Abstract:
According to the present disclosure, a circuit arrangement for operating semiconductor light sources includes: a power input for inputting an AC input voltage, an output having a first output terminal, and a second output terminal, which is designed to connect a string of semiconductor light sources, a control input for controlling the operation of the circuit arrangement with a control signal, a rectifier circuit for converting the AC input voltage into a rectified voltage, a converter circuit for transforming the rectified voltage into a current which is suitable for the semiconductor light sources, a first switch arranged between the converter circuit and the output, for the switching of the current through the semiconductor light sources, and a first diode arranged between the first switch and the output, or between the converter circuit and the first switch.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a national stage entry according to 35 U.S.C. §371 of PCT application No.: PCT/EP2016/051453 filed on Jan. 25, 2016, which claims priority from German application No.: 10 2015 202 370.2 filed on Feb. 10, 2015, and is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to a circuit arrangement for operating semiconductor light sources, having a power input for inputting an AC input voltage, an output having a first output terminal, and a second output terminal, which is designed to connect a string of semiconductor light sources, a control input for controlling the operation of the circuit arrangement with a control signal, a rectifier circuit for converting the AC input voltage into a rectified voltage, and a converter circuit for transforming the rectified voltage into a current which is suitable for the semiconductor light sources. 
       BACKGROUND 
       [0003]    The present disclosure proceeds from a circuit arrangement for the operation of semiconductor light sources, of the generic type described in the main claim. 
         [0004]    In many cases, state-of-the-art circuit arrangements for the operation of semiconductor light sources are not switched in the conventional manner, wherein they are turned on by the switching-in of the mains voltage and turned off by the switching-out of the mains voltage, but are permanently connected to the mains voltage, and are switched by means of a data bus such as, e.g. a DALI bus. The fact that these circuit arrangements are permanently connected to the mains voltage raises a problem which is known from the prior art. As a result of stray capacitances, the AC mains voltage can generate a small current in the semiconductor light sources, which causes the semiconductor light sources to glow, at least in part. Particularly in a dark environment, this glowing can be clearly perceived, and is undesirable. The current responsible for the glowing of semiconductor light sources is described hereinafter as the glow current I G . From the prior art, measures are known which are intended to attenuate the glowing of semiconductor light sources in a switched-out circuit arrangement. 
         [0005]      FIG. 2  shows a voltage U EWN  which, notwithstanding the switching-out of a circuit arrangement  100  for the operation of semiconductor light sources, is present on the LED string  55 , and results in the glowing of the LEDs  5  in the LED string  55 . This voltage flows via stray capacitances in the LED string  55 , although the circuit arrangement  100  for the operation of semiconductor light sources is not actively in service. This voltage can induce a small current in the light-emitting diodes  5  (typically of a value of 500 μA-1,000 μA), which causes the latter to glow. A glowing of the light-emitting diodes  5 , at least in darkness, is visible with effect from a light-emitting diode current of 1 μA. 
         [0006]    From  FIG. 3 , a known method is inferred for the reduction of the glowing of semiconductor light sources. 
         [0007]      FIG. 3  shows a circuit arrangement according to the prior art, which already reduces the glowing of the LEDs  5 .  FIG. 3  represents the output section of the circuit arrangement in the switched-out state, with the semiconductor light sources glowing. The two output conductors LED+ and LED− herein are short-circuited on the input side, on the grounds that, for the e.m.f. U EWN , the interconnection of the circuit arrangement at this point acts in the manner of a short-circuit. 
         [0008]    From the prior art it is known that, between a DC voltage converter and the output terminal of the circuit arrangement, a diode  1  is connected in series. In itself, this substantially reduces the glow current, as practically no more current can flow in the blocking direction of the diode. The diode must be appropriate for this function, and must show the smallest possible stray capacitance. 
         [0009]    In the light-emitting diode string  55 , a protective diode  7  is connected in an antiparallel arrangement with each light-emitting diode  5 , which is intended to protect the light-emitting diode  5  from excessively high blocking voltages. Light-emitting diodes are known to be highly sensitive to high blocking voltages, and can be easily destroyed as a result. Consequently, in practically every commercial light-emitting diode package, a protective diode  7  is connected to the LED chip  5  in an antiparallel arrangement. State-of-the-art light-emitting diodes are high-power modules which, on the grounds of their high power conversion capacity, generate substantial quantities of waste heat. As a result, these modules are customarily fitted to “metal-core printed boards”. These are printed circuit boards which are essentially comprised of a good thermally-conductive sheet metal, generally aluminum or copper. A very thin insulating layer is applied to this sheet metal to which, in turn, known printed conductors are applied. As a result of the limited thickness of the insulating layer, very good thermal conduction to the metal core, i.e. to the sheet metal, is provided. Waste heat generated on the light-emitting diodes  5  can thus be evacuated very effectively. However, this thermal advantage is also associated with an electrical disadvantage: as a result of the limited thickness of the insulating layer, the entire arrangement acts as a capacitor, and specifically as a Y-capacitor, as the sheet metal, in the majority of arrangements, is grounded. These stray capacitances are represented in the circuit diagram in  FIG. 3  as capacitors  9 . Via these capacitors  9 , a glow current can flow to ground, even with the circuit arrangement in the switched-out state. 
         [0010]    In order to further reduce the glow current flowing in the light-emitting diode string  55 , a MOSFET S 1  is arranged between the DC voltage converter and the output terminal  124  which, during the operation of the circuit arrangement for operating semiconductor light sources, is switched-in, and is likewise switched-out, when the circuit arrangement for operating semiconductor switches is switched-out. This MOSFET S 1  thus further suppresses the glow current in the forward direction of the light-emitting diodes  5 . The diode  3  represented in  FIG. 3  is the body diode of the MOSFET S 1 . A varistor  13  is connected in parallel with the drain-source gate of the MOSFET S 1 , in order to protect the MOSFET S 1  against overvoltage pulses. Between the MOSFET S 1  and the output terminal  124 , a Y-capacitor  11  is arranged in the ground connection, which likewise reduces the glowing of the light-emitting diodes  5 . 
         [0011]    However, even in this known circuit arrangement, a glow current I G , albeit weak, continues to flow in the light-emitting diodes  5 . This is essentially attributable to the drain-source capacitance of the MOSFET switch S 1  and, notwithstanding careful selection, also to the rather low resistance value and the high capacitance value of the varistor  13  which, even upon the application of a low voltage thereto, shows a rather low resistance value and a rather high stray capacitance. For technological reasons, the characteristic performance of available varistors is only conditionally suitable for the present application. 
       SUMMARY 
       [0012]    The object of the present disclosure is the disclosure of a circuit arrangement for operating semiconductor light sources, wherein the glow current is further reduced, such that it is no longer perceptible, even in a dark environment. 
         [0013]    This object is fulfilled according to the present disclosure by a circuit arrangement for operating semiconductor light sources, having a power input for inputting an AC input voltage, an output having a first output terminal, and a second output terminal, which is designed to connect a string of semiconductor light sources, a control input for controlling the operation of the circuit arrangement with a control signal, a rectifier circuit for converting the AC input voltage into a rectified voltage, a converter circuit for transforming the rectified voltage into a current which is suitable for the semiconductor light sources, a first switch arranged between the converter circuit and the output, for the switching of the current through the semiconductor light sources, and a first diode arranged between the first switch and the output, or between the converter circuit and the first switch. By the serial connection of the first switch and the diode, a four-quadrant switch is obtained which, advantageously, can effectively reduce glow currents flowing in the semiconductor light source string. As the diode  15  shows small stray capacitances, the glow current in the blocking direction of the diode is substantially reduced, and the glow current in the forward direction of the diode is reduced by the first switch. 
         [0014]    In a preferred form of embodiment, the circuit arrangement has a second switch, which is arranged between the converter circuit and the first output terminal, wherein the first switch is arranged between the converter circuit and the second output terminal. The second switch can advantageously further reduce the glow current flowing in the light-emitting diode string. 
         [0015]    In another form of embodiment, the circuit arrangement has a second diode, which is arranged between the converter circuit and the first output terminal, wherein the first switch is arranged between the converter circuit and the second output terminal. The second diode also advantageously reduces the glow current. 
         [0016]    In a specifically preferred form of embodiment of the circuit arrangement, the second switch is a MOSFET and the second diode is the body diode of the MOSFET. This has an advantage, in that the glow current is reduced, and efficiency can simultaneously be improved, as the body diode replaces the diode which would otherwise be present in this location and, when the transistor is switched-in, power losses in the diode are obviated accordingly. 
         [0017]    In a particularly advantageous form of embodiment of the circuit arrangement, a parallel-connected arrangement of a first Y-capacitor and a first resistor is connected between ground potential and one terminal of the first switch. The parallel-connected arrangement of the first Y-capacitor and the first resistor raises the potential of the terminal of the first MOSFET switch to a higher level, such that the stray capacitance thereof is reduced, thereby resulting in an advantageous reduction of the glow current. 
         [0018]    In another form of embodiment of the circuit arrangement, advantageously, a series-connected arrangement of a varistor and a voltage-dependent switching element is connected in parallel with the first switch. This results in a further reduction in the glow current, in comparison with the form of embodiment of a parallel varistor which is known from the prior art, on the grounds that, by means of the voltage-dependent switching element, the somewhat low impedance of the varistor does not come into effect, and the glow current is strongly suppressed by the varistor. 
         [0019]    In a particularly advantageous form of embodiment of the circuit arrangement, a parallel-connected arrangement of a second Y-capacitor and a second resistor is connected between ground potential and one terminal of the second switch. The parallel-connected arrangement of the second Y-capacitor and the second resistor raises the potential of the terminal of the second MOSFET switch to a higher level, such that the stray capacitance thereof is reduced, thereby resulting in an advantageous reduction of the glow current. 
         [0020]    In a further form of embodiment of the circuit arrangement, a series-connected arrangement of a second varistor and a second voltage-dependent switching element is connected in parallel with the second switch. This results in a further reduction in the glow current, in comparison with the form of embodiment of a parallel varistor which is known from the prior art, on the grounds that, by means of the voltage-dependent switching element, the somewhat low impedance of the varistor does not come into effect, and the glow current is thus strongly suppressed by the varistor. 
         [0021]    In another form of embodiment of the circuit arrangement, the voltage-dependent switching element is a SIDAC. SIDACs are rather cost-effective components, which are highly suitable for application in this context. 
         [0022]    In another form of embodiment of the circuit arrangement, the voltage-dependent switching element is a TVS diode. These components are also appropriate for the intended application, wherein they have a higher current and power transmission capacities than SIDACs. 
         [0023]    In another form of embodiment of the circuit arrangement, the voltage-dependent switching element is a spark gap. Spark gaps are exceptionally fast-acting and robust, and are thus highly appropriate for the intended application, but have disadvantages with respect to cost. 
         [0024]    In a particularly preferred form of embodiment of the circuit arrangement, the converter circuit incorporates a half-bridge comprised of two transistors, wherein the upper bridge transistor is controlled by means of a driver circuit and the second switch, according to this embodiment, is controlled by means of the same driver circuit. A further driver circuit can advantageously be omitted accordingly, thereby saving costs. 
         [0025]    In a further form of embodiment of the circuit arrangement, the second switch is controlled by means of the driver circuit, a diode and a sample-and-hold circuit. The sample-and-hold circuit assumes the desired switching device function of the second switch in a particularly advantageous manner, wherein the diode executes the requisite rectification. 
         [0026]    Other advantageous further developments and configurations of the circuit arrangement according to the present disclosure for the operation of semiconductor light sources proceed from further dependent claims, and from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the disclosed embodiments. In the following description, various embodiments described with reference to the following drawings, in which: 
           [0028]      FIG. 1  shows a schematic circuit diagram of one form of embodiment of the circuit arrangement for operating semiconductor light sources, 
           [0029]      FIG. 2  shows a voltage which, notwithstanding a switched-out LED module, is present on the LED string, thus resulting in the glowing of the LEDs  5  in the LED string  55 , 
           [0030]      FIG. 3  shows a circuit arrangement according to the prior art, which reduces the glowing of the LEDs  5 , 
           [0031]      FIG. 4  represents a stray voltage U GP , which induces a glow current I G  in the LEDs  5 , 
           [0032]      FIG. 5  shows the action of a resistor  10  arranged in parallel with the Y-capacitor  11 , resulting in a reduction of the glow current I G , 
           [0033]      FIG. 6  shows a diagram of the stray capacitance Coss of a MOSFET plotted against the drain-source voltage VDS thereof, 
           [0034]      FIG. 7  shows a first form of embodiment of the circuit arrangement according to the present disclosure for reducing the glow of a LED string, 
           [0035]      FIG. 8  shows a second form of embodiment of the circuit arrangement according to the present disclosure for reducing the glow of a LED string, 
           [0036]      FIG. 9  shows a control circuit for a MOSFET in the second form of embodiment of the circuit arrangement according to the present disclosure for reducing the glow of a LED string. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 1  shows a schematic circuit diagram of one form of embodiment of the circuit arrangement  100  for operating semiconductor light sources. The circuit arrangement  100  for operating semiconductor light sources has an input  110  for the inputting of an AC input voltage U E . The circuit arrangement  100  for operating semiconductor light sources is permanently connected to this AC input voltage U E , and is switched-in and switched-out by means of a control input  130 . Via the control input  130 , on a bus ST, in addition to switching commands, dimmer commands, for example, can also be transmitted to the circuit arrangement  100 . The input  110  is connected to a rectifier circuit  140 , which converts the AC input voltage U E  into a DC voltage. The DC voltage is transmitted to a DC voltage converter  150 , which converts the DC voltage into an appropriate direct current I B  for a light-emitting diode string which is connected to the circuit arrangement  100  for operating semiconductor light sources. This direct current I B  is fed via a first switch S 1  and a first diode  15  to the output  120  of the circuit arrangement  100  for operating semiconductor light sources. The light-emitting diode string  55  is connected between the first output terminal  122  and the second output terminal  124  of the output  120  of the circuit arrangement  100  for operating semiconductor light sources. The first diode  15  can thus be connected in series between the first switch S 1  and the output  120 , or between the DC voltage converter  150  and the first switch S 1 . However, the diode can also be arranged directly on the module of the light-emitting diode string  55 . Upon installation in a light fitting, the diode would then be arranged in said light fitting. The diode is advantageously connected in series between the first switch S 1  and the output  120 . Due to the fact that the circuit arrangement  100  for operating semiconductor light sources is permanently connected to the AC input voltage U E , the light-emitting diodes  5  can commence to glow, even though the circuit arrangement  100 , and thus also the DC voltage converter  150 , is switched-out by the control signal ST via the control input  130 . 
         [0038]      FIG. 4  shows the representation of a stray voltage U GP  plotted against time, which induces a glow current I G  in the LEDs  5 . By the application of the aforementioned known measures, notwithstanding the high stray voltage U GP , the glow current I G  is very small, but is nevertheless perceptible, particularly in a dark environment. The two current peaks of the glow current I G  can clearly be seen on the edge slopes of the stray voltage U GP . These are associated with two effects:
       1. A high glow current is generated by a large voltage variation in the stray voltage U GP , thereby reducing the impedance of the circuit considered, and thus increasing the current flow in the LEDs.   2. A high stray capacitance is present across the drain-source gate of the MOSFET S 1 , in the event of low voltages across this gate, as can be seen in  FIG. 6 . This high stray capacitance constitutes a not insignificant impedance, via which a glow current I G  can flow, thereby increasing the glow current which is already flowing in the varistor  13 .       
 
         [0041]    In one form of embodiment, a resistor  10  is arranged in parallel with the Y-capacitor  11 , in order to increase the voltage across the drain-source gate of the MOSFET S 1 . 
         [0042]      FIG. 5  illustrates the action of the resistor  10 , connected in parallel with the Y-capacitor  11 , which results in a reduction of the glow current I G . An increase in the voltage on the drain-source gate of the MOSFET S 1  from 0V to approximately 10V reduces the stray capacitance thereof from 5 nF to approximately 1.5 nF. The voltage ULP in  FIG. 5  is the voltage on the LED− terminal. In the course of the time characteristic, this voltage is raised by the resistor  10 . In the lower half of  FIG. 5 , the glow current I G  is illustrated. A drop in the glow current is clearly perceptible, from approximately 19 μA to approximately 13 μA. 
         [0043]      FIG. 6  shows a diagram of the stray capacitance COSS of a MOSFET, plotted against the drain-source voltage VDS of the MOSFET. It can clearly be seen that the capacitance of the drain-source gate becomes smaller, the greater the voltage across said gate. This results in the aforementioned drop in the glow current I G , as the impedance also becomes greater as the capacitance reduces. Repeated in other terms, as a result of the resistor arranged in parallel with the Y-capacitor, the voltage across the drain-source gate of the MOSFET S 1  rises, and the stray capacitance reduces accordingly. In consequence, the impedance of this drain-source gate rises, and the glow current associated with the latter reduces correspondingly. 
         [0044]      FIG. 7  shows a first form of embodiment of the circuit arrangement according to the present disclosure for the reduction of the glow of a LED string. This first form of embodiment has a second diode  1 , which is already known from the prior art, arranged between the LED+ terminal and the first output terminal  122 . In the first form of embodiment, the two problems described above have been addressed, in order to further reduce the glow current, in comparison with the known circuit arrangement from the prior art. According to the present disclosure, a first diode  15  is connected in series between the second output terminal ( 124 ) and the switch S 1 . By this measure, the glow current flowing from the switch S 1  in the direction of the LED terminal  124  is virtually suppressed. Consequently, a glowing of the LEDs  5  is no longer visible. 
         [0045]    As the first diode  15  also has a stray capacitance, a voltage across the other components described cannot be entirely ruled out either. 
         [0046]    Consequently, as a further measure, the aforementioned resistor  10  is connected in parallel with the Y-capacitor  11 . The Y-capacitor  11  is connected between ground potential and the connection point of the cathode of the diode  15  and the source terminal of the MOSFET S 1 . However, the Y-capacitor can also be connected between ground and the anode of the diode  15 . The resistor  10  results in the aforementioned voltage increase across the drain-source gate of the MOSFET S 1 , with a consequent reduction in the stray capacitance, thereby resulting in an increase in impedance. 
         [0047]    As a further measure, in the first form of embodiment, a SIDAC is connected in series with the varistor  13 , which is intended to reduce the current flowing in the varistor, as a result of the relatively low resistance of the varistor  13 . A SIDAC is a voltage-dependent switch, which is not conductive below a certain voltage threshold, such that no significant current can flow in the circuit thereof. In place of a SIDAC, another voltage-dependent switch, such as a TVS diode or a spark gap, can also be arranged. By this measure, the protective action in response to surge pulses is also improved, as the voltage-dependent switch is also capable of absorbing the energy of such a surge pulse. It is only important that the voltage-dependent switch, below its threshold voltage, should show the maximum possible impedance. 
         [0048]      FIG. 8  shows a second form of embodiment of the circuit arrangement according to the present disclosure for the reduction of the glow of a LED string. The second form of embodiment is similar to the first form of embodiment, in consequence whereof only the differences from the first form of embodiment will be described. 
         [0049]    As a result of the additional components for the reduction of the glow current flowing in the LEDs, additional losses occur in the circuit arrangement according to the present disclosure for the reduction of glow. These losses can be reduced by a second switch S 2 , also configured in the form of a MOSFET. The second switch S 2  is thus connected in parallel with the second diode  1 . However, this measure results in a significant increase in the glow current. With the converter switched-out, the second switch S 2  in the form of a MOSFET assumes a blocking state, thereby reducing the flux of a glow current I G . The MOSFET S 2  is connected between the DC voltage converter  150  and the light-emitting diode string  55 , such that the drain terminal of the MOSFET S 2  is coupled to the light-emitting diode string  55 , and the source terminal of the MOSFET S 2  is coupled to the DC voltage converter  150 . Thus, the body diode of the MOSFET S 2 , which is still present, becomes the second diode  1 . In service, the MOSFET S 2  is operated inversely, as the light-emitting diode current I B  flows from the DC voltage converter  150  to the light-emitting diode string  55 . The MOSFET, in comparison with the known second diode  1 , also improves the efficiency of the circuit arrangement, on the grounds that, at high currents, it generates significantly lower losses than the bipolar diode previously employed in this location. 
         [0050]    Here again, analogously to the MOSFET S 1 , a series-connected arrangement of a varistor  17  and a SIDAC  16  is connected in parallel with the drain-source gate, which protects the MOSFET S 2 , but which simultaneously permits no high stray current. 
         [0051]    In order to reduce the glow current associated with the stray capacitance of the MOSFET S 2 , the drain potential, as in the case of the MOSFET S 1  is likewise increased. To this end, between ground and the drain potential of the MOSFET S 2 , a resistor  18  is incorporated, which increases the voltage across the drain-source gate of the MOSFET S 2 . In parallel with the resistor  18 , a Y-capacitor  19  is again arranged, which reduces the voltage rise on the LED+ terminal  122 , in relation to the ground potential, thereby also reducing the glow current. 
         [0052]    In this form of embodiment, a problem arises, in that the MOSFET S 2  cannot be controlled in a simple manner, on the grounds that it is configured in an “overhead” arrangement, and the requisite potential can consequently not be generated by simple means. Consequently, a control circuit is employed in this form of embodiment, which eliminates this problem. 
         [0053]      FIG. 9  shows the complete power circuit of the second form of embodiment of the circuit arrangement according to the present disclosure. The relevant functional modules of the power circuit are briefly described hereinafter. 
         [0054]    The circuit arrangement is supplied with an AC mains voltage via the input terminals P 1 -A and P 1 -B. These constitute the power input  110 . The function of the fuse F 101  is the protection of the circuit arrangement against unacceptable states. The components L- 100 -A and L- 100 -B, together with the capacitor C 100 , constitute an input filter  115 , which serves for the conditioning of the AC voltage signal. The conditioned AC voltage is fed to a bridge rectifier  140  comprised of the diodes D 106  to D 109 . 
         [0055]    The rectified AC voltage is present on a power factor correction circuit  160  comprised of the components L 101 , Q 100 , D 105  and an intermediate circuit back-up capacitor C 110 . The resistor R 108  constitutes a shunt for the current measurement of the converter current on the power factor correction circuit  160 . The transistor Q 100  is controlled by means of a control circuit  162 , which measures the current flowing in the resistor R 108  as a parameter. The control circuit  162  controls the switch Q 100 , such that compliance with applicable standards for the power factor of the circuit arrangement is maintained. The power factor correction circuit  160  delivers an intermediate circuit voltage U ZKS . The intermediate circuit voltage U ZKS  is fed to a step-down half-bridge  170 , which steps down the intermediate circuit voltage U ZKS  and delivers a current I B  for the light-emitting diode string  55 . The step-down half-bridge  170  includes two half-bridge switches Q 200  and Q 201 , which are configured as MOSFETs. 
         [0056]    The source terminal of the lower MOSFET Q 201  is connected to ground. A current measuring shunt R 203  is connected to ground at one end. The other end of the resistor R 203  forms the first output LED− of the step-down half-bridge  170 . 
         [0057]    The two MOSFETs Q 200  and Q 201  are connected in series, and constitute a half-bridge mid-point M, which is connected to a filter choke L 201 . The other end of this filter choke L 201  constitutes the second output LED+ of the step-down half-bridge  170 . Between the first output LED- and the second output LED+, a capacitor C 205  is connected. The power factor correction circuit  160  and the step-down half-bridge  170 , in combination, constitute the converter circuit  150 . 
         [0058]    Between the first output LED- and the output terminal  124 , which is coupled to the light-emitting diode string  55 , the first switch S 1  is arranged, which is likewise configured as MOSFET. The first switch is controlled by a control circuit, which switches the MOSFET S 1  via a bipolar transistor Q 401 . To this end, an enable signal, supported by an auxiliary voltage signal VCCO is employed, which is generated by an auxiliary voltage supply which is not represented here. The resistors R 401  and R 402  constitute a voltage divider, which supplies the gate of the MOSFET S 1  with the requisite switching voltage. The bipolar transistor Q 401  is connected in parallel with this voltage divider, and can short-circuit the voltage divider, such that the MOSFET S 1  is switched-out. The function of the resistor R 403  is the decoupling of the auxiliary voltage supply VCCO. As the bipolar transistor Q 401 , with its emitter, is connected to the LED conductor, it can easily be switched, via its base, by means of the enable signal with a customary control level. The function of the resistor R 404  is the decoupling of this control level. A diode  15  is arranged between the first switch S 1  and the output terminal  124 . The enable signal is controlled by the control input  130  and, according to the dictates of the control signal ST (e.g. light-emitting diodes on/off), is switched accordingly. 
         [0059]    The diode  15  is connected such that its cathode is directed towards the cathode of the body diode of the MOSFET switch S 1 . The diode  15  is thus connected in an “antiserial” arrangement to the body diode of the MOSFET switch S 1 . This measure ensures a strong reduction in the glow current, as the resulting interconnection of S 1  and the diode  15  constitutes a four-quadrant switch. At the coupling point of the cathode of the diode  15  with the drain terminal of the MOSFET switch S 1 , a parallel-connected arrangement of a resistor  10  and a Y-capacitor  11  is connected. The other end of this parallel-connected arrangement is connected to ground. However, the parallel-connected arrangement can also be connected between the anode of the diode  15  and ground. The resistor  10 , as in the first form of embodiment, effects a rise in the potential of the drain-source gate of the MOSFET switch S 1 , such that the residual glow current of the circuit arrangement is further reduced as a result. 
         [0060]    Between the second output LED+ and the output terminal  122 , which is connected to the light-emitting diode string  55 , the second switch S 2  is arranged, which is also configured as a MOSFET. The function of the second switch is the bridging of the second diode  1 . Given that, particularly in the event of higher currents I B  flowing in the light-emitting diode string  55 , an increased power loss occurs on the diode  1 , the latter is bridged by means of the second switch S 2 , in order to reduce this power loss. As already described, the MOSFET S 2  is connected such that its source terminal is coupled to the LED+ terminal, and its drain terminal is coupled to the first output terminal  122 . Between the drain terminal and ground, a parallel-connected arrangement of a Y-capacitor  19  and a resistor  18  is connected. Here again, the resistor generates a rise in the potential of the source terminal of the MOSFET S 2 , in order to reduce the stray capacitance thereof. On the grounds of the connection thereof, the MOSFET S 2  is operated inversely. As the MOSFET S 2  is coupled to half-bridge mid-point, it can no longer be controlled by means of the customary ground-related low voltage level. The second form of embodiment of the circuit arrangement according to the present disclosure, for the control of the MOSFET S 2 , employs the circuit procedure described hereinafter. 
         [0061]    The step-down half-bridge  170 , for the control of the upper transistor Q 200 , requires a “high-side driver”, i.e. an auxiliary circuit which can actuate the upper transistor with the requisite potential for the switching thereof. As the upper MOSFET Q 200  carries the intermediate circuit voltage U ZKS , the control potential thereof must lie above this voltage. This auxiliary circuit is also employed in a simple and cost-effective manner for the control of the switch S 2 . The two half-bridge transistors Q 200  and Q 201  are controlled by an integrated circuit U 200 , via the resistors R 200  and R 201 . The high-side driver is integrated in this integrated circuit U 200 . The signal for the upper transistor Q 200  is delivered on the output HO of the integrated circuit U 200 . The signal for the lower transistor is delivered on the output LO of the integrated circuit U 200 . The half-bridge mid-point M is connected to the terminal VS of the integrated circuit U 200 . The integrated circuit U 200  is likewise supplied, by means of the auxiliary voltage supply which is not represented here, with the voltage VCCO. The components D 201  and C 203  constitute the external circuit elements of the high-side driver, in order to deliver the corresponding potential for the upper transistor Q 200 . The high-side driver thus includes the components U 200 , D 201  and C 203 . The components D 201  and C 203  are connected in series, and are arranged between the voltage VCCO and the half-bridge mid-point M. The node point between the cathode of the diode D 201  and the capacitor C 203  is coupled to the terminal VB of the integrated circuit U 200 . 
         [0062]    The output HO of the integrated circuit U 200 , according to the second form of embodiment, is coupled to a series-connected arrangement of a resistor R 405  and a diode D 402 . The anode of the diode D 402  is thus coupled to the resistor R 405 . The cathode of the diode D 402  is coupled to a sample-and-hold circuit, comprised of the components C 401 , D 401  and R 409 . “Sample-and-hold circuit” is the English term for “Abtast-Halte-Schaltung”. This circuit holds the voltage level of the rectified AC voltage of the high-side driver at a switching voltage which is sufficient for the MOSFET S 2 . The gate of the MOSFET S 2  is thus likewise connected to the cathode of the diode D 402  and the sample-and-hold circuit. 
         [0063]    By means of the diode D 402 , the AC voltage signal present on the output HO is rectified, and is applied to the sample-and-hold circuit. In the course of a plurality of full cycles on the step-down half-bridge, the capacitor C 401  is thus charged to a voltage, which is limited by the Zener diode D 401 . This voltage is now applied to the gate of the MOSFET S 2 , in order to switch-in the latter, provided that the half-bridge comprised of the MOSFETs Q 200  and Q 201  is in service. If the step-down half-bridge is switched-out, the capacitor C 401  is discharged via the resistor R 409 , and the MOSFET S 2  is switched-out. It should be observed that the transistor will only be switched-in after a number of operating cycles of the half-bridge. However, this does not constitute a disadvantage, on the grounds that, during these cycles, the body diode  1  is active, and carries the current flowing in the light-emitting diode string  55 . Although this is associated with an increased power loss, this only applies over a few cycles of the step-down half-bridge, and thus does not constitute a problem in practice. Depending upon the rating of the resistor R 409 , the MOSFET S 2  remains switched-in for some time after the switch-out of the step-down half-bridge, until the capacitor C 401  is discharged below the threshold voltage of the MOSFET S 2 . Again, in practice, only a very short time interval is involved, such that this does not pose any problem. By this arrangement, the transistor S 2  can be switched by simple and cost-effective means, without the requirement for a further and complex high-side driver. 
         [0064]    While the disclosed embodiments have been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the disclosed embodiments as defined by the appended claims. The scope of the disclosed embodiments is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           1  second diode 
           3  body diode 
           5  light-emitting diode 
           7  protective diode 
           9  stray capacitance 
           10  resistor 
           11  Y-capacitor 
           12  SIDAC 
           13  varistor for the protection of the MOSFET S 1   
           15  first diode 
           55  light-emitting diode string 
           100  circuit arrangement for operating semiconductor light 
         sources 
           110  power input for inputting an AC input voltage 
           115  input filter 
           120  output 
           122  first output terminal 
           124  second output terminal 
           130  control input 
           140  rectifier circuit 
           150  converter circuit 
           160  power factor correction circuit 
           162  control circuit of power factor correction circuit 
           170  step-down half-bridge 
         S 1  first switch, configured as a MOSFET 
         S 2  second switch, configured as a MOSFET 
         PE ground 
         LED+ positive LED conductor to first output terminal 
         LED− negative LED conductor to second output terminal 
         C 110  intermediate circuit back-up capacitor