GB2498060A

GB2498060A - LED lamp with current dependent colour temperature

Info

Publication number: GB2498060A
Application number: GB1222031.5A
Authority: GB
Inventors: Andrew Newman
Original assignee: Gerard Lighting Pty Ltd
Current assignee: Gerard Lighting Pty Ltd
Priority date: 2011-12-22
Filing date: 2012-12-07
Publication date: 2013-07-03
Also published as: AU2012100032A4; AU2012100032B4; AU2012261628A1

Abstract

An LED lamp 20 comprises at least three LED elements LD10 - LD33 connected in series between an anode terminal A and a cathode terminal K with at least two bypass resistors R20, R30 connected between at least one of the terminals and respective nodes defined in the series string of LED elements, wherein the LED elements are of differing colour temperatures. The arrangement allows the LED lamp to output light of variable colour temperature when supplied with different current levels by variable current source S1 so as to mimic changes in colour temperature output by an incandescent lamp upon dimming.

Description

LED lamp with current dependent colour temperature

FIELD OF THE INVENTION

This invention relates to an LED lamp, in particular an LED lamp in which the colour temperature varies with the drive current.

BACKGROUND TO THE INVENTION

LED technology has been steadily emerging as a viable alternative to traditional incandescent technologies for light sources. Efficiencies have reached a point where the typical efficiency of an LED lamp is well in excess that of incandescent lamps, halogen lamps and fluorescent lamps in some applications.

Traditional incandescent technologies (including halogen) are typically dimmed using phase-cut dimmers. This reduces the average input power level to the lamp, thereby reducing the amount of light that is emitted from the lamp. As the input power is reduced, the operating temperature of the filament is also reduced. The reduction in filament temperature results in a change in the colour of the light emitted from the lamp.

The colour of the light, referred to hereafter as the Colour Temperature, also reduces.

This results in the colour of the light shifting from a white light to a more orange/red light.

In certain applications, this change in colour Temperature as the lights are dimmed is desirable. In the hospitality industry, this property of traditional incandescing technologies is exploited to make an environment feel warmer at lower light levels, typically as the time of day becomes later and later. This aspect of incandescing technologies is part of creating "the atmosphere" of many establishments.

As LED lamps have become popular, it is becoming more apparent within the lighting industry that LED light sources do not significantly alter their Colour Temperature at different brightness levels; the colour of the emitted light remains almost the same. With the increasing drive to reduce the energy consumption of our society, partly enforced through government regulation, many establishments are renovating their facilities to take advantage of the higher energy efficiency of LED light sources compared to the current installed base of traditional incandescent technologies. The inherent fixed colour temperature property of LED technology has resulted in some owners of hospitality establishments losing an aspect of their interior design.

There are existing methods and patents to achieve colour temperature variation with respects to input power for LED lamps.

A first method uses microprocessors and multiple current sources to supply precisely controllable amounts of current to two different types of LEDs. A microprocessor can be placed within the LED supply and apportion current to the two different LED types as it sees fit. This method produces very accurate colour temperature control across the input power range; can be modified to suit other lamp types without hardware design changes; and is very efficient as little power is wasted. However such a method is expensive and time-consuming to implement and locks the lamp and driver together as a pair.

A second method uses a bypass resistor and two different types of LED to ensure that one of the two LED types is dominant at low input power levels. This method is very cheap to implement and gives a reasonable colour match at the ends of the dimming range but delivers quite inaccurate control of colour temperature as input power is varied and wastes some power in the lamp.

The object of this invention is to provide an LED lamp that alleviates the above problems, or at least provides the public with a useful alternative.

SUMMARY OF THE INVENTION

Therefore in one form of the invention there is proposed an LED lamp comprising: at least three LED elements connected in series between an anode input and a cathode input defining at least 2 nodes between successive said LED elements; at least two current sinks, wherein each said current sink is connected between one of said inputs and one of said nodes, wherein said LED elements comprise LED elements of at least two different colour temperatures.

In preference each said LED element comprises a series chain of at least one LED.

Preferably said current sinks are resistors.

It should be noted that any one of the aspects mentioned above may include any of the features of any of the other aspects mentioned above and may include any of the features of any of the embodiments described below as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various implementations of the invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings:

Figure 1 is a schematic of a prior art LED lamp;

Figure 2is a schematic of preferred embodiment of an LED lamp of the present invention; and Figure 3 is a schematic of a generic embodiment of an LED lamp of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The following detailed description of the invention refers to the accompanying drawings.

Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts. Dimensions of certain parts shown in the drawings may have been modified and/or exaggerated for the purposes of clarity or illustration.

The present invention provides an LED lamp that alters its colour temperature as the input power is varied. This is achieved by having multiple colour temperatures of LED within a single lamp, and then running those LEDs at varying power levels with respects to the amount of power being supplied to the lamp as a whole. The lamp delivers most of the benefits of a more complicated microprocessor solution whilst being almost as simple to implement as the resistor bypass method, improving the viability of colour temperature changing LED lamps.

The idea behind the invention is to vary the current delivered to different colour and/or white LEDs using as simple a method as possible, whilst delivering an approximation to the colour temperature characteristics of a dimming halogen lamp. The intention is to deliver as many of the possible benefits of full microcontroller-based power allocation methods to the LED strings whilst achieving this with a high degree of simplicity in the implementation.

The lamp exploits three well known characteristics of LEDs: an LED will not conduct unless a voltage exceeding the Turn On Threshold is placed across its terminals; the forward voltage drop across an LED does not vary significantly with the current passing through it; and the light emitted by an LED is closely correlated with the current passing through it. A typical LED used in lighting applications may have a Turn On Threshold of 2.8V and a forward voltage of 3.4V at a maximum rated current of i000mA.

The invention is implemented with a specific arrangement of white LED(s) of different colour temperatures, amber LED(s) and resistors. The principle method of operation is to arrange the different LEDs in a series string, and then tap into this string at various points to inject various currents. Some of the LED current that is being drawn by the lamp by-passes portions of the LED string in the process. The lamp retains a 2-wire connection, one each for the anode and cathode of the LED string allowing the lamp to be driven by existing LED drivers. The LED strings comprise one or more LEDs in series, the current sources can either be active or passive.

In order to appreciate the operation and benefits of the present invention it is first necessary to understand the operation and limitations of the closest prior art lamps which incorporate 2 LED5 of differing colour temperature in series and a bypass resistor in parallel with the LED of the higher colour temperature.

A prior art embodiment of an LED lamp is shown in Figure 1. The lamp 10 comprises: anode input A; cathode input K; a first LED LD1 of a first colour temperature corresponding to amber, in series with a second LED LD2 of a higher colour temperature corresponding to neutral white, and a bypass resistor R2 of 1800 in parallel with LD2. The lamp 10 is driven by a variable current source Si.

The operation of the prior art lamp 10 as the current source Si is varied can be readily understood. All of the current through the circuit will all pass through LDi. The light output from LD1 will thus be directly proportional to the current supplied by Si. As LD2 and R2 are in parallel with each other and in series with LD1, the current from Si witl pass through either LD2 or R2. At low currents the current will only pass through R2 as the voltage developed across R2 will be below the Turn On Threshold of LD2. For

S

example, at lOmA the voltage developed across R2 would be 1800 x l0mA = 1.8V which is below the 2.64V threshold required for LD2 to conduct. With a current of approximately 1 4.7mA the voltage across LD2 would reach 2.64V and LD2 would begin to conduct. Beyond 14.7mA any additional current would tend to flow through LD2 in preference to R2. The net effect is that at low currents only amber light is produced and beyond a certain threshold a mix of amber and white light is produced with the proportion of amber light reducing as the current is increased. The characteristics of this type of lamp is a reasonable approximation of an incandescing lamp at either extreme of the range of operation, being predominantly amber light at low currents and white light at high currents, particularly when LD2 consists of more than one LED.

However, between the two extremes the characteristics are a poor match. In particular, as the lamp transitions over a small current range from only LD1 being on to both LD1 and LD2 being on, the ratio of amber light to white light will change very rapidly (assuming both LEDs have similar light intensity versus current characteristics).

The present invention overcomes the limitations of the prior art by introducing a gradual bypass mechanism.

A preferred embodiment of the invention is shown in Figure 2. The lamp 20 comprises: a first LED LD1 0 of a first colour temperature, preferably corresponding to amber, in series with a second chain of LEDs LD21, LD22, LD23 of a second colour temperature preferably corresponding to warm white, in series with a third chain of LEDs LD31, LD32, LD33 of a third colour temperature, preferably corresponding to neutral white; a resistor R20 of 15000 in parallel with the second and third chains of LEDs; a resistor R30 of 6800 in parallel with the third chain of LEDs.

The lamp 20 of the invention behaves analogously to the lamp 10 of the prior art, however the transition between one LED and two (chains of) LEDs being in operation is gradual and a further transition between two (chains of) LEDs and three (chains of) LED5 is also provided to better approximate the characteristics of a traditional incandescing light source.

With the resistor values shown, for progressively increasing current, only the first LED LD1 will be on until the current reaches approximately 6mA at which stage the second chain of LEDs, LD21-LD23, will begin to turn on. As the series combination of LD21-LD23 and R30 is in parallel with R20, any current flowing through LD21 -LD23 and R30 will produce a voltage rise across R30 and in turn R20. This results in the voltage across R20 rising in proportion to the total current, albeit at a lesser rate than before.

This in turn causes a more gradual increase in the current flowing through the second chain of LEDs as opposed to the more rapid change in LED current in the prior art. As the current is increased the additional current passes through LD21-LD23 in preference to R20. Due to the relatively constant forward voltage characteristic of the LEDs, the voltage across the second LED string LD2I -LD23 does not increase significantly. The voltage across R20 however continues to rise as the additional LED current results in an increasing voltage drop across R30. As the current is further increased LD31-LD33 will turn on at approximately 25mA. By having a gradual turning on of the second chain of LEDs and also a third chain of LEDs the characteristics of an incandescing light are closely approximated.

The characteristics of the lamp 20 can be varied by changing the value of the resistors, the types of LEDs used and the number of LEDs in each section of the circuit.

The concept used in lamp 20 can be further expanded with additional chains of LEDs and bypass resistors. The resistors can also be replaced with other forms of current sinks as is well known in the art such as a MOSFET in combination with an operational amplifier. Such alternative current sinks may be desired in order to improve power efficiency or to attain specific operational characteristics. A resistor however would present the simplest and most economical implementation.

A generic embodiment of the invention is shown in Figure 3. The lamp 30 comprises: an anode input A; a cathode input K; a series chain of N LED strings LS1 through LSN between the inputs A and K where each LED string comprises one or more LEDs in series; and N-i current sinks Ii through l(N-i) connected between the anode input A and the junction between each LED string. Preferably the colour temperature of each LED string increases ri turn with LEDs of the lowest colour temperature being connected to the cathode input.

The invention and prior art example have been shown with the bypass element connected to the anode input. The circuit would also work with bypass elements connected instead to the cathode or even with a combination of bypass elements connected to the cathode and bypass elements connected to the anode. The arrangement of LEDs could also be varied with different colour temperature LED5 in / different positions in the chain in order to produce a desired colour temperature versus current characteristic.

The reader will now appreciate the benefits of the present invention in providing an LED lamp with current dependent colour temperature closely approximating the characteristics of a traditional incandescent lamp without the complexity or

inadequacies of prior art implementations.

Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus.

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the

common general knowledge in this field.

In the summary of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprising" is used in the sense of "including", i.e. the features specified may be associated with further features in various embodiments of the invention.

Claims

<claim-text>CLAIMS1. An LED lamp comprising: at least three LED elements connected in series between an anode input and a cathode input defining at least 2 nodes between successive said LED elements; at least two current sinks, wherein each said current sink is connected between one of said inputs and one of said nodes, wherein said LED elements comprise LED elements of at least two different colour temperatures.</claim-text> <claim-text>2. An LED lamp as in claim 1 wherein each said LED element comprises a series chain of at least one LED.</claim-text> <claim-text>3. An LED lamp as in claim 1 or claim 2 wherein said current sinks are resistors.</claim-text>