Lighting circuit

Abstract

The lighting circuit comprises a series connection of multiple lighting channels of different output colors driven by a current source, some or all of the lighting channels comprising an LED device, a shunt switch in parallel with the LED device, and a current leakage path in parallel with the LED device, the current leakage path being used to calibrate the current through the LED device, thereby taking into account the characteristics of the LED.

Description

[Technical field]

[0001] The present invention relates to a lighting circuit, in particular to a lighting circuit with multiple lighting channels. [Background technology]

[0002] Multi-channel lighting circuits are well known. By providing light sources with different colors (i.e. color points) in different channels, a desired overall color point can be achieved for the overall light output of the lighting circuit.

[0003] For example, it is well known to use shunt switching in multi-channel light sources using a pulse-width modulated (PWM) switching signal. A drive current is applied to the lighting load. Different channels are operated at different duty cycles to produce a desired mix of light output from the multiple channels. This approach has the advantage that the spread in drive current does not affect the color consistency of the overall light output, since all channels experience the same variations. As a result, the mixed color remains essentially the same, only the luminous flux varies.

[0004] However, the luminous flux (and color) variations of the LEDs of the different channels need to be compensated for in the controller software. For this purpose, information from LED bins (which define the characteristics of the specific LEDs used in a channel) needs to be sent to the controller or a calibration measurement needs to be performed. These bin characteristics are, among others, the luminous flux, color point and forward voltage of the different channels.

[0005] To implement this communication to the controller, the LED characteristics need to be recorded and managed. The need to transfer data between the LED devices (on the so-called L2 board) and the controller means that the controller and the L2 board need to be paired. This means that they cannot be exchanged independently. The need for data transfer from the L2 board to the controller also means that significant additional data storage, handling and processing is required. The alternative of calibration measurements is costly.

[0006] In this shunt driving scheme, color consistency is adjusted in software by adapting the PWM signal. This means that it is not possible to connect several L2 boards in parallel using the same shunt switch, since the color correction is at the level of each individual channel and switch of the L2 board.

[0007] Another known way of controlling a multi-channel light source is to connect all channels (even for multiple L2 boards) in parallel to a voltage bus. A single switch can be used to activate all channels of the same color. The current flowing to each channel in response to a common voltage can be adjusted by adding a series resistor to the LED string of each channel. Selecting the appropriate adjustment resistor means that the light output flux of a channel is constant, regardless of the flux bin of the LEDs in the channel. The effect of the channel flux on the color point of the resulting overall light output is larger than the variation of the color points of the individual channels, and therefore an acceptable overall color point variation is achieved.

[0008] This approach decouples the L2 board from the controller, but the circuit is less efficient than the current-driven approach and is more susceptible to bus voltage fluctuations. To prevent excessive losses, all channels must also have approximately the same string voltage, which imposes an additional design constraint. Summary of the Invention [Problem to be solved by the invention]

[0009] What is needed is an improved multi-channel lighting circuit that is energy efficient and eliminates the need for extensive calibration processes and associated computing power. [Means for solving the problem]

[0010] The invention is defined by the claims.

[0011] According to an example according to an aspect of the present invention, a light source device including a series or parallel connection of multiple lighting channels of different output colors; a current source for supplying a driving current to the light source device, Each lighting channel is LED device, and A lighting circuit is provided having current leakage paths in parallel with the LED devices, each current leakage path (Radd FW, Radd Lime, Radd CW) configured to bypass current from a corresponding LED device (12, 14, 16) such that the light output of each LED device (12, 14, 16) is approximately equal when each LED device (12, 14, 16) is driven by a substantially similar current.

[0012] When multiple LED devices are arranged in a series or parallel configuration, the light output may differ between each LED device when driven by the same current due to LED tolerances. These tolerances may be caused by, for example, the manufacturing process of the LEDs. These tolerances may be within a range of about ±10%. By placing a current leakage path in parallel with the LED devices, the light output (when the LED devices are supplied with the same current) can be approximated between the LED devices. Thus, instead of having a light output deviation of ±10%, the light output deviation can be significantly reduced. The current leakage path needs to be configured such that the current leakage path can bypass current from the corresponding LED device such that the light output of each LED device is approximately equal when each LED device is driven by a substantially similar current.

[0013] In a further example, the lighting system comprises a controller, the light source device comprises the series connection, and each lighting channel comprises a shunt switch in parallel with the LED device, the controller being configured to control the shunt switches.

[0014] In this example, the lighting circuit uses shunt switches to implement luminous flux control, which has the advantage that the same color point is maintained in a simple way at different brightness levels. However, in order to eliminate the need to provide bin data (i.e. the performance of the LEDs used in the channel) to the controller and to eliminate the need for a calibration phase, each current leakage path is used to adapt the response of at least some of the LED devices to the drive current supplied. These current leakage paths thus provide a hardware implementation of a compensation scheme to compensate for different LED characteristics, such as different LEDs being from different bins. The design of the current leakage paths is adapted to the characteristics of the associated LED device, without the need to relay any information to the controller or to perform calibration measurements. In this way, the advantages of known voltage driving systems (which do not require calibration) are combined with the advantages of shunt switching (which is more efficient and less dependent on the drive current or voltage).

[0015] Each and every LED device of the lighting channel may have a current leakage path. However, there may be some LED devices that do not need the leakage path (i.e. the current leakage path is effectively an infinite resistance, i.e. an open circuit), because some LED devices may represent the default (minimum luminous flux bin) for which compensation for all other LED devices is made.

[0016] Note that "different colors" is only intended to mean different color points: for example, warm white, cool white, and flame white can be considered to be different colors.

[0017] Each current leakage path comprises, for example, a resistor circuit. This provides a simple way of implementing current leakage paths. The resistor is chosen such that the remaining current reaching the LED device results in a predetermined (i.e. calibrated) light output flux.

[0018] The current leakage path may instead include a current regulation circuit, which is an alternative way to divert current away from the LED device so that the remaining current produces the desired light output. The current regulation circuit makes the bypass current more independent and more constant, acting as a constant current bypass circuit.

[0019] The controller may be adapted to apply different shunt switch PWM duty cycles for different luminous flux settings of the light source device, which is more efficient than a constant voltage driving scheme.

[0020] The controller may further be adapted to apply different drive currents for different luminous flux settings of the light source device, so that there may be not only PWM control but also amplitude control to set the luminous flux, e.g. for deep dimming.

[0021] For each LED device the current leakage paths are adapted, eg such that for a default drive current the luminous flux of the light output is calibrated to a desired value.

[0022] Thus, the light output flux is at a known level (for a given drive current) without the need for feedback to the controller.

[0023] The default drive current is based on, for example, an expected color point and luminous flux per channel, whereby to achieve a desired color point of the light source device with actual luminous flux matching the expected luminous flux. The default drive current is the drive current for the minimum required luminous flux for each of the channels for the lowest luminous flux bin of the light source device, i.e., the drive current at which the lowest luminous flux bin of each of the LED channels can still meet minimum illumination requirements.

[0024] For LED devices from the lowest flux bin, the bypass can be completely blocked with a very high resistance so that the entire current of the driver flows through these LEDs. Thus, for LED devices with the lowest flux bin, no current leakage path is needed since the entire current will pass through the LED device.

[0025] Thus, the current leakage paths ensure that for the lowest luminous flux bin and minimum light requirement (i.e., lowest luminance level), the various channels provide the required light output luminous flux to result in the desired overall color point and luminous flux. This ensures that the desired color point and luminous flux is produced down to the lowest light output (luminance) level.

[0026] The light source device may include a plurality of lighting boards, each having a respective series connection of a plurality of lighting channels of different output colors, each shunt switch being shared between the LED devices of a corresponding channel of the plurality of lighting boards.

[0027] Therefore, lighting boards (so-called L2 boards) may be added to the lighting circuit. The shunt switches are shared between all of the lighting boards, since the same duty cycle can be applied to the different boards, and compensation for differences in the L2 boards is performed in the board itself by the current leakage paths.

[0028] Each lighting board, for example, has a respective current leakage path in parallel with each LED device.

[0029] While each lighting board has its own current leakage path, the shunt switching is shared between boards.

[0030] The invention also provides a luminaire comprising a lighting circuit as defined above, said luminaire being, for example, a spotlight.

[0031] These and other aspects of the invention will be elucidated and elucidated with reference to the following embodiments.

[0032] In another example, the lighting system comprises: a first lighting circuit and a second lighting circuit, each lighting circuit comprising: a light source device including a series connection of a plurality of illumination channels of different output colors; a current source for supplying a drive current to the light source device; A controller, Each lighting channel is LED device, and A shunt switch in parallel with the LED device; Each lighting channel further comprises a current leakage path in parallel with the LED device; the controller is configured to control the shunt switch; Each current leakage path is configured to bypass current from a corresponding LED device such that when each lighting circuit is driven by a substantially similar current, the light output of the first lighting circuit and the light output of the second lighting circuit are approximately equal.

[0033] In this example, a first lighting circuit and a second lighting circuit may be provided. They may be the same lighting circuits, e.g. similar lamps, that also need to provide similar light output at a single given setpoint. By providing a shunt switch, the light output of each lighting circuit can be controlled to the desired light. A current leakage path is provided across each LED device to eliminate the need for calibration of the shunt switch on the corresponding LED device. The current leakage path is configured such that the light output of the first lighting circuit and the light output of the second lighting circuit are approximately equal when each lighting circuit is driven by a substantially similar current. This allows the light output of each lighting circuit to be close to each other without the need for a calibration step. Due to the omission of calibration, the controller can be simplified. [Brief description of the drawings]

[0034] For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which: [Figure 1] 1 illustrates a known current driven lighting circuit using a shunt switch. [Diagram 2] 1 shows a voltage driver comprising a voltage source supplying a bus voltage. [Diagram 3] 1 shows a lighting circuit according to an example of the present invention. [Figure 4] Currents and voltages are shown to illustrate the operation of the current leakage paths. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0036] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems and methods, are for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects and advantages of the devices, systems and methods of the present invention will become better understood from the following description, appended claims and accompanying drawings. It should 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 denote the same or similar parts.

[0037] The present invention provides a lighting circuit having a series connection of multiple lighting channels of different output colors driven by a current source. Each lighting channel has an LED device and a shunt switch in parallel with the LED device. Some or all of the lighting channels have a current leakage path in parallel with the LED device. The current leakage path is used to calibrate the current through the LED device, thereby taking into account the characteristics of the LED.

[0038] A known current driven lighting circuit using shunt switches is shown in Figure 1. The circuit comprises a current source 10 supplying current to a series connection of multiple (in this example three) lighting channels of different output colours.

[0039] Each lighting channel has a respective LED device 12, 14, 16 and a shunt switch 22, 24, 26 in parallel with the LED device.

[0040] The different colors in this example are cool white (CW, 6500K), flame white (FW, 2200K), and lime (with dominant wavelengths in the range of 550nm to 590nm). There is a forward voltage drop Vf through each LED device, and therefore V f,FW , V f,CW and V f,Lime There is.

[0041] There can be any number of individually addressable channels and any set of colors.

[0042] A PWM switching scheme is used to control the shunt switches. Different channels are operated at different duty cycles to provide a desired mix of light output from the multiple channels to create a desired color and luminous flux output. Flux (and color) variations of the LEDs in different channels are compensated for in software in a system controller (not shown). For this purpose, luminous flux and color point information from bins of LEDs are sent to the system controller, for example. The need for data transfer from the lighting board to the controller necessitates data storage, handling and processing requirements.

[0043] 2 shows a voltage driver comprising a voltage source 30 providing a bus voltage. The bus voltage can be provided to a number of lighting boards L2_1 and L2_2.

[0044] Each lighting board in this example has parallel lighting channels, each having an LED device 120, 140, 160 in series with a calibration resistor Radd FW, Radd Lime, Radd CW.

[0045] Each channel is also in series with a series switch 220, 240, 260. Thus, each series switch allows or prevents current from flowing through the channel. The series switches are shared between the channels of multiple lighting boards. This is possible because compensation for the characteristics of different LED devices is provided by the additional resistors. Thus, different currents can flow through different channels even with the same drive voltage and shared series switch.

[0046] This approach decouples the L2 board from the controller, but the circuit is less efficient than the current-driven approach and is more susceptible to bus voltage fluctuations. To prevent excessive losses, all channels must also have approximately the same string voltage, which imposes an additional design constraint.

[0047] Figure 3 shows a lighting circuit according to an example of the invention, which can be seen as a modification of the current driver circuit of Figure 1. The same reference numbers as in the circuit of Figure 1 are used.

[0048] Again, the lighting circuit comprises a series connection of multiple lighting channels of different output colors.

[0049] Each lighting channel has an LED device 12, 14, 16 and a respective shunt switch 22, 24, 26 in parallel with one of the LED devices. Each LED device may be a single LED or an LED string, and for clarity is represented in the example as a single LED symbol.

[0050] Current source 10 provides a drive current to the light source device. This can be any suitable current source, such as a switch mode current regulator (e.g., a hysteretic buck converter) that measures and regulates to keep the LED drive current at a quasi-constant level, regardless of the actual voltage level of the LED device.

[0051] The desired color output is achieved by the desired relationship between the PWM settings of the shunt switches. To control brightness, the controller may adjust only the PWM settings (synchronous with each other). However, in other examples, this PWM control may be combined with amplitude modulation. This may address the issue of PWM resolution at very deep dimming brightness settings (<2% brightness).

[0052] The shunt switches may be controlled in any desired timing arrangement. For example, the shunt switches may all be open at the same time so that drive current flows through all three channels simultaneously. The switches may then be closed at different times within a period of the duty cycle control to provide bypass paths and thereby turn off the channels at different times. For example, at the beginning of a duty cycle period, all the switches may be closed and they may open when the desired duty cycle is reached for each of the channels.

[0053] Alternatively, to reduce the period of no light output, some channels may be made to emit light at the beginning of the duty cycle and other channels may be made to emit light at the end of the duty cycle period.

[0054] The simplest implementation is to fire all channels at the same time, however it is preferable to stagger the on-periods not only for light continuity but also to limit voltage variations resulting from different LED forward voltages connected in the circuit, since it is more difficult to maintain a stable current when the voltage jumps are large.

[0055] 3 also shows a controller 40 for controlling the shunt switches 22, 24, 26. By adjusting the duty cycles proportionally to each other, the color point is maintained but the luminous flux is controlled, i.e., controlled to perform dimming. The controller 40 may also control the drive current level, for example during deep dimming, as explained above.

[0056] According to the present invention, some or all of the channels also have a current leakage path in parallel with the LED device. In the example shown, each channel has a current leakage path, each current leakage path being a bypass resistor Radd FW, Radd Lime, Radd CW. The current leakage paths eliminate the need to provide bin data (i.e., the performance of the LEDs used in the channel) to the controller and eliminate the need for a calibration phase.

[0057] There are common types of LEDs used on the lighting board for use in the lighting circuit. The controller software is set to the expected color point per channel based on the common LED type, as well as the luminous flux corresponding to the default drive current for the LED in the lowest expected luminous flux bin. Thus, the LEDs are assumed to have a certain range of possible characteristics defined by the luminous flux bin of the LED, with the lowest luminous flux bin being used as the default.

[0058] That is, in the case of the lowest flux bin, all the current needs to pass through the LED device to reach the default flux (since the lowest flux bin corresponds to the lowest flux for a given current), which may correspond to an infinite parallel resistance (open circuit).

[0059] The light output from the LED device from the lowest flux bin is Φ min More generally, this is the default drive current, Φ min is the default light output flux to which the other flux bins are calibrated.

[0060] At the same current I, the luminous flux Φ bin (I) brings about V f,bin The parallel resistance required to tune the LED string with the bin value that results in a forward voltage (for the entire string for that channel) under operating conditions of (I) can be determined.

[0061] Using the characteristic relationship between LED luminous flux and current, the required current is:

number

[0062] Therefore, the current I flowing through the LED device from this particular bin bin produces an output luminous flux that corresponds to the luminous flux produced by an LED from the bin with the lowest luminous flux.

[0063] This gives the required bin current. By definition, I bin is less than or equal to I (for the lowest bin, I=I bin ). The simplest relationship is I bin Although a more accurate estimate may be used, using a linear approximation, I bin teeth,

number

[0064] The resistance required is

number

[0065] The default current I, which in this case corresponds to the lowest luminous flux setting, is set by PWM control and / or amplitude modulation of the preset drive current. The required resistor values ​​are determined as part of the manufacture of the lighting circuit and are independent of the controller.

[0066] In FIG. 4 the current and voltage are shown.

[0067] The efficiency of the system is defined by the efficiency of the bin with the lowest luminous flux. The total system efficiency loss due to the bypass path depends on the PWM ratio of the channel and the total bin width.

[0068] For example, the range of luminous flux bins depends on the LED device, e.g., by the supplier. For example, for a typical bin value, an LED device may fall between a lowest luminous flux bin that is 10% below the typical luminous flux value and a highest luminous flux bin that is 10% above the typical luminous flux value.

[0069] This 20% range means that for the highest flux bin, about 20% of the current needs to be bypassed, so the resistors are then rated for 20% of the power dissipation for that color.

[0070] The present invention involves leaking a current from the main path. Using a resistive circuit (e.g., just a resistor) is the easiest and cheapest way to create a leakage current. However, other ways of achieving a leakage current are possible, for example, by adding a linear current regulator to the leakage path.

[0071] FIG. 3 shows a single L2 lighting board. However, the light source device may have multiple lighting boards, each with a respective series connection of multiple lighting channels of different output colors. However, the shunt switches may be shared between the LED devices of corresponding channels of the multiple lighting boards. Thus, multiple lighting boards may be controlled by the same controlled, shared set of shunt switches. The current leakage paths are implemented at the level of the individual lighting boards.

[0072] The number of LED boards connected determines the preset current. This preset current setting is generally not changed dynamically. Thus, the current level is configured as part of the installation procedure and may be set in software by changing a setting value, or in hardware by, for example, setting the drive current with a setting resistor.

[0073] In the above example, the PWM settings are used to change the brightness at the same color point. In more advanced control approaches, the PWM settings can be used to set different color points, or even different color points at different light output lumen levels, such that the color point is dynamically controlled as a function of the dimming level.

[0074] In the example shown, the LED devices are provided in a series configuration. Instead of the series configuration, the LED devices 12, 14, 16 can be arranged in a parallel configuration.

[0075] In the example shown, a shunt switch is provided in parallel with LED devices 12, 14, 16 in each lighting channel, however, it should be understood that a shunt switch is not required for the present invention to provide the desired effect of matching the light output of each LED device when the LED devices are each driven by a substantially similar current.

[0076] The current leakage paths shown in the examples may also be beneficial when multiple lighting circuits are desired to provide similar light outputs when given a single set point. The current leakage paths may provide matching of the light output produced by each lighting circuit when each lighting circuit is driven by a substantially similar current, e.g., by ensuring that the light output of a first lighting circuit and the light output of a second lighting circuit are approximately equal when given a single set point.

[0077] The lighting system is for example part of a lighting fixture, such as a spotlight.

[0078] Those skilled in the art can understand and effect variations to the disclosed embodiments in the practice of the claimed invention, from a study of the drawings, the specification and the appended claims. In the claims, the word "comprises" does not exclude other elements or steps, and the singular form "a" does not exclude a plurality.

[0079] A single processor or other unit may fulfill the functions of several items recited in the claims.

[0080] 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.

[0081] Please note that when the term "adapted to" is used in the claims or specification, the term "adapted to" is intended to be equivalent to the term "configured to."

[0082] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. a light source device including a series or parallel connection of a plurality of illumination channels; a current source for supplying a driving current to the light source device, Each lighting channel includes an LED device and a current leakage path in parallel with the LED device, each current leakage path including a resistor circuit or a linear current regulation circuit; The lighting circuit, wherein each current leakage path is configured to bypass current from a corresponding LED device such that when each lighting channel is driven by a substantially similar current, the light output of each LED device is approximately equal.

2. 2. The lighting system of claim 1, further comprising a controller, the light source device comprising the series connection, each lighting channel comprising a shunt switch in parallel with the LED device, the controller configured to control the shunt switches.

3. 3. The lighting circuit of claim 2, wherein the controller is adapted to apply different shunt switch PWM duty cycles for different luminous flux settings of the light source device.

4. 4. The lighting circuit of claim 3, wherein the controller is adapted to apply different current source drive currents for different luminous flux settings of the light source device.

5. 2. The lighting circuit of claim 1, wherein for each LED device, the current leakage paths are adapted such that the luminous flux of the light output is calibrated to a desired value for a default drive current.

6. 6. The lighting circuit of claim 5, wherein the default drive current is the drive current for a minimum required luminous flux for each of the channels for the lowest luminous flux bin of the light source device.

7. 2. The lighting circuit of claim 1, wherein the light source device includes a plurality of lighting boards, each having a respective series connection of a plurality of lighting channels of a different output color, and each shunt switch is shared between the LED devices of a corresponding channel of the plurality of lighting boards.

8. 8. The lighting circuit of claim 7, wherein each lighting board has a respective current leakage path in parallel with each LED device.

9. A lighting fixture comprising a lighting circuit according to claim 1.

10. 10. The luminaire of claim 9 comprising a spotlight.

11. 1. A lighting system having a first lighting circuit and a second lighting circuit, each lighting circuit comprising: a light source device including a series connection of a plurality of illumination channels of different output colors; a current source for supplying a drive current to the light source device; A controller, Each lighting channel is LED device, and a shunt switch in parallel with the LED device; each lighting channel further comprising a current leakage path in parallel with said LED device; the controller is configured to control the shunt switch; a lighting system, wherein each current leakage path is configured to bypass current from a corresponding LED device such that when each lighting circuit is driven by a substantially similar current, the light output of the first lighting circuit and the light output of the second lighting circuit are approximately equal.

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