An LED lighting circuit

Abstract

A mechanism for driving two parallel LED lighting arrangements. A power supply, for driving the two LED lighting arrangements, operates in a voltage control mode when both LED lighting arrangements are active. The power supply operates in a current control mode when only one of the LED lighting arrangements is active.

Description

[0001] An LED lighting circuit

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of LED lighting.

[0004] BACKGROUND OF THE INVENTION

[0005] The increasing use of artificial light is causing a greater demand for LED lighting circuits. In particular, there is a demand for LED lighting devices that maintain good characteristics during dimming procedures, e.g., maintains color mixing, power efficiency and / or reduced ripple or flicker.

[0006] United States Patent Application No. US 2010 / 001657 Al discloses one system for driving an LED. The system is configured to switch between a constant-current mode and a constant-voltage mode circuit in driving the same LED. This can be used to maintain a forward voltage of the LED and improve the power efficiency of driving the same LED.

[0007] Presently, in order to provide tunable color or color temperature, multiple LED arrangements with different color or color temperature are driven and their light outputs mixed to provide the desired color or color temperature. LED arrangements with different color or color temperatures usually have different electrical characteristics such as forward voltages and are not easy to be driven simultaneously, especially with a high efficiency and accuracy.

[0008] There is an ongoing desire to improve the performance of LED lighting circuits comprising multiple different LED arrangements.

[0009] SUMMARY OF THE INVENTION

[0010] The underlying idea of the present invention is to switch a power supply for a LED lighting circuit between a constant current mode and a constant voltage mode dependent upon which LED arrangement(s) draw from the supply power provided by the voltage supply. If only a first LED arrangement draws from the supply power, then the power supply operates in a constant current mode for improved efficiency. If both the first and a second LED arrangement draw from the supply power, then the power supply operates in a constant voltage mode for improving the current control in each LED arrangement so as to provide accurate light mixing and / or dimming performance. The proposed technique thereby balances the desire to provide a highly efficient LED lighting circuit (e.g., at high brightness levels) with the demand for good color mixing and dimming performance. This is achieved by switching in which mode the power supply operates responsive to or along with the different active LED arrangements.

[0011] The invention is defined by the claims.

[0012] According to examples in accordance with an aspect of the invention, there is provided an LED lighting circuit. The LED lighting circuit comprises: a power supply adapted to provide a supply power; a first LED arrangement having a first electrical characteristic; and a second LED arrangement connected in parallel to the first LED arrangement, the second LED arrangement having a second, different electrical characteristic.

[0013] The LED lighting circuit may be configured to operate the power supply: in a current control mode, to provide the supply power, when only one of the first and second LED arrangements draws from the supply power; and in a voltage control mode, to provide the supply power, when both of the first and second LED arrangements draw from the supply power.

[0014] The proposed approach switches a power supply from a voltage control mode to a current control mode when only one LED arrangement draws power. This facilitates improved efficiency of operating the LED lighting circuit when only one LED arrangement draws power. When multiple parallel LED arrangements need to be driven, the proposed approach switches the power supply to the voltage control mode for the parallel LED arrangements, thus ensuring all LED arrangements can be sufficient powered whilst still achieving good light mixing and dimming. The proposed approach thereby provides a more efficient LED lighting circuit without sacrificing output light characteristics.

[0015] The electrical characteristic may comprise a forward voltage, and the forward voltage of the second LED arrangement may be higher than the forward voltage of the first LED arrangement. In this way, the voltage of the supply power can be reduced when operating in the current control mode for the first LED arrangement at a more efficient operation.

[0016] The LED lighting circuit may comprise a first current regulator in series with the first LED arrangement and a second current regulator in series with the second LED arrangement. If present, each respective current regulator is configured to: be closed if the corresponding LED arrangement is powered and opened if the corresponding LED arrangement is not powered when the power supply is configured to operate in the current control mode; and be activated, to regulate a current through the respective LED arrangement, when the power supply is configured to operate in the voltage control mode. On one hand, the current regulator ensures the respective LED arrangement receives accurate current in the voltage control mode. In the context of the present disclosure, the term activate means the current regulator actively regulates the current. For example, it enters a linear mode wherein the current through it is controlled thereby. One the other hand, this approach closes or opens the current regulator(s) when the power supply operates in the current control mode. In the context of the present invention, the term “close” means the current regulator is closed in a full conduction mode (e.g., allows the passage flow of current therethrough). “Open” means that the current regulator is open so as to prevent the passage or flow of current therethrough, i.e., operates in a non-conduction mode. This avoids conflict between the current regulated by the current regulator and that of the power supply. Moreover, as current is controlled by the power supply, the active current regulation become unneeded during the current control mode, such that closing the current regulators by making it enter a full conduction mode saves power loss on the current regulator.

[0017] The power supply may comprise a switched-mode power supply. This provides a reliable and efficiency power supply for switching between a current control mode and a voltage control mode. Alternatively, the power supply (per se) can also be implemented by a linear component.

[0018] The LED lighting circuit may be configured to, as the switched-mode power supply transitions from the voltage control mode to the current control mode: at a first point in time, open the second current regulator. The LED lighting circuit may be further configured to, for a second period of time after the first point in time: operate the switched-mode power supply in the current control mode to output a peak current that gradually decreases from an initial level; and regulate the current through the first LED arrangement to substantially maintain a target average current through the first LED arrangement as the peak current is gradually decreased; after the second period of time, and when the voltage of the supply power has reached a forward voltage of the first LED arrangement: operate the switched-mode power supply in the current control mode to output the target average current; and make the first current regulator enter a full conduction mode such that the current through the first LED arrangement is defined by the switched-mode power supply.

[0019] This approach provides a technique for smoothly transitioning from the voltage control mode to the current control mode whilst maintaining a substantially same average current to the first LED arrangement that is still operating, and this produces the same light output level from the LED lighting circuit, e.g., to imperceptibly (to a viewer of the LED lighting circuit) change from the voltage control mode to the current control mode. In some examples, the LED lighting circuit is configured to, as the switched- mode power supply transitions from the voltage control mode to the current control mode: for a first period of time between the first point in time and the second period of time: obtain the target average current; maintain the switched-mode power supply in the voltage control mode and maintain a voltage of the supply power; regulate a current through the first LED arrangement by using the first current regulator to provide the target average current through the first LED arrangement, wherein the initial level in the current control mode in the second period of time is defined responsive to residual control parameters, of the switched mode power supply, generated in the voltage control mode in the first period of time.

[0020] When the second current regulator is opened, there may be a small rise in the output voltage of the power supply since the second LED arrangement strops drawing energy. Thus, this approach provides a (time) buffer within the voltage mode between deactivating the second LED arrangement (by opening the second current regulator) and beginning the full transition to the current control mode. This buffer allows for the (average) electrical current characteristics in the first LED arrangement to settle before beginning the transition, to reduce a risk of sudden changes or overshoots in adjusting the current through the first LED arrangement, thereby making the transition more imperceptible.

[0021] In some examples, the LED lighting circuit is configured to, during the second period of time, make the first current regulator enter a full conduction mode and apply a gradually increased duty cycle to the first current regulator to substantially maintain the target average current through the first LED arrangement as the peak current gradually decreases. This provides a technique for maintaining a current flow through the first LED arrangement (and thereby light output) during the transition, thereby reducing the power loss on the first current regulator.

[0022] The LED lighting circuit may be configured to, in the first period of time, regulate the current through the first LED arrangement to provide the target average current by controlling a peak current conducted by the first current regulator and a duty cycle of the first current regulator. Thus, the current through the first LED arrangement may be controlled using pulse width modulation. This approach provides high flexibility in facilitating the dynamic adaptation of current flow through the first LED arrangement, e.g., to respond to changes in lighting demand and / or power surges.

[0023] The LED lighting circuit may be configured to, as the switched-mode power supply transitions from the current control mode to the voltage control mode: for a third period of time: operate the switched-mode power supply in the voltage control mode; gradually increase the voltage of the supply power; regulate the current through the first LED arrangement by using the first current regulator to substantially maintain the average current through the first LED arrangement when the voltage gradually increases. After the third period of time and when the voltage of the supply power has reached the sum of the forward voltage of the second LED arrangement and a voltage drop of the second current regulator, the LED lighting circuit may: maintain the voltage of the supply power; and activate the second current regulator.

[0024] This embodiment provides a smooth transition from a current control mode to a voltage control mode without significant or sudden changes in (average) current drawn by the (first) LED arrangement. This maintains effectively the same light output from the LED lighting circuit during the transition.

[0025] The LED lighting circuit may be configured to, during the third period of time, gradually increase the peak current through the first LED arrangement via the first current regulator. This gradually increased peak current is for approximating the present output lumen to the new output lumen.

[0026] In some examples, during the third period of time, the LED lighting circuit may be configured to gradually decrease the duty cycle of the current through the first LED arrangement to substantially maintain the average current through the first LED arrangement as the peak current increases. This maintains a smooth transition.

[0027] In some examples, the first LED arrangement comprises one or more LED configured to emit white light; and / or the second LED arrangement comprises one or more LEDs configured to emit red, green and / or blue light.

[0028] The first LED arrangement may comprise a first branch of one or more LEDs configured to emit cool white light; and a second branch of one or more LEDs, connected in parallel to the first branch, configured to emit warm white light. This provides an approach for producing mixed white light. The forward voltage of warm and cool white LEDs is usually similar (not as different as between the white LEDs and RGB LEDs), such that it is possible to perform a same / similar operation to maintain a voltage across the cool white and warm white LED arrangements.

[0029] The LED lighting circuit may further comprise a switching circuit adapted to controllably close and open the first branch and second branches. The switching circuit is configured to: synchronously open the second branch and close the first branch; and synchronously open the first branch and close the second branch. Since the forward voltages of cool white LED and warm white LED are similar, they can be driven in a complementary manner even in the current control mode and the efficiency is optimized.

[0030] In some examples, the first LED arrangement comprises at least one LED configured to emit cool white light; and the second LED arrangement comprises at least one LED configured to emit warm white light. In this embodiment, the cool white LED and the warm white LED are also driven by the voltage control mode and the accuracy is optimized.

[0031] The LED lighting circuit may further comprise a microcontroller configured to control the operation of at least the power supply, e.g., control the mode in which the power supply operates. If present, the microcontroller may also and / or alternatively control the operation of the first and second current regulators.

[0032] There is also provided a luminaire comprising any herein disclosed LED lighting circuit.

[0033] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment s) described hereinafter.

[0034] BRIEF DESCRIPTION OF THE DRAWINGS

[0035] For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0036] Fig. 1 illustrates an existing LED lighting circuit;

[0037] Fig. 2 illustrates a proposed LED lighting circuits; and

[0038] Fig. 3 illustrates waveforms for the proposed LED lighting circuit.

[0039] DETAILED DESCRIPTION

[0040] The invention will be described with reference to the Figures.

[0041] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts. The invention provides a mechanism for driving two parallel LED lighting arrangements. A power supply, for driving the two LED lighting arrangements, operates in a constant voltage mode when both LED lighting arrangements are active. The power supply operates in a current control mode when only one of the LED lighting arrangements is active.

[0042] In the context of the present invention, the term “warm white” refers to a color having a correlated color temperature (CCT) below that of “cool white”. For instance, warm white may have a CCT of less than 3500K and cool white may have a CCT of more than 3500K. Other suitable definitions for warm white and cool white will be apparent to the skilled person.

[0043] In the context of the present invention, an LED is configured to output light of a particular color or mix of colors. A color adjective for an LED defines the color of light output or emitted by that LED. Thus, a red LED will emit red light, whereas a blue LED will emit blue light.

[0044] Figure 1 is a block diagram conceptually illustrating an existing LED lighting circuit 100. For the sake of illustrative clarity, power flow is illustrated using solid arrows, with control logic being illustrated using dashed arrows. The LED lighting circuit comprises a first power supply 111, a second power supply 112, a first LED arrangement 120, and a second LED arrangement 130.

[0045] The first power supply 111 is configured to drive the first LED arrangement 120. The second power supply 112 is configured to drive the second LED arrangement 130. The first power supply I l l is configured to operate in a constant current (i.e., CC) mode, i.e., to provide the first LED arrangement with a first supply power having a constant current. The second power supply 112 is configured to operate in a constant voltage (i.e., CV) mode, i.e., to provide the second LED arrangement with a second supply power having a constant voltage.

[0046] Each LED arrangement may comprise a plurality of LEDs. For instance, the first LED arrangement 120 may comprise at least a warm white LED LED1 and a cool white LED LED2. Similarly, the second LED arrangement 120 may comprise at least one red LED LED3, at least one blue LED LED4 and at least one green LED LED5.

[0047] The first LED arrangement 120 further comprises a dimming controller 125, configured to control a current flow through the LEDs of the first LED arrangement responsive to a demanded dimming or power level to be drawn by the first LED arrangement. The dimming controller may, for instance, increase the proportion of warm white light emitted by the first LED arrangement as the power drawn by the first LED arrangement increases (e.g., dimming level increases. The dimming controller 125 could be two complementary switches connected respectively in series with LED1 and LED2, so as to switch the current provided by the first power supply 111 to one of LED 1 and LED2.

[0048] The second LED arrangement may comprise a current regulator 135 configured to regulate the current through the second LED arrangement. The current regulator 135 could comprise three linear switches connected respectively in series with each LED LED3, LED4 and LED5 to provide a respective current through the LEDs from the voltage provided by the second power supply 112.

[0049] The operation of the dimming controller 125 and the current regulator 135 is controlled by a microcontroller 140. The microcontroller may be powered via a microcontroller driving arrangement 150, e.g., which converts a voltage output by the second power supply 112 (i.e., performs DC-DC voltage conversion).

[0050] Figure 2 is a block diagram conceptually illustrating a proposed LED lighting circuit 200. For the sake of illustrative clarity, power flow from the power supply 210 is illustrated using a solid arrow for constant current CC mode and a dot-dash arrow for constant voltage CV mode, with control logic being illustrated using dashed arrows. The LED lighting circuit comprises a power supply 210, a first LED arrangement 220, and a second LED arrangement 230. Please note that the power supply 210 may only have one physical output to output either the voltage or the current, and the different arrows for CC and CV in the figure are only for differentiating the two modes logically. Practically, all LED arrangements should be connected in parallel.

[0051] The power supply 210 is adapted to provide a supply power. The supply power is provided to the first 220 and second 230 LED arrangements, from which they are able to draw power.

[0052] In particular, the power supply may convert an input supply VIN to a supply power for the first and / or second LED arrangements. The input supply VIN may, for instance, be an AC supply, such as a mains supply. The supply power is a DC power for driving the first and / or second LED arrangements. Alternatively, the input supply VIN could also be a DC power supply, for example, an input supply provided by a battery or a solar panel.

[0053] The first LED arrangement 220 and the second LED arrangement 230 have at least one different electrical characteristic. In one particular example, the electrical characteristic comprises forward voltage. And in this embodiment, the forward voltage of the second LED arrangement is higher than the forward voltage of the first LED arrangement.

[0054] More particularly, the first LED arrangement 220 and the second LED arrangement 230 may have one or more different lighting characteristics. Examples of suitable lighting characteristics that may differ include: a color; a color temperature, a beam angle, a beam spread, a beam distribution, a glare rating and so on.

[0055] Each LED arrangement comprises one or more LEDs LED1, LED2, LED3 configured to output or emit light. Examples of suitable LED arrangements will be described later in this document.

[0056] The power supply 210 is configured to switch between operating in a current control mode (CC), i.e., a constant current mode, and a voltage control mode (CV), i.e., a constant voltage mode. For the sake of illustration only, Figure 2 illustrates these two different modes as different power flows, although it will be appreciated that, in practice, these power flows are provided along a same power line / bus (i.e., by simply adjusting the approach for defining the supply power).

[0057] As is well known, in a current control mode the magnitude of a current (e.g., an average magnitude of the current) of the supply power is maintained at a target current level, e.g., a target current. This can be achieved by using a current feedback signal, responsive to a current level, to control a high frequency switching of the power supply to as to control magnitude of the power in the supply power. The current level may be a current through an output of the power supply 210.

[0058] Similarly, in a voltage control mode, the magnitude of the voltage (e.g., an average magnitude of the voltage) of the supply power is maintained at a target voltage level, e.g., a target voltage. This can be achieved by using a voltage feedback signal, responsive to a voltage level, to control a high frequency switching of the power supply to as to control magnitude of the power in the supply power. The voltage level may be a voltage difference between the supply power and a ground or reference voltage.

[0059] Appropriate examples for a power supply 210 that can be selectively operated either in a voltage control mode or a current control mode are well known to the skilled person, and may include any switched-mode power supply (SMPS), such as any suitable SMPS having the topology of a buck converter, buck-boost converter or flyback converter. Other examples will be apparent to the skilled person. For example, the power supply 210 could also be implemented by a linear component.

[0060] The power supply 210 is configured to operate in the current control mode when only one of the first 220 and second 230 LED arrangements draws power from the power supply. The power supply 210 is configured to operate in the voltage control mode when both of the first 220 and second 230 LED arrangements draw power from the power supply. Thus, the power supply switches between operating in the current control mode and the voltage control mode based on how many of the LED arrangements draw power therefrom.

[0061] In particular examples, the power supply is configured to operate in the current control mode CC when only the first LED arrangement 220 draws power from the power supply.

[0062] The operation of the power supply may be controlled or defined by a microcontroller 240. Thus, the microcontroller 240 may receive feedback from the power supply and / or LED arrangement(s) and control the control mode of the power supply. In particular examples, e.g., where the power supply is a SMPS, the microcontroller 240 may control the switching of the power supply in order to define or control the operation of the power supply.

[0063] Thus, where any control operations are performed by the LED lighting circuit, then these control operations may in practice be performed by the microcontroller 240.

[0064] The first LED arrangement 220 may comprise a first current regulator 225. The second LED arrangement 230 may comprise a second current regulator 235. Each current regulator is configured to, when activated, regulate a current through the respective LED arrangement. Appropriate examples of current regulators include a BJT transistor or a MOSFET that operates works in a linear region so as to actively regulate a current. Other implementations are also well known in the art, such as small switched mode power supply regulators used as a variable impedance without the high power loss of linear transistor, and are not described in detail for the sake of conciseness. Moreover, the current regulator could also work in a fully conduction mode whereas the current through it is determined by how much current is provided by the power supply.

[0065] In particular, each current regulator 225, 235 may also control the current flow through its respective LED arrangement using a pulse width modulation (PWM) technique. Meaning that not only the peak current through the current regulator can be tuned, but also the duty cycle of turning on of the current regulator can be tuned. The average current through the current regulator could be regarded as the product of the peak current and the duty cycle.

[0066] Each current regulator 225, 235 is configured to be closed if the corresponding LED arrangement is powered and opened if the corresponding LED arrangement is not powered when the power supply operates in the current control mode, and be activated when the power supply operates in the voltage control mode. Thus, when the current regulator is closed, a current through the first and / or second LED arrangements is defined by the current of the power supply (which is controlled to be a constant current).

[0067] The proposed approach means that when only a single one of the LED arrangements draws power, constant current is supplied by the power supply for highly efficient operation. Use of a constant current power supply in this scenario also improves the power factor, total harmonic distortion and harmonics performance. The current regulator corresponding to the LED arrangement that is not to be driven enters an open circuit state (i.e., becomes open), at least for reduced power loss.

[0068] Similarly, when multiple LED arrangements draw power, then a constant voltage is supplied by the power supply, with the current through the LED arrangements being defined by their corresponding (dedicated) current regulators. This improves accuracy of current regulation in all of the LED arrangement and thus improves the (e.g., color) mixing of light emitted by the LED arrangements and the dimming performance of the LED arrangements, e.g., reduced flicker when a low light level is demanded (as the provision of a constant voltage avoids or reduces a likelihood that the LED arrangement will stop emitting light at low power levels).

[0069] Control over which of the LED arrangements draw power may be responsive to an external or internal trigger or control signal. Examples of suitable triggers or control signals include: a user input or request (e.g., at a user interface 290), a signal / trigger generated by a sensing device (such as a motion sensor), a timing schedule or a separate processing circuit. The proposed LED lighting circuit may be able to respond to the trigger or control signal and control which LED arrangements 220, 230 draw power and / or the operation of the power supply 210 appropriately.

[0070] The decision as to which LED arrangements draw power may be dependent upon desired lighting characteristics (e.g., indicated via the trigger or control signal). In particular, different LED arrangements may have different lighting characteristics (e.g., color, color temperature, beam spread, beam angle, glare ratings and so on). Mixing or selection of which LED arrangements draw power thereby allows for control over the lighting characteristics output by the LED lighting circuit, e.g., to match desired lighting characteristics.

[0071] In some examples, the LED lighting circuit is configured to control which LED arrangements draw power responsive to a demanded lighting level (or dimming level) of the LED lighting circuit. The demanded lighting level may, for instance, be a lighting level demanded by a user, e.g., at a user interface. In a preferred example, the LED lighting circuit is configured such that only the first LED lighting arrangement draws power for lighting levels above a predetermined threshold. Thus, if a demanded lighting level breaches a predetermined threshold, then the power supply 210 may operate in the current control mode and supply power (with a constant current) which is drawn by only the first LED lighting arrangement 220.

[0072] In a continuation of this example, both LED lighting arrangements may be permitted to draw power for lighting levels below the predetermined threshold. Thus, for lower lighting levels (i.e., higher dimming levels) then the power supply 210 may operate in the voltage control mode, and supply power (with a constant voltage) for both the first and second lighting arrangements 220, 230.

[0073] This ensures that, at high lighting levels, more efficient performance is achieved whereas at lower lighting levels, improved color mixing and dimming performance is achieved.

[0074] The LED lighting circuit 200 may further comprise a microcontroller driving arrangement 250 that supplies a power for the microcontroller 240. The microcontroller driving arrangement 250 may be independent of the power supply 210 and be configured to convert an input supply VIN to a microcontroller supply power.

[0075] Use of a separate driving arrangement 250 for the microcontroller can save standby power loss, e.g., as the power supply 210 can be switched off if no LED arrangement is to draw power from the voltage supply. Thus, the power supply may be further operable in a standby mode in which no supply power is produced. The microcontroller may operate the power supply in the standby mode responsive to a control signal to enter the power supply into the standby mode. Examples of suitable control signals have been previously provided.

[0076] In some examples, the first LED arrangement 220 comprises one or more LEDs LED1 configured to emit white light. In particular, the first LED arrangement 220 may comprise only one or more LEDs configured to emit white light (i.e., only one or more white light LEDs). Thus, in some examples, when the demanded lighting level is very high, then power is drawn by only the first LED arrangement 220 and only white light is emitted. This provides a highly efficient mechanism for outputting bright light.

[0077] In some examples, the second LED arrangement 230 comprises one or more LEDs configured to emit red, green and / or blue light.

[0078] In some examples, the first LED arrangement comprises at least one LED configured to emit cool white light (e.g., and no other LED). The second LED arrangement may comprise at least one LED configured to emit warm white light (e.g., and optionally a red, blue and / or green LED). In one variant, the first LED arrangement comprises: a first branch of one or more LEDs configured to emit cool white light; and a second branch of one or more LEDs, connected in parallel to the first branch, configured to emit warm white light.

[0079] In an example of this variant, the LED lighting circuit may further comprise a switching circuit adapted to controllably close and open the first branch and second branches, wherein the switching circuit is configured to: synchronously open the second branch and close the first branch; and synchronously open the first branch and close the second branch.

[0080] Preferably, the switching circuit is configured to be operational when the power supply is operating in the current control mode and the first LED arrangement draws from the supply power.

[0081] This approach recognizes that cool white (CW) and the warm white (WW) LEDs typically have similar forward voltages. Thus, if only white color with a certain color temperature is needed by mixing the cool white and warm white LEDs, two branches can be switched alternatively in complementary manner to achieve a desired white color temperature.

[0082] Of course, the second LED arrangement 230 may comprise one or more LEDs configured to emit red, green and / or blue light. Thus, if RGB (red, green blue) light is needed (e.g., for a certain lighting scenario), then the power supply may operate in the CV mode to drive both the RGB LED arrangement and the CW and / or WW LEDs.

[0083] The proposed LED lighting circuit 200 may form part of a luminaire, e.g., comprising additional components such as a housing, mounting equipment, sensing circuitry, driving circuitry, further processing circuitry and so on.

[0084] Further proposed embodiments provide a preferred approach for transitioning between (constant) voltage control and (constant) current control, to reduce flicker and / or power overshoot or undershoot during such transitions.

[0085] Figure 3 illustrates waveforms of various signals during a transition from a voltage control mode (e.g., constant voltage mode) to a current control mode (e.g., constant current mode) as well as the other way around for improved contextual understanding.

[0086] A first waveform 310 illustrates a control signal that indicates whether the power supply operates in a voltage control / constant voltage mode (CV) or current control / constant current mode (CC). A second waveform 320 illustrates the voltage of the supply power (i.e., a voltage across the first and second LED arrangements). A third waveform 330 illustrates a current through the first LED arrangement. A fourth waveform illustrates a current through the second LED arrangement. A number of time points are illustrated for the sake of explanation. There is an initial point in time to, a first point in time ti, a second point in time t2, a third point in time t3, a fourth point in time , a fifth point in time ts and a sixth point in time te.

[0087] Between the initial point in time to and the first point in time ti, the power supply operates in a voltage control mode. Between the third point in time ti and the fourth point in time t4, the power supply operates in the current control mode. A first transition (from voltage control mode to current control mode) takes place between the first and third points in time. A second transition (from current control mode to voltage control mode) takes place between the fourth and fifth points in time.

[0088] In the illustrated example, the LED lighting circuit is operating in an initial state in which both the first and the second LED arrangements draw power. The current through each LED arrangement is controlled or defined using a pulse width modulation technique in order to control the amount of light emitted by each LED arrangement. Current paths within any LED arrangement may be controlled independently, e.g., to control the color output by the particular LED arrangement.

[0089] To perform a first transition (from voltage control mode to current control mode), the LED lighting circuit may be configured to, at a first point in time ti, open the second current regulator. This stops a current flow through the second LED arrangement. The second current regulator may be opened, for instance, responsive to a user input or other trigger that indicates that there is a desire to prevent the second LED arrangement from emitting light.

[0090] Subsequently, for a first period of time ti - 12, the LED lighting circuit may be configured to maintain the power supply in the voltage control mode (and maintain the voltage of the supply power), and regulate a current through the first LED arrangement, using the first current regulator, to provide a target average current through the first LED arrangement.

[0091] In some examples, during the first period of time, the LED lighting circuit is configured to regulate the current through the first LED arrangement to provide the target average current by controlling a peak current conducted by the first current regulator and a duty cycle of the first current regulator. More specifically, the power supply may not be responsive to the deactivation of the second LED arrangement thus its output voltage 320 may have a small overshoot. As the first current regulator is still active (e.g., still works in a linear mode), the current through the first LED arrangement is still regulated without an overshoot.

[0092] Subsequently, for a second period of time t2 - t3, the LED lighting circuit is configured to operate the switched-mode power supply in the current control mode to output a peak current that gradually decreases from an initial level. Simultaneously, the current through the first LED arrangement is regulated (e.g., by the switched mode power supply or by the first current regulator) to maintain a / the target average current as the peak current is gradually decreased. In this way, the duty cycle of the current through the first LED arrangement increases in order to sustain the target average current. The target average current may be relevant with or defined by the control of a user / operator. In the second period of time, the initial level for the current control mode may be determined responsive to residual / remaining control parameters, of the switched mode power supply, generated in the voltage control mode in the first period of time.

[0093] In some examples, during the second period of time, the first current regulator may enter a full conduction mode (i.e., a 100% duty cycle) and the power supply may apply a gradually increased duty cycle to the first current regulator to substantially maintain the target average current through the first LED arrangement as the peak current gradually decreases. Thus, the current through the first LED arrangement may be controlled solely by the power supply during the second period of time.

[0094] Subsequently, once the voltage of the supply power has reached a forward voltage of the first LED arrangement (i.e., when the second period of time has ended and the third point ti in time has been reached), the power supply operates in the current control mode to output the target average current. The first current regulator then enters a full conduction mode with a 100% duty cycle, so that current through the first LED arrangement is defined (solely) by the power supply. In other words, the first current regulator is configured to be conductive and no longer provide any active contribution to the current through the first LED arrangement. Please note that in the if the LED lighting circuit is dimmed down when only the first LED arrangement is activated, the power supply 210 may decrease the output current, the first current regulator may decrease its duty cycle, or both.

[0095] The first period of time can also be used for the LED lighting circuit (e.g., the microcontroller thereof) to detect that only the first lighting arrangement is drawing power from the supply power. In particular, the microcontroller may be configured to detect that only the first lighting arrangement is drawing power from the supply power for more than a predetermined period of time, after which the power supply is controlled to operate in the current control mode.

[0096] The steps taken during the first period of time may, in some embodiments, be omitted. This is possible, for instance, when the same trigger that is used to open the second current regulator is also used to simultaneously or directly trigger the switching of the power supply to a current control mode. To perform a second transition (from current control mode to current control mode), the LED lighting circuit may be configured to, for a third period of time t4 — ts : operate the switched-mode power supply in the voltage control mode (and stop operating in the current control mode); gradually increase the voltage of the supply power; and regulate the current through the first LED arrangement by using the first current regulator to substantially maintain the average current through the first LED arrangement when the voltage gradually increases.

[0097] Subsequently, after the third period of time t4-ts and when the voltage of the supply power has reached the forward voltage of the second LED arrangement: the LED lighting circuit may then maintain the voltage of the supply power; and activate the current regulator.

[0098] In the illustrated example, the first LED arrangement’s light output is still needed thus the first current regulator may continue to draw current through the first current regulator. The precise operation of the first current regulator at this stage may depend upon the desired light output characteristics.

[0099] In some examples, during the third period of time, the LED lighting circuit is configured to gradually increase the peak current through the first LED arrangement via the first current regulator. In some further examples, during the third period of time, the LED circuit is configured to gradually decrease the duty cycle (using the first current regulator) of the current through the first LED arrangement to substantially maintain the average current through the first LED arrangement as the peak current increases.

[0100] This approach reduces a chance of flicker and / or sudden changes in amplitude of light output by the LED lighting circuit during transitions between a current control mode and a voltage control mode.

[0101] Embodiments may make use of a microcontroller to control the operation of the LED lighting circuit. The skilled person would be readily capable of developing a microcontroller for carrying out any such function. The microcontroller can be implemented in numerous ways, with software and / or hardware, to perform the various functions required. A processor is one example of a microcontroller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform the required functions. A microcontroller may however be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions. Examples of microcontroller components that may be employed 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).

[0102] 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 article “a” or “an” does not exclude a plurality.

[0103] Functions implemented by a microcontroller may be implemented by a single processor or by multiple separate processing units which may together be considered to constitute a “ microcontroller “. Such processing units may in some cases be remote from each other and communicate with each other in a wired or wireless manner (optional)

[0104] 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.

[0105] If the term “adapted to” is used in the claims or description, it is noted the term “adapted to” is intended to be equivalent to the term “configured to”. If the term “arrangement” is used in the claims or description, it is noted the term “arrangement” is intended to be equivalent to the term “system”, and vice versa.

[0106] Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An LED lighting circuit (200) comprising: a power supply (210) adapted to provide a supply power; a first LED arrangement (220) having a first electrical characteristic; and a second LED arrangement (230) connected in parallel to the first LED arrangement, the second LED arrangement having a second, different electrical characteristic, wherein the LED lighting circuit is configured to operate the power supply: in a current control mode (CC), to provide the supply power, when only one of the first and second LED arrangements draws from the supply power; and in a voltage control mode (CV), to provide the supply power, when both of the first and second LED arrangements draw from the supply power.

2. The LED lighting circuit of claim 1, wherein the electrical characteristic comprises a forward voltage, and the forward voltage of the second LED arrangement is higher than the forward voltage of the first LED arrangement.

3. The LED lighting circuit of claim 1 or 2, further comprising a first current regulator in series with the first LED arrangement and a second current regulator in series with the second LED arrangement, wherein each respective current regulator is configured to: be closed if the corresponding LED arrangement is powered and opened if the corresponding LED arrangement is not powered, when the power supply is configured to operate in the current control mode; and be activated, to regulate a current through the respective LED arrangement, when the power supply is configured to operate in the voltage control mode.

4. The LED lighting circuit of claim 3, wherein the power supply comprises a switched-mode power supply, the LED lighting circuit is configured to, as the switched-mode power supply transitions from the voltage control mode to the current control mode: at a first point in time, open the second current regulator;for a second period of time after the first point in time: operate the switched-mode power supply in the current control mode to output a peak current that gradually decreases from an initial level; and regulate the current through the first LED arrangement to substantially maintain a target average current through the first LED arrangement as the peak current is gradually decreased; after the second period of time, and when the voltage of the supply power has reached a forward voltage of the first LED arrangement: operate the switched-mode power supply in the current control mode to output the target average current; and make the first current regulator enter a full conduction mode such that the current through the first LED arrangement is defined by the switched-mode power supply.

5. The LED lighting circuit of claim 4, wherein the LED lighting circuit is configured to, as the switched-mode power supply transitions from the voltage control mode to the current control mode: for a first period of time between the first point in time and the second period of time: obtain the target average current; maintain the switched-mode power supply in the voltage control mode and maintain a voltage of the supply power; regulate a current through the first LED arrangement by using the first current regulator to provide the target average current through the first LED arrangement; wherein the initial level in the current control mode in the second period of time is defined responsive to residual control parameters, of the switched mode power supply, generated in the voltage control mode in the first period of time.

6. The LED lighting circuit of claim 4 or 5 configured to, during the second period of time, make the first current regulator enter a full conduction mode and apply a gradually increased duty cycle to the first current regulator to substantially maintain the target average current through the first LED arrangement as the peak current gradually decreases; and configured to, in the first period of time, regulate the current through the first LED arrangement to provide the target average current by controlling a peak current conducted by the first current regulator and a duty cycle of the first current regulator.

7. The LED lighting circuit of any of claims 4 to 6, wherein the LED lighting circuit is configured to, as the switched-mode power supply transitions from the current control mode to the voltage control mode: for a third period of time: operate the switched-mode power supply in the voltage control mode; gradually increase the voltage of the supply power; regulate the current through the first LED arrangement by using the first current regulator to substantially maintain the average current through the first LED arrangement when the voltage gradually increases; after the third period of time when the voltage of the supply power has reached the sum of the forward voltage of the second LED arrangement and a voltage drop of the second current regulator: maintain the voltage of the supply power; and activate the second current regulator.

8. The LED lighting circuit of claim 7 configured to, during the third period of time, gradually increase the peak current through the first LED arrangement via the first current regulator.

9. The LED lighting circuit of claim 8 configured to, during the third period of time, gradually decrease the duty cycle of the current through the first LED arrangement to substantially maintain the average current through the first LED arrangement as the peak current increases.

10. The LED lighting circuit of any of claims 1 to 9, wherein: the first LED arrangement comprises one or more LED configured to emit white light; and / or the second LED arrangement comprises one or more LEDs configured to emit red, green and / or blue light.

11. The LED lighting circuit of claim 10, wherein the first LED arrangement comprises: a first branch of one or more LEDs configured to emit cool white light; anda second branch of one or more LEDs, connected in parallel to the first branch, configured to emit warm white light.

12. The LED lighting circuit of claim 11, further comprising a switching circuit adapted to controllably close and open the first branch and second branches, wherein the switching circuit is configured to: synchronously open the second branch and close the first branch; and synchronously open the first branch and close the second branch.

13. The LED lighting circuit of any of claims 1 to 9, wherein the first LED arrangement comprises at least one LED configured to emit cool white light; and the second LED arrangement comprises at least one LED configured to emit warm white light.

14. The LED lighting circuit of any of claims 1 to 8, further comprising a microcontroller configured to control the operation of at least the power supply and / or the first and second current regulators.

15. A luminaire comprising the LED lighting circuit of any of claims 1 to 14.

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