Multi-Channel Driver with Switchable Bypass Capacitors - Patent application

Abstract

The present invention relates to a lighting unit comprising a power converter for supplying regulated power to a first LED load, the first LED load including at least one LED, the first LED load having a power converter coupled to the power converter and a series combination of a first capacitor and a first switch coupled in series with each other and coupled in parallel with the first LED load, the power converter being configured to operate in a first mode to supply a first amount of the regulated power to the first LED load, the first switch being closed, and in a second mode to supply a second amount of the regulated power to the first LED load, the second amount of the regulated power to the first LED load being less than the first amount of the regulated power to the first LED load, the first switch being open.

Description

[Technical field]

[0001] The present invention relates to a lighting unit, in particular a lighting unit offering improved dimming with reduced color point shift. [Background technology]

[0002] Nowadays, it is common practice to use light emitting diodes (LEDs) as light sources for general illumination. Currently, there are many different implementations in which LEDs are used to provide the desired light color and light intensity. It is desirable that the light intensity of an LED can be controlled to vary from a maximum light output to the lowest possible light output.

[0003] A common way to reduce the light output of an LED is to reduce the average current through the LED by modulating the current through it with pulse width modulation (PWM) or by reducing the amplitude of the current, called amplitude modulation. The disadvantage of using amplitude modulation is that at deep dimming, i.e. below about 10% of the maximum LED current, a color shift can occur in the color of the light produced by the LED. In some cases, at 5% light output, the color temperature can change by 450K. At higher light output intensities, this color shift is less severe. If a single LED string is used that emits light with a single color temperature, this color temperature will change when the intensity is reduced and amplitude modulation is applied.

[0004] A goal of the present invention is to provide an LED light source that has a more stable color temperature over the dimming range of the light source. Summary of the Invention [Problem to be solved by the invention]

[0005] It is an object of the present invention to provide a lighting unit that provides improved color consistency when the light output is dimmed, such that no change in color or color temperature is observed, or is less observed, by a user when the light output is reduced. [Means for solving the problem]

[0006] To solve this problem, in a first aspect of the present invention, a power supply for providing regulated power to a first LED load; - a first LED load including at least one LED, the first LED load being coupled to the power supply; a series combination of a first capacitor and a first switch coupled in series with each other, the series combination being coupled in parallel with the first LED load, The power source a first mode for providing a first amount of the regulated power to the first LED load, the first mode in which the first switch is closed; and configured to operate in a second mode of providing a second amount of the regulated power to the first LED load, the second amount of the regulated power to the first LED load being less than the first amount of the regulated power to the first LED load, and the first switch being open; A lighting unit is provided, wherein the power converter is configured to transition from the second mode to the first mode by controlling the switch in a linear mode of operation.

[0007] The lighting unit comprises an LED load, connected to a power supply that supplies power to the LED load, in parallel with the LED load is a capacitor for smoothing the current through the LED load.

[0008] It is the inventor's insight that color or color temperature shifts most from the desired color or color temperature, especially during deeper dimming of the light output. The color temperature or color temperature shift is further referred to as color point shift. It is desirable to provide PWM of the LED current at dimming levels to provide the lowest possible color point shift. This can be best done if there is no capacitance in parallel with the LED load, since a capacitor reduces the modulation depth of the PWM current. At deeper dimming levels, where full or significant PWM of the LED load current is desired, the present invention disconnects the capacitor by opening a switch in series with the capacitor. The power through the first LED load is measured or determined, and based on the power exceeding a threshold, it is determined whether to open or close the switch in series with the capacitor. The first switch is opened when the power to the first LED load is below a first threshold, e.g. indicating that dimming is occurring, and the first switch is closed when the power to the first LED load is above the first threshold, indicating that no dimming is occurring or that only a small amount of dimming is occurring. When the power converter transitions from the second mode to the first mode, the first LED load is shunted by the first capacitor, which is (partially) discharged. The first LED load does not emit light until the first capacitor is fully charged. This results in a dark period during the transition from the second mode to the first mode. This may be observed by a user as flickering, which is undesirable. To solve this problem, the present invention provides an improved control of the first switch. The first switch is operated in its linear operation mode during the transition from the second mode to the first mode. This prevents all the current provided by the power converter from flowing through the capacitor and allows a portion of the current to flow through the first LED load, thereby reducing or even eliminating the dark period.

[0009] When no dimming or a limited amount of dimming is implemented, it may be desirable to leave the capacitor in parallel with the LED so that the current through the LED load is filtered and smoothed. This can be beneficial in environments where high frequency flicker, such as the PWM frequency, needs to be avoided at high light output, for example when used near a barcode scanner.

[0010] In a further example, the lighting unit comprises: - a second switch for regulating a current through the first LED load, the first LED load being coupled in series with the power source and the second switch; - a second LED load including at least one LED, the power supply being configured to supply the regulated power to the second LED load; - a series combination of a second capacitor and a third switch coupled in series with each other, the series combination being coupled in parallel with the second LED load; - a fourth switch for regulating a current through the second LED load, the second LED load further comprising a fourth switch coupled in series with the power source and the fourth switch; The power supply is adapted to provide regulated power to the second LED load, the power supply comprising: a third mode of providing a first amount of the regulated power to the second LED load, the third mode being such that the third switch is closed; and and configured to operate in a fourth mode of supplying a second amount of the regulated power to the second LED load, the second amount of the regulated power to the second LED load being less than the first amount of the regulated power to the second LED load, and the third switch being open.

[0011] Two LED strings in parallel, with their own respective parallel capacitors, also suffer from color point shifts over the dimming range. In this example, each LED string has its own respective capacitor and its respective switch in series with the corresponding capacitor. When the power to the second LED load is below a second threshold, indicating, for example, that dimming is occurring, the third switch is opened, and when the power to the second LED load is above the second threshold, indicating that no dimming or little dimming is occurring, the third switch is closed. In this example, the first and second LED loads can independently decide whether to open or close their respective first and fourth switches. This can be very interesting if the control of the first switch is close to the first LED load and the control of the fourth switch is close to the second LED load. This can result in the control of the switch, the capacitor, the switch and the LED load for either the first or second LED load being grouped together, for example on the same printed circuit board, or at least in close proximity.

[0012] The use of the second switch and the fourth switch allows independent current control of the current through each LED load by modulating the current through the LED load.

[0013] Another possibility is that both the first switch and the third switch are controlled by a single controller, which may result in the first switch and the third switch being turned on and off simultaneously.

[0014] In a further example, the lighting unit further comprises a first unidirectional element, preferably a second unidirectional element, the first unidirectional element configured to supply the regulated power to the first LED load and the first capacitor, and the second unidirectional element configured to supply the regulated power to the second LED load and the second capacitor.

[0015] Using the first unidirectional element to provide the regulated power to the first LED load and the first capacitor allows for different string voltages between the first LED load and the second LED load, and without the first unidirectional element, for example, undesirable current may flow from the first capacitance coupled to the first LED load, which has a higher string voltage, to the second LED load, which has a lower string voltage.

[0016] In a further example, the power source is a current source configured to provide a regulated current.

[0017] Preferably, the power supply is a current source since the light output of the LEDs in the LED load is directly related to the amount of current flowing through the LEDs. The current through the first LED load and the second LED load can be used to determine if the first switch and / or the second switch needs to be opened or closed. The current through the first LED load and the second LED load can represent the power provided by the power supply to the first LED load and the second LED load.

[0018] In a further example, the lighting unit further comprises a fifth switch coupled between the output of the current source and a ground reference such that no regulated current is provided to the first LED load.

[0019] The addition of the fifth switch can enable deeper dimming of the light output of the first and second LED loads. The fifth switch can shunt the current provided by the current source away from the first and second LED loads.

[0020] In a further example, the first LED load comprises a red LED.

[0021] In a further example, the first LED load comprises a phosphor-converted LED.

[0022] In a further example, the second LED load comprises a phosphor-converted LED.

[0023] The first and second LED loads may comprise several types of LED loads: the examples shown are red, amber, green or blue LEDs, but also phosphor-converted LEDs to create a warm or cold white effect.

[0024] In a further example, a string voltage of the first LED load is greater than a string voltage of the second LED load.

[0025] In a lighting circuit according to the invention it is possible to have the first string voltage greater than the second string voltage, allowing greater design freedom when designing the LED load.

[0026] In a further example, the lighting unit comprises a controller for controlling the first switch and the second switch.

[0027] A single controller can be used to control the first and second switches. The controller can also be used to control the third, fourth and fifth switches. It is also possible that the controller comprises several building blocks, each building block being used to control a single switch. This allows for local integration of the building blocks near the corresponding switch.

[0028] In a further example, the controller is configured to provide dimming of the light output of the first LED load, and the controller is configured to control the second switch to modulate a current through the LED load.

[0029] The controller may be used to receive a dimming signal that enables the lighting unit to implement the dimming function.

[0030] In a further example, the controller is further configured to control the third switch and the fourth switch.

[0031] In a further example, the controller is configured to provide dimming of the light output of the second LED load, and the controller is configured to control the third switch to modulate a current through the LED load.

[0032] As well as providing dimming functionality to a single LED load, the controller can also be used to receive a dimming signal that enables the lighting unit to implement the dimming functionality for both the first LED load and the second LED load.

[0033] In a further example, the power supply is adapted to provide a maximum output power and the power supply is configured to operate in the second mode when the regulated power is less than 25% of the maximum output power, preferably less than 10% of the maximum output power.

[0034] Below 25% of the maximum output power of the power supply, a threshold may be reached indicating that the power supply should operate in the second mode, which allows the capacitor in the LED load to be disconnected by opening the first and / or the fourth switch.

[0035] Preferably, this threshold is less than 10% of the maximum output power of the power supply, but could be in the range of 10% to 25%, such as 15%, 20%, of the maximum output power of the power supply.

[0036] In a further example, the first switch comprises a semiconductor switch.

[0037] Preferably, the first switch is a semiconductor switch, such as a bipolar transistor or a metal oxide field effect transistor (MOSFET). The second, third, fourth and fifth switches may also be semiconductor switches. [Brief description of the drawings]

[0038] 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 an example of a current implementation of a lighting unit. [Diagram 2] 1 shows an embodiment of a lighting unit according to the invention; [Diagram 3] 1 shows an embodiment of a lighting unit according to the invention; [Figure 4] 4 shows a further refinement of an embodiment of a lighting unit according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] The present invention will be described with reference to the drawings.

[0040] 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 also 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.

[0041] FIG. 1 shows an example of a lighting unit as currently implemented. A power supply 1 is used to convert a primary power source into regulated power for the load. The primary power source can be, for example, a normal mains power source operating at 230V 50Hz, or 120V 60Hz. In another example, the primary power source can be a DC power source such as a battery or a DC grid. The regulated power to the load is provided to an LED load LED1 and a capacitor C1. A capacitor C1 is provided in parallel with the LED load LED1 to filter and remove any ripple current that would otherwise flow through the LED load LED1. A current flows through the LED load LED1, which results in a certain light output. To reduce the light output, the current through the LED load LED1 needs to be reduced. This can be done by lowering the amplitude of the current generated by the power converter 1. The amplitude of the current can be lowered in at least two ways, although more ways are possible. The first way is to lower the amplitude of the current by amplitude modulation. In that case, the amplitude of the current is gradually reduced until the desired current amount is reached. This amplitude modulation has a significant disadvantage. The lower the current level, the greater the color point shift of the light output by the LED load LED1. The color temperature of the light output is determined primarily by the type of LED used. Changing the amplitude of the current results in a color point shift. Using pulse width modulation on the current through the LED load LED1 results in a constant amplitude of the LED current, but the pulse width determines the average current supplied to the LED load LED1. It is therefore preferable to use a pulse width modulated current for the LED load LED1 to achieve dimming of the light output. The use of the term pulse width modulation is intended in its broadest sense, such that the on-off ratio of the pulses over time determines the average LED current, and does not necessarily have to have, for example, a fixed frequency pulse width modulation. As a result, pulse width modulation is intended herein to include, for example, pulse frequency modulation, pulse duration modulation, pulse position modulation, binary code modulation.In general, pulse width modulation includes any method of dimming the LED current with reduced color shift compared to amplitude dimming of the LED current. A pulse width modulated current can be generated by a power converter 1. This pulse width modulated current is fed to a capacitor C1 and an LED load LED1. The capacitor C1 is used to filter and remove ripples in the current going to the LED load LED1 so that a stable LED current is generated. This is beneficial in situations where flickering should be prevented. Cameras and barcode scanners may react to light with a flicker frequency generated by pulse width modulation. When dimming is implemented, the capacitor C1 makes the pulse width modulated current behave more like amplitude modulation, resulting in a shift in the color point of the light emitted by the LED load LED1. An improved lighting unit is desired that can provide less flicker to provide good operation where cameras or barcode scanners are used, and can provide reduced color shift when the light output is dimmed.

[0042] FIG. 2 shows an example of an embodiment according to the present invention. The power converter 1 supplies power to the LED load LED1 and the capacitor C1. In series with the capacitor C1 is a first switch SW1. The series combination of the capacitor C1 and the first switch SW1 is coupled in parallel with the LED load LED1. The power converter 1 is configured to provide dimming of the light output of the LED load LED1 by lowering the current to the LED load LED1. In a first operating mode of the power converter 1, the power converter 1 supplies a pulse width modulated current to the LED load LED1 and the capacitor C1. In this first operating mode, the first switch SW1 is closed. The first operating mode is, for example, an operating mode in which the power converter 1 supplies power to the LED load LED1 such that the light output of the LED load LED1 is in a range between maximum light output and 25% light output.

[0043] When the light output is reduced, for example below 25%, the power converter 1 can operate in a second operating mode. In this second operating mode, the current to the LED load LED1 is further reduced. In this second operating mode, the first switch SW1 is opened so that the capacitor C1 cannot receive any power provided by the power converter 1. The filtering and stabilization of the current through the LED load LED1, which is performed by the capacitor C1 in the first operating mode, is not performed since in the second operating mode the capacitor C1 cannot filter and stabilize the current through the LED load LED1. Instead, the pulse-width modulated current generated by the power converter 1 is directly supplied to the LED load LED1, which leads to flickering in the light output. Since the light output is dimmed, the effect of the light flickering is no longer significant, for example for a camera or a barcode scanner.

[0044] The pulse width modulated current can maintain a constant amplitude over the entire dimming range, and the average current through the LED load LED1 can be controlled by controlling the duty cycle of the current. In other examples, the amplitude of the current may be varied over the dimming range in conjunction with controlling the duty cycle. This combination of amplitude modulation and pulse width modulation still provides reduced color point shift.

[0045] The first switch SW1 may be controlled by a controller 2 that determines whether the power converter 1 operates in a first mode of operation or a second mode of operation. If the controller 2 determines that the power converter 1 operates in the first mode of operation, the controller 2 closes the first switch SW1. If the controller 2 determines that the power converter 1 operates in the second mode of operation, the controller 2 closes the first switch SW1.

[0046] In figure 3 another example of a lighting unit according to the invention is provided. The lighting unit comprises a power converter 1 supplying power to a first load and to a second load. The first load may be a first LED load LED1 and the second load may be a second LED load LED2.

[0047] Further, the power converter 1 may supply power to a first capacitor C1 and a second capacitor C2. In series with the first capacitor C1 is a first switch SW1. The series combination of the first capacitor C1 and the first switch SW1 is coupled in parallel with the first LED load LED1. The power converter 1 is configured to supply stable power to the first LED load LED1 and the second LED load LED2. It is desirable to regulate the current through the first LED load LED1 and the current through the second LED load LED2 independently of each other. The second switch SW2 is used to regulate the current through the first LED load LED1. The second switch SW2 is arranged in series with the power converter 1 and the first LED load LED1.

[0048] In series with the second capacitor C2 is a third switch SW3. The series combination of the second capacitor C2 and the third switch SW3 is coupled in parallel with the second LED load LED2. The fourth switch SW4 is used to regulate the current through the second LED load LED2. The fourth switch SW4 is disposed in series with the power converter 1 and the second LED load LED2.

[0049] Preferably, the current regulation provided by the second switch SW2 and the fourth switch SW4 is achieved by controlling the switches such that the current regulation is achieved by pulse width modulating the current through the LED load.

[0050] Again, the power converter can operate in a first and a second operating mode. In the first operating mode of the power converter 1, the power converter 1 supplies current to the first LED load LED1 and the first capacitor C1. In this first operating mode, the first switch SW1 is closed.

[0051] The first operating mode is, for example, an operating mode in which the power converter 1 supplies power to the first LED load LED1 such that the light output of the first LED load LED1 is within a range between maximum light output and 25% light output, preferably 10% light output.

[0052] When the light output of the first LED load LED1 is reduced, for example to less than 25%, the power converter 1 may operate in a second operating mode, in which the current to the first LED load LED1 is further reduced, and in which the first switch SW1 is opened so that the first capacitor C1 cannot receive any power provided by the power converter 1.

[0053] The power converter 1 is further adapted to operate in a third and a fourth mode of operation.

[0054] In a third operating mode of the power converter 1, the power converter 1 supplies current to the second LED load LED2 and the second capacitor C2. In this third operating mode, the third switch is closed.

[0055] The third operating mode is, for example, an operating mode in which the power converter 1 supplies power to the second LED load LED2 such that the light output of the second LED load LED2 is within a range between maximum light output and 25% light output, preferably 10% light output.

[0056] When the light output of the second LED load LED2 is reduced, for example to less than 25%, the power converter 1 may operate in a fourth mode of operation, in which the current to the second LED load LED2 is further reduced, and in which the third switch SW3 is opened so that the second capacitor C2 cannot receive any power provided by the power converter 1.

[0057] In the illustrated example, the power to the first LED load LED1 and the second LED load LED2 is controlled in four operating modes, which allow, for example, to control only the first switch SW1 or the third switch SW3. The operating modes may be combined such that the first switch SW1 and the third switch SW3 are simultaneously opened or closed. As an example, the light output of both the first LED load LED1 and the second LED load LED2 may be reduced such that the power converter 1 operates in the second and fourth operating modes. In this example, the first switch SW1 and the third switch SW3 are simultaneously opened.

[0058] The first controller 2 may be used to control the first switch SW1 and the second switch SW2. The second controller 3 may be used to control the third switch SW3 and the fourth switch SW4. The first controller 2 may be located near the first switch SW1 and the second switch SW2. The second controller 3 may be located near the third switch SW3 and the fourth switch SW4. This may allow the integration of the switches and the controllers into an integrated circuit.

[0059] In another example, a single controller 2 can be used to control the first switch, the second switch, the third switch and the fourth switch. The single controller 2 can also be used to control the power converter 1. The single controller 2 can allow easier control of the switches and the power converter 1 since all control parameters are sent to a single control unit.

[0060] The first controller 2, the second controller 3 or the single controller 2 may be, but is not limited to, a microcontroller, a Field Programmable Gate Array (FPGA), or a discrete control solution.

[0061] Preferably, the lighting unit comprises a first unidirectional element D1 that allows regulated power from the power converter 1 to be supplied to the first LED load LED1. This first unidirectional element D1 is arranged between the output of the power converter 1 and the input of the first LED load LED1. The input of the first LED load LED1 may be the anode of the LED.

[0062] Preferably, the string voltage of the first LED load LED1 is greater than the string voltage of the second LED load LED2. The first unidirectional element D1 prevents current from flowing from the first capacitor C1 to the second LED load LED2, which would otherwise flow due to the voltage difference.

[0063] Preferably, the lighting unit also has a second unidirectional element D2 that allows the regulated power from the power converter 1 to be supplied to the second LED load LED2. This second unidirectional element D2 is arranged between the output of the power converter 1 and the input of the second LED load LED2. The input of the second LED load LED2 may be the anode of an LED. The function of the second unidirectional element D2 is similar to that of the unidirectional element D1, but is combined with the second LED load LED2 instead of the first LED load LED1.

[0064] The use of first and second unidirectional elements allows the user to mount any type of LED load without having to take into account differences in string voltages.

[0065] Although the first unidirectional element D1 and the second unidirectional element D2 are shown as diodes, MOSFETs could also be used, which block current in a reverse biased situation and allow current to flow in a forward biased situation, which may reduce losses in the unidirectional elements.

[0066] Preferably, a unidirectional element is used when any of the first switch, the second switch, the third switch or the fourth switch is a MOSFET, which has an inherent body diode that can cause undesired current to flow, for example, from the first capacitor C1 to the second LED load LED2.

[0067] Figure 4 shows an improvement of the example lighting unit shown in Figure 3. Features with the same reference numbers as in Figure 3 are identical. An additional fifth switch SW5 is provided. This fifth switch SW5 is arranged between the output of the power converter 1 and a return path, e.g., a ground reference. The fifth switch SW5 essentially shunts the first LED load LED1 and the second LED load LED2 such that no current flows from the power converter 1 to the first LED load LED1 and the second LED load LED2 when the fifth switch SW5 is closed.

[0068] In the illustrated example, the LED load LED1 is shown and described as a single LED, but it should be understood that multiple LEDs can be used as the LED load LED1. The LEDs can be coupled in various configurations, such as in a series or parallel configuration, or a combination thereof.

[0069] In the example shown, the second switch SW2 and the fourth switch SW4 are located below the LED loads. It should be understood that the second switch SW2 and the fourth switch SW4 can be located above the LED loads so that the current through the first LED load LED1 and the second LED load LED2 can be adjusted. The shown position of the second switch SW2 and the fourth switch SW4 is a preferred position because in this position the switches can be easily controlled.

[0070] In the example shown, any of the first switch, second switch, third switch and fourth switch may be, but is not limited to, a semiconductor switch, such as a MOSFET or a bipolar transistor, or an electromechanical switch.

[0071] In the example shown, the power converter 1 may be a current source or a voltage source. A current source is a power converter 1 that regulates the current at the output of the power converter 1. A current source tries to keep the generated current at a constant level, and the voltage at the output of the current source is a result of the impedance of the load and the amount of regulated current. A voltage source is a power converter 1 that regulates the voltage at the output of the power converter 1. A voltage source tries to keep the generated voltage at a constant level, and the current at the output of the voltage source is a result of the impedance of the load and the amount of regulated voltage.

[0072] In the example shown, the power converter 1 is preferably configured to transition from the second mode to the first mode by controlling the first switch SW1 in a linear mode of operation.

[0073] It may also be preferred that the power converter 1 is configured to transition from the second mode to the first mode by controlling the third switch SW3 in a linear mode of operation.

[0074] In the illustrated example, the first controller 2, the second controller 3 or the single controller 2 may be used to control the second switch SW2 to provide a modulation of the current through the first LED load LED1 to provide a dimming function of the light output of the first LED load LED1. Additionally or alternatively, the first controller 2, the second controller 3 or the single controller 2 may be used to control the fourth switch SW4 to provide a modulation of the current through the second LED load LED2 to provide a dimming function of the light output of the second LED load LED2.

[0075] Those skilled in the art can understand and achieve other 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 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. Any reference signs in the claims should not be interpreted as limiting the scope.

Claims

1. a power converter for providing regulated power to the first LED load; the first LED load including at least one LED, the first LED load coupled to the power converter; a series combination of a first capacitor and a first switch coupled in series with each other, the series combination coupled in parallel with the first LED load; The power converter, a first mode for supplying a first amount of the regulated power to the first LED load, the first mode being such that the first switch is closed; and configured to operate in a second mode of providing a second amount of the regulated power to the first LED load, the second amount of the regulated power to the first LED load being less than the first amount of the regulated power to the first LED load, and the first switch being open; The lighting unit, wherein the power converter is configured to transition from the second mode to the first mode by controlling the first switch in a linear mode of operation.

2. a second switch for regulating a current through the first LED load, the first LED load being coupled in series with the power converter and the second switch; a second LED load including at least one LED, the power converter being configured to supply the regulated power to the second LED load; a series combination of a second capacitor and a third switch coupled in series with each other, the series combination being coupled in parallel with the second LED load; a fourth switch for regulating a current through the second LED load, the second LED load further comprising a fourth switch coupled in series with the power converter and the fourth switch; The power converter is adapted to provide regulated power to the second LED load, the power converter comprising: a third mode of supplying a first amount of the regulated power to the second LED load, the third mode being such that the third switch is closed; and 2. The lighting unit of claim 1, configured to operate in a fourth mode of supplying a second amount of the regulated power to the second LED load, the second amount of the regulated power to the second LED load being less than the first amount of the regulated power to the second LED load, and the third switch being open.

3. 3. The lighting unit of claim 2, further comprising a first unidirectional element, preferably a second unidirectional element, the first unidirectional element configured to supply the regulated power to the first LED load and the first capacitor, and the second unidirectional element configured to supply the regulated power to the second LED load and the second capacitor.

4. 10. The lighting unit of claim 1, wherein the power converter is a current source configured to provide a regulated current.

5. 5. The lighting unit of claim 4, further comprising a fifth switch coupled between the output of the current source and a ground reference such that no regulated current is provided to the first LED load.

6. 10. The lighting unit of claim 1, wherein the first LED load comprises a red or amber LED.

7. The lighting unit of claim 1 , wherein the first LED load comprises a phosphor-converted LED.

8. The lighting unit of claim 2 , wherein the second LED load comprises a phosphor-converted LED.

9. The lighting unit of claim 2 , wherein a string voltage of the first LED load is greater than a string voltage of the second LED load.

10. 3. The lighting unit of claim 2, further comprising a controller for controlling the first switch and the second switch.

11. 11. The lighting unit of claim 10, wherein the controller is configured to provide a dimming function for a light output of the first LED load, the controller being configured to control the second switch to modulate a current through the LED load.

12. 11. The lighting unit of claim 10 when dependent on claim 2, wherein the controller is further configured to control the third switch and the fourth switch.

13. 13. The lighting unit of claim 12, wherein the controller is configured to provide a dimming function for a light output of the second LED load, the controller being configured to control the fourth switch to modulate a current through the LED load.

14. 2. The lighting unit of claim 1, wherein the power converter is adapted to provide a maximum output power and wherein the power converter is configured to operate in the second mode when the regulated power is lower than 25% of the maximum output power, preferably lower than 10% of the maximum output power.

15. 15. A lighting unit according to any one of the preceding claims, wherein the first switch comprises a semiconductor switch.

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