Active circuit for detecting a light source current and method for detecting a parameter representing the current through a light source

Abstract

Driver circuit for operating a light source, preferably at least one LED, comprising a galvanically isolated converter switched on the primary side by a control unit (SE', SE'') by means of at least one controlled switch unit (S), the switch unit (S) with a first inverter switch (Q1) and a second inverter switch (Q2), which feeds a rectifier (D1, D2) from which the light source can be supplied, wherein a detection circuit (E', E'') for indirect detection of the current on the secondary side of the converter comprises a transformer (L3_1, L3_3) with at least one primary-side winding (L3_1), wherein the detection circuit (E', E'') includes an active detection rectifier (AG, AG'), wherein the control unit (SE', SE'') activates / deactivates a first diagonal of the active rectifier (AG, AG') synchronously with the first inverter switch (Q1), and wherein the control unit (SE', SE'') activates / deactivates a second diagonal of the active rectifier (AG', AG'') synchronously with the second inverter switch (Q2), and wherein only the first or the second diagonal is active at any given time, wherein the secondary winding (L3_3) of the transformer (L3_1, L3_3) is part of each of the diagonals of the active sensing rectifier (AG, AG'), where, by switching the tap of the secondary winding (L3_3), the fault-inducing "reverse current" is measured as a negative current, so that the lamp current is controlled by the control unit (SE'') by a corresponding control of the switch unit (S) on the primary side of the circuit, wherein the parameter representing the current through the light source is detected by means of an integration circuit (I), wherein a feedback signal is detected at the midpoint of a voltage divider formed from two resistors (R1, R2) by the control unit (SE''), where the voltage divider (R1, R2) is connected in parallel with a capacitor (C3), and wherein the rectified output signal from the active detection rectifier (AG') is supplied to the parallel circuit at the higher potential terminal of the capacitor (C3).

Description

[0001] The invention relates to an active circuit, in particular a driver circuit, for detecting a current through a lighting circuit with at least one LED. In particular, the invention relates to an active circuit that is divided by a potential-isolating and / or galvanically insulating barrier (for example, a safety extra-low voltage barrier (SELV barrier)) into a primary side supplied by a supply voltage and a secondary side supplying the lighting circuit.

[0002] It is known from the prior art to detect the current through the light source on the secondary side of the current transformer and return it to the primary side via a galvanic barrier. CN102636676A discloses a bridge circuit for current detection in which a current transformer is used in conjunction with controllable switches and a measuring resistor to detect the current in a full bridge circuit. This solution requires additional switching elements and transmission components, making the circuit complex and expensive.

[0003] Furthermore, EP2493063A1 describes a power converter with an integrated current transformer, which has multiple uses: firstly, for controlling synchronous rectifiers depending on the measured winding current, and secondly, for providing an auxiliary power supply in overload situations. The control is adaptive with respect to the winding current in order to optimize switching times and reduce losses. However, here too, an additional circuit on the secondary side is required.

[0004] EP2770623A1 discloses a resonant converter for LED drivers in which feedback of output information to the primary side is achieved via a separate signaling transformer. While this solution replaces the optocoupler, it requires additional windings and components on the secondary side, thus increasing the circuit complexity.

[0005] US5574338A relates to a control system for gas discharge lamps with a transformer that has separate starting and operating windings. The switching between the high-frequency starting signal and the low-frequency operating signal takes place on the primary side; however, current sensing is provided on the secondary side, which is fed back via a galvanic barrier.

[0006] US 2011 / 0037414 A1 describes an LED driver circuit with a resonant converter, in which the output signal is detected via an additional transformer or auxiliary winding and transferred to the primary side. A secondary-side detection circuit is also required here.

[0007] WO 2013 / 092662 A1 discloses driver circuits for LEDs in which the LED current is detected on the secondary side and fed back to the primary side via galvanic isolation. This requires additional components such as optocouplers and increases the complexity and cost of the circuit.

[0008] Furthermore, the publication by Yan-Fei Liu et al., “A New Current Sensing Scheme for Zero-Voltage Switching Phase-Shifted Bridge Converter” (INTELEC 2000) reveals various current sensing concepts for resonant converters, which, however, require either a complicated resetting of current converter cores or additional amplifier stages.

[0009] These known solutions have the disadvantage that the detection circuit is located on the secondary side and a transmission element to the primary side is required. This leads to increased circuit complexity, additional components, and potential sources of error.

[0010] It is also known that energy transfer occurs in such circuits from the primary to the secondary side. Specifically, this transfer from the primary to the secondary side of the circuit across the barrier is achieved by means of a converter, particularly a transformer, which induces a voltage or current on the secondary side via electromagnetic coupling. Thus, it is possible to supply power to the secondary side across the barrier.

[0011] To determine the current through the light source, or the section of the light source that can be used to control the light source brightness, it is known to measure the current on the secondary side. This is shown, for example, in WO 2014 / 060899 A2. It is also known that, as exemplified in the Fig. Figure 1 shows a control circuit SE that directly or indirectly controls a switching unit S with a clocked switch, e.g. via a driver unit.

[0012] The switching unit S can, for example, comprise a single switched-mode switch or multiple switched-mode switches. In particular, the switching unit S can include an inverter with a high-side switch and a low-side switch.

[0013] Starting from the switch unit S, a converter winding L2_1 (inductor, coil) is supplied. This converter winding L2_1 can be part of an LLC circuit and, for example, be fed from a midpoint of the inverter. In particular, a converter (e.g., resonant converter, LLC converter) is supplied, from which the secondary side of the circuit is fed via electromagnetic coupling of the primary-side converter winding L2_1 and the secondary-side converter windings L2_2, L2_3 (here shown as separate windings L2_2, L2_3) across a barrier B.

[0014] Via the diodes D1 and D2, which form a rectifier, a DC voltage smoothed by a smoothing capacitor C2 is supplied to the LED terminals LED+ and LED- for the operation of the light source.

[0015] An auxiliary winding Lh is provided with a sensing circuit E on the secondary side of the circuit, wherein the auxiliary winding Lh is electromagnetically coupled to the primary winding L2_1. The sensing circuit E then uses the auxiliary winding Lh to detect a current induced on the secondary side of the circuit, or a parameter representing this current. The corresponding parameter representing the current is determined by the sensing circuit E and fed back to the control unit SE via a bridging element X, which can be, for example, an optocoupler or another transformer, across the barrier B. The control unit SE can then evaluate the parameter as the actual value for the current through the light source and use it to control the light source. In particular, the control unit SE can compare the actual value for the current with a setpoint and control the switching unit S accordingly.

[0016] The in Fig. However, the circuit shown in Figure 1 has the disadvantage that, on the one hand, the detection circuit E must be provided on the secondary side of the circuit, and furthermore, a bridging element X is required to bridge the barrier B, which leads to relatively high circuit costs and complicated circuit arrangements.

[0017] Therefore, an objective of the invention is to provide a circuit that enables primary-side detection of a parameter representing the current through the light source. In particular, the object of the invention is to provide a means of detecting a parameter representing the current through the light source on the primary side of the circuit while avoiding detection errors.

[0018] A solution to this problem is described below and is achieved by the device according to claim 1 and the method according to claim 10. Further developments of the invention are the subject of the dependent claims.

[0019] In a first aspect of the invention, a circuit, in particular a driver circuit for operating a light source, preferably at least one LED, is provided, comprising a galvanically isolated converter, the primary side of which is clocked by a control unit via at least one controlled switching unit, and which feeds a rectifier from which the light source can be supplied, wherein the switching unit is configured with a first inverter switch and a second inverter switch. A sensing circuit for indirectly sensing the current on the secondary side of the converter comprises a transformer with at least one primary-side winding.

[0020] The detection circuit features an active detection rectifier, where the control unit activates / deactivates a first diagonal of the active rectifier synchronously with the first inverter switch, and where the control unit activates / deactivates a second diagonal of the active rectifier synchronously with the second inverter switch, with only the first or the second diagonal being active at any given time. The secondary winding of the transformer forms part of each diagonal of the active detection rectifier, and by switching the tap of the secondary winding, the fault-inducing "reverse current" is measured as a negative current, so that the lamp current is controlled by the control unit by appropriately activating the switch unit on the primary side of the circuit.The parameter representing the current through the light source is detected using an integration circuit. A feedback signal is detected at the midpoint of a voltage divider formed by two resistors by the control unit. The voltage divider is connected in parallel with a capacitor. The rectified output signal from the active detection rectifier is fed to the parallel circuit at the higher potential terminal of the capacitor.

[0021] A parameter representing the current through the light source can be fed back to the control unit from an output of the detection circuit. In particular, the output can be connected directly or indirectly to a measurement input of the control unit.

[0022] The switching unit can be a half-bridge or full-bridge inverter.

[0023] The at least one primary-side winding can in particular be part of a resonant converter, an LLC circuit, a flyback converter, a push-pull forward converter, a boost converter or a buck converter.

[0024] The control unit can control the rectifier switches of the active detection rectifier of the detection circuit depending on the activation of at least one switch.

[0025] The switching unit can include at least one transistor, in particular a FET or MOSFET. The other switches in the circuit can also be designed as transistors (FET, MOSFET, etc.).

[0026] The first and second diagonals can each have at least one, preferably two, rectifier switches. Preferably, each rectifier switch can be connected to a terminal of the secondary winding of the transformer.

[0027] Each diagonal can contain a rectifier switch and a diode.

[0028] The control unit can only activate one diagonal of the active detection rectifier after a certain time, i.e., after a dead time following the deactivation of the other diagonals of the active detection rectifier.

[0029] In a further aspect, the invention provides a method for detecting a parameter representing the current through a light source, preferably at least one LED, wherein a galvanically isolated, pulsed converter, controlled on its primary side by a control unit via at least one switching unit, feeds a rectifier from which the light source is supplied. The switching unit is configured with a first inverter switch and a second inverter switch. A detection circuit can indirectly detect a current on the secondary side of the converter at a transformer having at least one primary-side winding on the secondary side of the converter.The detection circuit (E', E'') includes an active detection rectifier (AG, AG'), wherein the control unit (SE', SE'') activates / deactivates a first diagonal of the active rectifier (AG, AG') synchronously with the first inverter switch (Q1), and wherein the control unit (SE', SE'') activates / deactivates a second diagonal of the active rectifier (AG', AG'') synchronously with the second inverter switch (Q2), and wherein only the first or the second diagonal is active at any given time. The secondary winding (L3_3) of the transformer (L3_1, L3_3) is part of each of the diagonals of the active detection rectifier (AG, AG'), whereby by switching the tap of the secondary winding (L3_3) the fault-inducing "reverse current" is measured as a negative current, so that the lamp current is controlled by the control unit (SE'') by a corresponding control of the switch unit (S) on the primary side of the circuit.The parameter representing the current through the light source is detected by means of an integration circuit (I), whereby a feedback signal is detected at the midpoint of a voltage divider formed by two resistors (R1, R2) by the control unit (SE''). The voltage divider (R1, R2) is connected in parallel with a capacitor (C3), and the rectified output signal from the active detection rectifier (AG') is supplied to the parallel circuit at the higher potential terminal of the capacitor (C3).

[0030] The invention is also described with reference to the figures. These show: Fig. 1 schematically a known embodiment according to the prior art. Fig. 2 schematically a first embodiment according to the invention. Fig. 3. An exemplary, more detailed circuit arrangement according to the first embodiment of the invention. Fig. 4 exemplary curves of recorded parameters. Fig. 5 schematically a second embodiment according to the invention. Fig. 6. An exemplary, more detailed circuit arrangement according to the second embodiment of the invention. Fig. 7 states of circuit components under specific control conditions.

[0031] Fig. Figure 2 now shows a first embodiment of the inventive circuit, in which the same reference numerals designate essentially the same circuit parts as in Fig. 1. As from Fig. As can be seen in Figure 2, a diode is connected in series with at least one secondary-side converter winding L2_2, L2_3 (two converter windings can also be provided, as shown). This diode is in turn connected in series with a primary winding L3_1 of a transformer. The primary winding L3_1 of the transformer is located in the supply path of the LED+ light source connection.

[0032] The primary winding L3_1 of the transformer can be electromagnetically coupled to a secondary winding L3_3 of the transformer.

[0033] In Fig. Figure 2 shows, in a dashed frame, an exemplary additional primary-side winding L3_2 of the transformer, connected in series with a diode D2, which may be optionally provided.

[0034] The secondary winding L3_3 of the transformer is connected to a detection circuit E' which includes a passive detection rectifier PG. This rectifies the voltage transmitted, in particular alternately, through the primary windings L3_1, L3_2 of the transformer to a DC voltage.

[0035] At one output of the passive sensing rectifier PG, the parameter representing the current through the light source can be measured, e.g., via a current-sensing resistor. This parameter is then fed back to a control unit SE', which preferably evaluates it as the actual value for the current through the light source (not shown) and performs corresponding control. For connecting the light source, which preferably has at least one LED, the following connections are required, as shown in Fig. 1. The connections (terminals) LED- and LED+ are provided.

[0036] An exemplary embodiment of Fig. 2 is in Fig. 3 shown. Here too, the same reference symbols denote the same circuit parts as in Fig. 1 and Fig. 2.

[0037] In particular, in Fig. 3. A control unit SE' is provided, which controls an inverter with a first inverter switch Q1 and a second inverter switch Q2. The first inverter switch Q1 is a "high-side switch", i.e., the switch "above" the load or the switch with a higher potential, and is controlled via a driver T, while the second inverter switch Q2 is a "low-side switch" (switch "below" the load or switch with a lower potential).

[0038] Starting from a midpoint between the first and second inverter switches Q1, Q2 of the illustrated inverter half-bridge, a resonant converter (here LLC converter) with a capacitance C1, an inductance L1 and a primary converter winding L2_1 is fed.

[0039] The secondary side of the circuit is essentially identical to that of the one from Fig. 2. Here too, one of the primary windings L3_1, L3_2 of the transformer can be optionally provided.

[0040] Furthermore, the passive detection rectifier PG' is designed as a diode full bridge on the primary side of the circuit as part of the detection circuit E', and the control unit SE' detects a parameter representing the current through the light source at its output via a measuring resistor R.

[0041] With the orders from the Fig. 2 and Fig. 3. The parameter representing the current through the light source can now be measured on the primary side of the circuit. Since both half-waves supplied to the light source are transformed, corresponding current values ​​for both half-waves can also be measured.

[0042] The problem here, however, is that the diodes D1, D2, which are typically used on the secondary side of the circuit as rectifiers, exhibit a "reverse current" at high temperatures, i.e., that there is no immediate switch-off through the diodes D1, D2, but a current flows through the diodes D1, D2 of the rectifier in the reverse direction of the diodes D1, D2.

[0043] Due to the transformer coupling and rectification by the passive detection rectifier PG', there is a potential for error which, especially at high temperatures, strongly disturbs the signal detected by the control unit SE'.

[0044] This is demonstrated by the examples in the Fig. 4a and Fig. The curves shown in 4b are clearly visible, with the Fig. Figure 4a illustrates a curve progression at high temperatures. In the diagram, the black curve, initially descending from left to right, represents the current through diode D1, while the gray curve, also descending from left to right, represents the signal detected at the measuring resistor R.

[0045] However, the passive sensing rectifier PG' is not able to correctly output the negative "reverse current" of the diode, which leads to a shift in the measured signal that is detected at the measuring resistor R.

[0046] In Fig. 4b shows corresponding curves for lower temperatures. Fig. As shown in Figure 4b, the lower the temperature, the less influence the reverse current flowing in the reverse direction at diode D1 has. Therefore, the detection error is reduced at low temperatures. The detected and erroneously measured positive reverse current is much smaller at these low temperatures. The corresponding range is shown in Fig. 4b marked.

[0047] It can be understood that a corresponding signal waveform can also result for diode D2, so that a corresponding error addition results for each half-wave induced in the secondary transformer winding L3_3 by the at least one primary transformer winding L3_1, L3_2.

[0048] Furthermore, it is to be understood that in addition to an LLC converter, as used in the Fig. 1 and Fig. As shown in Figure 2, other converters, such as flyback converters, buck or boost converters, and in particular other topologies of other clocked, potentially isolated converters, can also be used. Overall, the invention relates to the detection of the LED current, whereby this current is determined starting from an AC voltage that is rectified by means of diodes D1 and D2. The LED current is thus detected indirectly via the transformer, which has two primary windings L3_1 and L3_2, each of which is connected in series with one of the diodes D1 and D2.

[0049] The in Fig. The circuit shown in Figure 5 now allows for the detection of an LED current on the primary side of the circuit while simultaneously avoiding the detection errors caused by the "reverse current".

[0050] Compared to Fig. 2 or Fig. 3 the passive detection rectifier PG, PG' is replaced by an active detection rectifier AG in a detection circuit E'', in which the secondary winding of the transformer L3_3 is connected with at least two rectifier switches, but in particular four rectifier switches.

[0051] The rectifier switches are controlled in such a way that one diagonal of the rectifier is active, i.e., switched to conduction, so that ultimately a tap on the secondary winding is alternately performed. Preferably, the control unit SE'' controls the rectifier switches, which also controls the switch unit S.

[0052] Also in Fig. Figure 5 denotes essentially the same elements as in the preceding figures.

[0053] In Fig. Figure 6 is an exemplary embodiment of the design from Fig. 5 shown. This is based on the in Fig. 3 shows an exemplary circuit arrangement according to the first embodiment. Fig. The three circuit components, which have essentially already been shown, are labelled accordingly.

[0054] In Fig. Figure 6 shows in particular the active detection rectifier AG', wherein the secondary winding L3_3 of the transformer is connected to a first rectifier switch Q3 and a second rectifier switch Q6 and forms a first diagonal, while the secondary winding L3_3 of the transformer is also connected to a third rectifier switch Q4 and a fourth rectifier switch Q5 and thus forms a second diagonal.

[0055] As in Fig. Figure 6 shows, in particular, that the first rectifier switch Q3 and the second rectifier switch Q6 of the first diagonal are switched synchronously with the low-side inverter switch Q2, while the third rectifier switch Q4 and the fourth rectifier switch Q5 can be controlled synchronously with the high-side inverter switch Q1. Other control configurations for the rectifier switches are also possible. However, it is essential that the control unit SE'' knows the current polarity on the secondary side of the circuit and controls the rectifier switches Q3, Q4, Q5, and Q6 of the two diagonals accordingly.

[0056] In comparison to the passive rectifiers PG, PG' from the Fig. 2 and Fig. Thus, by switching the tap of the secondary winding L3_3, the fault-inducing "reverse current" is measured as a negative current, so that the control unit SE'' can control the lamp current by appropriately controlling the switch unit S on the primary side of the circuit. The "reverse current" can then be compensated for by such appropriate control.

[0057] The parameter representing the current through the light source can then be detected using an integration circuit I, in particular an RC integration circuit. For example, a feedback signal can be detected at the midpoint of a voltage divider formed by resistors R1 and R2 by the control unit SE''. The voltage divider R1 and R2 is connected in parallel with a capacitor C3. The rectified output signal from the active detection rectifier AG' is fed to the parallel circuit at the higher potential terminal of capacitor C3.

[0058] It should be understood that in both the first and second diagonals, a rectifier switch can be replaced by a diode.

[0059] The Fig. 7a and Fig. Figure 7b now shows measurement circuit diagrams for different states of the active rectifier: If the active detection rectifier AG, AG' is operated by the control circuit SE'' such that only one diagonal is active at a time, positive and negative currents are fed directly to the RC integration circuit I, consisting of capacitor C3 and the voltage divider with resistors R1 and R2 connected in parallel. The polarity of the secondary winding L3_3 is differentiated, which is synchronous with the control of the inverter switches Q1 and Q2.

[0060] If the sensing rectifier AG, AG' is operated close to the resonant frequency of the converter (here LLC), current still flows even during the inverter's dead time, i.e., when neither of the inverter switches Q1 and Q2 is active. This current then flows through the body diodes of the rectifier switches Q3-Q6 and thus also into the RC integration circuit I. However, this only occurs with very high loads, and the resulting error is marginal compared to the error that occurs when using a passive rectifier PG, PG'.

[0061] The invention relates generally to the acquisition of a measurement signal at the primary windings L3_1, L3_2 of a transformer, which are connected in series with diodes D1, D2, wherein the secondary winding L3_3 of the transformer is connected as the diagonal of an active bridge circuit (in particular a full bridge circuit). Preferably, the rectifier switches Q3 - Q6 are controlled by directly using the control signals of the at least one clocked switch S of the converter supplying the secondary side. Thus, no further acquisition input is required at the control unit SE', SE'', and the rectifier switches Q3 - Q6 can be controlled without a separate driver circuit. The primary windings L3_1, L3_2 are therefore connected directly in the power supply paths of the lighting circuit.

[0062] A dead time is provided between activating the two diagonals, during which neither diagonal is active, i.e., switched on. As in Fig. As shown in Figure 7b, during this interim period the so-called body diodes of the rectifier switches take over the rectifier functions, so that during this period there is no active, but a passive detection rectifier in the circuit.

[0063] It should be understood that the described switches can be implemented as transistors (e.g., FET, MOSFET, ...). The control unit SE, SE'' can be implemented as a microcontroller, IC, or ASIC, for example.

Claims

[1] Driver circuit for operating a light source, preferably at least one LED, comprising a galvanically isolated converter switched on the primary side by a control unit (SE', SE'') by means of at least one controlled switch unit (S), the switch unit (S) with a first inverter switch (Q1) and a second inverter switch (Q2), which feeds a rectifier (D1, D2) from which the light source can be supplied, wherein a detection circuit (E', E'') for indirect detection of the current on the secondary side of the converter comprises a transformer (L3_1, L3_3) with at least one primary-side winding (L3_1), wherein the detection circuit (E', E'') includes an active detection rectifier (AG, AG'), wherein the control unit (SE', SE'') activates / deactivates a first diagonal of the active rectifier (AG, AG') synchronously with the first inverter switch (Q1), and wherein the control unit (SE', SE'') activates / deactivates a second diagonal of the active rectifier (AG', AG'') synchronously with the second inverter switch (Q2), and wherein only the first or the second diagonal is active at any given time, wherein the secondary winding (L3_3) of the transformer (L3_1, L3_3) is part of each of the diagonals of the active sensing rectifier (AG, AG'), where, by switching the tap of the secondary winding (L3_3), the fault-inducing "reverse current" is measured as a negative current, so that the lamp current is controlled by the control unit (SE'') by a corresponding control of the switch unit (S) on the primary side of the circuit, wherein the parameter representing the current through the light source is detected by means of an integration circuit (I), wherein a feedback signal is detected at the midpoint of a voltage divider formed from two resistors (R1, R2) by the control unit (SE''), where the voltage divider (R1, R2) is connected in parallel with a capacitor (C3), and wherein the rectified output signal from the active detection rectifier (AG') is supplied to the parallel circuit at the higher potential terminal of the capacitor (C3). [2] Driver circuit according to claim 1, wherein a parameter representing the current through the light source is fed back to the control unit (SE', SE'') from an output of the detection circuit (E', E''), and in particular the output is directly or indirectly connected to a measuring input of the control unit (SE', SE''). [3] Driver circuit according to one of the preceding claims, wherein the switching unit (S) is a half-bridge or full-bridge inverter. [4] Driver circuit according to one of the preceding claims, wherein the at least one primary-side winding (L3_1) is part of a resonant converter, an LLC circuit, a flyback converter, a push-pull forward converter, a boost converter or a buck converter. [5] Driver circuit according to one of the preceding claims, wherein the control unit (SE', SE'') controls rectifier switches (Q3-Q6) of the active rectifier (AG) of the detection circuit (E', E'') depending on the control of the at least one switch unit (S). [6] Driver circuit according to one of the preceding claims, wherein the switching unit (S) comprises at least one switch, in particular a transistor, FET or MOSFET. [7] Driver circuit according to one of the preceding claims, wherein the first and the second diagonal each have at least one, preferably two, rectifier switches (Q3, Q4, Q5, Q6), wherein preferably one rectifier switch (Q3, Q4, Q5, Q6) is connected to a terminal of the secondary winding (L3_3) of the transformer. [8] Driver circuit according to one of the preceding claims, wherein each diagonal has a rectifier switch (Q3, Q4, Q5, Q6) and a diode. [9] Driver circuit according to one of the preceding claims, wherein the control unit (SE', SE'') activates one diagonal of the active rectifier only after a certain time, i.e. after a dead time following the deactivation of the other diagonals of the active sensing rectifier (AG, AG'). [10] Method for detecting a parameter representing the current through a light source, preferably at least one LED, wherein a potential-isolated, primary-side controlled switched-mode converter (S) supplied by a control unit (SE', SE'') via at least one switch unit (S), the switch unit (S) with a first inverter switch (Q1) and a second inverter switch (Q2), feeds a rectifier (D1, D2), from which the light source is supplied, wherein a detection circuit (E', E'') indirectly detects a current on the secondary side of the converter at a transformer (L3_1, L3_3) with at least one primary-side winding (L3_1) on the secondary side of the converter, wherein the detection circuit (E', E'') has an active detection rectifier (AG, AG'), wherein the control unit (SE', SE'') activates / deactivates a first diagonal of the active rectifier (AG, AG') synchronously with the first inverter switch (Q1), and wherein the control unit (SE', SE'') activates / deactivates a second diagonal of the active rectifier (AG', AG'') synchronously with the second inverter switch (Q2), and wherein only the first or the second diagonal is active at any given time, wherein the secondary winding (L3_3) of the transformer (L3_1, L3_3) is part of each of the diagonals of the active sensing rectifier (AG, AG'), where, by switching the tap of the secondary winding (L3_3), the fault-inducing "reverse current" is measured as a negative current, so that the lamp current is controlled by the control unit (SE'') by a corresponding control of the switch unit (S) on the primary side of the circuit, wherein the parameter representing the current through the light source is detected by means of an integration circuit (I), wherein a feedback signal is detected at the midpoint of a voltage divider formed from two resistors (R1, R2) by the control unit (SE''), where the voltage divider (R1, R2) is connected in parallel with a capacitor (C3), and wherein the rectified output signal from the active detection rectifier (AG') is supplied to the parallel circuit at the higher potential terminal of the capacitor (C3).

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