EP2866526A1 - LED switching system and method for operating a LED switching system - Google Patents

Abstract

An LED circuit arrangement having at least one LED array (110) has a plurality of LED strands (120) connected in parallel, wherein the LED array (110) has a protective circuit which is designed, depending on operating parameters of the LED array ( 110) to bridge all the LED strings (120) of the array (110). The operating parameters are the height of the current flowing through at least one of the LED strings (120) and / or a temperature in the region of the LEDs (125).

Description

The present invention relates to an LED circuit arrangement for operating a plurality of LEDs, preferably by a common operating device. Furthermore, the invention relates to a method for operating a corresponding circuit arrangement.

LEDs are replacing more and more classic light sources in modern lighting technology. There are a variety of different LED types available, which differ in terms of their performance and in terms of the emitted light or the color or color temperature. Depending on the field of application of a luminaire in which the LEDs are used, that is, for example, depending on the size of the luminaire, its light exit surface, the optical system used and the like, the use of a few so-called. High-performance LEDs or a plurality of LEDs with low or medium power of advantage.

A problem in this context, however, is that in comparison to the widely available LEDs required for operating the LEDs LED operating devices are available only to a limited extent, which in turn requires esp. In the event that a plurality of LEDs with low or medium power to be operated, they must be connected in a suitable manner. In practice, LEDs have become established in so-called serial-parallel arrays, as shown schematically in FIG Figures 1 and 2 is shown.

In this known from the prior art circuit variant all LEDs are powered by a common operating device 200. A single array 210 consists in each case of a plurality of parallel-connected LED strings 220, in each of which the LEDs 225 are connected in series. In the variant according to the Figures 1 and 2 Thus, for example, n parallel LED strands 220 are present, each of which has m LEDs 225, so that an array 210 consists altogether of nxm LEDs 225. How further FIG. 1 In this case, a plurality of such arrays 210 can also be connected in series with one another, wherein the operating device 200 then preferably supplies the overall resulting arrangement with a constant current. A parallel connection of such LED arrays 210 would be possible.

A serial-parallel LED array 210, as in the FIGS. 1 and 2 is shown to have certain advantages in terms of ease of construction, the associated low cost and despite all achievable high efficiency. If identical LEDs 225 are used, which have a substantially identical forward voltage, then the current I _ballast provided by the operating device 200 is equally divided between the individual LED strands 220 with only small tolerances. The following relationship applies: $${\mathrm{I}}_{\mathrm{branch}}\cong {\mathrm{I}}_{\mathrm{ballast}}/\mathrm{n}$$

wobei V_{f_LED} der Vorwärtsspannung der LEDs 225 entspricht, die wie bereits erwähnt vorzugsweise für jede LED 225 etwa gleich ist. Die Gesamtleistung des Arrays 210 bestimmt sich wie folgt: $${\mathrm{P}}_{\mathrm{tot}}=\mathrm{m}\cdot {\mathrm{V}}_{\mathrm{f\_LED}}\cdot {\mathrm{I}}_{\mathrm{ballast}}$$

In this case, I _branch corresponds to the current within a single LED strand 220, whereas I _ballast corresponds to the constant current provided by the operating _device 200. For the voltage drop V _{f_array} via the LED array 210, the following relationship also results: $${\mathrm{V}}_{\mathrm{f\_array}}=\mathrm{m}\cdot {\mathrm{V}}_{\mathrm{f\_LED}}$$

wherein V _{f_LED corresponds to} the forward _voltage of the LEDs 225, which as already mentioned is preferably approximately the same for each LED 225. The overall performance of the array 210 is determined as follows: $${\mathrm{P}}_{\mathrm{dead}}=\mathrm{m}\cdot {\mathrm{V}}_{\mathrm{f\_LED}}\cdot {\mathrm{I}}_{\mathrm{ballast}}$$

The in the Figures 1 and 2 illustrated circuit arrangement according to the prior art thus has many advantages in terms of their simple structure and despite all achievable thereby high efficiency. However, there is a problem that, in the case of a defect of at least one LED of the corresponding array, the distribution of the current provided by the constant current source is no longer uniform to all the LED strings but instead an imbalance occurs, resulting in large differences in the intensity of the emitted light can lead. Furthermore, there is a risk that the uneven distribution of the current leads to the permissible maximum value of the current or the maximum permissible operating temperature being exceeded in some areas, which may ultimately result in further failures of LEDs and / or defects of other components of the circuit arrangement.

Shown is this situation in the FIGS. 3a and 3b showing two different types of LED defects.

sowie $${\mathrm{I}}_{\mathrm{rest}}<{\mathrm{I}}_{\mathrm{branch}}$$

So is in FIG. 3a the most common defect in LEDs is shown, in which there is a short shot of the corresponding LED. In this case, therefore, the LED string 230 having the defective LED 235 effectively has one LED less in comparison with the other LED strings 220. Due to the parallel connection of all LED strings, this has the consequence that the current I _ballast provided by the constant current _source 200 is now divided such that a higher current I _SC flows through the LED string 230 with the defective LED 235 than through the other strands (I _rest ), wherein this current I _SC in particular is higher than that provided for the normal operation current I _branch , which is evenly distributed to all LED strands. Furthermore, this imbalance also leads to the fact that in the other, non-defective LED strands 220, the current I _{rest is} below the normally present current I _branch . It therefore applies: $${\mathrm{I}}_{\mathrm{SC}}>{\mathrm{I}}_{\mathrm{branch}}$$

such as $${\mathrm{I}}_{\mathrm{rest}}<{\mathrm{I}}_{\mathrm{branch}}$$

Another LED defect, on the other hand, can be found in the FIG. 3b illustrated situation in which the damaged LED 235 leads to an interruption of the corresponding strand 230. This therefore fails completely (I _INT = 0), so that now the current I _ballast provided by the operating _device 200 is distributed to the remaining n-1 LED strands 220, with the result that the resulting current I _{rest of} all active lines 220 is above the current intended for normal operation: $${\mathrm{I}}_{\mathrm{rest}}={\mathrm{I}}_{\mathrm{ballast}}/\left(\mathrm{n}-\mathrm{1}\right)>{\mathrm{I}}_{\mathrm{branch}}$$

As already mentioned, these imbalances in the current flow through the LED strands initially lead to the LEDs 225 of the array 210 emitting light with different brightness or intensity and accordingly no uniform appearance is obtained. More serious, however, is the problem that the at least partially resulting higher currents in the strands can lead to further damage to the still functional LEDs or even other components of the circuit can be damaged.

Another known problem in the operation of LED circuit arrangements is that they are operated only within a specific temperature window should or should be allowed. Lights with LED light sources often have optics made of plastic material in order to influence the light emitted by the individual LEDs in a suitable manner. For these optics PMMA is preferably used, since this material has very good optical properties with respect to the transmission coefficient and at the same time is UV-resistant. Furthermore, this material is relatively inexpensive and can be brought in a variety of forms in a simple manner. On the other hand, compared to other plastic materials, PMMA can only be used at relatively low maximum temperatures in the range of 90 ° to 95 ° C, as higher temperatures may otherwise damage the optics. This, in turn, means that very close attention must be paid to the prevailing temperatures, with a safety standard, for example, stipulating that under certain circumstances, maximum temperatures in the range between 65 ° and 70 ° C may be achieved.

Temperature rises in the field of LED light sources can be caused, inter alia, by external factors, for example, the use in environments with a relatively high temperature or the failure of the proposed cooling measures a luminaire. Also related to the FIGS. 3a and 3b however, LED faults can lead to current flows that have a negative effect on the temperature in the area of the LED board. However, an increased temperature in turn can not only lead to damage of the optical elements located in the immediate vicinity of the LEDs, but also lead to further LED defects.

Ultimately, this means that when operating LEDs, in particular in the case of the serial-parallel arrays described above, care should be taken that, on the one hand, the currents in the different LED strands are within permissible ranges and, on the other hand, there are no excessively high temperatures in the region of the LEDs.

The object of the present invention is to provide a solution for this which prevents the occurrence of such states in a simple but efficient manner.

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

The solution according to the invention is based on the idea of assigning a protection circuit to the LED array, which is designed to bridge the entire LED array in the event of certain fault conditions. In particular, in this case the protective circuit is designed to decide on the basis of the current flowing through at least one of the LED strings and / or on the basis of a temperature which is present in the region of the LEDs, whether the LED array is bypassed for safety reasons or not.

According to the present invention, an LED circuit arrangement with at least one LED array is accordingly proposed, which has a plurality of parallel-connected LED strands, wherein the LED array according to the invention comprises a protection circuit which is designed, depending on operating parameters of the LED array all Commonly bridge LED strands of the array, and wherein the operating parameters is the height of the current flowing through at least one of the LED strands and / or a temperature in the range of the LEDs.

It has been shown that the complete bridging of the LED array is the most efficient measure to reliably avoid further damage to the circuit arrangement in the event of an LED defect and / or the occurrence of excessive operating temperatures. This is the case in particular if, according to the illustration according to FIG FIG. 1 several similar LED arrays are connected in series with each other, since then in this case only the defective array is bridged, the other array, however, continue to be supplied unaffected with the current provided by the constant current source. As will be explained in more detail below with reference to the preferred exemplary embodiments, the solution according to the invention also has the additional advantage that it can be realized relatively simply and inexpensively, but at the same time represents a reliable safeguard against the problems described above.

Preferably, the protection circuit according to the invention is based on a thyristor-containing shunt, which is activated in the event of detection of certain fault conditions in order to permanently bridge the LED array. In this case, the thyristor may in particular be part of a so-called clamping circuit (so-called crowbar circuit). Such a clamping circuit is known from the prior art and is used here as protection against overvoltages in order to trigger a fuse which interrupts the power supply of a device. In a modification of this original approach, the present invention proposes Now, to use such a clamping circuit now for selective bridging defective LED arrays, such that, if necessary, further arrays unaffected thereof can continue to be supplied with power.

Preferably, the protection circuit is designed such that it monitors the current flow through each individual one of the LED strands of the array and bridges the array if the current is above a predetermined limit in at least one of the LED strands. That is, as soon as defects lead to such an imbalance in the power distribution, that at least in an LED string, the allowable limit is exceeded, the array is completely bypassed and thus deactivated.

As an alternative or in addition to this, as already mentioned, the protective circuit can also be designed to monitor the temperature. For this purpose, it preferably has at least one temperature sensor which is designed to activate the bridging of the array when a temperature above a predetermined limit value is detected. The temperature sensor can be based in particular on a temperature-dependent resistance (NTC), wherein preferably a plurality of distributed temperature sensors are provided. In the case of a shared use of current monitoring and temperature monitoring, it is provided, in particular, that the components of the current monitoring and the components of the temperature monitoring jointly control a corresponding shunt or thyristor in order to bridge the LED array, if necessary. That is, in the case of a combined power monitoring and temperature monitoring certain components that are responsible for performing the actual protective measures for bridging the LED array can be shared, so that the effort to realize an efficient safety circuit can be kept very low. Furthermore, the components of the current sensor and / or the temperature sensor are preferably designed such that they do not need their own or separate power supply source but instead are supplied by supply current for the LED array.

Figur 1

eine LED-Schaltungsanordnung gemäß dem Stand der Technik, bei der mehrere seriell-parallele LED-Arrays in Serie verschaltet und von einem gemeinsamen Betriebsgerät versorgt sind;

Figur 2

die Ansicht eines einzelnen LED-Arrays entsprechend dem Stand der Technik;

Figur 3a

beispielhaft den Fall eines LED-Defekts, der zu einem Kurzschluss der entsprechenden LED führt;

Figur 3b

beispielhaft den Fall eines LED-Defekts, der zu einer Unterbrechung des entsprechenden LED-Strangs führt;

Figur 4

den grundsätzlichen Gedanken der Stromüberwachung in einem LED-Array gemäß der vorliegenden Erfindung;

Figur 5

ein denkbares Ausführungsbeispiel einer erfindungsgemäßen Stromüberwachungsschaltung;

Figur 6

allgemein den Gedanken der Temperaturüberwachung gemäß der vorliegenden Erfindung und

Figur 7

ein denkbares Ausführungsbeispiel einer erfindungsgemäßen Temperaturüberwachungsschaltung.

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

FIG. 1

an LED circuit arrangement according to the prior art in which a plurality of series-parallel LED arrays are connected in series and supplied by a common operating device;

FIG. 2

the view of a single LED array according to the prior art;

FIG. 3a

for example, the case of an LED defect, which leads to a short circuit of the corresponding LED;

FIG. 3b

exemplified the case of an LED defect leading to an interruption of the corresponding LED string;

FIG. 4

the basic idea of current monitoring in an LED array according to the present invention;

FIG. 5

a conceivable embodiment of a current monitoring circuit according to the invention;

FIG. 6

in general, the idea of temperature monitoring according to the present invention and

FIG. 7

a conceivable embodiment of a temperature monitoring circuit according to the invention.

The procedure according to the invention is thus based on monitoring the currents present in the LED strings and, alternatively or additionally, on a temperature monitoring in the region of the LEDs.

Based on FIG. 4 In this case, the principle of the current monitoring according to the invention and the corresponding protection circuit should be explained in principle. Shown in turn is an LED array 110, which is supplied by the constant current _source 100 with the current I _ballast , wherein now in each individual LED strand 120 in series with the respective LEDs 125, a current detector 10 is arranged. These n current detectors 10 each generate an output signal which optionally activates a bridging element, a so-called shunt 50. This shunt 50 is arranged so that it completely bridges the LED array 110.

The activation of the shunt 50 by the current detectors 10 should take place when the respectively determined current is above a certain threshold I _LIM , this threshold preferably being set to be slightly higher than the LED current intended for normal operation, but still below the maximum allowable current value. The outputs of the current detectors 10 are logically linked with each other in an OR circuit. That is, as soon as at least one of the detectors 10 detects an unacceptably high current value, the shunt 50 is activated and effectively closes the entire LED array 110 short.

In a circuit arrangement in which a plurality of such LED arrays 110 are provided, a corresponding effect then results depending on the interconnection of the arrays 110 with one another. In the case where the arrays 110 are connected in series, as in FIG FIG. 1 is shown, so only the corresponding defective array 110 is bridged by the associated shunt 50. By contrast, the further LED arrays 110 are supplied unchanged by the current of the constant current source 100 and thus remain in operation. In contrast, in the case where the arrays 110 are connected in parallel, the corresponding shunt 50 would short-circuit all the arrays 110 together, which means that even in the case of a single impermissible current value, the entire circuit is deactivated. Accordingly, the serial connection of the arrays 110 is according to FIG FIG. 1 preferable.

When bridging the array 110 through the shunt 50, the condition that an unacceptably high current has been detected by at least one current detector 10 immediately disappears. Accordingly, the shunt 50 should preferably be configured to permanently maintain the lock-in state after appropriate activation to avoid uncontrolled oscillations of the system. Further, of course, the shunt 50 must be designed so that it is capable of cope with the now completely flowing through him current I _ballast in the case of a bridging while consuming as little power. It should therefore preferably have a very low impedance.

A preferred embodiment of the current protection circuit according to the invention, which fulfills the above-mentioned requirements, is in FIG. 5 shown. The shunt 50 has as its essential element a thyristor D _SCR , which is controlled by the current detectors 10 described below. These current detectors 10 in turn are formed by a circuit arrangement consisting of a transistor Q _pnp and two resistors R _set and R _b , which as already mentioned in series with the LEDs 125 of the respective strand 120 is connected. By appropriately dimensioning the resistors R _set and R _b , the threshold from which the thyristor D _{SCR is} activated can be set in a suitable manner.

The resistors R _set and R _{b are} usually dimensioned such that in a normal operation of the circuit, so if the current of the constant current source 100 is evenly distributed to all LED strands 120, a voltage drop across the Resistor R _set is present, which is just below the base-emitter voltage V _{BE of} the transistor Q _pnp . This means that the transistor Q _pnp blocks and accordingly the shunt 50 is opened.

wobei I_branch der Höhe des Stroms im Normalzustand der Schaltungsanordnung und I_LIM dem Stromwert entspricht, ab dem das LED-Array kurzgeschlossen werden soll.For the resistance R _set , the following relationship applies: $${\mathrm{I}}_{\mathrm{LIM}}={\mathrm{V}}_{\mathrm{BE}}/{\mathrm{R}}_{\mathrm{set}}>{\mathrm{I}}_{\mathrm{branch}}.$$

where I _branch corresponds to the magnitude of the current in the normal state of the circuit arrangement and I _LIM corresponds to the current value at which the LED array is to be short-circuited.

If, however, a fault condition now causes the corresponding current in the LED string 120 to increase, the voltage drop across the resistor R _set will exceed the base-emitter voltage V _{BE of} the transistor Q _pnp , which in turn results in a turning on of the transistor Q _pnp results, which then drives the thyristor D _SCR and thus closes the shunt 50. The thyristor D _SCR thus short-circuits the entire LED array 100 and then remains closed as long as current flows through it, even if the corresponding transistors Q _{pnp of} the current detectors 10 immediately block it again, since the LED strings 120 themselves do not Electricity flows more. As in FIG. 5 can also be seen, a filter consisting of a parallel resistor R _pd and a capacitor C _{flt is} formed at the output of the thyristor D _SCR to prevent the short-term fluctuations already to trigger the shunt 50 and thus a shorting of the LED array 110th to lead.

In the FIG. 5 The circuitry shown has proven to be extremely energy efficient, requiring less than 0.6V during normal operation, which, for example, in an array with 12 LEDs in series, represents a loss of only 1.6%. In the event that the shunt 50 is activated, too little power is consumed, since the voltage drop is typically less than 2V. Another advantage of the circuit arrangement shown is that it does not need its own power supply source but can use the power provided for operating the LED array available. Of course, it is also possible to realize the circuit with the aid of an npn transistor instead of the illustrated pnp transistor accordingly.

As an alternative or in addition to the current protection circuit just described, the circuit arrangement can also be designed with a temperature protection circuit, which is described below with reference to FIG FIGS. 6 and 7 is explained.

The basic principle here is comparable to that of the monitoring circuit according to FIG. 4 , That is, even in this case, a shunt 50 is provided, which is now controlled by temperature detectors 20 and bridges the entire LED array 110 in the event of detection of an impermissibly high temperature.

However, compared to current monitoring, it is not absolutely necessary to monitor each LED string 120 individually. Instead, it is sufficient if some temperature sensors or detectors 20 are distributed in the area of the LEDs 125 and here detect the corresponding temperatures. Again, the monitoring circuit is designed such that the shunt 50 bridges the LED array 110 as soon as at least one of the temperature detectors 20 triggers and activates the shunt 50. This in turn should remain permanently activated, even if, after bridging the LED array 110, the temperature detected by the detectors 20 drops again.

One way to realize this temperature protection circuit is in FIG. 7 shown, wherein it is initially recognizable that the shunt 50 is configured here in an identical manner as in the temperature monitoring circuit according to FIG. 5 , Differences exist only with regard to the realization of the temperature detectors 20, two of which are shown in the present case.

Central components of the temperature detectors 20 are now temperature-dependent resistors R _th (NTCs), which are arranged such that they - as illustrated by the dash-dotted lines - in thermal contact with the point to be monitored (for example, a corresponding LED) are. Through the resistors R _b , R _d , R _z and R _th , a voltage divider is formed, via which a corresponding limit value can be set. The voltage divider divides the voltage set by the zener diode D _z and drives the transistor Q _pnp . It is dimensioned such that in the event that the temperature is within the intended range, the resistance R _{th is} sufficiently large, which has the consequence that the voltage drop across R _{d is} below the base-emitter voltage V _BE . The transistor Q _pnp is closed in this case and the shunt 50 is opened.

Now increases the temperature, the resistance value R _th decreases until finally the base-emitter voltage V _{BE of} the transistor Q _{pnp is} exceeded. The transistor Q _pnp opens in this case and controls the thyristor D _SCR , so that the shunt 50 is closed and thus the LED array 100 is short-circuited. Again, the thyristor D _SCR remains activated as long as current flows through it, so that in turn avoided oscillating states of the entire circuit arrangement can be. The filter provided in the shunt 50 in turn serves to prevent inadvertent activation of the shunt 50 due to short-term fluctuations.

Also for the in FIG. 7 shown circuit arrangement arise in connection with FIG. 5 described advantages. So there is a very energy-efficient circuit, which can also be realized by relatively few components and in turn in turn requires no separate power source. Furthermore, since in both cases the shunts are configured in identical ways, it is easy to combine the idea of current monitoring and temperature monitoring, using a single shunt driven by both the current detectors and the temperature detectors. This represents a particularly advantageous embodiment, since certain elements can be used together in a synergistic manner-that is to say both within the scope of current monitoring and within the framework of temperature monitoring-and, accordingly, the total effort is further reduced.

Ultimately, the occurrence of false states in LED circuits is reliably avoided as using the circuit arrangement according to the invention.

Claims (15)

LED circuit arrangement having at least one LED array (110) which has a plurality of LED strands (120) connected in parallel,
wherein the LED array (110) has a protective circuit which is designed to bridge all the LED strings (120) of the array (110) depending on operating parameters of the LED array (110),
and wherein the operating parameters are the magnitude of the current flowing through at least one of the LED strings (120) and / or a temperature in the range of the LEDs (125). LED circuit arrangement according to claim 1,
characterized,
in that the protection circuit has a shunt (50) which is arranged parallel to all LED strings (120) and is driven by at least one current detector (10) and / or at least one temperature detector (20). LED circuit arrangement according to claim 2,
characterized,
in that the shunt (50) has a thyristor (D _SCR ), which is preferably part of a clamping circuit. LED circuit arrangement according to Claim 2 or 3,
characterized,
in that the shunt (50) is actuated both by at least one current detector (10) and by at least one temperature detector (20). LED circuit arrangement according to one of Claims 2 to 4,
characterized,
in that the current detector (10) and / or the temperature detector (20) are supplied with energy by a supply current for the LED array (110). LED circuit arrangement according to one of the preceding claims,
characterized,
that the protection circuit monitors the current flow through each of the LED strands (120) and adapted to bridge the array (110) if at least one of the LED strings (120) the current is above a predetermined limit value. LED circuit arrangement according to one of the preceding claims,
characterized,
in that the protection circuit has at least one temperature detector (20) which is designed to activate the bridging of the array (110) when a temperature above a predetermined limit value is detected. LED circuit arrangement according to claim 7,
characterized,
that the temperature detector (20) has a temperature-dependent resistor (R _th) has. LED circuit arrangement according to claim 7 or 8,
characterized,
in that a plurality of temperature detectors (20) arranged distributed are provided. LED circuit arrangement according to one of the preceding claims,
characterized,
that this has a plurality of LED arrays (110), each having its own protection circuit. LED circuit arrangement according to claim 10,
characterized,
in that the LED arrays (110) are connected in series with one another and are supplied by a common power supply source, preferably by a constant current source (100). Method for operating an LED circuit arrangement having at least one LED array (110) which has a plurality of LED strands (120) connected in parallel, wherein, depending on operating parameters of the LED array (110), all the LED strands (120) of the array (120) 110),
and wherein the operating parameters are the magnitude of the current flowing through at least one of the LED strings (120) and / or a temperature in the range of the LEDs (125). Method according to claim 12,
characterized,
is that the current flow is monitored by each of the LED strands (120) and bridges the array (110) if at least one of the LED strings (120) the current is above a predetermined limit value. Method according to claim 12 or 13,
characterized,
that the temperature is detected at several positions of the circuit arrangement and the array (110) is bridged, if at least at one position the temperature is above a predetermined limit value. Method according to one of claims 12 to 14,
characterized,
that the circuit arrangement comprises a plurality of LED arrays (110).

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