Power Demultiplexer for High-Efficiency Multi-Channel LED Drivers

Abstract

The present invention relates to a power stage for powering a first load, the first load being couplable between a first output and a second output, the power stage comprising: a first input for receiving a bus voltage; the first output coupled to the first input and configured to be couplable to the first load; a first capacitor coupled between the second output and a first further node and couplable in series with the first load; and a secondary power converter comprising: a controller for controlling the secondary power converter; a first inductive element and a first switching element coupled in series between the second input and a return node; a first unidirectional device coupled in series between the second output and the first further node, and a second inductive element inductively coupled to the first inductive element;

Description

[Technical Field]

[0001] The present invention relates to a power stage for powering a load.The present invention further relates to a power supply system. [Background technology]

[0002] To regulate the power supplied to a load, a power stage, also called a driver, is often used. The load may require a specific voltage for the load to operate properly. When the load or driver is replaced, broken, or upgraded, the load and driver may no longer be compatible in terms of supplied and required voltages. If the driver voltage is greater than the voltage required by the load, the voltage difference may result in additional losses. If the driver voltage is lower than the voltage required by the load, the load may not even start or function properly.

[0003]

[0003] Particularly in the field of lighting, a lighting load may be replaced independently of its driver. When a lighting load is replaced and a new combination of driver and lighting load is formed, the forward voltage of the lighting load may not exactly match the voltage supplied by the driver. This may result in additional power loss in the driver or a non-functional combination of driver and lighting load. It is desirable to be able to provide greater design freedom by allowing a wider range of drivers to be combined with more types of lighting loads. Summary of the Invention [Problem to be solved by the invention]

[0004] It is an object of the present invention to provide a power efficient device that allows the supply voltage from, for example, a driver to match the required voltage of the load. [Means for solving the problem]

[0005] To achieve this effect, in a first aspect of the present invention there is provided a power stage for powering a first load, said first load being couplable between a first output and a second output, said power stage comprising: - a first input for receiving a bus voltage; - the first output coupled to the first input and configured to be couplable to the first load; a first capacitor coupled between the second output and a first further node, the first capacitor being couplable in series with the first load; a secondary power converter, the secondary power converter comprising: - a controller for controlling the secondary power converter; a first inductive element and a first switching element coupled in series between the second input and a return node; - a first unidirectional device coupled in series between the second output and the first further node, and a second inductive element inductively coupled to the first inductive element; The secondary power converter is configured to supply a current to the first capacitor that is of opposite polarity to a current supplied to the first capacitor through the first load.

[0006] The power stage according to the invention has a first input capable of receiving a bus voltage. The bus voltage can be any kind of voltage, preferably a stable voltage. This voltage may have ripple such that the bus voltage is on average a stable voltage. A first output is provided to which a first load can be connected. The first output is also coupled to the first input.

[0007] A first capacitor is provided between the second output and a first further node, and the first load can also be coupled to the second output. When the first load is coupled between the first output and the second output, the first load and the first capacitor are in series. A secondary power converter is provided. The secondary power converter has a controller used to control the secondary power converter. A first inductive element and a first switching element are coupled in series with each other and coupled between the second input and a return node. The first inductive element is inductively coupled to a second inductive element. A first unidirectional device is coupled in series between the second output and the first further node.

[0008] The secondary power converter is configured to supply a current to the first capacitor that is of opposite polarity to a current supplied to the first capacitor through the first load.

[0009] The power stage according to the present invention allows the voltage of the first load to match the bus voltage. In this example, the sum of the voltages across the first load and the first capacitor equals the bus voltage. While the voltage across the first load is not easy to regulate, the voltage across the first capacitor can be regulated by the secondary power converter. The secondary power converter uses the first switching element to allow current to flow through the first inductive element. The inductive coupling between the first inductive element and the second inductive element effectively causes the first inductive element and the second inductive element to form a flyback transformer. Therefore, current flowing through the first inductive element is reflected to the second inductive element. The first unidirectional device is coupled to the second inductive element such that no current flows through the second inductive element when the first switching element is closed. Instead, when the first switching element is open, the current flowing through the first inductive element is mirrored in the second inductive element, and this current is then used to supply the first capacitor with a current of opposite polarity to the current flowing through the first load. For example, the current flowing through the first load also flows through the first capacitor. This current may discharge the first capacitor. The current supplied to the first capacitor via the second inductive element is of a polarity such that it charges the first capacitor. The balance between charging and discharging the first capacitor allows the voltage across the first capacitor, and therefore the current flowing through the first load, to be regulated. The duration of the on-time of the first switching element can be used to supply a regulated amount of energy to the first capacitor.

[0010] In a further example, the second input is directly coupled to the first input.

[0011] It is preferable to provide a direct coupling between the second input and the first input so that energy losses can be kept as low as possible, however, electrical components may be located between the second input and the first input if required, for example to provide additional functionality.

[0012] In a further example, the first further node is coupled to the return node.

[0013] Preferably, both the first capacitor and the series combination of the first inductive element and the first switching element are directly coupled to the return node, which allows for a simple and energy-efficient design of the power stage.

[0014] In a further example, the power stage further comprises a linear current regulator in series between the first further node and the return node.

[0015] A linear current regulator may be provided between the first further node and the return node, in which case the linear current regulator may effectively be connected in series with the first load and the first capacitor, and may be used to further reduce current ripple in the current through the first load.

[0016] In a further example, the current supplied through the first load is configured to discharge the first capacitor.

[0017] In another example, the current supplied through the first load is configured to charge the first capacitor.

[0018] In a further example, the current supplied through the second inductive element is configured to charge the first capacitor.

[0019] In another example, the current supplied through the second inductive element is configured to discharge the first capacitor.

[0020] In a further example, the secondary power converter is a flyback converter.

[0021] The use of a flyback converter for the secondary power converter allows an easy design of the inductive coupling between the first inductive element and the second inductive element to be realized.

[0022] In another example, a power supply system is provided, the power supply system comprising: a power stage according to any of the previous examples; a third output coupled to the first input and configured to be couplable to a second load, the second load being couplable between the third output and a fourth output; a second capacitor coupled between the fourth output and a second further node, the second capacitor being couplable in series with the second load; a tertiary power converter, the tertiary power converter comprising: - a second unidirectional device, a third switching element, and a third inductive element inductively coupled to the first inductive element, coupled in series between the fourth output and the second further node; the secondary power converter further comprising a second switching element in series with the first unidirectional device; The tertiary power converter is configured to supply a current to the second capacitor that is of opposite polarity to a current supplied to the second capacitor through the second load.

[0023] An additional second independent load may be powered by the same power supply system. The second load may be coupled in series with a second capacitor. A tertiary power source may receive power from the first inductive element. The tertiary power source includes a second unidirectional device, a third switching element, and a third inductive element inductively coupled to the first inductive element, coupled in series between the fourth output and the second further node. The tertiary power source may operate in the same manner as the secondary power source. The second unidirectional device enables current to be supplied to the second capacitor by the third inductive element when the first switching element is open. Furthermore, the tertiary power converter is configured to supply a current to the second capacitor that is opposite in polarity to the current supplied to the second capacitor via the second load. This means that if the current supplied via the load charges the capacitor, the current supplied by the third inductive element discharges the capacitor, and vice versa. The first inductive element supplies current to the second inductive element and the third inductive element. The secondary power converter has a second switching element in series with the first unidirectional device and the tertiary power converter has a third switching element in series with the second unidirectional device to determine power distribution between the second inductive element and the third inductive element, and control of the second switch and the third switch allows regulated current to flow to the first capacitor and the second capacitor, respectively.

[0024] In another example, the power supply system includes a first load and a second load.

[0025] In another example, the first load and / or the second load is a lighting load.

[0026] Preferably, at least one of the loads is a lighting load. The other load may also be a lighting load, but the other load may also be another type of load, such as a sensor.

[0027] In another example, the lighting load is a solid-state lighting load.

[0028] An example of a semiconductor lighting load may be an LED or a laser such as a laser diode or a vertical cavity surface emitting laser (VCSEL).

[0029] In a further example, the forward voltage of the first load is different from the forward voltage of the second load.

[0030] The power supply system according to the example makes it possible to compensate for differences between the forward voltages of the loads by supplying different voltages across the first capacitor and across the second capacitor.

[0031] In a further example, the power supply system further comprises a mains power converter adapted to convert a mains input voltage to the bus voltage.

[0032] The bus voltage may be an unregulated voltage but preferably the bus voltage is a regulated voltage. The mains power converter may regulate an unregulated voltage, such as a rectified mains voltage, to a regulated bus voltage. [Brief explanation of the drawings]

[0033] Examples of the invention will now be described with reference to the accompanying drawings, in which: [Figure 1] 1 shows an example of a power stage circuit diagram. [Figure 2] 10 shows another example of a power stage circuit diagram. [Figure 3] 10 shows another example of a circuit diagram of a power supply system. [Figure 4] 10 shows another example of a circuit diagram of a power supply system. [Figure 5] 1 shows an example of a power supply system. [Figure 6] 1 shows another example of a power supply system. DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention will be described with reference to the drawings.

[0035] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems, and methods, are for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the devices, systems, and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should also be understood that the figures are schematic only and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the figures to indicate the same or similar parts.

[0036] FIG. 1 shows an example of a power stage. The power stage has a first input, Input, configured to receive a bus voltage. The power stage also has a ground node for providing a return path for current supplied via the input, Input. This node is shown as ground. In this example, it may also be referred to as a return node or a first further node. A first inductive element, L1, and a first switching element, M1, are coupled in series between a second input, Input2, which in this example is directly coupled to the first input, Input1, and the return node.

[0037] A first load LED1 and a first capacitor C1 are coupled in series between the first input Input and a first further node, which in this example is directly coupled to the return node. The first load LED1 may form part of a power stage. The power stage therefore has a first output 1 and a second output 2 to which the first load LED1 is coupled. In this example, the first output 1 is directly coupled to the first input Input1. In this example, the second output 2 is directly coupled to the first capacitor C1 and the anode of the first unidirectional device D1. A first unidirectional device D1 and a second inductive element L2 are coupled in series between the second output 2 and the first further node. In this example, the secondary power converter includes a first inductive element L1, a first switching element M1, a first unidirectional device D1, and a second inductive element L2. The first inductive element L1 and the second inductive element L2 are inductively coupled such that the secondary power converter forms a flyback converter. The secondary power converter may be controlled by a controller. The first capacitor C1 is considered a load to the secondary power converter. The secondary power converter supplies current to the first capacitor C1 in the positive direction of the polarity of the first capacitor C1. The current through the first load LED1 flows through the first capacitor C1 in the negative direction of the polarity of the first capacitor C1. The insight of the present invention is that the voltage of the first load LED1 needs to be matched to the bus voltage so that the current through the first load LED1 can also be regulated. In this example, the bus voltage is lower than the voltage required by the first load LED1. The voltage across the first capacitor is negative compared to the voltage across the first load LED1, effectively "raising" the bus voltage relative to the required load voltage. Therefore, the secondary power converter is used to raise the bus voltage level so that the voltage across the first load LED1 is large enough to allow the regulated desired current amplitude to flow through the first load LED1.

[0038] As an example, the bus voltage may be 20V and the voltage required for the first load LED1 may be 25V.

[0039] The bus voltage is equal to the sum of the voltage of the first load LED1 and the voltage of the first capacitor C1. The secondary power converter is then controlled to generate a voltage across the first capacitor of -5V, V bus =V load +V c This results in 20V = 25V - 5V.

[0040] To provide a more stable bus voltage, this bus voltage can be buffered by a buffer capacitor C10. The bus voltage can have a significant double-line-frequency voltage ripple because the secondary power converter compensates for the ripple and keeps the load voltage constant.

[0041] FIG. 2 shows an improved example of the power stage as shown in FIG. 1. The power stage has a first input, Input1, configured to receive the bus voltage. The power stage also has a ground node for providing a return path for the current supplied via the input, Input. This node is shown as ground. In this example, it may also be referred to as the return node. A first inductive element L1 and a first switching element M1 are coupled in series between a second input, Input2, which in this example is directly coupled to the first input, Input1, and the return node. A first load LED1 and a first capacitor C1 are coupled in series between the first input, Input1, and a first further node. The first load LED1 may form part of the power stage. Thus, the power stage has a first output 1 and a second output 2 to which the first load LED1 is coupled. In this example, the first output 1 is directly coupled to the first input, Input1. In this example, the second output 2 is directly coupled to the first capacitor C1 and the anode of the first unidirectional device D1. A first unidirectional device D1 and a second inductive element L2 are coupled in series between the second output 2 and the first further node. In this example, the secondary power converter includes a first inductive element L1, a first switching element M1, a first unidirectional device D1, and a second inductive element L2. The first inductive element L1 and the second inductive element L2 are inductively coupled so that the secondary power converter forms a flyback converter. The secondary power converter can be controlled by a controller. A first capacitor C1 is considered a load for the secondary power converter. The secondary power converter supplies a current to the first capacitor C1 in the positive direction of the polarity of the first capacitor C1. The current through the first load LED1 flows through the first capacitor C1 in the negative direction of the polarity of the first capacitor C1. The insight of the present invention is that the voltage of the first load LED1 needs to be matched to the bus voltage so that the current through the first load LED1 can also be regulated. In this example, the bus voltage is lower than the voltage required by the first load LED1. The voltage across the first capacitor is negative compared to the voltage across the first load LED1, effectively "boosting" the bus voltage relative to the required load voltage.Therefore, the secondary power converter is used to increase the bus voltage level so that the voltage across the first load LED1 is large enough to allow the regulated desired current amplitude to flow through the first load LED1.

[0042] As an example, the bus voltage may be 20V and the voltage required for the first load LED1 may be 25V.

[0043] The bus voltage is equal to the sum of the voltage of the first load LED1 and the voltage of the first capacitor C1. The secondary power converter is then controlled to generate a voltage across the first capacitor of -5V, V bus =V load +V c This results in 20V = 25V - 5V.

[0044] To provide a more stable bus voltage, this bus voltage can be buffered by a buffer capacitor C10. Additional circuitry is provided to reduce ripple in the current through the first load LED1. This is a linear current regulator. A transistor Q1 with a current sensing circuit R1 can be provided in series between the first further node and the return node. A feedback circuit 5 can receive a signal from the current sensing circuit R1 representing the current through the transistor Q1. In this example, the current sensing circuit R1 is a resistor. The feedback circuit 5 uses this signal to control the base of the transistor Q1 so that the desired current flows through it. The feedback circuit 5 can also receive an additional signal to set the desired current amplitude. This can be a dimming signal, such as a PWM dimming signal, which can be provided by a controller controlled by an external device, such as a remote control.

[0045] It should be noted that linear current regulators can be implemented in any of the illustrated examples, not just the example of Figure 2. Each capacitor in series with a corresponding load can also be coupled in series with a corresponding linear current regulator. In such a case, the further node is not directly coupled to the return node because the linear current regulator is disposed therebetween.

[0046] FIG. 3 shows an example of a power supply system. The power supply system may include a power stage according to an example of the present invention. The power supply system has a first input, Input1, configured to receive a bus voltage. The power supply system also has a ground node for providing a return path for the current supplied via the input, Input1. This node is shown as ground. In this example, it may also be referred to as a return node or a first further node. A first inductive element L1 and a first switching element M1 are coupled in series between a second input, Input2, which in this example is directly coupled to the first input, Input1, and the return node. A first load LED1 and a first capacitor C1 are coupled in series between the first input, Input1, and the first further node, which in this example is directly coupled to the return node. The first load LED1 may form part of the power supply system. The power supply system therefore has a first output 1 and a second output 2 to which the first load LED1 is coupled. In this example, the first output 1 is directly coupled to the first input, Input1. In this example, the second output 2 is directly coupled to the first capacitor C1 and the anode of the first unidirectional device D1. The first unidirectional device D1, the second switching element M2, and the second inductive element L2 are coupled in series between the second output 2 and the first further node. In this example, the secondary power converter comprises the first inductive element L1, the first switching element M1, the first unidirectional device D1, the second switching element M2, and the second inductive element L2.

[0047] A second load LED2 and a second capacitor C2 are coupled in series between the first input Input1 and a second further node, which in this example is directly coupled to the return node. The second load LED2 may form part of a power supply system. The power supply system therefore has a third output 3 and a fourth output 4 to which the second load LED2 is coupled. In this example, the third output 3 is directly coupled to the first input Input1. In this example, the fourth output 4 is directly coupled to the second capacitor C2 and the anode of the second unidirectional device D2. A second unidirectional device D2, a third switching element M3, and a third inductive element L3 are coupled in series between the fourth output 4 and the second further node. In this example, the tertiary power converter includes the second unidirectional device D2, the third switching element M3, and the third inductive element L3. A controller may also be used to control the tertiary power converter. This means that the controller may be used, for example, to control the third switching element M3. The first inductive element L1, the second inductive element L2, and the third inductive element L3 are inductively coupled so that the secondary power converter forms a flyback converter. The main difference from a conventional flyback converter is that switching elements are arranged on the secondary and tertiary sides, i.e., on the side of the second inductive element L2 and the third inductive element L3, to allow or prevent current from flowing to the corresponding first capacitor C1 or second capacitor C2. The current through the first inductive element L1 is reflected to the second inductive element L2 and the third inductive element L3. The second switching element M2 and the third switching element M3 can be controlled to control the current flowing from the second inductive element L2 and the third inductive element L3 to the first capacitor C1 and the second capacitor C2, respectively.

[0048] Similar to the example shown in FIGS. 1 and 2, current through the load discharges the series capacitor, and current provided by the inductive element charges the capacitor. In this example, current through the first load LED1 discharges the first capacitor C1, and current provided by the second inductive element L2 charges the first capacitor C1. Current through the second load LED2 discharges the second capacitor C2, and current provided by the third inductive element L3 charges the second capacitor C2. Similar to the example shown in FIGS. 1 and 2, the voltages across the first capacitor C1 and the second capacitor C2 are negative, and the voltages required by the first load LED1 and the second load LED2 are greater than the bus voltage. By controlling the switching elements M1, M2, and M3, the voltages across the first capacitor C1 and the second capacitor C2 can also be controlled independently of each other. This allows the voltage required by the first load LED1 to be different from the voltage required by the second load LED2. The advantage in this situation is that a single bus voltage can be supplied to multiple loads even if the bus voltage does not match the voltage of the loads.

[0049] Preferably, the first switching element M1, the second switching element M2, and the third switching element M3 are controlled by a single controller.To provide a more stable bus voltage, the bus voltage can be buffered by a buffer capacitor C10.

[0050] FIG. 4 shows another example of a power supply system. This power supply system is almost the same as the power supply system of FIG. 3. The main difference is that the polarities of the first capacitor C1 and the second capacitor C2 are reversed. This means that the currents supplied by the second inductive element L2 and the third inductive element discharge the first capacitor C1 and the second capacitor C2, respectively. The currents flowing through the first load LED1 and the second load LED2 charge the first capacitor C1 and the second capacitor C2, respectively. In this example, the capacitor voltages can be positive. In that case, the bus voltage can be greater than the required load voltage.

[0051] As an example, considering only the first load LED1 for clarity, the bus voltage may be 25V and the voltage required for the first load LED1 may be 20V.

[0052] The bus voltage is equal to the sum of the voltage of the first load LED1 and the voltage of the first capacitor C1. The secondary power converter is then controlled to generate a voltage across the first capacitor of 5V, V bus =V load +V c This results in 25V = 20V + 5V.

[0053] To provide a more stable bus voltage, this bus voltage can be buffered by a buffer capacitor C10.

[0054] 5 shows another example of a power supply system. The power supply system has a first input Input1 configured to receive a bus voltage. The power supply system also has a ground node for providing a return path for the current supplied via the input Input. This node is shown as ground. In this example, it may also be called the return node or the first further node.

[0055] In this example, the bus voltage is provided by a mains power converter. The mains power converter may be configured to rectify the mains input voltage. Typical mains voltages are 230V at 50Hz or 120V at 60Hz. This rectified mains voltage is converted to the bus voltage by the mains power converter. In this example, the mains power converter comprises an inductor L10, a switching element M10, and a unidirectional device D10, and is configured to operate as a boost converter. The mains power converter may also be configured as any other type of switch-mode power converter, such as a boost converter, a buck converter, a buck-boost converter, a flyback converter, or an LLC converter. The main purpose of the mains power converter is to convert the input voltage, in this case the mains voltage, into a regulated bus voltage at input Input1.

[0056] A first inductive element L1 and a first switching element M1 are coupled in series between a second input Input2, which in this example is directly coupled to the first input Input1, and a return node. A first load LED1 and a first capacitor C1 are coupled in series between the first input Input1 and a first further node, which in this example is directly coupled to the return node. The first load LED1 may form part of a power supply system. The power supply system therefore has a first output 1 and a second output 2 to which the first load LED1 is coupled. In this example, the first output 1 is directly coupled to the first input Input1. In this example, the second output 2 is directly coupled to the first capacitor C1 and the anode of the first unidirectional device D1. A first unidirectional device D1, a second switching element M2, and a second inductive element L2 are coupled in series between the second output 2 and the first further node. In this example, the secondary power converter includes a first inductive element L1, a first switching element M1, a first unidirectional device D1, a second switching element M2, and a second inductive element L2.

[0057] A second load LED2 and a second capacitor C2 are coupled in series between the first input Input1 and a second further node, which in this example is directly coupled to the return node. The second load LED2 may form part of a power supply system. The power supply system therefore has a third output 3 and a fourth output 4 to which the second load LED2 is coupled. In this example, the third output 3 is directly coupled to the first input Input1. In this example, the fourth output 4 is directly coupled to the second capacitor C2 and the anode of the second unidirectional device D2. A second unidirectional device D2, a third switching element M3, and a third inductive element L3 are coupled in series between the fourth output 4 and the second further node. In this example, the tertiary power converter includes the second unidirectional device D2, the third switching element M3, and the third inductive element L3. A controller may also be used to control the tertiary power converter. This means that the controller may be used, for example, to control the third switching element M3. The first inductive element L1, the second inductive element L2, and the third inductive element L3 are inductively coupled so that the secondary power converter forms a flyback converter. The main difference from a conventional flyback converter is that switching elements are arranged on the secondary and tertiary sides, i.e., on the side of the second inductive element L2 and the third inductive element L3, to allow or prevent current from flowing to the corresponding first capacitor C1 or second capacitor C2. The current through the first inductive element L1 is reflected to the second inductive element L2 and the third inductive element L3. The second switching element M2 and the third switching element M3 can be controlled to independently control the current flowing from the second inductive element L2 and the third inductive element L3 to the first capacitor C1 and the second capacitor C2, respectively.

[0058] By coupling the series combination of the first inductive element L1 and the first switching element M1 between the first input and the return node, a regulated bus voltage can be provided and therefore control of the first switching element M1 can be simplified. If the bus voltage is lower than the voltage supplied to the mains power converter, the components may have lower voltage requirements allowing smaller and / or cheaper components to be selected.

[0059] In this example, the mains power converter is associated with two loads. It should be understood that a mains power converter can be associated with any number of loads. Power supply systems for supplying a single load, for example using a power stage according to Figures 1 or 2, can also benefit from a mains power converter.

[0060] FIG. 6 shows another example of a power supply system. This power supply system is substantially the same as the power supply system of FIG. 5. The same configuration is used to control the voltage across the first capacitor C1 and the voltage across the second capacitor C2. This power supply system differs from the power supply system of FIG. 5 in that a series combination of a first inductive element L1 and a first switching element M1 is located at the second input Input2 and the return node. In the previous example, the second input Input2 was directly coupled to the first input Input1. In this example, the second input Input2 is coupled to the outputs of rectifiers D11, D12, D13, and D14. Therefore, the mains power converter does not need to provide the power conversion required by the second power converter, or the second power converter and the tertiary power converter. Instead, power can be drawn directly from the mains power supply. This may result in more power-efficient power conversion for the power supply system.

[0061] The terms positive and negative voltage are referenced to ground, the return node, which means that the voltage across the capacitor can be considered to be positive with respect to ground or negative with respect to ground.

[0062] In the illustrated example, charging and discharging the capacitor can also be understood as supplying a positive or negative voltage to the capacitor. For example, if the average current supplied by the load is greater than the current supplied by the inductive element, the capacitor will effectively charge over time, and the voltage across the capacitor will increase and become positive. For example, if the average current supplied by the load is less than the current supplied by the inductive element, the capacitor will effectively discharge over time, and the voltage across the capacitor will decrease and even become negative.

[0063] In the example shown, some loads may require a voltage greater than the bus voltage, while other loads may require a voltage less than the bus voltage. The polarity of the corresponding capacitors can be adjusted so that the greater and lesser voltages can be matched to the bus voltage.

[0064] In the example shown, the switching elements can be any type of semiconductor switch, such as a transistor or a MOSFET.

[0065] In the example shown, the inductive elements may be coils of a transformer. In a preferred example, a single transformer is used in which all inductive elements for the secondary power converter, or for the secondary and tertiary power converters, form a single inductor and are therefore all inductively coupled to each other.

[0066] In the example shown, the unidirectional device may be, for example, a diode. A transistor that is controlled to allow conduction in one direction and block conduction in the other direction can also be used.

[0067] Those skilled in the art can understand and effect other variations to the disclosed embodiments in practicing the claimed invention, from a study of the drawings, the specification and the appended claims. In the claims, the word "comprises" does not exclude other elements or steps, and the singular does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be interpreted as limiting the scope.

Claims

1. A power stage for powering a first load, said first load being couplable between a first output and a second output, said power stage comprising: the second output, a first further node, a second input, and a return node; a first input for receiving a bus voltage; the first output coupled to the first input and configured to be couplable to the first load; a first capacitor coupled between the second output and the first further node, the first capacitor being couplable in series with the first load; a secondary power converter, the secondary power converter comprising: a controller for controlling the secondary power converter; a first inductive element and a first switching element coupled in series between the second input and the return node; a first unidirectional device coupled in series between the second output and the first further node, and a second inductive element inductively coupled to the first inductive element; A power stage configured such that the secondary power converter supplies a current to the first capacitor that is of an opposite polarity to a current supplied to the first capacitor through the first load.

2. The power stage of claim 1 , wherein the second input is directly coupled to the first input.

3. A power stage as claimed in claim 1 or 2, wherein the first further node is coupled to the return node.

4. 3. A power stage as claimed in claim 1 or 2, further comprising a linear current regulator in series between the first further node and the return node.

5. 5. A power stage according to any preceding claim, wherein the current supplied through the first load is arranged to discharge the first capacitor.

6. 5. A power stage according to any preceding claim, wherein current supplied through the first load is arranged to charge the first capacitor.

7. A power stage according to any preceding claim, wherein the current supplied through the second inductive element is arranged to charge the first capacitor.

8. 7. A power stage according to any one of claims 1 to 4 or 6, wherein the current supplied through the second inductive element is arranged to discharge the first capacitor.

9. A power stage according to any one of claims 1 to 8, wherein the secondary power converter is a flyback converter.

10. A power stage according to any one of claims 1 to 9; a third output coupled to the first input and configured to be couplable to a second load, the second load being couplable between the third output and a fourth output; a second capacitor coupled between the fourth output and a second further node, the second capacitor being couplable in series with the second load; a tertiary power converter, the tertiary power converter comprising: a second unidirectional device, a third switching element, and a third inductive element inductively coupled to the first inductive element, coupled in series between the fourth output and the second further node; the secondary power converter further comprising a second switching element in series with the first unidirectional device; A power supply system wherein the tertiary power converter is configured to supply a current to the second capacitor that is of opposite polarity to a current supplied to the second capacitor through the second load.

11. The power supply system according to claim 10 , further comprising the first load and the second load.

12. The power supply system according to claim 10 or 11, wherein the first load and / or the second load is a lighting load.

13. The power supply system according to any one of claims 10 to 12, wherein the lighting load is a semiconductor lighting load.

14. The power supply system of claim 13 , wherein a forward voltage of the first load is different from a forward voltage of the second load.

15. 15. A power supply system according to any one of claims 10 to 14, further comprising a mains power converter adapted to convert a mains input voltage to the bus voltage.

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