Method for controlling lighting device, lighting control circuit and lighting system

Abstract

A lighting device, such as an LED lamp, includes a light source and an associated current driver, or a set of at least two parallel light sources and associated current drivers, powered by a DC voltage from a voltage-regulated power converter. A feedback circuit provides a temperature-dependent control input to the control loop of the power converter, causing the DC voltage to be adjusted depending on the sensed temperature. In this way, the required voltage headroom can be reduced, and efficiency gains can be achieved.

Description

Technical Field

[0001] The present invention relates to the control of lighting systems, such as (but not exclusively) multi-channel lighting systems. For example, multiple channels can provide color mixing and color temperature control, but other effects can also be obtained by using multiple light sources. Background Art

[0002] Various multi-channel LED light sources are known. One possible arrangement utilizes different color channels connected in parallel. Each channel can, for example, independently provide a different color output. Alternatively, different LEDs can be connected in series, and bypass switches can be used to select which LEDs are activated, thereby controlling the output color.

[0003] For example, such a system can generate white light by independently driving red, green, and blue LEDs. Note that in practice, the green LED can utilize a native blue LED and a green phosphor layer. White light can be generated at different color temperatures, for example, by having separate LED strings generate cool white or warm white from a single fixture. Alternatively, such a system can provide full output color control.

[0004] Furthermore, multi-channel LED drivers are also encountered in LED modules or LED luminaires, in which different channels are used to generate separate light beams for general lighting or task lighting.

[0005] In current systems, separate drivers are used for different LEDs (or LED strings) of a module. This may be necessary, for example, due to different load dependencies of different channels.

[0006] Typically, in a multi-channel system, each channel does not always run at maximum power. In many cases, pulse width modulation (PWM) is used to control the power on one channel, meaning that power on that channel is only drawn during a certain portion of the time.

[0007] Figure 1 A conventional multi-channel lighting system driver circuit is shown in schematic form. Three LED loads 10, 11, and 12 are shown, which may, for example, have three different color outputs. Each LED load is driven by a corresponding driver 20, 21, and 22, which essentially comprises a switched-mode power supply (SMPS) or linear driver implementing PWM control. A global AC-DC converter 14, which may, for example, include power factor correction, is present, along with a global controller 16, which is located remote from the actual light sources themselves. The global controller 16 provides commands to the local drivers 20, 21, and 22 to control the operation of the LED loads.

[0008] This approach uses a two-stage driver concept. One driver stage is used to convert the mains voltage into an intermediate DC voltage, and the other driver stage is used to convert the intermediate DC voltage into LED current. Multiple LED channels can then be controlled independently of each other.

[0009] Figure 2 Shown Figure 1 An example of an embodiment of the apparatus schematically shown in FIG. It includes an AC-DC converter 14 that delivers a constant voltage output Vout, and each individual driver is shown as a DC current source I1 to I3, with all channels connected in parallel. Each current source has a current sensing resistor R1 to R3. The current sources are controlled using pulse width modulation using control signals PWM1 to PWM3. The current sensing resistors are used to set a fixed current level. This is a low-cost solution, with each channel having a linear driver.

[0010] For example, LED devices have different color points. They can be different types of white (warm white, cool white, flame white...) or different colors.

[0011] One drawback of this arrangement is efficiency. Specifically, the bus voltage (output voltage Vout) needs to be higher than the forward voltage of the LED string with the highest forward voltage. This maximum voltage is determined by the voltage bin, the number of LEDs, the drive current, and the temperature.

[0012] The losses in each channel are then proportional to the voltage difference between the set bus voltage and the LED (specifically, LED string) forward voltage.

[0013] To optimize driver efficiency, the voltage drop across current sources I1 to I3 should be minimized by operating the bus voltage at the lowest possible level. It is therefore necessary to keep the bus voltage as low as possible. Summary of the Invention

[0014] According to an example according to one aspect of the present invention, a lighting control circuit for controlling a lighting device is provided, the lighting control circuit comprising:

[0015] a power converter for delivering a DC voltage to a current driver, wherein the current driver is connected in series with the lighting device to drive current through the lighting device, and wherein the power converter has a control loop for controlling the level of the DC voltage;

[0016] a temperature sensing element for sensing a temperature at or near the lighting device; and

[0017] A feedback circuit is provided for providing a temperature-dependent control input to a control loop of the power converter such that the DC voltage is adjusted in dependence on the sensed temperature.

[0018] The present invention is based on the recognition that when using a fixed DC voltage (often referred to as the bus voltage), efficiency is temperature-dependent. When the LEDs are cold at startup, the bus voltage should be high enough to ensure sufficient voltage headroom for proper operation of the current driver, which can include, for example, a linear current source. As the lamp warms, the headroom increases because the LED forward voltage decreases, and thus the overall losses increase.

[0019] The present invention is based on the use of a feedback circuit in a voltage control loop of a power converter to regulate the DC voltage.

[0020] Preferably, the DC voltage is adjusted inversely with respect to temperature. In this way, at high temperatures, the bus voltage is adjusted to a lower voltage level to increase efficiency. At low temperature startup, the control loop increases the DC voltage to compensate for the cold LEDs.

[0021] In this way, efficiency is improved by dynamically adjusting the voltage headroom to match the operating temperature.

[0022] The feedback circuit comprises, for example, a resistor network, and the temperature sensing element comprises at least one temperature-sensitive resistor in the resistor network.

[0023] This provides a simple and low-cost way to generate appropriate feedback control signals for controlling the power converter.

[0024] A power converter for delivering a DC voltage comprises, for example, a switch-mode power converter. It may have a voltage-regulated output and may, for example, have a rectified mains input. The power converter comprises, for example, a buck converter.

[0025] The present invention further provides a lighting circuit, comprising:

[0026] A lighting control circuit as defined above;

[0027] current driver; and

[0028] A lighting device comprises a light source connected in series with a current driver, the series combination being supplied with a DC voltage, and the light source comprising an LED device through which a current is driven by the current driver.

[0029] This provides a combination of a lighting control circuit, and a lighting device (which in this case may be a single LED string light source) and a current driver driven by the lighting control circuit.

[0030] The present invention further provides a lighting circuit, comprising:

[0031] A lighting control circuit as defined above;

[0032] a collection of parallel current drivers; and

[0033] A lighting device includes a set of parallel LED devices, wherein each LED device is connected in series with a corresponding current driver that drives current through the LED device, and each series combination is supplied with a DC voltage.

[0034] In this case, the lighting control circuit is used to control a lighting device comprising a set of at least two light sources (together forming a lighting device), the set comprising a first light source and a second light source connected in parallel, wherein a power converter is used to deliver a DC voltage to the set of parallel current drivers. The DC voltage is a power supply for the current drivers and their associated LED devices.

[0035] For example, the current drivers each include a constant current source circuit. The current drivers can each include a pulse width modulation control input. In this way, the drive level of the light source is adapted in a time-division manner. The frequency of the pulse width modulation is high enough so that no visible flicker is perceived, for example in the kHz range (e.g., 500 Hz to 10 kHz).

[0036] A corresponding current sensing resistor is connected in series with each current driver, for example. This is used to set the current level of the current driver.

[0037] When multiple LED devices are used in parallel, they can have different color points, for example. They can be different types of white (warm white, cool white, flame white...) or different colors.

[0038] The temperature sensing element of the lighting control circuit is preferably located in close proximity to, or thermally coupled to, one of the LED devices in the LED arrangement. This provides for optimal operation of the feedback control. The one LED device in the LED arrangement is, for example, the LED device with the highest forward voltage. This is the LED device for which headroom control is most critical.

[0039] The present invention also provides a method for controlling a lighting device, the method comprising:

[0040] delivering a DC voltage to the current driver using a power converter, the power converter having a control loop for controlling a level of the DC voltage;

[0041] sensing temperature at or near the light source;

[0042] providing a temperature-dependent control input to a control loop of the power converter such that the DC voltage is adjusted in dependence on the sensed temperature; and

[0043] A current driver is used to drive the light source.

[0044] The method is for example used to control a lighting device comprising a set of at least two light sources, the set comprising a first light source and a second light source connected in parallel, wherein the method comprises: using a power converter to deliver a DC voltage to the set of parallel current drivers, and using the current drivers to drive the light sources.

[0045] The method comprises, for example, inversely regulating the DC voltage with respect to the temperature.

[0046] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] For a better understanding of the present invention, and in order to show more clearly how the same may be put into practice, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0048] Figure 1 A known lighting control architecture for driving multiple lighting channels is shown in schematic form;

[0049] Figure 2 Shown Figure 1 An implementation of a lighting control architecture;

[0050] Figure 3 The lighting control circuit of the present invention is shown, which is applied to Figure 2 Circuit architecture;

[0051] Figure 4 Shown by Figure 3 The efficiency improvement obtained by the circuit of

[0052] Figure 5 A method of controlling a lighting device is shown. DETAILED DESCRIPTION

[0053] The present invention will be described with reference to the accompanying drawings.

[0054] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems, and methods, are intended for illustrative purposes 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, the appended claims, and the accompanying drawings. It should be understood that the drawings are merely schematic and not drawn to scale. It should also be understood that the same reference numerals are used throughout the drawings to indicate the same or similar parts.

[0055] The present invention provides a lighting device, such as an LED lamp, comprising a light source and an associated current driver, or a set of at least two parallel light sources and associated current drivers, powered by a DC voltage from a voltage-regulated power converter. A feedback circuit provides a temperature-dependent control input to the control loop of the power converter, causing the DC voltage used to power the current driver and the light source to be adjusted depending on the sensed temperature. In this way, the required voltage headroom can be reduced, and efficiency gains can be achieved.

[0056] exist Figure 2 In known arrangements of , a current sensing resistor sets the current using an internal reference. The light output flux (and therefore the color point) is set by a PWM signal.

[0057] When the PWM signal is set to zero, the current source turns off and therefore stops generating current. When the PWM signal is non-zero, the amplitude of the current waveform is set by the current sense resistor and the internal reference, while the duty cycle and therefore the average current is set by the PWM signal.

[0058] The current sources are linear drivers, which means there is no high frequency switching mode. Instead, they are implemented by transistor current source circuits with a sense resistor for setting the current.

[0059] When a fixed bus voltage is used as a voltage source, a major issue to address is temperature-dependent efficiency. When the LEDs are cold at startup, the bus voltage needs to be high enough to ensure sufficient headroom (voltage) for proper operation of the current source. However, as the lamp and driver warm up, the headroom will increase because the LED forward voltage decreases, and therefore the total losses will increase.

[0060] This problem can occur for individual current drivers or collections of current drivers connected in parallel.

[0061] Figure 3 A lighting control circuit according to an example of the present invention is shown, which is applied to Figure 2 circuit architecture. Figure 3 The entire circuit is a lighting circuit.

[0062] Figure 3 The lighting control circuit is shown coupled to a single current driver and a light source. There may be only a single current driver, or the lighting control circuit may be implemented as a circuit for controlling a lighting device, the circuit comprising a set of at least two current drivers, the set comprising a first current driver and a second current driver connected in parallel. The (or each) current driver is connected in series with its associated light source. The current driver drives current through the associated light source. The one or more light sources then form a lighting device.

[0063] The power converter delivers a DC voltage Vout to the current driver or a set of parallel current drivers, and the power converter has a control loop for controlling the level of the DC voltage Vout. The DC voltage Vout is the power supply for the (or each) series-connected current driver and the light source.

[0064] The input Vin to the power converter is, for example, from a circuit comprising a full-bridge diode rectifier and an EMI filter (ie Figure 1 and Figure 2 The AC / DC converter 14 receives the signal. Thus, the converter circuit shown typically receives a rectified mains input and is itself a DC / DC converter. Alternatively, of course, the rectifier and EMI filter could be considered part of the overall AC / DC converter architecture.

[0065] Figure 3 The current driver shown in FIG comprises a first current source I1 and an associated first current sensing resistor R1, and is connected in series with a first light source D1, which itself is an LED device (represented as a single diode D1 for simplicity).

[0066] The circuit includes a constant voltage power converter 30. Figure 3 In the example of FIG, the power converter is implemented as a buck converter. The buck converter includes an energy storage inductor L1, a diode d1, and a storage capacitor C1. A switch S1 is provided between the cathode of the diode d1 and the input Vin.

[0067] The voltage feedback point for controlling switch S1 is shown as control pin Ctrl. It is provided to power converter controller IC 34, which then controls the timing of operation of main switch S1 to provide a regulated output voltage. Controller 34 implements the control loop of the power converter.

[0068] The switch S1 and the controller 34 may be part of a power converter integrated circuit, such as a buck PFC constant voltage regulator circuit.

[0069] The circuit further comprises a feedback circuit 32, in particular in the form of a resistor network R2, R3, R4, RS. The resistor RS is a temperature sensing element, in particular in the form of a temperature sensitive resistor.

[0070] The resistor RS is the only temperature-dependent component and is formed as a negative temperature coefficient (NTC) resistor. The resistors R3, R4 and R2 form a voltage feedback circuit for generating the voltage feedback Ctrl and thus for controlling the output voltage.

[0071] Without resistor RS, conventional voltage control is implemented. Resistor RS means that the control input Ctrl to controller 34 becomes temperature-dependent. Thus, temperature dependency is added to the control feedback used to control the timing of operation of power converter main switch S1. The feedback resistor circuit is designed so that temperature has the desired effect on the regulated voltage control.

[0072] A control input Ctrl is supplied to the controller 34 and thereby introduced into the control loop of the power converter such that the DC voltage is adjusted in dependence on the sensed temperature.

[0073] The DC voltage is adjusted inversely with respect to temperature. In this way, at high temperatures, the bus voltage is adjusted to a lower voltage level to improve efficiency. At low temperature startup, the control loop increases the DC voltage to compensate for the cold LEDs.

[0074] The linear driver I1 is a fixed current source as explained above, for example, with a 1 kHz PWM input. More generally, one or more current drivers each include a pulse width modulation control input, for example. In this way, the drive level of the light source is adapted in a time-division manner. The frequency of the pulse width modulation is high enough so that no visible flicker is perceived, for example in the kHz range (e.g., 500 Hz to 10 kHz).

[0075] The LED load (ie the light source D1 ) is, for example, a single LED string, such as a single string of 6 medium power LEDs. During the reverse period, the voltage across the inductor L1 is a measure of the output voltage.

[0076] At higher temperatures, the resistance of RS decreases and, therefore, the output voltage at the control pin Ctrl will increase. This corresponds, for example, to a lower output voltage, since the reference level of the control pin is reached earlier, giving a shorter on-period.

[0077] Conversely, at lower temperatures, the resistance increases and therefore the Vctrl pin voltage decreases. This corresponds to an increase in the output voltage, as it takes longer to reach the reference voltage at the CONTROL pin, giving a longer on-period.

[0078] Therefore, the present invention is based on using a feedback circuit in the voltage control loop of a power converter to regulate the DC voltage.

[0079] Figure 4 The standard method (plot 40) and Figure 3 A plot of efficiency versus temperature for an embodiment of (plot 42 ).

[0080] Without temperature compensation, the efficiency of the current source drops to 91% at 100 degrees, while with the above temperature compensation, an improvement of 5% is obtained.

[0081] Figure 3 Only one current source D1 is shown, but the lighting control circuit may include a collection of parallel current drivers and associated light sources (such as Figure 2 ), each powered by a DC voltage. The DC voltage is the power supply for the current driver. A corresponding current sensing resistor is connected in series with each current driver, for example.

[0082] The temperature sensing element of the lighting control circuit is preferably located in close proximity to, or thermally coupled to, one of the LED devices in the LED arrangement. This provides for optimal operation of the feedback control. The one LED device in the LED arrangement is, for example, the LED device with the highest forward voltage. This is the LED device for which headroom control is most critical.

[0083] Figure 5 A method for controlling a lighting device is shown, the lighting device comprising, for example, a set of at least two light sources, the set comprising a first light source and a second light source connected in parallel, the method comprising:

[0084] In step 50, a DC voltage is delivered to a current driver or a set of parallel current drivers using a power converter, the power converter having a control loop for controlling the level of the DC voltage;

[0085] In step 52, a temperature at or near the light source is sensed;

[0086] In step 54 , providing a temperature-dependent control input to a control loop of the power converter such that the DC voltage is adjusted in dependence on the sensed temperature; and

[0087] In step 56, one or more light sources are driven using a current driver.

[0088] The method comprises, for example, inversely regulating the DC voltage with respect to the temperature.

[0089] The invention is primarily useful for parallel switched current sources. However, it can be applied to lamps with a single current source, such as monochrome (white) lamps.

[0090] Variations on the disclosed embodiments may be understood and effected by those skilled in the art in practicing the claimed invention by studying 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 fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. If the term "suitable for" is used in a claim or in the specification, it is noted that the term "suitable for" is intended to be equivalent to the term "configured to". Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A lighting control circuit for controlling a lighting device, the lighting control circuit comprising: Current driver (I1); a power converter (30) for delivering a DC voltage to the current driver (I1), wherein the current driver (I1) is arranged to be connected in series with the lighting device between the power converter (30) and ground for driving current through the lighting device, and wherein the power converter has a control loop for controlling the level of the DC voltage, the power converter (30) for delivering the DC voltage comprises an energy storage inductor (L1) and a switch mode power converter (30) having a switch (S1), the switch (S1) being connected between an input (Vin) and the energy storage inductor (L1) and being disconnected from ground, the control loop having a power converter controller IC (34) being connected to the switch (S1) and being disconnected from ground; a temperature sensing element (RS) for sensing temperature at or near the lighting device and arranged for use in a feedback circuit (R2, R3, R4, RS); and The feedback circuit (R2, R3, R4, RS) is connected across an energy storage inductor (L1) of the power converter and is separated from ground, is arranged to receive a voltage across the energy storage inductor (L1) of the power converter, and is arranged to provide a temperature-dependent control input to a power converter controller IC (34) of the control loop of the power converter so that the DC voltage is adjusted in dependence on the sensed temperature, wherein the DC voltage is adjusted inversely with respect to the temperature.

2. The lighting control circuit according to claim 1 , for controlling a lighting device comprising a set of at least two light sources, the set comprising a first light source and a second light source connected in parallel, wherein the power converter is configured to deliver a DC voltage to the set of parallel current drivers.

3. The lighting control circuit according to any one of claims 1 to 2, wherein the feedback circuit comprises a resistor network, wherein the temperature sensing element (RS) comprises at least one temperature-sensitive resistor in the resistor network.

4. The lighting control circuit according to claim 1 or 2, wherein: The switch-mode power converter comprises a buck converter.

5. A lighting circuit comprising: The lighting control circuit according to any one of claims 1 to 4; as well as The lighting device comprises a light source connected in series with the current driver, this series combination being supplied with the DC voltage, and the light source comprises an LED device through which current is driven by the current driver.

6. A lighting circuit comprising: The lighting control circuit according to any one of claims 1 to 4; a collection of parallel current drivers (I1, I3, I3); as well as The lighting device comprises a set of parallel-connected LED devices, wherein each LED device is connected in series with a respective current driver that drives current through the LED device, and each series combination is supplied with the DC voltage.

7. The lighting circuit according to claim 6, wherein the temperature sensing element (RS) of the lighting control circuit is in close proximity to one of the LED devices or is thermally coupled to one of the LED devices.

8. The lighting circuit of claim 7, wherein the one of the LED devices is an LED device having a maximum forward voltage.

9. A lighting circuit according to any one of claims 5 to 8, wherein the or each current driver comprises a constant current source circuit, wherein the or each current driver comprises a pulse width modulation control input (PWM1, PWM2, PWM3).

10. A lighting circuit according to claim 9, further comprising a respective current sensing resistor (R1, R2, R3) connected in series with the or each current driver.

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