Constant current to multichannel converter with dimming interface for deep

Abstract

The invention relates to a driver circuit comprising a first light string comprising a series arrangement of a first light source and a first switch, a second light string comprising a series arrangement of a second light source and a second switch, an inductor arranged to be couplable between an output of a dimmable constant current source and a node coupled to the first light string and the second light string and a controller arranged to receive a control signal, determine an on time of the first switch based on the control signal, determine an on time of the second switch based on the control signal, determine a dimming signal for the dimmable constant current source based on the control signal, provide the dimming signal for the dimmable constant current source to an output adapted to be connectable to a dimming input of the dimmable constant current source.

Description

[0001] 2024PF80444

[0002] 1

[0003] CONSTANT CURRENT TO MULTICHANNEL CONVERTER WITH DIMMING INTERFACE FOR DEEP

[0004] FIELD OF THE INVENTION

[0005] The invention relates to a driver circuit. The invention further relates to a lighting system. The invention further relates to a method of controlling a driver circuit.

[0006] BACKGROUND OF THE INVENTION

[0007] Connected lamps, i.e. lamps that can be controlled via an external signal such that they can perform different actions such as dimming or color changing, more than one LED string is used. Each string is driven by an independent driver so that the two or more strings can be controlled in an optimized way. This requires additional drivers per added string and is costly. Another way to drive the LED strings is to provide an AC / DC converter followed by a DC / DC current source. The LED strings are then placed in parallel and each string is controlled with a series switch for achieving the desired light setting such as a color point. This provides more flexibility with the design of the driver when additional LED strings are attached. For an optimal drive without color spread, the length, or at least the forward voltage, of the LED strings should be equal. When there is a voltage spread between the LED strings, the color point shifts because there is an imbalance between the strings since the DC / DC converter generates only one voltage level that is buffered by a large buffer capacitor. This effect is undesired. An inductor is placed between the independent driver and the strings. Such inductor provides an effective buffer between the independent driver and the strings, effectively preventing a large voltage fluctuation to be present at the output of the independent driver. At deep dimming however, the output voltage of the driver may become too low, effectively resulting in an unstable control of the independent driver. It is desired to allow the independent driver to be used in a stable way at deep dimming.

[0008] SUMMARY OF THE INVENTION

[0009] It is an objective of the invention to provide a driver circuit that improves the current distribution between light sources and allow a good dimming performance.2024PF80444

[0010] 2

[0011] To provide such solution, in a first aspect of the invention a driver circuit is provided.

[0012] The driver circuit comprises:

[0013] a first light string comprising a series arrangement of a first light source and a first switch;

[0014] a second light string comprising a series arrangement of a second light source and a second switch;

[0015] an inductor arranged to be couplable between an output of a dimmable constant current source and a node coupled to the first light string and the second light string;

[0016] a controller arranged to:

[0017] receive a control signal;

[0018] determine a first on time of the first switch based on the control signal; determine a second on time of the second switch based on the control signal; control the first switch based on the determined first on time and the second switch based on the second on time;

[0019] determine a dimming signal for the dimmable constant current source based on the control signal,

[0020] provide the dimming signal for the dimmable constant current source to an output terminal of the driver circuit, wherein the output of the driver circuit is adapted to be connectable to a dimming input terminal of the dimmable constant current source.

[0021] A driver circuit may have a first light string that has a first light source in series with a first switch. The driver circuit further has a second light string that has a second light source in series with a second switch. The first and second light strings are coupled to each other at a node. An inductor is provided between the node and an output of a dimmable constant current source. This dimmable constant current source can be a current source or a current regulated voltage source. The inductor allows a current distribution to the light sources and smoothens the voltage fluctuations at the dimmable constant current source that may be caused by switching between the light strings. The voltage fluctuation is present at the node between the inductor and the two strings, while at the other side, the voltage is relatively constant. The dimmable constant current source therefore provides a constant current at a relatively constant voltage. To provide dimming the light intensity for both the light sources, it is desired to lower the current amplitude that is provided by the dimmable constant current source. Therefore, the controller is provided. The controller receives a control signal. This control signal may be provided via a wired or wireless interface, such as2024PF80444

[0022] 3

[0023] 1 - 10 V, phase-cut dimmer, DALI, Zigbee, WiFi, Bluetooth, Thread. The control signal may also be used for other functions such as color or color temperature setting. The control signal is then processed by the controller and translated into three signals. The first signal is a control signal for the first switch. The second signal is a control signal for the second switch. Both control signals allow the control of the ratio of current between the light sources. The controller also generates a third signal, a dimming signal. This signal is generated based on the control signal to allow the amplitude of the current generated by the dimmable constant current source to be set. Based on the control signal, the amplitude of the current provided by the dimmable constant current source needs to be set at a specific amplitude. This allows light with a specific color temperate with a specific light intensity to be generated by the first and second light sources. The dimming signal is provided to the dimmable constant current source, which adjusts its output current based on this signal. The dimmable constant current source has a dimming interface to which the dimming signal is provided.

[0024] In a further example, the driver circuit further comprises a first capacitor coupled in parallel with the first light source and a second capacitor coupled in parallel with the second light source.

[0025] To prevent abrupt current changes through the light sources, capacitors are provided in parallel to them. The capacitors serve as energy buffers that will store any current spike in its capacitance that would otherwise flow through the light source. Such current spike may occur when the switches switch such that the current provided by the dimmable constant current source flows from one light source to another light source. Such current spike may case the light output to deviate from the actual expected or regulated light output and may even potentially cause damage to the light sources.

[0026] In a further example, the driver circuit comprises:

[0027] a series arrangement of a first capacitor and a third switch, coupled in parallel with the series arrangement of the first light source and the first switch;

[0028] a series arrangement of a second capacitor and a fourth switch, coupled in parallel with the series arrangement of the second light source and the second switch;

[0029] a first diode coupled between the node and a node coupled to the series arrangement of the first capacitor and the third switch, coupled in parallel with the series arrangement of the first light source and the first switch, and

[0030] a second diode coupled between the node and a node coupled to the series arrangement of the second capacitor and the fourth switch, coupled in parallel with the series arrangement of the second light source and the second switch,2024PF80444

[0031] 4

[0032] wherein the controller is arranged to control the third switch and the fourth switch.

[0033] The driver circuit has a series arrangement of a first capacitor coupled in series with a third switch. This series arrangement is coupled in parallel with the series arrangement of the first light source and the first switch. The driver circuit also has a series arrangement of a second capacitor coupled in series with a fourth switch. This series arrangement is coupled in parallel with the series arrangement of the second light source and the second switch. A first diode is coupled between the node and a node coupled to the series arrangement of the first capacitor and the third switch, coupled in parallel with the series arrangement of the first light source and the first switch. A second diode is coupled between the node and a node coupled to the series arrangement of the second capacitor and the fourth switch, coupled in parallel with the series arrangement of the second light source and the second switch. The diodes prevent the capacitors to discharge into the other channels. The controller is arranged to further control the third and fourth switch. The control of these switches allows the current waveforms through the light sources to be shaped. Changing the current waveforms may be done in such a way that the average current over time through the light sources does not change.

[0034] In a further example, the driver circuit comprises:

[0035] a first capacitor coupled in parallel with the series arrangement of the first light source and the first switch;

[0036] a second capacitor coupled in parallel with the series arrangement of the second light source and the second switch;

[0037] a third switch coupled in series with an arrangement of the first capacitor coupled in parallel with the series arrangement of the first light source and the first switch;

[0038] a fourth switch coupled in series with an arrangement of the second capacitor coupled in parallel with the series arrangement of the second light source and the second switch;

[0039] a first diode coupled between the node and a node coupled to the first capacitor coupled in parallel with the series arrangement of the first light source and the first switch;

[0040] a second diode coupled between the node and a node coupled to the second capacitor coupled in parallel with the series arrangement of the second light source and the second switch;2024PF80444

[0041] 5

[0042] wherein the controller is arranged to control the third switch and the fourth switch.

[0043] The driver circuit has a first capacitor coupled in parallel with the series arrangement of the first light source and the first switch. The driver circuit further has a second capacitor coupled in parallel with the series arrangement of the second light source and the second switch.

[0044] The third switch is coupled in series with an arrangement of the first capacitor coupled in parallel with the series arrangement of the first light source and the first switch. The fourth switch is coupled in series with an arrangement of the second capacitor coupled in parallel with the series arrangement of the second light source and the second switch. The first diode is coupled between the node and a node coupled to the first capacitor coupled in parallel with the series arrangement of the first light source and the first switch. The second diode is coupled between the node and a node coupled to the second capacitor coupled in parallel with the series arrangement of the second light source and the second switch. The diodes prevent the capacitors to discharge into the other channels. The controller is arranged to further control the third and fourth switch. The control of these switches allows the current waveforms through the light sources to be shaped. Changing the current waveforms may be done in such a way that the average current over time through the light sources does not change.

[0045] In a further example, the controller is arranged to control the first switch and the second switch using pulse width modulation, PWM.

[0046] Controlling the first and second switch using PWM technique allows an accurate control of the light generated by the light sources.

[0047] In a further example, the controller is arranged to control the third switch and the fourth switch using pulse width modulation, PWM.

[0048] The control of the third and fourth switch allow the charging and discharging of the capacitors to be regulated. Using a PWM technique, this can be done efficiently.

[0049] In a further example, the controller is arranged to control a ratio of current between the first light source and the second light source by controlling duty cycles of the first switch and the second switch.

[0050] Controlling the duty cycles of the first and second switches allow the ratio of currents between the lights sources to be controlled. Preferably, the sum of duty cycles of the first and second switches, and therefore the currents through the first and second light sources, is equal to or greater than one.2024PF80444

[0051] 6

[0052] In a further example, the controller is arranged to control an amplitude of current through the first light source and the second light source by controlling duty cycles of the third switch and the fourth switch.

[0053] The control of the duty cycles of the third and fourth switch, combined with the control of the first and second switch allow the amplitude and duration of the current through the light sources to be regulated.

[0054] In a further example, the controller is arranged to regulate a total light intensity generated by the first light source and the second light source by providing the dimming signal to the dimmable constant current source.

[0055] Providing the dimming signal to the dimmable constant current source allows the overall light intensity of the light emitted by the light sources to be adjusted. The current amplitude provided by the dimmable constant current source then determines the overall light intensity of the light emitted by the light sources.

[0056] In a further example, the controller is arranged to close the first switch when the third switch is open and vice versa and wherein the controller is arranged to close the second switch when the fourth switch is open and vice versa.

[0057] Preferably, the first switch and the third switch are operated in a complementary manner when regulating the current through the light source. When the first switch is open, the third switch is closed and vice versa. When no current is supposed to flow to either the first light source or first capacitor, both the first switch and third switch may be opened. The current that is provided by the dimmable constant current source then either flows through the light source or through the first capacitor. Alternatively, the current provided by the dimmable constant current source flows through the first capacitor when the third switch is closed. When the first switch is closed, the first capacitor discharges in the first light source.

[0058] In a further example, forward voltage of the first light source differs from a forward voltage of the second light source.

[0059] When the first light source has a forward voltage, or operating voltage, that differs from the forward voltage of the second light source, the voltage difference may cause a voltage fluctuation at the output of the dimmable constant current source. This is however prevented by the introduction of the inductor.

[0060] In a further example, the first switch and the second switch operate at an identical frequency.2024PF80444

[0061] 7

[0062] It is desired to have the first switch and the second switch operate at an identical frequency. Preferably all switches operate at an identical frequency. This allows the controller to provide a simple control for the switches. Furthermore, this also allows for simpler electromagnetic interference, EMI, filtering.

[0063] In a further example, the first light source and the second light source comprise a semiconductor light source.

[0064] Preferably, the light sources have semiconductor light sources such as LEDs, laser diodes or vertical-cavity surface-emitting lasers, VCSELs.

[0065] In another example, a lighting system is provided. The lighting system comprises the driver circuit according to any of the examples and the dimmable constant current source.

[0066] A lighting system that has the driver circuit and the dimmable constant current source can provide an efficient way of dimming the light output while allow a good current distribution between the light sources.

[0067] In a further example, a method of controlling a driver circuit is provided. The driver circuit comprises a first light string comprising a series arrangement of a first light source and a first switch, a second light strip comprising a series arrangement of a second light source and a second switch, and an inductor arranged to be couplable between an output of a dimmable constant current source and a node coupled to the first light string and the second light string. The method comprises:

[0068] receiving a dimming signal;

[0069] determining a first on time of the first switch based on the dimming signal; determining a second on time of the second switch based on the dimming signal;

[0070] controlling the first switch based on the determined first on time and the second switch based on the second on time;

[0071] determining a dimming signal for the dimmable constant current source based on the control signal,

[0072] providing the dimming signal for the dimmable constant current source to an output terminal of the driver circuit, wherein the output of the driver circuit is adapted to be connectable to a dimming input terminal of the dimmable constant current source.2024PF80444

[0073] 8

[0074] BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Examples of the invention will now be described with reference to the accompanying drawings, in which:

[0076] Fig. 1 shows an example of a conventional lighting system.

[0077] Fig. 2 shows an example of a driver circuit.

[0078] Fig. 3 shows another example of a driver circuit.

[0079] Fig. 4 shows another example of a driver circuit.

[0080] Fig. 5 shows another example of a driver circuit.

[0081] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0082] The invention will be described with reference to the Figures.

[0083] 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 also 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.

[0084] Figure 1 shows an example of a lighting system as is commonly implemented. The lighting system has a driver circuit, which has a first driver stage 1 that can be a constant current source. An inductor LI is coupled to the output of the first driver stage 1 on one side of the inductor LI . A switch M5 is used to shunt the inductor to the return of the first driver stage 1 when the switch M5 is closed. A first LED light source LED1 is coupled in series with a first switch ML A second LED light source LED2 is coupled in series with a second switch M2. Both series arrangements are in parallel with each other and are coupled to the other side of the inductor LI. A controller 2 is used to control the three switches. The controller 2 receives a control signal Dim in the form of a dimming signal. The controller 2 translates this into two control signals for the first switch Ml and the second switch M2 so to set the desired light output color or color temperature. A third control signal is generated for the switch M5, which provides control of the overall intensity of light provided by the light sources. The switch M5 is controlled with a PWM control. The higher the duty cycle of the switch M5, the more current from the first driver stage 1 will be directly provided back to the2024PF80444

[0085] 9

[0086] return of the first driver stage 1 without powering any of the light sources. This allows the overall current to the light sources to be reduced, effectively resulting in dimming of the light output. This way of dimming however impacts the behavior of the lighting system in a negative way. The use of the switch M5 for bypassing the current away from the light sources results in additional energy losses. Furthermore, the deeper the dimming is performed, the more the average voltage at the output of the first driver stage 1 will be reduced. This may result in a voltage to be too low for the first driver stage 1 to be maintained properly. This may result in an unstable control of the first driver stage 1 and / or additional energy losses.

[0087] Figure 2 shows an example of an improved lighting system. The lighting system has a driver circuit and a dimmable constant current source 1. The dimmable constant current source 1 receives an AC input voltage Vmains. In the examples, this may be a mains voltage. The dimmable constant current source 1 is arranged to generate a regulated steady current at its output. The amplitude of the current can be regulated by providing a dim signal at a dim input of the dimmable constant current source 1. An inductor LI is coupled on one end to the output of the dimmable constant current source 1 and on the other end to a node. A first series arrangement, or first light string, is provided where a first light source LED1 is coupled in series with a first switch Ml. The first series arrangement is coupled to the node. A second series arrangement, or second light string, is provided where a second light source LED2 is coupled in series with a second switch M2. The second series arrangement is also coupled to the node. A controller 2 is used to control the first switch Ml and the second switch M2 based on a determined first on time for the first switch Ml and a determined second on time for the second switch M2. The controller 2 receives a control signal Dim. The control signal Dim may be a signal that uses a commonly known protocol such as 1 - 10 V, phase-cut dimmer, DALI, Zigbee, WiFi, Bluetooth, Thread. The control signal Dim may be used by the controller 2 for multiple functions. The control signal Dim may be used to dim the light output of the lighting system. In addition, the control signal Dim may be used to determine the current ratio between the first light source LED1 and the second light source LED2. The controller 2 provides a signal to the first switch Ml to activate the first switch Ml with a determined on-time. The controller 2 provides another signal to the second switch M2 to activate the second switch M2 with a determined on-time. The controller 2 further provides a dimming signal that is provided to the dimmable constant current source 1. The driver circuit may have an output terminal to which the dimming signal is provided. The dimmable constant current source 1 has an input for receiving a dimming signal. Such input2024PF80444

[0088] 10

[0089] may be a physical terminal when the dimmable constant current source l is a separate unit with its own housing. The output terminal of the driver circuit may then be coupled to the dimming input terminal of the dimmable constant current source. The dimming signal may be a signal that uses a commonly known protocol such as 1 - 10 V, phase-cut dimmer, DALI, Zigbee, WiFi, Bluetooth, Thread. The advantage is that a standard dimmable constant current source 1 can be used in combination with the driver circuit. By providing a dimming command and simultaneously controlling the first switch Ml and the second switch M2, the dimming performance of the lighting system is improved. The reduction of the current provided by the dimmable constant current source 1 allows a dimming without additional losses. In this example, it may be desired that the switches are controlled in a complementary manner. This allows at least one switch to be active in a moment in time, ensuring a current path between the output of the dimmable constant current source land the return path. The duty of the first switch Ml combined with the current amplitude provided by the dimmable constant current source 1 determine the total light emitted by the first light source LED1. The duty of the second switch M2 combined with the current amplitude provided by the dimmable constant current source 1 determine the total light emitted by the second light source LED2.

[0090] Figure 3 shows an example of an improved lighting system. A similar basic circuit is shown as in Figure 2 with additional components. In parallel with the first light source LED1, a first capacitor Cl is placed. The first switch Ml therefore regulates the charging of the first capacitor Cl, as well as a current flowing through the first light source LED1. In parallel with the second light source LED2, a second capacitor C2 is placed. The second switch M2 therefore regulates the charging of the second capacitor C2, as well as a current flowing through the second light source LED2. To prevent the first capacitor Cl to discharge into the second capacitor C2 or vice versa, diodes are provided. Between the node and the first light string, a first diode DI is provided. Between the node and the second light string, a second diode D2 is provided. The capacitors are provided to prevent abrupt changes in the voltage at the node to result in undesired current spikes flowing to the light sources. The first capacitor Cl is used for smoothing and buffering the voltage across the first light source LED1. The second capacitor C2 is used for smoothing and buffering the voltage across the second light source LED2. This may be especially desired if the forward voltage of the first light source LED1 differs from the forward voltage of the second light source LED2.

[0091] Figure 4 shows a further improvement of the lighting system. A similar basic circuit is shown as in Figure 2 with additional components. In parallel with the first light2024PF80444

[0092] 11

[0093] source LED1 and the first switch Ml, a series arrangement of a first capacitor Cl and a third switch M3 is placed. The activation of the first switch Ml is used to regulate the current flowing through the first light source LED1. The activation of the third switch M3 is used to regulate the charging of the first capacitor Cl. In parallel with the second light source LED2 and the second switch M2, a series arrangement of a second capacitor C2 and a fourth switch M4 is placed. The activation of the second switch M2 is used to regulate the current flowing through the second light source LED2. The activation of the fourth switch M4 is used to regulate the charging of the second capacitor C2. To prevent the first capacitor Cl to discharge into the second capacitor C2 or vice versa, diodes are provided. Between the node and the first light string, a first diode DI is provided. Between the node and the second light string, a second diode D2 is provided.

[0094] The controller 2 controls the first switch Ml, the second switch M2, the third switch M3 and the fourth switch M4 in relation to a desired ratio of currents between the first light source LED1 and the second light source LED2. An example will be provided to explain how the switches are controlled. The controller 2 receives a control signal Dim that is a representation of a ratio of 1 :3 between the current for the first light source LED1 and second light source LED2. The controller translates this ratio into a duty cycle of 0.25 for the first light source LED1 and a duty cycle of 0.75 for the second light source LED2. The sum of duty cycles is in this example equal to 1. This means that 25 % of the current provided by the dimmable constant current source 1 is to be provided to the first light source LED1 and 75 % of the current provided by the dimmable constant current source 1 is to be provided to the second light source. In this example, the sum of duty cycles of the first switch Ml and the third switch M3 equals to 0.25. The sum of duty cycles of the second switch M2 and the fourth switch M4 equals to 0.75.

[0095] The duty cycle ratio between the first switch Ml and the third switch M3 determines the current amplitude provided to the first light source LED1 compared to the current amplitude provided by the dimmable constant current source 1. The duty cycle of 0.25 can be divided into two parts. A first part, where the first switch Ml is active and a second part where the third switch M3 is active. As an example, the controller 2 may regulate the duty cycle of the first switch Ml to be 0.10 and the duty cycle of the third switch M3 to be 0.15. The total duty cycle then still corresponds to the duty cycle of 0.25. In this example, and for the sake of simplicity, it is assumed that no current flows to the second capacitor C2 and the second light source LED2.2024PF80444

[0096] 12

[0097] When the third switch M3 is closed and the first switch Ml is opened, the current provided by the dimmable constant current source 1 will be provided to the first capacitor Cl and not the first light source LED1. This allows the first capacitor Cl to charge such that the first capacitor Cl can discharge the stored charge into the first light source LED1 when the first switch Ml is conductive. This also prevents the first light load LED1 to receive a current with the amplitude of the current provided by the dimmable constant current source 1. Especially at deep dimming, the current amplitude provided by the dimmable constant current source 1 is low. A current with a relatively low current amplitude that is provided to the first light source LED1 may cause the color point of the first light source LED1 to shift from its expected color point. It is therefore desired to keep the current amplitude high and reduce the duty cycle at which the LEDs receive this current so as to keep the current over time constant while also preventing a color shift. This can be achieved by pre-charging the first capacitor Cl before closing the first switch Ml. In this example, the pre-charging of the first capacitor Cl occurs only during the on-time of the third switch M3, which is 15 % of the total time.

[0098] When the first switch Ml is closed and the third switch M3 is opened, current provided by the dimmable constant current source 1 and current provided by the first capacitor Cl will be provided to the first light source LED1. This will only be during the on-time of the first switch Ml, which is 10 % of the total time. The current through the first light source LED1 is therefore larger than the current provided by the dimmable constant current source 1. To improve the efficiency, it may be desired to close the third switch M3 when the first switch Ml is closed within this 10 % of the total time. This may for example be done to allow the first capacitor Cl to discharge more efficiently into the first light source LED1, and after the discharge has been completed the third switch M3 will be opened again to prevent the first capacitor Cl to be charged again. The current through the first light source LED1 only flows 10 % of the time instead of 25 % of the time. By discharging the first capacitor Cl into the first light source LED1, the current amplitude is increased. This may result in that the first light source LED1 will emit the same amount of light during 10 % of the time as it would have done without the pre-charged first capacitor Cl, and therefore a lower current, during 25 % of the time.

[0099] A similar reason can be applied for the second capacitor C2 and the second switch M2.

[0100] The duty cycle ratio between the second switch M2 and the fourth switch M4 determines the current amplitude provided to the second light source LED2 compared to the2024PF80444

[0101] 13

[0102] current amplitude provided by the dimmable constant current source 1. The duty cycle of 0.75 can be divided into two parts. A first part, where the second switch M2 is active and a second part where the fourth switch M4 is active. As an example, the controller 2 may regulate the duty cycle of the second switch M2 to be 0.25 and the duty cycle of the fourth switch M4 to be 0.5. The total duty cycle then still corresponds to the duty cycle of 0.75. In this example, and for the sake of simplicity, it is assumed that no current flows to the first capacitor Cl and the first light source LED1.

[0103] When the fourth switch M4 is closed and the second switch M2 is opened, the current provided by the dimmable constant current source 1 will be provided to the second capacitor C2 and not the second light source LED2. This allows the second capacitor C2 to charge such that the second capacitor C2 can discharge the stored charge into the second light source LED2 when the second switch M2 is conductive. This also prevents the second light load LED2 to receive a current with the amplitude of the current provided by the dimmable constant current source 1. In this example, the pre-charging of the second capacitor C2 occurs only during the on-time of the fourth switch M4, which is 50 % of the total time.

[0104] When the second switch M2 is closed and the fourth switch M4 is opened, current provided by the dimmable constant current source 1 and current provided by the second capacitor C2 will be provided to the second light source LED2. This will only be during the on-time of the second switch M2, which is 25 % of the total time. The current through the second light source LED2 is therefore larger than the current provided by the dimmable constant current source 1. To improve the efficiency, it may be desired to close the fourth switch M4 when the second switch M2 is closed within this 25 % of the total time. This may for example be done to allow the second capacitor C2 to discharge more efficiently into the second light source LED2, and after the discharge has been completed the fourth switch M4 will be opened again to prevent the second capacitor C2 to be charged again. The current through the second light source LED2 only flows 25 % of the time instead of 75 % of the time. By discharging the second capacitor C2 into the second light source LED2, the current amplitude is increased. This may result in that the second light source LED2 will emit the same amount of light during 25 % of the time as it would have done without the precharged second capacitor C2, and therefore a lower current, during 75 % of the time.

[0105] Figure 5 shows another improvement of the lighting system. A similar basic circuit is shown as in Figure 2 with additional components. A first capacitor Cl is placed in parallel with the first light source LED1 and the first switch Ml. A third switch M3 is provided in series with the arrangement of the first capacitor Cl placed in parallel with the2024PF80444

[0106] 14

[0107] first light source LED1 and the first switch Ml The activation of the first switch Ml is used to regulate the current flowing through the first light source LED1. The activation of the third switch M3 is used to regulate the current flowing through the first capacitor Cl. A second capacitor C2 is placed in parallel with the second light source LED2 and the second switch M2. A fourth switch M4 is provided in series with the arrangement of the second capacitor C2 placed in parallel with the second light source LED2 and the second switch M2 The activation of the second switch M2 is used to regulate the current flowing through the second light source LED2. The activation of the fourth switch M4 is used to regulate the current flowing through the second capacitor C2. To prevent the first capacitor Cl to discharge into the second capacitor C2 or vice versa, diodes are provided. Between the node and the first light string, a first diode DI is provided. Between the node and the second light string, a second diode D2 is provided.

[0108] The controller 2 controls the first switch Ml, the second switch M2, the third switch M3 and the fourth switch M4 in relation to a desired ratio of currents between the first light source LED1 and the second light source LED2. An example will be provided to explain how the switches are controlled. The controller 2 receives a control signal Dim that is a representation of a ratio of 1 :3 between the current for the first light source LED1 and second light source LED2. The controller translates this ratio into a duty cycle of 0.25 for the first light source LED1 and a duty cycle of 0.75 for the second light source LED2. The sum of duty cycles is in this example equal to 1. This means that 25 % of the current provided by the dimmable constant current source 1 is to be provided to the first light source LED1 and 75 % of the current provided by the dimmable constant current source 1 is to be provided to the second light source. In this example, the sum of duty cycles of the first switch Ml and the third switch M3 equals to 0.25. The sum of duty cycles of the second switch M2 and the fourth switch M4 equals to 0.75. The duty cycle ratio between the first switch Ml and the third switch M3 determines the current amplitude provided to the first light source LED1 compared to the current amplitude provided by the dimmable constant current source 1. The duty cycle of 0.25 can be divided into two parts. A first part, where the first switch Ml is active and a second part where the third switch M3 is active. As an example, the controller 2 may regulate the duty cycle of the first switch Ml to be 0.10 and the duty cycle of the third switch M3 to be 0.15. The total duty cycle then still corresponds to the duty cycle of 0.25. In this example, and for the sake of simplicity, it is assumed that no current flows to the second capacitor C2 and the second light source LED2.2024PF80444

[0109] 15

[0110] When the third switch M3 is closed and the first switch Ml is opened, the current provided by the dimmable constant current source 1 will be provided to the first capacitor Cl and not the first light source LED1. This allows the first capacitor Cl to charge such that the first capacitor Cl can discharge the stored charge into the first light source LED1 when the first switch Ml is conductive. This also prevents the first light load LED1 to receive a current with the amplitude of the current provided by the dimmable constant current source 1. Especially at deep dimming, the current amplitude provided by the dimmable constant current source 1 is low. A current with a relatively low current amplitude that is provided to the first light source LED1 may cause the color point of the first light source LED1 to shift from its expected color point. It is therefore desired to keep the current amplitude high and reduce the duty cycle at which the LEDs receive this current so as to keep the current over time constant while also preventing a color shift. This can be achieved by pre-charging the first capacitor Cl before closing the first switch Ml. In this example, the pre-charging of the first capacitor Cl occurs only during the on-time of the third switch M3, which is 15 % of the total time.

[0111] When the first switch Ml is closed and the third switch M3 is opened, current provided by the first capacitor Cl will be provided to the first light source LED1. This will only be during the on-time of the first switch Ml, which is 10 % of the total time. The current through the first light source LED1 is larger than the current provided by the dimmable constant current source 1. The current through the first light source LED1 only flows 10 % of the time instead of 25 % of the time. By discharging the first capacitor Cl into the first light source LED1, the current amplitude is increased. This may result in that the first light source LED1 will emit the same amount of light during 10 % of the time as it would have done without the pre-charged first capacitor Cl, and therefore a lower current, during 25 % of the time.

[0112] A similar reason can be applied for the second capacitor C2 and the second switch M2.

[0113] The duty cycle ratio between the second switch M2 and the fourth switch M4 determines the current amplitude provided to the second light source LED2 compared to the current amplitude provided by the dimmable constant current source 1. The duty cycle of 0.75 can be divided into two parts. A first part, where the second switch M2 is active and a second part where the fourth switch M4 is active. As an example, the controller 2 may regulate the duty cycle of the second switch M2 to be 0.25 and the duty cycle of the fourth switch M4 to be 0.5. The total duty cycle then still corresponds to the duty cycle of 0.75. In2024PF80444

[0114] 16

[0115] this example, and for the sake of simplicity, it is assumed that no current flows to the first capacitor Cl and the first light source LED1.

[0116] When the fourth switch M4 is closed and the second switch M2 is opened, the current provided by the dimmable constant current source 1 will be provided to the second capacitor C2 and not the second light source LED2. This allows the second capacitor C2 to charge such that the second capacitor C2 can discharge the stored charge into the second light source LED2 when the second switch M2 is conductive. This also prevents the second light load LED2 to receive a current with the amplitude of the current provided by the dimmable constant current source 1. In this example, the pre-charging of the second capacitor C2 occurs only during the on-time of the fourth switch M4, which is 50 % of the total time.

[0117] When the second switch M2 is closed and the fourth switch M4 is opened, current provided by the second capacitor C2 will be provided to the second light source LED2. This will only be during the on-time of the second switch M2, which is 25 % of the total time. The current through the second light source LED2 is larger than the current provided by the dimmable constant current source The current through the second light source LED2 only flows 25 % of the time instead of 75 % of the time. By discharging the second capacitor C2 into the second light source LED2, the current amplitude is increased. This may result in that the second light source LED2 will emit the same amount of light during 25 % of the time as it would have done without the pre-charged second capacitor C2, and therefore a lower current, during 75 % of the time.

[0118] In the examples provided, the dimmable constant current source 1 is represented as a standalone component that has its own dimming input. The dimmable constant current source 1 may any type of power converter that is arranged to output a regulated current. Examples of such converters are a constant current source and a current regulated voltage source.

[0119] In the examples provided the sum of the duty cycles of the switches is equal to or greater than 1. This allows the light sources to be utilized in the most efficient manner.

[0120] In the examples provided, the light sources can be any kind of light source that benefits from a regulated current. Preferably, the light sources have semiconductor light sources such as LEDs, laser diodes or vertical-cavity surface-emitting lasers, VCSELs.

[0121] In the examples provided, the switches are represented by MOSFETs but can be any type of switch that can receive an electrical signal for controlling the opening and closing of the switch. The switch may be any solid-state switch such as an IGBT, MOSFET, bipolar transistor.2024PF80444

[0122] 17

[0123] Other 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. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

2024PF8044418CLAIMS:

1. A driver circuit comprising:a first light string comprising a series arrangement of a first light source and a first switch;a second light string comprising a series arrangement of a second light source and a second switch;an inductor arranged to be couplable between an output of a dimmable constant current source and a node coupled to the first light string and the second light string;a controller arranged to:receive a control signal;determine a first on time of the first switch based on the control signal; determine a second on time of the second switch based on the control signal; control the first switch based on the determined first on time and the second switch based on the second on time;determine a dimming signal for the dimmable constant current source based on the control signal,provide the dimming signal for the dimmable constant current source to an output terminal of the driver circuit, wherein the output terminal of the driver circuit is adapted to be connectable to a dimming input terminal of the dimmable constant current source.

2. The driver circuit according to claim 1 further comprising a first capacitor coupled in parallel with the first light source and a second capacitor coupled in parallel with the second light source.

3. The driver circuit according to claim 1, further comprising:a series arrangement of a first capacitor and a third switch, coupled in parallel with the series arrangement of the first light source and the first switch;a series arrangement of a second capacitor and a fourth switch, coupled in parallel with the series arrangement of the second light source and the second switch;2024PF8044419a first diode coupled between the node and a node coupled to the series arrangement of the first capacitor and the third switch, coupled in parallel with the series arrangement of the first light source and the first switch, anda second diode coupled between the node and a node coupled to the series arrangement of the second capacitor and the fourth switch, coupled in parallel with the series arrangement of the second light source and the second switch,wherein the controller is arranged to control the third switch and the fourth switch.

4. The driver circuit according to claim 1, further comprising:a first capacitor coupled in parallel with the series arrangement of the first light source and the first switch;a second capacitor coupled in parallel with the series arrangement of the second light source and the second switch;a third switch coupled in series with an arrangement of the first capacitor coupled in parallel with the series arrangement of the first light source and the first switch;a fourth switch coupled in series with an arrangement of the second capacitor coupled in parallel with the series arrangement of the second light source and the second switch;a first diode coupled between the node and a node coupled to the first capacitor coupled in parallel with the series arrangement of the first light source and the first switch;a second diode coupled between the node and a node coupled to the second capacitor coupled in parallel with the series arrangement of the second light source and the second switch;wherein the controller is arranged to control the third switch and the fourth switch.

5. The driver circuit according to any of the preceding claims, wherein the controller is arranged to control the first switch and the second switch using pulse width modulation, PWM.

6. The driver circuit according to claim 3 or 4, wherein the controller is arranged to control the third switch and the fourth switch using pulse width modulation, PWM.2024PF80444207. The driver circuit according to any of the preceding claims, wherein the controller is arranged to control a ratio of current between the first light source and the second light source by controlling duty cycles of the first switch and the second switch.

8. The driver circuit according to any of the claims 3 or 4, wherein the controller is arranged to control an amplitude of current through the first light source and the second light source by controlling duty cycles of the third switch and the fourth switch.

9. The driver circuit according to any of the preceding claims, wherein the controller is arranged to regulate a total light intensity generated by the first light source and the second light source by providing the dimming signal to the dimmable constant current source.

10. The driver circuit according to any of the claims 3 or 4, wherein the controller is arranged to close the first switch when the third switch is open and vice versa and wherein the controller is arranged to close the second switch when the fourth switch is open and vice versa.

11. The driver circuit according to any of the preceding claims, wherein a forward voltage of the first light source differs from a forward voltage of the second light source.

12. The driver circuit according to any of the preceding claims, wherein the first switch and the second switch operate at an identical frequency.

13. The driver circuit according to any of the preceding claims, wherein the first light source and the second light source comprise a semiconductor light source.

14. A lighting system comprising the driver circuit according to any of the preceding claims and the dimmable constant current source.

15. A method of controlling a driver circuit comprising a first light string comprising a series arrangement of a first light source and a first switch, a second light strip comprising a series arrangement of a second light source and a second switch, and an2024PF8044421inductor arranged to be couplable between an output of a dimmable constant current source and a node coupled to the first light string and the second light string, the method comprising:receiving a dimming signal;determining a first on time of the first switch based on the dimming signal; - determining a second on time of the second switch based on the dimming signal;controlling the first switch based on the determined first on time and the second switch based on the second on time;- determining a dimming signal for the dimmable constant current source based on the control signal,- providing the dimming signal for the dimmable constant current source to an output of the driver circuit, wherein the output terminal of the driver circuit is adapted to be connectable to a dimming input terminal of the dimmable constant current source.

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