An LED device and a method for controlling an LED device

Abstract

An LED device has LED strings connected in parallel and smoothing capacitors. A controllable voltage limiter can limit the voltage across the LED strings to selectively prevent the LED strings from outputting light. When the LED device is turned on, the controllable voltage limiter can be activated so that the smoothing capacitors can be charged with a high current without activating the LED strings.

Description

Technical Field

[0001] The present invention relates to the field of LED devices, and more particularly to controlling the operation of an LED device during startup. Background Art

[0002] Dimmable LED devices are increasingly being used in lighting applications, such as as a replacement for existing halogen bulbs. There has been a continuing desire to maximize the efficiency of such LED devices.

[0003] Typically, an LED device includes an LED string that can be connected to a rectified mains output or other voltage source, such as a battery or cell. A controllable current source can control the current drawn through the LED string in order to control the brightness level output by the LED string. For example, pulse width modulation (PWM) techniques can be used to control the current. In some applications, to improve the power factor and efficiency of the LED device, a smoothing capacitor is connected in parallel with the LED string in order to perform mains current shaping.

[0004] However, the inventors have recognized that the introduction of this smoothing capacitor results in an inherently slow response of the LED device, such as during startup, to changes in the desired brightness intensity. Specifically, such LED devices are particularly slow to respond to changes from an "off" or "standby" state (i.e., no light output) to a low-intensity light output (i.e., a low brightness level).

[0005] Accordingly, it is desirable to reduce the effect of the smoothing capacitor on the speed at which the brightness level output by the LED string is changed. Summary of the Invention

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

[0007] According to an example of an aspect of the present invention, there is provided an LED device for connection to a voltage source. The LED device includes: an LED string including one or more LEDs and adapted to receive power from the voltage source; a controllable current source connected in series with the LED string; a capacitor connected in parallel with the LED string; a resistive load connected in parallel with the LED string; a controllable voltage limiter adapted to selectively limit the voltage across the LED string to no greater than a first voltage level, where the first voltage level is lower than the forward voltage of the LED string; and a controller adapted to control the controllable current source and the controllable voltage limiter in response to an indication signal that at least indicates a change in the desired output brightness level of the LED string. The controller is adapted to: in response to an indication signal indicating a desire to switch the brightness level of the LED string from a first brightness level to a second brightness level: during a first time period, control the controllable voltage source to limit the voltage across the LED string to no greater than the first voltage level, and control the controllable current source to regulate the charge stored by the capacitor; and after completion of the first time period, control the controllable voltage limiter to stop limiting the voltage across the LED string to no greater than the first voltage level, and control the controllable current source to attempt to draw the current of the LED string passing through from the voltage source such that the brightness of the LED string is at the second brightness level.

[0008] The present invention provides a way to appropriately and quickly charge or discharge a capacitor to have a suitable amount of charge for providing a second brightness level via the LED string without activating the LED string, thereby avoiding a flash when turning on the LED device or avoiding dimming ("fade out") when turning off the LED device.

[0009] In the turn-on scenario, i.e., during startup, by setting the voltage across the LED string to be less than the forward voltage of the LED string, the charge of the capacitor can be quickly adjusted (e.g., using a very large current) without driving the LED string with said large current. Thereby, this can increase the turn-on speed and reduce the length of the turn-on or startup process.

[0010] In the turn-off scenario, i.e., during shutdown, by setting the voltage across the LED string to be less than the forward voltage of the LED string, the LED string can be quickly turned off (i.e., stop emitting light) by allowing any residual charge on the capacitor to discharge through the resistive load without passing through the LED string (since the voltage across the LED string is less than the forward voltage). Thereby, this can increase the turn-off speed and reduce the length of the turn-off or shutdown process.

[0011] Thereby, the embodiment provides an LED device that responds more quickly to a desired change in brightness level, particularly to a desired response to switching the LED device between the "ON" state and the "OFF" state.

[0012] The controller can be adapted to control the current drawn by a controllable current source using pulse width modulation techniques after a first time period. This provides a simple and intuitive way to control the brightness level of the light output by the LED string to achieve a second brightness level.

[0013] The controller can be adapted such that the average current drawn by the controllable current source during the first time period is greater than the average current drawn by the controllable current source after the first time period.

[0014] This ensures that the capacitor will be charged (discharged) more quickly during the first time period than in other cases where the proposed method is not used (e.g., only charging the capacitor with the current required to achieve the second brightness level).

[0015] The first brightness level can be less than the second brightness level. The present invention is particularly advantageous when used to switch from a low brightness level to a higher brightness level, as rapid charging of the capacitor (to the charge required for the higher brightness level) can be achieved.

[0016] In one embodiment, the first brightness level is zero and the second brightness level is non - zero. In other embodiments, the first brightness level is non - zero and the second brightness level is zero. Thus, in some embodiments, the first brightness level is zero and the second brightness level is non - zero, and vice versa.

[0017] In some embodiments, the first brightness level is zero and the second brightness level is non - zero and does not exceed half or a quarter of the maximum (possible) brightness level of the LED string. It has been described how slow charging of the capacitor is particularly prevalent when a small current is required to pass through the LED string (to reach the second brightness level). Thus, the embodiments are particularly advantageous when used to switch from the off state to an on state with low dimming.

[0018] In some embodiments, if the second brightness level is greater than a predetermined fraction of the maximum possible brightness level of the LED string, e.g., greater than half or a quarter of the maximum possible brightness level, the first time period can be skipped (e.g., the controllable voltage source can be not used).

[0019] The length of the first time period can depend on the magnitude of the second brightness level. At low dimming levels, the change in charge required at the capacitor (e.g., to switch between the first and second brightness levels) is less than the change in charge required at the capacitor at high dimming levels. Thus, the length of the first time period can be different considering the required change in charge.

[0020] In some embodiments, a controllable voltage regulator includes: a first impedance device formed by a series of one or more impedance elements connected in parallel with the LED string; a second impedance device connected between the first impedance device and ground or a reference voltage, the first impedance device and the second impedance device being arranged to form a voltage divider between a voltage source and ground or a reference voltage; and a switching device arranged to controllably bypass one or more impedance elements of the first impedance device in response to a controller, thereby controlling the effective impedance of the first impedance device and thereby controlling the voltage across the LED string.

[0021] The switching device may include a first transistor having a base controlled by a controller and a collector and an emitter connected between two sides of at least one impedance element of the first impedance device.

[0022] Thus, the first transistor is capable of selectively bypassing at least one impedance element of the first impedance device through appropriate control of the base by the controller. Of course, the collector and / or emitter may be connected to one side of at least one impedance element of the first impedance device through one or more additional impedance elements (e.g., having a combined resistance different from that of the bypassed impedance element of the first impedance device).

[0023] In at least one embodiment, the first impedance device includes at least a first impedance element and a different second impedance element; the collector and emitter of the first transistor of the switching device are respectively connected between a first side and a second side of the first impedance element; and the switching device further includes a second transistor having: a base connected to the second side of the first impedance element and the first side of the second impedance element; a collector connected to the first side of the first impedance element; and an emitter connected to the second side of the second impedance element.

[0024] The LED device may further include a resistive load connected in parallel with the LED string.

[0025] According to an example of an aspect of the present invention, a method of controlling an LED device formed by an LED string, a controllable current source, and a capacitor is provided. The LED string includes one or more LEDs and is adapted to receive power from a voltage source. The controllable current source is connected in series with the LED string, and the capacitor is connected in parallel with the LED string. The method includes: in response to a desired indication signal indicating a switch of the brightness level of the LED string from a first brightness level to a different second brightness level: during a first time period: limiting the voltage across the LED string to a first voltage level and controlling the current drawn from the voltage source to charge the capacitor; and after completion of the first time period, removing the limitation on the voltage across the LED string and controlling the current passing through the LED string drawn from the voltage source such that the brightness of the LED string is at the second brightness level.

[0026] The average current drawn from the voltage source during a first time period can be greater than the average current drawn from the voltage source after the first time period. The first brightness level can be less than the second brightness level. The second brightness level can be no more than half of the maximum brightness level of the LED string.

[0027] With reference to the (one or more) embodiments described below, these and other aspects of the present invention will become apparent and be elucidated. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] For a better understanding of the present invention and to more clearly show how the present invention may be implemented, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0029] Figure 1 An LED device according to a known example is shown;

[0030] Figure 2 An LED device according to a first embodiment is shown;

[0031] Figure 3 An LED device according to a second embodiment is shown;

[0032] Figure 4 A waveform for triggering an understanding of the second embodiment is shown; and

[0033] Figure 5 A method according to one embodiment is shown.

[0034] Specific implementation method

[0035] The present invention will be described with reference to the accompanying drawings.

[0036] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for illustrative purposes only and are not intended to limit the scope of the present invention. These and other features, aspects and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, the appended claims and the drawings. It should be understood that the drawings are merely schematic and are not drawn to scale. It should also be understood that in all the drawings, the same reference numerals are used to indicate the same or similar parts.

[0037] According to the concepts of the present invention, an LED device having LED strings connected in parallel and a smoothing capacitor is proposed. A controllable voltage limiter can limit the voltage across the LED string to selectively prevent the LED string from outputting light. When the LED device is turned on, the controllable voltage limiter can be activated so that the smoothing capacitor can be charged with a high current without activating the LED string.

[0038] Embodiments are at least partially based on the recognition that smoothing capacitors in LED devices can cause slow turn-on and turn-off times of the LED devices. This is because the smoothing capacitor needs to store or remove a certain amount of charge before the LED string can be activated or deactivated, respectively. It has been recognized that limiting the voltage across the LED string during the turn-on or turn-off process or sequence can prevent the LED string from being activated (i.e., output light) while the capacitor is being properly charged / discharged. This enables the turn-on sequence or turn-off sequence to be performed more quickly.

[0039] For example, illustrative embodiments can be used in LED devices or LED systems and are particularly useful in multi-channel and single-channel lamp drivers when lighting.

[0040] Figure 1 An LED device 100 according to a known example is shown, and the device is provided to illustrate the advantages of the present invention.

[0041] The LED device 100 includes an LED string D and a capacitor C1 connected in parallel. The LED string D is adapted to draw power from a voltage source V. The LED device further includes a resistive load R1 connected in parallel with the LED string and the capacitor C1.

[0042] A controllable current source CS is connected in series with the LED string D (and thus in series with the capacitor C1 and the resistive load R1) and includes a known configuration for the controllable current source. The controllable current source CS includes a controllable voltage source V CS , a current source Cs CS , a sense resistor R sense and an operational amplifier A, which are appropriately connected such that the control of the controllable voltage source V CS controls the (average) current drawn by the controllable current source.

[0043] The function of the current source Cs is to deliver a fixed current to the LED independent of the voltage across the current source. According to Ohm's law, the current (I LED ) through the LED is defined by the following formula:

[0044]

[0045] The operational amplifier A acts as a control element to maintain the voltage across the sense resistor R sense at a control voltage V CS , thereby maintaining the current.

[0046] In the example shown, the controllable current source is controllable or switchable between two states (i.e., a first state in which it draws current and a second state in which it does not draw any current). Appropriate control of the state of the control current source (e.g., using pulse width modulation techniques) allows control of the average current drawn by the controllable current source.

[0047] The voltage source V can include, for example, a rectified mains power supply or a battery power supply. Thus, the voltage source V can be modeled as a DC (direct current) power supply. The voltage source provides the power for driving the LED string.

[0048] The brightness level of the light output by the LED string D is controlled by the controllable current source CS. Specifically, the controllable current source CS can control the average current drawn through the LED string, thereby controlling the brightness level of the light output by the LED string D.

[0049] The LED device 100 is thus switchable between at least an off or standby state (i.e., where light output from the LED string D is not desired) and an on state (i.e., where light output from the LED string D is desired). This can be performed by controlling the current drawn by the controllable current source S. In the off state, no current is drawn through the LED string. In the on state, current is drawn through the LED string. When operating in the on state, the magnitude of the current drawn by the controllable current source can be controlled in order to control the brightness level of the light output by the LED string D.

[0050] (Smoothing) capacitor C1 helps to smooth the current drawn through the LED device, thereby improving the power factor, efficiency and reducing flicker. When the voltage across the LED string D drops below the forward voltage of the LED string D (e.g., if the LED device switches from the on state to the off state), the resistive load R1 provides a current path (for discharging the capacitor C1).

[0051] Existing LED devices (such as the LED device 100) suffer from slow turn-on times (i.e., switching from the off or standby state to the on state) due to the time required to charge the capacitor to an appropriate level. This is particularly problematic when switching from the off / standby state to the on state with a low brightness level.

[0052] Consider a scenario where the desired brightness level is switched from zero to a low illumination level. In this scenario, to achieve the low illumination level, the (average) current drawn through the LED string will cause the capacitor to take a relatively long time to charge appropriately, resulting in a slow turn-on time.

[0053] One way to overcome this problem could be to initially draw a large current through the LED device in order to quickly charge the capacitor. However, this would disadvantageously result in "flicker" as the large current through the LED string would cause the LED string to output bright light.

[0054] The present invention overcomes these two problems by using a controllable voltage limiter to limit the voltage across the LED string to a level below the forward voltage (i.e., turn-on voltage) of the LED string while rapidly charging a capacitor. This prevents the LED string from outputting strong light while reducing the time required to charge the capacitor to an appropriate level.

[0055] Similarly, existing LED devices also suffer from the problem of slow turn-off times because the capacitor will store a certain amount of charge that can continue to dissipate through the LED string (thereby causing it to emit light) for a period of time after the controllable current source is turned off. This effect is sometimes referred to as "fade".

[0056] The controllable voltage limiter of the present invention enables the voltage across the LED string to be rapidly limited to a voltage below the forward voltage level, thereby preventing the LED string from outputting light and quickly switching the LED device to the off state.

[0057] Figure 2 An LED device 200 according to an embodiment of the present invention is shown, which includes an LED string D, a capacitor C1, and a controllable current source CS as described above.

[0058] The LED device 200 further includes a controllable voltage limiter VL1. The controllable voltage limiter VL1 is configured to controllably limit (i.e., constrain and unconstrain) the voltage V across the LED string D D to a first predetermined voltage level. The first predetermined voltage level is less than the forward voltage of the LED string D. The forward voltage of the LED string is well known to those skilled in the art and is sometimes referred to as "voltage drop".

[0059] In other words, the controllable voltage limiter is capable of switching between a first mode and a second mode. In the first mode, the controllable voltage limiter constrains the voltage V across the LED string D D to not be greater than the first predetermined voltage level. In the second mode, the controllable voltage limiter does not limit the voltage V across the LED string D D to not be greater than the first predetermined voltage level. Therefore, when operating in the second mode, the voltage V across the LED string D D can reach or exceed the forward voltage of the LED string.

[0060] The illustrated embodiment of the controllable voltage limiter VL1 of the LED device 200 includes a first impedance device R1, a second impedance device R2, and switching devices Q1, R3.

[0061] The first impedance device R1 is connected in parallel with the LED string D. The second impedance device R2 is connected between the LED string (i.e., the first impedance device) and ground or a reference voltage GND. Thus, the first impedance device and the second impedance device actually form a voltage divider.

[0062] When the voltage across the LED string D drops below the forward voltage of the LED string D (e.g., if the LED device switches from an on state to an off state), the first impedance device R1 also acts as a resistive load for providing a current path.

[0063] The switching devices Q1, R3 include a switch, such as a transistor (e.g., MOSFET or BJT), adapted to controllably bypass the first impedance device R1 so as to control the effective impedance of the first impedance device R1. Specifically, the switching device Q1 is configured to effectively switch the impedance between the first impedance device R1 and the impedance R3.

[0064] Thus, the switching devices Q1, R3 can effectively modify the voltage across the LED string D by switching between a first mode and a second mode (i.e., opening or closing the switch Q1 respectively). In the first mode, the first impedance device R1 and the second impedance device R2 act as a voltage divider, thereby fixing the voltage across the LED string to be no greater than a specific voltage level. In the second configuration, the first impedance device R1 and the impedance R3 are connected in parallel, thereby modifying the fixed voltage level across the LED string D.

[0065] The impedance values of the components R1, R2, and R3 are appropriately selected (with due reference to the voltage level provided by the voltage source V) such that when the switching device is in the second mode and the LED device is in a steady state, the voltage across the first impedance device R1 and the LED string D is less than the forward voltage of the LED string.

[0066] The impedance values of the components R1, R2 are appropriately selected (with due reference to the voltage level provided by the voltage source V) such that when the switching device is in the first mode and the LED device is in a steady state, the voltage V across the LED string D D is greater than or equal to the forward voltage of the LED string.

[0067] In this way, the controllable voltage limiter VL1 can be controlled to selectively fix (in a steady state) the voltage level V across the LED string D D at a first predetermined voltage level that is less than the forward voltage of the LED string.

[0068] The impedance R3 is optional and may be omitted in some embodiments. This will effectively cause the switching device to short-circuit the LED string (i.e., there is virtually no voltage drop across the LED string except for any collector / emitter or source / drain voltage drop). In this scenario, the first predetermined voltage level is virtually zero (or equal to the voltage drop from the collector to the emitter or from the source to the drain of the switch Q1).

[0069] The LED device 200 further includes a controller 250. The controller 250 is adapted to control the controllable current source CS and the controllable voltage limiter VL1. This can be performed using a first control signal s 1 and a second control signal s 2 respectively.

[0070] The controller 250 responds to an indication signal s i and the indication signal s i indicating at least a desire to change the intensity of the light output by the LED string D. This can be provided by a user interacting with a user interface (such as a dimmer or a switch) or another device (such as a timer, a scheduler, or a router).

[0071] The controller 250 is adapted to perform certain steps in response to an indication signal indicating a desire to switch the brightness level of the LED string from a first brightness level to a different second brightness level.

[0072] In response to an indication of the desired switch, the controller controls the controllable voltage source to limit the voltage across the LED string to a first voltage level during a first time period, and controls the controllable current source to draw current through the controllable voltage limiter to charge the capacitor.

[0073] After the first time period is completed (i.e., when the first time period ends), the controller 250 controls the controllable voltage limiter VL1 to release the voltage limitation across the LED string D, and controls the controllable current source CS to draw current through the LED string D such that the brightness of the LED string is at the second brightness level.

[0074] By limiting the voltage across the LED string to the first voltage level, the voltage across the LED string is low enough to prevent the LED string from being activated, thereby preventing flickering, while allowing the capacitor to be quickly charged (or discharged).

[0075] Thus, in the turn-on scenario, the capacitor can be quickly charged during the first time period without causing LED flickering due to high current, thereby reducing the turn-on time of the LED device. Similarly, in the turn-off scenario, the capacitor can be quickly discharged during the first time period, and the LED device is prevented from outputting light while the capacitor is discharging.

[0076] Preferably, during the first time period, the controller controls the controllable current source to draw the maximum possible average current to quickly charge or discharge the capacitor.

[0077] In some embodiments, the controllable current source is designed to be controllable using pulse width modulation techniques. Accordingly, the controller 250 can be adapted to control the controllable current source by providing a pulse width modulation signal employing pulse width modulation techniques.

[0078] Thus, after the first time period is completed, the controller 250 can use pulse width modulation techniques to control the controllable current source to control the average current through the LED string, thereby achieving a second brightness level (e.g., providing a pulse width modulation signal with a duty cycle less than 100% if a non-maximum amount of light is desired).

[0079] In another embodiment, during the first time period, the controller 250 can provide a pulse width modulation signal with a 100% duty cycle to the controllable current source. This ensures the fastest charging or discharging of the capacitor, thereby minimizing the time of the turn-on process or the turn-off process.

[0080] As previously mentioned, the inventors have recognized that the present invention is particularly advantageous when it is desired to switch the brightness level of the LED string from zero to a very low brightness level. Thus, in some embodiments, the second brightness level does not exceed half of the maximum brightness level of the LED string. In some additional embodiments, the second brightness level does not exceed one quarter of the maximum brightness level of the LED string.

[0081] The maximum brightness level is the brightness level output by the LED string when the controllable current source draws the current through the LED string at the maximum allowable average current (e.g., based on the components of the controllable current source or safety requirements, which may limit the maximum allowable average current).

[0082] The required length of the first time period can vary depending on the desired brightness level output by the LED string D. Specifically, the lower the brightness level, the less charge needs to be stored on the capacitor C for effective operation, which means the length of the first time period can be reduced.

[0083] Thus, in some embodiments, the length of the first time period depends on the magnitude of the second brightness level. In a specific example, the lower the magnitude of the second brightness level, the lower the length of the first time period.

[0084] Figure 3 An LED device 300 according to a second embodiment of the present invention is shown. The LED device 300 differs from the LED device 200 in that it includes a different controllable voltage limiter VL2.

[0085] The controllable voltage limiter VL2 of the LED device 300 also includes first impedance devices R4, R5 and second impedance devices R6. The controllable voltage limiter also includes switching devices Q2, Q3, Q4, R7, R8, R9, R10, R11, R12.

[0086] The first impedance device is connected in parallel with the LED string D. The second impedance device is connected between the LED string D and ground or a reference voltage.

[0087] The first impedance devices R4, R5 include a first impedance element R4 and a different second impedance element R5. Here, the second impedance device R6 includes a single impedance element R6, but in other embodiments may include more than one impedance element.

[0088] The switching device includes a first PNP transistor Q2. The collector of the first PNP transistor Q2 is connected (via a resistor R8, which is optional) to the first side of the first impedance element R4. The emitter of the first PNP transistor Q2 is directly connected to the second side of the first impedance element. In different embodiments, one or more additional resistors may be connected between the emitter / collector and the corresponding side of the first impedance element R4.

[0089] The switching arrangement further includes a second PNP transistor Q3. The base of the second PNP transistor is connected to the second side of the first impedance element R4 and the first side of the second impedance element R5 (i.e., between the first impedance element and the second impedance element). The collector of the second PNP transistor is connected to the first side of the first impedance element. The emitter of the second PNP transistor is connected to the second side of the second impedance element (i.e., the collector and emitter are connected between either side of the first impedance element). The base, collector, and / or emitter may be connected via one or more additional resistors (such as resistor R7), which are optional.

[0090] The base of the first PNP transistor Q2 is controlled by a first control signal s 1 to selectively control the voltage across the LED string D.

[0091] To facilitate proper control of the base of the first PNP transistor Q2, the switching device includes some additional (but optional) components that act as a level shifter.

[0092] The collector of the first NPN transistor Q4 (via an optional resistor R10) is connected to the base of the first PNP transistor Q2, and the emitter of the first NPN transistor Q4 is connected to ground or a reference voltage. The base of the first NPN transistor Q4 is controlled by a first control signal s 1 (here, the voltage at the base of the first NPN transistor is appropriately biased via a voltage divider R12, R11). Additionally, the base of the first PNP transistor is connected to a voltage source V via a resistor R9 (which may be replaced by more than one resistor).

[0093] The first NPN transistor Q4 and the resistors R11, R12, and R10 act as a level shifter. The first PNP transistor Q2 and the resistors R8, R9 act as an enable / disable circuit (controlled by the first control signal). The second PNP transistor Q3 and the resistors R4, R5, and R7 limit the voltage in response to the enable / disable circuit.

[0094] The component values of the first impedance devices R4, R5 and the additional resistor R7 define the LED limiting voltage level.

[0095] The LED device 300 includes a controller 350 that uses the first control signal s in a manner similar to the LED device of the second embodiment 1 to control the controllable voltage limiter VL2 and uses the second control signal s 2 to control the controllable current source CS (as described above). The operation of the controller will be further explained later.

[0096] The LED device 300 also includes a resistive load R13 that can act as a current path. In some embodiments, the resistive load R13 can be omitted because its function can be assumed by components of the controllable voltage limiter VL2 (such as resistors R4 and R5).

[0097] The resistive load R13 and the second impedance device R6 can be used to maintain the pre - bias voltage across the capacitor C1.

[0098] Thus, in some embodiments, the resistive load can form part of the controllable voltage limiter VL2.

[0099] Figure 4 Waveforms are shown to assist in explaining the operation of the controller 350. The waveforms are provided for a period of time when the LED device switches from the off or standby state to the on state (with a low desired brightness level output by the LED string D) (i.e., the "turn - on" sequence).

[0100] Figure 4 A first waveform representing the voltage V across the LED string D is provided D a second waveform representing the current I through the LED string D D a third waveform representing the first control signal s 1 s 1 and a fourth waveform representing the second control signal s 2 s 2 .

[0101] At a first time point t 1 , an indication signal (not shown) indicates the desire to switch the LED device from the off state to the on state. Thus, this indicates the desire to switch the brightness level output by the LED string D from a first brightness level (i.e., zero) to a greater second brightness level (which is non - zero).

[0102] At this first time point, the controller sets the first control signal s 1 such that the voltage V across the LED string is maintained not greater than a predetermined voltage level V D pd ​。The predetermined voltage level V pd is set to be less than the forward voltage of the LED string such that no current I D flows through the LED string.

[0103] At this first time point t 1 , the controller also sets a second control signal s 2 such that the controllable current source draws current. Control the second control signal s 2 such that the current drawn by the controllable current source is at a maximum value (e.g., if PWM control is used, the duty cycle is 100%).

[0104] The second time point t 2 is defined as the time after the expiration of the first time period t 1 following the first time point t p1 .

[0105] At the second time point t 2 , the first control signal s 1 is set such that it no longer restricts the voltage V across the LED string D , and the voltage V D thus rises to the forward voltage of the LED string.

[0106] At the second time point t 2 , the second control signal s 2 is set to control the (average) current through the LED string to achieve a desired brightness level of the light output by the LED string. As described above, this can be performed by controlling the current using pulse width modulation techniques (as shown in the figure).

[0107] Thus, the described method enables the controllable current source to draw the maximum current during the first time period t p1 , thereby enabling the capacitor C1 to be quickly charged to improve the "turn-on" time of the LED device.

[0108] Reference Figure 2 and Figure 3 , the LED devices 200, 300 proposed herein may suffer from a slow turn-off time (i.e., switching from the on state to the off state). This may be due to the fact that when the controllable current source is controlled not to draw current (i.e., the LED device is switched off), the charge across the capacitor C1 takes a certain amount of time to dissipate (i.e., through the LED string D and the second impedance devices R2, R6).

[0109] Therefore, the controllers 250, 350 can perform a "turn-off" sequence to reduce this effect.

[0110] Specifically, the controller may be adapted to respond to an indication signal s indicating a desire to turn off the LED device (i.e., the LED string does not emit output light) 1 , control the controllable voltage sources VL1, VL2 to limit the voltage across the LED string D to a first voltage level, and control the controllable current source not to draw current. This prevents the LED string D from emitting light (since the voltage across the LED string is less than the forward voltage), while allowing the capacitor C1 to discharge (e.g., through the resistive loads R1, R13, and / or the controllable voltage limiter).

[0111] After a certain period of time (e.g., the "second period") has elapsed, the charge stored on the capacitor will have sufficiently dropped to a level where little or no current may flow through the LED. Thus, after the second period has elapsed, it is not necessary to control the controllable voltage source to limit the voltage across the LED string. Here, it is not necessary to control the controllable voltage limiter to stop limiting the voltage across the LED string (except to minimize any power losses caused by operating the controllable voltage limiter).

[0112] The length of the second period may be fixed, dynamic (e.g., based on the brightness level or current prior to the second period), or indeterminate (e.g., until the first period is triggered again). The length of the second period is preferably selected such that the capacitor C1 has sufficient time to discharge to a level such that, if the controllable voltage limiter stops limiting the voltage, the voltage across the LED string will be less than the forward voltage of the LED string. A person skilled in the art will be able to appropriately select or define the length of the second period.

[0113] In any of the above embodiments, the LED device may be adapted to be connected to an AC (alternating current) voltage source (e.g., one that includes a rectifier itself), rather than to a DC voltage source. Such modifications are known to a person skilled in the art.

[0114] In any of the above embodiments, the reference to "capacitor" refers to any impedance device having an intentional (i.e., non-parasitic) capacitive element, such as a capacitor array, a single capacitor, etc.

[0115] In one embodiment, an LED system is provided that includes any embodiment of the LED device described herein and a user interface for defining the indication signal s i . The indication signal indicates a desired brightness level of the light output by the LED string such that the user interface acts as a dimmer. Specifically, the indication signal may respond to user input provided at the user interface.

[0116] Other methods of defining the indication signal will be apparent to the skilled person, such as using a further controller responsive to one or more of: a schedule, ambient light level, motion, wireless signal, infrared signal, etc. A LED system may be provided comprising any of the described LED devices and such a further controller.

[0117] Figure 5 A method 500 of controlling an LED device is shown.

[0118] The method is for use with an LED arrangement formed by an LED string comprising one or more LEDs and adapted to receive power from a voltage source, a controllable current source connected in series with the LED string, and a capacitor connected in parallel with the LED string.

[0119] The method 500 is performed in response to an indication signal indicating a desire to switch a brightness level of the LED string from a first brightness level to a second, different brightness level.

[0120] The method comprises a first step 501 performed during a first or initial time period. The first step 501 comprises limiting the voltage across the LED string to a first voltage level and controlling the current drawn from the voltage source to charge the capacitor. The first step is performed throughout the first / initial time period.

[0121] After the first time period is completed, a second step 502 is performed, which includes removing the limitation on the voltage across the LED string and controlling the current drawn from the voltage source through the LED string so that the brightness of the LED string is at a second brightness level.

[0122] The method may be suitably adapted to perform any of the functions of the controller described above.

[0123] Where reference is made to transistors, MOSFET and BJT are considered suitable choices, with relevant terminology used where appropriate. Thus, the term "base" may be interchanged with the term "gate", the term "collector" may be interchanged with the term "source", and the term "emitter" may be interchanged with the term "drain".

[0124] A skilled person will be able to easily develop a controller for performing any of the methods described herein. Thus, each step of the flow chart may represent a different action performed by the controller and may be performed by a corresponding module of the processing system. Similarly, a skilled person will be able to easily develop a method for performing the functions of any of the controllers described herein.

[0125] The embodiments utilize a controller. The controller can be implemented in software and / or hardware in various ways to perform the various required functions. A processor is an example of a controller that employs one or more microprocessors, which can be programmed using software (e.g., microcode) to perform the required functions. However, the controller can be implemented with or without a processor and can also be implemented as a combination of dedicated hardware that performs some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) that performs other functions.

[0126] Examples of controller components that can be used in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field programmable gate arrays (FPGAs).

[0127] In various implementations, the processor or controller can be associated with one or more storage media such as volatile and non-volatile computer memories, such as RAM, PROM, EPROM, and EEPROM. The storage media can be encoded with one or more programs that, when executed on one or more processors and / or controllers, perform the required functions. The various storage media can be fixed within the processor or controller or can be removable, such that one or more programs stored thereon can be loaded into the processor or controller.

[0128] It will be understood that the disclosed methods are preferably computer-implemented methods. Thus, the concept of a computer program including code components is also proposed, which, when the program runs on a processing system or controller, are used to implement any of the described methods.

[0129] From a study of the drawings, the disclosure, and the appended claims, those skilled in the art can understand and implement variations of the disclosed embodiments when practicing the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit can implement the functions of several items recited in the claims. 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. If a computer program is discussed above, it can be stored / distributed on a suitable medium, such as an optical storage medium or a solid-state medium provided together with or as part of other hardware, but can also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. If the term "adapted to" is used in the claims or the specification, it should be noted that the term "adapted to" is intended to be equivalent to the term "configured to". Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An LED device for connection to a voltage source (V), the LED device comprising: an LED string (D) including one or more LEDs and adapted to receive power from the voltage source (V); a controllable current source (CS) connected in series with the LED string (D); a capacitor (C1) connected in parallel with the LED string (D); controllable voltage limiters (VL1, VL2) adapted to selectively limit the voltage across the LED string (D) to no greater than a first voltage level, wherein the first voltage level is lower than the forward voltage of the LED string (D); and a controller (250) adapted to control the controllable current source (CS) and the controllable voltage limiters (VL1, VL2) in response to an indication signal, the indication signal at least indicating a change in a desired output brightness level of the LED string (D), wherein the controller (250) is adapted to: in response to the indication signal indicating a desire to switch the brightness level of the LED string (D) from a first brightness level to a second brightness level: during a first time period, control the controllable current source (CS) to limit the voltage across the LED string (D) to no greater than the first voltage level, and control the controllable current source (CS) to regulate the charge stored by the capacitor (C1); and after completion of the first time period, control the controllable voltage limiters (VL1, VL2) to stop limiting the voltage across the LED string (D) to no greater than the first voltage level, and control the controllable current source (CS) to attempt to draw current through the LED string (D) from the voltage source (V), such that the brightness of the LED string (D) is at the second brightness level, wherein the controllable voltage limiters (VL1, VL2) include: a first impedance device (R4, R5) formed by a series of one or more impedance elements connected in parallel with the LED string (D); a second impedance device (R6) connected between the first impedance device (R4, R5) and ground or a reference voltage, the first impedance device and the second impedance device being arranged to form a voltage divider between the voltage source (V) and the ground or the reference voltage; and switching means (Q2, Q3, Q4, R7, R8, R9, R10, R11, R12) arranged to controllably bypass one or more of the impedance elements of the first impedance device (R4, R5) in response to the controller (250), thereby controlling the effective impedance of the first impedance device (R4, R5), and thereby controlling the voltage across the LED string (D).

2. The LED device according to claim 1, wherein the controller (250) is adapted to control the controllable current source (CS) to draw current using pulse width modulation techniques after the first time period.

3. The LED device according to claim 1, wherein the controller (250) is adapted such that the average current drawn by the controllable current source (CS) during the first time period is greater than the average current drawn by the controllable current source (CS) after the first time period.

4. The LED device according to claim 2, wherein the controller (250) is adapted such that the average current drawn by the controllable current source (CS) during the first time period is greater than the average current drawn by the controllable current source (CS) after the first time period.

5. The LED device according to any one of claims 1 to 4, wherein the first brightness level is less than the second brightness level.

6. The LED device according to claim 5, wherein the first brightness level is zero and the second brightness level is non - zero.

7. The LED device according to claim 5, wherein the second brightness level does not exceed half of the maximum brightness level of the LED string (D).

8. The LED device according to any one of claims 1 to 4, wherein the length of the first time period depends on the magnitude of the second brightness level.

9. The LED device according to claim 1, wherein the switching means (Q2, Q3, Q4, R7, R8, R9, R10, R11, R12) includes a first transistor having a base controlled by the controller (250) and a collector and an emitter connected between two sides of at least one impedance element of the first impedance means (R4, R5).

10. The LED device according to claim 9, wherein: the first impedance means (R4, R5) includes at least a first impedance element and a different second impedance element; the collector and the emitter of the first transistor of the switching means (Q2, Q3, Q4, R7, R8, R9, R10, R11, R12) are respectively connected between a first side and a second side of the first impedance element; and the switching means (Q2, Q3, Q4, R7, R8, R9, R10, R11, R12) further includes a second transistor having: a base connected to the second side of the first impedance element and the first side of the second impedance element; a collector connected to the first side of the first impedance element; and an emitter connected to the second side of the second impedance element.

11. The LED device according to any one of claims 1 to 4, 9 and 10, further comprising a resistive load connected in parallel with the LED string (D).

12. A method of controlling an LED device formed by an LED string (D), a controllable current source (CS), and a capacitor (C1), the LED string (D) including one or more LEDs and being adapted to receive power from a voltage source (V), the controllable current source (CS) being connected in series with the LED string (D), the capacitor (C1) being connected in parallel with the LED string (D), the method comprising, in response to a desired indication signal indicating a switch of the brightness level of the LED string (D) from a first brightness level to a different second brightness level: During a first time period: controlling the controllable current source (CS) to limit the voltage across the LED string (D) to no greater than a first voltage level, and controlling the controllable current source (CS) to regulate the charge stored by the capacitor (C1); and After completion of the first time period, ceasing to limit the voltage across the LED string (D) to no greater than the first voltage level, and controlling the controllable current source (CS) to attempt to draw current through the LED string (D) from the voltage source (V) such that the brightness of the LED string (D) is at the second brightness level.

13. The method according to claim 12, wherein the average current drawn from the voltage source (V) during the first time period is greater than the average current drawn from the voltage source (V) after the first time period.

14. The method according to any one of claims 12 or 13, wherein the first brightness level is less than the second brightness level.

15. The method according to claim 14, wherein the second brightness level does not exceed half of the maximum brightness level of the LED string (D).

Images