EP2911480B1 - Circuit assembly and method for monitoring the flow of electricity through LEDs - Google Patents

Description

The present invention relates to a circuit arrangement with the aid of which the current flow through a string of serially connected LEDs is to be monitored.

The invention further relates to a method for monitoring the current flow through series-connected LEDs and a method for operating LEDs.

There are many different ways of operating LEDs that are used for lighting purposes, the circuit variants used also being dependent in particular on the LEDs used. It has been found that the simultaneous operation of a large number of LEDs is preferable to the use of a few high-power LEDs. On the one hand, high-power LEDs are still relatively expensive to manufacture, on the other hand, large numbers of luminous arrangements can be achieved better with a large number of LEDs controlled in the same way.

When operating a large number of LEDs, there is basically the possibility of supplying the LEDs with a constant voltage and then interconnecting them in a corresponding manner. However, a so-called constant current source is preferably used, which provides the LED arrangement with current of a certain fixed level. The LEDs are then either all connected in series or arranged in a so-called serial-parallel array. This array has a plurality of LED strings connected in parallel, in which in turn a certain number of LEDs are connected in series, the number of LEDs in the different strands connected in parallel being generally identical. Such an arrangement has proven to be extremely efficient in the past when operating with a constant current source.

A problem with the serial connection of LEDs - in particular in the form of the serial-parallel array mentioned above - is that a defect in individual LEDs can lead to a large imbalance in the current distribution between the different LED strings. Defects in LEDs can on the one hand lead to a short circuit of the corresponding LED, which means that this individual defective LED then fails within the LED string, but the other LEDs continue to be supplied with power and light up. On the other hand, however, LED defects can also lead to a complete interruption, with the result that no current then flows through the corresponding LED string at all can. In this case, the current made available by the constant current source is then distributed to the other LED strands of the arrangement, so that there is an increased current value here. The problem can arise here that the increased current flow in the further LED strings there leads to further LED defects, so that a defect of an individual LED can spread quasi avalanche-like to other LEDs of the arrangement.

To counteract such an effect, it is known from the prior art, for example, to arrange current limiters within the individual LED strands. Such a solution known from the prior art is, for example, in Figure 4 is shown, wherein it can be seen that each LED strand 110 of the serial-parallel array 100 is assigned a current limiter 115, which tries to limit the current to a certain value in the event of an increase in the current of the associated LED strand 110. This is to prevent a greatly excessive current in a single strand 110 from causing a defect in the LEDs 105 located therein.

In the Figure 4 The schematically illustrated solution has proven itself in practice, but nevertheless has some disadvantages. To implement the current limiters, relatively expensive components are used which, even in a very simple embodiment, form a current path running parallel to the LEDs. This also consumes additional energy during normal operation of the LEDs, that is to say if the current value is within a permissible range, as a result of which the efficiency of the circuit arrangement is not insignificantly reduced.

From the DE 10 2013 213 639 A1 , the US 2013/0063035 A1 , the WO 2012/077013 A2 and the DE 102 36 862 A1 circuit arrangements with LEDs, for example.

The object of the present invention is to provide a novel possibility for monitoring the current flow through an LED string, which is characterized by improved efficiency compared to previously known solutions.

The object is achieved by a circuit arrangement for monitoring the current flow through a strand of serially connected LEDs with the features of claim 1 and by a method for monitoring the current flow through an LED strand according to claim 10. Advantageous developments of the invention are the subject of the dependent claims.

The circuit arrangement according to the invention has a resistor connected in series with the LEDs as a sensor element for detecting the current flow, further components being provided to determine or evaluate the current flow through the LED strand on the basis of the voltage drop across the resistor and when a predetermined value is exceeded Output a signal.

In contrast to previously known solutions, in particular to that in Figure 4 Solution shown, should not be actively influenced by the circuit arrangement according to the invention on the current flow through the LED string. Instead, only the level of the current flowing through the LEDs is detected and, if necessary, a signal is output when it is determined that the current exceeds a permissible range. A major advantage of this solution is that a corresponding circuit arrangement can be implemented with very simple means and in particular can be designed in such a way that the consumption during normal operation of the circuit arrangement, that is to say when the current flow through the LEDs is within a permissible range, is minimal . In comparison to the solution according to the prior art described above, the solution according to the invention has almost no influence on the energy efficiency of LED operation.

Accordingly, a circuit arrangement for monitoring the current flow through a strand of series-connected LEDs is proposed, the circuit arrangement having a resistor connected in series with the LEDs and further components which are designed to increase the current flow through the LEDs based on the voltage drop across the resistor evaluate and output a signal when a predetermined current value is exceeded, the other components of the circuit arrangement being at least partially several transistors, the signal output when the predetermined current value is exceeded being output by one of the transistors and the circuit arrangement for driving the signal Output transistor has at least two control transistors, which are arranged mirrored to each other, a first control transistor with the input-side connection of the measuring resistor and a second control Tran sistor is connected to the output connection of the measuring resistor.

Furthermore, a method for monitoring the current flow through a string of LEDs connected in series is proposed, a measuring resistor connected in series with the LEDs and further components (22) being provided, the current flow through the LEDs from the further components based on the voltage drop across the measuring resistor is evaluated and a signal is output when a predetermined current value is exceeded, and the further components are at least partially a plurality of transistors, the signal output when the predetermined current value is exceeded being output by one of the transistors and for driving the signal output Transistors at least two control transistors are provided, which are arranged mirrored to one another, a first control transistor connected to the input-side connection of the measuring resistor and a second control transistor connected to the output-side connection of the measuring resistor is.

The dependent claims relate in particular to measures which relate to the arrangement of the further components of the monitoring circuit arrangement and which are intended to ensure that the desired minimum energy consumption is achieved during normal operation of the LEDs.

It is preferably provided that the signal output is a current flow through a voltage drop or a voltage drop across an output resistor. The second control transistor can be arranged in such a way that it bridges the base-emitter path of the transistor outputting the signal and accordingly accordingly causes the latter to output the corresponding signal at a given time, this signal then, as already mentioned, in a corresponding manner Current flow through or a voltage drop across an output resistor downstream of the transistor. In this case, at least one further control transistor can optionally be connected in parallel with the second control transistor, which makes it possible to precisely set the threshold at which the signal is output by the circuit arrangement according to the invention.

The circuit arrangement described above is preferably operated here by the forward voltage of some LEDs of the line to be monitored. That is, the control transistors of the arrangement are connected in parallel to some LEDs of the LED string. Accordingly, there is no need for a separate current or voltage source for the monitoring circuit, although as detailed below described - the energy consumption of the circuit arrangement can be kept to a minimum.

The monitoring circuit according to the invention is preferably used in an arrangement for operating a plurality of LEDs, the LEDs being connected in series with one another and supplied by a current source and the LEDs being assigned the circuit arrangement according to the invention for monitoring the current flow. With such an arrangement, it can then be provided in particular that it additionally has a circuit element for bridging, that is to say for short-circuiting the LED strand or for interrupting the power supply for the LEDs, this circuit element then becoming active on the basis of the signal output by the monitoring circuit according to the invention . In particular, the concept according to the invention can be used in a serial-parallel LED array.

Figur 1

das grundlegende Konzept der erfindungsgemäßen Schaltungsanordnung zum Überwachen des Stromflusses durch LEDs;

Figur 2

ein detailliertes Ausführungsbeispiel für die Realisierung der erfindungsgemäßen Schaltungsanordnung;

Figur 3

die Anwendung der erfindungsgemäßen Schaltungsanordnung in einem seriell-parallelen LED-Array, welches von einer Konstantstromquelle betrieben wird; und

Figur 4

eine aus dem Stand der Technik bekannte Lösung zum Begrenzen des Stromflusses durch einen LED-Strang;

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

Figure 1

the basic concept of the circuit arrangement according to the invention for monitoring the current flow through LEDs;

Figure 2

a detailed embodiment for the implementation of the circuit arrangement according to the invention;

Figure 3

the application of the circuit arrangement according to the invention in a serial-parallel LED array which is operated by a constant current source; and

Figure 4

a solution known from the prior art for limiting the current flow through an LED string;

The basic concept of the solution according to the invention is in Fig. 1 shown. Shown is an LED string provided with the reference number 10, which therefore has a plurality of LEDs 11 connected in series with one another. These LEDs 11 are usually supplied with current from a constant current source, not shown, which is designed in such a way that a suitable voltage drop and a corresponding current flow are established via the LEDs 11, in which the LEDs 11 can be operated optimally. If, however, the level of the current now rises above a permissible value, which can be the case, for example, in the case of defects in adjacent or parallel LED strings (not shown here), this should be reliably detected with the solution according to the invention and signaled in a corresponding manner .

According to the invention, it is therefore provided that a circuit arrangement 20 for monitoring the current flow is used, which initially has a resistor 21 as a measuring or sensor element for detecting the current flow. The resistor 21 is in turn connected in series with the LEDs 11 of the LED strand 10, so that the current flowing through the resistor 21 also corresponds to the LED current I _LED . The resulting voltage drop V _R across the measuring resistor 21 thus represents a measure of the magnitude of the current I _LED , which is why the circuit arrangement 20 according to the invention additionally has means 22 which evaluate the voltage drop across the measuring resistor 21 and then optionally output a corresponding signal, which provides information about whether there is an impermissibly high current or not.

As in Fig. 1 is shown schematically, these additional components 22 for evaluating the voltage drop are therefore connected to the input and the output of the measuring resistor 21. At the same time, these components are also connected to an additional connection point within the LED string 10, so that the forward voltage applied via these LEDs can be used as a quasi energy supply source for these components 22. As will be described in more detail below, the energy consumption can be minimized in this way.

Basically, a measuring resistor 21 with a very low resistance value should be used. Of course, this has the consequence that the voltage drop V _R across the measuring resistor 21 will also be very small and, for example, be in the range of a few millivolts or even less. One possibility for realizing the means 22 for evaluating the voltage drop, which despite everything enables reliable detection of the exceeding of an intended maximum current, is shown in Fig. 2 shown.

In the exemplary embodiment shown, the monitoring of the voltage drop via the measuring resistor 21 and the signal output dependent thereon are essentially realized with the aid of four transistors T1 to T4. The last transistor T4 is responsible for the output of the signal, which - as shown - is connected on the input side, ie in the case of the PNP transistor used here, with its emitter connection to the output of the measuring resistor 21. The output of the transistor T4, in the present case the collector connection, is connected to an output resistor R _A , the signal output by the transistor T4 in the present case being that in the event that an excessive LED current is present, a small current flow is generated via the resistor R _A , which is used as a signal, with the aid of which, for example, a switch 35 can be activated, by which the entire LED strand 10 may be bridged.

The control of the transistor T4 outputting the signal takes place with the aid of three control transistors T1, T2 and T3, the first control transistor T1 on the input side with the input connection of the measuring resistor 21 and on the output side via a resistor R ₁ with the additional connection point within the LED -Strangs 10 is connected. The base and emitter of this transistor T1 are connected to one another. Mirrored to this first control transistor T1, two further control transistors T2 and T3 are arranged in parallel, which are preferably identical - ie transistor T2 corresponds to transistor T3 - the inputs of which are the same as for transistor T4 with the output of measuring resistor 21 are connected. The outputs of these transistors T2 and T3 are in turn connected via a further resistor R ₂ to the connection point in the LED strand 10, but at the same time also to the base of the transistor T4.

The function of in Fig. 2 Circuit arrangement shown is then as explained below.

wobei Vbe1 die Basis-Emitters-Spannung des ersten Steuer-Transistors T1 und Vbe2 die Basis-Emitter-Spannung der zwei Steuer-Transistoren T2 und T3 darstellt. V_R ist der Spannungsabfall über den Messwiderstand 21, der also die Differenz zwischen den Basis-Emitter-Spannungen der Transistoren T1 und T2 (beziehungsweise T3) darstellt. Die Werte des Messwiderstands 21 sowie der weiteren Widerstände R₁ und R₂ sowie die Transistoren sind nun derart gewählt, dass im Normalbetrieb, d.h. I_LED < I_max, der erste Transistor T1 gesperrt ist, die beiden gespiegelten Transistoren T2 und T3 hingegen geöffnet sind. In diesem Fall fließt über die Transistoren T2 und T3 Strom, wodurch die Basis-Emitter-Spannung des Transistors T4 auf einem niedrigen Wert gehalten wird. In diesem Fall ist auch der Transistor T4 gesperrt, das heißt, es liegt kein Stromfluss über den Transistor T4 vor und dementsprechend fällt an dem Ausgangs-Widerstand R_A auch keine Spannung ab, die, wie bereits erwähnt, dem Signal für einen unzulässig hohen Strom entspricht.The relationship applies during normal operation, that is to say while the current flow through the LED strand 10 and thus also through the measuring resistor 21 is within a permissible range $$\mathrm{Vbe}1={\mathrm{V}}_{\mathrm{R}}+\mathrm{Vbe}\mathrm{2nd},$$

where Vbe1 is the base-emitter voltage of the first control transistor T1 and Vbe2 is the base-emitter voltage of the two control transistors T2 and T3. V _R is the voltage drop across the measuring resistor 21, which thus represents the difference between the base-emitter voltages of the transistors T1 and T2 (or T3). The values of the measuring resistor 21 and of the further resistors R ₁ and R ₂ and the transistors are now selected such that the first transistor T1 is blocked in normal operation, ie I _LED <I _max , but the two mirrored transistors T2 and T3 are open . In this case, current flows through transistors T2 and T3, thereby keeping the base-emitter voltage of transistor T4 low. In this case, the transistor T4 is also blocked, that is, there is no current flow through the transistor T4 and, accordingly, no voltage drops across the output resistor R _A , which, as already mentioned, the signal for an impermissibly high current corresponds.

If, however, the current flow I _LED in the present LED strand 10 would increase sharply due to a defect in a parallel LED strand, this would also result in a sharp increase in the voltage drop V _R across the measuring resistor 21. Compared to the transistors T2 and T3, the first control transistor T1 now has a significantly higher base-emitter voltage Vbe1, which means that current can now flow through this control transistor T1. In this case, since the voltage present at the base of the transistors T2 and T3 is increased and the base-emitter voltage for these transistors T2 and T3 is reduced accordingly, a current flow through the transistors T2 and T3 is now suppressed, which in turn has the consequence that there is also a higher voltage drop between the base and emitter of transistor T4. This opens accordingly and current can now flow through this transistor T4 flow, which is available as a measurable signal at the resistor R _A and can be used, for example, to activate the switch 35.

The return of the control transistors T1 to T3 via the resistors R ₁ and R ₂ to the connection point within the LED string 10 is, as already mentioned, necessary so that there is a sufficient voltage drop across the components of the monitoring circuit 22 according to the invention which is necessary for the Operation of the circuit as a whole is required. As shown, a point is preferably used here that bridges at least three LEDs 11, since in this case it is ensured that even if one of these LEDs fails, a sufficiently high voltage drop is still available in the area in between.

The threshold at which the circuit arrangement switches over such that there is a current flow at the output transistor T4 can be set by the choice of the resistors, in particular the measuring resistor 21, and by the number of second control transistors T2 and T3 connected in parallel. Varying the number of second control transistors makes sense insofar as generally only a limited number of resistors are available for use as measuring resistors, with the switching threshold being fine-tuned in spite of everything by adding additional control transistors can.

As already mentioned, an advantage of the solution according to the invention is that only a very low voltage drops across the measuring resistor and that only an extremely low current flows through the second control transistors even during normal operation. This results in a negligible influence on the efficiency of the LED string, a further advantage also being that no separate energy supply means are required for the different components and inexpensive elements can be used.

It is worth mentioning that in Fig. 2 The exemplary embodiment shown could of course also be realized with NPN transistors, which then results in a configuration of the circuit arrangement which is comparable in principle. For example, a MOS transistor or the like could also be used for the transistor T4 outputting the signal. The switch 35 shown could be designed, for example, as a transistor or as a thyristor, the signal output by the circuit of course also being able to be used for other purposes, for example for controlling the power supply source.

Fig. 3 finally shows a specific embodiment of the use of the circuit arrangement according to the invention in a serial-parallel LED array. An array is shown with three LED strands 10 connected in parallel, which are supplied by a common constant current source 5. In the present case, the measuring resistor 21 is connected in series with the third LED strand. This means that if an impermissibly high current flows through this third strand, the corresponding signal is output by the fourth transistor T4 of the monitoring circuit shown.

In the case of a serial-parallel LED array, a corresponding circuit arrangement for current monitoring does not necessarily have to be provided for each LED string. An excessive current flow in a single strand already provides a very specific indication that there is a problem within the circuit arrangement, which is then signaled by the output of the corresponding signal via the transistor. Of course, it would also be conceivable to assign each of the LED strings its own circuit arrangement according to the invention for monitoring the current flow.

Ultimately, the present invention creates a very simple, cost-effective and energy-efficient way of monitoring the current flow in LED circuit arrangements.

Claims (13)

1. A circuit arrangement (20) for monitoring the current flow through a string (10) of serially connected LEDs (11),
   wherein the circuit arrangement (20) has a measuring resistor (21) connected in series to the LEDs (11) and further components (22), which are designed to evaluate the current flow through the LEDs (11) by means of the voltage drop across the measuring resistor (21) and to output a signal when a predetermined current value is exceeded,
   characterized in
   that the further components (22) of the circuit arrangement (20) are at least partially a plurality of transistors (T1 to T4), wherein the signal output when the predetermined current value is exceeded is output by one of the transistors (T4) and the circuit arrangement (20) has at least two control transistors (T1, T2) for controlling the transistor (T4), which outputs the signal, which are arranged mirrored with respect to each other, wherein a first control transistor (T1) is connected to the input-side connection of the measuring resistor (21) and a second control transistor (T2) is connected to the output-side connection of the measuring resistor (21).
2. The circuit arrangement according to Claim 1,
   characterized in
   that the output signal is a current flow through a voltage drop or a voltage drop across an output resistor (R_A).
3. The circuit arrangement according to Claim 1 or 2,
   characterized in
   that at least one further control transistor (T3) is connected in parallel to the second control transistor (T2).
4. The circuit arrangement according to any one of the preceding claims,
   characterized in
   that the second control transistor or the second control transistors (T2, T3) bridge the basis-emitter path of the transistor (T4), which outputs the signal.
5. The circuit arrangement according to any one of the preceding claims,
   characterized in
   that said circuit arrangement is operated by the forward voltage of some of the LEDs (11) of the LED string (10).
6. An arrangement for operating a plurality of LEDs (11), wherein the LEDs (11) are connected to one another in series and are supplied by a current source (5), wherein the arrangement contains a circuit arrangement (20) for monitoring the current flow according to any one of the preceding claims.
7. The arrangement according to Claim 6,
   characterized in
   that said arrangement has a circuit element (35) for bridging the LEDs (11) or for interrupting the current supply for the LEDs (11), wherein the circuit element is controlled by the circuit arrangement (20).
8. The arrangement according to Claim 6 or 7,
   characterized in
   that the LEDs (11) are connected in a serial-parallel array.
9. The arrangement according to Claim 8,
   characterized in
   that the circuit arrangement (20) for monitoring the current flow is assigned to one of the LED strings (10).
10. A method for monitoring the current flow through a string (10) of serially connected LEDs (11),
    wherein a measuring resistor (21) connected in series to the LEDs (11) and further components (22) are provided, and
    wherein the current flow through the LEDs (11) is evaluated by the further components (22) by means of the voltage drop across the measuring resistor (21) and a signal is output when a predetermined current value is exceeded,
    characterized in
    that the further components (22) are at least partially a plurality of transistors (T1 to T4), wherein the signal output when the predetermined current value is exceeded is output by one of the transistors (T4) and least two control transistors (T1, T2) are provided for controlling the transistor (T4), which outputs the signal, which control transistors are arranged mirrored with respect to each other, wherein a first control transistor (T1) is connected to the input-side connection of the measuring resistor (21) and a second control transistor (T2) is connected to the output-side connection of the measuring resistor (21).
11. The method for operating LEDs (11), wherein the LEDs (11) are connected to each other in series and are supplied by a current source (5) and wherein the current flow through the LEDs (11) is monitored with the method according to Claim 10.
12. The method according to Claim 11,
    characterized in
    that with the aid of the output signal the current supply to the LEDs (11) is switched off or the LEDs (11) are bridged.
13. The method according to Claim 10 or 11,
    characterized in
    that the LEDs (11) are arranged in a serial-parallel array.

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