Circuit arrangement for operating at least a first and a second LED string from an AC or DC voltage source

Abstract

Circuit arrangement (10) for operating at least a first and a second LED string, comprising: - a rectifier (14) having a first (14a) and a second rectifier terminal (14b) for coupling to an AC voltage source (12) or a DC voltage source (11), and a third (14c) and a fourth rectifier terminal (14d) for providing a rectifier output voltage (Urect); - a voltage equalisation longitudinal impedance (24); - at least one first (100) and one second load subgroup (200), each having a first (102, 202) and a second terminal (104, 204), wherein a bridging element (120, 220) is connected between the first and the second terminal, and each having a coupling element (110, 210) designed as a four-pole with a first (106, 206) and a second primary terminal (108, 208), as well as with a first (112, 212) and a second secondary terminal (114, 214), wherein the first primary terminal (106, 206) is electrically connected to the first terminal (102, 202) and the second primary terminal (108, 208) is electrically connected to the second terminal (104, 204), wherein the first LED string (D100 to D155), which has a first number of LEDs connected in series, between the secondary terminals (112,114) of the coupling element (110) of the first load subgroup (100) and the second LED string (D200 to D227), which has a second number of LEDs connected in series,is connected between the secondary terminals (212,214) of the coupling element (210) of the second load subgroup (200); , - a control device (20) which is designed to control the bridging elements (120, 220) as a function of a voltage difference between the respectively associated first terminal (102, 202) and an auxiliary DC voltage source (22) related to the third rectifier terminal (14c), and as a function of a respective LED string voltage which is applied between the first (102, 202) and the second terminal (104, 204) when the respectively associated bridging element (120, 220) blocks a short-circuit current flow between the two terminals (102, 104, 202, 204); - wherein the second terminal of a higher load subgroup, which is not a lowest load subgroup, is electrically connected to the first terminal of a next lower load subgroup, wherein the first terminal of a highest load subgroup is electrically coupled to the fourth rectifier terminal (14d) and the voltage compensation longitudinal impedance (24) is connected between the second terminal of a lowest load subgroup and the third rectifier terminal (14c); - a voltage divider device (26) connected between the third (14c) and the fourth (14d) rectifier terminal for providing a control signal (28) to the voltage compensation series impedance (24), wherein the voltage divider device (26) is designed to control a current with a current value (Ireg) as a function of an instantaneous value of the rectifier output voltage (Urect) through the series-coupled load subgroups (100, 200) by means of the control signal (28);characterized in that the control device (20) is designed, when the circuit arrangement (10) is operated on a DC voltage source (11), to permanently control at least a first bridging element of the bridging elements of the load subgroups into a short-circuit state and to permanently control at least a second bridging element of the bridging elements of the load subgroups into a state in which the second bridging element blocks a short-circuit current flow between the first and the second terminal of the associated load subgroup.;

Description

[0001] The invention relates to a circuit arrangement for operating at least a first and a second LED string according to the preamble of patent claim 1.

[0002] Light-emitting diode (LED)-based lighting sources have matured within just a few years into a competitive alternative to traditional lighting sources, such as incandescent lamps, halogen lamps, or (compact) fluorescent lamps. Various circuit concepts are available for operating these LEDs, which operate with a comparatively low forward voltage, on an AC power grid with, for example, a nominal voltage of 240 volts, depending on the requirements of the respective application. High efficiency values ​​of the light sources can be achieved, particularly in applications where electrical isolation of the LEDs from the power grid is not required (non-SELV).

[0003] Document WO 2013 / 107894 A1 describes an optoelectronic component comprising a first and a second group of optoelectronic components for providing electromagnetic radiation of different colors. A phase dimmer controls the groups so that different color components are provided in different operating modes. This is intended to provide a technically simple way to realize dimmable, color-changing LED lighting, with the color components varying during operation.

[0004] Document DE 10 2013 201 439 A1 describes a circuit arrangement for operating LED cascades that are operated alternately by controlling them according to the instantaneous value of the AC supply voltage. A voltage divider and a linear regulator enable more efficient control of the LEDs. An auxiliary voltage supply is generated from the voltage drop across the linear regulator.

[0005] Document DE 10 2013 222 226 B3 describes a circuit arrangement for operating LED cascades with different numbers of LEDs, which are operated alternately by controlling them according to the instantaneous value of the supply voltage. A linear regulator and a voltage divider contribute to efficient control. An auxiliary voltage is used to efficiently control the different LED cascades.

[0006] DE 10 2013 201 439 A1 discloses a generic circuit arrangement for operating at least a first and a second cascade of LEDs. The LED cascades have a different number of LEDs. These are operated alternately by suitable control logic, adapted to the instantaneous value of the rectified AC supply voltage. The LED cascades are assigned to LED units, with each LED unit comprising a control device for controlling the respective LED cascade. The LED units are coupled in series between the two input terminals, with the input terminals being formed by the output of a rectifier. A linear regulator is provided in series with the LED cascades and is controlled via a voltage divider coupled between the two input terminals.To operate the respective control devices, an auxiliary voltage supply is required, which is generated from the voltage drop across the linear regulator during operation of the circuit arrangement.

[0007] Another circuit arrangement of this type is known from DE 10 2013 222 226 B3. For adaptation to different AC supply voltages, at least the highest LED cascade is assigned a switching device that is designed to connect all LEDs of the LED cascade in series in a first state and to connect a first half of the LEDs of the LED cascade in parallel with a second half of the LEDs of the LED cascade in a second state.

[0008] Due to its high efficiency values, particularly due to the non-isolated, transformerless circuit topology, this type of circuit arrangement would be ideal for emergency lighting operation. In a centralized emergency power system for luminaires, a DC voltage of 275 volts to 176 volts is switched on in the event of a mains power failure. This DC voltage is typically provided by a central battery. However, practice has shown that lamps with this type of circuit arrangement cannot be reliably operated with a DC voltage. Therefore, safe emergency lighting operation with such a highly efficient lamp cannot currently be achieved.

[0009] It is therefore an object of the present invention to further develop a generic circuit arrangement in such a way that it enables safe emergency lighting operation.

[0010] This object is achieved by a circuit arrangement having the features of patent claim 1. Advantageous developments of the present invention are the subject of the dependent claims.

[0011] The invention is based on a circuit arrangement for operating at least a first and a second LED string, comprising a rectifier with a first and a second rectifier terminal for coupling to an AC voltage source or a DC voltage source, and a third and a fourth rectifier terminal for providing a rectifier output voltage.The generic circuit arrangement further comprises a voltage compensation longitudinal impedance, at least a first and a second load subgroup, each with a first and a second terminal, wherein a bridging element is connected between the first and the second terminal, and each with a coupling element designed as a four-pole network with a first and a second primary terminal, as well as with a first and a second secondary terminal, wherein the first primary terminal is electrically connected to the first terminal and the second primary terminal is electrically connected to the second terminal. The first LED string, which has a first number of LEDs connected in series, is connected between the secondary terminals of the coupling element of the first load subgroup, and the second LED string, which has a second number of LEDs connected in series, is connected between the secondary terminals of the coupling element of the second load subgroup.The circuit arrangement further comprises a control device which is designed to control the bridging elements as a function of a voltage difference between the respectively associated first terminal and an auxiliary DC voltage source related to the third rectifier terminal, as well as as a function of a respective LED string voltage which is present between the first and the second terminal when the respectively associated bridging element blocks a short-circuit current flow between the two terminals.In each case, the second terminal of a higher load subgroup, which is not a lowest load subgroup, is electrically connected to the first terminal of a next lower load subgroup, wherein the first terminal of a highest load subgroup is electrically coupled to the fourth rectifier terminal, and the voltage equalization longitudinal impedance is connected between the second terminal of a lowest load subgroup and the third rectifier terminal.Furthermore, the circuit arrangement comprises a voltage divider device which is connected between the third and the fourth rectifier terminal for providing a control signal to the voltage compensation series impedance, wherein the voltage divider device is designed to control a current having a current value depending on an instantaneous value of the rectifier output voltage through the series-coupled load subgroups by means of the control signal.

[0012] The circuit arrangement is further developed according to the invention in that the control device is designed to permanently control at least a first bridging element of the bridging elements of the load subgroup into a short-circuit state when the circuit arrangement is operated on a DC voltage source and to permanently control at least a second bridging element of the bridging elements of the load subgroup into a state in which the second bridging element blocks a short-circuit current flow between the first and the second terminal of the associated load subgroup.

[0013] The invention is based on the finding that previously known circuit arrangements (LED modules) of this generic type are not suitable for emergency power supply, as they are only designed for alternating voltage and, accordingly, when connected to a direct voltage, the bridging elements of the individual load subgroups (switching stages) bounce in an undefined manner. This results in flickering light, i.e., an uneven luminous flux profile over time. In the worst case, operation with direct voltage can result in the destruction of the LED module. In particular, the relevant standards for emergency lighting and safety lighting systems must be met. Thus, a predefined luminous flux can now be provided even under these fundamentally changed input voltage conditions.

[0014] In particular, it can be provided that a constant luminous flux value is provided across a changing DC input voltage. Such functionality can advantageously be provided by a program-controlled computing unit (e.g., a microprocessor / microcontroller). In order to operate the illustrated circuit topology on a DC voltage, it is necessary to actively intervene in the switching logic of the bridging elements of the load subgroups. This is achieved by activating or deactivating the bridging elements, for example, in the form of switching transistors, when a DC voltage is detected. The inventors have recognized that stable operation on a DC voltage source is not possible with the generally known automatic control of the bridging elements of the individual load subgroups in mutual dependence on the voltages applied across the respective load subgroups.The control devices expediently used for a generic circuit arrangement utilize an energy storage device assigned to the respective load subgroup, preferably in the form of a capacitor, to provide the control energy required to control a bridging element. Charging such an energy storage device may then require switching phases in which the voltage across the respective load subgroup is not zero or is close to zero, i.e., the bridging element is permanently short-circuited in this state. In a permanent short-circuit condition, as can occur when supplied from a DC voltage source, the energy storage device is therefore not periodically recharged. This also means that the respective bridging elements are no longer fully or predominantly supplied from the corresponding energy storage devices of the associated load subgroups.As a result, a bridging element may deviate from the switching state intended by the respective currently applied rectifier output voltage, resulting in undesirable interaction with the other load subgroups or their bridging elements. The result is uncontrolled, unstable, flickering operation of the individual LED strings of the respective load subgroups. A further, even more serious effect may occur: the respective bridging element, whose supplying energy storage device no longer has sufficient energy, fails to reach a short-circuit state due to inadequate control, meaning it is unable to provide a low-impedance, fully conducting current path.With a bipolar transistor used as a bridging element, the required base current can no longer be provided, and with a MOSFET used as a bridging element, the required gate-source voltage can no longer be achieved. This can lead to thermal destruction of the bridging element if the current determined by the voltage compensation series impedance flows through the bridging element and, at the same time, the voltage across the bridging element is outside the permissible limits for safe operation due to insufficient control.

[0015] Preferably, the uppermost LED string can be permanently short-circuited by the associated bridging element. The operating voltage of the LED strings is thus significantly reduced, and the remaining voltage drops across the voltage compensation longitudinal impedance. This type of control enables operation with consistent brightness, i.e., constant luminous flux, across an entire expected input voltage range of 275 volts to 176 volts.

[0016] Another option is to deactivate the two lowest load subgroups. This results in a higher forward voltage than in the previous case. The remaining, now lower, voltage drops across the voltage compensation series impedance. The advantage of this is lower power loss at the voltage compensation series impedance and overall improved circuit efficiency in DC operation. This can reduce the input current required to generate a given luminous flux, which in turn can reduce the load on a central battery.A preferred embodiment of the circuit arrangement, in particular for AC operation at 230 volts, 50 hertz, is constructed with three LED strings, wherein the LED string of the uppermost load subgroup has a forward voltage of approximately 160 volts, the LED string of the middle load subgroup has a forward voltage of approximately 80 volts, and the LED string of the lowest load subgroup has a forward voltage of approximately 40 volts.

[0017] In a further preferred embodiment, the control device comprises a modular structure, with, for each load subgroup, a series circuit comprising a first capacitor and a first diode, which is connected between the first and the second terminal, a first electronic switching element with a first reference electrode and a first working electrode and a first control electrode, wherein the first reference electrode is electrically coupled to the first capacitor and the first working electrode is electrically coupled via a first resistance element to a control terminal of the bridging element, and a series circuit comprising a second diode and a second resistance element, which is connected between the first control electrode and the auxiliary DC voltage source.In this way, a particularly simple and cost-effective control of the switching stages, i.e. the bridging elements of the respective load subgroups, can be achieved. The respective switching stages are controlled automatically and alternately depending on the respective instantaneous value of the currently applied rectifier output voltage. Particularly preferably, the first electronic switching element is designed as a bipolar transistor with an emitter as the reference electrode, a collector as the working electrode, and a base as the control electrode. Such a control device is particularly suitable for being designed for the inventive intervention in the switching logic. The first capacitor represents the aforementioned energy storage device, which prevents the stable operation of a load subgroup in a permanently short-circuited state.

[0018] In an advantageous development, the circuit arrangement comprises a detection device for distinguishing between DC operation, in which the circuit arrangement is electrically coupled to the DC voltage source, and AC operation, in which the circuit arrangement is electrically coupled to the AC voltage source, wherein the detection device is designed to generate a switching signal depending on the presence of DC operation. A threshold switch in the form of a transistor, in particular a MOSFET, can preferably be used here. The threshold switch can advantageously be coupled to the auxiliary DC voltage source, so that the switching signal, in an activated state, has the level of the auxiliary DC voltage source. In a non-activated state, the switching signal can assume the level of the reference potential.

[0019] Alternatively, the detection device can also be implemented on the basis of a programmable processing unit (microcontroller, microprocessor). This particularly advantageously results in a wide range of design options for selecting the load subgroups to be short-circuited, in particular for controlling these dynamically during operation by providing switching signals separately for several of the load subgroups, in particular all of the load subgroups. In particular, this makes it possible to optimally adapt the total forward voltage of the operating LED strings to the available voltage for the load subgroup to be short-circuited, depending on the voltage level of the applied DC voltage source.

[0020] In an advantageous development, the detection device is designed to determine an AC voltage component of the rectifier output voltage using a high-pass filter and to feed the AC voltage component to a threshold switch via a low-pass filter. Particularly preferably, the input of the low-pass filter can be electrically coupled to an output of the high-pass filter via a peak-value rectifier, in particular a rectifier diode. Such a circuit arrangement can be implemented with a small number of, in particular, discrete components.

[0021] In a further advantageous embodiment, the control device comprises at least one override stage which has a second electronic switching element with a second reference electrode and a second working electrode as well as a second control electrode, wherein the second working electrode is electrically coupled to a control terminal of the bridging element of one of the load subgroups via a third resistance element, wherein the second reference electrode is electrically coupled to the third rectifier terminal via a fourth resistance element, and wherein the second control electrode is electrically coupled to a switching signal for activating the override stage depending on the presence of DC operation via a fifth resistance element.By means of such a “level shifter” circuit, an intervention in the corresponding switching stage can be realized in an advantageous manner in such a way that the bridging element of the corresponding load subgroup is controlled into a low-resistance conductive short-circuit state independently of the modules of the control device which operate automatically in interaction with the other load subgroups.

[0022] In a preferred embodiment, one of the load subgroups is a second-lowest load subgroup, between whose second terminal and the voltage equalization longitudinal impedance a lowest load subgroup is connected, wherein the second working electrode is electrically coupled to a control terminal of the bridging element of the lowest load subgroup via a third diode and a further third resistance element. In this way, no further second electronic switching element, for example in the form of a bipolar transistor, which is to be designed as a high-voltage transistor (for example, 500 volts), is necessary; instead, the third diode can be used for simplification, whereby the two lowest load subgroups can be particularly expediently controlled together into a permanent short-circuited state.

[0023] In a further advantageous embodiment, the voltage divider device is designed to output the control signal depending on a switching signal that signals the presence of DC operation, deviating from a control characteristic present in the case of AC operation. For example, it can be provided that in a voltage range intended for DC operation, in particular a voltage range between 176 volts and 275 volts, the control signal is specified, regardless of the instantaneous value of the rectifier output voltage, such that a constant current value is set by the voltage compensation longitudinal impedance.Furthermore, in DC operation, particularly in emergency lighting operation, it can be provided that the load subgroups to be short-circuited are selected such that the forward voltage of the actively controlled LEDs, i.e., the LEDs remaining in the non-short-circuited LED strings, is preferably in a voltage range between 80 and 95 percent of the available DC voltage, thereby achieving high efficiency in generating the required luminous flux for the emergency lighting application. The required luminous flux can advantageously be adjusted by adjusting the control signal.

[0024] Furthermore, it can be provided that when the DC voltage as the supply voltage, for example from a central battery, drops, the selection of the LED string(s) to be short-circuited is adjusted in order to achieve operation with a forward voltage of the non-short-circuited LED strings within the preferred voltage range of 80 to 95 percent of the available DC voltage. The control signal can then be adjusted accordingly in order to generate the same required luminous flux overall. To avoid a perceptible jump in light (both integrally and in relation to the spatial distribution), it can be provided to generate the switching from one load subgroup to be short-circuited to another load subgroup to be short-circuited by means of a pulse-width modulated controlled transition between the two short-circuit signals.The respective control signals of the overdrive stages can be complementary, meaning that only one of the two overdrive signals is active at a time during crossfading. However, it can also be provided to insert a predeterminable dead time between the two overdrive signals between which a crossfade occurs, during which the control device automatically returns to the state corresponding to the currently applied rectifier output voltage. Such control can advantageously be implemented in a programmable processing unit.Through the clever combination of the two controllable intervention variables, namely the selection of the LED strings to be short-circuited on the one hand and thus determining the forward voltage of the LED strings remaining in the current path and the current value of the current through the series-connected LED strings on the other, a constant luminous flux can be generated even with a rapidly changing input voltage, while at the same time maintaining high efficiency, thus enabling particularly long operation from a single battery. The LED strings can be switched without any noticeable change in brightness; a sudden change in brightness, which is not permitted in Germany due to the regulations applicable to emergency lighting applications, can thus be reliably avoided. Level shifter transistors of the override stages can thus be controlled directly, for example via a program-controlled processing unit (microcontroller / microprocessor).

[0025] In an advantageous development, the voltage divider device is designed to control a current with a higher current value during DC operation than specified for AC operation. The voltage divider device has, in particular, a tapping point within a voltage divider chain, which is electrically coupled to the switching signal via the series connection of a fourth diode and a sixth resistance element. Since the setpoint for the current changes when switching to DC operation, it may be necessary to adjust it accordingly. An increase in the current setpoint can be achieved in this way with very little additional circuitry effort.

[0026] In an alternative embodiment, the voltage divider device is designed to control a current during DC operation with a lower current value than specified for AC operation. The voltage divider device has, in particular, a tapping point within a voltage divider chain, which is electrically coupled to the third rectifier terminal via a shunt regulator, and a reference terminal of the shunt regulator is electrically coupled to the center point of a voltage divider, to which the switching signal is supplied and which, like the shunt regulator, is referenced to the third rectifier terminal. The reduction in the current value can thus also be achieved with minimal circuitry. The shunt regulator can, for example, be a TL431 component, which is offered by various manufacturers.Reducing the current value is particularly advantageous because it allows the circuit's power consumption to be reduced to the required minimum (plus any safety margins). This allows for a long operating time on a central battery in emergency lighting mode.

[0027] In a further advantageous embodiment, the circuit arrangement has a temperature control function, by means of which the current is reduced in AC operation to a lower current value than in nominal operation if a predeterminable temperature threshold is exceeded at a temperature measuring point of the circuit arrangement, whereby the current reduction is suppressed in DC operation. In normal operation with an alternating voltage source (AC operation), a correspondingly specified service life is required, for example, 50,000 hours. However, the predicted service life can only be achieved if the circuit arrangement is operated within the specified temperature limits; exceeding the permissible temperatures can lead to premature deterioration or even total failure of LEDs.For this reason, a temperature control system is provided that reduces the power in the LED strings at high ambient temperatures, ensuring that overall temperatures remain within the permissible range. However, such operating behavior is undesirable in emergency lighting operation if external environmental conditions (fire) cause elevated temperatures. In such a case, the luminous flux specified for emergency lighting operation must not be reduced, let alone the circuitry and thus the lamp shut down. Suppressing the temperature control system can be advantageously achieved with the aid of the detection device.

[0028] In a further advantageous embodiment, the voltage compensation longitudinal impedance has a seventh resistance element, which is electrically connected to the third rectifier terminal and through which current flows through the load subgroups. The potential reference to the third rectifier terminal thus provides a particularly simple way of controlling the other functional groups relevant for controlling the circuit arrangement, for example, the voltage divider device or the control device, with a common potential reference to the third rectifier terminal.The seventh resistance element advantageously serves in a first function as a negative feedback resistor of the voltage compensation longitudinal impedance and in a second function as a measuring resistor for providing a voltage value proportional to the current through the voltage compensation longitudinal impedance, which voltage value can be provided to a higher-level control for adjusting the current through the voltage compensation longitudinal impedance.

[0029] In a further advantageous embodiment, a current mirror is formed by the voltage compensation longitudinal impedance in conjunction with the voltage divider device. In particular, a current flowing through a base resistor of the voltage divider device, which is electrically connected to the third rectifier terminal, i.e., in this case, the reference potential, can be mapped in the ratio of the base resistor to a base resistor in the voltage compensation longitudinal impedance. This makes it particularly easy to determine the dependence of the controlled current through the voltage compensation longitudinal impedance on the rectified AC supply voltage.

[0030] The features and combinations of features mentioned above in the description as well as the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the respective combination specified, but also in other combinations or on their own, without departing from the scope of the invention. Thus, embodiments are also to be regarded as encompassed and disclosed by the invention that are not explicitly shown and explained in the figures, but which arise and can be produced by separate combinations of features from the explained embodiments. Further advantages, features and details of the invention emerge from the claims, the following description of preferred embodiments and from the drawings. In the figures, the same reference symbols designate the same features and functions.

[0031] They show: Fig. 1 shows a simplified schematic representation of an embodiment of a circuit arrangement according to the invention, Fig. 2 shows a schematic representation of a section of the circuit arrangement according to Fig. 1 with specific preferred embodiments of coupling elements and bridging elements of the individual load subgroups as well as the control device with override stages according to the invention, Fig. 3 shows a schematic representation of an alternative embodiment of the override stages according to the representation in Fig. 2, Fig. 4 shows a schematic representation of a preferred embodiment of a detection device for distinguishing DC operation from AC operation, Fig. 5 a schematic representation of a preferred embodiment of a voltage divider device of a voltage compensation longitudinal impedance and a temperature control, and Fig. 6 a schematic representation of the functional groups according to the representation in Fig. 5 with an alternative embodiment of the voltage divider device.

[0032] In the Fig. 1 shows a circuit arrangement 10 for operating a first, a second, and a third LED string, wherein the circuit arrangement 10 can be selectively coupled to a DC voltage source 11 or an AC voltage source 12. The circuit arrangement 10 comprises a rectifier 14 with four rectifier terminals 14a, 14b, 14c, 14d, wherein the first rectifier terminal 14a and the second rectifier terminal 14b can be coupled to the AC voltage source 12 or the DC voltage source 11. The first rectifier terminal 14a and the second rectifier terminal 14b thus represent the input terminals of the rectifier 14.The third rectifier terminal 14c and the fourth rectifier terminal 14d serve to provide a rectifier output voltage Urect. In the preferred embodiment shown, the third rectifier terminal 14c forms a negative terminal and the fourth rectifier terminal 14d forms a positive terminal. The positive terminal has a positive potential 16, and the negative terminal has a negative potential 18. The negative potential 18 is simultaneously defined as the reference potential GND of the circuit arrangement 10.

[0033] The circuit arrangement 10 comprises three load subgroups, namely a first load subgroup 100, a second load subgroup 200, and a third load subgroup 300. The first load subgroup 100 has a first terminal 102 and a second terminal 104, with a bridging element 120 connected between the first terminal 102 and the second terminal 104. Furthermore, the first load subgroup 100 has a first LED string consisting of 56 series-connected LEDs D100 to D155, which is electrically coupled to the first terminal 102 and the second terminal 104 via a coupling element 110. The coupling element 110 is designed as a four-pole network with a first primary terminal 106 and a second primary terminal 108, as well as with a first secondary terminal 112 and a second secondary terminal 114.The first terminal 102 is electrically connected to the first primary terminal 106, and the second terminal 104 is electrically connected to the second primary terminal 108. The first secondary terminal 112 is electrically connected to an anode terminal of the LED D155, and the second secondary terminal 114 is electrically connected to a cathode terminal of the LED D100. Thus, the first LED string—comprising the LEDs D100 to D155—is connected between the secondary terminals 112, 114 of the coupling element 110.

[0034] In a simple embodiment, for example, an electrically conductive connection between the first primary terminal 106 and the first secondary terminal 112, as well as an electrically conductive connection between the second primary terminal 108 and the second secondary terminal 114, can be realized within the coupling element. Within the coupling element 110, the first primary terminal 106 can be arranged so as to be electrically insulated from the first primary terminal 108.

[0035] A second load subgroup 200 and a third load subgroup 300 are constructed in the same way as the first load subgroup 100. The corresponding elements are identified by corresponding reference numerals, with the hundreds place of the reference numerals indicating the corresponding load subgroup. For example, a first connection of the second load subgroup 200 is designated 202, and a first secondary connection of a third coupling element 310 of the third load subgroup 300 is designated 312. The construction of the second load subgroup 200 differs from the first load subgroup 100 in that the number of LEDs in the second LED string is only half as large, i.e., 28 LEDs D200 to D227 are arranged between the two connections 212 and 214. In the same way, the number of LEDs in the third load subgroup 300 is again reduced by half.Here, 14 LEDs D300 to D313 are coupled between the first secondary terminal 312 and the second secondary terminal 314.

[0036] The circuit arrangement 10 further comprises a control device 20, which is coupled to the three load subgroups 100, 200, 300. Parts of the control device 20 can simultaneously also be part of one of the load subgroups 100, 200, 300. The effect of the control device 20 on the bridging elements of the individual load subgroups 100, 200, 300 in the form of the first bridging element 120, the second bridging element 220, and the third bridging element 320 is indicated by corresponding arrows. The three load subgroups 100, 200, 300 are electrically connected in series, wherein the second terminal 104 of the first load subgroup 100 is electrically connected to the first terminal 202 of the second load subgroup 200 and a second terminal 204 of the second load subgroup 200 is electrically connected to a first terminal 302 of the third load subgroup 300.

[0037] In the illustrated embodiment, the positive terminal, which carries the positive potential 16, is electrically connected to the first terminal 102 of the first load subgroup 100.

[0038] The circuit arrangement further comprises an auxiliary DC voltage source 22, which is coupled at its negative terminal to the negative potential 18 and at its positive terminal to the control device 20. A voltage compensation series impedance 24, through which a current with a current value Ireg flows, is coupled between the second terminal 304 of the third load subgroup 300 and the negative potential 18 serving as the reference potential GND. A voltage divider device 26, which provides a control signal 28 to the voltage compensation series impedance 24, is coupled between the positive potential 16 and the negative potential 18.

[0039] Fig. Figure 2 shows a specific first embodiment of a part of the circuit arrangement 10, namely the three load subgroups 100, 200, 300 as well as a first override stage 221 and a second override stage 231, which are each part of the control device 20. The bridging element 120 is implemented by two transistors Q12, Q13, which are arranged in a Darlington circuit. The transistors Q12, Q13 are preferably designed as PNP transistors, wherein the emitter of the transistor Q12 is electrically connected to the first terminal 102 and the collector of the transistor Q12 is electrically connected to the collector of the transistor Q13 and also to the second terminal 104. Furthermore, the base of the transistor Q12 is electrically connected to the emitter of the transistor Q13.A resistor element R14 is arranged between the emitter and base of transistor Q12, and a resistor element R13 is arranged between the emitter and base of transistor Q13. The base terminal of transistor Q13 is coupled to the control device 20.

[0040] In a preferred embodiment, the coupling element 110 has a diode D13, which is coupled between the first primary terminal 106 and the first secondary terminal 112. A capacitor C13 is coupled between the first secondary terminal 112 and the second secondary terminal 114. Thus, the capacitor C13 is connected in parallel to the first LED string with the LEDs D100 to D155. A series circuit comprising a capacitor C12 and a diode D12 is coupled between the first terminal 102 and the second terminal 104 of the first load subgroup 100. The diode D12 is oriented such that, when the bridging element 120 is blocked, the capacitor C12 charges to the voltage determined by the forward voltage of the forward-biased LEDs D100 to D155.The anode of diode D12, which is coupled to capacitor C12, is also electrically coupled to the emitter of a transistor Q11, which is embodied as an NPN transistor. The collector of transistor Q11 is coupled to the control terminal of bridging element 120 via a resistor element R12. A capacitor C11 can optionally be connected between the control terminal (base terminal of transistor Q13) and the second terminal 104. A series circuit comprising a diode D11 and a resistor element R11 is connected between the base of transistor Q11 and the auxiliary DC voltage source 22. The specific forms of the second load subgroup 200 and the third load subgroup 300 are correspondingly identical in construction, with the specific components that correspond to one another being identified by corresponding reference numerals, with the tens digit of the reference numerals indicating the corresponding load subgroup.For example, the components comprised by the coupling element 310 are the diode D33 and the capacitor C33.

[0041] In a preferred embodiment, the control device 20 comprises a first override stage 221 and a second override stage 231. The first override stage 221 comprises a transistor Q502, which is embodied as an NPN transistor, whose collector is coupled to the control terminal of the bridging element 220 via a resistor element R509. The emitter of the transistor Q502 is coupled to the reference potential GND via a resistor element R522. A capacitor C504 is also coupled between the base of the transistor Q502 and the reference potential. A switching signal CENTRAL_OFF is coupled to the base of the transistor Q502 via a resistor element R508.During normal operation, i.e., particularly during AC operation, the bridging element 220 is controlled into a short-circuit state when the potential at the second terminal 204 is correspondingly lower than the potential provided by the auxiliary DC voltage source 22 at the diode D21, so that a base current flows through the transistor Q21, which subsequently enables a collector current through the transistor Q21, which supplies the control current for the Darlington transistors Q22, Q23. The control current for the bridging element 220 is taken from the capacitor C22. Control of the bridging element 220, comprising the two transistors Q22 and Q23, in a low-resistance conducting state or short-circuit state can therefore only occur as long as a sufficient charge is available in the capacitor C22.The override stage 221 now provides the possibility of permanently driving the bridging element 220 into a short-circuit state. A current path for the base current of the bridging element 220, supplied by the transistors Q22, Q23, is provided via the transistor Q502 when the switching signal CENTRAL_OFF assumes a positive value, for example, of the auxiliary DC voltage source 22, which leads to the transistor Q502 being switched on. The auxiliary voltage to which the switching signal is switched in the activated state is designated VAUX. In this way, the bridging element 220 can be switched on independently of the switching state of the transistor Q21 and the charge state of the capacitor C22.

[0042] The second, identically constructed overdrive stage 231 comprises the resistor elements R511, R512, R523, as well as a capacitor C505 and an NPN transistor Q504. The second overdrive stage 231 is coupled to the control terminal of the third bypass element 320, i.e., to the base of transistor Q33.

[0043] Since the voltage in emergency power operation is a direct voltage of 275 volts to 176 volts, it must be ensured that the LEDs shine with constant brightness at every voltage level. However, since the total forward voltage of the LED strings is normally above 176 volts, one or more LED strings must be deactivated. This is done by intervening in the base current of the respective switching transistor (Q23 or Q33) with a resistor element (R509 or R512), which is connected to the reference potential GND via a "level-shift transistor" (Q502 / Q504). This causes the transistor Q23 / Q33 to become conductive. A typical voltage distribution of the individual LED strings at 230 volts mains voltage is 160 volts / 80 volts / 40 volts. Deactivation of the second LED string and the third LED string, i.e. activation of the bridging elements 220, 320 in the short-circuit state, thus leads to a forward voltage of approximately 160 volts.The remaining voltage (mains voltage minus 160 volts) drops across the voltage compensation impedance, which can be designed as a linear regulator.

[0044] Alternatively, what is in the Fig. 2 is not shown - the bridging element 120 can be controlled into the short-circuit state by providing the transistors Q12, Q13 with a permanently available base current of sufficient magnitude. For this purpose, an overdrive stage according to the structure of the overdrive stages 221, 231 is coupled to the control terminal of the bridging element 120, i.e., the base of the transistor Q13. This results in the uppermost and thus the longest LED string being bridged. The second LED string and the third LED string remain, through which the current flows. This results in a forward voltage of approximately 120 volts across the second and third strings. The remaining voltage (mains voltage minus 120 volts) is dropped across the voltage compensation series impedance 24, which can be designed, for example, as a linear regulator.

[0045] Fig. 3 shows an alternative embodiment for a combination of the two override stages 221, 231, when the second override stage 231 is always to be controlled synchronously with the first override stage 221 via the common switching signal CENTRAL_OFF. Fig. The circuit part of the override stage 221 known from Figure 2 is present here unchanged for controlling the second bridging element 220. To control the third bridging element 320, the resistor element 512 and a diode D504, which replaces the transistor Q504, are sufficient in this case. The other components R511, R523, and C503 can thus be omitted. This eliminates the need for a level-shift transistor, which is usually designed as a high-voltage transistor, for example, 500 volts.

[0046] A detection device 40 for distinguishing DC operation from AC operation is provided in the Fig. 4. For this purpose, the rectified mains input voltage, i.e. the rectifier output voltage Urect, is analyzed via a high-pass filter formed by the components C501, R501, and R502. At a tapping point of the high-pass filter, an AC voltage component is tapped off by a diode D501. A capacitor C501 is coupled between the tapping point and the positive potential 16, and a series circuit consisting of a resistor element R501 and a resistor element R502 is connected between the tapping point and the reference potential GND. The diode D501 is electrically coupled to the tapping point by its anode. A series circuit consisting of a resistor element R504 and a resistor element R503 is connected to the cathode of the diode D501, which together with a capacitor C502 form a low-pass filter. The capacitor C502 is connected to the reference potential GND.A resistor element R506 and a Zener diode D502 are also connected in parallel to the capacitor C502. The Zener diode D502 is arranged in such a way that it prevents the capacitor C502 from being charged via the diode D501 above the Zener voltage of the diode D502. The tapping point of the capacitor C502 formed in this way is coupled to the gate of a P-channel MOSFET Q501. The source terminal of the MOSFET Q501 is coupled to the auxiliary voltage VAUX, which can be provided, for example, by the auxiliary DC voltage source 22. A capacitor C503 is connected between the drain terminal of the transistor Q501 and the reference potential GND. The switching signal CENTRAL _OFF can therefore be tapped via the capacitor C503. In parallel with the capacitor C503 there is - in the . Fig. 4 not shown, but from the Fig. 2 or Fig. 3 – at least the series connection of the resistor elements R508 and R522 as well as the base-emitter diode of transistor Q502. Thus, a base load is always present for the CENTRAL_OFF signal, and a defined signal level is maintained even when transistor Q501 is off. The AC voltage component is evaluated via a low-pass filter consisting of D501, D504, R503, C502, and R506, and transistor Q501. If the voltage falls below the voltage threshold U GS , the auxiliary voltage VAUX is switched through to the "CENTRAL_OFF" mains. A missing AC voltage component (in the case of DC voltage) in the mains voltage is detected. A typical voltage value for the auxiliary voltage VAUX can, for example, be in the range of 5 volts to 6 volts.

[0047] Fig. 5 shows preferred embodiments of the voltage compensation series impedance 24, the voltage divider device 26, and a temperature feedback control. The voltage compensation series impedance 24 comprises a linear regulator with two transistors Q51, Q52 in a Darlington configuration, wherein the two collectors of the transistors Q51 and Q52 are connected to one another and electrically coupled to the second terminal 304 of the lowest, namely the third, load subgroup 300. A series circuit comprising two resistance elements R56, R57 is connected between the emitter terminal of the transistor Q51 and the reference potential GND. A capacitor C51 is coupled to a connection point between the base of the transistor Q51 and the emitter of the transistor Q52, which capacitor is coupled on the other side to the reference potential. The base of the transistor Q52 is coupled to a tapping point of the voltage divider device 26.A series circuit consisting of the resistor elements R51, R52, and R53 is connected between the positive potential 16 and the tapping point of the voltage divider device 26. A double diode D51 and a resistor element R55 arranged in series with it are connected between the tapping point of the voltage divider device 26 and the reference potential GND.

[0048] Optionally, a series circuit consisting of a diode D71 and a parallel circuit consisting of a resistor R71 and a capacitor C71 can be connected in parallel to the series circuit consisting of the double diode D51 and the resistor R55, forming an RC element (R71, C71). This circuit branch serves to limit the inrush current, so that the linear regulator does not set its full rated current at the moment of switch-on. The base current of the linear regulator is thus slowly increased after switch-on, as charge flows into the aforementioned RC element.

[0049] A transistor Q66 is also connected to the collector of the temperature control circuit in parallel with the tapping point. The emitter of the transistor Q66 is connected to the reference potential GND. The base of the transistor Q66, which is an NPN transistor, is connected to a midpoint of a voltage divider consisting of resistor elements R66 and R67. The resistor element R67 is arranged in parallel with the base-emitter diode of the transistor Q66. The resistor element R66, which is characterized by a temperature-dependent resistance with NTC (negative temperature coefficient) behavior, is coupled between the base of the transistor Q66 and the auxiliary voltage VAUX.In the event of a temperature increase at the temperature-dependent resistance element R66, the resistance value of the resistance element R66 decreases, which leads to a voltage increase at the base of the transistor Q66 and thus to an increasing conduction of the transistor Q66. This reduces the voltage at the tapping point of the voltage divider device 26 and thus reduces the current value Ireg set by the voltage output series impedance 24, which flows through the resistance elements R57, R56 and subsequently also via the terminal 304. To deactivate the temperature regulation function in DC operation, a transistor Q503 with a base series resistor R510 is provided, which is coupled to the reference potential GND via its emitter. The collector of the transistor Q503, which is designed as an NPN transistor, is conductively connected to the base of the transistor Q66.The switching signal CENTRAL_OFF is fed to transistor Q503 via the base series resistor R510. To ensure a constant luminous flux in the event of a fire at excessively high temperatures, it is necessary to ensure that the temperature cutoff / temperature regulation is deactivated. This is achieved by transistor Q503. When the switching signal CENTRAL_OFF exceeds a positive value equal to the base-emitter voltage of transistor Q503, the base-emitter voltage of transistor Q66 is reduced via the switching-on transistor Q503, preventing intervention by the temperature-dependent voltage divider R66, R67 and thus deactivating the temperature regulation.

[0050] In the Fig. Figure 5 shows a preferred embodiment for potentially increasing the current value Ireg. The switching voltage CENTRAL_OFF is applied to the voltage divider at a feed point between the resistor elements R52 and R53 by means of a series connection of a resistor element R507 and a diode D503. This either has no effect on the voltage divider if the voltage at the cathode of D503 is higher than the switching voltage CENTRAL_OFF, or it increases the current value Ireg.

[0051] In the Fig. 6 shows a preferred embodiment for the possible reduction of the current value Ireg. The illustration largely corresponds to the illustration in the Fig.5, the difference lies in the circuit part that is applied to the voltage divider device 26. In this embodiment, the switching voltage CENTRAL_OFF is applied to a voltage divider formed by the resistor elements R517 and R515. The center tap of the voltage divider formed by the two resistor elements R515 and R517 is connected to a reference voltage input of a shunt regulator, preferably a TL431 component. If the reference voltage exceeds a value of 2.5 volts, the cathode voltage is also limited to 2.5 volts. This limits the voltage at the resistor element R53 and thus also the current through the linear regulator and thus through the LEDs. As long as the reference voltage is below 2.5 volts, the TL431 component is high-impedance and does not intervene in the voltage divider around R51, R52, R53.

[0052] The exemplary embodiments serve merely to explain the invention and are not limiting thereof. In particular, the specific configurations of the load subgroups 100, 200, 300 as well as the control device 20 and the voltage divider device 26 can be designed as desired without departing from the spirit of the invention. In the exemplary embodiments illustrated, the third rectifier connection 14c is designed as a negative connection and coupled to a reference potential GND. Of course, the third rectifier connection 14c can also be provided by a positive connection, in which case the reference potential is provided by the negative potential. With this swapped assignment, the directions of the diodes must then be adjusted accordingly. In this case, the polarity of the auxiliary DC voltage source 22 must also be changed.In this case, transistors are expediently replaced by their complementary transistors, i.e. NPN transistors are replaced by PNP transistors and vice versa, and N-channel MOSFETs are replaced by P-channel MOSFETs and vice versa.

[0053] For a more detailed description of the switching processes in AC operation, which can be performed with a circuit arrangement of this type, reference is made to an earlier work by one of the two inventors, which is presented in the aforementioned DE 10 2013 222 226 B3 and is incorporated herein by reference in its entirety. In particular, the part of the document relating to explanations and specifications of features of the present invention is incorporated herein by reference.

[0054] Thus, the above example demonstrates how LED strings with serially connected load subgroups can be operated from a central emergency power supply from a DC source. Compliance with standards regarding constant light output in the event of an emergency power failure is achieved both with decreasing DC voltage and in the event of excessive temperature.

Claims

[1] Circuit arrangement (10) for operating at least a first and a second LED string comprising: - a rectifier (14) having a first (14a) and a second rectifier terminal (14b) for coupling to an AC voltage source (12) or a DC voltage source (11), and a third (14c) and a fourth rectifier terminal (14d) for providing a rectifier output voltage (Urect); - a voltage equalisation longitudinal impedance (24); - at least one first (100) and one second load subgroup (200), each having a first (102, 202) and a second terminal (104, 204), wherein a bridging element (120, 220) is connected between the first and the second terminal, and each having a coupling element (110, 210) designed as a four-pole with a first (106, 206) and a second primary terminal (108, 208), as well as with a first (112, 212) and a second secondary terminal (114, 214), wherein the first primary terminal (106, 206) is electrically connected to the first terminal (102, 202) and the second primary terminal (108, 208) is electrically connected to the second terminal (104, 204), wherein the first LED string (D100 to D155), which has a first number of LEDs connected in series, between the secondary terminals (112,114) of the coupling element (110) of the first load subgroup (100) and the second LED string (D200 to D227), which has a second number of LEDs connected in series,is connected between the secondary terminals (212,214) of the coupling element (210) of the second load subgroup (200);, - a control device (20) which is designed to control the bridging elements (120, 220) as a function of a voltage difference between the respectively associated first terminal (102, 202) and an auxiliary DC voltage source (22) related to the third rectifier terminal (14c), and as a function of a respective LED string voltage which is applied between the first (102, 202) and the second terminal (104, 204) when the respectively associated bridging element (120, 220) blocks a short-circuit current flow between the two terminals (102, 104, 202, 204); - wherein the second terminal of a higher load subgroup, which is not a lowest load subgroup, is electrically connected to the first terminal of a next lower load subgroup, wherein the first terminal of a highest load subgroup is electrically coupled to the fourth rectifier terminal (14d) and the voltage compensation longitudinal impedance (24) is connected between the second terminal of a lowest load subgroup and the third rectifier terminal (14c); - a voltage divider device (26) connected between the third (14c) and the fourth (14d) rectifier terminal for providing a control signal (28) to the voltage compensation series impedance (24), wherein the voltage divider device (26) is designed to control a current with a current value (Ireg) as a function of an instantaneous value of the rectifier output voltage (Urect) through the series-coupled load subgroups (100, 200) by means of the control signal (28); characterized bythat the control device (20) is designed, when the circuit arrangement (10) is operated on a DC voltage source (11), to permanently control at least a first bridging element of the bridging elements of the load subgroups into a short-circuit state and to permanently control at least a second bridging element of the bridging elements of the load subgroups into a state in which the second bridging element blocks a short-circuit current flow between the first and the second terminal of the associated load subgroup. [2] Circuit arrangement (10) according to claim 1, characterized by that the control device (20) comprises a modular structure, with for each load subgroup: - a series circuit comprising a first capacitor (C12, C22) and a first diode connected between the first (102, 202) and the second terminal (104, 204), - a first electronic switching element (Q11, Q21) having a first reference electrode and a first working electrode as well as a first control electrode, wherein the first reference electrode is electrically coupled to the first capacitor (C12, C22) and the first working electrode is electrically coupled via a first resistance element (R12, R22) to a control terminal of the bridging element (120, 220), and - a series circuit comprising a second diode (D11, D21) and a second resistance element (R11, R21) connected between the first control electrode and the auxiliary DC voltage source (22). [3] Circuit arrangement (10) according to claim 1 or 2, characterized bya detection device (40) for distinguishing between DC operation, in which the circuit arrangement (10) is electrically coupled to the DC voltage source (11), and AC operation, in which the circuit arrangement (10) is electrically coupled to the AC voltage source (12), wherein the detection device (40) is designed to generate a switching signal (CENTRAL_OFF) depending on the presence of DC operation. [4] Circuit arrangement (10) according to claim 3, characterized by that the detection device (40) is designed to determine an AC voltage component of the rectifier output voltage (Urect) by means of a high-pass filter (C501, R501, R502) and to feed the AC voltage component to a threshold switch (Q501) via a low-pass filter (D501, R504, R503, C502, R506). [5] Circuit arrangement (10) according to one of the preceding claims, characterized byin that the control device (20) comprises at least one override stage (221, 231) which has a second electronic switching element (Q502, Q504) with a second reference electrode and a second working electrode as well as a second control electrode, wherein the second working electrode is electrically coupled to a control terminal of the bridging element (120, 220) of one of the load subgroups (100, 200) via a third resistance element (R509, R512), wherein the second reference electrode is electrically coupled to the third rectifier terminal (14c) via a fourth resistance element (R522, R523), and wherein the second control electrode is electrically coupled to a switching signal (CENTRAL_OFF) for activating the override stage (221, 231) as a function of the presence of DC operation via a fifth resistance element. [6] Circuit arrangement (10) according to claim 5, characterized bythat one of the load subgroups (100, 200) is a second lowest load subgroup (200), between whose second terminal (204) and the voltage equalization longitudinal impedance (24) a lowest load subgroup (300) is connected, wherein the second working electrode is electrically coupled to a control terminal of the bridging element of the lowest load subgroup (300) via a third diode (D504) and a further third resistance element (R512). [7] Circuit arrangement (10) according to one of the preceding claims, characterized by that the voltage divider device is designed to output the control signal (28) as a function of a switching signal (CENTRAL_OFF) which signals the presence of DC operation, deviating from a control characteristic present in the case of AC operation. [8] Circuit arrangement (10) according to claim 7, characterized byin that the voltage divider device (26) is designed to control a current with a higher current value (Ireg) during DC operation than that specified for AC operation, wherein the voltage divider device (26) in particular has a tapping point within a voltage divider chain, which is electrically coupled to the switching signal (CENTRAL_OFF) via the series connection of a fourth diode (D503) and a sixth resistance element (R507). [9] Circuit arrangement (10) according to claim 7, characterized byin that the voltage divider device (26) is designed to control a current with a lower current value (Ireg) during DC operation than that specified for AC operation, wherein the voltage divider device (26) in particular has a tapping point within a voltage divider chain which is electrically coupled to the third rectifier terminal (14c) via a shunt regulator, wherein a reference terminal of the shunt regulator is electrically coupled to the center point of a voltage divider (R507, R515), to which the switching signal (CENTRAL_OFF) is supplied and which, like the shunt regulator, is related to the third (14c) rectifier terminal. [10] Circuit arrangement (10) according to one of the preceding claims, characterized bya temperature control by means of which the current is reduced in AC operation to a lower current value (Ireg) than in nominal operation when a predeterminable temperature threshold is exceeded at a temperature measuring point of the circuit arrangement (10), wherein the reduction of the current is suppressed in DC operation. [11] Circuit arrangement (10) according to one of the preceding claims, characterized by that the voltage compensation longitudinal impedance (24) has a seventh resistance element (R57, R56) which is electrically conductively connected to the third rectifier terminal (14c) and through which current flows through the load subgroups (100,200). [12] Circuit arrangement (10) according to one of the preceding claims, characterized by that a current mirror (R57, R56, Q52, Q51, R55, D51) is formed by the voltage compensation longitudinal impedance (24) in conjunction with the voltage divider device (26).

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