WO2013171625A2

WO2013171625A2 - Lamp driver and method for power supply voltage drop compensation

Info

Publication number: WO2013171625A2
Application number: PCT/IB2013/053701
Authority: WO
Inventors: Peter Henricus Gerardus BEELEN
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-05-15
Filing date: 2013-05-08
Publication date: 2013-11-21
Also published as: WO2013171625A3

Abstract

The present invention relates to a driver for a light source, such as a driver for an outdoor lighting system comprising a group of light sources which are provided with a respective driver connected to a common power source. The driver comprises a first circuitry and a control unit comprising a second circuitry coupled to the first circuitry for controlling the power supplied to the light source at an operational value in relation to a voltage reference value V-ref; a third circuitry comprising a memory for storing the voltage reference value V-ref; and a fourth circuitry coupled to the memory for monitoring the line voltage supplied by the power supply to the first circuitry and for adjusting the voltage reference value V-ref to correspond to a maximum value of the line voltage supplied by the power supply to the first circuitry during a time period. According to the invention, voltage drops due to e.g. power line impedance giving rise to lower maximum line voltage for certain drivers may be compensated by locally storing information in the driver specifying the maximum line voltage seen by the driver and by using this information when controlling the light source.

Description

Lamp driver and method for power supply voltage drop compensation

FIELD OF THE INVENTION

The present invention relates to a driver for a light source and a method for driving a light source. For example, the invention relates to outdoor lighting applications wherein a number of lamps, each comprising a driver and a light source, are connected to the same electrical power source. The invention also relates to a lighting system comprising an electrical power source for supplying a line voltage to a group of light sources, wherein each light source comprises a driver including a control unit for controlling the light output of the respective light sources. BACKGROUND OF THE INVENTION

In the field of lighting systems, for example for the outdoor lighting market, it is known to use different light sources, such as high-intensity discharge (HID) lamps, which are controlled by locally arranged conventional or electronic drivers, or ballasts, connected to a common mains voltage supply. In order to operate an outdoor lighting system in an efficient manner, the common line voltage that is supplied to the drivers of the separate lamps of the lighting system may be lowered when conditions allow it to do so. Such conditions may for instance be during dusk or dawn, when a sufficient amount of daylight is present, or when traffic on a road is limited. In order to operate properly, the electronic driver is provided with logic functions, for example formed of solid state electronic circuits, which control the light source in an appropriate manner in relation to the supplied voltage. During operation, electronic drivers typically change the frequency of the power source from standard mains frequencies to a drive frequency, such as 20,000 Hz, or higher, which reduces the flickering and the weight of the driver unit. A higher frequency allows for a more efficient and cooler operation of the driver in relation to the heavier line-frequency magnetic ballasts.

For outdoor application, such as street lighting, it is common to use HID lamps, such as low-pressure sodium lights, which generate a monochromatic light and which may be operated by conventional drivers/ballasts. Examples of HID lamps providing non- monochromatic light include: ceramic discharge metal halide lamps, hydrargyrum medium- arc iodide (HMI) lamps, mercury- vapor lamps, etc. For these types of lamps, which do not generate monochromatic light but light with a range of wavelengths, a change in the power consumed by the lamp results in a change in the color of the light being generated. This may lead to a problem when the power supply lines between a mains voltage power source and the lamp driver are long, since the power supply lines have a certain impedance which may lead to a voltage drop or line voltage fluctuations.

For example, this problem may occur in a street lighting system arranged along a road and which comprises a plurality of lamps separated by e.g. 50 meters, wherein lamps within a predetermined section, or group, having a length of e.g. 1 km, are connected to one cabinet comprising an electrical power source. In more detail, due to the impedance of the electric lines connecting the lamps in the group, each lamp driver is supplied with a different line voltage, wherein the line voltage drops when the distance between the respective lamp driver and the power source increases. This may lead to the undesired effect that the light from different lamps in the group gradually decreases and/or that the color content of the generated light changes from lamp to lamp. Also, an occurring mains voltage drop may lead to the fact that lamps at the end of the section may turn off, or that the dimming functionality, such as mains dimming by lowering the mains voltage at the power source, provided in the separate drivers dims different lamps in an inconsistent manner.

In WO 2008/155714, a driver unit for controlling a light source, such as a HID light source, is suggested to improve the driver dimming functionality and to provide a more consistent dimming in a lighting system, comprising a group of separate light sources and driver units. In more detail, this is achieved by specifying the mains voltage ranges, where each voltage range is defined between two voltage threshold values stored in a memory in the driver, wherein the driver is configured to control the light output of the lamp according to a set light-output reference value associated with a certain voltage range. Hence, all driver units in a group that are supplied with different line voltages due to a voltage drop, but which line voltages are within the same voltage range, will control the lamps according to the same light output reference value, and thereby reducing the occurrence of inconsistent light levels. However, if the voltage drop within a lighting system section provided with this type of driver results in that the line voltage supplied to a far-away driver enters a different voltage range in relation to a driver being closer to the power source, the light output or dimming level of the different lamps will be inconsistent. Also, for severe voltage drops, lamps may turn off in an undesired manner. SUMMARY OF THE INVENTION

An object of the present invention is to at least alleviate the problems discussed above. In particular, an object is to provide an improved driver for a light source, and an improved method for driving a light source.

These and other objects are achieved by a driver for light source, a lighting system, and a method according to the independent claims. Preferred embodiments of the invention are presented in the dependent claims.

According to a first aspect thereof, the present invention relates to a driver for a light source, comprising input terminals for connection to a power supply, output terminals for connection to the light source, a first circuitry, which is coupled to the input terminals, for generating a current through the light source out of a line voltage supplied by the power supply, and a control unit. The control unit comprises a second circuitry coupled to the first circuitry for controlling the power supplied to the light source at an operational value in relation to a voltage reference value V-ref; a third circuitry coupled to the input terminals and the second circuitry, comprising a memory for storing the voltage reference value V-ref; and a fourth circuitry coupled to the memory, for monitoring the line voltage supplied by the power supply to the first circuitry, and for adjusting the stored voltage reference value V-ref to correspond to a maximum value of the line voltage supplied by the power supply to the first circuitry during a time period.

The present invention is based on the insight that improved controlling by a driver of a light source may be provided by locally monitoring and storing a maximum line voltage reference value in a memory in the driver and using this value to adapt the controlling of the power supplied to the light source. Hence, voltage drops due to e.g. power line impedance giving rise to a lower maximum line voltage for certain drivers may be compensated by locally storing information in the driver by means of specifying the maximum line voltage seen by the driver and by using this information when controlling the light source.

The driver for a light source according to the present invention is further advantageous in that it provides an improved and more durable dimming functionality, such as when the supplied line voltage is lowered by the power source in order to instruct the local driver to dim the light output. In particular, an undesired mains voltage drop may be compensated by comparing the present line voltage with the stored maximum reference value, such that it may be locally determined in the driver if the lower line voltage arises from the power line voltage drop or dimming instructions from the power source. For example, by comparing the present line voltage value with the stored maximum voltage value, the driver can generate consistent light output by adapting the driving power to the light in an appropriate manner, such as altering the driving voltage, current and/or frequency, or other lamp drive power characteristics, including modulated drive power content.

According to an exemplifying embodiment, the second circuitry is arranged to control the power supplied to the light source based on an operational control value S-op determined by the control unit. Furthermore, according to an embodiment, the second circuitry is arranged to control the power supplied to the light source depending on the voltage reference value V-ref, the determined operational control value S-op, and the voltage level supplied by the power supply.

In an embodiment, the operational control value S-op is indicative of a percentage of a maximum light source power. The percentage may for example be about 40 %, 60 %, 80 % and 100 % of the desired maximum light output.

According to an embodiment, the operational control values S-op represents a light source dimming level according to a light source dimming curve. For example, the dimming curve may be a stepwise curve that is indicative of different light output levels, or an essentially continuous dimming curve, locally stored or configured in the driver. The dimming curves may further be adapted for the operating range of a specific light source type. Also different dimming curves for different light sources and operational modes of the driver may be stored locally in the driver unit, and dimming curve adaptation steps for different dimming curves based on the monitored and stored voltage reference values may be defined locally in the light source driver.

According to a further embodiment, the control unit of the driver is configured to identify a maximum voltage supply time period during which the power supply provides maximum line voltage to the driver, wherein the voltage reference value is adjusted to correspond to a maximum value supplied by the power supply during the maximum voltage supply time period. For example, a cabinet controller forming the electric power source may be configured to perform a start-up power sequence during which a maximum line voltage, or a reference line voltage, is supplied to the driver and light source. In the case of light sources producing light by means of an electric arc, such as HID lamps, a start-up sequence may be used for starting the arc and heating up the lamp in order to reach a desired or optimum operating efficiency level.

In various embodiments, the light source is an HID lamp, a low-pressure mercury discharge lamp, a LED lamp, or an array of LEDs and/or HIDs. Furthermore, the HID lamp may be a mercury vapor lamp, a metal halide (MH) lamp, a ceramic MH lamp, a sodium vapor lamps, a xenon short-arc lamp, or similar.

According to another aspect thereof, the present invention relates to a lighting system comprising an electrical power source for providing an output voltage, a group of light sources, the group comprising two or more light sources, wherein each light source is provided with a driver according to the first aspect of the invention, wherein each driver is connected to the power source. For example, the lighting system is a centrally controlled lighting system, wherein operation of the group of light sources in the system is controlled centrally, depending on the desired operation.

In an embodiment of the system the electrical power source is arranged to provide an adjustable output voltage, thereby allowing for a dimming functionality that is controlled locally in each driver. For example, the dimming functionality may be provided using mains dimming, involving lowering a mains voltage RMS amplitude or phase-cut dimming, which involves cutting away parts of an alternating sine-shaped mains voltage supply.

According to another aspect thereof, the present invention relates to a method for driving a light source by a light source driver connected to a mains supply, which supplies a line voltage, the method comprising:

monitoring the line voltage over a time period,

- determining a maximum value of the line voltage associated with the time period,

determining a voltage reference value V-ref corresponding to the maximum value,

storing the voltage reference value V-ref in a memory, and

- controlling the power supplied to the light source at an operational value in relation to the stored voltage reference value V-ref.

The method is advantageous in similar manners as described in relation to the first aspect of the present invention. In particular, the method provides an improved controlling by a driver of a light source by monitoring a maximum line voltage reference value in a memory and using this value to adapt the controlling of the drive power supplied to the light source such that, e.g. inconsistent dimming by using mains dimming due to voltage drops can be reduced or avoided.

According to an exemplifying embodiment, the method further comprises adjusting the voltage reference value V-ref stored in the memory to correspond to an updated maximum value associated with a second time period. For instance, during the second time period, the maximum line voltage supplied by the electric power source may be changed due to the changed operating mode of the electric power source, or the voltage drop over the power line may have changed, for example, due to the physical change or long time degradation of the power lines, or due to other types of alterations causing power line impedance fluctuations over time.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled addressee will realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing example embodiments of the invention, wherein:

Fig. 1 illustrates a schematic block diagram of an embodiment of the light source driver according to the present invention.

Fig. 2 illustrates a schematic chart of an exemplifying operational area of an embodiment of the light source driver specified in the light output PI versus the mains voltage Vm, and exemplifying light source dimming curves.

Fig. 3 illustrates a schematic view of an embodiment of a lighting system according to the present invention, in which the lighting system comprises a group of light sources each provided with a light source driver.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled addressee. Like reference characters refer to like or corresponding elements throughout. In Fig. 1, a schematic block diagram of an embodiment of an electric light source driver 1 is illustrated, in which driver 1 is connected with input terminals (7) to a mains supply 3, formed by an alternating voltage source, via power lines 3', and connected with output terminals (8) to a lamp or light source 2 to which the electric driver 1 is configured to operate. The driver 1 further comprises a first circuitry 4 that is formed by an input circuit 4a connected to the mains supply 3, a converter circuit 4b, and a drive circuit 4b connected to the light source 2. In addition, the driver 1 comprises a control unit 5 comprising operational circuits configured to control and monitor the operation of the light source 2, which circuits may be separately arranged or integrated with each other in the control unit of the driver 1. The first circuitry 4 may also be formed integrally with the control unit 5 in the driver 1.

As shown, the control unit 5 comprises a second 5 a, a third 5b and a fourth 5 c circuitry which are coupled to each other and to the first circuitry 4. In more detail, the second circuitry 5a is coupled to the first circuitry 4 for controlling the power supplied to the light source 2. The third circuitry 5b is coupled to the input terminals (not shown) and the second circuitry 5b and comprises a memory 6 for storing the voltage reference value V-ref. For example, the memory 6 is a writeable persistent storage device, such as an EEPROM or writeable FLASH memory. The fourth circuitry 5c is coupled to the memory 6, and is arranged for monitoring the line voltage supplied by the mains supply 3 to the first circuitry 4, and for adjusting the stored voltage reference value V-ref to correspond to a maximum value of the line voltage supplied by the mains supply 3 to the first circuitry 4 during a time period.

During operation, the driver 1 is controlled by the control unit 5 to drive the light source 2 based on the V-ref value which is stored in the memory 6 such that the light output level of the light source correspond to a desired level. In more detail, the driver 1 is configured to monitor the line voltage supplied by the mains supply 3 and to store the maximum level of the line voltage seen by the driver over a time period as a the V-ref value in the memory device. This enables the driver 1 to remember the maximum line voltage value and determine at any time if a measured line voltage level which is lower than the theoretical maximum line voltage delivered by the mains supply occurs due an intended voltage lowering of the mains supply created by the electrical power source for e.g. dimming functionality, or if the measured lower line voltage occurs due to a voltage drop due to e.g. impedance problems or failure of the power lines 3 ' . In Fig. 2, a schematic chart of an exemplifying operational area 20 of an embodiment of the light source driver 1 is illustrated. The operational area is specified in light output PI versus mains voltage Vm. As shown, between mains voltage levels A and B, the potential light output generated by the driver 1 is physically restricted and cannot reach 100 %. However, between the mains voltage levels B and C, the driver 1 may control the light output level of the light source 2 between 50 % and 100 %. As further shown, the driver 1 may be configured to control the light output of the light source 2 according to

predetermined dimming curves represented by an upper level 21a corresponding to 100 % light output, a lower level 21b corresponding to 60 % light output, and first and second intermediate dimming curves 23a and 23b, wherein the light output level of the dimming curve depend on the mains supply Vm, i.e. the line voltage level supplied by the mains supply 3. The first intermediate dimming curve 23 a represents a first example of a stepwise dimming curve according to which the driver 1 generates a light output of approximately 80 % when the mains voltage is between levels VI and V2, such as between 210 V and 195 V. The second intermediate dimming curve 23b represents a second example of a continuous linear dimming curve according to which the driver 1 , substantially, continuously lowers the light output between 100 % and 60 % when the mains voltage is lowered from level VI to V2. It should be noted that the range and value levels of the dimming curves and operational area only represent exemplifying embodiments and may be configured differently. Also, alternative intermediate dimming curves comprising e.g. a plurality of different steps, shapes, and/or inclinations are contemplated, including linear, non-linear, continuous, and/or non- continuous curve portions.

For example, during operation, the control unit 5 of the driver 1 determines an operational control value S-op that represents a light output level to be outputted by the lamp, where the S-op value is determined based on a selected dimming curve and on a measured line voltage. Furthermore, the control unit compares the measured line voltage with the stored V-ref value in order to determine if the measure line voltage is offset by an undesired voltage drop. In case the presence of an undesired voltage drop is determined, the S-op value, or the complete dimming curve, is adapted in order to compensate for the voltage drop. For example, the compensation of the S-op value due to a voltage drop experienced by the driver 1 can be represented by shifting the dimming curve to the left in the chart in Fig. 2, i.e. in a negative direction along the x-axis as exemplified by the shifted intermediate dimming curve 23b'. In Fig. 3, a schematic view of an embodiment of a lighting system 100 according to the present invention is illustrated. The lighting system 100 comprises an electrical power source 101 arranged in a cabinet for providing an output mains voltage via power lines 3'. Furthermore, the system comprises a group of light sources 102 formed of a plurality of outdoor street lights 110 and 111, each connected to the cabinet via power lines 3'. Each street light is further provided with a respective light source driver 1 arranged to control a respective lamp 2. Each driver 1 is configured as described with reference to Fig. 1 and Fig. 2, and is arranged to control the associated lamp 2 based on a locally monitored and stored V-ref value. Furthermore, each driver 1 may be configured for the dimming functionality based on a preconfigured dimming curve being dependent on the local experience of the line voltage supplied to each driver 1. As schematically shown, the group of street lights 102 comprises a street lights 110 which are located close to the electrical power source 101, thereby being substantially unaffected by a voltage drop due to the voltage line impedance. Street light 111, however, being located on a further away distance from the electrical power source, such as more than 200 meters, 500 meters, or 1000 meters, may be subjected to a noticeable voltage drop due to the impedance of the power lines 3'. However, the light output from the street light 111 will be adapted by the driver 1 based on the monitored and locally stored V-ref.

For example, in an exemplifying case wherein a line voltage of e.g. 215 V supplied by the electrical power source is reduced to 208 V at street light 111 due to the power line impedance, a conventional light source driver arranged for the dimming functionality may interpret the lower line voltage value as an instruction to dim the light to a 90 % light output level according to a predetermined dimming curve. Hence, street light 111 will, in an undesired manner, output light of a different output level compared to street lights 110. In comparison, the light source driver 1 according to the present invention allows for comparison of the measured line voltage with the stored V-ref value such that the driver 1 may locally determine that the measure line voltage substantially equals the maximum line voltage reaching street light 111 and that the light source in street light 111 should be operated at a 100 % level. In addition, the driver may compensate different dimming steps, or the operational S-op value determining the light output level, such that the same level is provided for different light sources in the system, independent of the voltage drop.

For example, the time period for monitoring during which the control unit is configured to monitor and adjust the V-ref value comprises the time during which the lighting system or light source is activated. The stored V-ref value may further be reset each time the light source driver 1 , or lighting system, is restarted, or by a remote reset, or according to a predetermined time schedule. Alternatively, the time period for monitoring comprises a maximum voltage supply time period, such as a start-up sequence for starting the light source, during which a maximum line voltage is supplied by the mains supply. A start- up sequence may for example be identified by the driver 1 each time the mains supply is turned on from an off-state by the centrally arranged electrical power source. A maximum voltage supply time period may also be identified for the light source driver 1 via a remotely controlling communication network, connecting the driver 1 with the control unit by means of the electrical power source.

Furthermore, in order to prevent the adjustment of the light output, i.e. the output power of the drive circuit to the light source 2, in case of a temporary, undesired fluctuation of the line voltage, the control unit in the driver 1 may be configured to implement a predetermined hysteresis function. Temporary fluctuation may, for example, include transient, swells, sags, or interruptions, occurring, e.g., within a predetermined time interval, or within a predetermined voltage range.

Each driver 1 in the lightning system 110 described with reference to Fig. 3, may further be remotely controlled, for example via the power lines 3', via additional control signal lines, and/or via wireless communication network/nodes. Thereby, the control settings relating to the light source operation and the dimming functionality may be modified and updated centrally. For example, the electronic driver 1, or electronic ballast, may allow for remote control via networks, such as digital addressable lighting interfaces, such as DALI.

Even though the invention has been described with reference to the specific exemplifying embodiment thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. For example, the driver may be used for many different kinds of light sources and lighting system, both for outdoor and indoor applications, in different mains voltage ranges.

Variations to the disclosed embodiments can be understood and effected by the skilled addressee in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Furthermore, in the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

Claims

CLAIMS:

1. A driver (1) for a light source (2), the driver comprising:

input terminals (7) for connection to a power supply (3),

output terminals (8) for connection to the light source,

a first circuitry (4), coupled to the input terminals, for generating a current through the light source out of a line voltage supplied by the power supply, and

a control unit (5), comprising

a second circuitry (5 a) coupled to the first circuitry for controlling the power supplied to the light source at an operational value in relation to a voltage reference value V- ref,

- a third circuitry (5b) coupled to the input terminals and the second circuitry, comprising a memory (6) for storing the voltage reference value V-ref, and

a fourth circuitry (5c) coupled to the memory, for monitoring the line voltage supplied by the power supply to the first circuitry, and for adjusting the stored voltage reference value V-ref to correspond to a maximum value of the line voltage supplied by the power supply to the first circuitry during a time period.

2. The driver according to claim 1, wherein the second circuitry is arranged to control the power supplied to the light source based on an operational control value S-op determined by the control unit (5).

3. The driver according to claim 2, wherein the second circuitry is arranged to control the power supplied to the light source in dependence on the voltage reference value V-ref, the operational control value S-op, and the voltage level supplied by the power supply.

4. The driver according to any one of claims 2 to 3, wherein the operational control value S-op is indicative of a percentage of a maximum light source power.

5. The driver according to any one of claims 2 to 4, wherein the operational control value S-op represents a light source dimming level according to a light source dimming curve.

6. The driver according to any one of claims 2 to 5, wherein the operational control value S-op is determined by the control unit by comparing the voltage level supplied by the power supply with the voltage reference value V-ref stored in the memory.

7. The driver according to any one of the preceding claims, wherein the control unit is configured to identify a maximum voltage supply time period during which the power supply provides a maximum line voltage to the driver, and wherein the voltage reference value V-ref is adjusted to correspond to a maximum value supplied by the power supply during the maximum voltage supply time period.

8. A lighting system (100), comprising:

an electrical power source (101) for providing an output voltage, power lines (3') connected to the electrical power source,

a group (102) of light sources (2), the group comprising two or more light sources (2), wherein each light source (2) is provided with a driver (1) according to any one of the preceding claims, and wherein each driver is connected to the power source (101).

9. The lighting system (100) according to claim 8, wherein the electrical power source (101) is arranged to provide an adjustable output voltage.

10. A method of driving a light source by a light source driver (1) connected to a power supply supplying a line voltage, comprising:

monitoring the line voltage over a time period,

determining a maximum value of the line voltage associated with the time period,

- determining a voltage reference value V-ref corresponding to the maximum value,

storing the voltage reference value V-ref in a memory, and

controlling the power supplied to the light source at an operational value in relation to the stored voltage reference value V-ref.

11. The method of driving a light source according to claim 10, further comprising adjusting the voltage reference value V-ref stored in the memory to correspond to an updated maximum value associated with a second time period.

12. The method of driving a light source according to any one of claims 10 to 11, further comprising controlling the power supplied to the light source at the operational value based on the stored voltage reference value V-ref, an operational control value S-op, indicative of a percentage of a maximum light source power, and the voltage level supplied by the power supply.

13. Use of the method according to any one of claims 10 to 12 for compensating the light output due to a voltage drop in a mains voltage.