Patent Grant
9629212
The present invention relates to lighting systems using Light Emitting Diodes.
At present, in architectural and entertainment lighting applications more and more solid state lighting based on Light Emitting Diodes (LED) is used. LED's or LED units have several advantages over incandescent lighting, such as higher power to light conversion efficiency, faster and more precise lighting intensity and color control. In order to achieve this precise control of intensity and color from very dim to very bright light output, it is necessary to have accurate control of the forward current flowing through the LED's.
In order to provide said forward current through the LED or LED's, a converter (or a regulator such as a linear regulator) can be used. Examples of such converters are Buck, Boost or Buck-Boost converters. Such converters are also referred to as switch mode power sources. Such power sources enable the provision of a substantially constant current to the LED unit. When such a LED unit comprises LED's of different color, the resulting color provided by the LED unit can be modified by changing the intensity of the different LED's of the unit. This is, in general, done by changing the duty cycles of the different LED's. Operating the LED's at a duty cycle less than 100%, can be achieved by selectively (over time) providing a current to the LED's, i.e. providing the LED's with current pulses rather than with a continuous current.
As more and more conventional lighting systems such as halogen lighting or light bulbs are replaced by lighting systems using Light Emitting Diodes, it is important to operate such a lighting system efficiently in order to minimize the power consumption associated with it. In general, a lighting system is applied to operate over a range of illumination (or lighting) conditions (e.g. the brightness of lighting system may be set within a certain range). By merely considering the efficiency of the lighting system at e.g. a nominal operating point rather than over the entire operating range or part of the operating range, the power losses of known lighting systems may be important when operating under certain conditions (e.g. a reduced brightness compared to a nominal brightness).
It is therefore an object of a first aspect of the present invention to improve the efficiency of a lighting system using LED's.
It has been described to drive a plurality of LED's by means of time based modulation techniques, such as pulse width modulation, duty cycle modulation algorithms etc. Thereby, the LED's may be divided in groups, wherein each group of LED's e.g. has its own color of light, each group of LED's being driven by a suitable modulation technique with a certain duty cycle. An example thereof is provided in WO2006107199 A2, wherein LED's or groups of LED's are connected in series, the LED's or groups of LED's each being provided with its own switching device connected in parallel to the group or to each LED. A current source is provided to generate a current through the series connection of LED's or groups of LED's. Closing the parallel switch will bypass the LED or group of LED's so as to switch it off.
At a lower intensity, a change in the intensity by an increase or decrease of the duty cycle becomes relatively larger, the smaller the duty cycle. As an example, assuming a 16 bit duty cycle information, a decrement from FFFF (hexadecimal) to FFFE (hexadecimal) provides percentagewise a small reduction, thus enabling a smooth dimming, while a decrement of for example 0009 to 0008 provides percentagewise a large reduction. This effect may be emphasized by a sensitivity of the human eye, which is commonly assumed to have a logarithmic or similar characteristic. Hence, at low intensity levels and low duty cycles, an increment or decrement in duty cycle will result in a relatively more noticeable change than at large duty cycles. Hence, at low intensities, a possibly less smooth change in intensity can be obtained as compared to more large intensities.
Accordingly, an object of a second aspect of the invention is to provide a higher dimming resolution at lower intensities.
According to a first aspect of the invention, there is provided a control unit for a LED assembly comprising a first and second LED unit, said LED units being serial connected, the LED assembly, in use, being powered by a switched mode power supply, the control unit being arranged to
Within the present invention, a LED unit is understood as comprising one or more light emitting diodes. In case the LED unit comprises more than one light emitting diode, said diodes can either be connected in series or in parallel, or a combination thereof.
A LED assembly is understood as comprising more than one LED unit.
The control unit according to the present invention is arranged to receive an input signal representing a desired characteristic of the LED assembly. Such input signal can e.g. be an analogue signal or a digital signal. Such signal can e.g. be generated by a user interface such as a dimmer or push button. The desired characteristic of the LED assembly can e.g. be defined in any suitable way, e.g. optical or electrical, examples being a desired brightness/intensity or color.
The control unit according to the present invention can be applied to a LED assembly comprising multiple LED units, in particular a LED assembly comprising LED units connected in series. Said serial connection of LED units can e.g. be powered by a switched mode power supply such as a buck converter or a boost converter or any other switching power supply. In use, said power supply can provide a current to the serial connected LED units.
Each of the LED units is individually driven by the control unit, so as to operate the one or more LED's of each unit simultaneously. The control unit according to the present invention is further arranged to determine the required duty cycles of the LED units for obtaining the desired characteristic of the LED assembly, given the nominal current of the power supply. These duty cycles of the LED units can be represented as the percentage or the fraction of time that a current is provided to the LED unit (e.g. 50% or 0.5).
In order to operate at e.g. a reduced brightness, known control units merely reduce the duty cycle of the different LED units of the LED assembly. Thereby, a current level of the switched mode power supply is kept at its nominal level. This may result in a situation were the switched mode power supply, at certain levels of brightness, operates at a relatively low power efficiency. According to the invention, a current (or other relevant output characteristic) of the switched mode power supply is adjusted in such a way that an output current (or other relevant output characteristic) is provided which is adapted to meet the circumstances. As an example, reducing the output power of the LED units according to the state of the art may be achieved by reduction of the duty cycle with which the LED units are driven, while the current is kept at its nominal level. According to the invention however, a value is chosen for the current (or other relevant output characteristic) of the switched mode power supply and for the duty cycle, which results in the desired brightness (or other relevant output characteristic), however, at more power efficient working conditions of the e.g. switched mode power supply and/or other components involved. Due to the serial connection of the LED units, the same current may be applied in order to operate each of the LED units. Therefore, the operating current (or other relevant output characteristic) may be determined, taking into account a value of it as would be required by the different LED units. Thereto, the power supply may be set to such a level so as to provide an output current (or other relevant output characteristic), which has a sufficiently high value in order to be able to drive the LED unit which requires such value. For each of the LED units, a duty cycle is now selected or amended, in order to reflect the changed output current (or other relevant output characteristic) of the switched mode power supply. This may be illustrated by a simple example: Assume that three LED units are driven by the power supply, the LED units being serially connected. Assume that, at nominal operating current of the power supply, a duty cycle for the first, second and third units would be set at 10%, 1% and 1% resp. By reducing the output current of the power supply to e.g. 1/10th of its nominal value, and increasing the duty cycles of the units by a factor 10, the same brightness level would be obtained, thereby operating the power supply at a low current which may achieve a more favourable power efficiency thereof. In general, reducing the current (or other relevant output characteristic) of the power supply by a factor N may be combined with an increase of the duty cycle of each of the units by that same factor. The factor N is determined from the largest one of the duty cycles of the LED units. Reducing the output current (or other relevant output characteristic) of the power supply may be performed stepwise or as a continuous value within a certain operating range. In general, the reduced current will be set so as to keep the duty cycle of the LED unit requiring the largest duty cycle to a value below or equal to 100%. Depending on an implementation, a maximum effect may be achieved by reducing the current such that it substantially corresponds to the nominal current multiplied with the largest duty cycle. Thereby, the LED unit requiring the largest duty cycle is then operated at substantially 100% duty cycle. It is noted that the term duty cycle may refer to a periodic part of any type of time period, e.g. continuous time, time slots, etc. 100% duty cycle may thus be interpreted so as to comprise 100% of continuous time or 100% of any (e.g. repetitive) time slot. It can be noted that the steps as performed by the control unit can be performed in any suitable time order. It is for example possible that the step of determining the reduced current based on the at least largest duty cycle may equally applied when the adjusted duty cycles are already determined, e.g. based on the largest duty cycle.
When the LED assembly and power supply are thus operated based on the reduced current and adjusted duty cycles, rather than based on the nominal current and the duty cycles associated with this current, an improved efficiency can be observed either with the LED units of the LED assembly or with the power supply, as will be detailed further below.
The control unit as applied in the present invention can e.g. comprise a programmable device such as a microprocessor or microcontroller or another processing unit, the programmable device being programmed with suitable program instructions in order to provide the functionality as described in this document. Further solutions are imaginable too, such as analogue hardware or electronic circuits. The output data provided by the control unit for obtaining the desired characteristic can be in any suitable form e.g. as a data stream on a data bus, a data stream in any digital format, as separate signals for the duty cycle and the switched mode power supply, e.g. Pulse Width Modulation, as an analogue voltage level, or as any other information. The output data may comprise single signals or multiple signals. Where in this document signal or signals are applied, this is to be understood as to comprise any form of output data.
According to a second aspect of the invention, there is provided a control unit for a LED assembly comprising a first and second LED unit, said LED units being serial connected, the LED assembly, in use, being powered by a switched mode power supply, the control unit being arranged to
Thereby, in addition to the duty cycle dimming as known from the art, a further mechanism for dimming may be made available. Hence, at low intensities, where the resolution of the duty cycle dimming may set a limit to the obtainable brightness resolution, the power supply current may be reduced allowing a larger duty cycle hence allowing a higher brightness resolution. Furthermore, power efficiency may be increased as described above.
A lighting system comprising a LED assembly that comprises a first and second LED unit and the control unit for controlling the LED assembly may further comprise a feedback circuit to feed a signal representative of the power supply current to a feedback input of the switched mode power supply, the feedback circuit comprising a digital potentiometer, the control unit having a control output connected to the digital potentiometer for controlling the power supply current. By using a (microprocessor controllable) digital potentiometer, e.g. in a feedback circuit of an amplifier, in a resistive level shifter, in a resistive attenuator or otherwise, an accurate, fast, low cost control of the current may be obtained, while enabling a convenient interfacing with the control unit.
The power supply current may further be controlled by controlling the power supply current to a first value in a first part of a cycle time and to a second value in a second part of the cycle time, to thereby obtain an effective power supply current between these values, thereby allowing e.g. a further increase in brightness resolution.
According to a further aspect of the invention, there is provided a circuit for driving a LED assembly comprising at least one LED illumination device, the circuit comprising
a switch,
an inductor, in a series connection with the switch, the switch to in a conductive state thereof charge the inductor,
a current measurement element to measure a current flowing through at least one of the inductor and the LED illumination device,
the switch, inductor and current measurement element being arranged to establish in operation a series connection with the LED illumination device,
the circuit further comprising:
a reference signal generator for generating a reference signal;
a comparator to compare a signal representing the current measured by the current measurement element with the reference signal, an output of the comparator being provided to a driving input of the switch for driving the switch, and
a controller to control an operation of at least one of the reference signal generator and the comparator.
In order to obtain a desired characteristic of a lighting system comprising a LED unit, several variables are available for obtaining this characteristic. As an example, when powered by a switched mode power supply such as a buck converter, the required characteristic can be obtained by providing a current I to the LED unit having a certain duty cycle. In case the duty cycle required to provide the desired characteristic, the desired characteristic may also be obtained by selecting a smaller current, combined with an increased duty cycle. This is illustrated in
Providing a current I with a certain duty cycle to a LED unit can be realised in different ways.
As an example, when a LED unit is e.g. supplied from a buck converter, a certain duty cycle can be realised by switching the converter resulting in a certain ON time and OFF time. The duty cycle can then be defined as the percentage ON time.
Alternatively, a current I with a certain duty cycle can be realised by providing a substantially constant current I by the power supply, e.g. a buck converter, and controlling a switch provided in parallel to the LED unit. When such switch is closed, the current provided by the converter is redirected from the LED unit to the closed switch. A lighting system according to the present invention that enables both methods of providing a current I to a LED unit is schematically depicted in
In order to provide a desired output characteristic of the LED assembly, each of the LED units can be driven at a certain duty cycle. The control unit 400 is arranged to receive an input signal 110 that may represent a desired characteristic (e.g. a certain brightness or color) of the LED assembly. The power supply 300 is known as a buck converter and comprises a switching element 2, an inductance 3 and a diode 4. A controller 6 controls the switching of the switching element 2, e.g. based on a reference input 5 and a feedback of the LED assembly. A voltage over the resistance 90 of the LED assembly can e.g. be applied as a feedback for the actual current 7 provided by the power supply. The control unit 400 can further be arranged to provide an output signal 120 to the power supply 300 for controlling the output of the power supply. Designated by reference number 1 is the supply voltage of the power supply (e.g. 16 or 24 V), designated by reference number 8 is the output voltage of the power supply which substantially corresponds to the sum of the voltages over the multiple LED units, also referred to as the forward voltage over the LED units.
In accordance with the present invention, the control unit 400 is arranged to provide a control signal to the LED assembly. As such, the switches 80 can be controlled and the different LED units can be arranged to operate at a certain duty cycle.
In order to illustrate this,
Assuming that the duty cycles for the LED units as shown in
In accordance with the present invention, it has been observed that it may be advantageous to operate a lighting system by applying a reduced current (compared to the nominal current of the power supply) in combination with increased duty cycles for driving the LED units of the LED assembly of the lighting system. Applying a reduced current, will in general, as illustrated in
In an embodiment of the present invention, the reduced current substantially corresponds to the nominal current multiplied with the largest duty cycle. By doing so, an adjusted duty cycle of approx. 100% will be obtained for the LED unit having the largest duty cycle. As the duty cycle of the LED units cannot be more than 100%, the reduced current as obtained in this way corresponds to the smallest current that enables the provision of the desired characteristic of the LED assembly.
Note that the current reduction as described in the previous paragraph assumes a linear correspondence between the output of the LED unit and the current. In case this is not true, a correction can be applied to the reduced current to ensure that the desired characteristic of the LED assembly is met. This is illustrated in
The control unit according to the present invention can advantageously be applied for controlling a LED assembly comprising two or more LED units that are connected in series.
As explained above, the determination of the duty cycles for the multiple LED units using a control unit according to the present invention may result in an improvement of the efficiency of the power supply powering the LED units. In general, adjusting the duty cycles of the LED units as described above may result in the application of larger duty cycles in order to compensate for the application of a reduced current. It has been observed that the application of a larger duty cycle for a LED unit may have a further advantage in that it may reduce flicker. Flicker of a LED assembly may occur as either visible flicker or non-visible flicker, the latter may e.g. cause nausea. When a LED unit is e.g. operated at a duty cycle of 90%, a smaller occurring flicker can be observed compared to a duty cycle of e.g. 10%.
According to an other aspect, the present invention provides in an improved way of powering a LED assembly comprising a plurality of LED units, arranged in parallel, each LED unit being powered by a different power supply, e.g. a switched mode current supply such as a buck or boost converter.
To illustrate the improved way of powering, assume the LED assembly to comprise two LED's connected in parallel, each provided with a switched mode current supply for providing a current to the LED. The light emitted by the LED's having substantially the same color.
In such case, in order to realise a desired brightness from the LED's taken together, the conventional way is to adjust the duty cycles of the different LED's in the same manner.
As such, a desired brightness of 50% of the nominal (or maximal) brightness, can be realised by controlling both LED's substantially at a duty cycle of 50%. Note that a correction as discussed in
In accordance with an aspect of the present invention, an alternative way of operating the different LED's (or LED units) is proposed:
It has been observed that the efficiency of a switched mode power source may vary, depending on the load to be powered (i.e. the LED's or LED units) or the operating conditions (e.g. the current to be supplied, the duty cycle of the load). As explained above, losses in the switcher element or diode of the power supply may vary with these conditions.
Rather than controlling the different LED's in substantially the same way (i.e. have them operate at the same duty cycle), the present invention proposed to take the actual efficiency characteristic of the power supplies into account. In the example as discussed, a brightness of 50% may equally be realised by operating one of the LED's at 100% duty cycle and the other LED at 0% duty cycle. As the efficiency of the power supply when powering a LED at a 50% duty cycle may be lower than the efficiency at a 100% duty cycle, the application of different duty cycles may prove advantageous. Assuming the efficiency characteristic of the power supplies is known, a control unit can be arranged to determine which combination of duty cycles provide for the best efficiency for a given desired characteristic of the LED assembly. An efficiency characteristic of a power supply can e.g. be determined experimentally or based on theoretical considerations.
In the concept of duty cycle dimming, a brightness resolution is therefore limited by the duty cycle resolution.
The above may be illustrated by a simple example: if at nominal power supply current t3 would be 0003 (Hex) and t4 0002 (Hex), then this minimum step of 0001 (Hex) would reduce the duty cycle by 33%, hence providing a brightness step of 33%. In case the current would be reduced by a factor 4, and hence the duty cycle would be increased by the same factor 4, then starting at a new value for t3: 4×0003 (Hex) providing 000C (Hex), would allow to increase or decrease the duty cycle in steps of 0001 (Hex), hence providing a brightness step of approximately 8%, thereby allowing a more smooth dimming.
Generally speaking, the concept of dimming the LEDs by a combination of duty cycle dimming and reducing the power supply current may, depending on the configuration, implementation, dimensioning, and other factors, provide for one of more of the below effects:
Smooth dimming may render a comparably lower amount of noise and flickering:
Noise:
EMI:
Flickering:
Unnoticeable color shifts:
A voltage drop across the R1 through R3 resistance is fed back to the current source at a feedback input FB of the buck converter, thereby enabling control of an amplitude of the current. Duty cycle is controlled by the microcontroller μC, which, in response to a setpoint at a corresponding setpoint input, controls switches, such as in this example switching transistors, connected in parallel to each of the LEDs or LED groups. In order to take account of possible potential differences, the switches are controlled by the microcontroller via respective level converters.
As explained above, the current source in this example controls its output current by controlling the voltage present at input FB to a fixed value. By changing the total R1 through R3 resistance, f.e. by mounting different values for R2 and/or R3 or even leaving them out altogether, different current values can be set that will deliver the same voltage at pin FB. In this manner the nominal current Inom can be set to different values, e.g. for different applications.
The digital potentiometer may be controllable by the microcontroller uC (as indicated by the dotted line) and thus by a suitable software programming and may form an integral part of the brightness and color control algorithm in the microcontroller uC, especially the very flexible set of algorithms as described in WO2006107199 A2. Making use of such algorithms, very smooth take-over profiles can be achieved when changing the Inom (and consequently time duty cycle settings).
Note that the Rs resistance typically is very small and that potentiometers in general have larger values. A more practical arrangement will be described below.
A more practical arrangement (though still a principle schematic) is provided in the highly schematic circuit diagram in
In the circuit depicted here, the voltage across the (possibly very low ohmic) series resistor Rs is amplified by an amplifier circuit comprising in this example an operational amplifier and potentiometer P2 as a voltage feedback network, and level-shifted by potentiometer D1 connected between an output of the amplifier circuit, a reference voltage (indicated in
The above principles can be used for multiple LED chains, either by using complete double circuitry, by sharing the microcontroller uC, by sharing the microcontroller uC and the current source etc. An example is illustrated in the highly schematic circuit diagram of
In other words, a plurality of parallel branches may be provided, each comprising at least one LED unit, a respective switched mode power supply being provided for each of the branches, the control unit being arranged for determining a power supply current for each of the power supplies, depending on the desired output characteristic for the respective LED unit, and for providing output data for each of the power supplies.
In
In this figure, it is shown that, given a certain average LED parameter (f.e. Brightness), different settings can be chosen to achieve that average brightness. For example, one could choose the values used in
This freedom in alternative settings can be used to trade-off between avoiding visible frequencies, smoothness of the control, circuit cost and limitations, software complexity, electromagnetic interference, noise, etcetera. (For example, the higher frequency content in a 2.5% pulse is generally higher than in a 10% pulse given the same period T.)
Thereby, possibly at the “cost” of some ultimate brightness resolution, an effective, low current may be achieved without the above mentioned color shift or instability problems as the momentary current in the duty cycle part T4 is kept above the minimum value.
The switching off may be obtained by appropriate setting the Potentiometer ratio (in a suitable feedback circuit configuration) or by closing the parallel switches during a certain part of the duty cycle time.
It is remarked that, because of the likely higher step in the current value, the importance of trading off between visible flickering and the choices for T and t increases. Given the many variables available now: duty cycle dimming, current dimming, current duty cycling, etc, many variables are available to be able to obtain a good tradeoff.
In this configuration, it is even possible to change the current during each part of the cycle time to a value that matches the desired output characteristic of the respective LED unit that is to be operated in that part of the cycle time. Thus, in case R, G and B are to be operated at a low brightness level while W is to be operated at a high brightness level, the current can be set to a low value in the cycle time parts corresponding to R, G and B, thereby allowing to drive the respective LEDs at a relatively high time duty cycle within that cycle part, while in the cycle time part corresponding to W, a higher power supply current is set.
In this way, it is also possible to avoid the low frequency components (f.e. having 8096 us as base frequency in a cycling scheme of 8 time periods of 1024 microseconds each) that would arise when trying to achieve high brightness resolutions using the above referred, known algorithm at maximum Inom. Using e.g. such known algorithm to achieve high resolution would imply for example to set the duty cycle in 7 of the 1024 us periods for Red to 128 us/128 us while setting it to 125.5/130.5 in the eight one of the 1024 us periods. This would provide a slightly lower brightness, thus achieving a high brightness resolution, however it would introduce a brightness ripple, namely a 125 Hz frequency component, as only in one of the 8 time periods of 1024 us the brightness of the LED is different.
By lowering the Inom (either by lowering the current, or by duty cycling the current in each of the time periods) and thereby keeping the LED current behaviour the same in each of the 1024 us time periods, the above described low frequency effects may be avoided.
It is remarked that, at very high brightnesses, the eyes' sensitivity becomes less and lower frequency components needed to achieve 100% brightness may have less impact.
Hence, the various embodiments as depicted and described with reference to
Reverting to
Further variants are depicted with reference to
By a corresponding setting of the value of the reference Vref, an amplitude of the pulse may be set. As the pulses may provide for a comparatively lower effective current then a continuous current, a resolution may be further increased by combinations of parts of the cycle during which a continuous current is provided, and parts of the cycle during which the current is pulsed. Thereby, by a corresponding setting of the reference, different values of the continuous and/or the pulsed current may be obtained within a cycle. Calibration of the pulses may be performed in various ways, e.g. timing a pulse width by a timer, filtering a sequence of pulses by a low pass filter, measuring a pulse shape using sub-sampling techniques. Also, feedback mechanisms such as optical feedback (brightness measurement) may be applied.
It will be understood that, although the above explains the controlling of the reference (so as to set the current) and the pulsing in a free running configuration as depicted in
In another embodiment, asynchronous sampling is used by the microprocessor in order to determine a time of switching off the comparator. Thereto, the microprocessor samples an analogue signal representing the current through the inductor and LED's, e.g. by sampling the signal at the output of the amplifier AMP for amplifying the signal measured by Rsens. Due to the free running character of the hysteretical or other converter, an asynchronous sampling is provided enabling to determine the waveform and hence the switching on and/or off of the comparator with a comparably high resolution. For this purpose, the current may be sampled and/or the output of the comparator. In order to provide a low average current through the LED's, the microprocessor may now disable the hysteretical converter (or other type of converter) by either setting after a time (e.g. prior to the finalisation of the cycle of oscillation of the converter itself) the value of the reference source back to zero, by overriding or by disabling the comparator or by any other suitable means to force the switch SW to the desired state. As a result, comparably short current pulses are created, shorter than could have been provided by letting the oscillator run on its own motion, the current pulses having such short time duration enable a low level and/or high resolution dimming. A frequency of repetition of the pulses may be determined by the microprocessor by the time until a following enabling of the converter (by e.g. a following setting of the reference generator and/or a following enabling of the comparator. Thereby, current pulses may be generated e.g. 1, 2, 3 of N (N being an integer) times per cycle time. Furthermore, it is possible to synchronise the switching of the converter to cycle times of the operation of the microprocessor by the described interaction by the microprocessor on the comparator.
The above principle may be applied in a method for dimming of the LED current provided by a driver. The method comprises:
dimming an effective current by disabling the converter (e.g. a hysteretical converter) during a part of cycle time; this may be performed until a level of for example ¼ or ⅛ of the maximum (i.e. 100%) current level. Then, further dimming is provided by dividing a cycle time of the operation in cycle time parts, an example of a cycle frequency could be 300 Hz, as it is a multiple of 50 Hz and 60 Hz mains frequencies and a multiple of common video image capturing frequencies. The cycle time could then for example be divided in 128 parts so as to provide sufficient resolution. Dimming may be performed by during each cycle time part, enabling the converter at a beginning of the cycle time part and disabling the converter during the end of the cycle time part. Prior to the disabling, the value of the reference is increased, so as to force the comparator to switch on the switch, thereby providing for a defined switching off behaviour, a reduction of jitter by the effects of the asynchronous operation of the converter with respect to the cycle time and cycle time parts, and hence a more defined dimming behaviour. A gradual transition towards the situation where the current is increased at the end of each cycle may be obtained by gradually activating this higher current during 1, then 2, then 3, etc cycle time parts of each cycle. With progressed dimming, the part of the cycle time part during which the converter is enabled is made that short that only the part remains where the reference is increased. Further dimming may then be provided by decreasing (e.g. per cycle time part) the value of the reference, and still further dimming may be obtained by keeping the converter shut down during some of the cycle time parts.
The above process is illustrated in
A further embodiment will be explained with reference to
The dimming as described with reference to
In an embodiment, there is provided a control unit for a LED assembly comprising a first and second LED unit, said LED units being serial connected, the LED assembly, in use, being powered by a switched mode power supply. As illustrated by the flow diagram depicted in
In an embodiment, there is provided a control unit for a LED assembly comprising a first and second LED unit, said LED units being serial connected, the LED assembly, in use, being powered by a switched mode power supply. As illustrated by the flow diagram depicted in
In an embodiment, there is provided a circuit for driving a LED assembly comprising at least one LED illumination device, the circuit comprising
Aspects of the invention are described in the following numbered clauses which form part of the description.
1. A control unit for a LED assembly comprising a first and second LED unit, said LED units being serial connected, the LED assembly, in use, being powered by a switched mode power supply, the control unit being arranged to
receive an input signal representing a desired output characteristic of the LED assembly,
determine a first and second duty cycle for the respective first and second LED units associated with a nominal current of the switched mode power supply, for providing the desired output characteristic,
determine the largest of the first and second duty cycles for respective LED units,
determine a reduced current based on at least the largest of the duty cycles,
adjust the first and second duty cycle for respective LED units based on the reduced current or the largest of the duty cycles,
provide output data for the LED assembly and the switched mode power supply based on the adjusted first and second duty cycles and the reduced current.
2. The control unit according to clause 1 wherein the reduced current substantially corresponds to the nominal current multiplied with the largest duty cycle.
3. The control unit according to clause 1 or 2 wherein the reduced current is based on a brightness characteristic of the LED unit.
4. A lighting system comprising a LED assembly that comprises a first and second LED unit and a control unit according to any of clauses 1 to 3 for controlling the LED assembly.
5. The lighting system according to clause 4 further comprising a switched mode power supply for powering the LED assembly.
6. The lighting system according to clause 5 wherein the switched mode power supply comprises a buck converter.
7. A control unit for a LED assembly comprising a first and second LED unit, said LED units being serial connected, the LED assembly, in use, being powered by a switched mode power supply, the control unit being arranged to
receive an input signal representing a desired output characteristic of the LED assembly,
determine a power supply current of the switched mode power supply from the received input signal,
determine a first and second duty cycle for the respective first and second LED units from the determined power supply current and the input signal, the combination of duty cycle and power supply current being set for providing the desired output characteristic,
provide output data for the LED assembly and the switched mode power supply based on the determined first and second duty cycles and the determined power supply current.
8. The control unit according to clause 7, being arranged to control the power supply current to a first value in a first part of a cycle time and to a second value in a second part of the cycle time.
9. The control unit according to clause 7 or 8, being arranged to provide the output data such as to sequentially operate the LED units each in a respective part of a cycle time, and to set in each of the parts of the cycle time, the power supply current of the power supply to a value that matches the desired output characteristic of the respective LED unit that is to be operated in that part of the cycle time.
10. A lighting system comprising a LED assembly that comprises a first and second LED unit and a control unit according to any of clauses 7-9, for controlling the LED assembly.
11. The lighting system according to clause 10, further comprising a feedback circuit to feed a signal representative of the power supply current to a feedback input of the switched mode power supply, the feedback circuit comprising at least one of a digital potentiometer and a digital to analog converter, the control unit having a control output connected to the at least one of the digital potentiometer and the digital to analogue converter, for controlling the potentiometer ratio respectively the digital to analogue converter output, thereby controlling the power supply current.
12. The lighting system according to clause 10 or 11, wherein a plurality of parallel branches is provided, each comprising at least one LED unit, a respective switched mode power supply being provided for each of the branches, the control unit being arranged for determining a power supply current for each of the power supplies, depending on the desired output characteristic for the respective LED unit, and for providing output data for each of the power supplies.
13. A circuit for driving a LED assembly comprising at least one LED illumination device, the circuit comprising
a switch,
an inductor, in a series connection with the switch, the switch to in a conductive state thereof charge the inductor,
a current measurement element to measure a current flowing through at least one of the inductor and the LED illumination device, the switch, inductor and current measurement element being arranged to establish in operation a series connection with the LED illumination device, the circuit further comprising:
a reference signal generator for generating a reference signal;
a comparator to compare a signal representing the current measured by the current measurement element with the reference signal, an output of the comparator being provided to a driving input of the switch for driving the switch, and
a controller to control an operation of at least one of the reference signal generator and the comparator.
14. The circuit according to clause 13, wherein the controller is arranged to control the reference signal generator so as to generate a first reference signal value during a first part of a cycle time and a second reference signal value during a second part of the a cycle time.
15. The circuit according to clause 13 or 14, wherein the controller in arranged to disable the comparator during at least a part of the cycle time.
16. The circuit according to clause 15, wherein the controller is arranged to enable the comparator at least once during the cycle time to allow a generation of at least one short current pulse during the cycle time.
17. The circuit according to any of clauses 13-16, wherein the controller is arranged to:
provide enable pulses to enable the comparator in at least two cycle time parts of a cycle time; wherein a pulse length of the enable pulses is varied within each cycle time.
This application is the National Stage of International Application No. PCT/NL2010/000065, filed Apr. 9, 2010, which claims the benefit of U.S. Provisional Application No. 61/175,242, filed May 4, 2009, the contents of which is incorporated by reference herein.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NL2010/000065 | 4/9/2010 | WO | 00 | 1/17/2012 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2010/128845 | 11/11/2010 | WO | A |
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| 20120104974 A1 | May 2012 | US |
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