Method of controlling a lighting arrangement, a lighting control circuit and a lighting system

Abstract

A lighting arrangement such as a LED lamp comprises a light source and associated current driver, or a set of at least two parallel light sources and associated current drivers, supplied by a DC voltage from a voltage-regulated power converter. A feedback circuit provides a temperature-dependent control input to a control loop of the power converter, such that the DC voltage is adjusted in dependence on a sensed temperature. In this way, the voltage headroom required can be reduced, and efficiency gains are made.

Claims

1. A lighting control circuit for controlling a lighting arrangement, the lighting control circuit comprising: a power converter for delivering a DC voltage to a current driver, wherein the current driver is arranged to be connected in series with the lighting arrangement for driving a current through the lighting arrangement, and wherein the power converter has a control loop for controlling the level of the DC voltage; a temperature sensing element for sensing a temperature at or near the lighting arrangement and arranged to be used in a feedback circuit; and the feedback circuit arranged to receive the measured voltage and arranged to provide a temperature-dependent control input to the control loop of the power converter, such that the DC voltage is adjusted in dependence on a sensed temperature, wherein the DC voltage is adjusted inversely with respect to sensed temperature.

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

3. The lighting control circuit as claimed in claim 1, wherein the feedback circuit comprises a resistor network, and wherein the temperature sensing element comprises at least one temperature sensitive resistor in the resistor network.

4. The lighting control circuit as claimed in claim 1, wherein the power converter for delivering a DC voltage comprises a switch mode power converter.

5. The lighting control circuit as claimed in claim 4, wherein the power converter for delivering a DC voltage comprises a buck converter.

6. A lighting circuit comprising: a lighting control circuit as claimed in claim 1; the current driver; and the lighting arrangement, which comprises a light source in series with the current driver, the series combination supplied with the DC voltage, and the light source comprising a LED arrangement through which current is driven by the current driver.

7. The lighting circuit as claimed in claim 6, wherein the current driver comprises a constant current source circuit, wherein the or each current driver comprises a pulse width modulation control input.

8. The lighting circuit as claimed in claim 7, further comprising a current sense resistor in series with the current driver.

9. A lighting circuit comprising: a lighting control circuit as claimed in claim 1; a set of parallel current drivers; and the lighting arrangement, comprising a set of parallel LED arrangements, wherein each LED arrangement is in series with a respective current driver which drives current through the LED arrangement, and each series combination is supplied with the DC voltage.

10. The lighting circuit as claimed in claim 9, wherein the temperature sensing element of the lighting control circuit is in close proximity to one of the LED arrangements or thermally coupled to one of the LED arrangements.

11. The lighting circuit as claimed in claim 10, wherein the one of the LED arrangements is the LED arrangement with the largest forward voltage.

12. A method of controlling a lighting arrangement, comprising: delivering a DC voltage to a current driver using a power converter, the power converter having a control loop for controlling the level of the DC voltage; sensing a temperature at or near a light source; providing a temperature-dependent control input based on the sensed temperature and on a measured voltage over an inductor of the power converter to the control loop of the power converter, such that the DC voltage is adjusted in dependence on a sensed temperature; and driving the light source using the current driver, and adjusting the DC voltage inversely with respect to sensed temperature.

13. The method as claimed in claim 12 for controlling a lighting arrangement comprising a set of at least two light sources comprising a first light source and a second light source in parallel, wherein the method comprises delivering the DC voltage to a set of parallel current drivers using the power converter and driving the light sources using the current drivers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 shows in schematic form a known lighting control architecture for driving multiple lighting channels;

(3) FIG. 2 shows one implementation of the lighting control architecture of FIG. 1;

(4) FIG. 3 shows a lighting control circuit of the invention, applied to the circuit architecture of FIG. 2;

(5) FIG. 4 shows the efficiency improvements obtained by the circuit of FIG. 3; and

(6) FIG. 5 shows a method of controlling a lighting arrangement.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) The invention will be described with reference to the Figures.

(8) It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

(9) The invention provides a lighting arrangement such as a LED lamp comprising a light source and associated current driver, or a set of at least two parallel light sources and associated current drivers, supplied by a DC voltage from a voltage-regulated power converter. A feedback circuit provides a temperature-dependent control input to a control loop of the power converter, such that the DC voltage used to power the current driver(s) and light source(s) is adjusted in dependence on a sensed temperature. In this way, the voltage headroom required can be reduced, and efficiency gains are made.

(10) In the known arrangement of FIG. 2, the current sense resistor sets the current making use of an internal reference. The light output flux (and hence color point) is set by the PWM signal.

(11) When the PWM signal is set to zero, the current source is turned off and hence stops generating current. When the PWM signal is non-zero, the amplitude of the current waveform is set by the current sense resistor and internal reference, whereas the duty cycle, and hence average current, is set by the PWM signal.

(12) The current sources are linear drivers, by which is meant there is no high frequency switching mode. Instead, they are implemented by a transistor current source circuit, with a sense resistor to set the current.

(13) A major issue to solve is the temperature dependent efficiency when a fixed bus voltage is used as the voltage source. When LEDs are cold at start-up, the bus voltage needs to be high enough to guarantee enough headroom (voltage) for correct operation of the current sources. However, when the lamp and driver is warmed up, the headroom will increase because the LED forward voltage decreases and hence the total losses will increase.

(14) This issue arises for an individual current driver or for a set of current drivers in parallel.

(15) FIG. 3 shows a lighting control circuit in accordance with one example of the invention, applied to the circuit architecture of FIG. 2. The overall circuit of FIG. 3 is a lighting circuit.

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

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

(18) The input Vin to the power converter is for example a signal received from a circuit including a full bridge diode rectifier and EMI filter (i.e. the AC/DC converter 14 of FIGS. 1 and 2). Thus, the converter circuit as shown typically receives a rectified mains input and is itself a DC/DC converter. Of course, the rectifier and EMI filter may instead be considered to be part of an overall AC/DC converter architecture.

(19) The current driver shown in FIG. 3 comprises a first current source I1 and associated first current sense resistor R1, and the current driver is connected in series with a first light source D1, which itself is a LED arrangement (represented as a single diode D1, for simplicity).

(20) The circuit comprises a constant voltage power converter 30. In the example of FIG. 3, this power converter is implemented as a buck converter. The buck converter comprises an energy storage inductor L1, diode d1 and storage capacitor C1. A switch S1 is provided between the cathode of the diode D1 and the input Vin.

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

(22) The switch S1 and controller 34 may be part of a power converter integrated circuit, such as a buck PFC constant-voltage regulator circuit.

(23) 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.

(24) Resistor RS is the only temperature dependent element, 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 hence for controlling the output voltage.

(25) Without the resistor RS, conventional voltage control is implemented. The resistor RS means the control input Ctrl to the controller 34 becomes temperature dependent. Thus, a temperature dependency is added to the control feedback used to control the timing of operation of the power converter main switch S1. The feedback resistor circuit is designed such that temperature has the desired influence on the regulated voltage control.

(26) The 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.

(27) The DC voltage is adjusted inversely with respect to temperature. In this way, at high temperature the bus voltage is adjusted to a lower voltage level to increase the efficiency. At start-up with low temperature, the control loop increases the DC voltage to compensate for the cold LEDs.

(28) The linear driver I1 is a fixed current source as explained above, for example with a 1 KHz PWM input. More generally, the current driver or drivers for example each comprise a pulse width modulation control input. In this way, the drive level to the light sources is adapted in a time division manner. The frequency of the pulse width modulation is high enough that visible flicker is not perceived, for example in the kHz range (e.g. 500 Hz to 10 kHz).

(29) The LED load, i.e. the light source D1, is for example a single string of LEDs, such as 6 mid-power LEDs. The voltage across the inductor L1 at the reverse period is a measure of the output voltage.

(30) The resistance of RS decreases at higher temperatures and hence the output voltage at the control pin Ctrl will increase. This for example corresponds to a lower output voltage because the reference level of a control pin is reached earlier, giving a shorter conduction period.

(31) Conversely, at lower temperatures the resistance increases, so the Vctrl pin voltage decreases. This corresponds to an increase in the output voltage because the reference voltage of the control pin takes longer to be reached, giving a longer conduction period.

(32) The invention is thus based on regulating the DC voltage using a feedback circuit in the voltage control loop of the power converter.

(33) FIG. 4 shows a plot of efficiency versus temperature for the standard approach (plot 40) and for the implementation of FIG. 3 (plot 42).

(34) Without the temperature compensation, the efficiency of the current source drops to 91% at 100 degrees, whereas an improvement of 5% is obtained with the temperature compensation described above.

(35) FIG. 3 shows only one current source D1, but the lighting control circuit may comprise a set of parallel current drivers and associated light sources (as shown in FIG. 2), each supplied by the DC voltage. The DC voltage is the power supply to the current drivers. A respective current sense resistor is for example in series with each current driver.

(36) The temperature sensing element of the lighting control circuit is preferably in close proximity to one of the LED arrangements or thermally coupled to one of the LED arrangements. This provides optimum operation of the feedback control. The one of the LED arrangements is for example the LED arrangement with the largest forward voltage. This is the LED arrangement for which the headroom control is most critical.

(37) FIG. 5 shows a method of controlling a lighting arrangement, which for example comprises a set of at least two light sources comprising a first light source and a second light source in parallel, the method comprising:

(38) in step 50, delivering a DC voltage to the current driver or to the set of parallel current drivers using a power converter, the power converter having a control loop for controlling the level of the DC voltage;

(39) in step 52, sensing a temperature at or near a light source;

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

(41) in step 56, driving the light source or light sources using the current driver(s).

(42) The method for example comprises adjusting the DC voltage inversely with respect to temperature.

(43) The invention is of primary interest for parallel switched current sources. However, it may be applied to a lamp with a single current source such as a one color, white, lamp.

(44) 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. If the term “adapted to” is used in the claims or description, it is noted the term “adapted to” is intended to be equivalent to the term “configured to”. Any reference signs in the claims should not be construed as limiting the scope.