WO2008155714A1

WO2008155714A1 - Lamp driver, lighting system and method

Info

Publication number: WO2008155714A1
Application number: PCT/IB2008/052372
Authority: WO
Inventors: Gerlach C. P. M. Emmen
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2007-06-20
Filing date: 2008-06-17
Publication date: 2008-12-24
Anticipated expiration: 2009-12-20

Abstract

The present invention provides a lamp driver, a lighting system and a method. The method comprises the steps of measuring a line voltage; determining a voltage range between two threshold voltages wherein the measured line voltage lies; determining a reference signal Sref corresponding to the voltage range, the reference signal indicating a percentage of the maximum light source power; and controlling the light source power at a value that substantially equals the reference signal Sref.

Description

Lamp driver, lighting system and method

BACKGROUND OF THE INVENTION

The present invention is concerned with a lamp driver and a method for driving lamps. Applications include for instance outdoor lighting, wherein a number of lamps, each provided with a lamp driver, is connected to the same electrical power source. In the outdoor market, both conventional drivers as well as electronic drivers are used in combination with high-intensity discharge (HID) lamps. The HID lamps commonly used in outdoor applications such as street-lighting are low pressure sodium lamps. The low pressure sodium lamps generate monochromatic (orange) light and the lamps are operated with conventional drivers. Examples of HID lamps providing non-monochromatic light include ceramic discharge metal halide lamps, hydrargyrum medium-arc iodide (HMI) lamps, mercury- vapor lamps, metal halide lamps (MH), high pressure sodium vapor lamps, xenon arc lamps, and sulfur lamps. Regarding examples of structural features of high-pressure gas discharge lamps, reference is made to EP-0587238-A1. The light-producing element of HID lamps is a well- stabilized arc discharge contained within a refractory envelope (arc tube) with wall loading in excess of 3 W/cm² (19.4 W/in.²). Compared with fluorescent and incandescent lamps, HID lamps produce a far higher quantity of light per unit area of lamp package.

To lower the energy consumption of the lamps, the mains voltage supplied to the conventional drivers may be lowered when conditions allow. Such conditions include for instance dusk or dawn, when a comparatively large amount of daylight is present, or when traffic at a road is limited.

When the mains voltage Vm decreases below a threshold voltage Vt, the lamp power Pl starts to decrease following a predetermined slope (fade), as shown in Fig. 1. When the mains voltage is above the threshold voltage Vt, the lamp power Pl is at a maximum, nominal value Pl,max. If the mains voltage drops below a minimum voltage Vmin, the lamp extinguishes, i.e. the lamp turns off.

If the conventional drivers are replaced by electronic lamp drivers, lowering of the mains voltage does not automatically result in dimming of the lamp. Lowering the mains voltage could result in extinguishing of the lamp or damaging of the driver. Therefore the electronic lamp drivers are equipped with circuitry that monitors the mains voltage and adjusts the power consumed by the lamp in dependency of the mains voltage.

An electronic lamp driver may include solid state electronic circuitry to provide the proper starting and operating electrical condition to power one or more fluorescent lamps, or more in particular HID lamps.

Electronic drivers or ballasts usually change the frequency of the power from the standard mains frequency (for instance 50 Hz in Europe or 60 Hz in the U.S.) to a drive frequency. The drive frequency is for instance 20,000 Hz or higher, substantially eliminating the stroboscopic effect of flicker (about 100 or 120 Hz, twice the line frequency) associated with fluorescent lighting. Because of the high frequency of operation, electronic ballasts are generally smaller, lighter, and more efficient (and thus run cooler) than line frequency magnetic ballasts.

Electronic ballasts are often based on SMPS topology, first rectifying the input power and then chopping it at a high frequency. Electronic ballasts may allow dimming via for instance pulse-width modulation or controlling the amplitude of a drive signal.

Electronic ballasts may allow remote control and monitoring via networks such as Lon Works, DALI, DMX-512, DSI, via analog control using a 0-1 OV DC brightness control signal, or using radio frequency (rf) control signals. Examples of structural features of electronic ballasts are disclosed in for instance WO-01/82658-A1. Regarding dimming of HID lamps using electronic lamp drivers and the construction of electronic lamp drivers, reference is made to the non- limiting examples disclosed in the following documents.

WO-2004/057934-A1 of Koninklijke Philips Electronics N.V. discloses a lamp driver for multiple-state operation of an HID lamp. A modulation is selected to generate a driving signal based on a determined power mode control selection.

The driver allows a variable setting of the transition between the low-end and the high-end of the HID lamp illumination range. The driver includes a mode control system including, for example, a discrete circuit with user inputs, a microcontroller, and a software module. The microcontroller may have an instruction set able to configure the mode control system for multiple state low or high power operation with a variety of HID lamps of different construction and output ratings. A user interface may be included for making adjustments to the mode control system. An oscillator circuit is contained in at least one of the various components of the ballast for generating high frequency square waves. In a high-frequency mode, two signals A and B are square waves of identical frequency having 180 degrees of phase lag. Generally, signals A and B have frequencies on the order of 20-110 kHz. For some applications it is beneficial to approach frequencies of 1 MHz or greater. In low frequency mode, two signals A and B comprise periodic pulse trains of high power high-frequency square waves. Short pulse trains appear at regular intervals depending on the desired drive frequency, and decline to zero during intervals between the pulse trains. The pulse trains of signal A and signal B alternate. The pulse trains generally consist of several cycles of a high-frequency square wave followed by a period of zero output equal to several times the period of the high-frequency square wave. Application of the signals to the driver circuit produces a low- frequency output signal across the HID lamp. Generally, in low- frequency mode, the resultant driving signal frequencies are on the order of 40-1000 Hz.

The lamp driver circuit therefore is capable of multiple-state power drive for the HID lamp providing both low- frequency high-power operation, and high-frequency low- power operation.

WO-02/34015-A1 provides an electronic driver for a gas discharge lamp. The driver comprises a full-bridge circuit having four field effect transistors Sl to S4. A DC voltage is applied to the input of the full bridge circuit, the output of the full bridge circuit being formed by a shunt resistance connected with ground. Only the gas discharge lamp is connected as a load. A switching over between the two bridge diagonals is effected by means of two driver circuits Tl and T2 which control the four field effect transistors Sl to S4 in a suitable manner. The regulation of the lamp brightness is effected through control of the field effect transistors S2 and S4 arranged in the bridge diagonals as regulatable constant current sources. The two field effect transistors S2, S4 are operated by an operational amplifier in their dynamic range. The transistors S2, S4 form a resistance which is connected in series with the lamp for defining an operating point for the lamp.

PROBLEMS OR DISADVANTAGES OVERCOME BY THE INVENTION In case the HID lamps supplied by the electronic ballasts are HID lamps that do not generate monochromatic light but light with a range of wavelengths, a change in the power consumed by the lamp is accompanied by a change in the color of the light generated by the lamp. Since the power supply lines have a certain impedance, the line voltage is generally lower when the power supply lines between the power supply plant and the lamp driver are longer. As a consequence, not only the amount of light decreases but also the color of the light changes when the power supply lines become longer. The above is for instance disadvantageous in street lighting. The street lighting along a highway or freeway comprises for instance one lamp every 50 m along the road. Lamps within predetermined sections, each section having a length of for instance 1 km, are connected to one electrical power source. Every section comprises a number of lamps, or in other words a group of lamps. In the present example, one group comprises about 20 or 21 lamps, wherein each lamp is provided with a lamp driver.

Due to the impedance of the electrical power lines connecting the lamps of the group to the respective power source of that group, each lamp driver is supplied with a different line voltage. The line voltage drops with increasing distance between the respective lamp driver and the power source. The amount of light that is generated by each of the lamps gradually decreases from lamp to lamp, while the color of the generated light changes from lamp to lamp as well.

Although in practice this effect may possibly be unnoticeable in the light generated by neighboring lamps, it will generally be noticeable and experienced as unpleasant in the light generated by lamps that are at a greater distance from each other. Networks such as Lon Works or DALI for controlling the lamp drivers would need additional infrastructure, such as one or more control lines, in addition to the electrical power lines. However, adding infrastructure to existing street lighting is expensive and labor- intensive.

THE FEATURES OF THE INVENTION

The present invention provides a driver for a light source, comprising: input terminals for connection to a mains supply, a first circuit, coupled to the input terminals, for generating a current through the light source out of a line voltage supplied by the mains supply; - a control circuit, comprising: a second circuit coupled to the first circuit and the light source for controlling the power supplied to the light source at a value that substantially equals a reference signal Sref; a third circuit, coupled to the input terminals and the second circuit, comprising a memory for storing a number of threshold voltages and a number of reference signal values, each reference signal value being associated with a voltage range determined by two subsequent threshold values; - a fourth circuit coupled to the memory for adjusting the value of the reference signal Sref to the reference signal value that is associated with the voltage range between two subsequent threshold voltages in which the line voltage lies.

The driver for a light source provides a cost efficient solution to control the output power of the light source at a predetermined level (and thus the color and intensity of the emitted light), using the line voltage as a control signal. The driver is suitable for application in street lighting and obviates the need to add additional infrastructure for networks such as DALI.

In an embodiment, the reference signal Sref indicates a percentage of the maximum light source power. The percentage is for instance about 40%, 60%, 80% or 100%. In another embodiment, the control circuit implements a hysteresis. The hysteresis may include a predetermined time interval and/or a predetermined voltage drop with respect to one of the threshold voltages. The hysteresis prevents unwanted adjustment of the output power, for instance in case of a temporary line voltage drop or fluctuation.

According to another aspect, the present invention provides a lighting system, comprising: an electrical power source for providing an adjustable output voltage; power lines connected to the electrical power source; a group of light sources, the group comprising two or more light sources, wherein each light source is provided with a driver and wherein each driver is connected to the power lines; the driver comprising: input terminals for connection to the power lines; a first circuit, coupled to the input terminals, for generating a current through the light source out of a line voltage supplied by the mains supply; and - a control circuit, comprising: a second circuit coupled to the first circuit and the light source for controlling the power supplied to the light source at a value that substantially equals a reference signal Sref; a third circuit, coupled to the input terminals and the second circuit, comprising a memory for storing a number of threshold voltages and a number of reference signal values, each reference signal value being associated with a voltage range determined by two subsequent threshold voltages; - a fourth circuit coupled to the memory for adjusting the value of the reference signal Sref to the reference signal value that is associated with the voltage range between two subsequent threshold voltages in which the line voltage lies.

The lighting system provides a cheap solution to control the output power of street lighting and obviates the need to add additional infrastructure. The lamps of one group substantially emit light having the same color and intensity.

In an embodiment, the power source of the lighting system comprises an adjustable voltage transformer. The adjustable transformer is relatively robust and has a relatively long live expectancy. The lighting system thus provides a cheap solution to control the output power of (existing) street lighting. According to still another aspect, the present invention provides a method for driving a light source, comprising the steps of: measuring a line voltage; determining a voltage range between two threshold voltages wherein the measured line voltage lies; - determining a reference signal Sref corresponding to the voltage range, the reference signal indicating a percentage of the maximum light source power; and controlling the light source power at a value that substantially equals the reference signal Sref.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an exemplary diagram of lamp power Pl versus mains voltage Vm; and

Figure 2 shows a diagram of an embodiment of a lamp provided with a lamp driver according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The electric lamp driver 10 according to the present invention comprises an input circuit 12, a converter circuit 14, a drive circuit 16 and a control circuit 18 (Fig. 2). The control circuit 18 is connected to the input circuit 12, the converter circuit 14 and the drive circuit 16. The input circuit 12 is electrically connected to an AC voltage source 20 via electrical power lines 22, 24 having a certain length and impedance. A lamp 26 is connected to the drive circuit 16.

The input circuit 12 may comprise a rectifier circuit, such as a diode bridge. If necessary, a power factor correction (PFC) circuit may be included in for instance the input circuit 12. The converter circuit 14 may comprise circuitry for boosting a DC voltage provided by the input circuit to another, higher DC voltage.

The drive circuit 16 comprises for instance a half bridge or a full bridge switch circuit, comprising two or four electronic switches respectively. The electronic switches include for instance transistors or field effect transistors (FET's). The control circuit 18 is arranged for controlling a switching frequency of the switches.

The control circuit 18 measures the voltage over the electrical power lines 22,

24, at the input circuit 12. The control circuit 18 may also be connected to the drive circuit for measuring the lamp current and lamp voltage, for determining the lamp power. Measuring the voltage over the electrical power lines for instance includes measuring the RMS value or the amplitude of the supplied AC voltage.

According to the invention, the control circuit 18 keeps the lamp power at a substantially constant, first level when the mains voltage fluctuates within a first predetermined range. The control circuit changes the lamp power to another, second level if the line voltage changes to a voltage within a second predetermined range, etc. Thus, the control circuit controls and changes the lamp power stepwise as a function of the line voltage.

The number of ranges is optional, and for instance depends on the mains voltage range, user requirements, and/or a dimming range of the lamp power. The dimming range is for instance about 20%, 40%, or 60% up to 100% of maximum power. The control circuit 18 includes a second circuit, a third circuit and a fourth circuit (not shown).

The second circuit is coupled to the input circuit 12 and to the drive circuit for controlling the power supplied to the light source at a value that substantially equals a reference signal Sref. The third circuit is coupled to the input terminals of the input circuit 12 and to the second circuit. The third circuit comprises a memory for storing the threshold voltages and a number of corresponding reference signal values. Each reference signal value is associated with a voltage range determined by two subsequent threshold voltages. The fourth circuit is coupled to the memory of the third circuit. The fourth circuit is adapted to adjust the value of the reference signal Sref to the reference signal value that is associated with the respective voltage range between the two subsequent threshold voltages.

Each reference signal Sref corresponds to a percentage of the maximum lamp power. The second circuit of the control circuit controls the lamp power in accordance with the reference signal provided by the third circuit.

An example of voltage ranges is provided below. Please note that other voltage ranges are conceivable, within the scope of the present invention. In the example below, the voltages 198V, 210V, 230V and 240V are subsequent threshold voltages. If the line voltage exceeds 240 V, the control circuit could for instance control the drive circuit to drive the lamp at 100% of the nominal value of the lamp power.

Example of voltage ranges

(European mains voltage: 198V - 264V)

Line voltage range Lamp power (Pl)

1 ^st 198 V - 21 OV 60% of nominal value of lamp power

2^nd 210V- 230V 80% of nominal value of lamp power

3^rd 230V- 240 V 100% of nominal value of lamp power

Examples of changing the lamp power to another level using an electronic driver are provided above. The control circuit for instance drives the electronic switches of the drive circuit at two frequencies, i.e. at a first high frequency, and at a second, lower frequency.

The first frequency is a drive frequency. Depending on the type of lamp, the drive frequency for an HID lamp may for instance be in the following ranges:

Low Frequent Square Wave (LFSW) 50Hz to 500Hz;

Medium Frequency (MF) 500Hz to 10kHz;

High Frequency (HF) 1 OkHz to 10OkHz;

Very High Frequency (VHF) 10OkHz to 50OkHz; Ultra High Frequency (UHF) 50OkHz and higher.

The second frequency is for instance a square wave that switches the first drive frequency on and off. A cycle of the second frequency is in the order of milliseconds, for instance 100 Hz to 5 kHz. Dimming the lamp may involve increasing the off-time of the square wave of the second frequency. The present invention however is independent of the drive frequency of the lamp. The control circuit of the present invention adjusts the output power of the drive circuit in case the line voltage Vm reaches one of the threshold voltages. By changing the output power, the control circuit changes the power Pl consumed by the lamp. The threshold voltages determine the boundary conditions of the voltage ranges. The threshold voltages determining the voltage ranges are set within the normal working range of the respective driver.

To prevent adjustment of the output power of the drive circuit in case of a temporary, undesired fluctuation of the line voltage, the control circuit may implement a predetermined hysteresis. A temporary fluctuation of the line voltage includes for example transients, swells, sags, or interruptions. The hysteresis may include a predetermined time interval, and/or a predetermined voltage drop with respect to one of the threshold voltages. I.e., the control circuit only changes the output power if the line voltage is within another voltage range longer than the predetermined time interval, and/or further than the predetermined voltage drop. The time interval is for instance in the range of 1 to 10 seconds, or up to 60 seconds. The predetermined voltage drop is for instance in the range of 1 to 5 Volt.

All lamps of a group of lamps preferably emit light of the same intensity and the same color. Therefore, the power source connected to a group of lamps preferably supplies the power lines with a voltage that is near the higher voltage threshold value of the respective voltage range. In the above example, near the higher voltage threshold value indicates for instance a voltage between about 205V-210V (1^st range), 225V-230V (2^nd range), or 235V-240 (3^rd range) respectively. As a consequence, the amount of light as well as the color of the light generated by the group of lamps is identical. Thus all lamps within a group emit light at the same intensity and color. Color spread is substantially prevented. As all the lamps of the group emit light at the same intensity, the amount of light (lux) on the road is constant along the road.

The line voltage is monitored in the control circuit of the driver. When the line voltage has a value that is in a voltage range between two subsequent threshold voltages, the lamp power is adjusted to a predetermined level corresponding to that voltage range. As a consequence, over a line with a voltage drop, the lamp power (and therefore also the color of the light) is maintained at the same value as long as the line voltage is within the range between two subsequent threshold voltages. The above described lamp driver and method of the present invention allow changing, i.e. controlling the light intensity and color of street lighting by controlling the voltage of the power lines, without additional infrastructure needed to implement control networks such as DALI. The voltage of the power lines may be changed using an adjustable voltage transformer. The transformer could for instance be arranged to adjust the power line voltage in steps corresponding to the above described voltage ranges. All lamps of a group of lamps can be remotely controlled by changing the line voltage. Information may thus be transferred via the existing power lines. The robustness of the adjustable voltage transformer provides the additional advantage of long life expectancy. The present invention is not limited to the above described embodiments. The described embodiments may be appropriately scaled according to the respective application. Modifications of the embodiments are conceivable within the scope of the appended claims.

Claims

CLAIMS:

1. Driver for a light source, comprising: input terminals for connection to a mains supply (20), a first circuit, coupled to the input terminals, for generating a current through the light source (26) out of a line voltage supplied by the mains supply; - a control circuit (18), comprising: a second circuit coupled to the first circuit and the light source for controlling the power supplied to the light source at a value that substantially equals a reference signal Sref; a third circuit, coupled to the input terminals and the second circuit, comprising a memory for storing a number of threshold voltages and a number of reference signal values, each reference signal value being associated with a voltage range determined by two subsequent threshold voltages; a fourth circuit coupled to the memory for adjusting the value of the reference signal Sref to the reference signal value that is associated with the voltage range between two subsequent threshold voltages in which the line voltage lies.

2. Driver of claim 1, wherein the reference signal Sref indicates a percentage of the maximum light source power.

3. Driver of claim 2, wherein the percentage is about 40%, 60%, 80% or 100%.

4. Driver of any of claims 1-3, wherein the control circuit (18) implements a hysteresis.

5. Driver of claim 4, wherein the hysteresis include a predetermined time interval, and/or a predetermined voltage drop with respect to one of the threshold voltages.

6. Driver of claim 5, wherein the control circuit (18) is arranged to change the output power of the light source if the line voltage is within another voltage range longer than the predetermined time interval.

7. Driver of claim 5 or 6, wherein the time interval is in the range of about 1 to

60 seconds, for instance about 10 seconds.

8. Driver of claim 5 or 6, wherein the control circuit (18) is arranged to change the output power of the light source if the line voltage is within another voltage range further than the predetermined voltage drop.

9. Driver of claim 8, wherein the predetermined voltage drop is in the range of about 1 to 5 Volt.

10. Driver of any of the previous claims, wherein the light source comprises an

HID lamp, a low pressure mercury discharge lamp, or an array of LED's.

11. Driver of any of the previous claims, wherein the first circuit comprises: an input circuit (12) for converting the line voltage to a DC voltage via power factor correction (PFC); and a drive circuit (16) for converting the DC voltage to an AC voltage over the light source.

12. Lighting system, comprising: - an electrical power source (20) for providing an adjustable output voltage; power lines (22, 24) connected to the electrical power source (20); a group of light sources, the group comprising two or more light sources, wherein each light source (26) is provided with a driver (10) and wherein each driver is connected to the power lines (22, 24); - the driver (10) comprising: input terminals for connection to the power lines (22, 24), a first circuit, coupled to the input terminals, for generating a current through the light source (26) out of a line voltage supplied by the mains supply; and a control circuit (18), comprising: a second circuit coupled to the first circuit and the light source for controlling the power supplied to the light source at a value that substantially equals a reference signal Sref; a third circuit, coupled to the input terminals and the second circuit, comprising a memory for storing a number of threshold voltages and a number of reference signal values, each reference signal value being associated with a voltage range determined by two subsequent threshold voltages; a fourth circuit coupled to the memory for adjusting the value of the reference signal Sref to the reference signal value that is associated with the voltage range between two subsequent threshold voltages in which the line voltage lies.

13. Lighting system of claim 12, wherein the power source comprises an adjustable voltage transformer.

14. Method for driving a light source, comprising the steps of: measuring a line voltage; determining a voltage range between two threshold voltages wherein the measured line voltage lies; determining a reference signal Sref corresponding to the voltage range, the reference signal indicating a percentage of the maximum light source power; and controlling the light source power at a value that substantially equals the reference signal Sref.