WO2005022957A1

WO2005022957A1 - Reduced emi led circuit

Info

Publication number: WO2005022957A1
Application number: PCT/CH2004/000555
Authority: WO
Inventors: Artur Seibt
Original assignee: Ilumera Group Ag
Priority date: 2003-09-02
Filing date: 2004-09-02
Publication date: 2005-03-10

Abstract

The invention concerns an electronic circuitry for driving at least one light-emitting diode with dc current. The circuitry comprises in series connection in a current path for the LED a switch and a choke. The circuitry comprises a capacitor in parallel connection to the LED and a diode in parallel connection to the current path for the LED. An integrated circuit is operatively connected to the switch in order to provide a signal for controlling the brightness of the LED.

Description

REDUCED EMI LED CIRCUIT

BACKGROUND OF THE INVENTION

[0001 ] Light-emitting diodes (LEDs) are gaining importance and market acceptance for lighting purposes since high-power units became available entering now competition with established lighting gear such as incandescent lamps, halogen lamps, low and high pressure gas discharge lamps, and fluorescent lamps which still excel in efficiency.

[0002] The use of LEDs for lighting is markedly different from their use in signaling lights (e.g. in automobiles' rear brake signals) and requires the utmost attention to their effects on humans, and, in addition, to the fulfillment of applicable safety and emi laws. Whatever may be acceptable for a brake signal on a car which is only activated for a few moments and always viewed from an appreciable distance will be definitely unacceptable in a high power lighting system destined for use in homes, offices etc.

[0003] Most lighting systems are fed from the mains supply which is ac. This means that the light emitted is modulated more or less by twice the ac frequency which is 50 or 60 Hz. The light will thus flicker with 100 or 120 Hz, the extent of this flicker will be determined by the amount of filtering involved. The filaments of incandescent lamps, due to their thermal mass, can hardly follow the 100 resp. 120 Hz, thus these lamps emit a very constant light. Fluorescent lamps feature a phosphor with a fairly long decay time so that their flicker is reduced but still noticeable to such an extent that people who are sensitive to this may be adversely affected. Epileptic seizures may be provoked which is opulently documented in the respective medical literature. Such a seizure may lead to serious personal injury and thus make the manufacturer of such gear responsible with all legal implications. In any case the flicker will cause discomfort the cause of which will hardly be identified.

[0004] High-pressure gas discharge lamps such as metal halide lamps can not rely on any filtering, and their light is thus fully modulated by twice the line frequency. Hence use of these lamps with conventional lamp ballasts is restricted to applications where humans are not subjected to them for any appreciable length of time. Electronic lamp ballasts have been designed which feed the lamps with (effectively) pure dc so they never extinguish and reignite during operation, thus yielding totally flicker free light. Those ballasts are considerably more expensive yet indispensible in all lighting installations for homes, offices etc. where concern for the health of people is of para- mount importance.

[0005] Of all lamps known LEDs are those with the fastest response to their drive current, the response time is usually in the area of nanoseconds, consequently, they follow any drive signal and reproduce its current waveform faithfully as light output. The flicker of ac- or pulse-driven LEDs is hence by far the worst of all lamp types. Consequently, the problems well known and documented caused by fluorescent lamps will be much worse with LEDs.

[0006] The prior art uses exclusively any of a variety of pulse-drive methods. Cost considerations and some technical aspects may recommend at first sight pulse operation. Typically, the LEDs are driven by a fairly low-frequency pulse train the duty cycle of which is modulated in order to achieve brightness control. At full rated current the light output of modern LEDs is extremely high, unbearable and harmful to the unprotected eye. This is aggravated by the fact that the angle of the light beam is rather narrow.

[0007] In the most cost-conscious designs an unregulated high dc voltage is chopped such that the output voltage equals input voltage times duty- cycle. The regulation loop will respond to the average LED current which determines the brightness desired. However, although the eye will register a brightness corresponding to the duty cycle the LED will be always on at its peak brightness level, be it only for a fraction of the switching frequency period. Thus the light will flicker between fully on at peak intensity and fully off.

[0008] Even if the LEDs may appear innocently dim, the viewer will be unaware of the fact that he is looking into an LED radiating at peak brightness. The viewer will be misled to assume that the LED operates at very low current and that it was harmless to look straight into it. What he does not know is the fact that he receives extremely intense although short light pulses. The integration (averaging) to the apparent low brightness level is left to the limited response speeds of eyes and brain. An individual would never look into the same LED at continuous maximum intensity as his reflexes would cause him to look away as quickly as possible. Indeed the manufacturers of such LEDs include a warning in their data sheets that these LEDs are harmful. Nothing is said about the harmfulness if the maximum intensity is duty cycle pulsed so that the apparent brightness is lowered. It is known that there are very strict laws for lasers, these will unquestionably be extended to . lighting LEDs.

[0009] Other pulse drive circuits operate the LEDs using a buck converter with a filtering choke so that at 100 % duty cycle the LEDs will receive average dc with a rather high ripple content still causing flicker. The flicker will become more intense as soon as the duty cycle is reduced.

[0010] One might be tempted to raise the pulse drive frequency in order to reduce the effects on people, but there is no evidence nor proof that a higher frequency would be safe for customers.

[001 1 ] It will probably take years of extensive medical research before sufficient knowledge and evidence will have been accumulated to answer this question so that a manufacturer of LED light- ing gear might dare to offer pulsed light to the public without running a high risk to be caught in litigation.

[0012] Thus it must be concluded that LEDs for lighting purposes in environments where people are present must definitely not be pulse-driven, thus another drive method had to be found.

OBJECTS AND SUMMARY OF THE INVENTION

[0013] Briefly stated, methods, apparatus, and electronic circuitry for driving light-emitting diodes (LEDs) as singles or arrays are disclosed. According to this invention the LEDs are driven by dc current generators at all brightness levels thus preventing harmful flicker which can cause personal discomfort and injury.

[0014] A method which guarantees flicker-free operation of LEDs in lighting applications and thus ensures customer safety, freeing the manufacturer of all concerns that he may become the object of customer personal injury claims. Also, the clean, flicker-free light will be much more agreeable to customers.

[0015] Emi-free operation. All pulse drive methods generate more or less severe electromagnetic interference (EMI). Hence it is virtually impossible to place the LED drive electronics far away from the LEDs proper as the conductors would radiate unacceptable emi. As these electronics must be included with the LEDs the design, size and cooling problems of the lighting apparatus become severely impaired. The gear must be well shielded including the window from which the light exits (wire mesh). This raises system costs thus overcompensating by far the cost advantage claimed for the pulse drive. [0016] Minimum system cost. In order to minimize costs in a further embodiment of this invention the LED drive electronics can be integrated with the power supply in one unit; the connections from this unit to the LED lighting gear consist of standard low-cost unshielded wires which may extend several hundred meters if necessary without any effect on the LED currents and which may be installed by unskilled personnel. As the lighting apparatus proper only contains the LEDs and no electronics it may be more elegantly designed, become much smaller and less costly. The unit may comprise also a microcomputer and/or a bus interface.

[0017] The LED drive method described herein delivers very precise and constant currents allowing an operating current close to the maximum limits thus achieving maximum brightness.

[0018] In a further embodiment of the invention means are provided to detect the zero value of the brightness control signal in whatever electrical representation it may be applied to the input and to turn off the current generator completely, because even high power LEDs will emit visible light at currents as low as 10 μA, i.e. 14 ppm of rated current.

[0019] In a further embodiment of the invention the necessary control of the LEDs' temperature is effected by installing a calibrated temperature sensor onto the same mounting surface (e.g. aluminum-clad etched circuit board) as the LED(s) and connecting this sensor to the current generators) electronics such that rather than turning off the lighting system abruptly and totally upon detection of excessive temperature the current generator output current(s) will be gradually re^¬ duced once a first predetermined temperature level will be passed thus establishing a constant temperature regulation loop for the duration of the insufficient heat removal interval. For safety reasons a second temperature level is provided, if this should be reached the resp. all current gen^¬ erators) will be turned off completely. [0020] In a further embodiment of the invention it is recognized that ideally the LEDs' junction temperature(s) rather than that of the mounting surface should be controlled. According to the invention the LEDs themselves are used as their own and best junction temperature sensors, automatically limiting the individual junction temperatures to predetermined safe levels consistent with long life and high reliability. The method includes self-calibration at system start-up in order to compensate for component tolerances.

[0021 ] In a further embodiment of the invention automatic self-calibration of zero and nominal maximum current levels is used.

[0022] In a further embodiment of the invention the system will measure and store output current and voltage levels at start-up and subsequently in predetermined intervals in order to continuously monitor all components so that it may detect and take proper action in case of any component or other failure.

[0023] E.g. LEDs will preferably be operated in parallel series strings of four, if one LED fails by opening all parallel strings would be overloaded and fail also soon. If an LED fails by shorting the string affected would draw current away from the other strings causing failure of the 3 remaining LEDs. Considering as well the importance of a lighting system as well as the still very high cost of high-power LEDs a single component failure should neither cause an expensive repair by destroying more components nor a complete blackout.

[0024] LED dc operation is described with features of special electronics circuitry. [0025] Several applications specify lighting devices with processors. The present system does not need a processor or other electronics incorporated with the LEDs in lighting devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Fig. 1 is a simple block diagram of the present invention;

[0027] Fig. 2 is a circuit block diagram of a dc current generator;

[0028] Fig. 3 is a circuit diagram illustrating processing of a brightness control input signal and a zero control signal recognition circuit according to the present invention; and

[0029] Fig. 4 is a circuit diagram illustrating temperature processing and protection.

DETAILED DESCRIPTION

[0030] The structure and operation of a preferred embodiment will be described. Those skilled in the art will understand that there are many more ways of practicing the invention, the embodiments described herein are exemplary and not limiting.

[0031] Fig. 1 shows the basic principle. It is understood, that there will be used as many such circuits as there are different (color) LEDs in a lighting system. A precision controllable dc current generator 100 which, as will be shown later, will mostly consist of a SMPS (Switch Mode Power Supply) with extremely low residual ac ripple on its dc output current. The input signal can control the output current over its full range of 0 to 100 %. Even high power LEDs with nominal currents lo e.g. 700 mA will emit visible light already at about 10 μA. Because of the low ripple content of the output current this drive method does allow to set the 100 % level close to the maximum permissible LED current thus taking full advantage of its maximum light output. In contrast all pulse drive methods must observe the maximum peak current specification and thus limit the average current which determines the light output.

[0032] There is a theoretical disadvantage of dc current operation because the voltage-current characteristic of LEDs becomes nonlinear at low levels. If the LED is pulsed always with a constant amplitude, and the brightness varied by duty cycle control the linearity of the light output at low levels will be better. However, due to the logarithmic behaviour of the eye this is rather academic. Also, this could be easily linearized by a microcomputer, if desired. In reality, such low levels are of no practical use in a lighting system.

[0033] Only dc operation is completely emi-free, so conductors of any length and type may be used between the current generator and the LEDs. Preferably, the small size of the LEDs can best be taken advantage of if the lighting fixture containing them will be small, elegant and light. It is a problem anyway to conduct the LEDs' dissipation away, if electronics would have to be included in the fixture it would become more clumsy, heavy, expensive, also the additional heat would have to be taken care of. All pulse drive methods generate more or less hefty emi levels, so they must be incorporated with the LEDs, any long and unshielded wires would radiate unacceptably. Only with dc operation is it possible to use fixtures without any emi shields on the light output window.

[0034] Fig. 2 shows a block diagram of a preferred embodiment. Because of the high power levels characteristic of modern LEDs (1 W resp. 5 W per piece) all kinds of linear dc current generators are out of the question. Emi filter 101 prevents the pulse ripple on capacitor 102 to exit into the conductors leading to the dc power supply, the circuit is basically a buck converter consisting of switch 122, a p-channel mosfet, storage choke 123, free-wheeling Schottky diode 124, filter ca^¬ pacitor 125 and the control integrated circuit 120. The latter is a standard SMPS control ic UC2842B made by ON Semiconductor and many more manufacturers. The data sheet of this product may be used as a reference. It contains a precise reference voltage supply (5 V 1 %), an oscillator which runs typically here at 125 KHz, pulse steering logic, a feedback operational amplifier which is internally connected to half the reference, i.e. 2.5 V, a npn totem-pole output stage, and a current sense input to a comparator which terminates the switch on time. Vital to the invention is among other items the inclusion of the high value capacitor 125 (e.g. 100 μF for a 1.4 A output current) together with a high switching frequency and a high inductance choke, the combination of which results in an almost pure dc current with only a few percent of 125 KHz ripple.

[0035] The input of the feedback amplifier FB on the ic is a virtual ground due to feedback from output to input (not shown) at node 150. In operation, i.e. with functioning current regulation, this node will rest at 2.5 V, the sum of control currents into and out of this node must always equal zero. The output current will cause a voltage drop across precision shunt 125. The output voltage from block 200, the control signal processing explained later, will be 5 .. 0 V for 0 .. 100 % output current In order to set the output current to zero an offset current must flow through resistor 127 through shunt 126 to ground which generates exactly 2.5 V across resistor 127 (neglecting the shunt). The loop will thus be in equilibrium. This offset current will come partly via resistor 128 out of Vref and partly via resistor 129 out of block 200. The portion contributed by block 200 will go from maximum at 5 V through zero at 2.5 V; below 2.5 V current will be drawn out of node 150 into block 200, so that at 0 V from block 200 the maximum output current will be reached.

[0036] Block 300 contains the overtemperature protection circuitry with a ntc sensor on the LED mounting board, explained later, when it becomes operative, typically above 80 degrees C LED board temperature, its output will rise from zero, send current through diode 131 and resistor 130 and reduce the maximum current level gradually to zero at a predetermined temperature above 80°C depending on the LED types used and the thermal impedances involved. There is also a ptc overtemperature sensor (not shown) inside the electronics which turns them off in case of over- heating. [0037] Block 400 is a necessary zero control recognition circuit. As mentioned even 700 mA LEDs emit visible light already at 10 μA, hence it is necessary to ensure complete turn-off for zero control input irrespective of small offsets.

[0038] In a further embodiment block 500 will be present, this is a autozero circuit.

[0039] Fig. 3 shows the details of block 200. Although the control of the output current could easily be effected by directly applying a dc input signal, e.g. within the standard range of 0 .. 10 V, it is advantageous to bridge greater distances using a duty cycle modulated pulse train. Hence it is necessary in such case to convert the duty cycle information into a linearly corresponding dc voltage. Inverter 201 which is preferably a CMOS Schmitt-trigger inverter, is fed directly from the pre- cise Vref output of 120. Taking advantage of the internal structure of such a CMOS circuit it is used here as a precision analog switch between ground an Vref = 5 V. Its output will thus be a pulse train of input frequency with an exact amplitude of 5 V. RC filter 202 plus 203 extracts the average value leaving a small percentage of input frequency ripple the amount of which depends on the duty cycle and to a small extent on the supply voltage. 204 is a preferably CMOS opera- tional amplifier which translates the voltage on capacitor 203 to its low impedance output where it is converted by resistor 129 into the control current for the current generator.

[0040] The RC network 401 and 403 at the input of inverter 404 is designed for the input frequency used, each pulse keeps capacitor 403 discharged into inverter 201 through diode 402. For true zero input duty cycle zero equals zero pulse width and thus the disappearance of the input signal altogether, if that happens resistor 401 will be allowed to charge capacitor 403 up to the upper hysteresis level of Schmitt-trigger 404, when it switches the output of inverter 405 will rise to 5 V causing current to flow through diode 133 and resistor 130 into node 150 sufficient to suppress operation of the current generator irrespective of offsets which maybe present. [0041] As with all semiconductors it must be guaranteed that the maximum junction temperature of the LEDs will never be reached, moreover, acceptable life and reliability dictate that the mean operating temperature is kept substantially below maximum. LEDs can not be exchanged by the customer like ordinary lamps, a defect will mean an expensive repair, also, one of the advantages claims for LEDs is their long life, however, this is known to depend exponentially on the mean junction temperature. A simple solution would be an overtemperature sensor like a ptc (passive thermal control) which would turn off the light. This is a highly undesirable alternative for the customer and must thus be ruled out.

[0042] In the scope of this invention a different solution is presented: Below a first temperature limit, e.g. 80°C, there is no influence of block 300 on the programmed LED current. Beginning at this temperature the maximum current of the generator will be linearly and thus gradually reduced, resulting in a true LED board resp. LED junction maximum constant temperature regulation loop. This loop will only become active if the combination of the total input power to all LEDs protected by the temperature sensor connected to block 300 and the ambient temperature is such that the heat generated can no more be adequately removed. The light output will decrease, but there will be no abrupt blackout. As cooling improves the reduction will automatically be gradually removed. Also, a monitoring and controlling microcomputer which receives the temperature signal could step in and modify the control signals in order to reduce the power dissipation.

[0043] One way of sensing could be a ntc sensor mounted on the board to which the LEDs are attached. However, the real object of the measurement is not the board but the junction temperature. In a further embodiment of the invention the LEDs, either single or in strings, are used as their own junction temperature sensors by measuring and continuously monitoring their single or combined voltage. In order for this to be precise enough and take part tolerances into account as well as aging the system will measure the voltages of all strings at system start-up and store these values. As a temperature reference an absolute sensor will be included. This can e.g. be a precision ntc or a temperature measuring ic of which there is an abundance on the market. In a further embodiment the hardware block 300 may then be removed, its function will be taken over by software residing in a control and monitoring microcomputer.

[0044] Fig. 4 shows the details inside block 300 for the case that a ntc is used. A precision current generator 301 generates a voltage across precision ntc which is a calibrated representation of the ntc temperature. Follower 304 is a low impedance source for the inverting control amplifier 308. Resistors 305 and 307 determine the amplification factor which in turn determines the slope of current reduction. V2 is a^' voltage corresponding to the first temperature limit, e.g. 80°C, so that amplifier 308 remains in its low voltage output state as long as the ntc temperature stays below that value. If the temperature rises above that first limit the amplifier output will rise, as soon as its voltage exceeds about 3.1 V current will flow via diode 131 and resistor 130 into node 150 and reduce the current generator's maximum current. In the case that several LEDs of different colors are employed with their respective current generators the output of 308 will influence the other current generators exactly the same way via diodes and resistors as depicted. The choice of 305, 307 and 130 is such that at a second temperature limit, e.g. 95 C the current generators will be completely cut off. Comparator 309 monitors the voltage drop across a small resistor in series with the ntc, in case of a missing or broken sensor the comparator will switch its output to low and pull the minus input of amplifier 308 low via diode 310; this will cause the output to rise cutting all current generators off. [0045] In order to cope with inevitable offsets in amplifiers etc. block 500 is included in a further embodiment of the invention. Whenever block 400 recognizes zero control input it will activate block 500, the autozero circuit. The voltage across the shunt 126 should be zero, if not the error will be amplified and stored in an analog memory inside block 500, a correction current will be sent via resistor 134 into node 150 such that the voltage across 126 is returned to and kept at zero. In case of negative offsets, however, a so called dead zone would appear near zero, i.e., the input control voltage must be increased beyond a certain minimum value before an output current will start to flow. This can be taken into account by declaring a small positive offset across 126 as „zero"

[0046] In a still further embodiment Block 500 may also contain a microcomputer with an a/d converter for measurement of the shunt voltage and a d/a converter for outputting the correction signal into node 150. This microcomputer may also be the control element; in that case hardware blocks 200, 300, 400 can be done away with, all signals feeding into node 150 would come from d/a converter outputs.

[0047] hi a still further embodiment the microcomputer can also perform autocalibration in addi- tion to autozero which has to precede autocalibration by applying full scale input signal and measuring again the voltage across shunt 126, if it deviates from the nominal value the control signal outputted to node 150 will be multiplied by an appropriate compensation factor.

[0048] hi order to minimize total system costs in a preferred embodiment the complete LED drive circuitry is be integrated with the power supply. Also a control and monitoring microcomputer can be integrated with both, so there will be just one unit from which standard unshielded conductors of any length are run to the LED lighting fixture. Other than a temperature sensor (unless the LEDs are taken as sensors) the fixture will contain just the LEDs and their cooling hardware. This allows the smallest, most elegant and least costly fixture design. It should be noted that most other competing lamp types do not require any cooling. Also the efficiency of present high power LEDs is still only a fraction of those of metal halide (1/4) or fluorescent lamps (1/5). hi other words: a lighting fixture with a metal halide lamp will be smaller than another one with LEDs for the same light output, so anything which would further enlarge the volume and mass of a LED light fixture is highly undesirable. It is mainly the ability to create various colors with two or more LED types where LEDs shine as superior. [0049] A further embodiment applies to a standardization of power supply/drive circuit/control units and their connectors such that a plurality of LED lighting fixtures can be hooked up to such combination units. Their connectors can be coded or may contain a memory so that the unit can identify the fixture and output the correct current levels. This allows customers to buy units and fixtures separately and mix them at will.

[0050] According to the present invention, several embodiments are disclosed that include a power supply, at least one LED, a source of a brightness control signal, a regulated or unregulated programmable current generator which feeds the LED(s) with dc current in all operating modes and at all brightness levels.

[0051 ] The light apparatus current generator is a SMPS type current generator delivering dc current with extremely low superimposed ripple (few percent) yielding essentially flicker-free high quality and customer-safe light at all brightness levels.

[0052] The light apparatus current generator consists of a SMPS followed by a linear current regulator.

[0053] The light apparatus current generator is a linear current generator.

[0054] The light apparatus brightness control signal for the current generator is a dc voltage.

[0055] The apparatus control signal received is a duty cycle modulated pulse train the average dc content of which is converted into a proportional dc signal which controls the current generator.

[0056] The apparatus control signal received is in digital series or parallel representation which is converted, e.g. by a d/a converter, into a dc voltage for control of the current generator. [0057] The apparatus permits detecting the zero value of the input brightness control signal and where means are provided for shutting the current generator completely off when such zero input signal is detected.

[0058] The apparatus permits suppressing residual high frequency ac are. provided at the power supply input and/or at the current output terminals feeding the LED(s) in order to meet emi regulations.

[0059] The apparatus has a buck converter type SMPS that is used to deliver essentially clean dc current with only a few percent ripple by a suitable combination of high operating frequency, high inductance of the storage choke and high capacity of the output capacitor.

[0060] The apparatus permits monitoring and control of the LED(s)' temperature(s) are provided such that a calibrated temperature sensor is mounted onto the same surface as the LED(s), that it is connected to the aforementioned current generator(s), that upon transgression of a first predetermined temperature level the output current(s) of the resp. all current generator(s) is/are gradually reduced so that a temperature regulation loop is established , further, that a second predeter- mined temperature level is provided, the transgression of which will completely shut off the/all current generator(s).

[0061 ] The apparatus output signal representing the temperature as sensed by said sensor will be generated and available at a terminal for an external microcomputer.

[0062] The apparatus permits monitoring and control of the LED(s) junction temperature(s) by using the current-controlled LED(s) voltage(s) as temperature signals in conjunction with a calibrated temperature sensor. Component tolerances and aging effects are taken into account by measuring all LEDs' voltages at system start-up, i.e., when the junction temperatures are known and storing these values in a microcomputer memory. The characteristics and the temperature coefficients of the various types (colors) of LEDs used in a system are likewise stored and used to continuously measure the LED voltages and to compare them with the start-up values and to calculate the actual junction temperature(s) from these information. According to the invention the microcomputer will take corrective action by reducing gradually the currents of those LEDs which become too hot. As this would in general cause a shift in the desired color of a multi - LED lighting apparatus the microcomputer will change the currents of all the other LEDs in such a way that the color balance will not change.

[0063] The apparatus microcomputer is programmed so that for all combinations of the different color LEDs' currents it will calculate the eventual junction temperatures in advance and set the individual currents so that no junction will reach an excessive temperature thus preventing any annoying dimming later on.

[0064] The apparatus brightness decrease with rising temperature will be compensated for by raising the current(s) of the respective LED(s); this being counterproductive to limiting the junction temperatures to safe levels according to the invention the microcomputer will take both phenomena into account and calculate in advance how far the brightness decrease may be compensated by increasing the currents without running the danger of reaching unacceptable temperature levels.

[0065] The apparatus permits automatic zero current correction such that whenever the bright- ness control input signal is zero it is recognized by the aforementioned zero recognition means, a compensation loop with a memory is closed which reduces any zero offset to zero and stores the correction value until the next zeroing. [0066] The apparatus microcomputer with a/d and d/a converters performs the autozero operation.

[0067] The apparatus microcomputer also performs automatic calibration after autozero, i.e. at system start-up, by setting the input signal to the current generator to 100 % and checking the voltage drop across the (precision) shunt, if it deviates from nominal the microcomputer will calculate a correction factor by which the brightness control signal is multiplied.

[0068] The apparatus series string of LEDs in a paralleled set of series strings is individually returned to the current generator and its current monitored such that monitoring those currents will indicate an open failure of an LED in any string causing the current generator to reduce its maxi- mum current to the value which can be safely taken by the remaining strings thus preventing burn-out of more LEDs and eventual complete blackout. In case of a short failure of a LED the string affected would draw current away from the others and thus destroy the remaining LEDs in that string eventually shorting the current generator and thus causing blackout. Overcurrent detected in any string will then cause the current generator to reduce its output current to such ex- tent that the current in the affected string will stay below the maximum limit thus preventing blackout.

[0069] The apparatus is integrated with the power supply in one unit into which the ac mains or other supply feeds and which has an output connector for connection to the LED(s), and as many current generators are contained as there are different (color) LEDs in the lighting system. The unit will either contain a control microcomputer or connect to a bus system via an interface. This is only possible if essentially pure dc current(s) are used according to this invention, standard wiring of any length may be used, and, according to this invention preferably the lighting fixtures proper will only contain the LEDs and a temperature sensor, no electronics. [0070] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Claims

1 Electronic circuitry for driving at least one light-emitting diode (LED, 900) with dc current, - wherein the circuitry comprises in series connection in a current path for the LED (900) a switch (122) and a choke (123), - wherein the circuitry comprises a capacitor (125) in parallel connection to the LED (900), - wherein the circuitry comprises furthermore a diode (124) in parallel connection to the current path for the LED (900) and - wherein an integrated circuit (120) is operatively connected to the switch (122) in order to provide a signal for controlling the brightness of this LED (900).

2 Electronic circuitry according to claim 1 comprising an EMI filter (101) preventing pulse ripple in the current path for the LED (900).

3 Electronic circuitry according to claim 1 or 2, wherein a temperature sensor arrangement (300) measures the temperature of the at least one LED.

4 Electronic circuitry according to claim 3, wherein the temperature sensor arrangement (300) comprises a negative temperature coefficient probe or a temperature measuring integrated circuit.

5 Electronic circuitry according to one of the preceding claims, wherein the LED (900) is used as a temperature probe.

6 LED circuit according to claim one of the claim 3 to 5, wherein an absolute sensor is in- eluded that measures a reference temperature.