Branching linear LED driving device

Abstract

The invention relates to a branching linear LED driving device. The apparatus includes a first LED module, a charge storage device, and at least one other LED module. The first LED module includes a set of LEDs connected in series and a current controller. Each other LED module includes an LED set and a switching device connected in parallel. The charge storage device is connected in parallel to the first LED module. A current controller controls the current flowing through the LED set of the first LED module to facilitate a response to the LED set of any other LED module being bypassed by its respective switching device.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of LED modules and in particular to LED driving arrangements utilizing a tapped linear driver. BACKGROUND

[0002] The use of artificial light is increasing, resulting in an increasing demand for LED devices, for example for automotive applications.

[0003] Existing forms of LED driving arrangements include tapped linear drivers. Typically, such a driver comprises a string of series connected light emitting diodes (LEDs). Typically, the cathode terminal of each LED (except one LED in the string) is controllably coupled to the anode terminal of that LED or to ground or a reference voltage by a respective switch, in order to control the current through the subsequent LEDs in the string. The current through each respective LED in the string is controlled in response to the available power for driving the string. International patent application number WO 2012 / 168827 A2 provides one example of an LED module having a tapped linear driver.

[0004] US patent number US 8,330,390 B2 provides another example which also utilizes a charge storage device in the form of a capacitor. The charge storage device is selectively connected to the LED string when the mains power supply is below a predetermined value, otherwise the charge storage device is charged by the mains power supply.

[0005] There is a continuing desire to improve the performance of LED driving arrangements, in particular those having a tapped linear driver. SUMMARY

[0006] The present invention is defined by the claims.

[0007] According to an example in accordance with one aspect of the present invention, there is provided a tapped linear LED driving arrangement comprising: a first input terminal and a second input terminal for receiving an input voltage from an external power supply; a first LED module comprising a first set of LEDs comprising one or more LEDs connected in series and a first current controller connected in series to the first set of LEDs; a second LED module connected in series to the first LED module and comprising a second set of LEDs comprising one or more LEDs connected in series and a first switching device connected in parallel to the second set of LEDs; and a charge storage device connected in parallel to the first LED module.

[0008] The first LED module and the second LED module are connected in series between the first input terminal and the second input terminal.

[0009] The first current controller is configured to, in response to the magnitude of the input voltage rising above a first threshold, reduce or prevent current from flowing through the first set of LEDs such that current flows from the input terminal through the charge storage device to the second LED module; and in response to the magnitude of the input voltage falling below the first threshold, increase the current flowing through the first set of LEDs such that current flows through the first set of LEDs from the charge storage device.

[0010] The first switching arrangement is configured to, in response to the magnitude of the input voltage falling below a second threshold, bypass the second set of LEDs, thereby preventing the second set of LEDs from conducting current; and in response to the magnitude of the input voltage rising above the second threshold, prevent the bypassing of the second set of LEDs, thereby allowing the second set of LEDs to conduct current, wherein the first current controller is configured to control the maximum current flowing through the first set of LEDs such that the first set of LEDs emits the same or almost the same amount of light as the amount of light emitted by the second set of LEDs when the input voltage is above the second threshold.

[0011] The proposed LED driving arrangement provides a novel tap linear driver with LED modules of different forms. In particular, a charge storage device is connected in parallel to (only) one of the LED modules, and a current controller is used to control the current flowing from the charge storage device through the set of LEDs of the aforementioned LED module. This facilitates light emission even in the absence of sufficient power from an external power source, thereby improving current control in the LED driving arrangement. Furthermore, the tap linear LED driving arrangement can generate more uniform light over time, as the first LED module and the second LED module provide approximately similar light output over time.

[0012] The proposed method allows the current through the first set of LEDs to be controlled, for example, in response to changes in the current flowing through any other set of LEDs caused by standard operation of the tap linear driver. This allows any reduction in light emission due to bypassing of other sets of LEDs (as typically occurs in TLDs) to be easily compensated for.

[0013] The proposed mechanism therefore provides a system by which the current controller is able to independently allocate the total current provided via an external power source between a charging current for the charge storage device or a charging current for powering the set of LEDs. This facilitates compensation for any output power fluctuations of the LED driving arrangement, for example, to achieve constant or near constant light output for a pulsating input voltage.

[0014] The second threshold can be less than or equal to the first threshold. In this way, the first set of LEDs is able to immediately act on the second set of LEDs to stop current conduction. This ensures that the LED driving arrangement will continuously output light (when powered by an external power source).

[0015] The second threshold can be equal to a sum of the forward voltage of the first set of LEDs and the forward voltage of the second set of LEDs.

[0016] The first current controller can comprise a controllable current source.

[0017] The apparatus can further comprise a controller configured to control operation of the first current controller and the first switching device.

[0018] The first set of LEDs can comprise a first LED and a second LED connected in series. The first set of LEDs can further comprise an LED bypass device connected in parallel to the second LED, the LED bypass device configured to: in response to a magnitude of a voltage across the first LED or a magnitude of a voltage across the charge storage device falling below a first LED threshold, bypass the second LED, thereby preventing the second LED from conducting current; and in response to the magnitude of the voltage across the first LED or the magnitude of the voltage across the charge storage device rising above the first LED threshold, prevent bypassing of the second LED, thereby allowing the second LED to conduct current.

[0019] The apparatus can further comprise a second current source controller connected in series with the second LED module, the second current source controller for controlling a maximum current flowing through the first LED module and the second LED module. This approach allows the current through the second LED module to be controlled so that the total light output by the driving apparatus can be maintained or kept at a consistent level.

[0020] The apparatus can further comprise a third LED module connected in series with the second LED module, and comprising: a third set of LEDs comprising one or more LEDs connected in series; and a second switching device connected in parallel to the third set of LEDs, the second switching device configured to: in response to a magnitude of the input voltage falling below a third threshold, bypass the third set of LEDs, thereby preventing the third set of LEDs from conducting current; and in response to the magnitude of the input voltage rising above the third threshold, prevent bypassing of the third set of LEDs, thereby allowing the third set of LEDs to conduct current. The third threshold is different to the second threshold.

[0021] In some examples, the first current controller is configured to: in response to the magnitude of the input voltage falling below a fourth threshold, increase the current flowing through the first set of LEDs; and in response to the magnitude of the input voltage rising above the fourth threshold, decrease the current flowing through the first set of LEDs.

[0022] The fourth threshold is less than the first threshold.

[0023] The first current controller can be further configured such that the current flowing through the first set of LEDs when the magnitude of the input voltage is between the fourth threshold and the first threshold is greater than the current flowing through the first set of LEDs when the magnitude of the input voltage is above the first threshold.

[0024] The fourth threshold value can be greater than or equal to the third threshold value.

[0025] It is also proposed an LED driving system comprising any of the tap linear LED driving devices disclosed herein; and a rectifier connected to the first input terminal and the second input terminal, the rectifier being configured to rectify a mains power supply to generate an input voltage for the first input terminal and the second input terminal.

[0026] These and other aspects of the application will become apparent and elucidated from the embodiments described hereinafter. BRIEF DESCRIPTION OF DRAWINGS

[0027] For a better understanding of the present application, and to show how it can be put into effect, reference will now be made, purely by way of example, to the accompanying drawings in which:

[0028] Figure 1 A proposed LED driving system is shown;

[0029] Figure 2 Waveforms in a proposed LED driving system are shown;

[0030] Figure 3 Another proposed LED driving system is shown;

[0031] Figure 4 Waveforms in another proposed LED driving system are shown; and

[0032] Figure 5 Another proposed LED driving system is shown. DETAILED DESCRIPTION

[0033] The application will be described with reference to the accompanying drawings.

[0034] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of apparatuses, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the present application. These and other features, aspects, and advantages of the apparatuses, systems and methods of the present application will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the drawings are only schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the drawings and that these are used to indicate the same or similar components from drawing to drawing.

[0035] A tapped linear LED driving apparatus is provided. The apparatus comprises a first LED module, a charge storage device and at least one further LED module. The first LED module comprises a set of LEDs connected in series and a current controller. Each further LED module comprises a set of LEDs connected in parallel and a switching device. The charge storage device is connected in parallel to the first LED module. The current controller controls the current through the set of LEDs of the first LED module to facilitate a response of the sets of LEDs of any further LED module being bypassed by their respective switching device.

[0036] Figure 1 An LED driving system 10 comprising a tapped linear LED driving apparatus 100 according to an embodiment is shown.

[0037] The LED driving system 10 further comprises a rectifier 101 configured to rectify an AC mains supply V AC , i.e. an alternating current supply, to produce an input voltage V IN for the tapped linear LED driving apparatus. Suitable examples of rectifiers are well known in the art and include a bridge rectifier. Thus, the input voltage V IN is a rectified version of a sinusoidal signal, i.e. a pulsating signal pulsating at twice the frequency of the AC mains supply.

[0038] The tapped linear LED driving apparatus 100 comprises a first input terminal 508 and a second input terminal 509. The input terminals receive the input voltage V IN from an external supply, in the shown example the rectifier 101.

[0039] The tapped linear LED driving apparatus 100 further comprises a first LED module 201, 403 coupled to the first input terminal 508.

[0040] The first LED module comprises a first set of LEDs 201 comprising one or more LEDs connected in series. In the shown example, the first set of LEDs comprises only a single LED, but the skilled person will understand that this can easily be replaced by a series / parallel connection of two or more LEDs.

[0041] The first LED module further comprises a first current controller 403 connected in series with the first set of LEDs 201. Thereby, the first current controller 403 is able to control the current through the first set of LEDs.

[0042] The tapped linear LED driving apparatus 100 further comprises a second LED module 202, 301 connected in series with the first LED module. More specifically, the first LED module and the second LED module are connected in series between the first input terminal 508 and the second input terminal 509.

[0043] The second LED module comprises a second set of LEDs 202 (comprising one or more LEDs) and a first switching arrangement 301. The first switching arrangement is capable of selectively bypassing the second set of LEDs. In the illustrated example, the second set of LEDs 202 comprises only a single LED, but the skilled person will appreciate that this can readily be replaced by a series / parallel connection of two or more LEDs.

[0044] The tapped linear LED driving arrangement 100 further comprises a charge storage device 402, e.g. a capacitor, connected in parallel to the first LED module. Thus, the charge storage device 402 is similarly connected in series with the second LED module.

[0045] The operation of the first current controller 403 and the first switching arrangement 301 can be performed / controlled by a controller 405. The controller 405 can be configured to receive a signal (not shown) representative of the magnitude of the input voltage and / or other feedback signals for controlling the current (e.g. a signal representative of the magnitude of the current through the first set of LEDs and / or the second set of LEDs).

[0046] The first current controller 403 is configured to control the current flowing through the first set of LEDs 201 in response to the magnitude of the input voltage V IN . In this way, as a direct consequence of Kirchhoff’s current law, the first current controller 403 is capable of controlling the current flowing to / from the energy storage device 402, i.e. the charging current.

[0047] More specifically, the first current controller 403 is configured to control whether the first set of LEDs 201 draws power from the charge storage device 402, or to prevent the first set of LEDs 201 from drawing such power, such that the charge storage device is charged (and current is caused to flow to the second LED module 202, 301).

[0048] The first current controller 403 is configured to, in response to the magnitude of the input voltage rising above a first threshold, reduce or prevent current from flowing through the first set of LEDs 201, such that (at least some) current flows from the input terminal 508 through the charge storage device 402 to the second LED module 202, 301.

[0049] Thus, when the input voltage is sufficiently high (e.g. sufficient to drive the second set of LEDs), the current through the first set of LEDs is reduced, and current can flow to the second LED module.

[0050] Similarly, the first current controller 403 is configured to, in response to the magnitude of the input voltage V IN falling below a first threshold, increase the current flowing through the first set of LEDs, such that current flows from the charge storage device through the first set of LEDs.

[0051] In this way, the charge storage device 402 can be controlled by the current controller 403 to selectively drive or power the first LED set 201 based on whether the input voltage exceeds the first threshold. The current controller thereby also controls the current flow to and through the charge storage device 402.

[0052] The first switching arrangement 301 is configured to selectively bypass the second LED set 202 in response to whether the magnitude of the input voltage V IN drops below a second threshold. When below the second threshold, the first switching arrangement 301 bypasses the second LED set (i.e. such that it no longer conducts current). When above the second threshold, the first switching arrangement 301 permits or allows the second LED set to conduct current.

[0053] In a simple example, the first and second thresholds are the same. Thus, only one of the first and second LED sets conducts current at a time. This helps to sustain the light emission.

[0054] In other examples, the first and second thresholds are different, e.g. the first threshold is greater than or equal to the second threshold. This will cause the second LED set to start conducting current before the current through the first LED set is reduced or prevented (as the magnitude of the input voltage rises). Similarly, this will cause the first LED set to start conducting current before the second LED set is bypassed (as the magnitude of the input voltage decreases). This can advantageously avoid any drop in light output below a certain light threshold.

[0055] The first current controller 403 is able to effectively apportion the current between the charge storage device 402 and the first LED set 201. This allows the first current controller 403 to compensate for any fluctuations or changes in the output power of any other LED set 202 (e.g. due to the bypassing of the LED set(s) described above due to insufficient voltage), whilst still providing a mechanism for charging the charge storage device.

[0056] Figure 2 Waveforms are shown representing the voltage (V) or power (P) of various signals over time (t). Figure 2 A method for controlling the tap linear LED driver apparatus 100 described in Figure 1 is described in relation to a simple example (where the thresholds are the same).

[0057] The first waveform 210 shows the voltage provided to the input terminals 508, 509. The second waveform 220 shows the power drawn by the first LED set 201. The third waveform 230 shows the power drawn by the second LED set, and the fourth waveform 240 shows the power storage of the charge storage device 402 over time.

[0058] Figure 2It is shown how the current through the first LED set is stopped, so that the first LED set does not draw or draws negligible power, when the voltage provided to the input terminals (i.e. the input voltage) reaches a threshold value T1 at a first point in time t1 (and subsequently rises). Thereafter, current flows through the charge storage device (which charges the charge storage device as shown) and is provided to the second LED set.

[0059] When the voltage is higher than this threshold value T1, the first switch means is controlled to prevent the bypassing of the second LED set. The second LED set is thus able to conduct the current provided by the charge storage device. The second LED set thereby outputs light.

[0060] When the voltage is lower than the threshold value T1 (i.e. after a second point in time t2), the first switch means is controlled to bypass the second LED set, so that the second LED set is prevented from conducting current.

[0061] Furthermore, when the voltage is lower than the threshold value T1, the current through the first LED set is increased, so that the first LED set draws power and emits light. As Figure 2 shown, the current through the first LED set is provided by a combination of the power at the input terminals 508, 509 (when high enough) and the power discharged from the charge storage device.

[0062] As mentioned before, in this simple example, the first threshold value and the second threshold value (for the input voltage) are the same. The value of the threshold value can be equal to the forward voltage of the second LED set (e.g. plus any headroom voltage required to drive any optional further components).

[0063] Figure 1 And Figure 2 It is also shown an advantageous embodiment, wherein the tapped linear LED driver 100 further comprises a second current source controller 102 connected in series with the second LED module 202, 301, for controlling the maximum current flowing through the first LED module and the second LED module.

[0064] In particular, the second current source controller 102 can be configured to control the maximum current flowing through the second LED set (when the second LED set conducts current) so that the amount of light emitted by the second LED set does not exceed a certain threshold value. Any excess power provided to the input terminals 508, 509 will be stored by the energy charge storage device 402.

[0065] Similarly, the first current source controller 403 can be configured to control the maximum current flowing through the first LED set (when the first LED set conducts current) such that it emits the same or almost the same amount of light as emitted by the second LED set when the second LED set conducts current. This advantageously provides consistent or constant total light output by the tapped linear LED driver 100.

[0066] In other words, the combination of the power drawn by the first LED set and the second LED set (assuming both LED sets comprise similar components) is maintained by appropriate current control.

[0067] Figure 3 A more advanced embodiment of the LED driving system 30 is shown, in which the tapped linear LED driver 300 further comprises a third LED module 203, 302 connected in series with the second LED module.

[0068] The third LED module comprises a third LED set 203 comprising one or more LEDs connected in series. The third LED module further comprises a second switching device 302 connected in parallel to the third LED set.

[0069] The second switching device 302 is configured to, in response to the amplitude of the input voltage falling below a third threshold, bypass the third LED set 203, thereby preventing the third LED set from conducting current; and in response to the amplitude of the input voltage rising above the third threshold, prevent the bypassing of the third LED set, thereby allowing the third LED set to conduct current.

[0070] The third threshold is different from the second threshold. This effectively allows the second LED set and the third LED set to be turned off at different points of the cycle of the input voltage. This follows standard tapped linear driver operation.

[0071] In some examples, the first current controller 403 is configured to, in response to the amplitude of the input voltage falling below a fourth threshold, increase the current flowing through the first LED set; and in response to the amplitude of the input voltage rising above the fourth threshold, decrease the current flowing through the first LED set.

[0072] The fourth threshold is less than the first threshold.

[0073] The first current controller 403 is further configured such that the current (i.e. the magnitude of the current) flowing through the first LED set 201 when the amplitude of the input voltage is between the fourth threshold and the first threshold is greater than the current flowing through the first LED set when the amplitude of the input voltage is above the first threshold.

[0074] Thus, the current through the first set of LEDs can be effectively staged. When the magnitude of the input voltage is below a fourth threshold, the current is a first value. When the magnitude of the input voltage is between the fourth threshold and a first (higher) threshold, the current is at a second value less than the first value. When the magnitude of the input voltage is above the first threshold, the current is at a third value (e.g. 0 or negligible) less than the second value.

[0075] The technique allows the current through the first set of LEDs to be controlled to reflect the power drawn by the second and third sets of LEDs.

[0076] Thus, the first threshold can control whether the first set of LEDs is allowed or prevented from conducting current; the second threshold can control whether the second set of LEDs is bypassed; the third threshold can control whether the third set of LEDs is bypassed; and the fourth threshold can control whether the first set of LEDs conducts a first non-zero current or a different second non-zero current.

[0077] The thresholds can be selected or configured so that the total current or power drawn by all the sets of LEDs remains effectively constant. This ensures or achieves a constant light output (assuming that sets of LEDs drawing the same current emit the same amount of light).

[0078] Figure 4 Waveforms showing voltage (V) or power (P) representing various signals as a function of time (t) are shown. Figure 4 for controlling a tapped linear LED driver according to examples. Figure 3 Methods of operation of the tapped linear LED driver 300 described in the Background section.

[0079] A first waveform 410 shows the voltage provided to the input terminals 508, 509. A second waveform 420 shows the power drawn by the first set of LEDs 201. A third waveform 430 shows the power drawn by the second set of LEDs 202 and the third set of LEDs 203, and a fourth waveform 440 shows the power storage of the charge storage device 402 over time.

[0080] Figure 4 A scenario is shown in which the voltage 410 across the input terminals (i.e. the input voltage) is initially zero or negligible at a zero time point to. Initially, both the second and third sets of LEDs are bypassed by their respective switching devices, and the first set of LEDs is controlled to conduct the power 420 (provided by the energy charge storage device).

[0081] As the voltage at the input terminal rises, it reaches a first voltage threshold T1 at a first time point t1 (and subsequently rises). The first voltage threshold T1 occurs when the voltage provided at the input terminal is sufficient to drive only one of the second LED set 202 and the third LED set 203, but not both, e.g. below the forward voltage of the first, second and third LED sets.

[0082] At this point, one of the second and third LED sets is no longer bypassed, while the other is bypassed. Therefore, the power 430 drawn by the second and / or third LED set increases.

[0083] At this point, the current through the first LED set also decreases, so that the first LED set draws less power 420. In particular, the current through the first LED set can be reduced so that the combined power drawn by all LED sets in the tapped linear LED driver 300 remains constant or close to constant. This maintains the overall light level output by the LED driver.

[0084] It will be clear that the first time point t1 occurs when the amplitude of the input voltage reaches or exceeds the second threshold or the third threshold, whichever is smaller. At this point, one of the second and third LED sets is no longer bypassed, i.e. is allowed to conduct current.

[0085] It will also be clear that the first time point t1 occurs when the amplitude of the input voltage reaches or exceeds the fourth threshold. At this point, the current drawn by the first LED set decreases.

[0086] In the illustrated scenario, the fourth threshold is equal to the second threshold or the third threshold. This facilitates the ease of control.

[0087] However, this is not essential. For example, the fourth threshold can be greater than one of the second threshold or the third threshold (but not greater than the other) to allow the first LED set to maintain its power draw for a non-zero period of time after one of the second and third LED sets is no longer bypassed. This can avoid or reduce any sudden drop in light output by the LED driver 300.

[0088] As the voltage at the input terminal continues to rise, it will reach a second voltage threshold T2 at a second time point t2. The second voltage threshold T2 occurs when the voltage provided at the input terminal is sufficient to drive both the second LED set 202 and the third LED set 203, i.e. above the forward voltage of the first, second and third LED sets.

[0089] At this point, neither the second LED set nor the third LED set is bypassed. Therefore, the power 430 drawn by the second and / or third LED set increases.

[0090] At this time, the current through the first set of LEDs is further reduced as well, so that the first set of LEDs draws less power 420. In particular, the current through the first set of LEDs can be reduced so that the combined power drawn by all sets of LEDs in the tapped linear LED driving device 300 remains constant or close to constant. This maintains the overall light level output by the LED driving device.

[0091] It is clear that the second point in time tl occurs when the amplitude of the input voltage reaches or exceeds the second threshold and the third threshold. At this time, neither the second set of LEDs nor the third set of LEDs is bypassed, i.e. both sets of LEDs are allowed to conduct current.

[0092] It is clear that the second point in time t2 occurs when the amplitude of the input voltage reaches or exceeds the first threshold. At this time, the current drawn by the first set of LEDs is reduced, e.g. to zero or a negligible value.

[0093] In the illustrated scenario, the first threshold is equal to the larger of the second threshold or the third threshold. This facilitates the ease of control.

[0094] However, this is not necessary. For example, the fourth threshold can be greater than either of the second threshold or the third threshold to allow the first set of LEDs to maintain its power draw for a non-zero period of time after both the second set of LEDs and the third set of LEDs are no longer bypassed. This can avoid or reduce any sudden drop in light output by the LED driving device 300.

[0095] The same process occurs in reverse when the power 410 at the input terminal drops.

[0096] In particular, the voltage 410 provided to the input terminal, i.e. the input voltage, again drops to the second voltage threshold T2 at a third point in time t3. At this time, one of the second set of LEDs and the third set of LEDs is bypassed, which allows the other to continue conducting power and emitting light. As a result, the power drawn by the second set of LEDs or the third set of LEDs is reduced. To offset the reduction in light emission, the current through the first set of LEDs is increased.

[0097] The voltage 410 provided to the input terminal, i.e. the input voltage, then again drops to the first voltage threshold Tl at a fourth point in time t4. At this time, both the second set of LEDs and the third set of LEDs are bypassed. To offset the reduction in light emission by the second set of LEDs and the third set of LEDs, the current through the first set of LEDs is again increased.

[0098] Throughout the process, the current through the sets of LEDs can be effectively controlled so that the total light output by the device 300 remains effectively constant.

[0099] This technique helps to avoid or reduce the perception of pulsating light output by the apparatus.

[0100] More specifically, the current through the first set of LEDs can be controlled to draw a power equal to the total average power (at the input terminal) minus the (predicted) instantaneous power drawn by the second and third sets of LEDs. This can achieve constant or near-constant light output by the overall apparatus.

[0101] Figure 5 Another example of an LED driving system 50 is shown, which comprises another example of a tapped linear LED driving apparatus 500. In this example, the first set of LEDs 201 is effectively configured to comprise another tapped linear LED driving apparatus, i.e. a slave tapped linear LED driving apparatus.

[0102] In this way, the first set of LEDs 2011 comprises a first LED LD1 and at least one second LED LD2, LD3 connected in series. For each second LED, the first set of LEDs further comprises an LED shunt device SD1, SD2 connected in parallel to its respective second LED.

[0103] Hence, each LED shunt device SD1, SD2 is connected in parallel to a respective second LED LD2, LD3 of the first set of LEDs.

[0104] Each LED shunt device is configured to, in response to a magnitude of a voltage across the first LED or a magnitude of a voltage across the charge storage device falling below a respective LED threshold, shunt the respective second LED, thereby preventing the respective second LED from conducting current.

[0105] Each LED shunt device is further configured to, in response to the magnitude of the voltage across the first LED or the magnitude of the voltage across the charge storage device rising above the respective LED threshold, prevent shunting of the respective second LED, thereby allowing the second LED to conduct current.

[0106] The respective LED threshold of each LED shunt device is different. In particular, for each LED threshold, the minimum difference from any other LED threshold is preferably equal to the forward voltage of the LED.

[0107] Hence, the LED driving system 50 effectively operates with a nested TLD, i.e. provides an additional TLD which allows for further voltage swing across the charge storage device.

[0108] The above and shown examples make use of a plurality of LED modules, one of which is connected in parallel to the charge storage device. Although the shown examples only show up to three LED modules, the skilled person will understand that the principles outlined herein can be applied to tapped linear LED driving arrangements with more than three LED modules connected in series.

[0109] The arrangement comprises a first LED module and at least one other LED module.

[0110] Each other LED module (except the first LED module) can comprise a respective set of LEDs connected in parallel to a respective switching device. Each of these LED modules is also associated with a respective different threshold value. For each LED module, the switching device is configured to bypass the set of LEDs when the input voltage falls below the respective threshold value. The value of each threshold value can represent the sum of the forward voltage(s) of a different number of sets of LEDs. This represents standard operation of a tapped linear LED driving arrangement.

[0111] The first LED module is connected in parallel to the charge storage device and comprises a first set of LEDs and a current controller. The current controller is configured to control the current through the first set of LEDs to compensate for any reduction in power drawn by the other sets of LEDs (caused by a reduction in the input voltage) to maintain the output light level.

[0112] In some variants of any of the above disclosed systems, the driving arrangement further comprises an additional switching device configured to controllably bypass the first LED module (and thus also the charge storage device). This facilitates flexibly changing the conduction order of the LED modules, if required, and can be used to control the total harmonic distortion of the driving arrangement, e.g. using a switched mode power supply mechanism.

[0113] In such embodiments, to prevent dissipation of the charge stored at the charge storage device, the driving arrangement can comprise a diode connected between the additional switching device and the charge storage device (e.g. between the first input terminal and the charge storage device).

[0114] Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality.

[0115] The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0116] If the word "adapted" is used in the claims or specification it should be noted that "adapted" is intended to mean "configured" or "arranged." If the term "means" is used in the claims or specification it should be noted that the term "means" is intended to refer to a means-plus-function claim. If the term "apparatus" is used in the claims or specification it should be noted that the term "apparatus" is intended to refer to a system or corresponding hardware specific device.

[0117] The use of any of the following terms in the claims or specification is not intended to limit the scope, unless otherwise explicitly recited therein.

Claims

1. A tapped linear LED driving apparatus (100, 300, 500), comprising: a first input terminal and a second input terminal for receiving an input voltage from an external power source; a first LED module (201, 403) comprising a first set of LEDs (201) and a first current controller (403) connected in series to the first set of LEDs, the first set of LEDs (201) comprising one or more LEDs connected in series; a second LED module (202, 302) connected in series with the first LED module and comprising a second set of LEDs (202) and a first switching device (301) connected in parallel to the second set of LEDs, the second set of LEDs comprising one or more LEDs connected in series; and a charge storage device (402) connected in parallel to the first LED module, wherein: the first LED module and the second LED module are connected in series between the first input terminal and the second input terminal; the first current controller (403) is configured to: - in response to an amplitude of the input voltage rising above a first threshold, reduce or prevent current flow through the first set of LEDs, such that current flows from the input terminal through the charge storage device to the second LED module; and - in response to an amplitude of the input voltage falling below the first threshold, increase current flow through the first set of LEDs, such that current flows from the charge storage device through the first set of LEDs, and the first switching device (301) is configured to: - in response to an amplitude of the input voltage falling below a second threshold, bypass the second set of LEDs, thereby preventing the second set of LEDs from conducting current; and - in response to an amplitude of the input voltage rising above the second threshold, prevent bypassing of the second set of LEDs, thereby allowing the second set of LEDs to conduct current, wherein the first current source controller (403) is configured to control a maximum current flow through the first set of LEDs (201) when the first set of LEDs (201) conducts current, such that the first set of LEDs (201) emits the same or substantially the same amount of light as emitted by the second set of LEDs (202) when the first set of LEDs (201) conducts current.

2. The tapped linear LED driving apparatus of claim 1, wherein the second threshold is less than or equal to the first threshold.

3. The tapped linear LED driving apparatus of claim 1 or 2, wherein the second threshold is equal to a sum of a forward voltage of the first set of LEDs and a forward voltage of the second set of LEDs.

4. The tapped linear LED driving apparatus of any of claims 1 to 3, wherein the first current controller comprises a controllable current source.

5. The tapped linear LED driving apparatus of any of claims 1 to 4, further comprising a controller configured to control operation of the first current controller and the first switching device.

6. The tapped linear LED driving apparatus of any one of claims 1 to 5, wherein: the first LED set (201) comprises a first LED (LD1) and a second LED (LD2) connected in series; the first LED module comprises an LED shunt device (SD1) connected in parallel to the second LED (LD2), the LED shunt device being configured to: - shunt the second LED in response to a magnitude of a voltage across the first LED or a magnitude of a voltage across the charge storage device falling below a first LED threshold, thereby preventing the second LED from conducting current; and - prevent shunting of the second LED in response to the magnitude of the voltage across the first LED or the magnitude of the voltage across the charge storage device rising above the first LED threshold, thereby allowing the second LED to conduct current.

7. The tapped linear LED driving apparatus of any one of claims 1 to 6, further comprising a second current source controller (102) connected in series with the second LED module, the second current source controller being for controlling a maximum current flowing through the first LED module and the second LED module.

8. The tapped linear LED driving apparatus of any one of claims 1 to 7, further comprising a third LED module (203, 302) connected in series with the second LED module, and comprising: a third LED set (203) comprising one or more LEDs connected in series; and a second switching device (302) connected in parallel to the third LED set, the second switching device being configured to: - shunt the third LED set in response to a magnitude of the input voltage falling below a third threshold, thereby preventing the third LED set from conducting current; and - prevent shunting of the third LED set in response to the magnitude of the input voltage rising above the third threshold, thereby allowing the third LED set to conduct current, wherein the third threshold is different from the second threshold.

9. The tapped linear LED driving apparatus of claim 8, wherein: the first current controller is configured to: - increase a current flowing through the first LED set in response to the magnitude of the input voltage falling below a fourth threshold; - decrease the current flowing through the first LED set in response to the magnitude of the input voltage rising above the fourth threshold, the fourth threshold being smaller than the first threshold; and the first current controller is further configured such that the current flowing through the first LED set when the magnitude of the input voltage is between the fourth threshold and the first threshold is greater than the current flowing through the first LED set when the magnitude of the input voltage is above the first threshold.

10. The tapped linear LED driving apparatus of claim 9, wherein the fourth threshold is greater than or equal to the third threshold.

11. An LED driving system comprising: a tapped linear LED driving apparatus according to any one of claims 1 to 10; and a power supply for supplying the input voltage to the tapped linear LED driving apparatus. a rectifier connected to the first input terminal and the second input terminal configured to rectify a mains power supply to generate the input voltage for the first input terminal and the second input terminal.

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