step. In addition to the working electrode, an electrochemical cell comprises a counter electrode and an electrolyte, and optionally a separator, such as a microporous polyethylene film, between the working electrode and counter electrode. Suitable materials for the counter electrode include lithium-containing or lithium-intercalated material, such as a lithium metal oxide, wherein the metal may be a transition metal such as Co, Fe, Ni, V, or Mn, or combination thereof. Some examples of counter electrode materials include lithium cobalt oxide (LiCoO2) lithium nickel manganese cobalt oxide (NMC, 008698920 LiNiMnCoO2, e.g., LiNi0.6Co0.2Mn0.2O2), lithium vanadium fluorophosphate (LiVPO4F), lithium nickel cobalt aluminium oxide (NCA, LiNiCoAl2), lithium iron phosphate (LFP, LiFePO4) and manganese-based spinels (e.g. LiMn2O4). In one embodiment, the counter electrode is substantially free of binders. In an alternative embodiment, the counter electrode is admixed with a binder or adhesive. Some examples of binders or adhesives include PVDF, PTFE, CMC, PAA, PMMA, PEO, SBR and co-polymers thereof. The counter electrode may be fixed to a current collecting substrate, such as an aluminium plate. The electrochemical cell typically comprises an electrolyte. The electrolyte facilitates ion transport between the cathode and anode. The electrolyte comprises lithium salts, such as lithium (bis(trifluoromethane)sulfonimide (LiTFSI), LiPF6, LiBF4, LiCIO4, lithium triflate (LiTF), or lithium bis(oxalate)borate (LiBOB). The electrolyte may be a liquid electrolyte, such as a liquid at ambient temperature, for example at 25°C. The electrolyte may be a non-aqueous electrolyte. The electrolyte may comprise a polar aprotic solvent, such a cyclic or linear carbonate, such as ethylene carbonate, dimethyl carbonate, or ethyl methyl carbonate. Suitable solvents include carbonate solvents. For example propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), chloroethylene carbonate, fluorocarbonate solvents (e.g., fluoroethylene carbonate and trifluoromethyl propylene carbonate), as well as the dialkylcarbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC). Suitable solvents also include sulfone solvents. For example methyl sulfone, ethyl methyl sulfone, methyl phenyl sulfone, methyl isopropyl sulfone (MiPS), propyl sulfone, butyl sulfone, tetramethylene sulfone (sulfolane), phenyl vinyl sulfone, allyl methyl sulfone, methyl vinyl sulfone, divinyl sulfone (vinyl sulfone), di phenyl sulfone (phenyl sulfone), dibenzyl sulfone (benzyI sulfone), vinylene sulfone, butadiene sulfone, 4-methoxyphenyl methyl sulfone, 4- chlorophenyl methyl sulfone, 2-chlorophenyl methyl sulfone, 3,4-dichlorophenyl methyl sulfone, 4-(methylsulfonyl)toluene, 2-(methylsulfonyl) ethanol, 4-bromophenyl methyl sulfone, 2-bromophenyl methyl sulfone, 4-fluorophenyl methyl sulfone, 2-fluorophenyl methyl sulfone, 4-aminophenyl methyl sulfone, a sultone (e.g., 1,3-propanesultone), and sulfone solvents containing ether groups (e.g., 2-methoxyethyl(methyl)sulfone and 2- methoxyethoxyethyl(ethyl)sulfone). Suitable solvents also include silicon-containing solvents such as a siloxane or silane. For example, hexamethyldisiloxane (HMDS), 1,3-divinyltetramethyldisiloxane, the polysiloxanes, and polysiloxane-polyoxyalkylene derivatives. Some examples of silane solvents include methoxytrimethy lsilane, ethoxytrimethy lsilane, dimethoxydimethylsilane, methyltrimethoxysilane, and 2-(ethoxy)ethoxytrimethylsilane. 008698920 Typically, an additive may be included in the electrolyte to improve performance. For example vinylene carbonate (VC), vinyl ethylene carbonate, allyl ethyl carbonate, t-butylene carbonate, vinyl acetate, divinyl adipate, acrylic acid nitrile, 2-vinyl pyridine, maleic anhydride, methyl cinnamate, ethylene carbonate, halogenated ethylene carbonate, α-bromo-γ- butyrolactone, methyl chloroformate, 1,3-propanesultone, ethylene sulfite (ES), propylene sulfite (PS), vinyl ethylene sulfite (VES), fluoroethylene sulfite (FES), 12-crown-4 ether, carbon dioxide (CO2), sulfur dioxide (SO2), and sulfur trioxide (SO3). The electrochemical cell may also include a separator, such as a solid porous membrane positioned between the working and counter electrodes. The solid porous membrane may partially or completely replace the liquid electrolyte. The solid porous membrane may comprise a polymer (e.g., polyethylene, polypropylene, or copolymer thereof) or an inorganic material, such as a transition metal oxide (e.g., titania, zirconia, yttria, hafnia, or niobia) or main group metal oxide, such as silicon oxide, which can be in the form of glass fiber. The solid non-porous membrane may comprise a lithium-ion conductor. For example, LLZO (garnet family), LSPO (LISICON family), LGPS (thio-LISICON family), LATP / LAGP (NASICON family), LLTO (perovskite family) and phosphide / sulfide glass ceramics. Use In another aspect there is provided a use of a control module of the fourth aspect for controlling a DC-DC buck-boost converter, wherein: when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; when the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, when the voltage demand requires the DC-DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%. In a further aspect there is provided a use of the buck-boost converter system of the fifth aspect, for controlling the DC-DC buck-boost converter. In another aspect there is provided a use of the buck-boost converter system of the fifth aspect, for controlling power transfer between a first DC source and a second DC source using a DC-DC buck-boost converter system as described in the first aspect, comprising: transferring power between the first DC source and the second DC source using the DC-DC buck-boost converter, and 008698920 controlling the rate of power transfer using the control module, by controlling the relative voltage of the first DC-source compared to the second DC-source, using a method according to the first aspect. In another aspect there is provided a use of the buck-boost converter system of the fifth aspect, for transitioning a DC-DC buck-boost converter between a step-up mode and a step- down mode, comprising transitioning from a step-down mode to a step-up mode, wherein the first PWM signal duty cycle is increased from less than 100% to 100% while the second PWM signal maintains a duty cycle of 0%, and subsequently the second PWM signal duty cycle is increased from 0% to greater than 0% while the first PWM signal maintains a duty cycle of 100%; or transitioning from a step-up mode to a step-down mode, wherein the second PWM signal duty cycle is decreased from greater than 0% to 0% while the first PWM signal maintains a duty cycle of 100%, and subsequently the first PWM signal duty cycle is decreased from 100% to less than 100% while the second PWM signal maintains a duty cycle of 0%. The methods and systems described herein may be used to control power transfer from a battery of a mobile charging device, a static charging device, an electric vehicle or a power tool. Preferably, the methods and systems may be used to control power transfer from the battery of an electric vehicle or a power tool. The methods and systems described herein may be used to improve the efficiency of a buck- boost converter in a transition region. In particular, the invention may be used to improve the efficiency at high currents and / or high-rates, such as a C-rate of 3C or more, preferably 5C or more, more preferably 10C or more. The methods and systems described herein may be used to improve the efficiency of the DC- DC buck-boost converter. The use may be when the voltage demand requires the DC-DC buck-boost converter to maintain voltage. The methods and systems described herein may be used to reduce switching in a buck-boost converter in a transition region. A reduction in switching may refer to a reduced rate of switching, based on all switches in the buck-boost converter. A reduction in switching may refer to a lower total number of switches operating at a time in the buck-boost converter. Examples The following examples are provided to further illustrate the present invention and are not intended to limit the scope of the invention. 008698920 Figure 1A and 1B have been partially described in the Background but will now be described in further detail. Figure 1A shows a four-switch bi-directional buck-boost converter. Figure 1B shows a two-switch buck-boost converter, which is typically mono-directional. In Figures 1A and 1B, the circuit comprises two Phase Legs: Phase Leg A

[0013] and Phase Leg B

[0014] . Each Phase Leg [13, 14] comprises a connection to a device [11, 12] which may be a source or a sink, since a circuit arranged as shown in Figure 1A may be bidirectional. In Figure 1A, each phase leg [13, 14] comprises two switches, with an inductor 17 connected between the switches, to a form first and second half-bridges. The first half bridge is shown by Phase Leg A

[0013] with the first high-side switch A, and the second half bridge is shown by Phase Leg B

[0014] with the second low-side switch B.̅ In addition, the first half-bridge includes a low-side switch Ā, and the second half bridge includes a high-side switch B. The switches in either phase leg are never switched on simultaneously, as the switches are switched synchronously. A and Ā are never both on or off simultaneously. B and B̅ are never both on simultaneously. Although the two switches in a single phase leg will never be simultaneously closed, though they may be simultaneously open where the system is not active and transferring power. They are all shown open in the Figure for clarity. When Switch A is “on” (and Switch Ā is “off”) the voltage at the midpoint of A and Ā

[0015] is the full voltage provided by Source / Sink A

[0011] . When Switch A is “off” (and Switch Ā is “on”) the voltage at the midpoint

[0015] is 0. Controlling the relative “on times” of Switch A allows the average voltage at their midpoint

[0015] to be accurately controlled from 0 up to the Source / Sink A voltage. Increasing the on time of Switch A increases the voltage at

[0015] . Phase Leg B

[0014] operates similarly. When Switch B is “on” (and Switch B is “off”), the voltage at the midpoint of B and B

[0016] is the same as the voltage at

[0015] . Decreasing the on time of Switch B increases the voltage at

[0016] . Controlling the relative “on times” of Switches B and B allows the average voltage at their midpoint

[0016] to be accurately increased (or stepped up). The inductor

[0017] between the phase legs buffers current flow to force a net zero average voltage across the inductor, so the current flow will be such that the net power flow is from the side with the higher average voltage at the midpoint [15 / 16] even if the difference is only slight. The behaviour of the circuit is controlled by the behaviour of the switches in each phase leg [13, 14], which set the voltage at the midpoint [15 / 16] of each phase leg [13, 14], i.e. at the point where each phase leg [13, 14] is connected to the other via the inductor

[0017] . Considering Phase Leg A

[0013] , when Switch A is on (and Switch Ā is off), the voltage at the 008698920 midpoint

[0015] of Phase Leg A

[0013] is the full voltage provided by Source / Sink A

[0011] . When Switch Ā is on (and Switch A is off) the voltage at the midpoint

[0015] is 0. Controlling the relative “on times” of switches A and Ā using a demand signal allows the average voltage at the midpoint

[0015] of Phase Leg A

[0013] to be accurately controlled. Phase Leg B operates similarly and controlling the relative “on times” of switch B̅ and B using a demand signal allows the average voltage at the midpoint

[0016] of Phase Leg B

[0014] to be accurately controlled. The inductor

[0017] stores energy in a magnetic field and acts as a filter to the high frequency switching waveforms, allowing the system to react to the average voltage at each midpoint [15 / 16], produced as described above, rather than reacting from moment to moment. The inductor

[0017] will, by its nature, support current flow to force a net zero average voltage across it. If the average voltage at the midpoint

[0015] of Phase Leg A

[0013] is greater than the average voltage at the midpoint

[0016] of Phase Leg B

[0014] by an amount sufficient to overcome the parasitic resistance of the inductor

[0017] and associated circuitry then DC inductor current flow will be such that net power flow is from Phase Leg A

[0013] to Phase Leg B

[0014] . If the average voltage at the midpoint

[0015] of Phase Leg A

[0013] is less than the average voltage at the midpoint

[0016] of Phase Leg B

[0014] by an amount sufficient to overcome the parasitic resistance of the inductor

[0017] and associated circuitry then DC inductor current flow will be such that net power flow is from Phase Leg B

[0014] to Phase Leg A

[0013] . This control of current and direction of power flow is possible irrespective of the relative voltages of the Source / Sink A

[0011] and Source / Sink B

[0012] . In Figure 1B, each phase leg [13, 14] comprises a switch and a diode, with the inductor

[0017] connected between, to a form the first and second half-bridges. The first half bridge is shown by Phase Leg A

[0013] with the first high-side switch A and a first diode D1, and the second half bridge is shown by Phase Leg B

[0014] with the second low-side switch B̅ and a second diode D2. The two-switch buck-boost converter is operated as described above for the four-switch converter of Figure 1A except that only the switches A and B̅ are operated to control the voltage conversion. However, these conventional systems are flawed in the case where the input voltage from the source and the output voltage to the sink are similar: a situation that often arises during battery charging, in which a battery acts as the sink. As a battery is charged, its voltage rises and may surpass that of the source, causing the buck-boost converter to switch from “buck” mode, in which it lowers the voltage supplied to the sink, to “boost” mode, in which it raises the voltage. During this transition, the switches in both phase legs will be operational, causing a loss of efficiency. 008698920 Figure 2 shows a high-level view of an example system comprising two devices: Device A

[0021] and Device B

[0022] . Each device [21, 22] contains a battery [24, 26] and a battery controller [25, 27]. The devices [21, 22] are connected to a charging device

[0023] which contains the system of the invention: a control module

[0029] and a buck-boost converter

[0028] . The batteries [24, 26] in the devices [21, 22] are connected to the buck-boost converter

[0028] in the charging device

[0023] . In the example shown in Figure 2, the battery

[0024] in Device A

[0021] acts as a source and the battery

[0026] in Device B

[0012] acts as a sink. Accordingly, the power flows from Device A

[0011] to Device B

[0012] across the charging device, as indicated by the direction of the arrows in Figure 2. In another example, this power flow could be reversed. The battery controllers [25, 27] in the devices [21, 22] are connected to the control module

[0029] in the charging device

[0023] to provide appropriate system variables such as the health and state of charge of their internal batteries [24, 26]. Solid lines indicate electrical power transfer connections and dashed lines indicate signalling connections. Figure 3 shows a more detailed block diagram of the components of the charging device

[0023] . The internal batteries [24, 26] of the two devices [21, 22] (Battery A and Battery B) and the control module

[0029] are shown connected to the buck-boost converter

[0028] as mentioned with reference to Figure 2. Again, dashed lines indicate signalling connections and solid lines indicate electrical power transfer connections. The buck-boost converter

[0028] comprises two Phase Legs [13, 14] such as those described in Figure 1A and 1B. Battery A

[0024] is connected to Phase Leg A

[0013] and, as in the example shown in Figure 2, acts as a source such that electrical power flows from Battery A

[0024] to Phase Leg A

[0013] , then to Phase Leg B

[0014] and to Battery B

[0026] . Accordingly, Phase Leg A

[0013] is connected to Phase Leg B

[0014] and Phase Leg B

[0014] is connected to Battery B

[0026] . The buck-boost converter

[0028] further comprises two modulator blocks, Modulator A

[0031] and Modulator B

[0032] , which control the demand inputs to the phase legs [13, 14] that determine the duty cycles of the switches. Accordingly, they have signalling connections to the control module

[0029] and the phase legs [13, 14]. Figure 4 shows a detail of the modulators [31, 32] and their control mechanisms. As previously described, the battery controllers [25, 27] in Device A

[0021] and Device B

[0023] are connected to the control module

[0029] in the intermediary block

[0023] and supply system variables to it. The control module

[0029] is connected to the two modulation blocks [31, 32], which are in turn connected to the phase legs [13, 14] such that Modulation Block A

[0031] is connected to Phase Leg A

[0013] and Modulation Block B

[0032] is connected to Phase Leg B

[0014] . The charging device

[0023] also contains a correction input

[0041] and combiner

[0042] connected to the modulation block

[0032] that controls Phase Leg B

[0014] (Modulation Block B). The control module

[0029] receives system variables from the battery controllers [25, 27] in the devices [21, 22]. These variables can include, for example, the voltages of their respective 008698920 batteries [24, 26], the state of charge of the respective batteries [24, 26], the temperatures of the devices [21, 22], any fault conditions, etc. The control module

[0029] may use these variables to generate a voltage demand signal indicating the rate at which power should be transferred through the buck-boost converter

[0028] , (e.g., a target current) which is determined by the voltages at the midpoints [15, 16] of the phase legs [13, 14], as previously described. The control module

[0029] then outputs the demand signal to the modulation blocks [31, 32]. Alternatively, the demand signal may be a further variable supplied to the control module

[0029] by an external engine not shown in Figure 4. The modulation blocks [31, 32] each comprise a saturation block [43, 44] and a modulator [45, 46]. The saturation block [43 / 44] is arranged to limit the range of values reaching its respective modulator [45 / 46] to a threshold range. For example, the range may be between 0 and 1, but it may also be between 0 and 100 or any other appropriate value. The upper limit of the threshold range corresponds to a duty cycle of 100%. The lower limit of the threshold range corresponds to a duty cycle of 0%. This limitation is applied to the demand signal received from the control module

[0029] , so that, for example, where the saturation block [43 / 44] applies a range of 0 to 1: ^ The saturation block [43 / 44] receives a demand value of 0.5; no change is required and the modulator [45 / 46] receives a value of 0.5. ^ The saturation block [43 / 44] receives a demand value of 1.5; the value passed to the modulator [45 / 46] is limited to 1. The modulated demand value is then used to generate a PWM signal for each of the phase legs [13, 14]. The PWM signal has a duty signal corresponding to the modulated demand value. For example, if the modulated demand value is 0.5, then the duty cycle is 50%. If the modulated demand value is 1, then the duty cycle is 100%. If the modulated demand value is 0, then the duty cycle is 0%. The PWM signal is then used to control the switches in the respective phase legs [13, 14] as previously described. The correction input and combiner are used to amend the demand signal supplied to the modulation block

[0032] that controls Phase Leg B

[0014] . Accordingly, the correction input

[0041] comprises a value equal to the additive inverse of the maximum value of the range used by the saturation blocks [43, 44]. For example, if the range applied by the saturation blocks [43, 44] is 0 to 1, the value used for the correction input

[0041] is -1 and if the range applied by the saturation blocks [43, 44] is 0 to 100 the value used for the correction input

[0041] is -100. In either case, the magnitude of the correction input

[0041] , e.g. -1 or -100, is that which would result in full modulation depth of the phase leg. This value is added to the demand signal supplied to the modulation block

[0032] connected to Phase Leg B

[0014] to amend the demand signal before saturation, so the demand signals 008698920 supplied to the phase legs [13, 14] are different. For example, in a system that uses a range from 0 to 1: ^ The control module

[0029] outputs a demand value of 0.5. Modulation Block A

[0031] receives a demand value of 0.5 which is not altered by the saturation block

[0043] , as previously described, but is used unchanged as the demand signal for Phase Leg A

[0013] ; Modulation Block B

[0032] receives -0.5, but this value is limited to 0 by its saturation block

[0044] and this is the value used for Phase Leg B

[0014] . ^ The control module

[0029] outputs a demand value of 1. Modulation Block A

[0031] receives a demand value of 1 and this is used unchanged as a demand signal for Phase Leg A

[0013] ; Modulation Block B

[0032] receives a demand value of 0 and this is used unchanged as a demand signal for Phase Leg B

[0014] . ^ The control module

[0029] outputs a demand value of 1.5. Modulation Block A

[0031] receives a value of 1.5, but this value is limited to 1 by its saturation block

[0043] and this is the value passed to the modulator

[0045] for use as a demand signal for Phase Leg A

[0013] ; Modulation Block B

[0032] receives a demand value of 0.5 and this is used unchanged as a demand signal for Phase Leg B

[0014] . In each modulation block [31, 32], the modulator [45, 46] converts the demand signal to a PWM signal for its respective phase leg [13, 14], producing a result as described with reference to Figure 1A. The inclusion of the correction input

[0041] and combiner

[0042] allows the same demand signal output by the control module

[0029] to be used for both phase legs [13, 14] and also eliminates overlap between the switching behaviour as elaborated in Figure 5. Figure 5 shows the process of the system in a use case where Source / Sink A

[0011] and Source / Sink B

[0012] are both batteries and Source / Sink A (Battery A) [11 / 24] is being used to charge Source / Sink B (Battery B) [12 / 26], similarly to the situation described in Figure 2. The reference numbers from Figure 1A and the reference numbers from Figure 2 will be used together. In this part of the description, the following shorthand will be used: ^ “VbattA” means the voltage of Battery A [11 / 24] ^ “VbattB” means the voltage of Battery B [12 / 26] ^ “VmidA” means the average voltage at the midpoint

[0015] of Phase Leg A

[0013] ^ “VmidA” means the average voltage at the midpoint

[0016] of Phase Leg B

[0014] The figures for time periods and voltages in this part of the description are chosen for clarity and in practice these values and the differences between them may be significantly different, for example a difference in voltage of less than 1V. 008698920 At Step S51, the charging process begins and VbattA is greater than VbattB. The control module

[0029] in the charging device

[0023] receives information from the battery controllers [25, 27] in the respective devices [21, 22] indicating the current voltages of the batteries [11 / 24, 12 / 26] and use this information to generate a single demand signal. In a system such as that in the example used so far, the control module

[0029] generates a demand signal below 1. For example, 0.3. The single demand signal may Instead be generated by a further engine external to the control module

[0029] , in which case the control module

[0029] will simply receive such demand signal and use it as described below. At Step S52, the demand signal generated or received by the control module

[0029] is converted to switching signals that control the behaviour of the phase legs [13, 14]. The demand signal is passed directly to Modulation Block A

[0031] , but before it is passed to Modulation Block B

[0032] the value of -1

[0041] is added to it, as previously described, leading to a value of -0.7. The demand signal sent to Modulation Block A

[0031] is unchanged by the saturation block

[0043] and the modulator

[0045] converts it into switching signals that give a duty cycle of 0.3 for Switch A. This means that Switch A is “on” for 3µs in each ten microseconds and off for the remaining 7µs, when Switch Ā is “on”. If VbattA is 100V, VmidA will be 30V. The demand signal sent to Modulation Block B

[0032] is amended to 0 by the saturation block

[0044] and the modulator

[0046] converts it into a switching signal that keeps Switch Ḃ off constantly and Switch B on constantly. This means that Phase Leg B is not switching and VmidB will be equal to VbattB – say, 0V. This means that current will flow across the inductor

[0017] and therefore Battery A [11 / 24] will discharge and Battery B [12 / 26] will be charged. At Step S53, the control module

[0029] determines based on the signals from the connected devices [21 / 22] whether VbattA is equal – or similar within a predetermined tolerance – to VbattB. If not, and therefore VbattA is still greater than VbattB, it returns to Step S51 and these three steps are repeated with appropriate alterations to the demand signal produced by the control module

[0029] to maintain an appropriate ratio between VmidA and VmidB. Alternatively, the control module may determine whether VbattA is equal to VbattB based on the demand signal generated by an external engine as mentioned at Step S51. For example, it may determine that VbattA is greater than VbattB based on whether the demand signal equals – or is within a predetermined tolerance of – 1. If VbattA is equal to VbattB, the process progresses to Step S54. 008698920 At Step S54, the control module

[0029] produces or passes on a demand signal of 1. As previously described, this is passed directly to Modulation Block A

[0031] , which produces switching signals that give a duty cycle of 1 for Switch A and therefore a duty cycle of 0 for Switch Ā. This means that Switch A is constantly “on” and Switch Ā is constantly “off” and therefore Phase Leg A

[0013] does not carry out any switching. The demand signal produced or received by the control module

[0029] is also passed to Modulation Block B,

[0032] but as previously described the combiner adds the value of -1 to it. This results in a value of 0 being passed to Modulation Block B

[0032] , meaning that there continues to be no switching in Phase Leg B

[0014] . Accordingly, VmidA will be equal to the value of VbattA and VmidB will still have a value of VbattB, leading to a continued flow of current across the inductor, so Battery A [11 / 24] continues to charge Battery B [12 / 26]. At Step S55, the control module

[0029] determines whether VbattA is now less than VbattB. If not, the process returns to Step S54 and that part of the process is repeated until VbattA is less than VbattB. When VbattA is less than VbattB, the process proceeds to Step S56. At Step S56, the control module

[0029] produces – or receives as previously described – a demand signal above 1. For example, 1.3. This value is passed unchanged to Modulation Block A

[0031] as previously described and is also passed to the combiner

[0042] , which adds the value of -1 and produces a demand signal of 0.3, which is then passed to Modulation Block B

[0032] for use in generating switching signals for Phase Leg B

[0014] . When Modulation Block A

[0031] receives the value 1.3, it is reduced to value 1 by the saturation block

[0043] and Modulation Block A

[0031] continues to produce switching signals leading to a duty cycle of 1 for Switch A and 0 for Switch Ā. This means that phase leg A does not carry out any switching . If VbattA is 45V, VmidA will also be 45V. When Modulation Block B

[0032] receives the value 0.3, it is not affected by the saturation block

[0044] and Modulation Block B

[0032] produces switching signals leading to a duty cycle of 0.3 for Switch Ḃ and 0.7 for Switch B of Phase Leg B

[0014] . This means that if VbattB is, for example, 55V, VmidB will be 38.5V. As a result, regardless of the voltages of the two batteries [11 / 24, 12 / 26], VmidB continues to be less than VmidA and therefore current continues to flow across the inductor

[0017] ; Battery A [11 / 24] continues to charge Battery B [12 / 26]. The values given in this description are examples only and the difference between VmidA and VmidB may be very slight, depending on the design of the inductor and the parasitic resistance of the circuitry. 008698920 The modulation blocks [31, 32], combiner

[0042] , and controller may be separate engines as suggested by the Figures, or they may be embodied in one or more software modules, or a combination of such embodiments may be used. Experimental Results Figure 6 shows oscilloscope plots of the switching behaviour of the phase legs [13, 14] in a current-art system: the LM5118 Wide Voltage Range Buck-Boost Controller from Texas Instruments®. The upper trace represents the behaviour of Phase Leg A

[0013] (the first half bridge) and the lower trace represents the behaviour of Phase Leg B

[0014] (the second half bridge). For clarity, the two traces are shown on different Y axis scales, representing the midpoint voltage values of the respective phase legs. Phase Leg B

[0014] uses the values as written on the chart while Phase Leg A

[0013] uses a zero point at 40V. Throughout the process, the control module

[0029] outputs a required output voltage of 12V at a current of 1A. Figure 6A shows the known system’s behaviour with an input of 6V. As can be seen from the plots, the midpoint voltages of both phase legs [13, 14] are rapidly changing; as previously mentioned, this is due to the behaviour of the switches and indicates that both phase legs [13, 14] are switching simultaneously. Figure 6B shows the known system’s behaviour with an input of 12V. As can be seen from the plots, the midpoint voltages of both phase legs [13, 14] continue to rapidly change. As previously mentioned, this means that both phase legs [13, 14] are switching simultaneously. Figure 6C shows the known system’s behaviour with an input of 16V. As can be seen from the plots, the midpoint voltage

[0063] of Phase Leg A

[0013] is rapidly changing, indicating that it is switching, while the midpoint voltage

[0062] of Phase Leg B

[0014] remains steady, indicating that it has stopped switching. The system of the invention avoids some of this excess switching, resulting in greater energy efficiency. This is shown in Figure 7. Figure 7 shows an oscilloscope plot of the switching behaviour of the phase legs [13, 14] in a system according to the invention. Here, both plots are shown on a single scale and measurements are taken over a longer period of time, resulting in a more concentrated view of the switching waveforms since the system used a switching period of 20µs. The lighter- coloured plot

[0071] shows the switching behaviour of Phase Leg A

[0013] (high-side switch of the first half bridge) and the darker-coloured plot

[0072] shows the switching behaviour of Phase Leg B

[0014] (low-side switch of the second half bridge). The single line below the two plots

[0073] shows the output current, which remains steady at 10A across the transition region. In this experiment, the input voltage was reduced manually to force the system to transition from switching in one leg to the other. During the first phase

[0074] , Phase Leg A

[0013] is 008698920 switching and Phase Leg B

[0014] is not. The buck-boost converter is therefore operating in the step-down (buck) mode. If Phase Leg B

[0014] were switching as in Figure 6 with the current-art system, the two waveforms would be indistinguishable. During the second phase

[0075] , neither phase leg [13, 14] is switching. The buck-boost converter is therefore operating in the maintaining mode. This is shown by the fact that there is very little activity shown in the plot during this section. During the third phase

[0076] , only Phase Leg B

[0014] is switching. The buck-boost converter is therefore operating in the step-up (boost) mode. During the transitions there is no change to the output current

[0073] shown in the plot. In a current-art system, if there were no overlap between switching in the phase legs [13, 14] there would be a visible change in the current output when the system transitions between phases. The system of the invention only requires one phase leg [13, 14] to be switching at a time, and as a result the system of the invention has a greater conversion efficiency than a comparable current-art system, while also providing excellent current control. The system of the invention is equally responsive and controllable and does not result in any interruption in the power supplied. Other Preferences Each and every compatible combination of the embodiments described above is explicitly disclosed herein, as if each and every combination was individually and explicitly recited. Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure. “and / or” where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example “A and / or B” is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein. Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described. Certain aspects and embodiments of the invention will now be illustrated by way of example and with reference to the figures described above. References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these 008698920 references are provided below. The entirety of each of these references is incorporated herein. (1) https: / / www.ti.com / lit / ds / symlink / lm5118.pdf. (2) US 2023 / 0144906 008698920

Claims

Claims 1. A method of controlling a DC-DC buck-boost converter system, the DC-DC buck-boost converter system comprising: a DC-DC buck-boost converter comprising a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first pulse-width modulation (PWM) signal, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges; and a control module for controlling the DC-DC buck-boost converter, the control module comprising a PWM signal generator; the method comprising: generating the first PWM signal and the second PWM signal with the signal generator based on a voltage demand, wherein the voltage demand determines if the DC-DC buck- boost converter steps-up, steps-down, or maintains the voltage of the first DC source relative to the second DC source, wherein: when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; when the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, and when the voltage demand requires the DC-DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%.

2. The method of claim 1, wherein the step of generating the first PWM signal and the second PWM signal from the voltage demand comprises: calculating a first demand signal and a second demand signal, wherein the first and the second demand signals differ by at least a threshold value equal to the difference between a minimum threshold value and a maximum threshold value, wherein the minimum threshold value corresponds to a duty cycle of 0% and the maximum threshold value corresponding to a duty cycle of 100%; generating a first PWM signal from the first demand signal; and generating a second PWM signal from the second demand signal.

3. The method of claim 2, wherein the calculating step comprises: comparing the first demand signal and the second demand signal to both the minimum threshold value and the maximum threshold value; and 008698920if the first and / or second demand signal is greater than a maximum threshold value, setting the demand signal to the maximum threshold value, if the first and / or second demand signal is less than a minimum threshold value, setting the demand signal to the minimum threshold value, and if the first and / or second demand signal is between the minimum threshold value and the maximum threshold value, leaving the demand signal unchanged.

4. The method of either claim 2 or claim 3, where in the calculating step, the first or second demand signal corresponds to the voltage demand and the other of the first or second demand signal corresponds to the difference between the voltage demand and the threshold value.

5. The method of any one of claims 1 to 4, wherein the step-down in voltage is proportional to the duty cycle of the first PWM signal at all duty cycles, and the step-up in voltage is proportional to the duty cycle of the second PWM signal at all duty cycles.

6. A method of controlling power transfer between a first DC source and a second DC source using a DC-DC buck-boost converter system as described in claims 1 to 5; the method comprising: transferring power between the first DC source and the second DC source using the DC-DC buck-boost converter, and controlling the rate of power transfer using the control module, by controlling the relative voltage of the first DC-source compared to the second DC-source using a method according to any one of claims 1 to 5.

7. The method of claim 6, further comprising the steps of: sending status data on the power transfer between the first DC source and the second DC source to a receiver of the control module; and adjusting the voltage demand based on the status data.

8. The method of claim 7, wherein the status data comprises status data on the rate of the power transfer between the first DC source and second DC source, such as current and / or voltage supplied to or delivered from the first DC source or second DC source.

9. The method of claim 7 or claim 8, wherein the step of adjusting the voltage demand comprises: comparing the status data to a target range; and adjusting the voltage demand value in response to the comparison of the status data to the target range, such that subsequent status data is maintained within the target range or is moved into the target range.

10. The method of claim 9 wherein the target range is a pre-set range. 00869892011. The method of claim 9 wherein the target range is a variable target range, and the variable target range is provided by the first and / or the second DC source, such as by a battery management system of an electric vehicle.

12. The method of claim 11, wherein a first variable target range is provided by the first DC source and a second variable target range is provided by the second DC source, and the target range used is equal to the region of overlap between the first and second variable target range.

13. The method of any one of claims 7 to 12, wherein the step of adjusting the voltage demand based on the status data uses a control loop, such as a PID loop.

14. The method of any one of claims 7 to 13, wherein the adjustment step is carried out on a controller programmed with a suitable algorithm.

15. The method of any one of claims 7 to 14, wherein the adjustment step is repeated at a rate of 20 kHz or more, preferably from 20 to 100 kHz, more preferably from 40 to 60 kHz, yet more preferably about 50 kHz.

16. A method of transitioning a DC-DC buck-boost converter between a step-up mode and a step-down mode using a DC-DC buck-boost converter system as described in any one of claims 1 to 5; the method comprising: controlling the DC-DC buck-boost converter according to the method of any one of claims 1 to 5, and transitioning from a step-down mode to a step-up mode, wherein the first PWM signal duty cycle is increased from less than 100% to 100% while the second PWM signal maintains a duty cycle of 0%, and subsequently the second PWM signal duty cycle is increased from 0% to greater than 0% while the first PWM signal maintains a duty cycle of 100%; or transitioning from a step-up mode to a step-down mode, wherein the second PWM signal duty cycle is decreased from greater than 0% to 0% while the first PWM signal maintains a duty cycle of 100%, and subsequently the first PWM signal duty cycle is decreased from 100% to less than 100% while the second PWM signal maintains a duty cycle of 0%.

17. The method of claim 16, wherein during the transition: (i) when the first PWM signal duty cycle is a duty cycle of less than 100%, the duty cycle is greater than 80 % and less than 100%, preferably greater than 90% and less than 100%, more preferably greater than 95% and less than 100%, yet more preferably greater than 99% and less than 100%; and / or (ii) when the first PWM signal duty cycle is a duty cycle of greater than 0%, the duty cycle is greater than 0 % and less than 20%, preferably greater than 0% and less than 10%, 008698920more preferably greater than 0% and less than 5%, yet more preferably greater than 0% and less than 1%.

18. A control module for controlling a DC-DC buck-boost converter, the DC-DC buck-boost converter comprising: a first half-bridge connectable to a first DC source, the first half-bridge comprising a high-side first switch controllable by a first pulse-width modulation (PWM) signal, a second half-bridge connectable to a second DC source, the second half-bridge comprising a low-side second switch controllable by a second PWM signal, and an inductor for buffering electrical energy wherein the inductor electrically connects the first and second half bridges, the control module comprising: a signal generator, wherein the signal generator is configured to generate the first PWM signal and the second PWM signal based on a voltage demand, wherein the voltage demand determines if the DC-DC buck-boost converter steps-up, steps-down, or maintains the voltage of the first DC source relative to the second DC source, wherein: when the voltage demand requires the DC-DC buck-boost converter to step-down voltage, the first PWM signal has a duty cycle less than 100%, and the second PWM signal has a duty cycle of 0%; when the voltage demand requires the DC-DC buck-boost converter to maintain voltage, the first PWM signal has a duty cycle of 100% and the second PWM signal has a duty cycle of 0%, when the voltage demand requires the DC-DC buck-boost converter to step-up voltage, the first PWM signal has a duty cycle of 100%, and the second PWM signal has a duty cycle of greater than 0%.

19. The control module of claim 18, wherein the control module further comprises: a receiver configured to receive status data on the power transfer between the first DC source and second DC source, and a controller configured to adjust the voltage demand in response to the status data.

20. The control module of claim 19, wherein the controller comprises a microcontroller programmed with an algorithm to carry out the adjustment to the voltage demand.

21. A DC-DC buck-boost converter system comprising: the control module of any one of claims 18 to 20, and a DC-DC buck-boost converter as described in claim 18.

22. A charging device comprising: the DC-DC buck-boost converter system of claim 21, a first DC source, wherein the first DC source is in electrical connection with the first half-bridge, and 008698920a power connector for releasably connecting the DC-DC buck-boost converter system to a second DC source, wherein the power connector is in electrical connection with the second half-bridge of the DC-DC buck-boost converter system.

23. The charging device of claim 22, wherein the first DC source comprises an electrochemical cell, and preferably the second DC source comprises an electrochemical cell, such as the electrochemical cell of an electric vehicle.

24. The charging device according to claim 23, wherein the first DC source comprises an electrochemical cell having a maximum operable charge and / or discharge rate of 1C or more, preferably 3C or more, more preferably 5C or more, yet more preferably 10C or more, even more preferably 20C or more.

25. The method of claims 1 to 17, the control module of claim 18 to 20, the DC-DC buck- boost converter system of claim 21, or the charging device of claims 22 to 24, wherein the buck-boost converter is a four-switch buck-boost converter wherein: the first half-bridge comprises the high-side first switch and a low-side synchronous switch configured to switch inversely to the high-side first switch, and the second half-bridge comprises the low-side second switch and a high-side synchronous switch configured to switch inversely to the low-side second switch.

26. A use of the control module of claim 18 to 20, the DC-DC buck-boost converter system of claim 21, or the charging device of claims 22 to 24, to improve the efficiency of the DC-DC buck-boost converter, such as to improve the efficiency of the DC-DC buck-boost converter, when the voltage demand requires the DC-DC buck-boost converter to maintain voltage. 008698920

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