Improvement of high frequency PFC converter

Abstract

The present invention relates to a driver for driving a load, the driver having a first node adapted to be coupled to a varying voltage, a second node adapted to be coupled to a stable voltage, a switch mode power converter configured to convert the varying voltage to the stable voltage or to convert the stable voltage to the varying voltage, a first ceramic capacitor coupled to the first node, and a second ceramic capacitor coupled between the first node and the second node.

Description

[Technical Field]

[0001] The present invention relates to a driver. The present invention also relates to a luminaire comprising said driver. [Background technology]

[0002] Many applications use drivers with switch-mode power converters to power electronic devices. Laptops, cell phones, and lighting applications all require power supplies that can deliver regulated power while preventing or minimizing electrical noise and electromagnetic interference (EMI) from being emitted onto the grid. Capacitors are commonly used to provide high-frequency filtering of EMI. For this function, capacitors are typically ceramic or film capacitors because these types of capacitors are well-suited to filtering high-frequency components from voltages. Ceramic capacitors come in many varieties, ranging from low-quality X5R types to X7R, or Class 2, ceramic capacitors, and even NP0, or Class 1, ceramic capacitors. NP0 capacitors are considered the most technologically interesting capacitors because they offer tight capacitance tolerances and are the least sensitive to the voltage applied to them. However, NP0 capacitors are very expensive and have limited capacitance. A small capacitance drop occurs when the voltage across the capacitor is large. If a larger capacitance value is required, multiple NP0 capacitors are required, which significantly increases the cost and size of the power supply. X5R capacitors have a large tolerance in capacitance value and are also strongly affected by the voltage applied to the capacitor. When the voltage across the capacitor is large, a large capacitance drop occurs. Therefore, to provide a large total capacitance at the rated voltage of a multilayer ceramic capacitor (MLCC), many capacitors must be used.

[0003] FIG. 1 shows an example of the relationship between the capacitance value of a capacitor and the voltage across it. On the X-axis, the voltage across the capacitor is defined from 0 V to the maximum voltage, vCmax, that can be applied across the capacitor. On the Y-axis, the relative capacitance of the capacitor is defined. C defines the actual capacitance of the capacitor, and C0 defines the absolute capacitance of the capacitor. The maximum relative capacitance is 1, in which case C is equal to C0. The minimum capacitance is the capacitance value, Cmin. Therefore, the relative minimum capacitance is defined as Cmin / C0. Cmin / C0 is defined at the maximum voltage, vCmax. Preferably, the voltage across the capacitor may not exceed this voltage. The capacitance of the capacitor is highest at the lowest voltage. An increase in the voltage across the capacitor significantly reduces the capacitance, almost exponentially. At higher voltages, the filtering function is significantly reduced due to the lower capacitance value. Therefore, to provide good filtering at higher voltages, the capacitors need to be dimensioned so that the capacitance is still high enough at the highest voltage. This can lead to over-sizing the capacitors, which results in capacitors that are larger in volume and / or cost. It is desirable to provide a driver in which the total capacitance of the capacitors can be better utilized without providing capacitors that are more expensive or larger in volume. Summary of the Invention [Problem to be solved by the invention]

[0004] The object of the present invention is to provide a driver with improved capacitance utilization, which means using the same amount of capacitor as used in conventional solutions, for example placing two capacitors in parallel to double the effective capacitance, but achieving a higher effective capacitance, especially as the voltage across the capacitor increases. [Means for solving the problem]

[0005] A first aspect of the present invention is a driver for driving a load, - a first node adapted to be coupled to a fluctuating voltage; - a second node adapted to be coupled to a stable voltage; - a switched mode power converter configured to convert the varying voltage to the stable voltage or to convert the stable voltage to the varying voltage; a first ceramic capacitor coupled to the first node; and A driver is provided having a second ceramic capacitor coupled between the first node and the second node, the second ceramic capacitor being arranged to provide a dominant capacitance between the first node and the second node, and the first ceramic capacitor being arranged to provide a dominant capacitance to the first node.

[0006] The driver includes a switch-mode power converter configured to convert a fluctuating voltage into a stable voltage. This stable voltage can be supplied to the load. The load may be any type of load requiring stable power. The load may also be another switch-mode power converter that uses the stable voltage to convert it into another voltage for the other load. The operation of the switch-mode power converter generates EMI, i.e., noise, which needs to be filtered out. A capacitor is used to reduce noise from the switch-mode power converter to the input, e.g., mains voltage connection. It is generally known that capacitors are used to filter and remove high-frequency noise. The first ceramic capacitor is coupled to the first node. At this first node, an input power supply is received. The input voltage is a fluctuating voltage. This may be a rectified mains voltage. The rectified mains voltage is still fluctuating. Therefore, the first ceramic capacitor is used to provide a filtering function at the first node. The second ceramic capacitor is coupled between the first node and the second node. Therefore, the second ceramic capacitor provides a filtering function at the first node to the second node. The voltage at the second node is a stable voltage generated by the switch-mode power converter. A stable voltage may be understood as a voltage having a steady DC voltage value with a ripple superimposed as a result of the operation of the switch-mode power converter. The second ceramic capacitor is therefore arranged between a stable voltage at one end and a fluctuating voltage at another end. The second ceramic capacitor is arranged to provide a dominant capacitance between the first node and the second node. The first ceramic capacitor is arranged to provide a dominant capacitance at the first node. This means that no other capacitors, e.g., electrolytic capacitors, provide any significant capacitance at these nodes.The capacitance of these other capacitors at these nodes, e.g., in parallel with the first or second capacitor, is negligible in the capacitive behavior of the driver. Arranging the two capacitors in this manner allows them to provide an improved capacitance value at the input of the switch-mode power converter over the entire range of the varying voltage. The relationship between the total capacitance value and the voltage across the two capacitors is modified in a defined manner that increases, i.e., improves, the total capacitance. When the varying voltage is low, the capacitance of the first ceramic capacitor is high. In that case, the voltage across the second ceramic capacitor is relatively large, and therefore, the capacitance is lower. This will also be explained in more detail in the detailed description of the embodiments. However, the total capacitance remains relatively large, i.e., the total capacitance remains larger than when the second ceramic capacitor is arranged in parallel with the first capacitor, especially at higher voltage levels of the varying voltage.

[0007] In a further example, the ratio between the capacitance of the first ceramic capacitor and the capacitance of the second ceramic capacitor is based on the ratio between the peak voltage of the fluctuating voltage and the amplitude of the stable voltage.

[0008] The ratio between the capacitance of the first ceramic capacitor and the capacitance of the second ceramic capacitor can be determined based on the ratio between the peak voltage of the fluctuating voltage and the amplitude of the stable voltage, which allows using optimized capacitance values ​​based on the type of the switched-mode power converter.

[0009] In a further example, the switch-mode power converter is a boost converter, the first node is coupled to an input of the switch-mode power converter, and the second node is coupled to an output of the switch-mode power converter and the load.

[0010] In one example, the switch-mode power converter is a boost converter. In that case, the first node is used as an input for the switch-mode power converter and is arranged to receive the varying voltage, which may be a mains supply or a rectified mains supply. In that case, the second node is used as an output of the switch-mode power converter. In that case, the second ceramic capacitor is coupled between the input and the output of the switch-mode power converter.

[0011] In a further example, the switch-mode power converter is a buck converter, the second node is coupled to an input of the switch-mode power converter, and the first node is coupled to an output of the switch-mode power converter and the load, and the boost converter can be configured to provide the stable voltage at the second node.

[0012] The switch-mode power converter may be a buck converter rather than a boost converter. The buck converter may receive a stable voltage. The second node is the input to the switch-mode power converter. The first node is the output of the switch-mode power converter. The voltage at the output of the switch-mode power converter, i.e., the buck converter, may fluctuate. This fluctuation is provided to allow for changes in the power supplied to the load. An increase in the voltage may result in an increase in power to the load, and vice versa. The increase or decrease in voltage is defined within a range of the driver's output voltage, also known as the driver's operating window. Such a driver is sometimes called a window driver. The fluctuation may not be as large as the fluctuation of the mains voltage. Therefore, the effect obtained with the first and second ceramic capacitors may be smaller, but still provides an improvement over a conventional capacitor arrangement, i.e., two capacitors in parallel.

[0013] In a further example, the driver has a third node adapted to be coupled to a further varying voltage, the switch-mode power converter is a two-stage converter, a first stage being a boost converter and a second stage being a buck converter, the output of the boost converter is coupled to the input of the buck converter, the first node is coupled to the input of the boost converter, the second node is coupled to the output of the boost converter and the input of the buck converter, the third node is coupled to the output of the buck converter and the load, the boost converter is adapted to supply the stable voltage to the second node, and the buck converter is adapted to supply the further varying voltage to the third node.

[0014] In a further example, the driver includes a third capacitor coupled between the second node and the third node.

[0015] The driver may be a two-stage driver. The first stage of the driver is a boost converter that converts a fluctuating voltage into a stable voltage. The second stage is a buck converter that converts the stable voltage into a further fluctuating voltage. The definition of this fluctuating voltage may be the same as that already defined for the buck converter. The presence of two stages requires some adaptation to the capacitor configuration. The first ceramic capacitor is coupled to the first node. The second ceramic capacitor is coupled between the first node and the second node. The second node is the output of the boost converter and the input of the buck converter. A third capacitor may be placed between the second node and the third node. The third node is the output of the buck converter. The two-stage driver receives a fluctuating voltage, such as a mains supply or a rectified mains supply, at the first node. The boost converter converts this fluctuating voltage into a stable voltage at the second node. The stable voltage is supplied to the buck converter. The boost converter converts the stable voltage into a further fluctuating voltage and provides it at the third node. This varying voltage is supplied to the load, where it is used to provide variable power to the load, for example to provide dimming if the load is a lighting load. The two-stage driver allows both power factor correction and good power regulation for the load.

[0016] In another example, the voltage variation of the fluctuating voltage is greater than the voltage variation of the further fluctuating voltage. Preferably, the fluctuating voltage may be a mains voltage and may have a voltage variation of 0 V to, for example, 325 V. The voltage variation of the further fluctuating voltage may be significantly smaller and may depend on the operating window of the driver. The operating window is defined as the voltage range that can be generated by the driver. In that case, this is also the range of the further fluctuating voltage. The range may be, for example, a window between 40 V and 100 V. Nevertheless, the desired effect of the present invention is achieved.

[0017] In a further example, the first ceramic capacitor and the second ceramic capacitor are multilayer ceramic capacitors (MLCC capacitors).

[0018] The use of MLCC capacitors allows the use of capacitors that are very well suited to filtering and removing high frequency noise. Furthermore, these types of capacitors have a large deviating capacitance value that is highly dependent on and varies with changes in voltage across the capacitor.

[0019] In a further example, the first ceramic capacitor and the second ceramic capacitor are of the X7R type.

[0020] The use of X7R types provides a cost-effective solution. Using alternative materials such as NP0 may be too expensive for some applications, making X7R a suitable alternative in this invention. X5R materials may be even cheaper, but offer significantly worse performance.

[0021] In a further example, the switch mode power converter is configured to provide power factor correction.

[0022] Preferably, the switch-mode power converter is capable of power factor correction, which improves the power factor of the driver. The mains voltage supplied to the driver may be rectified and supplied to the switch-mode power converter. To provide good power factor correction, the rectified mains voltage may not be buffered, for example by an electrolytic capacitor. Therefore, the voltage fluctuations of the fluctuating voltage are as large as possible. The voltage across the first ceramic capacitor varies between 0 V and the peak voltage of the mains voltage.

[0023] In a further example, the switch mode power converter is a synchronous switch mode power converter.

[0024] The switched mode power converter is more energy efficient as it is a synchronous switched mode power converter. It also allows the switched mode power converter to use negative currents which are used to enable soft switching, resulting in lower EMI levels.

[0025] In a further example, the first ceramic capacitor and the second ceramic capacitor have substantially the same capacitance.

[0026] Using the same type and / or value capacitors for the first and second ceramic capacitors allows the same components to be used, and therefore allows a cheaper driver to be made, as some components, when required in double quantity, result in a cheaper component cost per component.

[0027] In another example, the first ceramic capacitor and the second ceramic capacitor have substantially different capacitances from each other.

[0028] In a further example, the driver comprises a rectifier circuit adapted to rectify an alternating current (AC) voltage into a rectified voltage, the rectified voltage being the varying voltage.

[0029] Using a rectifier circuit to rectify an AC voltage provides a rectified DC voltage that has similar fluctuations as the AC voltage. This voltage can be used for effective power factor correction. The rectified voltage is then supplied to the first node, making the fluctuating voltage equal to the rectified voltage.

[0030] In another example, a system is provided, the system comprising a driver according to the present invention and the load.

[0031] In another example, the load is a solid state lighting load and the system is a lighting fixture or lamp.

[0032] Preferably, the load is a solid state lighting load such as an LED or laser diode load. Preferably, the system is a luminaire or lamp. [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 illustrates an example of the relationship between the voltage across a capacitor and the capacitance of the capacitor. [Figure 2] An example of a driver circuit is shown. [Figure 3] 10 shows an example of the relationship between the voltage across a capacitor and the capacitance of the capacitor according to an improved configuration. [Figure 4] 10 shows another example of a driver circuit. [Figure 5] 10 shows a further example of a driver circuit. 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. 2 shows an example of a driver. The driver includes a switch-mode power converter 2. In the example shown, the switch-mode power converter 2 is a boost converter. A first node N1 is coupled to the input of the switch-mode power converter 2. A second node N2 is coupled to the output of the switch-mode power converter 2. The switch-mode power converter 2 may include an inductor L1, a switch M1, and another switch M2. A controller 1 is used to control the switch M1. The other switch M2 may be a diode. If the other switch M2 is a switch such as a transistor or a MOSFET, the other switch may also be controlled by the controller 1. If the other switch M2 is a transistor or a MOSFET, the switch-mode power converter 2 may be operated as a synchronous converter. A first ceramic capacitor C1 is coupled at one end to the first node N1. The other end of the first ceramic capacitor C1 is preferably coupled to a return path such as a ground reference. A second ceramic capacitor C2 is coupled between the first node N1 and a second node N2. The second ceramic capacitor C2 is arranged to provide a dominant capacitance between the first node N1 and the second node N2, and the first ceramic capacitor C1 is arranged to provide a dominant capacitance at the first node N1. A rectifier circuit RECT may be provided to rectify the input voltage V1. The input voltage may be any type of voltage, preferably a commonly used mains voltage, such as 230 V at 50 Hz or 120 V at 60 Hz. An optional capacitor C3 may be used to buffer the output of the switch-mode power converter 2 so that the voltage can be more reliably and easily stabilized. The optional capacitor C3 may be an electrolytic capacitor. A stabilized voltage is supplied to the second node N2 and the load LED by the boost converter. The first ceramic capacitor C1 is exposed to the fluctuating voltage present at the first node N1. Here, an example is provided to illustrate the relationship between the voltages at the first node N1 and the second node N2 and the capacitance values ​​of the first ceramic capacitor C1 and the second ceramic capacitor C2.As can be seen from FIG. 1, when the fluctuating voltage is low, e.g., 0 V, the capacitance of the first ceramic capacitor C1 is at its maximum, and point C / C0 is at 1 on the Y-axis in FIG. 1. The stable voltage at the second node N2 is adjusted to a steady voltage of, e.g., 400 V. The differential voltage across the second ceramic capacitor C2 is 400 V, i.e., 400 V relative to 0 V. This means that the capacitance of the second ceramic capacitor C2 is at its minimum. This is shown in FIG. 1, where the value of C / C0 is Cmin / C0. In this example, vCmax is 400 V. Now, as the fluctuating voltage increases, if the fluctuating voltage reaches, e.g., 200 V, the voltage across the first ceramic capacitor C1 is 200 V. The voltage across the second ceramic capacitor C2 is 400 V - 200 V = 200 V. In the example shown, the polarity of the voltage across the capacitors is irrelevant in determining the capacitance value. The amplitude is considered to be the determining factor for the voltage. The voltage across the first ceramic capacitor C1 and the voltage across the second ceramic capacitor C2 are identical. This results in the capacitance being identical. The fluctuating voltage increases even further, to a maximum voltage of, for example, 325 V. The voltage across the first ceramic capacitor C1 is 325 V. The voltage across the second ceramic capacitor C2 is 400 V - 325 V = 75 V. The voltage across the first ceramic capacitor C1 is relatively large, resulting in a low capacitance value. The voltage across the second ceramic capacitor C2 is relatively low, resulting in a relatively large capacitance value. The total capacitance available to filter and remove noise generated by the switch-mode power converter 2 is greater than if the capacitances C1 and C2 were coupled in parallel at the first node N1. This means that for the same capacitor, a higher overall minimum capacitance can be achieved, effectively improving the utilization of the capacitor.

[0037] Preferably, the first ceramic capacitor C1 is the only capacitor coupled to the first node N1. Preferably, the second ceramic capacitor C2 is the only capacitor coupled between the first node N1 and the second node N2. Preferably, the first ceramic capacitor C1 is coupled between the first node N1 and the return path. By having only ceramic capacitors, the total capacitance at the input of the driver is relatively low. This is mainly because the capacitance value of a ceramic capacitor is significantly lower than the capacitance value of an electrolytic capacitor or a film capacitor. Therefore, the lower the input capacitance of the driver, the better the power factor can be corrected, and therefore power factor correction can be performed in an efficient manner. Since ceramic capacitors are well suited for high-frequency operation, this also allows the driver to operate at high frequencies. Preferably, the driver can be operated at frequencies above 500 kHz.

[0038] FIG. 3 shows an example of a graph illustrating the relationship between the capacitance value of the combined capacitance of the first ceramic capacitor C1 and the second ceramic capacitor C2 and a varying voltage, e.g., a mains input voltage or a rectified mains input voltage. On the X-axis, the varying voltage at the first node N1 is defined from 0 V to a maximum voltage vCmax. On the Y-axis, the relative capacitance of the capacitors is defined. Cin defines the actual capacitance of the capacitor, and C0 defines the absolute capacitance of the capacitor. Since there are two capacitors where Cin of both capacitors is equal to C0, the maximum relative capacitance is 2. The minimum capacitance that can be achieved in conventional applications is a capacitance value of 2Cmin. Therefore, the minimum relative capacitance is defined as 2Cmin / C0. 2Cmin / C0 is defined at the maximum voltage vCmax. Preferably, the voltage across the capacitors may not exceed this voltage. In the example shown, for simplicity, the first ceramic capacitor C1 and the second ceramic capacitor C2 are assumed to be identical. If the second ceramic capacitor C2 is placed in parallel with the first ceramic capacitor C1 rather than between the first node N1 and the second node N2, the dashed line shows the relationship also shown in FIG. 1. The capacitance at the lowest voltage level is twice the capacitance value of a single capacitor because the sum of the capacitances of the first ceramic capacitor C1 and the second ceramic capacitor C2 is equal to twice the capacitance of a single capacitor. An approximately exponential decrease in capacitance occurs with increasing voltage levels across the capacitors. At the maximum supply voltage vCmax, the total capacitance is reduced to 2Cmin / C0. However, the solid line shows the total capacitance of the first ceramic capacitor C1 and the second ceramic capacitor C2 according to the present invention. The capacitance at the lowest voltage is lower than that of the conventional configuration, i.e., 1+Cmin / C0. An increase in the fluctuating voltage level causes the capacitance of the first ceramic capacitor C1 to decrease approximately exponentially and the capacitance of the second ceramic capacitor C2 to increase approximately exponentially.Initially, the capacitance of the first ceramic capacitor C1 decreases faster than the capacitance of the second ceramic capacitor C2 increases, and therefore the total capacitance decreases as the voltage increases. This is because the voltage level increases beyond the threshold.

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[0039] In the example shown, at the maximum available voltage vCmax, the total capacitance is 1 + Cmin / C0. The total capacitance may be larger or smaller depending on the driver design choices. The minimum capacitance value is always larger than 2Cmin / C0; in this example, the minimum capacitance value is 4Cmin / C0. In the example shown, the minimum capacitance value is twice as large as in conventional solutions. The minimum capacitance value achieved with the present invention, defined as 4Cmin / C0, is preferably between 50% and 20% of the maximum possible voltage, defined as C0. More preferably, the minimum capacitance value is between 40% and 30%.

[0040] FIG. 4 shows another example of a driver. The driver includes a switch-mode power converter 2 configured as a buck converter. The buck converter has an input coupled to a second node N2. The switch-mode power converter 2 may include an inductor L1, a switch M1, and another switch M2. A controller 1 is used to control the switch M1. The other switch M2 may be a diode. If the other switch M2 is a switch such as a transistor or a MOSFET, the other switch may also be controlled by the controller 1. If the other switch M2 is a transistor or a MOSFET, the switch-mode power converter 2 may be operated as a synchronous converter. A second ceramic capacitor C2 is coupled to the second node N2. Preferably, the second node N2 is coupled to an optional capacitor C3, which provides a buffer to keep the voltage at the second node N2 stable. A rectifier circuit RECT may be provided to rectify the input voltage V1. The input voltage may be any type of voltage, preferably a commonly used mains voltage, such as 230 V at 50 Hz or 120 V at 60 Hz. A first node N1 is coupled to the output of the buck converter. A first ceramic capacitor C1 is coupled to the first node N1. The buck converter supplies a fluctuating voltage to the first node N1, which then supplies the load LED. This can be defined as the operating window of the buck converter. The fluctuating voltage does not have to fluctuate over a short period of time; the voltage can remain constant throughout the period. The controller 1 of the buck converter can be provided with a configuration that allows the voltage supplied by the buck converter to vary. The voltage can be increased or decreased within the operating window of the buck converter. In this case, this can be identified as an additional fluctuating voltage. The capacitance value must be guaranteed throughout the entire operating window. Therefore, a second ceramic capacitor C2 is coupled between the first node N1 and the second node N2. The effect described in the description for FIG. 3 is obtained in a similar manner. The effective minimum capacitance value is increased.

[0041] FIG. 5 shows another example of a driver. The driver has a switch-mode power converter 2 configured as a two-stage converter. The first stage is shown as a boost converter. The boost converter has the same function as the boost converter shown in FIG. 2. A fluctuating voltage, such as a mains voltage, is supplied to the driver's input. The fluctuating voltage is rectified by a rectifier circuit RECT. The rectifier circuit RECT provides a fluctuating voltage that is supplied to a first node N1. The boost converter supplies a stable voltage to a second node N2. The voltage may be buffered by an optional capacitor C3. The stable voltage is supplied to a second stage, a buck converter. The buck converter receives the stable voltage at the second node N2. The buck converter supplies a further fluctuating voltage to a third node N3 and a load LED. The load LED is also coupled to the third node N3. A capacitor C5 may be provided to stabilize the voltage at the output of the buck converter, allowing the buck converter to operate at a stable voltage within a voltage window. The second ceramic capacitor C2 is coupled between the first node N1 and the second node N2. A third capacitor C4 may be disposed between the second node N2 and the third node N3. In this case, the total minimum capacitance at the input of the boost converter and the total minimum capacitance at the output of the buck converter are improved. Optionally, only one of the second ceramic capacitor C2 and the third ceramic capacitor C4 is used. In some examples, the second ceramic capacitor C2 is disposed and the third ceramic capacitor C4 is not disposed.

[0042] In the example shown, the total variation in capacitance value is reduced. Although the maximum capacitance value is reduced, this is not a problem for the driver design. Rather, the absolute minimum capacitance value is the crucial parameter. With the present invention, the absolute minimum capacitance value is increased, in the example shown by a factor of two, which allows for an improvement in the overall design of the driver.

[0043] In the example shown, the capacitance of the first ceramic capacitor C1 and the capacitance of the second ceramic capacitor C2 are assumed to be substantially the same in value, and the capacitance of the third capacitor C4 is also assumed to be substantially the same in value.

[0044] A driver according to any of these examples can be used in many applications where a load should be supplied with regulated power. An example of a load that can be powered by the driver may be a load that can be USB-C powered. An example of a load that can be powered by the driver may be, but is not limited to, a laptop, a mobile phone, a lighting load such as an LED or laser diode, a monitor, or a television.

[0045] Preferably, the switch-mode power converter is of the non-isolated type: a buck converter or a boost converter are the preferred topologies, therefore no isolation transformer is used, making the design of the switch-mode power converter simpler.

[0046] Preferably, the driver is integrated into the luminaire or lamp, but the driver may also be a stand-alone driver.

[0047] In the examples, the capacitor is shown as a single capacitor. It should be understood that more capacitors of the same type may be used in various configurations to achieve, for example, a desired capacitance value or voltage rating. Thus, the capacitor may consist of multiple capacitors in series and / or parallel.

[0048] 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 driver for driving a load, a first node adapted to be coupled to a varying voltage; a second node adapted to be coupled to a stable voltage; a switched mode power converter configured to convert the varying voltage to the stable voltage or to convert the stable voltage to the varying voltage; a first ceramic capacitor coupled to the first node; and A driver having a second ceramic capacitor coupled between the first node and the second node, the second ceramic capacitor arranged to provide a dominant capacitance between the first node and the second node, and the first ceramic capacitor arranged to provide a dominant capacitance to the first node.

2. 2. The driver of claim 1, wherein the ratio between the capacitance of the first ceramic capacitor and the capacitance of the second ceramic capacitor is based on the ratio between the peak voltage of the fluctuating voltage and the amplitude of the stable voltage.

3. 3. The driver of claim 1, wherein the switch-mode power converter is a boost converter, the first node is coupled to an input of the switch-mode power converter, and the second node is coupled to an output of the switch-mode power converter and to the load.

4. 3. The driver of claim 1, wherein the switch-mode power converter is a buck converter, the second node is coupled to an input of the switch-mode power converter, and the first node is coupled to an output of the switch-mode power converter and to the load.

5. 3. The driver of claim 1, wherein the driver has a third node adapted to be coupled to a further varying voltage, the switch-mode power converter is a two-stage converter, a first stage being a boost converter and a second stage being a buck converter, the output of the boost converter is coupled to the input of the buck converter, the first node is coupled to the input of the boost converter, the second node is coupled to the output of the boost converter and the input of the buck converter, the third node is coupled to the output of the buck converter and the load, the boost converter is adapted to supply the stable voltage to the second node, and the buck converter is adapted to supply the further varying voltage to the third node.

6. 6. The driver of claim 5, further comprising a third capacitor coupled between the second node and the third node.

7. 7. A driver as claimed in claim 5 or 6, wherein the voltage variation of the fluctuating voltage is greater than the voltage variation of the further fluctuating voltage.

8. 8. The driver according to claim 1, wherein the first ceramic capacitor and the second ceramic capacitor are multilayer ceramic capacitors.

9. 9. A driver according to any one of the preceding claims, wherein the first and second ceramic capacitors are of the X7R type.

10. 10. A driver as claimed in any preceding claim, wherein the switch mode power converter is configured to provide power factor correction.

11. 11. A driver according to any one of the preceding claims, wherein the switch mode power converter is a synchronous switch mode power converter.

12. 12. The driver of claim 1, wherein the first ceramic capacitor and the second ceramic capacitor have substantially the same capacitance.

13. 13. A driver according to any one of the preceding claims, further comprising a rectifier circuit adapted to rectify an alternating voltage into a rectified voltage, said rectified voltage being said varying voltage.

14. A system comprising a driver according to any one of claims 1 to 13 and said load.

15. 15. The system of claim 14, wherein the load is a solid-state lighting load and the system is a lighting fixture or lamp.

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