WO2014001987A2

WO2014001987A2 - Light engine module, related power supply unit and lighting system

Info

Publication number: WO2014001987A2
Application number: PCT/IB2013/055106
Authority: WO
Inventors: Francesco Angelin; Paolo De Anna; Bernd Rudolph
Original assignee: Osram GmbH; Osram SpA
Current assignee: Osram GmbH; Osram SpA
Priority date: 2012-06-25
Filing date: 2013-06-21
Publication date: 2014-01-03
Anticipated expiration: 2014-12-25
Also published as: EP3413690A8; US20170111966A1; CN104412708A; US20150195877A1; EP3413690B1; WO2014001987A3; WO2014001360A1; CN104412710A; US20150195884A1; US9554430B2; EP2865239A1; WO2014001342A1; CN104412710B; CN105960056A; EP3413690A1; EP2865238A1; US9872350B2; US9445466B2; US9609700B2; US20150145437A1

Description

"Light Engine Module, related Power Supply Unit and lighting system"

-k -k -k

Field of the Invention

The invention relates to the field of Solid State

Lighting, and describes an interface for a Light Engine Module to its Power Supply Unit and the Light Engine Module respective the Power supply unit. The present invention generally relates to a Power Supply Unit for driving one or more Light Engine Modules, in particular Light Engine Modules with light-emitting diode (LED) light sources, and a lighting unit including a Power Supply Unit and at least one Light Engine Module. More particularly, various inventive methods and apparatus disclosed herein relate to a self-adjusting Power Supply Unit for driving one or more Light Engine Mod^¬ ules with light-emitting diode (LED) light sources, and an LED-based lighting unit including a self-adjusting Power Supply Unit and at least one Light Engine Module.

Technical Background

Illumination devices based on semiconductor light sources, such as light-emitting diodes (LEDs) , offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, longer expected lifetime, lower operating costs, and many others.

In some applications, an LED-based lighting unit may include a Power Supply Unit which supplies an LED driving current to a plurality of Light Engine Modules, each including one or more LEDs. For example, an Light Engine Module may include a circuit board (e.g., a printed circuit board) having one or more LEDs mounted thereon. Such circuit boards may be plugged into slots in a lighting fixture, or a motherboard, on which the Power Supply Unit may be provided.

In various applications and installations, an LED- based lighting unit may include different numbers of LEDs and/or Light Engine Modules. For example, the number of LEDs and Light Engine Modules may be changed depending on the light output requirements, e.g. lu^¬ mens, for a particular installation.

In general, the magnitude or level of the LED driving current output by a Power Supply Unit will need to be changed according to the number of LEDs and Light Engine Modules to which it is connected and which it drives. This means that if a single Power Supply Unit is going to be employed in a variety of LED-based lighting units with different numbers of LEDs and/or Light Engine Modules, then the Power Supply Unit will have to include a means or provision for adjusting the LED driving current to match the current driving re^¬ quirements for the different Light Engine Modules ac^¬ cording to the different numbers of light sources that they include. Meanwhile, the number of LEDs and Light Engine Modules to be included in a particular LED-based lighting unit is determined at the time of manufactur^¬ ing that LED lighting unit. Thus, if the same Power Supply Unit is to be employed in a variety of LED lighting units with different numbers of Light Engine Modules, then the power supply unit would have to be programmed at the time of manufacturing for each dif^¬ ferent LED lighting unit so that its output LED driving current is appropriate for the particular number of Light Engine Modules that are included in that LED lighting unit.

This problem has been addressed by means of inter^¬ facing between Power Supply Unit and Light Engine Mod^¬ ule .

Interfacing means that the Light Engine Module provides the Power Supply Unit with some information, regarding its needed current to fulfil flux specifica^¬ tion and/or its working temperature, in order to reduce the supplied current level when a certain limit is exceeded. There are several ways in the Art to inter^¬ change this information between the Light Engine Module and the Power Supply Unit. Buses can be used to inter^¬ change such information. Known in the art are analog buses like the 0..10V bus or digital buses like the DALI (Digital Adressable Light Interface) bus. Also known in the Art are simple Resistor networks that can be measured by the Power Supply Unit and tell the Power Supply Unit the current requirements of the Light En^¬ gine Modules. DE 100 51 528 discloses such an interface where a specific Resistor is connected between a third wire and the negative supply line. If several Light Engine Modules are connected to one Power Supply Unit, the resistors are connected in parallel or serial, so a sum signal is given into the Power Supply Unit to de- fine the current requirements. The german patent appli^¬ cation 102011087658.8 discloses also resistors to de^¬ fine the current requirement of each Light Engine Mod^¬ ule .

The bus solutions have the disadvantage of two ex- tra wires needed. The resistor solutions only need one extra wire, but the evaluation of the resistor network and the current adjustment can be very complex.

Since complete Power Supply Unit and Light Engine Module systems have appeared on the market, different companies have tried to fix a way to make the two parts communicate; also some digital protocols have been used for the more complex and high-end systems, but this latter technique is out of the present invention' s background, and have to be considered apart.

For instance, the company OSRAM has already pro- posed a three extra-wire interface, able to supply also power to an active Light Engine Module onboard circui^¬ try which provides thermal derating. In this interface type a Light Engine Module onboard resistor forms a divider with a Power Supply Unit pull-up resistor, in order to develop a voltage which sets the Power Supply Unit output current. An operational amplifier on the Light Engine Module then starts to limit this voltage (so reduces the current) when the module overheats.

The company Philips has proposed a different ex^¬ tra-three wire interface, where one wire is connected to the current setting resistor, while another one is connected to a temperature sensing resistor, and the derating is done by the Power Supply Unit itself, not involving any active part on the Light Engine Module.

Both interfaces include a third extra wire for the common signal ground return, and use a voltage devel^¬ oped by the Light Engine Module resistor to set the current, in such a way that the greater voltage causes the greater output current.

Recently, the company OSRAM has come out with a slightly different interface, that actually is a 0..10 V one customized with a precise current source in the Power Supply Unit to enable the Light Engine Module to use just a resistor to set the current.

Now a new request rises from the market, i.e. the capability of paralleling different modules to be sup^¬ plied by the same Power Supply Unit. Obviously the Power Supply Unit's outputted current must be the sum of each Light Engine Module nominal value, and the thermal derating capability must be kept even for a multiple Light Engine Module arrangement.

As well, the market is asking for a cost cut, ac^¬ tually pointing to a wire number reduction. Bus-based interfaces normally need 4 wires, two for the power supply of the Light Engine Modules and two for the bus.

So a couple of new features to satisfy the needs have been postulated:

- Multiple modules must be allowed to be connected in parallel using the same interface (of course the different modules are supposed to be identical, or at least to have the same string voltage) .

- The setting interface must have a reduced number of wires, and must be as simple as possible in order to reduce costs, especially at the Light Engine Module side .

All the known interfaces proposed up to now are not able to support multiple Light Engine Module con^¬ nections, a new interface is proposed in order to ful- fill all the newest requirements.

Summary of the Invention

In the present invention, the way to provide the current requirement information is analog, i.e. by the magnitude of some electrical parameters directly set by the onboard circuitry of the Light Engine Module: these parameters are then recognized by the Power Supply Unit which adjusts its output current as demanded.

Hereafter both a concept and a possible implementation of a "one wire" analog interface are proposed, with "one wire" meaning that only one extra wire is need besides the two power wires.

Various embodiments of the invention are directed to a Light Engine Module comprising:

- a plurality of series connected LEDs

- a positive power supply line

- a common ground line

- a communication line where signals on the commu^¬ nication line are measured against the common ground line,

- a current set resistor with its value propor- tional to the current demand of the Light Engine Mod^¬ ule,

- an amplifier responsive to a temperature sensi^¬ tive input current,

- a temperature sensitive resistor responsible for the temperature sensitive input current.

This Light Engine Module is easy and cheap to build and can handle different current demands and thermal derating over only one extra line.

The conductance of the current set resistor is preferably direct proportional to the current demand of said Light Engine Module. This eases the processing of the measured signal.

Additionally the Light Engine Module is preferen- tially comprising a threshold set resistor responsible for a threshold level for thermal derating. This has the advantage of thermal derating only taking place at higher temperatures so no light will be lost in nominal operation .

The Light Engine Module comprises preferably also:

- a voltage source with the negative output con^¬ nected to common ground,

- a series connection of the temperature sensitive resistor and the threshold set resistor connected in parallel to the voltage source,

- a transistor where the base is connected to the node between the temperature sensitive resistor and the threshold set resistor, the collector is connected to the node between the positive output of the voltage source and the temperature sensitive resistor, the emitter is connected to the communication line via a emitter resistor.

This current source is capable of conducting a du^¬ ration of the supplied current to substantially zero to protect the Light Engine Module. The Light Engine Module comprises preferably in an alternative solution:

- a voltage source with the negative output con^¬ nected to common ground,

- a transistor where the base is connected to the node between the temperature sensitive resistor and the threshold set resistor, the emitter is connected via a emitter resistor to the node between the positive out^¬ put of the voltage source and the temperature sensitive resistor, the collector is connected to the communica^¬ tion line.

In another embodiment the voltage source is de^¬ rived from the two power supply lines.

In a further embodiment the voltage source is con^¬ nected with common ground and tapped between a portion of the plurality of series connected LEDs .

Various embodiments of the invention are also di^¬ rected to a Power Supply Unit comprising:

- an output providing electrical power between a positive power supply line and a common ground line,

- a communication line where signals on the commu- nication line are measured against the common ground line,

- an adjustable current generator responsive to an internal measurement signal generating an output cur^¬ rent at the output,

- a voltage source coupled to the communication line,

- a current measurement unit generating the inter^¬ nal measurement signal.

This allows an easy and cheap Power Supply Unit with adequate precision. In a first embodiment the current measurement unit comprises a current measurement resistor and an opera^¬ tional amplifier.

In a second embodiment the Power Supply Unit has the following attributes:

- the voltage source is connected between common Ground and the positive input of the operational am^¬ plifier,

- the current measurement resistor is connected between the negative input of the operational amplifier and the output of the operational amplifier,

- the communication line is connected to the nega^¬ tive input of the operational amplifier,

- the output of the operational amplifier outputs the internal measurement signal.

This enhances the precision and leads to a cheap and precise power source.

In a further embodiment the voltage source is ad^¬ justed in a way, that every single Light Engine Module can decrease the supplied current to substantially zero. This protects the Light Engine Modules.

In a further embodiment the Power Supply Unit con^¬ ducts linear characteristic between the internal mea^¬ surement signal and the output current of the adjusta- ble current generator. This leads also to a easy and cheap measurement unit.

In a still further embodiment the linear characte^¬ ristic takes place only over a portion of the total range of the internal measurement signal. This gene- rates a threshold for the temperature derating.

In a still further embodiment the beginning of the portion is defined by an offset voltage of the internal measurement signal, and below this offset voltage no current will be provided by the adjustable current generator. This also is good for the temperature degra- dation .

In another embodiment the offset voltage is iden^¬ tical to the value of the voltage source. This leads to an easy implementation.

In another further embodiment the value of the current measurement resistor is defined by the maximum current output of the adjustable current generator. This helps to scale the Power Supply Unit in regard to the current capabilities.

Various embodiments of the invention are also con^¬ ducted to a lighting system, comprising:

- a Power Supply Unit;

- at least one Light Engine Module;

wherein the Power Supply Unit and the Light Engine Module have an interface as described above, providing information to the Power Supply Unit on the current demand of the connected Light Engine Modules.

In a first embodiment of the lighting system the Power Supply Unit includes an identification voltage source supplying a Light Engine Module identification voltage and measuring the corresponding identification current. This leads to an easy and cheap interface between the Power Supply Unit and the Light Engine Module .

In a second embodiment all Light Engine Modules are connected in parallel. This leads to a very simple handling for the customer.

As anticipated, the features of this analog inter^¬ face are :

- To tell the Power Supply Unit which is the no^¬ minal supply current of the Light Engine Module.

- To reduce the supplied current e.g. when the temperature is higher than expected. This is sometime referred as "thermal derating", and it is an event caused by unpredictable reasons. Brief Description of the Drawings

Further advantages, features and details of the invention will be apparent from the following description of exemplary embodiments and the attached drawings in which identical or functionally identical elements are designated by identical reference numerals. In the drawings show:

- Fig. 1: The paralleling concept of current set resistors .

- Fig. 2: A simple solution for thermal derating.

- Fig. 3: The complete concept of the invention with the thermal derating unit TDU.

- Fig. 4: A very simple TDU implementation.

- Fig. 5: A simple system implementation. V_out is the internal voltage representing the output current.

- Fig. 6: A simulation graph of the circuit of Fig. 5.

- Fig. 7: A schematic circuit of how to model the cable voltage drop due to LED current.

- Fig. 8: A characteristic of the Current Genera^¬ tor

- Fig. 9: A characteristic of the Current Mea^¬ surement Unit

Description of preferred embodiments

In the following, several embodiments of the in^¬ ventive concept will be described. The inventive con^¬ cept always deals with a three wire interface, where several Light Engine Modules can be connected in paral^¬ lel to a Power Supply Unit and the current requirements of every Light Engine Module match.

First embodiment: Analog Circuit

The basic idea of having a resistor to set the current has been kept, but the inventive concept of using it is different. Fig. 1 shows the general paral- leling concept of current set resistors. Three Light Engine Modules LEM connected to a Power Supply Unit PSU are shown. The connection consists of three lines: A supply line LED+, a common ground line LED- and a com^¬ munication line CL . Each Light Engine Module contains at least one LED string. The LED string consists of a plurality of LEDs . A plurality in the light of the invention means that there are at least three LEDs connected in series. Each Light Engine Module also contains a current set Resistor Rset. The current set Resistors are connected between the common ground line LED- and the communication line CL . This leads to a parallel connection of each current set Resistor Rsetl, Rset2, Rsetm, so the Power Supply Unit PSU measures the equivalent resistance of that parallel connection. The concept is to have the Power Supply Unit PSU reading not a voltage as in the prior art, but a current repre^¬ sentative for the resistance value. Then an inverse law is applied to the resistance value to set the Power Supply Unit's output current. The law is as follows:

Output

S^e>

Kv has the dimension of a voltage.

By doing so, the Power Supply Unit's output cur^¬ rent is inversely proportional to the Light Engine Module current set resistor value Rset, i.e. the small- er the resistance, the higher the output current of the Power Supply Unit PSU.

This intrinsically satisfies the requirement of having a final current equal to the sum of each single Light Engine Module one, according to the well known Ohm' s law .

Fig. 2 shows a concept schematic of an interface with a thermal derating capability. This adds a very simple thermal derating by putting a PTC element in series with Rset. As the temperature of the Light En- gine Module LEM rises, the value of the PTC also rises leading to a smaller current for that module. The dis^¬ advantage of such an arrangement is that it won't be adequate for a multiple Light Engine Module connection, because a single PTC action would take away from the sum of the parallel connected resistors Rset only that member's contribute, and this could be not enough to reduce the suffering Light Engine Module's temperature enough .

Anyhow this solution could be kept for very low- cost applications, when a partial current reduction in the event of overheating is still acceptable.

Furthermore, a simple temperature element in se^¬ ries with the current setting resistor has the disad- vantage of continuously derating the current, without having a precise starting point for the derating itself (even if some PTC elements have a very steep behaviour around the trigger temperature) . So the "nominal" cur^¬ rent setting would be corrupted by a "parasitic" effect of the derating element.

Fig. 3 shows the inventive concept of an interface with a thermal derating unit TDU.

The concept relies on a different approach, by adding an extra current generator TDU onboard the Light Engine Module. This current generator is temperature controlled by a sensing element, and takes power di^¬ rectly from the Light Engine Module's power line, in order to avoid extra wires for the interface. The cur^¬ rent generator comprises a temperature sensitive resis- tor generating an input current and an amplifier amplifying that input current to the generated current I_TDU- The generator is arranged with a threshold which inhi^¬ bits any current injection until a certain over- temperature of the Light Engine Module is achieved. Then the slope of current versus temperature (gain of I_TDU) is high enough for the system to try to stabilize the max working temperature of the Light Engine Module, but not so to trigger instabilities due to heat trans^¬ mission time lags. The current generator is able to override completely the signal generated by the paral^¬ leled resistors Rset: in such a way it can safely pro^¬ tect the whole system and especially its own Light Engine Module even in case of multiple Light Engine Module connection together with a very concentrated overheating.

With the temperature dependent current generator a new problem arises. It is necessary to measure Rset independent of the actual temperature of the module and therefore independent of the provided current of the current generator. The way to measure Rset out must be fixed in order to make the action of the current gene^¬ rator predictable.

In various embodiments, the invention uses a fixed voltage generator Vk to measure the resistance value, by putting this voltage across the resistor Rset (or their parallel) and then reading the current flowing through it. This in turn makes the current generator TDU directly interacting with the current fixed by Vk on Rset, resolving the final behavioural law.

Fig. 4a shows a first embodiment of the Light En^¬ gine Module providing the inventive interface, with just one bipolar transistor, an NTC element and a couple of added resistors.

The circuit contains a voltage source VI, which is derived from the supply line LED+ of the LED module. LEDs have a quite stable flux voltage, so this can serve as a voltage source "good enough". Dependent of the supply voltage needed for TDU, the voltage source VI, always connected to common ground LED-, can be tapped between a portion of the plurality of series connected LEDs . This means, the voltage VI can be ad^¬ justed in a way that it represents a multiple value of a single LED flux voltage. In parallel to this voltage VI there is a series connection of the NTC and a thre- shold resistor Rthr . The base of a NPN Bipolar Junction Transistor (BJT) Ql is connected to the node between the NTC and Rthr. The collector of Ql is connected to the voltage VI. The Emitter of Ql is coupled to the communication line via an emitter resistor Rtg. All these components of Fig.4a described above are forming the thermal derating unit TDU.

The current set Resistor Rset is connected between the rail-wise positioned CL and common ground LED- lines of the power supply.

In this circuit the potential of Ql's emitter is referred to a forced voltage (by definition Vk) in the Power Supply Unit PSU that realizes the threshold below which no current I_TDU is injected into the CL line. When the temperature rises, the NTC starts to raise the base potential, until moving Ql into the active region. Now the emitter resistor Rtg sets the gain of the circuit TDU, and fixes the slope of the injected current I_TDU versus temperature.

The resistor Rthr, together with the NTC at the temperature trigger specified for the TDU, sets the thermal derating starting point in relation to the voltages VI and Vk.

A further advantage of this arrangement is the good linearity of the current I_TDU versus temperature achievable.

One of the most interesting advantages of the in^¬ vention, besides the easiness of the implementation on the Light Engine Module side, is its capability to be used in different quality grade systems, by adjusting the wanted accuracy and features only by scaling the Power Supply Unit interface's circuitry complexity. In other words, it's possible to build the reading inter^¬ face on the Power Supply Unit side according to the requested accuracy and/or extended features needed.

Fig. 4b shows a second embodiment of the Light En^¬ gine Module LEM interface with a dual implementation. Here a PNP-Type Transistor Q2 is used together with a PTC. A PTC is a temperature sensitive resistor with a positive temperature coefficient. The voltage VI is derived from either the whole number of series con^¬ nected LEDs or a portion of the series connected LEDs . In contrary to the embodiment of Fig. 4a, the collector of Q2 is providing the current source characteristic producing the current I _DU/ and is connected to CL . Thus the temperature derating threshold is not depending on Vk but only on VI and the values of the voltage divider formed by the temperature sensitive resistor PTC and the threshold resistor Rthr .

Thus, in the embodiments discloses in the forgoing the Light Engine Module LEM comprises a set resistor Rset to specify the nominal current and a thermal de^¬ rating unit TDU, which generate a current as a function of the temperature of the Light Engine Module LEM. Specifically, in the embodiments shown in Figures 4a and 4b, a bipolar transistor is used to amplify the current through a temperature sensitive resistor. How^¬ ever, in general, also other variable current genera^¬ tors could be used.

Moreover, the current generated by the unit TDU could also depend on other parameters. For example, also other sensors could be used to vary the current generated by the unit TDU, e.g.:

- a light sensor configured to detect the ambient luminosity; e.g. in order vary the light generated by the Light Engine Module LEM as a function of the am- bient light, e.g. in order to keep the total light quantity substantially constant;

- a twilight light sensor, e.g. for activating or deactivating the Light Engine Module LEM as a function of the ambient light;

- a movement sensor, such as a Passive InfraRed (PIR) sensor, e.g. for activating or deactivating the Light Engine Module LEM only in the presence of humans; and/or

- a wireless receiver for receiving a signal form a remote control.

The unit TDU may also comprise a plurality of such sensors in order to implement different functions. For example, in an embodiment, the unit TDU may comprise a digital processor, such as a microprocessor, wherein at least one sensor is connected to the processor via an analog to digital converters and the processor gene^¬ rates the output current via a programmable current generator. For example in this way, more complex con- trol functions could be implemented.

Finally, instead of performing a tapping between a portion of the plurality of series connected LEDs, the supply signal for the unit TDU may also be obtained in a different manner. For example, the supply signal for the unit TDU may be generated by a current or voltage generator connected directly to the supply line LED+. Moreover, in principle, the supply signal could also be derived from the voltage Vk.

Thus, the Light Engine Module LEM disclosed herein comprises a passive component (i.e. the resistor Rset) , which specifies the nominal current requirements, and an active current generator, which generates a compensation current, thus modifying the current I_CL flowing through the control line CL . Specifically, in various embodiments, this compensation current is determined as a function of certain operation conditions detected by means of one or more sensors. Due to the fact that the power supply unit PSU varies the supply current for the Light Engine Module LEM as a function of the current I_CL flowing through the control line CL, the current generator of the unit TDU may modify the current re^¬ quirements of the Light Engine Module LEM.

Fig. 5 shows an embodiment of the Power Supply Unit's PSU interface. This is a very simple circuit for cheaper Power Supply Units, where no high accuracy is needed .

Due to the requirement of reduced connection lines and the concept of a common ground line LED-, the prob^¬ lem of voltage drop on that common Ground line LED- due to the Light Engine Module current (s) for the LEDs arises. The embodiment adopts a very simple circuit based on a single operational amplifier, without any compensation of the ground line offset due to the Light Engine Module current. The Power Supply Unit interface comprises an operational Amplifier OpAmp, where its negative input is connected to the communication line CL . The output generates an internal measurement signal Vout, which is used to adjust the current lout provided at the output of the Power Supply Unit. The output of the Power Supply Unit is connected to LED+ and LED- of the Light Engine Module. A current measurement resistor Rfb is connected between the output and the negative input of the operational amplifier OpAmp, thus forming its mandatory negative feedback. A voltage source Vk is connected between the positive input of the operational amplifier OpAmp and the common ground line LED-, thus forming the reference for the PSUs interface.

Actually, just by choosing an adequate value for Vk, the measuring error can be reduced until a reasona- ble value for the application. For example, stating a 50 mV max voltage drop on the ground path (1 A on a 50 mOhm connection) , a 5V voltage is the minimum value for Vk to have an error due to the voltage drop of under

1 9- To achieve a better accuracy, different compensat^¬ ing techniques for that common ground line offset may be applied. One of the most simple is of course to switch-off the Light Engine Module string before to read out Rset: this can be done at the system start-up by a simple machine based on a sample & hold system.

It must be noticed that when the Light Engine Mod^¬ ule string is turned off by removing the supply on the LED+ wire, the current level on the communication line CL is not affected by the temperature signal. This is not a disadvantage, because this information is not needed when the Light Engine Modules are completely turned off, rather it is a way to read the Rset value not only with a better accuracy, but also without any deviation due to a possible overheating, respective without any deviation due to the Light Engine Module temperature .

On the other hand, also the opposite way is via^¬ ble. This means that the pure temperature information is available by simply separating the reference voltage Vk from the OpAmps positive input. Doing so makes the voltage on the third wire be a function of solely the Light Engine Module temperature (the highest one in case of multiple connection), even in case it's lower than the derating threshold. This makes the Power Supply Unit able to derate itself the current to the Light Engine Module (s), according its proper law, and allows to know the working temperature of the Light Engine Module (s) even when not overheated (of course Rset must be known to achieve the best temperature accuracy) . Fig. 6 shows a derating curve of the inventive Power Supply Unit. The curve shows the internal control voltage Vout of the Power Supply Unit over the tempera^¬ ture of the Light Engine Module (s) . The multiple curves relate to the different current requirements of the connected Light Engine Module (s) . It can be seen that the derating starts at a temperature of about 93°C until about 100°C to 104°C the power is shutdown com^¬ pletely.

The function of the inventive interface will be explained in the following with the help of a practical example .

As can be seen in the figure, an output current of 1 A results in an internal measurement signal Vout of 10 V. The interface shall be designed in a way, that a conductance of lmS for Rset results in an Output cur^¬ rent of 1A. According to the figure, the voltage source Vk is adjusted to 5 V. This means, that 5V are applied to Rset (see Fig. 5) . The operational Amplifier works in a way to minimize the signal Level on its inputs, so it will work until the level at the positive input is the same like the level at the negative input. So if Vk has 5V, this means that 5V will also be at the negative input of the operational amplifier. This leads to 5V at the current set resistor Rset, resulting in a current through the communication line CL of 5 V / 1 kOhm = 5 mA. 5 mA through the communication line CL means that these 5 mA also flow through the current measurement resistor Rfb, because the input of the operational amplifier has a high impedance and therefore no current consumption As the voltage of the internal measurement signal Vout shall be 10 V according to Fig. 6, the voltage over the current measurement resistor Rfb has also to be 5 V resulting in a current measurement re- sistor Rfb with a value of also 1 kOhm respective 1 mS . According to this example, a Light Engine Module with a current requirement of 2 A would have a current set resistor Rset of 2 mS, that is 500 Ohms.

As mentioned above, the inventive three wire in- terface with the concept of the measuring current re^¬ turning through the common ground line together with the LED current has the disadvantage of corrupting the measuring signal with the voltage drop on the common ground line LED- due to the Light Engine Modules' cur- rent flowing through it, but with a proper strategy it is possible to compensate this effect in order to re^¬ trieve the true value for the Power Supply Unit.

Fig. 7 shows a schematic circuit of how to model the cable voltage drop Vo due to LED current.

The general method to compensate the voltage drop is to vary the Vk voltage of the voltage source in the Power Supply Unit. The voltage drop can be cleared out by a linear equation system based on two different values of Vk. Raising the Vk voltage beyond VI inside the Light Engine Module makes the Rset value uncor- rupted by the temperature information (whichever it could be) without turning off the Light Engine Module power .

As shown in Fig. 7, the voltage drop on the common return LED- can be modelled as a voltage generator Vo in series with Rset: The circuit is similar to the circuit in Fig. 5 with the temperature section left out and added offset generator Vo, which is representing the voltage drop on the cable.

Now the circuit's equation is formulated by simply considering both Op Amp inputs are at the same voltage: γ out -Y k _ γ k -Y o

Rfb ^Rset

or, equivalently, [1]

R

Now, calling

R_^

K

^R p ' we can solve [1] into V₀ (constant), and apply two different values for V^:

(^o ^{= —} K_R V_out^— V_ki )

It is possible to solve this linear system by equ^¬ ation comparison, finally having:

+ V_{k 2}) [ 3 ] This equation can also be written in terms of dif^¬ ferences AV = V_l-V₂ and solved into K_R:

K_R = ^ [ 4 ]

This expresses the ratio between the known and the unknown resistors as a ratio of superimposed (Vk) and measured (Vout) voltage differences.

As can be seen, the voltage drop Vo can be compu^¬ tationally eliminated by two measurements and some mathematics .

Fig. 8 shows a characteristic of the Current Gene- rator according to the example of Fig. 6. The graph shows the input of the Current Generator CG, the inter^¬ nal measurement signal Vout, against the output current of the Current Generator CG lout. It can be seen, that under a certain voltage, here called Vsilent, no Output current is provided. At the maximum of the internal measurement signal VoutMax, the maximum specified out^¬ put current of the Current Generator CG is provided. Vsilent is the voltage up to where no current flows on the communication line CG. This can be due to the vol- tage Vk or due to the Temperature Derating Unit TDU creating a current I_TDU similar to the current created by Vk, but in the opposite direction. So this current creates a voltage over Rset similar to VK, therefore no current flows over the communication line CL .

Under normal circumstances, a lighting system would be designed in a way that no current is provided by the Current Generator CG if no current flows over the communication line CL . This is because if the con^¬ dition of a miswiring or a weak contact exists, no power should be provided from the Power Supply Unit PSU to the Light Engine Modules LEM. But under certain circumstances, this provision can be amended.

For normal circumstances, if no power should be provided from the Power Supply Unit PSU to the Light Engine Modules LEM, when no current flows on the commu^¬ nication line CL, the Voltage Vsilent is the same as the Voltage Vk.

Fig. 9 shows a characteristic of the Current Mea^¬ surement Unit CMU. A main part of the Current Measure- ment Unit CMU is the current measurement resistor Rfb . The characteristic shows the output of the Current Measurement Unit CMU, the internal measurement signal Vout, against the normalized current measurement resis^¬ tor Rfb/RsetMin. RsetMin is the minimal Value leading to the maximal specified output current IoutMax of the Power Supply Unit PSU. So at the value 1, when Rfb=Rsetmin, the Power Supply Unit provides maximal current and Power at its output, and the internal mea^¬ surement signal Vout is 2*Vk as described in the exam- pie of Fig. 6.

Summary :

In various embodiments of the inventive interface allows to acquire:

- A composite information from the Light Engine Module, i.e. a nominal current derated by over- temperature, or

- A split information about nominal current and working temperature by properly switching the different generators inside the Power Supply Unit. This of course

5 involves a logic circuit, and it's not as simple as reading a composite, non-compensated value from the communication line CL .

- These are different approaches to read the Light Engine Module communication line CL, but the electronic) ics inside the module stays the same.

These and other advantages of the invention are summarized in the following:

- The inventive interface uses only a simple re^¬ sistor to set the required current.

15 - Only one extra wire is required besides the pow^¬ er connection to the Light Engine Modules.

- More Light Engine Modules are allowed to be con^¬ nected in parallel on the same bus interface.

- The thermal derating can be realised by only 20 adding a simple PTC or four cheap components.

- The auxiliary supply for thermal derating is simply derived from a Light Engine Module string tap^¬ ping .

- The interface is intrinsically fail-safe, in the 25 sense that, if Rset is broken or the communication line disconnected (the most likely fault events) , the output current is switched off.

- In case of short-circuit fault between Light En^¬ gine Module + and the third wire (could be a wrong

30 connection) , the output current is intrinsically switched off, so also preserving the interface circui^¬ try itself.

- The Thermal derating unit doesn't drain current from Light Engine Module's supply until Light Engine

35 Module overheating. - The current used to read out Rset can be varied according to the Power Supply Unit rating, in order to limit its ranging (and improve accuracy) according to the expected applied load.

- The inverse Ohm law allows to keep a constant percentage resolution of output current.

- The accuracy on reading out Rset depends on the complexity of the Power Supply Unit side interface, which can be arranged according to expected system quality grade. Furthermore the reading of Rset may be ratiometric to a reference resistor inside the PSU, without requiring accurate voltage or current sources as in the prior art.

- The invented interface may provide different in^¬ formation according the applied stimulus, ranging from a single thermal derated current to two independent and accurate values of nominal current and working tempera^¬ ture .

Reference Character List

PSU Power Supply Unit

LEM Light Engine Module

CMU Current Measurement Unit

TDU Temperature Derating Unit

PGU Pulse Generation Unit

PG Pulse Generator

CL communication line

CG Current Generator

Vout internal measurement signal

Rset current set resistor

Rthr threshold set resistor

Rtg emitter resistor

Rfb current measurement resistor

Rp Pulse Resistor

LED+ positive power supply line

LED- common ground line

VI voltage source

Vk voltage source

Vout internal measurement signal

S pulse switch

Claims

1. A Power Supply Unit (PSU) comprising:

- an output providing electrical power between a positive power supply line (LED+) and a common ground line (LED-) ,

- a communication line (CL)

- an adjustable current generator (CG) responsive to an internal measurement signal (Vout) generating an output current (lout) at the output,

- a voltage source (Vk) coupled to the communica^¬ tion line,

- a current measurement unit (CMU) , measuring a current through the communication line (CL) and gene- rating the internal measurement signal (Vout) depending on the measured current through the communication line (CL) .

2. The Power Supply Unit (PSU) of claim 1, where the current measurement unit (CMU) comprises a current measurement resistor (Rfb) and an operational amplifier (OpAmp) .

3. The Power Supply Unit (PSU) according to claim 2, where:

- the voltage source (Vk) is connected between common Ground (LED-) and the positive input of the operational amplifier (OpAmp) ,

- the current measurement resistor (Rfb) is con^¬ nected between the negative input of the operational amplifier and the output of the operational amplifier (OpAmp),

- the communication line is connected to the nega^¬ tive input of the operational amplifier (OpAmp) ,

- the output of the operational amplifier (OpAmp) outputs the internal measurement signal (Vout) .

4. The Power Supply Unit (PSU) according to one of the preceding claims, where a linear characteristic exists between the internal measurement signal (Vout) and the output current of the adjustable current gene^¬ rator (CG) .

5. The Power Supply Unit (PSU) according to claim

4, where the linear characteristic is valid only over a portion of the total range of the internal measurement signal (Vout) .

6. The Power Supply Unit (PSU) according to claim 5, where the beginning of the portion is defined by an offset voltage of the internal measurement signal (Vsi- lent) , and below this offset voltage no current will be provided by the adjustable current generator (CG) .

7. The Power Supply Unit (PSU) according to claims 5 and 6, where the offset voltage is identical to the value of the voltage source (Vk) .

8. The Power Supply Unit (PSU) according to one of preceding claims, wherein the Power Supply Unit (PSU) is configured to power at least one Light Engine Module (LEM) comprising:

- a plurality of series connected LEDs connected between said positive power supply line (LED+) and said common ground line (LED-) ,

- a current set resistor (Rset) with its value in- dicative for the current demand of the Light Engine

Module, wherein said current set resistor (Rset) is connected between said communication line (CL) and said common ground line (LED-) , and

- a current generator (TDU) connected to said com- munication line (CL) .

9. The Power Supply Unit (PSU) according to claim 8, wherein said at least one sensor comprises:

- a temperature sensor (NTC, PTC) configured to detect the temperature of said Lighting Engine Module (20) and/or said plurality of series connected LEDs (L) ;

- a light sensor configure to detect the ambient luminosity;

- a twilight light sensor;

- a movement sensor; and/or

- a wireless receiver for receiving a signal form a remote control.

10. The Power Supply Unit (PSU) according to claim 8 or claim 9, wherein said variable current generator (TDU) comprises:

- an amplifier responsive to a temperature sensi^¬ tive input current,

- a temperature sensitive resistor (NTC, PTC) re^¬ sponsible for the temperature sensitive input current.

11. The Power Supply Unit (PSU) according to one of the preceding claims 8 to 10, wherein said Power Supply Unit (PSU) is configured to determine the vol^¬ tage drop on said common ground line by applying via said voltage source (Vk) two different voltages to said communication line (CL) .

12. The Power Supply Unit (PSU) according to one of the preceding claims 8 to 11, wherein said current generator (TDU) is powered from said positive power supply line (LED+) , and wherein said Power Supply Unit (PSU) is configured to read the value of said current set resistor (Rset) when said output current (lout) at said output is turned off.

13. The Power Supply Unit (PSU) according to one of the preceding claims 8 to 12, wherein said current generator (TDU) is powered from said positive power supply line (LED+) , wherein said Power Supply Unit (PSU) is configured to measure the current generated by said current generator (TDU) of said Light Engine Mod^¬ ule (LEM) when said output voltage of said voltage source (Vk) is deactivated.

14. The Power Supply Unit (PSU) according to one of the preceding claims 8 to 13, where the voltage source (Vk) is adjusted in a way, that every single Light Engine Module can decrease the supplied current (I_CL) to substantially zero.

15. A Light Engine Module comprising:

- a plurality of series connected LEDs,

- a positive power supply line (LED+) ,

- a common ground line (LED-) ,

- a communication line (CL) ,

- a current set resistor (Rset) coupled to the communication line, with its value of conductance being indicative for the current demand of the Light Engine Module,

- a variable current generator (TDU) connected to said communication line (CL) , said variable current generator being responsive to at least one measurement signal provided by at least one sensor.

16. The Light Engine Module according to claim 15, wherein said variable current generator (TDU) compris^¬ es :

- an amplifier responsive to a temperature sensi^¬ tive input current,

- a temperature sensitive resistor (NTC, PTC) re- sponsible for the temperature sensitive input current.

17. The Light Engine Module according to one of the preceding claims 15 to 16 additionally comprising a threshold set resistor (Rthr) responsible for a thre^¬ shold level for thermal derating.

18. The Light Engine Module according to one of the preceding claims 15 to 17, comprising:

- a voltage source (VI) with the negative output connected to common ground (LED-) ,

- a series connection of the temperature sensitive resistor and the threshold set resistor (Rthr) con- nected in parallel to the voltage source (VI),

- a transistor (Ql) where the base is connected to the node between the temperature sensitive resistor and the threshold set resistor (Rthr) , the collector is connected to the node between the positive output of the voltage source (VI) and the temperature sensitive resistor (NTC) , the emitter is connected to the commu^¬ nication line (CL) via a emitter resistor (Rtg) .

19. The Light Engine Module according to one of the preceding claims 15 to 18, comprising:

- a voltage source (VI) with the negative output connected to the common ground (LED-) ,

- a transistor (Ql) where the base is connected to the node between the temperature sensitive resistor and the threshold set resistor (Rthr) , the emitter is connected via a emitter resistor (Rtg) to the node between the positive output of the voltage source (VI) and the temperature sensitive resistor (PTC) , the collector is connected to the communication line (CL) .

20. Light Engine Module according to one of the preceding claims 15 to 19, where the voltage source (VI) is derived from the two power supply lines (LED+; LED-) .

21. Light Engine Module according to one of the preceding claims 15 to 20, where the voltage source (VI) is connected with common ground (LED-) and tapped between a portion of the plurality of series connected LEDs .

22. Light Engine Module according to one of the preceding claims 15 to 21, wherein said at least one sensor comprises:

- a light sensor configure to detect the ambient luminosity;

- a twilight light sensor;

- a movement sensor; and/or

- a wireless receiver for receiving a signal form a remote control.

23. A lighting system, comprising:

- a Power Supply Unit (PSU) according to one of the preceding claims 1 to 14; and

- at least one Light Engine Module (LEM) according to one of the preceding claims 15 to 22.

24. The lighting system according to claim 23, where all Light Engine Modules are connected in paral^¬ lel.