EP2534927A1

EP2534927A1 - Method for operating at least one light-emitting diode and lighting device for carrying out the method

Info

Publication number: EP2534927A1
Application number: EP11773206A
Authority: EP
Inventors: Tobias Frost; Stefan Lorenz
Original assignee: Osram GmbH
Current assignee: Osram GmbH
Priority date: 2010-08-26
Filing date: 2011-08-02
Publication date: 2012-12-19
Also published as: US20130147388A1; CN103081565A; DE102010039827A1; WO2012025345A1; DE102010039827B4; US9137875B2

Abstract

The invention relates to a method for operating at least one light-emitting diode (4a, 4b, 4c), wherein the at least one light-emitting diode (4a, 4b, 4c) is operated at least in areas by means of a pulse number modulation, in which a number (n_dim) of uniform current pulses (p_red) is adjusted within a predetermined interval (T) according to a light energy desired value, in particular a dimming level. The lighting device (1) is equipped with at least one light-emitting diode (4a, 4b, 4c) and at least one driver (3a, 3b, 3c) for feeding the at least one light-emitting diode (4, 4b, 4c), wherein the lighting device (1) is designed to carry out the method.

Description

description

Method for operating at least one light emitting diode and lighting device for carrying out the method

The invention relates to a method for operating or feeding at least one light-emitting diode. The invention also relates to a lighting device with at least one light-emitting diode and at least one driver for feeding the at least one light-emitting diode.

In order to control the brightness of light emitting diodes (LEDs) over a wide range, varying the LED current (current level modulation) operating the LEDs is inappropriate, since at low and high currents the brightness of the LEDs is different from an otherwise well-balanced linear current / brightness Ratio may vary and also the LEDs can be damaged. Therefore, LEDs are usually operated by means of a pulse-width modulation (PWM), in which the LEDs for a predetermined duty cycle tl (also referred to as on ^{¬ time} ) ^¬ operated with a constant current I and then for a predetermined switch-off t2 (also as Shutdown time called) are de-energized. A ^duty ratio tl / T with a period T = (tl + t2) corresponds to approximately one target dimming level. If the period T = (tl + t2) is set too long (typically longer than 10 ms), this PWM dimming method can be perceived by a human observer by a flicker. The current or the current I, integrated over the duty cycle tl, defines the light energy or amount of light generated in the period T.

The PWM control or the PWM dimming method makes dynamic demands on a current source used. Ideally, a constant current is provided for the on-time tl, and no current flows during the off period t2 (ie, the switching operation is negligible is carried out quickly). Only the duty ratio tl / (tl + t2) is directly proportional to an effective Hellig ^¬ ness or intensity of the LED (s). However, with current sources having a large rise time constant t_PSU (ie, the duration the current source takes to raise its current value from the off state to a predefined fraction of the nominal value) and low dimming levels, this is no longer guaranteed so that the dimming behavior is linear Behavior deviates.

In a (color) mixture of light of different colored LEDs with different required intensity ratios or individual duty cycles (eg tli for i different LEDs) in order to set a certain Sumfarbfarbort, it depends on the intensity ratio or brightness ratio between the different LEDs on to hold a certain value. At too low a degree of dimming (ratio of the actual current to a maxi ^¬ paint current), the luminance ratio is, for example, but see be- two differently colored LEDs are no longer proportional to tla / TLB, and the effective mixed color changes significantly.

Thus, the minimum, practically usable degree of dimming has been substantially determined by the rise time constant t_PSU or Ge ^¬ speed of the power source and the period T ^¬ Festge sets. In practice, therefore periods T of approximately 0.1 ms are not exceeded what the degree of dimming adversely be ^¬ borders. The dimming by means of the pulse width modulation is consequently used, in particular, for period lengths T in a range of 10 ms>T> 0.1 ms. At such periods T no flicker is perceived.

One way to address these drawbacks is to use fast power sources, which are relatively expensive. Also, the flicker at low dimming levels and or color variations at low dimming levels be taken into account, but this is adversely affected by a user.

It is the object of the present invention to at least partially overcome the disadvantages of the prior art and in particular to improve a luminous behavior of at least one light-emitting diode for low dimming levels, in particular to linearize them more. This object is achieved according to the features of the independent claims. Preferred embodiments are insbesonde ^¬ re the dependent claims.

The object is achieved by a method for operating at least one light emitting diode, wherein the at least one light emitting diode is operated at least in regions by means of a Pulszahlmo ^¬ modulation, wherein a number of uniform current pulses within a predetermined interval duration in response to a target value for the radiated by at least one light emitting diode Light energy ("light energy setpoint"), in particular a degree of dimming, is set. Thus, in other words, the number of current pulses depending on the desired light energy setpoint variable ^¬ bar, and the setpoint can be approximated in particular by the smallest deviation resulting number.

The novel pulse-number modulation makes it possible, in a simple manner, to improve in particular a linearity between the set light-energy setpoint value and an actually achieved actual value, in particular for low degrees of dimming. Here, the pulse number modulation benefit that the Lichtener ^¬ energy is due to the uniformity of the current pulses generated per current ^¬ pulse from the at least one light emitting diode well known, even without knowing the shape of the current pulses. In other words, the degree of dimming can be varied in stages in a precisely defined manner and enables a greatly reduced ^nonlinearity . In particular, a relative error can between the light energy setpoint, in particular a desired dimming level, and an associated light energy actual value, for example the actual dimming level, can be considerably reduced. A Stufig ^¬ resistance and a remaining non-linearity can be easily adjusted by a height or a duty cycle of the current pulses. The pulse-number modulation can also be used with slow current sources.

The degree of dimming may in particular giewert as a proportional Lichtener-, current value and / or pro rata brightness value in Be ^¬ train to a maximum current value for supplying the at least one light emitting diode or brightness value of light emitted from the at least one light-emitting diode can be considered. However, the light energy setpoint may also comprise another parameter, for example an electrical power.

It is an embodiment that the method involves switching between a pulse width modulation (in which, in particular, a switch-on duration of a current pulse is set as a function of a desired degree of dimming), in particular at higher dimming levels or at higher dimming levels, and pulse-number modulation, in particular at lower ones Dimming levels or lower dimming levels. Thereby, the mode can be adapted to different operating areas of the light-emitting diode, in particular under ^¬ schiedliche dimming ranges. The use of the Pulsweitenmodu ^¬ lation for higher Dimmgrade, for example of 10% or more, there enables a particularly simple activation or energization of the at least one light source, namely, to a undimmed state.

In particular can be switched between the pulse-width modulation and the pulse number modulation so that the Pulswei ^¬ width modulation and the pulse number modulation or the like on an associated switching point at least approximately an equal luminous intensity, luminous energy, brightness and / or achieve equal Genauig ^¬ ness. It is still an embodiment that the switching is performed when a degree of dimming is reached or exceeded, which corresponds to a minimum duty cycle of the current pulse width modulation pulse at which the current pulse reaches its rated current. Pulse width modulation can thus be used in particular above this Dimmgrads ^¬ the pulse number and the modulation is below this Dimmgrads. It is exploited that linearity between the target dimming level and the actual dimming level in Pulsweitenmodula ^¬ tion considerably deteriorated when the rated current of the associated single current pulse is no longer achieved, although an absolute deviation is lower. It has been found, however, that the use of several, in particular, comparatively short or small current pulses of Pulszahlmodu ^¬ lation improves linearity.

It is also an embodiment that a current source for supplying the at least one light emitting diode has a rise time constant which is shorter than the period of the pulse width modulation, in particular 0.01 times to 0.1 times shorter. This simple, variable pulse width modulation can be used with one of ^¬ sufficiently good linearity for a wide range of Dimmgrade. The case that the rise time constant is 0.01 times to 0.1 times shorter than the period of the pulse width modulation has the advantage that then within the period a steady state after a period corresponding to usually about three rise time constant can be achieved.

In particular, the rise time constant may be the duration required by the current source to reach a current value of 0.5 times the rated current or, preferably, a current value of (1-1 / e), corresponding to approximately 0.63 times the current value Rated current to reach. It is further an embodiment that the method comprises switching between current level modulation at higher dimming levels and pulse number modulation at lower dimming levels. This results in the advantage that the at least one light emitting diode can be operated with dimming levels at which a (continuously applied) current level of a current level modulation would be below its lower threshold value. It is exploited that many light-emitting diodes for a damage-free operation require a supply with a current level with at least the lower threshold. However, the heart rate modulation can be carried out at higher values of the current level or even dimming in which the pulse number modulation greater accuracy made ^¬ light than the current level modulation.

It is also an embodiment that a pulse duration or duty cycle of the at least one current pulse of Pulszahlmo ^¬ dulation is smaller than or equal to a rise time constant of a power source for powering the at least one light-emitting diodes de such as a driver. With significantly longer a ^¬ switching period, the number of pulses per interval or Perio ^¬ de and thus the (discrete) Number of Dimmgrade significantly ^¬ a side set. It is still an embodiment that a duty cycle of the at least one current pulse pulse modulation pulse is between about 0.1 times a rise time constant of a current source for supplying the at least one light emitting diode of the power source and about 10 times the rise time constant, in particular between see 0.3 times the rise time constant and 1.5 times the An ^¬ rise time constant. With a significantly longer duty cycle, the number of pulses per interval or period and thus the (discrete) number of dimming levels are considerably limited. With significantly shorter duty cycle or duty cycle, the switching requires a fast skid, which can cause a high cost and high-frequency noise. It is yet a further embodiment, the interval of time between about 1 ^¬ \ is and about 10 ms, in particular between about 0.5 ms and about 10 ms, including the end values.

It is also an embodiment that the switching is performed at a switching point with a continuous brightness or light energy of the at least one light-emitting diode. As a result of the switching over, no jump in brightness is caused. Thus, a viewer does not notice the transition between the two types of control.

It is yet an embodiment that the pulse duration or duty cycle is at least one current pulse of Pulszahlmo- dulation based on a least fixed at the time of switchover number of this at least one current pulse defines the Festge ^¬. In other words, at first the number of current pulses of the pulse number modulation for switching can be fixed and be determined knowing the number of current pulses whose duty cycle or pulse duration so that the transition is as continuous as possible. A suitable pulse duration can be determined, for example, by means of suitable characteristic curves. The pulse duration may alternatively be set by means of a sensor (for example, a light sensor) and eg a Regelschlei- fe, that the transition is made possible continu ously ^¬.

It is an alternative embodiment that during the transition between the pulse width modulation and the pulse number modulation a duty cycle or pulse duration of the current pulses of the pulse number modulation is fixed and the number of current pulses is set so that the transition as possible kontinu ^¬ ierlich or with the least possible jump , A suitable number of current pulses may be, for example, under the knowledge of a brightness or light energy at the transition point and with knowledge of the light energy generated by a current pulse by a multiplication of the current pulses caused light energy and the number of current pulses are determined as a variable.

The switch-on duration or pulse duration and the number of current pulses of the pulse-number modulation can alternatively be determined as a best combination or variation of both variables, possibly within predefined limits for at least one of the parameters. In particular, the number of current pulses of Pulszahlmodula ^¬ tion per period at the transition point is the maximum number of current pulses. Another dimming reduces the number of current pulses. The maximum number of current pulses thus ent ^¬ speaks the number of adjustable by the pulse number modulation levels of brightness or brightness levels.

It is also an embodiment that the method is performed in parallel for several groups of light emitting diodes of different colors. Each of the groups may have one or more light emitting diodes of the same color. In this case, in particular, the (mixed) light emitted by the light-emitting diodes can have a desired sum color location. Due to the individual adjustment of control mode to various operating states, particularly distinguishable by different areas of the light energy target value, particular Dimmgrads can ver ^¬ be simplified compliance of the total color locus on low Dimmgrade.

The object is also achieved by a lighting device with at least one light emitting diode and at least one driver for supplying the at least one light emitting diode, wherein the light ^¬ device for implementing the method as described above is set up. In the following figures, the invention will be described schematically with reference to exemplary embodiments. there may be provided with the same reference numerals for clarity, identical or equivalent elements ^¬ Ele.

Fig.l shows a plot of normalized to a nominal current pulse width modulated current I by at least one light-emitting diode as a function of time t ei ^¬ ner in arbitrary units of time for a comparatively slow current source;

2 shows a plot of normalized to a nominal current pulse width modulated current I by at least one light-emitting diode as a function of time t ei ^¬ ner in arbitrary units of time for a comparatively rapid current source;

3 shows a plot of normalized to a nominal current pulse width modulated current I by at least one light-emitting diode as a function of time t ei ^¬ ner in arbitrary units of time for the comparatively slow power source having a length ^¬ ren duty cycle;

4 shows a plot of a normalized to a rated current pulse width modulated current I through at least one light emitting diode as a function of egg ^¬ ner time t in any time units for the comparatively slow power source with a shorter duty cycle;

FIG. 5 shows the plot from FIG. 4 with its pulse width modulation together with an alternative pulse-number modulation for switch-on periods of the current source above the rise-time constant t_PSU; 6 shows a plot of the pulse width modulation together with the alternative pulse-number modulation for switch-on durations of the current source below the rise time constant t_PSU; and

7 shows an LED lighting device for performing the pulse number modulation. Fig.l shows a plot of a rated current In = 1 normalized pulse-width modulated current I through an LED or multiple LEDs (o.Abb.) As a function of time t in be ^¬ arbitrary units of time, namely for a comparatively slow current source. The PWM period or period T of a single current pulse P here is about 10 Zeiteinhei ^¬ th. The period T may in particular be between 0.5 ms and 10 ms. A switch-on duration t 1 of the current pulse P as well as a switch-off duration t 2 (= T-t 1) have a setpoint value of exactly 5 time units, so that the setpoint duty cycle t 1 / T or t 1 / (t 1 + t 2) corresponding to the desired value Dimming degree ^¬ speaks, ideally at 50%.

Due to the non-negligible (rise) time constant of the power source from here t_PSU 0.5 time units the effective (actual) degree of dimming is, however, only 46%, which corresponds to a relati ^¬ ven error of about 8%. Due to the inertia of the power source, the degree of dimming is reduced by approx. 4%. 2 shows a view similar to Fig.l plot for a relatively fast power source with t_PSU of approximately 0.1 times ^¬ units. The relative error with respect to the degree of dimming is then negligibly small. 3 shows a plot analogous to FIG. 1 for the comparatively slow current source, wherein now the setpoint duty cycle t1 is eight time units, corresponding to a target dimming level of 80%. However, the actual (actual) dimming level is only 80% - 4% = 76%, which corresponds to a relative error of approx. 5%.

FIG. 1 shows a plot analogous to FIG. 1 for the comparatively slow current source, wherein now the setpoint duty cycle t1 is two time units, corresponding to a target dimming level of 20%. However, the actual (actual) dimming level is only 20% - 4% = 16%, which corresponds to a relative error of about 20%. The reduction of the degree of dimming by 4%, which is essentially constant for the examples described above, is due to the constant shape of the flanks of the current pulse there, if the rated current is reached at 1 = 1.

If the dimming level is reduced to less than 20%, the rated current is no longer reached and the edge becomes steeper. This redu ^¬ decorates the absolute error to below 4%, but increases the relative error.

If, for example, a switch-on duration t 1 of one time unit corresponds to a setpoint dimming level of 10%, the actual dimming level is 7%, which corresponds to a deviation of 3% but a relative error of 30%. Analogously, a ^¬ A switching duration tl of 0.5 or 0.2 units of time (desired degree of dimming 5% and 2%) an actual degree of dimming of 3% and 1%. The associated relative error is 40% or 50%. 5 shows a plot of Figure 4 to the similar Appli ^¬ supply with the arrangement shown there conventional type of pulse-width modulation with the single current pulse P with the parameters: tl_min = 1.9 units of time (corresponding to a desired dimming level of 19%), an actual dimming level of 15% and a relative error of 21%. The tl_min is the smallest duty cycle for a single current pulse P at which the current pulse still reaches the nominal value of the current In = 1.

In addition, dashed lines show a novel pulse number modulation (PZM). Within the original period T of 10 units of time, the pulse-number modulation has a plurality of 'reduced' current pulses p_red, which do not reach the rated current In and, as a result, a relatively steep slope. The period T of the pulse width modulation corresponds to an interval duration of the pulse-number modulation. A pulse width or duty cycle tl_LLD as well as a number n_dim the reduced current pulses pressure P_Red within the ur ^¬ sprünglichen period T is chosen such that in the 'crossover point' t = tl_min the pulse width modulation and the pulse number modulation equal the actual degree of dimming of 15% with an equal have a relative error of 21%. The behavior, in particular a brightness behavior, of the LED (s) is thus the same at the transition point tl_min, so that a change between the two types of modulation at the transition point tl_min (eg PWM for tl> tl _min or for tl> = tl _min and PZM for tl <= tl_min or tl <tl_min) is not perceivable by the brightness of an observer.

In the example shown, the same behavior is achieved at the transition point tl_min by n_dim = 10 reduced current pulses which each have a pulse width or duty cycle tl_LLD = t_PSU and thus a current level I of approximately 0.63 of the rated current value In. If, in particular, the switch-on duration t1 of the individual pulse P of the pulse width modulation drops below tl_min, the degree of dimming is thus reduced to very low degrees of dimming, the use of the reduced pulse-pulse modulation current pulses significantly improves the linearity between the nominal dimming level and the actual dimming level. 6 shows a conventional individual PWM current pulse P with an on-time tl = 0.95 <tl_min = 0.5 to 1.9 time units (corresponding to half of tl_min in FIG. 5), which corresponds to an actual dimming level of 6 , 2% and a relative error of 35%. The single current pulse P reaches now only about 0.85% of the rated current.

The number of reduced current pulses p_red of the pulse-number modulation reduced accordingly to n_dim = 0.5 × 10 = 5, which in their form resembles the reduced current pulses p_red shown in FIG. 5 (ie are dimming-degree-invariant), results in an improved actual value. Dimming level of 7.6% and an improved relative error of 25%. Because of the pulse rate the reduced current pulses p_red for n_dim> 1 is higher than that of the individual current pulse P in the pulse width modulation, flicker is imperceptible. It is also possible to replace the previously used one PWM current pulse P by two different numbers n_dim redu ^¬ cated current pulses p_red. For example, with a single PWM current pulse P of tl = 0.4 time units (corresponding to a target dimming level of 4%), an actual dimming level of 2% and a relative error of 50% can be achieved. Using n_dim = 2 reduced current pulses p_red of the form described above results in an actual dimming level of 3% and a relative error of 25%. Using n_dim = 3 reduced current pulses p_red results in an actual dimming level of 4.5% and a relative error of only 13%.

The linearity between the desired degree of dimming, and the actual degree of dimming or brightness of the LED (s) may thus be as ^¬ improved by that starting from a pulse width modulation for tl> approx tl_min with a single current pulse P at ca. tl_min is switched on approximately tl_min a pulse number modulation for tl <, wherein the pulse width modulation ge ^¬ Mäss pressure P_Red has the second mode, depending on the degree of dimming one or more re ^¬-induced current pulses, and vice versa.

In general, a duty cycle can tl_LLD = t_PSU the reduced current pulses are pressure P_Red set tl_min by the requirement for a continuous transition of the brightness or light energy at the transition point and following the desired degree of dimming n_dim by the best number and / or turn ^¬ duration tl_LLD the reduced current pulses pressure P_Red be approximated. The amount of light emitted from the LED or the LEDs is proportional to n_dim · tl_LLD. As can be seen from the examples, the relative error at low dimming levels can be significantly reduced by the use of pulse-number modulation. The linearis So at low dimming levels can be significantly increased.

For a plurality of different types of LEDs, in particular differently colored LEDs, a pulse-number modulation can be carried out separately. The control of the various types of LEDs can in particular be synchronized or coordinated. Insbeson ^¬ particular may be the same for the various ^¬ denartigen LEDs an interval duration.

Among other things, the pulse rate modulation achieves the following advantages:

- Even at very low dimming levels (0% <t / T <10%) no flicker is visible. Even dimming levels below 1% can be achieved with sufficiently accurate linearity.

The time constant t_PSU of the current source can be longer than the PWM duty cycle tl_LLD required to achieve low dimming levels (tl / T <10%, in particular <2%).

- Even with a large time constant t_PSU is a linear dimming behavior to less than 10% dimming degree (in particular between

1% and 10%), which is particularly advantageous for a color mixing control.

7 shows a possible LED lighting device 1 for carrying out the pulse number modulation and in particular the method described above. The LED lighting device 1 has a Net zanschluss 2, which here exemplifies three Trei ^¬ over 3a, 3b, 3c supplied with power. The three drivers 3a, 3b, 3c each feed a light-emitting diode 4a, 4b or 4c, which emit light of different colors, for example red, green or blue. The drivers 3a, 3b, 3c can their associated light emitting diode 4a, 4b and 4c respectively drive by the above-described methods be ^¬. The drivers 3a, 3b, 3c can be coordinated with one another in such a way that they control the light emitting diodes 4a, 4b or 4c assigned to them in each case in such a way that a desired summation color locus of a light source through the light emitting diode approximated the mixed light 4a, 4b and 4c generated or provided a ^¬, in particular regulated, is.

The lighting device 1 may e.g. a lamp or a light.

Of course, the present invention is not limited to the embodiments shown.

For example, it may be advantageous for a particularly effective approach to the target dimming level that tl_LLD <t PSU.

Reference sign list

1 LED light device

2 mains connection

3a driver

3b driver

3c driver

4a LED

4b LED

4c light emitting diode

I electricity

In rated current

T period

P current pulse

t time

tl switch-on duration t2 switch-off duration

11 min transition point t_PSU rising time constant p red reduced current pulse n dim number of current pulses tl LLD duty cycle

Claims

claims

1. A method for operating at least one light emitting diode (4a, 4b, 4c), wherein the at least one light emitting diode (4a, 4b, 4c) is operated at least partially by means of a pulse number ^¬ modulation, in which a number (n_dim) uniform current pulses (pressure P_Red) is within a vorbe ^¬ agreed interval duration (T) in response to a light energy setpoint, in particular degree of dimming, set Default.

2. The method of claim 1, wherein the method comprises switching between a pulse width modulation at higher dimming levels and pulse number modulation at lower dimming levels.

3. The method of claim 2, wherein said switching is performed, when a dimming level is reached or überschrit ^¬ th which a smallest duty cycle (tl_min) corresponds to the current pulse (P) of pulse width modulation, wherein the current pulse (P) its rated ^¬ current (In) reached.

4. The method according to any one of claims 2 to 3, wherein a power source for powering the at least one light emitting diode, a rise time constant (t_PSU) which is kür ^¬ be shorter than a period (T) of the Pulsweitenmodula ^¬ tion, in particular from 0.01 times to 0.1 times shorter.

5. The method of claim 1, wherein the method comprises switching between current level modulation at higher dimming levels and pulse width modulation at lower dimming levels.

6. The method according to any one of the preceding claims, wherein a duty cycle (tl_LLD) of the at least one current pulse pulse modulation pulse less than or is equal to a rise time constant (t_PSU) of a current ^¬ source for feeding the at least one light emitting diode.

Method according to one of the preceding claims, wherein a duty cycle (tl_LLD) of the at least one current pulse pulse number modulation between about 0.1 times a time cons tante (t_PSU) a current source for supplying the at least one light emitting diode and about 10 times the Ans constant time constant (t_PSU), in particular between 0.3 times the initial time constant (t_PSU) and 1.5 times the rise time constant (t_PSU).

Method according to one of the preceding claims, in which the interval duration (T) is between approximately 1 μs and approximately 10 ms, in particular between approximately 0.5 ms and approximately 10 ms.

Method according to one of the preceding claims, wherein the switching at a switching point having a continu ous ^¬ brightness of the at least one light emitting diode is performed.

The method according to claim 9, wherein the switch-on duration (tl_LLD) of the at least one current pulse pulse modulation pulse is determined on the basis of a fixed number (n_dim) of this at least one current pulse at the time of switchover.

Method according to one of the preceding claims, wherein the process (4c 4a, 4b) of different color is performed in parallel for several groups of Leuchtdi ^¬ oden, wherein the of the light emitting diodes (4a, 4b, 4c) abge ^¬ incident light a target Summenfarbort having. 12. Lighting device (1) with at least one light-emitting diode (4a, 4b, 4c) and at least one driver (3a, 3b, 3c) for supplying the at least one light-emitting diode (4a, 4b, 4c), wherein the lighting device (1) is arranged to carry out the method according to one of the preceding claims.