WO2014067893A1

WO2014067893A1 - Method for operating a light source with a plurality of leds or led groups

Info

Publication number: WO2014067893A1
Application number: PCT/EP2013/072491
Authority: WO
Inventors: Thomas Steffens
Original assignee: Zumtobel Lighting Gmbh
Priority date: 2012-10-30
Filing date: 2013-10-28
Publication date: 2014-05-08
Also published as: EP2915412A1; DE102012219817A1

Abstract

The invention relates to a method for operating a light source which has at least two separate LEDs or LED groups (101, 102), wherein pulse-modulated currents (I_LED1, ILED2) are supplied to each of the LEDs and/or LED groups (101, 102). The currents (ILED1, ILED2) supplied to the LEDs and/or LED groups (101, 102) are temporally matched to each other such that, at any point, a maximum of one of the LEDs and/or LED groups (101, 102) is supplied with current (ILED1, ILED2).

Description

Method for operating a light source with multiple LEDs or LED groups

The present invention relates to a method according to the preamble of

Claim 1 for operating a light source, which has at least two separate LEDs or LED groups, wherein the LEDs or LED groups respectively

Pulse width modulated currents are supplied. The term "LEDs" is intended to mean both semiconductor materials based light sources and so-called organic LEDs (OLEDs).

There are various possibilities for setting the brightness of a light source based on LEDs. The current most common approach is to provide the LEDs with pulse width modulated currents. In this case, so the LEDs are repeatedly turned on and off, whereby by changing the ratio between duty and off duration, the average

Whole brightness is set so the light source can be dimmed.

Compared with procedures in which the amplitude of the current supplied to the LED is continuously changed, the procedure based on pulse-width-modulated currents has the advantage that during the switch-on phase of the LED, it can be operated with the optimum current for LED operation. So here is a very efficient LED operation feasible, although in spite of a basically stepless adjustment of the brightness is possible. This procedure for changing the brightness of an LED or an LED group can also be used, in particular, when a light source has different LEDs or LED groups which, for example, emit light of a different color or color temperature, for example. By appropriately adjusting the brightness of the respective LED or LED group, the total emitted light can then be changed with respect to its color or color temperature. In such a case, each LED or LED group is then supplied with a pulse width modulated current required to achieve the individual brightness. Based on this prior art, the present invention is based on the object to further optimize the approach in operating a light source based on LEDs, on the one hand in terms of efficiency and on the other hand in terms of quality of light output.

This object is achieved by a method for operating a light source with the features of claim 1 and by a circuit arrangement for operating an LED light source according to claim 10. Advantageous developments are the subject of the dependent claims.

The inventive method is based on the idea to operate the different LEDs or LED groups in turn with pulse width modulated currents. It is now provided, however, that the individual LEDs or LED groups supplied currents are timed to each other, that to each

Time maximum one of the LEDs or LED groups, ie a so-called. Channel is supplied with power.

Accordingly, according to the invention, a method for operating a

Light source, which has at least two separate LEDs or LED groups, proposed, wherein the LEDs or LED groups each pulse width modulated

Currents are supplied and the currents are timed to each other in such a way that at any time maximum one of the LEDs or LED groups is supplied with power. Compared to the previously known procedure, in which the switch-on cycles for the different LED groups were started substantially simultaneously, the inventive method proposed here has several advantages. So now can no longer be the case that all LED groups are activated at the same time and must be supplied with power accordingly. In the procedure according to the invention, a maximum of a single channel or a single LED group is always supplied with power at any time, so that significantly lower current or voltage peaks occur here. While in the known approach, the maximum power consumption is often a multiple of the average power consumption of the light source remains at the the current consumption of the invention substantially constant. This has the consequence that the requirements for the means for power supply of the LEDs or LED groups in the inventive solution are significantly lower. Another advantage is that in the solution according to the invention a light emission by the LEDs or LED groups over a longer period of time takes place than in the prior art. In the known solution, especially for achieving a low total brightness often very long off phases are present in which no one of the LEDs is activated, resulting in flicker phenomena that are particularly perceived when the light source or illuminated by the light source area with a camera is recorded. In the case of very long shutdown phases would be such

Flicker phenomena even recognizable to the human eye. In the

The solution according to the invention, however, the light output over a longer period of a single drive cycle for the light sources away is distributed, so that there is a more homogeneous light output in terms of time. The quality of the emitted light is accordingly much better in terms of its temporal homogeneity. Of course, the inventive method also allows a change in the color or color temperature of the total emitted light and a dimming of the total brightness. Depending on the intended overall brightness, a period may be provided in particular in which none of the LEDs or LED groups is supplied with power.

A particularly simple procedure for realizing the solution according to the invention is that after switching off a first LED or LED group, a different LED or LED group is switched on only at a predetermined time or after a predetermined switch-off period. The time interval between the switching off of the first group and the switching on of the following group is permanently constant in this case, and after switching off the second group - if there are no other other LEDs or LED groups - it is still possible to connect a switch-off period, which, as mentioned above, serves to adjust the overall brightness and can be variable in duration. Alternatively, however, the time interval between switching off a first LED or LED group and switching on a second LED or LED group could also be variable. As a result, the light output with respect to its time course is even more variable or homogeneous, in particular, it may be provided that the Gesamtausschaltdauer a drive cycle

is divided equally between the off periods. The duration of a

Switch-off period or a minimum duration for a switch-off period can be in the range of a few μβ, in particular about ΙΟμβ. The different LEDs or different LED groups may in particular be light sources that emit light of a different color or color temperature. By adjusting the individual brightnesses, the total light output can then be adjusted with regard to its color or color temperature. In this case, the inventive concept is by no means limited to only two LEDs or LED groups but can be extended in principle to any number of different LEDs or LED groups.

The invention will be explained with reference to the accompanying drawings. Show it:

1 shows schematically a circuit arrangement according to the invention

Operating several LEDs or LED groups;

Figure 2 shows the time course of the currents for two different LED groups according to a known from the prior art solution;

Figure 3 shows the time course of the LED currents according to the invention

Solution; FIG. 4 shows the individual steps in a first variant of the invention

Driving the LEDs and

FIG. 5 shows the individual steps in a second variant of the invention

Activation of the LEDs. FIG. 1 very schematically shows an operating device 1 for operating a LED-based light source 100, which in the illustrated embodiment consists of two LED groups 101 and 102. The operating device 1 is connected on the input side to the general power supply and internally has a control unit 5 which is designed to convert the mains supply voltage UNetz into supply voltages or currents I _LED I and I _LED 2 for operating the two LED groups 101 and 102 , For this purpose, the control unit 5 has means internally known from the prior art for carrying out a corresponding conversion of the mains supply voltage UNetz.

The currents I _LED I and I _LED 2 supplied to the LED groups 101 and 102 are-as schematically indicated-pulse-width-modulated, that is to say the LEDs of the two groups 101, 102 are switched on and off alternately, whereby

Changing the time ratio between the switch-on duration and the switch-off duration

Brightness of the respective group 101, 102 can be changed. In this case, the control circuit 5 can be designed, in particular, to convert external control signals received via an interface 2 into suitable operating currents I _LED I and I _LED 2 to ensure that the total light output realized by the light source 100 has a desired mixed color or a desired color temperature , In addition, by appropriately setting the switch-off duration (s) and the

Total light output can be changed in their brightness, ie a dimming

be made. As indicated, this principle is applicable not only to the two illustrated LED groups 101 and 102 but can in principle be used with any number of LED groups. For example, in particular RGB arrangements, ie light sources with the colors red, green and blue are common, which allow the emission of light of almost any hue. The hitherto predominantly conventional procedure for driving the two LED groups 101 and 102 is shown schematically in FIG. This figure shows the course of the respective LED currents I _LED I and I _LED 2, wherein so far at a time t ₀ , which represents the beginning of a drive cycle, both LED groups were turned on simultaneously. In the illustrated embodiment is provided that the first group 101 remains switched on for a period τι, while on the other hand, the second group 102 is switched on only for a shorter period τ ₂ . The first LED group 101 is, in this case, a larger proportion to the total light output at as the second group 102. After the time period τι a further period of time includes x on _off, in which both groups of LEDs 101, 102 are inactive. The duration of this third time period _from x is then calculated in how high the intensity of the total light output should be. That is, over the change of this period x _from a dimming of the total light output can be made, while on the other hand via a change in the ratio of the times τι and τ _2, an adjustment of the color or color temperature is possible. After the third switch-off phase x _off , a new actuation cycle begins, that is, both LED groups 101 and 102 are again supplied with power.

It is immediately apparent that in the procedure known from the prior art according to FIG. 2 at the beginning of a drive cycle, in particular during the time period τ ₂ to which both LED groups 101, and 102 are active in the illustrated example, a high total power consumption and / or high voltage peaks exist. The control unit 5 must be designed so that these maximum current spikes or surges are covered, which is not one

inconsequential expense. This problem can be avoided with the aid of the procedure according to the invention, which will be described in more detail below.

In this case, it is provided that the currents I _LED I and I _LED 2 supplied to the LED groups 101, 102 are matched in time to one another in such a way that at most one maximum of the two groups 101, 102 is active. A first possibility for this is shown in FIG.

In this case, a drive cycle for the two LED groups 101, 102 begins again at the time t ₀ , wherein, however, initially only the first group 101 is switched on, and in turn for the period τι. Only after completion of this switch-on phase for the first LED group 101, the activation of the second group 102 for the period τ ₂ , which is strictly speaking after switching off The first group 101 is first waited for a short period τ _{0 &} before the second LED group 102 is turned on. This time separation for the phase x _0ff ensures that there is actually no overlap in terms of the power supply. It is actually a conscious one

Waiting for the delay time x _0ff and not just a systemic delay that occurs when another channel is immediately activated after disabling one channel. The delay time τ _{0 &} is therefore in the range of a few μβ, for example about ΙΟμβ, so that in fact there is a clear separation of the switch-on of the different color channels.

The second group 102 again remains active over the period τ ₂ and is then deactivated again. The first group remains inactive over the period, which corresponds in terms of its duration to the period x _of FIG. 2, so that the

Period τ ₂ another turn-off consisting of the period Tbi _ac k and the delay time τ _{0 followed &} then again at time ti the next drive cycle begins.

A comparison of Figures 2 and 3 shows directly that - due to the unaltered switch-on τι and τ ₂ and the identical total duration of a drive cycle - both LED groups 101, 102 in a comparable manner

Contribute total light output. That is, the light output realized in the procedure according to FIG. 3 is identical to the light output in the procedure of the prior art with respect to its hue or its color temperature as well as its overall brightness. As a result, however, that never both LED groups 101, 102 are active simultaneously, now the maximum power consumption is much lower. This means that no such high current and voltage peaks can occur, as is the case in the prior art procedure. The measures for powering the LED groups 101, 102 can be realized accordingly with a significantly lower cost.

Another difference is that in the approach of Figure 2 over a considerably longer period of time, namely over the entire period T _a us neither of the two LED groups 101, 102 is active. So here is a relatively long time Phase in which no light is emitted at all, which could be noticeable in a flickering. While this may not be apparent to the human eye, it would be more pronounced if the

Light source or illuminated by the light source area is recorded with a digital camera.

In contrast, in the procedure according to FIG. 3, the switch-off duration T _out for the first LED group 101 is used to realize the light output of the second LED group 102. The periods τ ₀ & and Tbiack, in which there is no light output, are thus significantly shorter, whereby obviously the following relationship applies:

2 * Toff ^ black ^ out ^- ^ 2

Of course, the time period Tbi _ac k can be kept variable or adjusted to make a dimming of the total brightness, but it is important that at comparable light output in the procedure according to the invention over a much shorter period of time no light is emitted as in the case of the The prior art solution is the case. Thus, in terms of time, a more homogeneous light output is achieved by which any flicker effects or the like can be avoided.

In this case, according to an advantageous variant, it would also be possible to vary the period of the phase x ₀ ff between switching off the first group 101 and switching on the second group 102. Ultimately, this would mean the total switch-off time Tbi _ac k - preferably evenly - over several

Divide sections, which would lead to a more homogeneous light output.

A first flow diagram of a conceivable variant for driving the LED groups according to the invention is shown in FIG. It is assumed that there are altogether n different LED groups or n so-called channels. In this case, in a first step Si by the control unit 5, the turn-on τι to τ _η for the different channels as well as the duration of it possibly subsequent complete switch-off phase Tbi _ac k determined, these time periods in particular on the basis of the externally supplied control signals over which, for example, a total color or color temperature and a total brightness is specified, are calculated. In the following step S ₂ , a channel counter value i is set to the start value 1.

In step S ₃ , a check is made as to whether the channel corresponding to the channel counter value i is to be activated at all, ie whether the corresponding LED group should contribute its color to the light output. If this is the case, all channels except the channel i are first turned off in step S ₄ (at the beginning of the cycle all channels are switched off anyway) and in the following step S ₅ the

Delay time x _0ff waited. Subsequently, the channel i is activated, that is supplied with power (step S ₆ ). After the expiration of the corresponding period of time τ in step S ₇ ; has been detected for the activation of this channel i, the channel counter value i is increased by the value 1 in step S ₈ .

In step S, it is then checked whether the channel counter value i is less than or equal to the total number n, ie whether the last channel has already been activated or whether there are other channels. If there are still other channels, the method is continued with step S ₃ and checked accordingly whether the new

Channel counter value i corresponding channel to be activated. Possibly. Then, in the manner described above, steps S ₄ to S _{8 are} again carried out for the next channel. If, on the other hand, the last channel has already been reached, it is checked in step S ₁₀ whether the brightness of the total light output should be 100% or whether the total light output should be dimmed. If dimming is required, all channels are deactivated in step Sn and then (step S ₁₂ ) the period Tbiack is awaited. This is then a drive cycle is completed and the process begins again with step S _ls which in turn is checked by the control unit 5, if - eg

due to a change in the externally supplied control signals - the switch-on phases τ; have to be modified for the different channels. The above description of the procedure according to the invention for driving the LED channels makes it clear that the method according to the invention can be implemented relatively simply or with little effort. Instead of using a microprocessor with a corresponding PWM module or an external PWM module as usual, a timer can be used in a simple manner, whereby in any case usually several such timers are present in a microprocessor. The various channels can then be controlled via the corresponding digital outputs of the microprocessor. As already mentioned, the method according to the invention can also be modified in such a way that the switch-off period Tbi _ac k is subdivided and, for example, is distributed uniformly over the delay times x _0ff . A corresponding

Diagram is shown in Figure 5 and will be explained below. In this second variant, in the first step S _101, depending on the

desired mixed color and the desired overall brightness first calculates the number n _on on the separated channels and the total break ibiackGesamt during a cycle. Further, for each channel, the individual

Duty cycle τι to τ _η calculated.

In the second step S102, it is then checked first whether the number of

channels to be switched _{on is} greater than 0, ie whether a channel is to be activated at all. If this is not the case, all channels are deactivated in step S103 and the cycle begins again.

If, on the other hand, at least one channel is to be switched on, then in step S104 the

Total switch-off time ibiackTotal during a cycle by the number of

channels to be switched _{on are} divided so that the proportionate switch-off time Tbiack results. In the course of a subsequent control step S105, it is checked whether the proportionate switch-off time Tbi _ac k calculated in this way is below a predetermined mill-off switch-off duration τ _{0 &} . If this is the case, a correction of the switch-off time to the minimum duration x _0ff takes place in step S106. This completes the calculation of the switch-on durations for the various channels and the proportionate switch-off times and the actual activation of the channels begins, the count index i being initially set to 1 as in the method of FIG. 4 in a first step (step S ₁₀₇ ) , This is followed by the check whether the corresponding channel is to be activated (S ₁₀₈ ). If this is not the case, the program jumps directly to step S ₁₁₂ and increments the counting index i by one. If, on the other hand, the corresponding channel is to be activated, a check is made in step S10 as to whether the proportional switch-off duration is greater than zero. If, on the other hand, the current channel is to be activated, all channels are deactivated in step Sno. Furthermore, the proportional switch-off time Xbiack is waited for and the channel i finally for the calculated on-time x; activated. If, on the other hand, Xbiack is not greater than 0, which is the case when only a single channel, ie a single color, is to be switched on at 100%, then in step Sm all channels except channel i are first deactivated. After waiting for the minimum delay time x _Qff , the channel i is finally activated for the start time calculated at the beginning.

The method is finally continued with step S ₁₁₂ , in which therefore the count index i is increased. If the subsequent check S ₁₁₃ reveals that i is less than or equal to n, ie other channels exist, then the method is continued with step Sios. When the last channel has been reached, the drive cycle is completed and the process starts again with step _S101 .

In this second variant, therefore, uniform turn-off times are achieved over the entire cycle, so that overall a more homogeneous light output is produced.

Another development of the method according to the invention can also be to vary the PWM frequencies. Thus, for example, it may be provided that when adding a specific color component, the corresponding LED group is initially operated, in principle, at the minimum pulse length, and therefore the

Total shutdown Xbkck is reduced. By contrast, at higher dimming levels, ie at a higher intensity of the corresponding color, the switch-on duration for the corresponding LED or LED group can then be increased and the switch-off period x _b i _ac k remains constant at a small value. This can be continued until at maximum brightness the switch-off period Tbi _ac k completely disappears.

Ultimately, therefore, by the solution according to the invention the driving of for generating a desired mixed color or mixed color temperature and overall brightness is significantly optimized. On the one hand, the effort for driving can be reduced, on the other hand, the quality of the light output is improved in terms of temporal homogeneity.

Claims

claims

1. A method for operating a light source, which has at least two separate LEDs or LED groups (101, 102), wherein the LEDs or LED groups (101, 102) each pulse-modulated currents (I _LED I, I _LED 2) supplied become,

characterized,

that the LEDs or LED groups (101, 102) supplied currents (I _LED I, I _LED 2) are timed to each other such that at any time a maximum of one of the LEDs or LED groups (101, 102) Power (I _LED I, I _LED 2) is supplied.

2. The method according to claim 1,

characterized,

in that, depending on an intended overall brightness over a period of time (x _b i _ack ), none of the LEDs or LED groups (101, 102) is supplied with power.

3. The method according to claim 1 or 2,

characterized,

a delay time (x _0ff ) is waited between switching off a first LED or LED group (101) and switching on a second LED or LED group (102).

4. The method according to claim 3,

characterized,

the duration of the delay time (x _0ff ) is constant.

5. The method according to claim 3,

characterized,

that the duration of the delay time (x _0ff ) is variable, in particular in

Depending on a planned total brightness is selected.

6. The method according to claim 5,

characterized, the duration of the delay time (x _0ff ) is selected such that a

Total _{turn-off of} a drive _cycle evenly distributed to the delay times (x _0ff ).

7. The method according to claim 5 or 6,

characterized,

the delay time (x _0ff ) has a minimum duration.

8. The method according to any one of claims 4 to 7,

characterized,

that the delay time (x _off ) or its minimum duration is in the range of a few μβ, in particular about ΙΟμβ.

9. Method according to one of the preceding claims,

characterized,

the LEDs or LED groups (101, 102) emit light of a different color or color temperature.

10. Circuit arrangement for operating a light source, which has at least two separate LEDs or LED groups (101, 102), wherein the

Circuit arrangement comprises power supply means (5), which are designed to supply respectively to the LEDs or LED groups (101, 102) pulse-modulated currents (ILEDI, ILED2),

characterized,

in that the power supply means (5) adjusts the currents (ILEDI, ILED2) supplied to the LEDs or LED groups (101, 102) in such a way that at most one maximum of one of the LEDs or LED groups (101, 102) Power (ILEDI, ILED2) is supplied.

1 1. Circuit arrangement according to Claim 10,

characterized,

in that, depending on an intended overall brightness over a period of time (x _bkck ), none of the LEDs or LED groups (101, 102) is supplied with power.

12. Circuit arrangement according to claim 10 or 11,

characterized,

13. Circuit arrangement according to claim 12,

characterized,

the duration of the delay time (x _0ff ) is constant.

14. Circuit arrangement according to claim 13,

characterized,

that the duration of the delay time (x _0ff ) is variable, in particular in

Depending on a planned total brightness is selected.

15. Circuit arrangement according to claim 14,

characterized,

the duration of the delay time (x _0ff ) is selected such that a

16. Circuit arrangement according to claim 14 or 15,

characterized,

the delay time (x _0ff ) has a minimum duration.

17. Circuit arrangement according to one of claims 13 to 16,

characterized,

that the delay time (x _0ff ) or its minimum duration is in the range of a few μβ, in particular about ΙΟμβ.

18. Lighting arrangement with a light source, which has at least two separate LEDs or LED groups (101, 102), and a circuit arrangement according to one of claims 10 to 17.

19. Lighting arrangement according to claim 18,

characterized,