Lighting circuit

Abstract

The lighting circuit comprises a first light source device and a second light source device of different types in series. A driver supplies a controllable current with an adjustable DC component and an adjustable modulation component, e.g. a superimposed pulse width modulation component with an adjustable duty cycle, to the light source circuit. This allows control of the contribution to the overall light output flux from the different types of light sources.

Description

[Technical field]

[0001] The present invention relates to a lighting circuit, in particular to a lighting circuit that combines different types of light sources. [Background technology]

[0002] LED lighting is replacing traditional forms of lighting in almost all lighting applications. LED lighting is energy efficient and offers easy control of light output color, directional control, and static and dynamic lighting effects.

[0003] However, laser diodes (LDs) still provide increased light output flux with very small beam divergence for high brightness illumination systems.

[0004] In addition to LEDs and laser diodes, another type of solid-state semiconductor light source is the superluminescent diode (SLED). These combine the features of laser diodes with those of light-emitting diodes. Like laser diodes, SLEDs are high-brightness light sources with small beam divergence, but they exhibit a broader spectral composition and lower temporal coherence, which manifests as a much less noticeable speckle compared to lasers.

[0005] In contrast to laser diodes, superluminescent diodes have no cavity mirrors in the active region, and in the case of SLEDs, spontaneous emission light is amplified by stimulated emission during propagation through the active layer. This light is called superluminescence, and since the spontaneous emission light is a seed light with random phase and broad spectrum, the superluminescence is also low-coherence light with a broadband spectrum. SLEDs, like laser diodes, exhibit threshold-like behavior in the light-current-voltage (LIV) characteristic.

[0006] There are also hybrid lighting systems that combine the advantages of different types of light sources, such as laser diodes and LEDs, allowing for high brightness and color tunable lighting systems.

[0007] When different types of light sources need to be combined, the standard approach is to control their currents separately with separately controllable current drivers. This is a common approach since different types of light sources have different luminous flux vs. drive current characteristics.

[0008] Figure 1 shows a graph of light output luminous flux (y-axis) versus drive current (x-axis) for an LED (plot 10) and a laser diode (plot 12). The luminous flux values ​​(Φ_LED_rel and Φ_LD_rel) are shown as relative values ​​based on the normalized (and therefore equal) maximum luminous flux values ​​of the two light sources.

[0009] Laser diodes exhibit a threshold current i_th_LD, i.e., the current below which the laser diode produces almost no light, which does not exist in the LED characteristic. Laser diode characteristics also have a less pronounced droop, i.e., the luminous flux versus current diagram of an LED becomes flatter at increased drive levels compared to the laser diode characteristic. Summary of the Invention [Problem to be solved by the invention]

[0010] It is desirable to be able to control the contribution to the overall light output flux from different types of light sources, for example to be able to control the output color that results from the combination of light outputs. However, it would be desirable to be able to achieve this with a combined driver for a single string configuration, and therefore with only two connectors at the end of the string. [Means for solving the problem]

[0011] The invention is defined by the claims.

[0012] According to an example according to an aspect of the present invention, a light source circuit including a first light source device and a second light source device of a different type from the first light source device in series; A lighting circuit is provided having a driver for supplying a controllable current to the light source circuit, the driver being configurable to set a DC component of the controllable current and to set a modulation of a superimposed modulation component of the controllable current.

[0013] This lighting circuit uses a series connection (string) of a first and a second light source type and therefore has only two connectors. The drive current used is a modulated drive current with a DC offset. The DC offset and the modulation are adjustable.

[0014] Preferably, the controllable current supplied to the light source circuit provides a light output distribution between the first light source device and the second light source device.

[0015] In one set of examples, the first light source device is an LED device and the second light source device is a laser diode device and / or a superluminescent diode device.

[0016] This lighting circuit uses, for example, a series connection (string) of LEDs and laser diodes and therefore has only two connectors.

[0017] The driver sets the modulation, for example by setting a duty cycle of a superimposed pulse width modulated component of the controllable current. The driver may optionally set a frequency of the pulse width modulation. The driver may optionally set an amplitude of the pulse width modulated component. Thus, in general, the driver may be configurable to set the frequency and / or amplitude of the superimposed modulation component.

[0018] The lighting circuit exploits the differences between the two light source types, and in particular between the different light source technologies: the first difference is the threshold current characteristic, and the second difference is the luminous flux-current characteristic.

[0019] The DC component exploits, for example, a different threshold current of the laser diode device or the superluminescent diode device compared to the LED device in order to change the ratio of light output flux between the LED and the laser diode or the superluminescent diode. The larger the DC component (compared to the modulation amplitude), the more pronounced the light output of the LED becomes, since LEDs have no threshold current.

[0020] The resulting modulation pattern applied by the driver allows, to a certain extent, independent control of the respective emissions from the two different types of light sources.

[0021] The driver can be configured, for example, to set a DC current component below the threshold current of the laser diode device or the superluminescent diode device. Thus, the LED device can be operated below the laser threshold current to effectively control the LED device separately from the laser diode device or the superluminescent diode device. This results in a first operating point of the circuit where only the LED device emits light.

[0022] The modulation component and the DC component may have different relative sizes. For example, the maximum amplitude of the superimposed pulse width modulation component (or unitary amplitude if the amplitude is not adjustable) may be, for example, up to 5 times the maximum amplitude of the DC component. The amplitude of the superimposed modulation component may be set to zero, for example, if only LED illumination is desired.

[0023] Different types and colors of light sources have different threshold currents (e.g., laser diodes may have threshold currents in the range of 0.1 A to 1 A) and different (continuous) maximum currents (e.g., in the range of 1 A to 4 A). High power blue laser diodes are available with continuous wave 5 A currents, but maximum current values ​​are increasing over time as the wattage of newly developed diode chips increases. For short pulse operation, these maximums can be overcome by several factors. For ns pulses, driving 10 A or more can be achieved.

[0024] The lasing current threshold of a laser diode is temperature dependent and is typically 0.1 to 0.3 times the rated DC current.

[0025] LED studies have shown that droop becomes visible at DC currents of about 0.5 times the rated DC current, with an upper amplitude limit of, for example, about 2 times the rated DC current to avoid significantly compromising the LED's lifespan or driving it under inefficient operating conditions.

[0026] Conventional components can operate at currents up to about 4A, whereas components utilizing an internal parallel configuration can operate at higher currents, for example up to 20A.

[0027] The maximum amplitude of the DC component may be, for example, 20 A or less, and the amplitude of the DC component may be controlled, for example, within a range from 0 A to a maximum amplitude. Similarly, the maximum amplitude of the superimposed pulse width modulated component (or a single amplitude if the amplitude is not adjustable) may be, for example, 20 A or less. The driver may be further configurable to set the amplitude of the superimposed pulse width modulated component. In such a case, the amplitude of the superimposed pulse width modulated component may be controlled, for example, within a range from 0 A to a maximum amplitude.

[0028] Amplitude and duty cycle control allows for an increased range of possible combinations of LED and laser diode flux.

[0029] The lighting circuit may further comprise a capacitor circuit in parallel with the LED device.

[0030] The capacitor circuit makes the characteristics of the entire circuit frequency dependent so that frequency control can be used to produce different responses from the LED device and the laser diode device, or even between different LED devices. In the latter case, a parallel capacitor can be used to change the electrical characteristics of a particular LED type, thereby providing additional luminous flux modulation.

[0031] Among other things, the non-linear flux-current characteristic of the LED means that the average current (when operated at a high frequency) gives a different light output than a slowly circulating current of a lower frequency. The capacitor circuit also smooths out high peaks with short intervals into lower peaks with longer intervals, which can be beneficial for the lifetime of the LED.

[0032] The driver is then preferably configurable to set the frequency of the superimposed pulse width modulated component.

[0033] The driver may be configurable to selectively set the frequency of the superimposed pulse width modulated component higher or lower than a frequency corresponding to a time constant of the combination of the LED device and the capacitor circuit.

[0034] In a parallel capacitor configuration, reducing the PWM frequency below the frequency corresponding to the resulting time constant may thereby shift the relative light output toward the laser diode device because the droop phenomenon is less pronounced in the laser diode device.

[0035] This phenomenon occurs at high light levels, whereas at low light levels the threshold-related phenomenon explained above (utilizing the DC current component) dominates.

[0036] The LED device may have at least two sets of LEDs in series and the capacitor circuit may have a respective capacitor circuit in parallel with each set of LEDs, which may then define different LEDs with different frequency behavior.

[0037] In another example, the LED device may again have at least two sets of LEDs in series, with the capacitor circuit having a capacitor circuit in parallel with only a subset of the sets of LEDs.

[0038] In a particular example, the driver generates summed individual outputs, i.e., a DC output and a modulated (e.g., pulsed) output, which may be obtained by individual power converters or by a compound power converter.

[0039] The driver, for example, comprises a first part for providing the DC component and a second part for providing the modulation component.

[0040] These and other aspects of the invention will be elucidated and elucidated with reference to the following embodiments. [Brief description of the drawings]

[0041] 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 shows a graph of light output flux (y-axis) versus drive current (x-axis) for LEDs and laser diodes. [Diagram 2] The lighting circuit is shown. [Diagram 3] 1 illustrates how the relative contributions of the LED device and the laser diode device to the output luminous flux can be adjusted. [Figure 4] 4 illustrates a modulated current waveform operating at a sequence of operating points as shown in FIG. [Diagram 5]The effect of the capacitor circuit is illustrated by showing a modulated current waveform operating at the further operating point shown in FIG. [Figure 6] An example of a driver is shown below. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] The present invention will be described with reference to the drawings.

[0043] 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.

[0044] The present invention provides a lighting circuit having a first light source device and a second light source device of different types in series. A driver supplies a controllable current with an adjustable DC component and an adjustable modulation component, e.g. a superimposed pulse width modulation component with an adjustable duty cycle, to the light source circuit. This allows control of the contribution to the overall light output flux from the different types of light sources.

[0045] The present invention applies to different types of technologies (e.g. laser diodes, LEDs and SLEDs), and therefore the term "different types" refers to different electrical responses due to the inherent electrical characteristics (e.g. threshold behavior) of the technology types.

[0046] By way of example only, the invention will be described with reference to a light source circuit combining an LED and a laser diode, the laser diode being replaced by a superluminescent diode or a combination of a laser diode and a superluminescent diode.

[0047] 2 shows a lighting circuit including a light source circuit 20 having in series an LED device 22 (which comprises a series string of LEDs) and a laser diode device 24 (which comprises a series string of laser diodes). The light source circuit is connected between a first, anode, terminal 26 and a second, cathode, terminal 28.

[0048] The LED device and / or laser diode device may include more complex circuit configurations, for example with multiple parallel branches.

[0049] The driver 30 generates a current i_str to be supplied to the light source circuit 20 .

[0050] The current has a DC component and a superimposed modulation component, such as a pulse width modulated component, but the driver can adjust the DC component (including generating a current with no DC component) and can adjust the modulation component, for example, by adjusting the duty cycle of the superimposed pulse width modulated component (which may have a zero duty cycle, i.e., no pulse width modulated component).

[0051] Thereby, the string current i_str is modulated by the driver 30.

[0052] In the preferred embodiment shown, a capacitor circuit, represented as a single capacitor Cp, is connected in parallel with the LED of the LED device 22.

[0053] Figure 3 shows the LED luminous flux (Φ_LED_rel) on the y-axis and the laser diode luminous flux (Φ_LD_rel) on the x-axis, again with relative flux values ​​shown having the same maximum flux (at point B).

[0054] FIG. 3 is used to represent the main range over which the relative contribution of the LED and laser diode devices to the output luminous flux can be adjusted as region 40. This adjustment is by setting the level of the DC component of the controllable current and by setting the modulation of the superimposed modulation component of the controllable current. Points on the line x=y thus represent operating points where the luminous flux from the two types of light sources is the same. Above the line the LED light output is relatively more dominant, below the line the laser diode light output is relatively more dominant. Thus, by allowing operation away from the line x=y the light output mix is ​​controlled.

[0055] The dotted area is discussed further below.

[0056] FIG. 3 shows various operating points to explain the operation of the circuit.

[0057] Point A The current has only a DC component and is below the threshold current of the laser diode device (i_th_LD in FIG. 1). Therefore, DC operation at point A results in only LED emission while the DC current remains below the laser diode threshold current (i_th_LD). The LED luminous flux is Φ_th.

[0058] point B Here, a pulse-width modulated current with a given (e.g., fixed) amplitude and a variable duty cycle is superimposed on a DC current. Point B is at the maximum duty cycle dmax.

[0059] In region 40, the frequency of the pulse width modulated component is kept much higher than the frequency corresponding to the time constant formed by the capacitor circuit Cp and the dynamic resistance Rdyn of the LED device (the time constant is Cp*Rdyn).

[0060] Thus, while the laser diode device is being pulse width modulated, the LED of the LED device still receives a DC current. At point B, the amplitude and maximum duty cycle result in a rated maximum current.

[0061] Changing the duty cycle from zero to the maximum duty cycle (dmax) at point B while maintaining the maximum DC current i_th_LD moves the operation from point A to point B.

[0062] point C Point C represents the removal of the DC current component while maintaining the maximum duty cycle dmax. Removing the DC offset affects the light output of the LED device more than the light output of the laser diode.

[0063] point D Between points C and O (the origin), the duty cycle is reduced from its maximum value dmax to zero. Along this trajectory, the light from the laser diode device dominates the light output of the LED device.

[0064] Region 40 thus represents an operating window bounded by maximum and zero levels of DC current and maximum and zero duty cycle ratios. By controlling these two parameters, any point within operating window 40 can be selected.

[0065] Point B' and Point C': If the frequency of the pulse width modulation component is reduced, the light output of the LED device can be further dimmed somewhat relative to the light output of the laser diode device. In particular, the frequency can be reduced to below the frequency corresponding to the time constant of the LED circuit and its parallel capacitor (i.e., frequency 1 / Cp*Rdyn). The parallel capacitor may also be formed in whole or in part by the parasitic capacitance of the LED.

[0066] The droop phenomenon is more pronounced in LEDs than in laser diodes, which show little or no deviation from the linear flux-current characteristic. As will be explained further below, frequency control can therefore adjust the brightness of LEDs relative to that of laser diodes.

[0067] Thus, the driver may be able to adjust the frequency of the superimposed pulse width modulated component. The effect is to reduce the maximum LED luminous flux while maintaining the maximum luminous flux of the laser diode device. Thus, region 40 is distorted as shown.

[0068] The current through an LED device is no longer essentially constant, but instead oscillates between high and low flux states. The non-linear flux-current characteristic means that the droop described above changes the average flux.

[0069] FIG. 4 shows the modulated current waveforms at points A to D.

[0070] At point A there is only a DC current at the threshold i_th_LD of the laser diode device.

[0071] At point B there is a high frequency superimposed pulse width modulated component at maximum duty cycle.

[0072] At point C, the DC component is removed but the maximum duty cycle is maintained.

[0073] At point D, the DC component is still removed and the duty cycle is reduced.

[0074] FIG. 5 illustrates the effect of the capacitor circuit, showing points B and B'.

[0075] At point B, the superimposed high frequency pulse width modulation components (greater than 1 / Cp*Rdyn) at maximum duty cycle produce only small periodic oscillations in the LED current i_LED, and the current remains essentially DC because the time constant of their charge cycle is much larger than the period of the pulse width modulation oscillations.

[0076] At point B', the frequency of the superimposed pulse-width modulated component is reduced (below 1 / Cp*Rdyn). Now the time constant of the charge cycle of the LED device current i_LED is shorter than the period of said oscillations. Thereby, the current i_LED does not have only small periodic fluctuations, but rises and falls between a minimum and a maximum value. As a result, the average luminous flux is reduced, since the periods of high light output when the current is highest are subject to a droop in the luminous flux-current characteristic.

[0077] In other words, the light output resulting from a constant average current will not be the same as the average light output from a current that varies in time between a minimum and a maximum value.

[0078] The high and low frequencies are, for example, 10 kHz and 1 kHz, or 100 kHz and 10 kHz.

[0079] Thus, the higher frequency can be between 5 and 100 times the lower frequency.

[0080] If the light output at point B is taken to be the maximum level of both light sources, then the emission from one light source can be altered by a maximum amount, for example within the range of 10% to 20% of its maximum value, while leaving the emission from the other light source unchanged.

[0081] Thus, a significant range of adjustment is achievable even with a single current signal supplied to the series arrangement of LED device 22 and laser diode device 24.

[0082] The above examples show a string of LEDs in an LED device with a single capacitor circuit in parallel (a single capacitor in the example shown). The LED device may instead have at least two sets of LEDs (e.g. of different colors) in series. In that case, the capacitor circuit may have a respective capacitor circuit in parallel with each set of LEDs, or there may be a capacitor circuit in parallel with only a subset of the set of LEDs.

[0083] Thus, parallel capacitors may be connected across only some of the LEDs, or different capacitors may be used for subsections of the LEDs. In this manner, the luminous flux from different sets of LEDs can be adjusted somewhat relative to each other.

[0084] Laser diodes, unlike LEDs, are generally high current devices, so a laser diode unit may be in series with several parallel branches of LEDs to provide appropriate current levels for different device types.

[0085] The pulsed operation achieved by the pulse width modulated components can be used for modulated light applications such as coded light or LiFi.

[0086] By way of example, the amplitude of the superimposed pulse width modulated component may be, for example, in the range of 0 to 5 times the amplitude of the DC component.

[0087] The maximum amplitude of the DC component is, for example, 20 A or less, or for typical components, 4 A or less. Similarly, the maximum amplitude of the superimposed pulse width modulated component is, for example, 20 A or less, or for typical components, 4 A or less.

[0088] The amplitude of the superimposed pulse width modulation components is controllable, for example, within a range of 0 A to a maximum value (0 A meaning that one type is not activated). Similarly, the amplitude of the DC component is controllable, for example, within a range of 0 A to a maximum value (0 A meaning that one type is not activated).

[0089] A driver that may be used has a simple structure, for example two power supplies connected in series, one supplying a DC current and the other supplying a pulsed current.

[0090] 6 shows in simplified form a driver having a first section 60 for providing a DC component of a controllable current and a second section 70 for providing a modulated component of the controllable current. In this example, the two sections provide currents that are summed at a summing node. Each has a respective secondary winding 62, 72 of an isolation transformer 80 at the output of a switch mode power converter 82.

[0091] The first section 60 has a storage capacitor 64 and provides a DC component to a node N1 which functions as a ground terminal for the second section 70. The output N1 of the first section forms the ground for the second section. The second section 70 has a storage capacitor 74 and provides a modulation component to a node N2 which connects to the light source circuit 20. Node N2 is a current summing node of the DC current and the modulation current.

[0092] A diode 84 is connected between nodes N1 and N2. Diode 84 is thus in parallel with second section 70 and provides a conduction path for current from first section 60 when the second section is not applying a current pulse. As a result of diode 84, the output from the second section cannot flow back into the first section.

[0093] The above example is based on pulse width modulation for the modulation component of the controllable current. Instead of the PWM signal, a sinusoidal waveform with a DC offset current may be applied, where the amplitude and DC offset are adjusted to achieve a similar result as the PWM signal, which may improve ENC (electromagnetic compatibility). Any other waveform may be used as long as it utilizes a combination of DC offset and amplitude modulation to vary the radiation output of the laser diode device and the LED device.

[0094] The lighting circuit described above may be used in a lighting device that emits white light, for example, having a color temperature in the range of 2000K to 6000K, a color rendering index of at least 80, and / or a color point within 8 steps of standard deviation color matching from the blackbody locus.

[0095] The light source may comprise a laser diode (optionally with a phosphor conversion layer) and an LED, emitting in different spectral ranges, as in the above example. The DC content, frequency and duty cycle of the driver's superimposed pulse modulated current are adjusted to tune the output of the light source to a certain color point, for example in the range of 2700K to 7000K with a color rendering index in the range of 80 to 95.

[0096] The lighting device may, for example, be incorporated into a lamp (e.g., having an electrical connector base and an envelope for at least partially enclosing the light source unit) or into a lighting fixture having a housing with a fixing for fixing the lighting fixture to a wall or ceiling.

[0097] The invention is particularly interesting for lighting systems with tunable color point, for example by using a blue laser in combination with a red LED and a green-yellow phosphor to enhance the color rendering index. The invention can be used for laser-based light sources with tunable MDER (melanopic daylight efficacy ratio), for example using a cyan LED.

[0098] In the illustrated example, the light source circuit may only receive current from the driver 30. This may result in no other current source providing current to any of the light source devices. In that case, the light source circuit may only receive a single current from the driver 30, where the single current is a controllable current having a DC component and a superimposed modulation component.

[0099] 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.

[0100] 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.

[0101] 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."

[0102] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. a light source circuit including a first light source device and a second light source device of a different type from the first light source device in series; 1. A lighting circuit comprising: a driver for supplying a controllable current to the light source circuit, the driver being configured to set a DC component of the controllable current and to set a modulation of a superimposed modulation component of the controllable current, the controllable current supplied to the light source circuit providing a light output distribution between the first light source device and the second light source device, A lighting circuit, wherein the light source circuit receives only a current from the driver, such that the light source circuit is adapted to receive only a single current from the driver.

2. 2. The lighting circuit according to claim 1, wherein the first light source device is an LED device and the second light source device is a laser diode device and / or a superluminescent diode device.

3. 3. The lighting circuit of claim 2, wherein the driver is configured to set a DC current component below a threshold current of the laser diode device and / or the superluminescent diode device.

4. 2. The lighting circuit of claim 1, wherein the driver is configured to set the modulation by setting a duty cycle of a superimposed pulse width modulated component of the controllable current.

5. 5. A lighting circuit as claimed in claim 4, wherein the amplitude or maximum amplitude of the superimposed pulse width modulated component is at most 5 times the maximum amplitude of the DC component.

6. The lighting circuit of claim 1 , wherein the driver is configured to set a frequency of the superimposed modulation component.

7. The lighting circuit of claim 1 , wherein the driver is configured to set an amplitude of the superimposed modulation component.

8. 2. The lighting circuit of claim 1, wherein the amplitude or maximum amplitude of the superimposed modulation component is up to 20A.

9. 2. The lighting circuit of claim 1, wherein the maximum amplitude of the DC component is up to 20A.

10. 2. The lighting circuit of claim 1, further comprising a capacitor circuit in parallel with the first light source device, the first light source device being an LED device.

11. 11. The lighting circuit of claim 10, wherein the driver is configured to selectively set a frequency of the superimposed modulation component higher or lower than a frequency corresponding to a time constant of the combination of the LED device and the capacitor circuit.

12. 11. The lighting circuit of claim 10, wherein the LED device comprises at least two sets of LEDs in series, and the capacitor circuit comprises a respective capacitor circuit in parallel with each set of LEDs.

13. 11. The lighting circuit of claim 10, wherein the LED device comprises at least two sets of LEDs in series, and the capacitor circuit comprises a capacitor circuit in parallel with only a subset of the sets of LEDs.

14. 2. The lighting circuit of claim 1, wherein the driver has a first portion for providing the DC component and a second portion for providing the modulated component.

15. A lamp or luminaire comprising a lighting circuit according to claim 1.

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