WO2009000369A2

WO2009000369A2 - High voltage led lighting system

Info

Publication number: WO2009000369A2
Application number: PCT/EP2008/003843
Authority: WO
Inventors: Pedro De Smit
Original assignee: Lioris B.V.
Priority date: 2007-06-22
Filing date: 2008-05-14
Publication date: 2008-12-31
Also published as: TW200926883A; WO2009000369A3; WO2009000282A1

Abstract

The present invention relates to a lighting system comprising light emitting diodes (LEDs) wherein the LEDs are operated at high voltages. Specifically, the present invention provides a lighting system comprising: a power supply unit having an input connected to a mains network, and an output providing a rectified voltage; a carrier comprising a first layer, which is substantially electrically isolating, and a second layer, which is electrically conducting and non-floating; a lighting group comprising a plurality of serially connected light emitting diodes (LEDs) connected to said power supply and mounted to said first layer of said carrier; wherein said first layer has a critical electric field substantially larger than the root mean square voltage of said mains network divided by the smallest distance between any of said LEDs and said second layer.

Description

HIGH VOLTAGE LED LIGHTING SYSTEM

The present invention relates to a lighting system comprising light emitting diodes (LEDs) wherein the LEDs are operated at high voltages .

In recent years, light emitting diodes (LEDs) have become an attractive alternative in many lighting applications. This, amongst others, because they are more energy efficient than the conventional incandescent light bulb.

The energy efficiency of a light emitting device is related to its luminous efficacy, measured in lumen per Watt (lm/W) . This quantity takes into account the different sensitivity of the human eye to various wavelengths.

Conventional incandescent light bulbs have typical efficacies in the order of 15 lm/W which is roughly 10 times less than the effacies of recently reported Gallium Nitride based LEDs. In addition, LEDs provide very long lifetimes (100,000 hrs compared to 1000 hrs for incandescent lighting) and are very responsive to changes in drive current, which allows pulsed operation.

Nowadays, LEDs are available for the entire visible range. LEDs inherently emit monochromatic light. However, by using a combination of phosphors and an ultraviolet LED, made from Gallium Nitride, white LEDs have been fabricated. These aspects, as well as many others, drive the industry to implement LEDs in many different applications such as automotive, traffic signaling and general lighting. Because LEDs are manufactured from semiconductor material, LED based lighting systems usually require sufficient cooling to guarantee proper operation and lifetime . The design of the cooling system is especially important for high-power LEDs that can dissipate up to 3 Watt per LED. LEDs are typically mounted on a carrier, e.g. a PCB or a metal board, which in turn is connected to a housing. Because the thermal conductivity of these carrier materials is limited, they are not suited for high power operation unless the thickness of these boards is kept small.

Large lighting systems containing a plurality of LEDs, being for example mounted on a ceiling should preferably be light weight. Much of the weight of such a system is determined by the housing. Metals like aluminum or other metals or metal containing compounds are a cost effective solution that meet both the thermal and weight requirements . The use of a metal housing introduces an extra requirement for the carrier material. To ensure maximum safety, it is required that the conductive housing is electrically grounded. The carrier must therefore be able to withstand the electric field present between the LEDs it carries on the top and the conductive housing on the bottom.

Hence, carrier materials should be chosen that have a high thermal conductivity and a high critical electric field. In conventional LED systems, carrier materials are used that have thermal conductivities and critical electric fields in the order of 0.25 and 200 V, respectively.

Most LED systems operate internally at relatively low voltages in order of 10V to 30 V. Most carrier materials used today can withstand these voltages while still providing sufficient cooling. However, a drawback of this approach is related to the voltage transformation from the voltage of the mains network to the low internal voltage. If this step could be omitted, electric losses associated with this step could be avoided. Furthermore, for a given power output, a low voltage system has more ohmic losses than a high voltage system due to the higher currents that flow.

LED based lighting can also be employed for assimilation lighting in greenhouses to replace the conventional high-pressure Sodium lamps. Although these lamps currently have a higher efficacy, their spectral output is not optimized for example for the development of plants. A LED based lighting system has the advantage that an optimal choice in wavelengths can provided.

The high-pressure Sodium lighting solutions available today do not allow for efficient cooling. Large amounts of infrared radiation heat up different parts of the system in contrary to the LED based systems in which^' the heat is dissipated in the optically active junction in the semiconductor material of the LED. The latter system can therefore be more easily cooled.

The absence of suitable cooling means in conventional greenhouse lighting systems results in non-optimal growth conditions. In a greenhouse, temperature and lighting are factors that both need to be controlled. Any heat dissipated in the system could lead to a deviation from the optimal plant growth conditions.

For instance, if the ambient temperature outside the greenhouse is close to the optimal growth temperature, e.g. during the summer, conventional assimilation lighting cannot take place because the influence of the generated heat, both directly and indirectly due to infrared radiation, would cancel out the positive effect of the lighting. In a LED based system, this heat could be transferred by a cooling system allowing a prolonged time span in which the assimilation lighting can be employed. As a result, the production capabilities of such a greenhouse would increase while the energy costs would decrease.

Considering the above, it is an object of the present invention to provide an energy efficient LED based lighting system that can internally operate at high voltages and which can be equipped with a conductive housing.

A further object of the present invention is the use of such a system for assimilation lighting for the development of plants. These objectives are achieved by a system and use in accordance with the appended claims. Preferred embodiments are defined in the dependent claims.

According to a first aspect, the present invention provides a lighting system comprising a power supply unit, a carrier, and a lighting group comprising a plurality of LEDs mounted on said carrier.

The power supply has an input connected to a mains network. In this context the mains network corresponds to an electrical network present in most buildings, being in general a 230V or a 110V network.

An output of the power supply provides a rectified voltage. This voltage is preferably a rectified form of a voltage present at the input of the power supply.

The carrier comprises a first layer, which is substantially electrically isolating, and a second layer, which is electrically conducting. The second layer, or part thereof, has a well-defined electrical potential. In the art, such layer is generally referred to as an electrically non- floating layer. The lighting group comprises a plurality of serially connected light emitting diodes (LEDs) connected to the power supply and mounted to the first layer of the carrier. LEDs usually have a forward voltage drop in the order of around 1.2V.

A group of serially connected LEDs allows for a more efficient use of the available voltage headroom. The term LED usually refers to a packaged diode. To the person skilled in the art it is clear that such lighting group could also include a serial connection of discrete diodes on the same semiconductor material.

To be able to withstand the high electric fields, the first layer has a critical electric field that is substantially larger than the root mean square voltage of said mains network divided by the smallest distance between any of said LEDs and said second layer.

Because the lighting system can withstand voltages in the order of the maximum voltage of the mains network, there is no need for down converting the voltage thereby- eliminating the losses that would occur in circuitry typically employed for this purpose.

To further reduce the energy consumption, while still maintaining suitable lighting conditions, the principle of flicker-fusion is used.

This well known principle is based on the inability of biological organisms, like humans or plants, to discriminate between or show a different response to steady lighting conditions and rapidly alternating lighting conditions, provided that the frequency of the alternation is well above the so-called flicker-fusion frequency.

This frequency is different for different organisms, and can even vary between organisms belonging to the same species. Because the heat dissipated in the alternating condition is less than the steady condition, energy can be saved. In a preferred embodiment, the system further comprises an electronic control unit connected to the output of the power supply unit.

This unit modulates an electrical signal to the lighting group thereby modulating the light output of the lighting group. The frequency of this modulation is substantially larger than the operational frequency of said mains network and preferably substantially equal to or larger than the flicker-fusion frequency relevant for the given application.

For instance, if the system is used in domestic lighting the frequency would generally be in the order of 100 Hz or higher, which corresponds to the flicker-fusion frequency for human beings . In another preferred embodiment, the system further comprises an electrically conductive housing. The housing provides, amongst others, cooling and mechanical stability to the system. The carrier is preferably in direct contact with the housing to ensure maximum heat exchange between carrier and housing.

In a further embodiment, the power supply unit comprises an AC-DC converter to produce a DC voltage at the output of said unit.

Preferably, the AC-DC converter comprises a full-wave rectifier followed by a smoothing filter to achieve a DC voltage .

Additional filters to suppress harmonics re-injection into the mains network and to improve the power correction factor can be included. These circuits are well-known in the art and therefore not further detailed.

Having no voltage down conversion, the voltages in the system are typically in the order of 1.4 times the root mean square voltage of the mains network. In a preferred embodiment, the maximum voltage between any terminal of any of said LEDs and said second layer is at least HOV.

In order to be compatible with the different voltages typically encountered in the various mains networks, the first layer of the carrier preferably has a suitable critical electric field and thermal conductivity.

In another preferred embodiment, the electronic control unit modulates an electrical signal to the lighting group at a frequency substantially equal to or larger than the flicker-fusion frequency of a plant under illumination. This embodiment is specifically suited for assimilation lighting used in greenhouses.

Further, to adjust the spectral distribution of the emitted light to the spectral sensitivity of the plant it is efficient to use LEDs with associated wavelengths ranging from 375 to 500 nm, 475 to 575 nm and/or 550 to 660.

In a preferred embodiment, the first and second layer of the carrier comprise an Aluminum layer and an Aluminum oxide layer, respectively. Aluminum oxide is a very good insulator that has a high thermal conductivity.

Also the housing is preferably constructed using Aluminum. To comply with safety regulations the housing can be electrically grounded. It may be connected to the neutral of the mains network but is preferably connected to the earth ground.

Although the housing may _.provide sufficient cooling, it can be advantageous to use additional cooling means that are connected to the housing. An example of such means is a water cooling system. Such systems are well known, and a detailed description can therefore be omitted. Such a system can transport the heat extracted from the lighting system to a different system where it can be re-used or stored. For instance, energy stored in a buffer during the day can be used to heat up the greenhouse during the night.

In a preferred embodiment, LEDs with different wavelengths are used in the same group. Such an arrangement can be an appealing light source for different applications, e.g. domestic lighting.

For application in a greenhouse, the LEDs in a lighting group can be chosen so that their wavelength distribution, both in number of LEDs for a given wavelength and the relative power emitted by these LEDs, corresponds to the optimal spectral illumination for plants.

The present invention does not exclude the use of a combination of phosphors in a white LED to achieve the same goal.

To increase the total light output several lighting groups can be placed in parallel. These groups do not need to have the same wavelength distribution. On the contrary, if some of the groups can be individually controlled by the electronic control unit, the total spectral distribution of the system can be controlled.

For instance, by using a different modulation frequency and/or voltage swing, the system can operate in a mode optimal for the growth process or in a mode optimal for the flowering process of plants.

Similarly, in domestic lighting, such an application could be used to tune the lighting conditions, e.g. light intensity or color. The tuning could even be done automatically depending on ambient conditions like temperature, sound levels or the amount of available sun light.

Preferably, all the LEDs are mounted on the same carrier. The electronic control unit and power supply unit do not require the same cooling as the LEDs because much less heat is dissipated in these elements. It is therefore cost effective to fabricate these elements on a separate carrier. Conventional low-cost PCBs can be used for this purpose. In a further embodiment, the frequency or amplitude are externally adjustable. In this way, the light intensity of wavelength distribution can be adjusted by a user without having to open the system.

It is possible within the context of the present invention to change the settings corresponding to all the lighting groups as a whole or for each group or set of groups individually.

Several methods can be used to change the settings of the system including wired, wireless or infrared. Other means achieving the same goal are not excluded.

Hereafter, embodiments of the present invention will be discussed in more detail under reference to the accompanied drawings, in which:

Figure 1 illustrates an exploded cross-section of a LED-Al₂O₃ carrier-cooling unit assembly according to the invention;

Figure 2 shows a perspective view of a LED array mounted on a cooling unit;

Figure 3 illustrates a cross-sectional view of the cooling unit of figure 2;

Figure 4 illustrates part of the electrical circuit for the LED array of figure 2.

In figure 1, a LED 1 is mounted on a Al₂O₃ carrier 2. The LED is provided with connecting pads 3,3' for electrical connection with the Al₂O₃ carrier 2. Similarly, the Al₂O₃ carrier 2 is provided with corresponding pads 4,4'. In addition, the LED and Al₂O₃ carrier may be provided with thermal pads, 5 and 5¹, respectively, that provide a thermal connection that can be electrically active.

The Al₂O₃ carrier with LEDs is mounted on a Aluminum plate 6. Due to the excellent isolation properties of the Al₂O₃ carrier, sufficient electrical isolation between the LED and the Aluminum can be obtained, while at the same time providing adequate thermal conduction. Consequently, high voltage operation (>230V) can be employed reducing the need for down-conversion of the voltage, thereby eliminating losses during such conversion. The Aluminum plate 6 can be mounted in grooves 11' in a cooling unit 11, as will be discussed in conjunction with figure 3.

Figure 2 demonstrates a LED array mounted on the Aluminum cooling unit 11. The LED array comprises four subarrays 7-7 ' ' ' , each having 12 LEDs connected in series. Each subarray is fed by a corresponding electrical wire 8- 8 ' ' ' . For sake of clarity only the connection points of the wires to the subarrays are shown. A common return path is provided by electrical wire 9. The electrical wires are connected to an electronic driver 10, see figure 4.

Figure 3 illustrates a cross-section of the cooling unit 11 of figure 2. In this figure the Al₂O₃ carrier 2 with LEDs mounted on the Aluminum plate 6, which can be inserted in grooves 11' of a cooling unit 11, has been left out for clarity. As shown, heat generated by the LEDs is transported to sidewalls of the cooling unit. The electrical driver 10 is incorporated in the same housing.

Figure 4 shows part of the electrical driver 10. The driver is connected to the mains (L = phase, N = null, E = earth) of the electrical network. The voltage from the mains network is rectified by rectifying bridge Bl. A common mode coil TRl, together with capacitor Cl, is used to isolate any switching transients generated in the electrical driver from the mains network. Capacitors C2 and C3 are used to level off the rectified voltage, whereas inductors Ll and L2 are used to compensate for the phase shift that these capacitors induce. The combination of C2, C3, Ll and L2 increase the power factor.

The resulting voltage is fed to inductance L3 which is connected in series to a plurality of LEDs (LED-I ... LED- 12) . A fly-back diode Dl is placed in parallel to this series connection such that the corresponding diodes, e.g. LED-I and Dl, are anti-parallel. At the cathode side of LED-12, indicated by CON4, the series-parallel combination is connected to n-channel MOSFET Tl. Resistor Rl is in series with Tl and is used during the "on" state of Tl to measure the current through LED-I, as will be discussed later. The gate of Tl is driven by a transistor-driver ICl, e.g. the MLX10803 from Melexis or the HV9910 from Supertex. In the embodiment shown in figure 4, the MLX10803 is used and therefore only the relevant pin names of the MLX10803 are shown. Next, the operation of the lighting system will be discussed in detail.

As a starting condition it is assumed that Tl is open, e.g. low ohmic. A current will flow through L3, LED- 1.. LED-12, Tl and Rl to ground. Due to the nature of L3, this current will gradually increase thereby storing magnetic energy in the inductance. The current through LED-I is sensed using the voltage over Rl. This voltage is fed to the Rsense pin of ICl. Once the current has exceeded a certain predetermined limit, which is adjustable using a voltage applied to the Vref pin, the transistor driver switches off Tl. Consequently, the inductance will start to release its magnetic energy using a current that will flow through LED- 1.. LED-12, Dl back to L3. The maximum current as well as the time that Tl is put in the "off" state can be adjusted using components external to ICl and/or the voltage applied to pin Vref.

In figure 4, Zener diode D2, capacitor C4 and resistor R2 provide a 12V supply voltage for ICl. The external components C5, R3 and C6, R4 connected to Irefl and Iref2, respectively, can be adjusted to optimize the temperature behavior of the lighting system. With components C7, R5 the oscillation frequency of ICl can be chosen. In addition, diode D3 is used to discharge the gate of Tl. More details can be found in the application note of ICl and a more detailed description is therefore deemed unnecessary.

Finally, it should be noted that it is apparent for a skilled person in the art that various modifications and changes can be applied to the embodiments described in conjunction with the present invention without deviation from the scope of the invention as set forth in the appended claims .

Claims

1. A lighting system comprising:

A power supply unit having an input connected to a mains network, and an output providing a rectified voltage;

A carrier comprising a first layer, which is substantially electrically isolating, and a second layer, which is electrically conducting and non-floating;

A lighting group comprising a plurality of serially connected light emitting diodes (LEDs) connected to said power supply and mounted to said first layer of said carrier; wherein said first layer has a critical electric field substantially larger than the root mean square voltage of said mains network divided by the smallest distance between any of said LEDs and said second layer.

2. A system according to claim 1, further comprising an electronic control unit, connected to the output of said power supply unit, that modulates an electrical signal to said lighting group thereby modulating the light output of said lighting group, at a frequency substantially larger than the operational frequency of said mains network.

3. A system according to any of the claims 1 to 2, further comprising an electrically conducting housing connected to said second layer.

4. A system according to any of the claims 1 to 3, wherein the power supply unit comprises an AC-DC converter to produce a DC voltage at the output of said unit.

5. A system according to any of the claims 1-4, wherein the maximum potential difference between any terminal of any of said LEDs and said second layer is at least HOV.

6. A system according to any of the claims 2-5, wherein said electronic control unit modulates said electrical signal to said lighting group at a frequency substantially equal to or larger than the flicker-fusion frequency of a plant under illumination.

7. A system according to any of the claims 1 to 6, wherein said LEDs have associated wavelengths ranging from 375 to 500, 475 to 575 and/or 550 to 660 nm.

8. A system according to any of the claims 1 to 7 , wherein said first and second layer of said carrier comprise an Aluminum layer and an Aluminum oxide layer, respectively.

9. A system according to any of the claims 3-9, wherein said housing is made out of Aluminum and is electrically grounded.

10. A system according to any of the claims 3-10, further comprising cooling means connected to said housing.

11. A system according to claim 10, wherein said cooling means comprise a water cooling system.

12. A system according to any of the claimsl to 11, wherein LEDs with different associated wavelengths are used within a single lighting group

13. A system according to any of the claims 1 to 12, wherein parallel connected lighting groups are used.

14. A system according to any of the claims 1 to 13, wherein the wavelength distribution of at least two of said lighting groups is different.

15. A system according to any of the claims 1 to 14, wherein said electronic control unit controls at least two different lighting groups individually.

16. A system according to any of the claims 1 to 15, wherein the system has at least two different spectral distributions of the emitted light, corresponding to the different spectral distributions required for the growth and flowering process.

17. A system according to any of the claims 9-16, wherein said LEDs are mounted on a single carrier.

18. A system according to any of the claims 2-17, wherein said electronic control unit is mounted on a different carrier than the LEDs.

19. A system according to any of the claims 2-18, wherein the frequency or amplitude of said electronic signal is externally adjustable.

20. Use of a system according to any of the claims 1- 19 for stimulating or supporting the development of plants.