Patent Description:
Buck-boost converters typically provide DC-to-DC conversion. Where AC-to-DC conversion is required, a front-end bridge rectifier and bulk capacitor are generally provided. Bridgeless buck-boost converters that provide AC-to-DC conversion are known. However, the topologies and/or the control schemes are typically complex.

<CIT> discloses a power factor correction converter having low conduction losses.

The present invention provides a buck-boost converter comprising: input terminals for connection to a power supply supplying an alternating input voltage; output terminals for outputting an output voltage; a first group of devices and a second group of devices, each group of devices comprising: a first switch, a second switch, and a third switch or diode connected in series; a first node located between the first switch and the second switch; and a second node located between the second switch and the third switch or diode; an inductor; a capacitor; and a controller for controlling the switches of the first group of devices and the second group of devices, wherein: a first of the input terminals is connected to the first node of the first group of devices; a second of the input terminals is connected to the first node of the second group of devices; the inductor is connected between the second node of the first group of devices and the second node of the second group of devices; the first group of devices, the second group of devices, and the capacitor are connected in parallel across the output terminals; the first switch of each group of devices has an OFF state in which the switch is non-conductive in both directions; and the second switch of each group has an ON state in which the switch is conductive in both directions, the third switch or diode of each group of devices provides a path for inductive current when charging the capacitor; and the controller configures the switches in a configuration in which the first switch of one group of devices is OFF, the first switch of the other group of devices is ON, and the second switches of both groups of devices are ON, thereby causing the inductor to be energised by the power supply, a further configuration in which the first and second switches of one group of devices are OFF, and the first and second switches of the other group of devices are ON, thereby causing energy stored in the inductor to be transferred to the capacitor.

The converter therefore provides AC-to-DC conversion without the need for a rectifier bridge or front-end bulk capacitor. Moreover, in comparison to other bridgeless buck-boost converters, the topology of the converter is less complex. In particular, the converter is capable of providing AC-to-DC conversion using just six devices (i.e. six switches, or four switches and two diodes), an inductor and a capacitor. Other bridgeless buck-boost converters have a higher number of switches and/or additional inductors.

The first switches have an OFF state in which the switch is non-conductive in both directions. The switches can therefore be controlled such that (i) current does not flow directly from the power supply to the capacitor when the instantaneous value of the input voltage is greater than the output voltage, and/or (ii) current does not flow from the capacitor to the power supply when the instantaneous value of the input voltage is less than the output voltage.

The second switches have an ON state in which the state is conductive in both directions. The switches are therefore capable of conducting in a first direction when the polarity of the input voltage is positive and current flows through the inductor in one direction, and are capable of conducting in a second direction with the polarity of the input voltage is negative and current flows through the inductor in an opposite direction. The second switch may comprise a controllable device that conducts in both directions when ON. Alternatively, the second switch may comprise a controllable device that conducts in one direction only when ON, but includes an antiparallel diode that conducts in the other direction.

With the topology of the converter, the second switches are primarily responsible for controlling the energisation of the inductor and the subsequent transfer of inductive energy to the capacitor. Whilst the first switches provide a path for the inductive energy, they play no part in the energisation of the inductor. The controller may therefore switch the first switches at a lower frequency than that of the second switches. This is beneficial since the first switches, which are required to have an OFF state in which they do not conduct in either direction, may be comparatively more difficult to control and/or have a lower rated switching frequency. For example, the first switches may be double gated.

The controller may switch the first switches at a frequency of the input voltage. This then has the advantage that the first switches are switched at a relatively low frequency, e.g. around <NUM> or <NUM>.

The controller may control the switches such that the first switch of the first group of devices is OFF and the first switch of the second group of devices is ON when the polarity of the input voltage is positive, and the first switch of the first group of devices is ON and the first switch of the second group of devices is OFF when the polarity of the input voltage is negative. Accordingly, only one of the first switches is ON at any one time. Moreover, each of the first switches is ON for one half-cycle of the input voltage, and OFF for the other half-cycle.

The controller may control the switches such that the second switch of the second group of devices is ON and the second switch of the first group of devices is repeatedly switched when the polarity of the input voltage is positive, and the second switch of the first group of switches is ON and the second switch of the second group of switches is repeatedly switched when the polarity of the input voltage is negative. Accordingly, during each half-cycle of the input voltage, one of the second switches is ON throughout and the other of the second switches is repeated switched between ON and OFF. One of the second switches is therefore switched at a higher frequency than that of the input voltage.

The second switches and the third switches may be the same type of switch. With the topology of the converter, the second and third switches of both groups of devices effectively form an H-bridge. Accordingly, by using the same type of switch for the second and third switches, the converter may be implemented in part using commercially available H-bridge integrated circuits, thus reducing the cost.

The controller may switch the first switches at a frequency less than <NUM>, and the second switches at a frequency greater than <NUM>. The first and second switches are therefore switched at very different frequencies. The first switches are switched at a relatively low frequency, which has the benefit of lower switching losses. However, perhaps more importantly, the first switches, which are required to have an OFF state in which they do not conduct in either direction, may be comparatively more difficult to control and/or have a lower rated switching frequency. By switching at a frequency less than <NUM>, a simpler driver and/or switches having a low rated switching frequency may be used. In contrast, by switching the second switches at a frequency greater than <NUM>, an inductor of relatively low inductance may be employed. Additionally, ripple in the input current drawn from the power supply may kept be relatively low.

The first switches may have an ON state in which the switches are conductive in both directions. This then has the advantage that zero-voltage switching may be achieved when turning ON the second switches. Additionally or alternatively, the converter may be used for bidirectional power transfer, i.e. power may be transferred from the power supply to the capacitor (forward power transfer), and power may be transferred from the capacitor to the power supply (reverse power transfer).

The controller configures the switches in a configuration in which the inductor is energised by the power supply, and a further configuration in which energy stored in the inductor is transferred to the capacitor. Moreover, the controller may switch between the configuration and the further configuration throughout each half-cycle of the input voltage. This then simplifies the control of the switches. In particular, irrespective of the instantaneous value of the input voltage, the converter uses the same mode of operation in which the switches are switched between the two configurations. The converter does not, for example, have discreet boost and buck modes of operation, with each mode having a different set of configurations.

The controller may configure the switches such that (i) in the configuration the first switch of one group of devices is OFF, the first switch of the other group of devices is ON, and the second switches of both groups of devices are ON, and (ii) in the further configuration the first and second switches of one group of devices is OFF, and the first and second switches of the other group of devices is ON.

The present invention also provides a buck-boost converter comprising: input terminals for connection to a power supply supplying an alternating input voltage; output terminals for outputting an output voltage; a first group of devices and a second group of devices, each group of devices comprising: a first switch, a second switch, and a third switch or diode connected in series; a first node located between the first switch and the second switch; and a second node located between the second switch and the third switch or diode; an inductor; a capacitor; and a controller for controlling the switches of the first group of devices and the second group of devices, wherein: a first of the input terminals is connected to the first node of the first group of devices; a second of the input terminals is connected to the first node of the second group of devices; the inductor is connected between the second node of the first group of devices and the second node of the second group of devices; the first group of devices, the second group of devices, and the capacitor are connected in parallel across the output terminals; and the controller configures the switches in (i) a configuration in which the first switch of one group of devices is OFF, the first switch of the other group of devices is ON, and the second switches of both groups of devices are ON, thereby causing the inductor to be energised by the power supply, and (ii) a further configuration in which the first and second switches of one group of devices are OFF, and the first and second switches of the other group of devices are ON, thereby causing energy stored in the inductor to be transferred to the capacitor.

Embodiments will now be described, by way of example, with reference to the accompanying drawings in which:.

The buck-boost converter <NUM> of <FIG> comprises input terminals <NUM> for connection to a power supply <NUM> supplying an alternating input voltage VIN, and output terminals <NUM> for outputting an output voltage VOUT. The converter <NUM> further comprises an input filter <NUM>, a first group of devices <NUM>, a second group of devices <NUM>, an inductor L1, and capacitor C1, a gate driver <NUM> and a controller <NUM>.

The input filter <NUM> comprises an inductor L2 and a capacitor C1, and attenuates high-frequency ripple in the input current drawn from the power supply <NUM>. Whilst the input filter <NUM> has particular benefits and may be required for regulatory compliance (e.g. harmonics), the input filter <NUM> is not required for AC-to-DC conversion and could conceivably be omitted.

Each group of devices <NUM>,<NUM> comprises a first switch SW1,SW4, a second switch SW2,SW5, and a third switch SW3,SW6 or diode connected in series. Each group of devices <NUM>,<NUM> further comprises a first node <NUM>,<NUM> located between the first switch and the second switch, and a second node <NUM>,<NUM> located between the second switch and the third switch or diode.

Each of the first switches SW1,SW4 has an ON state in which the switch is conductive in one or both directions and an OFF state in which the switch is non-conductive in both directions. The switch therefore differs from, say, a MOSFET, which continues to conduct in one direction when in an OFF state owing to the inherent body diode. In the example illustrated in <FIG>, each of the first switches SW1,SW4 is a bidirectional switch having an ON state in which the switch conducts in both directions, and an OFF state in which the switch conducts in neither direction.

Each of the second switches SW2,SW5 has an ON state in which the switch is conductive in both directions, and an OFF state in which the switch is non-conductive in one or both direction. Unlike the first switch, there is no requirement for the second switch to have an open-circuit state when in the OFF state. The second switch may comprise a controllable device that conducts in both directions when turned on (e.g. MOSFET or GaN device). Alternatively, the second switch may comprise a controllable device that conducts in one direction only when turned on (e.g. IGBT) and includes an antiparallel diode that conducts in the other direction. The switch therefore has an ON state in which it conducts in both directions. Although there is no requirement for the switch to have an open-circuit state when in the OFF state, a bidirectional switch of the type described in the preceding paragraph may nevertheless be used.

Each group of devices <NUM>,<NUM> comprises a third switch or diode. In the example illustrated in <FIG>, each group comprises a third switch SW3,SW6. As explained below in more detail, the switch or diode provides a path for inductive current when charging the capacitor C1. Where a third switch is employed, the switch has a state in which it conducts in one direction only, i.e. functions as a diode. This then avoids a short-circuit during switching (described below) whilst providing a path for the inductive current. By way of example only, the switch may be a MOSFET or GaN device, which conducts in one direction only when in an OFF state. As a further example, the switch may be a bidirectional switch having a diode state, i.e. a state in which the switch conducts in one direction only. The use of a switch over a diode has the advantage of lower conduction losses, and may therefore be used to provide synchronous rectification.

One of the input terminals <NUM> is connected to the first node <NUM> of the first group of devices <NUM>, and the other of the input terminals <NUM> is connected to the first node <NUM> of the second group of devices <NUM>. The inductor L1 is connected between the second node <NUM> of the first group of devices <NUM> and the second node <NUM> of the second group of devices <NUM>. That is to say that the inductor L1 is connected at one end to the second node <NUM> of the first group <NUM> and at the opposite end to the second node <NUM> of the second group <NUM>. The first group of devices <NUM>, the second group of devices <NUM>, and the capacitor C1 are then connected in parallel across the output terminals <NUM>.

The controller <NUM> is responsible for controlling the operation of the converter <NUM> and generates control signals S1-S6 for controlling each of the switches SW1-SW6. The control signals are output to the gate driver <NUM>, which in response outputs gate signals for driving the switches.

Operation of the converter will now be described with reference to <FIG>.

<FIG> illustrates two different configurations of the switches when the polarity of the input voltage is positive. In the first configuration, shown in <FIG>, the first switch SW1 of the first group is OFF, the first switch SW4 of the second group is ON, the second switches SW2,SW5 of both groups are ON, and the third switches SW3,SW6 of both groups are OFF. As a result, the inductor L1 is energised by the power supply, with current flowing through the inductor L1 in a direction from left to right. In the second configuration, shown in <FIG>, the first switch SW1 of the first group is OFF, the first switch SW4 of the second group is ON, the second switch SW2 of the first group is OFF, the second switch SW5 of the first group is ON, the third switch SW3 of the first group is ON and the third switch SW6 of the second group is OFF. Energy stored in the inductor L1 is then transferred to the capacitor C1.

The only difference between the two configurations is the states of the second and third switches SW2,SW3 of the first group. In particular, the second switch SW2 is ON and the third switch SW3 is OFF in the first configuration, and the second switch SW2 is OFF and the third switch SW3 is ON in the second configuration. As noted above, the third switch could conceivably be a diode. In this instance, the only difference between the two configurations would be the state of the second switch of the first group. However, the provision of the third switch enables synchronous rectification to be achieved, thereby improving the efficiency of the converter.

There is a deadtime between turning OFF the second switch SW2 and turning ON third switch SW3, and vice versa. This then prevents a short circuit down through switches SW2,SW3 and up through switches SW6,SW5. This may be implemented by the controller <NUM> (i.e. by introducing a deadtime between changes in signals S2 and S3) or by the gate driver <NUM> (i.e. by introducing a deadtime between changes in the gate signals for switches SW2 and SW3). During the deadtime, a path is provided for the inductive current through the body diode of the third switch SW3. This then has the advantage that zero-voltage switching may be achieved when turning ON the third switch, thereby further improving efficiency.

The controller <NUM> repeatedly switches between the first configuration and the second configuration in order to transfer power from the power supply <NUM> to the capacitor C1. The controller <NUM> switches between the two configurations at a duty, which the controller <NUM> controls in order to regulate the output voltage VOUT whilst also shaping the current drawn from the power supply <NUM>.

<FIG> illustrates two further configurations of the switches when the polarity of the input voltage is negative. In the third configuration, shown in <FIG>, the first switch SW1 of the first group is ON, the first switch SW4 of the second group is OFF, the second switches SW3,SW5 of both groups are ON, and the third switches SW3,SW6 of both groups are OFF. As a result, the inductor L1 is energised by the power supply, with current flowing through the inductor L1 in a direction from right to left. In the fourth configuration, shown in <FIG>, the first switch SW1 of the first group is ON, the first switch SW4 of the second group is OFF, the second switch SW2 of the first group is ON, the second switch SW5 of the second group is OFF, the third switch SW3 of the first group is OFF and the third switch SW6 of the second group is ON. Energy stored in the inductor L1 is then transferred to the capacitor C1.

It will be apparent that third and fourth configurations mirror that of the first and second configurations. For example, the only difference between the third and fourth configurations is the states of the second and third switches SW5,SW6 of the second group. In particular, the second switch SW5 is ON and the third switch SW6 is OFF in the third configuration, and the second switch SW5 is OFF and the third switch SW6 is ON in the fourth configuration. Again, the third switch could conceivably be a diode, in which case the state of the second switch only changes.

Again, the controller <NUM> repeatedly switches between the third configuration and the fourth configuration in order to transfer power from the power supply <NUM> to the capacitor C1. And again, the controller <NUM> switches between the two configurations at a duty which the controller <NUM> controls so as to regulate the output voltage VOUT whilst also shaping the current drawn from the power supply <NUM>.

<FIG> illustrates the states of the switches SW1-SW6 over one cycle of the input voltage VIN.

The states of the first switches SW1,SW4 change only in response to a change in the polarity of the input voltage VIN. In particular, the first switch SW1 of the first group is OFF and the first switch SW4 of the second group is ON when the polarity of the input voltage is positive, and the first switch SW1 of the first group is ON and the first switch SW4 of the second group is OFF when the polarity of the input voltage is negative. The first switches SW1,SW4 are therefore switched at the frequency of the input voltage VIN. By contrast, the second switches SW2,SW5 are switched at a higher frequency.

During each half-cycle of the input voltage VIN, one of the second switches remains ON throughout and the other of the second switches is repeatedly switched between ON and OFF. In the particular example illustrated in <FIG>, the second switch of the second group SW5 is ON and the second switch of the first group SW2 is repeatedly switched when the polarity of the input voltage is positive (<FIG>), and the second switch of the first group SW2 is ON and the second switch of the second group SW5 is repeatedly switched when the polarity of the input voltage is negative (<FIG>). The second switches SW2,SW5 are therefore switched at a higher frequency than that of the input voltage VIN.

The second switches SW2,SW5 are responsible for controlling the transfer of power from the power supply <NUM> to the capacitor C1. In particular, when both switches are ON, the inductor L1 is energised, and when one of the second switches is subsequently turned OFF, energy stored in the inductor L1 is transferred to the capacitor C1. The controller <NUM> switches the second switches at a particular duty, which defines the ratio of the time spent energising the inductor L1 and the time spent charging the capacitor C1. As noted, the controller <NUM> controls the duty in order to regulate the output voltage VOUT whilst shaping the current drawn from the power supply <NUM>. The controller <NUM> may control the duty in the same or similar manner as that of a conventional PFC boost converter, in which the duty is controlled using an inner, faster current loop and an outer, slower voltage loop.

As the switching frequency of the second switches SW2,SW5 increases, the ripple in the inductor current decreases and therefore an inductor have a smaller inductance may be employed. The current drawn from the power supply <NUM> is discontinuous. In particular, when one of the second switches is OFF and energy is transferred from the inductor L1 to the capacitor C1 (e.g. <FIG> and <FIG>), no current is drawn from the power supply <NUM>. The input filter <NUM> then acts to smooth these gaps in the input current. By increasing the switching frequency of the second switches, a smaller input filter (i.e. having a smaller inductance and/or capacitance) may be used.

The frequency of the input voltage is likely to be <NUM> or <NUM>, although frequencies as high as <NUM> are not unknown. The switching frequency of the first switches SW1,SW4 may therefore be said to be less than <NUM>. By contrast, the second switches SW2,SW5 may be switched at frequencies of at least <NUM>. Indeed, switching frequencies of at least <NUM> are achievable with many MOSFET devices. Moreover, where GaN devices are employed, frequencies of at least <NUM> are possible. The size of the inductor L1 and/or the input filter <NUM> may therefore be reduced significantly through the use of GaN devices.

The converter <NUM> provides AC-to-DC conversion without the need for a rectifier bridge or front-end bulk capacitor. In comparison to other bridgeless buck-boost converters, the topology of the converter is less complex. In particular, the converter provides AC-to-DC conversion using just six devices (i.e. six switches, or four switches and two diodes), an inductor and a capacitor. Other bridgeless buck-boost converters have a higher total number of switches, a higher number of bidirectional switches (which are comparatively expensive) and/or additional inductors. Moreover, the bidirectional switches of other buck-boost converters are switched at relatively high frequencies. By contrast the first switches of the present converter, which may be bidirectional switches, are switched at the relatively low frequency of the input voltage.

The converter <NUM> does not have discreet boost and buck modes of operation. Instead, irrespective of the instantaneous value of the input voltage, the controller <NUM> uses the same mode of operation in which the switches are switched between two configurations (i.e. the first and second configurations when the input voltage is positive, and the third and fourth configurations when the input voltage is negative). Control of the switches is therefore simpler in comparison to that of other buck-boost converters which have discreet boost and buck modes, with each mode having a different set or sequence of switch configurations.

With the topology of the present converter, the second and third switches SW2,SW3,SW5,SW6 of both groups effectively form an H-bridge. Accordingly, the converter <NUM> may be implemented in part using commercially available H-bridge integrated circuits, thus reducing the cost.

In the example described above, the first switches SW1,SW4 are bidirectional. That is to say that the switches conduct in both directions when in an ON state. However, as will be apparent from <FIG> and <FIG>, each of the first switches SW1,SW4 is only ever required to conduct in one direction (i.e. upwards in <FIG> and <FIG>). However, employing bidirectional switches has at least two advantages, which will now be described.

The first advantage of having bidirectional switches is that zero-voltage switching may be achieved when turning ON the second switches SW2,SW5, i.e. when moving from the second configuration to first configuration, or when moving from the fourth configuration to the third configuration.

<FIG> illustrates a sequence of steps when moving from the second configuration to the first configuration. <FIG> corresponds to the second configuration shown in <FIG>, in which energy stored in the inductor L1 is transferred to the capacitor C1. As energy is transferred to the capacitor C1, current flowing through the inductor L1 decreases. When all energy stored in the inductor L1 has been transferred to the capacitor C1, the current through the inductor L1 is zero. At this stage, there is no self-induced voltage across the inductor L1 and therefore the capacitor C1 begins to energise the inductor L1, i.e. energy now transfers from the capacitor to the inductor, causing current to flow through the inductor in the opposite direction. This is the situation illustrated in <FIG>. Upon detecting a change in the polarity of the current through the inductor L1, the controller <NUM> switches from the second configuration to the first configuration. As already noted, there is a deadtime between turning OFF the third switch SW3 and turning ON the second switch SW2. There is therefore a period during which both switches are OFF. This is the situation illustrated in <FIG>. As a result of turning OFF the third switch SW3, a voltage is induced across the inductor L1 which cause current to be driven through the diode of the second switch SW2. At the end of the deadtime, switch SW2 is turned ON. However, with the diode of the second switch SW2 already conducting, the voltage across the switch is zero. With the second switch SW2 turned ON, the switches are now in the first configuration. This is the situation illustrated in <FIG>, which corresponds to <FIG>. Current in the inductor L1 quickly reverses and the inductor L1 is energised by the power supply <NUM>.

<FIG> illustrates the corresponding sequence of steps when moving from the fourth configuration to the third configuration. <FIG> corresponds to the fourth configuration shown in <FIG>, and <FIG> corresponds to the third configuration shown in <FIG>. Again, with the diode of the second switch SW5 conducting (<FIG>), zero-voltage switching is achieved when turning ON second switch SW5 (<FIG>).

The second advantage of having bidirectional switches is that the converter <NUM> may be used for bidirectional power transfer. That is to say that power may be transferred from the power supply <NUM> to the capacitor C1 in the manner described above (i.e. forward power transfer). Additionally, power may be transferred from the capacitor C1 to the power supply <NUM> (i.e. reverse power transfer), as will now be described.

<FIG> and <FIG> illustrate reverse power transfer (i.e. from the capacitor to the power supply) when the polarity of the input voltage VIN is positive (<FIG>) and negative (<FIG>).

<FIG> illustrates two different configurations of the switches. In the configuration of <FIG>, the first switch SW1 of the first group is OFF, the first switch SW4 of the second group is ON, the second switch SW2 of the first group is OFF, the second switch SW5 of the second group is ON, the third switch SW3 of the first group is ON and the third switch SW6 of the second group is OFF. As a result, the inductor L1 is energised by the capacitor C1, with current flowing through the inductor L1 in a direction from right to left. In the configuration of <FIG>, the first switch SW1 of the first group is OFF, the first switch SW4 of the second group is ON, the second switches SW2,SW5 of both groups are ON, and the third switches SW3,SW6 of both groups are OFF. As a result, energy stored in the inductor L1 is transferred to the power supply <NUM>.

<FIG> similarly illustrates two different configurations of the switches. In the configuration of <FIG>, the first switch SW1 of the first group is ON, the first switch SW4 of the second group is OFF, the second switch SW2 of the first group is ON, the second switch SW5 of the second group is OFF, the third switch SW3 of the first group is OFF and the third switch SW6 of the second group is ON. As a result, the inductor L1 is energised by the capacitor C1, with current flowing through the inductor L1 in a direction from left to right. In the configuration of <FIG>, the first switch SW1 of the first group is ON, the first switch SW4 of the second group is OFF, the second switches SW2,SW5 of both groups are ON, and the third switches SW3,SW6 of both groups are OFF. As a result, energy stored in the inductor L1 is transferred to the power supply <NUM>.

The configurations illustrated in <FIG> are the same as those of <FIG>. For example, the configuration of <FIG> corresponds to that of <FIG>, and the configuration of <FIG> corresponds to that of <FIG>. The same is true of the configurations of <FIG> and <FIG>. Accordingly, reverse power transfer may be achieved using the same switching configurations as that for forward power transfer. Moreover, zero-voltage switching of the second switches SW2,SW5 can be achieved in the same manner as that described above, i.e. by monitoring the inductor current and switching configurations (e.g. from <FIG>) when the polarity of the current changes.

Claim 1:
A buck-boost converter (<NUM>) comprising:
input terminals (<NUM>) for connection to a power supply (<NUM>) supplying an alternating input voltage;
output terminals (<NUM>) for outputting an output voltage;
a first group of devices (<NUM>) and a second group of devices (<NUM>), each group of devices comprising: a first switch, a second switch, and a third switch or diode connected in series; a first node (<NUM>, <NUM>) located between the first switch and the second switch; and a second node (<NUM>, <NUM>) located between the second switch and the third switch or diode;
an inductor;
a capacitor; and
a controller (<NUM>) for controlling the switches of the first group of devices (<NUM>) and the second group of devices (<NUM>),
wherein:
a first of the input terminals (<NUM>) is connected to the first node of the first group of devices (<NUM>);
a second of the input terminals (<NUM>) is connected to the first node of the second group of devices (<NUM>);
the inductor is connected between the second node of the first group of devices (<NUM>) and the second node of the second group of devices (<NUM>);
the first group of devices (<NUM>), the second group of devices (<NUM>), and the capacitor are connected in parallel across the output terminals (<NUM>);
the first switch of each group of devices has an OFF state in which the switch is non-conductive in both directions;
the second switch of each group has an ON state in which the switch is conductive in both directions;
the third switch or diode of each group of devices provides a path for inductive current when charging the capacitor; and
the controller configures the switches in (i) a configuration in which the first switch of one group of devices is OFF, the first switch of the other group of devices is ON, and the second switches of both groups of devices are ON, thereby causing the inductor to be energised by the power supply, and (ii) a further configuration in which the first and second switches of one group of devices are OFF, and the first and second switches of the other group of devices are ON, thereby causing energy stored in the inductor to be transferred to the capacitor.