Improved balance control for two-channel CCT dimming

Abstract

The present invention relates to a lighting system having a load including a parallel arrangement of a first LED load having a first forward voltage, a first switch coupled in series with the first LED load, and a second LED load having a second forward voltage different from the first forward voltage; and a switched mode power supply including a switching element, an inductor, a diode, a set of output pins for coupling to the load, and a capacitor coupled between the set of output pins, wherein the switched mode power supply is configured to supply a voltage between the set of output pins, and the lighting system has a further inductor coupled between one of the output pins and the load.

Description

[Technical field]

[0001] The present invention relates to a lighting system. [Background technology]

[0002] In connected lamps, i.e. lamps that can be controlled via external signals such that the lamp can perform various actions such as dimming or changing color, multiple LED strings are used. Each string is driven by an independent driver so that two or more strings can be controlled in an optimized manner. This requires an additional driver for each additional string, which is costly.

[0003] Another way to drive LED strings is to provide an AC / DC converter followed by a DC / DC current source. In that case, the LED strings are placed in parallel and each string is controlled with a series switch to achieve the desired light setting, such as the color point. This provides more flexibility in the driver design when additional LED strings are installed. For optimal driving without color spread, the length of the LED strings, or at least the forward voltage, should be equal. If there is a voltage spread between the LED strings, the color point will shift because there is an imbalance between the strings, since the DC / DC converter produces only one voltage level that is buffered by a large buffer capacitor. This effect is undesirable. Summary of the Invention [Problem to be solved by the invention]

[0004] It is an object of the present invention to provide an improved lighting system that provides an inexpensive solution for reducing the effects of color shift due to energy transfer between an LED string and a parallel capacitor. [Means for solving the problem]

[0005] In order to solve this problem, in a first aspect of the present invention, - a load including an input for receiving a voltage, said input comprising: - a first LED load having a first forward voltage, and a first switch coupled in series with the first LED load; and a load coupled in a parallel arrangement of a second LED load having a second forward voltage different from the first forward voltage; - a switched mode power supply, - switching elements, - inductor, - Diodes, - a set of output pins for coupling to said load; and - a switched mode power supply including a capacitor coupled between the set of output pins, the switched mode power supply configured to supply a voltage between the set of output pins, the lighting system having a further inductor coupled between one of the output pins and the input of the load.

[0006] The LED string can be understood as an LED load.

[0007] A major problem with two LED strings in parallel, at least one of which is coupled to a series switch, is that a voltage step is generated across the output capacitor of the switched-mode power supply when the first switch is opened or closed. This is because the output capacitor is connected to the LED strings with different string voltages at the output of the switched-mode power supply. This voltage step causes a current change from one string to another together with a current step, which negatively affects the light output and results in color spreading. This problem is solved by placing an inductor at the output of the switched-mode power converter, between the output and the LED load. This inductor allows the switched-mode power converter to act as a real current source, so it is not susceptible to sudden voltage changes. The capacitive behavior of the switched-mode power supply towards the load is reduced or counteracted. Sudden changes in load voltage do not affect the constant current behavior of the inductor. Therefore, no current step is generated that negatively affects the light output.

[0008] In a further embodiment of the present invention, the load comprises a second switch coupled in series with the second LED load.

[0009] This allows for improved control of both LED strings, as they can be controlled independently.

[0010] In another example of the present invention, the first switch is turned on in a complementary manner to the second switch.

[0011] Turning on the first switch in a complementary manner with the second switch means that at least one switch is on at any one time so that current flow from the driver to the load is always guaranteed.

[0012] In another example of the invention, the lighting system includes only one switched mode power supply.

[0013] The lighting system as described is optimized to operate with only one switched mode power supply.

[0014] In another example of the invention, the first switch is controlled using pulse width modulation.

[0015] Using pulse width modulation to control the first switches provides an efficient way to control the light output in the first string.

[0016] In another example of the invention, the switched mode power supply is configured to provide a constant current.

[0017] In another example of the invention, the switched mode power supply is configured as any one of a buck converter, a boost converter, a buck-boost converter, a flyback converter, an LLC converter, a SEPIC converter, and a Cuk converter.

[0018] In another example of the present invention, the load includes a third LED load and a third switch coupled in series with the third LED load.

[0019] Providing a third string in parallel with another load provides more control if color control needs to be implemented, in which case the three strings of LEDs might include red, green and blue LEDs.

[0020] In another example of the present invention, the light colors of the first LED load, the second LED load and the third LED load are different from each other.

[0021] In another example, the capacitor is a buffer capacitor for storing energy to drive the load.

[0022] The larger the capacitance at the output of the switched mode power supply, the more severe the effect of the current step, therefore when the capacitance is larger a parallel inductor provides better results than when the capacitance is relatively small.

[0023] In another example, the first switch includes an anti-parallel diode.

[0024] An example of a switch with an anti-parallel diode is a MOSFET, which has a low resistance path when turned on and is therefore preferred for use in the circuit.

[0025] In another example, the parallel configuration comprises: - a first diode coupled in series with the first LED load; - a second diode coupled in series with the second LED load; - a first capacitor coupled in parallel with the first LED load; and - a second capacitor coupled in parallel with the second LED load;

[0026] Placing a capacitor in parallel with the LED load reduces the current ripple through the LED load. To prevent unwanted discharge across the capacitor, an additional diode needs to be placed in series with the parallel arrangement of the LED load and the capacitor. [Brief description of the drawings]

[0027] Examples of the invention will now be described with reference to the accompanying drawings, in which: [Figure 1] 1 shows an example of a known lighting system. [Figure 2A] 4 shows a graphical representation of color shift. [Figure 2B] 4 shows a graphical representation of color shift. [Figure 2C] 4 shows a graphical representation of color shift. [Figure 2D] 4 shows a graphical representation of color shift. [Diagram 3] 1 illustrates an embodiment of the present invention. [Figure 4A] 1 shows a graphical representation of the effect of the present invention. [Figure 4B] 1 shows a graphical representation of the effect of the present invention. [Figure 4C] 1 shows a graphical representation of the effect of the present invention. [Figure 4D] 1 shows a graphical representation of the effect of the present invention. [Diagram 5] 1 shows an embodiment of the present invention with three LED strings. [Figure 6] 1 illustrates an embodiment of the present invention with improved ripple reduction. [Figure 7] 1 illustrates an example of operation of a lighting system in discontinuous mode. [Figure 8] 1 illustrates an example of an embodiment of the present invention in which compensation is provided for operating in discontinuous mode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention will be described with reference to the drawings.

[0029] 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.

[0030] FIG. 1 shows a lighting system as known in the art. The lighting system has a rectifier circuit for rectifying an AC input voltage into a rectified output voltage. This output voltage is fed to a switched mode power supply that provides a controlled output voltage and / or current. The output of the switched mode power supply is connected to an LED load. The LED load comprises a first string of LEDs and is coupled in series with a first switch. Another LED load is connected in series with a second switch, and this series arrangement is connected in parallel with the first LED load and the first switch. In this case, the forward voltages of both LED strings are different from each other. If only the first switch is closed, the current flows through the first switch and the corresponding LED load. If the second switch is closed and the first switch is open, the current flows through the second LED load. However, due to the difference in string voltages, the current to the load changes rapidly. If the forward voltage of the first LED string is higher than the forward voltage of the second string, a voltage that is too large for the second LED string is fed to the second string of LEDs. This results in an undesirable overcurrent through the second LED string. If the first forward voltage of the first LED string is lower than the forward voltage of the second LED string, the voltage at the output of the switched-mode power supply must increase before the second LED string begins to draw current, resulting in a period during which the second LED string remains off when it should be drawing current.

[0031] Both the increase in current and the periods when no current is drawn result in a spread of color or a change in the desired output color.

[0032] Fig. 2A shows a graph of the desired current through the first and second LED strings in a situation where the present invention is not implemented. In Fig. 2B, the LED current is shown in a situation where the forward voltage of the first LED string is equal to the forward voltage of the second LED string. The current through the first and second LED strings follows the desired current very closely.

[0033] 2C and 2D show examples of the first and second LED string currents when the forward voltages are not equal to each other. It can be observed that the currents through the first and second LED strings change relative to the desired current. This effect is especially noticeable when dimming of the light output is applied. This change in current results in a color shift perceived by the user, which is undesirable.

[0034] Fig. 3 shows an example of a lighting system according to the invention. The lighting system comprises an AC / DC converter 1 in the form of a rectifier bridge with diodes D1, D2, D3 and D4. Via a line wire L and a neutral wire N, an AC voltage is supplied to the AC / DC converter. The AC / DC converter supplies a rectified voltage at its output. The rectified voltage is supplied to a switched-mode power supply. In this example, the switched-mode power supply is designed as a boost converter 2. The boost converter 2 comprises an inductor L1, a diode D5, a capacitor C1 and a switch M1. It should be noted that in this example, a boost converter 2 is used, but other topologies can also be used. Other commonly known topologies are the buck converter, the buck-boost converter, the flyback converter, the LLC (resonant) converter, the SEPIC converter and the Chuk converter.

[0035] The boost converter 2 can provide a controlled output voltage or output current. In the example shown in Fig. 1, the boost converter 2 provides a constant voltage at its output. The capacitor C1 is used to provide an energy buffer at the output of the boost converter 2 to improve the stability of the voltage at the output of the boost converter 2.

[0036] This constant voltage is supplied to a load. The load comprises a first LED string LED1 in series with a switch M2 and a resistor R1. A second LED string LED2 in series with a second switch M3 and a resistor R2 is connected in parallel to the series connection of the first LED string LED1, the resistor R1 and the switch M2. The forward voltage of the first LED string LED1 is different from the forward voltage of the second LED string LED2. The controller 3 provides control signals to the first switch M2 and the second switch M3. In this example, the first switch M2 and the second switch M3 are controlled in a complementary manner. This means that when the first switch M2 is turned on, the second switch M3 is turned off and vice versa. The controller 3 can also control the first switch M2 independently of the second switch M3, as long as the switches are controlled in a non-overlapping manner, i.e., as long as both switches cannot be turned on at the same time.

[0037] To provide the simplest control method, the second switch M3 can be omitted. For correct operation of the load, it is required that the first LED string LED1 has a lower voltage than the second LED string LED2. When the first switch M2 is closed, the LED string with the lowest forward voltage starts to conduct. In this case, the first LED string LED1 conducts. When the switch is open, only the second LED string LED2 can conduct, so that in that case the LED string with the highest string voltage conducts. It is not possible to turn off both LED strings with one switch. This can only be done by turning off the boost converter 2.

[0038] The controller 3 may also provide a control signal to the switch M1.

[0039] The voltage generated by the boost converter 2 needs to be at least as large as the largest forward voltage of the two LED strings so that it can power the LED strings. In that case, the string with the lowest forward voltage will be driven, at least for a short period of time, with the same high voltage as the voltage used to drive the string with the higher forward voltage. This will result in a short-term increase in the current through the LED strings, resulting in an undesirable color spread.

[0040] This problem is solved by introducing an inductor L2 in series between the output of the switched mode power supply and the load.

[0041] Inductor L2 may have a preferred value of 20 μH, although other values ​​are possible based on design preference and power requirements.

[0042] Inductor L2 provides current stabilization because the current in the inductor cannot change quickly due to the instantaneous voltage changes that occur when first switch M2 is turned on or off. Instead, the voltage change across the inductor creates a current slope that depends on the inductance value and the amount of voltage drop across inductor L2.

[0043] If there is a sudden voltage change on the load side due to the switching of either switch M2 or M3, the switched-mode power supply 2 will not see this voltage change. Instead, only a relatively gradual slope of the output current can be observed, rather than a sudden change in the output current. If the switched-mode power supply is a constant output current source, the current can be easily regulated to a constant amount of current due to the gradual current variation provided by inductor L2.

[0044] The introduction of the inductor L2 makes it very easy to control the output voltage or current, so it is preferable to use only one switched mode power supply 2 to power the first LED string LED1 and the second LED string LED2.

[0045] The present invention as described has two LED loads. More LED loads may be connected in parallel with other LED loads along with additional switches.

[0046] Resistors R1 and R2 are used to limit the current through the LED loads LED1 and LED2. If the switched-mode power supply 2 is a constant current source, the resistors can be omitted since the current is limited by the switched-mode power supply 2.

[0047] The first switch M2 and the second switch M3 can be controlled by the controller 3. The first LED string LED1 and the second LED string LED2 can have different color temperatures. Controlling the first switch M2 and the second switch M3 allows control of the overall color temperature generated by the lighting system. An external signal can be provided via a user interface such as a remote control, a mobile device or a control button, allowing the user to select the desired lighting effect to be used by the controller 3. In this example, only the first switch M2 can be used to generate the lighting effect.

[0048] If more than two parallel LED strings are used, desired color effects can be produced, in which case the controller 3 can control all three LED strings.

[0049] Figure 4A shows a graph of the desired current through the first string of LEDs and the second string of LEDs in a situation where the present invention is not implemented. In Figure 4B, the LED current is shown in a situation where the forward voltage of the first string of LEDs is equal to the forward voltage of the second string of LEDs. The current through the first and second strings of LEDs follows the desired current very closely.

[0050] 4C and 4D show examples of the first and second LED string currents when the forward voltages are not equal to each other. It can be observed that the currents through the first and second strings of LEDs remain very close to the desired current levels. Therefore, there is virtually no color shift.

[0051] Figure 5 shows an example of a lighting circuit in which three LED strings are used to provide improved color mixing. An additional LED string LED3 is coupled in series with an additional switch M3. If current limiting is required by the application, a series resistor R3 can be added to limit the current.

[0052] FIG. 6 shows an example of a lighting circuit according to the invention in which further ripple reduction is achieved.

[0053] The main features of FIG. 6 are similar to those of FIG. 3. In parallel with the first LED string LED1, a capacitor C2 is placed. In parallel with the second LED string LED2, a capacitor C3 is placed. These capacitors filter out high frequency components present in the current provided by the switched mode power supply 2. Since the string voltages of the two LED strings are not equal, one capacitor may discharge into another. When switching from an LED string with a high forward voltage to an LED string with a low forward voltage, the capacitor connected to the LED string with the highest string voltage discharges into the capacitor connected to the LED string with the lower string voltage. To prevent this from happening, diodes are placed in series with the parallel arrangement of the LED loads and the corresponding capacitors. A first diode D6 is coupled in series with the parallel combination of the first LED load LED1 and the first capacitor C2. A second diode D7 is coupled in series with the parallel combination of the second LED load LED2 and the second capacitor C3.

[0054] FIG. 7 shows an example mode of operation where the current through inductor L2 is not continuous. This is caused by the boost converter because it is operated such that the current through inductor L2 is discontinuous. This may occur when the load is dimmed such that very low power is required. In another example, the inductance of inductor L2 may be chosen smaller such that the current through inductor L2 is discontinuous. The control signal V of switch M2 bypass is shown in the top graph of FIG. 7. This control signal is inverted and provided to switch M3. In this example, the first LED string LED1 is assumed to have a lower forward voltage than the second LED string LED2. The voltage of the LED strings is present on the right side of inductor L2. The voltage on the left side is based on the total amount of power provided by the switched-mode power supply 2. Since the switched-mode power supply 2 provides a constant current, the voltage on the left side of inductor L2 is equal to the total power provided divided by the current provided by the switched-mode power supply 2. This voltage level is located between the forward voltage level of the first LED string and the forward voltage level of the second LED string. When the first LED string LED1 is turned on, a positive voltage is present across inductor L2, resulting in an increase in the current through inductor L2, i.e., a positive di / dt. When the second LED string LED2 is turned on, a negative voltage is present across inductor L2, resulting in a decrease in the current through inductor L2, i.e., a negative di / dt. The current through inductor L2 is the current I L27, a fifty / fifty relation between the light output of the first LED string LED1 and the light output of the second LED string LED2 is desired. However, due to the discontinuous current through the inductor L2, this relation is disturbed as the first LED string LED1 emits more light than the second LED string LED2. In the bottom graph, the continuous line represents the current ILED1, which is the current through the first string LED1. The dashed line represents the current ILED2, which is the current through the second string LED2.

[0055] FIG. 8 shows how the relationship of light output between the first LED string LED1 and the second LED string LED2 can be corrected. As in FIG. 7, the control signal Vbypass for switch M2 is shown in the top graph. The duty cycle of switch M2 is lowered. This results in a shorter period during which the current is allowed to increase in inductor L2. This also results in a longer period during which the current through inductor L2 is zero, since the current returns to zero sooner. This allows the voltage on the left side of inductor L2 to increase across capacitor C1, since the current provided by the switched mode power supply now flows only through capacitor C1, instead of through inductor L2. Due to this increase in voltage,

number

[0056] In this example, a 50 / 50 relationship is preferred, however, it will be apparent that other relationships may also be used, such as, but not limited to, a 30 / 70 or 20 / 80 relationship.

[0057] In the example shown, the first LED string LED1 and the second LED string LED2 each have a switch coupled in series. For the invention to function, it is sufficient that only one of the LED strings has a switch coupled in series. To further improve the color controllability, additional switches may be introduced in the other LED string.

[0058] In the example shown, the first LED string LED1 may have a lower total forward voltage than the second LED string LED2.

[0059] To simplify control in situations where each string of LEDs has a switch coupled in series, the switches can be controlled in a complementary manner, meaning that when one switch is turned on, the other switch is turned off and vice versa.

[0060] This may be even more beneficial in situations where the switched mode power supply 2 is configured to supply a constant current, in which case one of the switches is always turned on so that the current always has a current path.

Claims

1. The lighting system is A load, an input for receiving a voltage; A parallel configuration, a first LED load having a first forward voltage and a first switch coupled in series with the first LED load; and a parallel arrangement of a second LED load having a second forward voltage different from the first forward voltage, the input being coupled to the parallel arrangement, the lighting system comprising:

1. A switched mode power supply configured to provide a controlled voltage to the load, comprising: Switching elements, Inductor, diode, a set of output pins for coupling to said load; and a switched mode power supply including a capacitor coupled between the pair of output pins; the switched mode power supply is configured to provide the voltage across the set of output pins; The lighting system comprises a further inductor coupled between one of the output pins and the input of the load.

2. A lighting system comprising: A load, an input for receiving a voltage; A parallel configuration, a first LED load having a first forward voltage and a first switch coupled in series with the first LED load; and a parallel arrangement of a second LED load having a second forward voltage different from the first forward voltage, the input being coupled to the parallel arrangement, the lighting system comprising:

1. A switched mode power supply, comprising: Switching elements, Inductor, diode, a set of output pins for coupling to said load; A controller, and a switched mode power supply including a capacitor coupled between the pair of output pins; the switched mode power supply is configured to provide the voltage across the set of output pins; the lighting system having a further inductor coupled between one of the output pins and the input of the load; The switched mode power supply is configured to operate such that the current through the further inductor is discontinuous, the controller comprising: Comparing a current through the first LED load and a current through the second LED load; A lighting system configured to control a duty cycle of the first switch using an error between a current through the first LED load and a current through the second LED load.

3. 3. The lighting system of claim 1 or 2, wherein the load comprises a second switch coupled in series with the second LED load.

4. The lighting system according to claim 3 , wherein the first switch is turned on in a complementary manner to the second switch.

5. 5. A lighting system according to any one of the preceding claims, wherein the lighting system comprises only one switched mode power supply.

6. 10. The lighting system of claim 1, wherein the first switch is controlled using pulse width modulation.

7. 7. A lighting system according to any preceding claim, wherein the switched mode power supply is arranged to provide a constant current.

8. 8. The lighting system of claim 1, wherein the switched mode power supply is configured as one of a Buck converter, a boost converter, a buck-boost converter, a flyback converter, an LLC converter, a SEPIC converter and a Cuk converter.

9. 2. The lighting system of claim 1, wherein the load comprises a third LED load and a third switch coupled in series with the third LED load, the third LED load and the third switch coupled in parallel with the load.

10. The lighting system of claim 9 , wherein the first LED load, the second LED load, and the third LED load have light colors different from each other.

11. 11. The lighting system according to claim 1, wherein the capacitor is a buffer capacitor for storing energy for driving the load.

12. 12. A lighting system according to any one of the preceding claims, wherein the first switch comprises an anti-parallel diode.

13. The parallel configuration includes: a first diode coupled in series with the first LED load; a second diode coupled in series with the second LED load; a first capacitor coupled in parallel with the first LED load; and 13. A lighting system according to any preceding claim, comprising a second capacitor coupled in parallel with the second LED load.

14. 14. The lighting system of claim 1, wherein a forward voltage of the first LED load is lower than a forward voltage of the second LED load.

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