Description

The present invention relates to a linear LED light source according to the subject-matter of claim 1.

The present invention specifically relates to a linear LED light source in which LED units are linearly arranged in an elongated, substantially cylindrical translucent lamp envelope, such as a glass tube. The present invention is applicable to conventional linear LED light sources in which multiple LEDs are used in the LED units, as well as to linear LED filament light sources.

LED filament light sources such as LED filament lamps or LED filament light bulbs produce light by LED filaments - multi-diode structures that resemble the filament of an incandescent light bulb. The LED filaments consist of multiple LEDs connected in series on a transparent substrate, allowing the light emitted by the LEDs to disperse evenly and uniformly. A coating of yellow phosphor in a resin binder material converts the blue light generated by the LEDs into white light. An example for a LED filament light source is disclosed in US 8,400,051 B2.

LED filaments are also used in linear LED light sources that have a translucent lamp envelope of essentially cylindrical tube shape and a plurality of LED filaments arranged along the longitudinal axis of the lamp envelope. The length of such linear LED filament arrangements is limited owing to problems of current imbalance and a consequent reduction in lamp performance and lifetime that occur when prior art techniques for powering the LED filaments are applied to longer lamps. Consequently it has not yet been possible to capitalise on the very high efficacy and full 360 degree light emission of LED filaments in linear light sources having lengths greater than approximately 500 mm.

When multiple LED filaments are disposed along a glass tube to form a linear light source, it is essential that the current flow through all LEDs and their operating temperature is substantially the same. Any discrepancies would cause the higher current LEDs to dissipate more power than the rest, evidenced not only by a visually undesirable variation in light output between the individual LEDs along the tube, but also by a technically undesirable variation in

temperature between individual LEDs.

An ideal situation would be to connect all LEDs electrically in series such that the current flow through the whole series chain would be identical. However in practice this is difficult to realise, owing to the fact that LED filaments inherently have a relatively high voltage. This arises from their construction in which each filament contains a plurality of LEDs arranged in linear configuration and connected in series, in which the voltage of the complete LED filament is the sum of the voltages of its individual LED. As such a typical filament has a voltage in the region of 50-100V.

It is desirable that tubular LED filament lamps should be produced in dimensions substantially the same as traditional linear light sources such as fluorescent discharge tubes and LED replacements for the same, i.e. having lengths between approximately 450 mm and 2400 mm. This calls for the use of between about 10 and 80 LED filaments per lamp, and if connected in series would result in lamp voltages of around 500-8000 Volts (V).

Rather inconveniently, it is also desirable that the voltage of such lamps should not exceed about 100 V, for reasons of electrical safety, compatibility with the creepage and clearance distances of standardised lamp caps and holders established for use with linear light sources, and compatibility with the output voltage ratings of typical LED drivers and power suppliers.

Therefore, when fabricating prior art linear LED filament lamps it is customary that the plurality of LED filaments within an individual lamp are connected in parallel - or at most with series-pairs of filaments which themselves are then connected in parallel.

This configuration of electrical connection is satisfactory for lamps of physically small dimensions. However in linear lamps of considerable length, it has been found that a significant voltage drop exists across the length of the electrical supply conductors which deliver electrical current to the LED filaments, while also mechanically supporting them. The voltage drop means that the LED filaments which are located furthest from the power supply end of the lamp receive less electrical current than those at the other end of the frame closest to the power supply.

This current imbalance limits the total power at which such a lamp can be operated. The current must be set sufficiently low to avoid over-running the LED filaments closest to the power supply end, and shortening their life. The LEDs at the other end of the tube are then under-run which limits light output of the lamp, and leads to the undesirable characteristics of not only a gradually decreasing luminous flux along the length of the lamp, but more importantly, a temperature gradient along the length of the lamp.

Owing to the gas-phase cooling system employed for thermal management of LED filaments, and the relatively high thermal resistance from one filament to its neighbours via the gaseous atmosphere (which would not be the case in conventional LED products in which the temperatures of the individual LEDs tend to be homogenised due to the thermal conductivity of the printed circuit boards to which they are soldered), very strong temperature differences can exist between the individual LED filaments. The forward voltage of an LED is influenced by its ambient temperature, and the thermal imbalance which is caused by the volt drop along the length of the frame materials triggers a change in the forward voltage of each LED filament.

This further exacerbates the problem and can lead to a situation of thermal runaway in which some LED filaments begin to dissipate exponentially more and more power until they are destroyed, whereupon the same total lamp current is dissipated over the surviving LEDs, which are driven still harder and will also fail one by one until the complete lamp has been destroyed.

In order to delay the onset of this failure mechanism, even in the commercially available short linear LED filament lamps it is necessary to under-run the LEDs at lower current than would be desirable, to avoid premature failures triggered by the troublesome voltage drop along their support frames.

In light of the above, it is an object of the present invention to provide a linear LED filament light source with good radiation characteristics of considerably greater length than the prior art which does not suffer premature failures and reliability issues as the ones described above. The above object is solved by a linear LED light source according to the subject- matter of claim 1. Preferred embodiments of the invention are indicated by the subject-matter of the dependent claims.

Specifically, the present invention provides a linear LED light source comprising : a plurality of LED units conductively connected to metallic support frames, wherein the metallic support frames are configured to serve as supply conductors through which electric power for driving the LED units is feedable to the LED units, wherein the metallic support frames are manufactured from an alloy and with a diameter such that they have an electrical resistance R/l between

50 itiW/m and 200 itiW/m.

It has been found that with the selection of alloys with specific properties for the construction of the support frames the problems mentioned above can be overcome. Conventionally, steel wires are used for the support frames of LED light sources. However, steel wires are characterised by a high electrical resistance, which causes an unfavorable voltage drop, leading to current imbalances between the different LED units. The steel wires have therefore been replaced by an alloy having greatly reduced electrical resistance, which allows the diameter of the wires to be minimised and the luminous flux and efficacy of the linear light source to be maximised.

The electrical resistance R/l as defined in the claims denotes electrical resistance per length unit with the unit itiW/m (milliohms per metre). It is calculated from the specific electrical resistance or electrical resistivity of the used alloy, p, which is a material specific constant and usually given in units of W-m (ohm-metres) at a temperature of the alloy of 20°C, and the cross-sectional area A of the metallic support frame, which is usually expressed in mm ² , according to the formula

R ₌ P

l A

With support frames having the characteristics according to the present invention, the voltage drop in the LED units can be dropped to acceptable levels of less than approximately 100 millivolts per metre. Thus, a linear LED light source with considerably greater length than in the prior art can be

manufactured. Preferably, the metallic support frames are manufactured from an alloy and with a diameter such that they have an electrical resistance between 50 itiW/m and 150 itiW/m, more preferably 90 itiW/m to 120 itiW/m.

It is preferred that the metallic support frames are manufactured from nickel or a nickel alloy, preferably a nickel-manganese alloy. These alloys have a very low specific electrical resistance and favorable mechanical properties. Materials such as copper and its alloys are known to be used as materials for the wiring in electric lamps, and specifically for the tracks of printed circuit boards to which traditional LEDs are normally attached. However, copper is a very soft metal which is not mechanically robust, and which is also very difficult to attach to the LED filaments by conventional techniques such as resistance welding. Nickel and its alloys overcome these problems, providing a metal alloy with low specific electrical resistance, high mechanical stability and good weldability. The high mechanical stability further enhances the reliability of the linear LED light source, since the support frame is less prone to breaking.

Preferably, the metallic support frames are manufactured from a metal alloy that consists of 1 to 3 wt% manganese (Mn), preferably 2 wt% manganese (Mn), the remainder being nickel (Ni) and inevitable impurities. This alloy has been found to be specifically suitable for attaining the object of the present invention due to its good mechanical and welding properties.

Regarding the metallic material used for the metallic support frames, it is preferred that the specific electrical resistance p of the metallic material is in a range between 5 mWoiti and 20 mWoiti, further preferably between 10 mWoiti and 15 mWati, further preferably between 11 mWoiti and 12 mWati. As specified above, the specific electrical resistance or electrical resistivity p is a material property hat quantifies how strongly that material opposes the flow of electric current. It is defined as the resistance of a centimeter cube of a material to the passage of an electric current perpendicular to the two faces. It is calculated from the resistance R of a sample of the material in question with a length / and cross- sectional area A according to the formula It is further preferred that the support frames are manufactured as wires with a diameter of 2 mm or less, preferably 1.5 mm or less, more preferably between 1 mm and 1.5 mm, more preferably between 1.1 mm and 1.3 mm. The value of 2 mm is an upper limit for the diameter of the support frames that is typically less than the diameter of the LED filaments. Thus, the effects of light-loss due to shadowing of the light-emitting LED units can be reduced. Due to the low specific electrical resistance of the used alloys, the diameter of the support frames can be reduced to such low values without encountering a significant voltage drop along the LED units.

The cross-sectional shape of the support frames is not particularly limited and may be circular if the support frames are to be manufactured from wires.

Alternatively, the support frames may be manufactured from metal strips or sheets having a non-circular cross section to further limit optical shadowing and increase mechanical strength.

Preferably, two support frames are provided, each being conductively connected to an electrical contact of the linear LED light source. Preferably, the LED units are connected between the support frames in parallel. This allows operating all LED units in parallel. With this, the voltage for operating the linear LED light source can be greatly reduced.

In a further preferred embodiment the linear LED light source comprises a sealed lamp envelope of essentially cylindrical shape, wherein the LED units are sequentially arranged along the longitudinal axis of the sealed lamp envelope. Such an arrangement provides good radiation characteristics, since light can be emitted along substantially the entire length of the longitudinal axis. It is preferred that the LED units are arranged substantially along the entire length of the longitudinal axis. Specifically, it is preferred that the distance between the ends of the sealed lamp envelope and the respective LED unit nearest to said end of the sealed lamp envelope is smaller than four times, preferably three times, more preferably twice the diameter of the sealed lamp envelope.

It is further preferred that the length of the linear LED light source is 500 mm or more. The fabrication of linear LED light sources of these lengths is made possible by use of the inventive support frames. In a further preferred embodiment the LED units are connected to the metallic support frames by metallic spacer components. The metallic spacer components are preferably manufactured from the same material as the metallic support frames. With this construction, a high mechanical reliability as well as a further improvement of the lifetime of the linear LED light source can be attained.

It is further preferred that the linear LED light source comprises buffer springs that are configured to support the support frames against the inner wall of the sealed lamp envelope. This increases the mechanical stability of the electrical components of the light source, thus improving the robustness and contributing to a prolongation of its lifetime.

Preferably, the linear LED light source further comprises isolating bridges that are provided between the support frames and are configured to maintain a fixed relative position between the metallic support frames. This serves to further improve the mechanical stability of the linear LED light source. Preferably, the isolating bridges are arranged adjacent to the longitudinal ends of the sealed lamp envelope, respectively. Thus, the isolating bridges can support the mechanical stability of the LED units whilst minimising the blockage of emitted light.

It is further preferred that the linear LED units are constituted by LED filaments. LED filaments have an essentially omnidirectional light emission pattern and are thus beneficial for obtaining good light emission characteristics in the linear LED light source. It should be emphasised, however, that the present invention is also applicable to conventional linear LED light sources in which the LED units are constituted by LEDs of all types of packages mounted on a printed circuit board or an equivalent carrier and arranged inside the sealed lamp envelope.

The above and further features and advantages of the invention will become more readily apparent from the following detailed description of preferred embodiments of the invention with reference to the accompanying drawings, in which like reference signs designate like features, and in which: Fig. 1 shows a schematic view of a linear LED filament light source according to an embodiment of the present invention;

Fig. 2 is a schematic view of the linear LED filament light source of Fig. 1 in which relevant dimensional parameters are specified.

Fig. 1 is a schematic view of a linear LED light source according to an

embodiment of the present invention. The linear LED light source comprises a sealed lamp envelope 11 of essentially cylindrical shape that is translucent and made of glass. A light source mount assembly is arranged inside the sealed lamp envelope 11. In the present embodiment, the light source assembly comprises multiple LED units 12 mounted to metallic support frames 13a, 13b optionally via metallic spacer components 14, isolating bridges 15 and buffer springs 16.

The LED units 12 of the present embodiment are constituted by LED filaments.

The LED units 12 are sequentially aligned along the longitudinal axis of the sealed lamp envelope 11 and disposed essentially along the entire length of the sealed lamp envelope 11.

The light source assembly 10 is connected to an electrical feedthrough

component 17. Specifically, the metallic support frames 13a, 13b are conductively connected, e.g. welded or soldered to the electrical feedthrough component 17.

The metallic support frames 13a, 13b which carry the LED units 12 are supported against the inner wall of the sealed lamp envelope 11 by buffer springs 16 which serve to maintain the LED units 12 and the support structure of the support frames 13a, 13b and the optional metallic spacer components 14 along the axis of the sealed lamp envelope 11. They also serve to prevent physical damage by absorbing mechanical shocks that may be experienced during handling and transportation of the linear LED light source.

The optional metallic spacer components 14 may serve to orientate the LED units 12 in a particular mechanical configuration - in the present embodiment, in a linear configuration extending over the most part of the length of the sealed glass envelope 11. However it will be appreciated that many different mechanical configurations of the LED units 12 are possible, which may or may not require the utilisation of metallic spacer components 14. In order to further stabilise the assembly of the LED units 12 and metallic spacer components 14 two electrically isolating bridges 15 are provided near the respective ends of the sealed lamp envelope 11 to maintain a fixed relative position between the metallic support frames 13a, 13b, and may also optionally be provided at intermediate locations.

The isolating bridges 15 may be formed, for instance, from a dielectric material such as glass or ceramic bearing electrically isolated metallic wires for convenient welding to the support frames 13a 13b. The buffer springs 16 may be combined into the same physical assembly as the isolating bridges 15, as can be seen at the right end of the sealed lamp envelope 11.

The sealed lamp envelope 11 is filled with a gas filling 18 that is preferably a gas of low atomic weight like hydrogen, helium or a mixture thereof. Preferably, the gas filling 18 consists of a thermally conductive gas of low atomic mass containing fewer than 50,000 ppm (parts per million) of impurities, preferably fewer than 10,000 ppm, more preferably fewer than 1,000 ppm, further more preferably fewer than 100 ppm. The gas filling 18 preferably consists of hydrogen or helium with the specified high chemical purity. It is furthermore preferred that the sum of the contents of oxygen, nitrogen, argon and hydrocarbon vapours in the gas filling 18 is 50,000 ppm or lower, preferably 10,000 ppm or lower, more preferably 1,000 ppm or lower, further more preferably 100 ppm or lower.

Surprisingly, it has been found that the premature failure of conventional linear LED light sources, particularly linear LED filament light sources, can be attributed to properties of the gas filling 18 of the lamps. With a gas filling in compliance with the above-mentioned limitations for the constituent components, the lifetime of the linear LED light source can be elongated.

The sealed lamp envelope 11 may optionally be capped by bases 19 at one or both ends. The left base 19 is equipped with electrical contacts 17a that are connected to the electrical feedthrough components 17. The bases 19 are attached to the sealed lamp envelope 11 by an adhesive 19a. Although Fig. 1 depicts a linear LED light source with a pair of electrical contacts 17a at the same end of the linear LED light source, it should be noted that the electrical contacts 17a may also be arranged with one electrical contact 17a at each end of lamp, or with a plurality of electrical contacts 17a at both ends of the lamp. The electrical feedthrough components 17 electrically connect the light source mount assembly 10 to the exterior of the sealed lamp envelope 11. The electrical feedthrough components 17 are hermetically sealed into the sealed lamp envelope 11 in a gas-tight fashion of sufficient quality to avoid leakage of the gas filling 18.

Electrical power is fed to the linear LED light source via the electrical contacts 17a, through the electrical feedthrough components 17 to the metallic support frames 13a, 13b. If present, the metallic spacer components 14 are connected to the metallic support frames 13a, 13b and provide a conductive connection between the metallic support frames 13a, 13b and the LED units 12. Alternatively the LED units 12 may be connected directly to the metallic support frames 13a, 13b without the use of intermediate metallic spacer components 14. The metallic spacer components 14 and LED units 12 are arranged such that the LED units 12 are connected in parallel between the metallic support frames 13a, 13b.

Thus, the metallic support frames 13a, 13b and the metallic spacer components 14 not only serve not only serve as mechanical support frame for the LED units 12, but also as supply conductors via which electrical power supplied from the electrical contacts 17a is fed to the LED units 12.

The metallic components of the light source mount assembly 10 and the electrical feedthrough component 17 can be connected in any suitable manner that ensures a conductive connection between them, e.g. by welding.

The metallic support frames 13a, 13b are manufactured from an alloy having an electrical resistance R/l in the region of 50 to 200 milliohms per metre (itiW/m).

In the preferred embodiment described herein the metallic support frames 13a, 13b are manufactured from nickel or a nickel alloy, more specifically a nickel- manganese alloy, still more specifically nickel having 2% manganese content by weight, in order to attain satisfactory mechanical and welding properties.

The metallic support frames 13a, 13b may be made from ordinary wires with an essentially circular cross-section and a diameter that does not exceed 1.5mm, in order to limit light absorption and shadowing effects. The metallic support frames 13a, 13b may also be manufactured from strips or sheet metal having a non- circular cross section to further limit optical shadowing and increase mechanical strength, and may be so formed as to integrate the function of the metallic spacers 14 and the buffer springs 15 into a single component.

The cross-sectional shape of the metallic support frames 13a, 13b is not particularly limited, as long as the metallic alloy for the metallic support frames 13a, 13b and the cross-sectional area of the metallic support frames 13a, 13b are chosen such that the electrical resistance R/l, which may be calculated by dividing the specific electrical resistance p of the used alloy by the cross-sectional area A of the metallic support frames 13a, 13b, lies within the specified range of 50 W/m to 200 itiW/m.

Fig. 2 serves to illustrate relevant dimensional parameters of the linear LED light source of Fig. 1. The outer diameter of the sealed glass envelope 11 and, thus, of the linear LED light source, is denoted by d. The inner diameter of the sealed glass envelope is denoted by d,. L designates the length of the linear LED light source, excluding the protruding electrical contact pins as is standard practice.

As can be seen in Fig. 2, the light-emitting source constituted by the sequentially arranged LED units 12 extends substantially over the entire length L of the linear LED light source. More precisely, the distance between the inner ends of the sealed glass envelope 11 and the nearest LED unit 12 is smaller than twice the outer diameter of the sealed lamp envelope 11. Thus, the length of the non- radiating zones at each end of the linear LED light source does not exceed two times the outer diameter d of the linear LED source.

Through the use of the inventive support frames 13a, 13b, it is possible to produce linear LED light sources with a length of 500 mm or more. The linear LED light source according to the present embodiment can be driven at a power density greater than 6 Watts or 1000 lumens per linear foot of tube length without suffering problems of thermal runaway of the LED units 12 and the associated premature failures.

Overall, the present invention allows the length of linear LED light sources, particularly that of LED filament light sources, to be considerably increased as compared to conventional linear LED light sources. In parallel, the present invention allows the efficacy and performance of LED linear light sources to be improved, and allows the uniformity of luminous flux and temperature along the length of the linear LED light sources to be improved such that higher- performance products can be realised.

It is noted that the present invention has been motivated in the context of LED filament light sources. It is, however, emphasized that the present invention is also applicable to conventional linear LED light sources in which the LED units 12 are constituted by LEDs mounted on a printed circuit board or an equivalent carrier and arranged inside the sealed lamp envelope 11. The LED units 12 may also be constituted by LED packages as defined in the International

Electrotechnical Vocabulary (IEC 60050). According to this definition, a LED package is an electric component comprising at least one LED die, and can include optical elements, light converters such as phosphors, thermal, mechanical and electric interfaces, as well as components to address ESD concerns.

List of Reference Signs

11 sealed lamp envelope

12 LED unit

13a, 13b support frame

14 spacer component

15 isolating bridge

16 buffer spring

17 electrical feedthrough component

17a electrical contact

18 gas filling

19 base

19a adhesive

L length

d outer diameter

di inner diameter

P specific electrical resistance

A cross-sectional area of support frame

R/l electrical resistance