Abstract:
The invention relates to an operating circuit for an LED series, having: a converter, particularly a DC-DC converter, having a controllable switch (S 1 ) and an inductor (L BUCK ) for converting an input voltage (Vin) fed to the operating circuit into a supply voltage for the LED series; —a control unit (SR) for driving the switch (S 1 ); —a secondary-side inductor (L 2 ) coupled to the inductor (LBUCK); —an envelope curve demodulator ( 30 ) for detecting the envelope curve of the voltage (V′LED) present at the secondary-side inductor (L 2 ); and—a compensating circuit ( 31 ) for compensating an error caused by the envelope curve demodulator ( 30 ) relating to the detection of the envelope curve.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a circuit for operating light-emitting diodes (LEDs) by means of switching regulators or respectively converters for supplying an operating voltage or respectively an operating current for the LEDs. 
     BACKGROUND OF THE INVENTION 
     The use of a step-down converter, also referred to as a buck-converter, for the control of LEDs is known in principle. As shown in  FIG. 1 , a switch S 1  is closed and opened in alternation, wherein, in its activated condition, a coil L Buck  is energised. In turn, in the deactivated condition of the switch S 1 , the energy accumulated in the coil L Buck  is discharged via the LED system. 
     The switch S 1  is clocked by a control unit (not shown). This control unit monitors the current through the switch S 1  during the activation phase of the switch S 1  via a measuring resistor R SHUNT  connected in series to the switch S 1 . As soon as the voltage which is picked up via the measuring resistor R SHUNT  reaches a given maximal value, the switch S 1  is opened. 
     Furthermore, an indirect detection of the voltage V LED  across the LED system is provided. The voltage detection is implemented in the freewheeling phase of the switch S 1 , that is, when the switch S 1  is open, that is, not conducting, wherein, in this phase, a current flows through the LED system, a diode D 2  and the coil L Buck  embodied as the primary side of a transformer T 1 . 
       FIG. 2  shows the characteristic of electrical parameters from the circuit according to  FIG. 1 . With closed switch S 1 , the following equation applies for the voltage V′ LED  across the secondary side L 2  of the transformer T 1 :
 
 V′   LED =( VIN−V   LED )/ r  
 
wherein VIN denotes the input voltage of the step-down converter, and r denotes the transformer ratio of the transformer T 1 .
 
     During the freewheeling phase, the following equation once again applies approximately:
 
 V′   LED   =V   LED   /r  
 
     According to the prior art, the coil L Buck  is embodied as a primary winding of the transformer T 1 , wherein the secondary winding L 2  of the transformer T 1  serves for the indirect detection of the voltage V LED  across the LED system. Accordingly, a secondary winding L 2  is coupled to the primary winding L Buck , by means of which the LED voltage can be measured in the freewheeling phase of the switch S 1 , because the LED voltage is fully present across this primary winding L Buck  in the freewheeling phase. 
     The secondary winding L 2  is connected, on the one hand, to ground and, on the other hand, to a resistor R CHG . An envelope-curve demodulator comprising a diode D 1 , a capacitor C 1  and a resistor R DISCHG  are connected in series to this resistor R CHG . These three components form an envelope-curve demodulator for the voltage V′ LED  of the secondary winding L 2 . The diode D 1  allows only one polarity of the high-frequency voltage V′ LED  to pass. The parallel configuration of the capacitor C 1  and of the resistor R DISCHG  forms a low-pass filter for the removal of the high-frequency signal. The corresponding characteristic of the voltage V ADC  present in this low-pass filter or respectively in the envelope-curve demodulator is shown in  FIG. 2 . 
     It is already known that this voltage V ADC  is supplied to the control unit in order to determine the activation time of the switch S 1 . More particularly, the voltage V ADC  reproduces the voltage V LED  across the LED system during the freewheeling phase of the switch S 1 , wherein the transformer ratio r and the voltage V F  across the diode D 1  must then also be taken into consideration. This diode voltage V F  depends upon the current through the diode D 1 . Since the current through the diode D 1  declines almost to zero with a charged capacitor C 1 , the diode voltage V F  is dependent upon the deactivation duration of the switch S 1 . 
     The transformer T 1  and the capacitor C 1  together form a resonant circuit which, in turn, can cause harmonics or respectively electromagnetic disturbances. Furthermore, the fact that the envelope-curve demodulator comprising the diode and the low-pass filter provides a temperature-dependent and operating-point-dependent voltage error is also problematic. As a result, considerable measurement errors occur, which cannot be corrected. 
     SUMMARY OF THE INVENTION 
     The invention is therefore based upon the object of providing a correspondingly improved operating circuit for at least one LED and a method for operating at least one LED. 
     The object is achieved by the features of the independent claims. The dependent claims develop the central idea of the invention further in a particularly advantageous manner. 
     According to the invention, an operating circuit for an LED system is proposed, comprising a converter, more particularly a DC voltage converter, with a controllable switch and an inductor for converting an input voltage supplied to the operating circuit into a supply voltage for the LED system. The operating circuit comprises a control unit for controlling the switch, a secondary-side inductor coupled to the inductor, and an envelope-curve demodulator for detecting the envelope curve of the voltage present in the secondary-side inductor. A compensation circuit is provided in order to compensate an error relating to the detection of the envelope curve caused by the envelope-curve demodulator. 
     By preference, a temperature-dependent and/or operating-point-dependent voltage error caused by the envelope-curve demodulator can be counteracted by means of the compensation circuit. 
     By preference, the envelope-curve demodulator can comprise a first diode for rectification of the voltage present in the secondary-side inductor. The compensation circuit can comprise a second diode for compensation of a detection error caused by the diode. 
     By preference, both diodes can be arranged in such a manner that the second diode counteracts an offset voltage introduced by the first diode. 
     By preference, the second diode can compensate the temperature dependence of the first diode in that both diodes preferably comprise an identical or similar temperature dependence of the forward voltage. 
     By preference, the first diode and the second diode can be of identical construction, preferably Schottky diodes. 
     By preference, the compensation circuit can comprise an operational amplifier at the positive input of which the output voltage of the envelope-curve demodulator is connected and to the negative input of which the second diode is connected. 
     By preference, the envelope-curve demodulator can comprise a low-pass filter. 
     By preference, a capacitor can be connected upstream of the envelope-curve demodulator in order to counteract an oscillation of the current flowing through the inductor caused by a resonance circuit comprising the secondary-side inductor. 
     By preference, a level-matching circuit can be connected downstream of the compensation circuit. 
     By preference, the secondary-side inductor, the envelope-curve demodulator and the compensation circuit can form a sensor unit for detecting the voltage present in the inductor. 
     By preference, the control unit can control the switch dependent upon the envelope curve of the voltage present in the secondary-side inductor. 
     By preference, starting from the envelope curve of the voltage present in the secondary-side inductor, the control unit can therefore infer the voltage present in the LED system or in the inductor and control the switch. 
     According to the invention, a lamp is provided, comprising a lighting means, more particularly an LED system and such an operating circuit. 
     Further properties, advantages and features are now presented for the person skilled in the art on the basis of the following extensive description of the invention and with reference to the drawings accompanying the FIGS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit arrangement according to the prior art; 
         FIG. 2  shows a diagram with the time characteristic of electrical parameters according to the prior art; 
         FIG. 3  shows a schematic presentation of an exemplary embodiment of the operation according to the invention of an LED system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The schematic circuit arrangement illustrated in  FIG. 3  serves for the operation of at least one LED or respectively one LED system. In the illustrated exemplary embodiment, one LED is provided. Of course, several series connected and/or parallel connected LEDs can also be operated by the circuit arrangement. The LED or respectively the several series connected and/or parallel connected LEDs form a so-called LED system. 
     An input voltage Vin is supplied to the circuit, which can be a previously rectified alternating voltage or respectively mains voltage, preferably processed by a power-factor correction circuit. 
     As an alternative to a pulsed, rectified alternating voltage, the input voltage Vin can also be a constant voltage, for example, supplied from a battery. Such a constant voltage originating from a battery is provided, for example, in an emergency lighting device. 
     The LED system is connected in series to an inductor L Buck  and a switch S 1 . Furthermore, the circuit arrangement provides a diode D 2 , which is connected in parallel to the LED system and to the inductor L 1 . The cathode of the diode D 2  is connected to the anode of the LED or respectively to the anode of at least one LED of the LED system. The anode of the diode D 2  is connected in turn to the connecting point between the inductor L Buck  and the switch S 1 . In parallel with the LED system, a capacitor (not shown) can be connected. The input voltage Vin is applied at the connecting point between the diode D 2  and the LED system. 
     The switch S 1  is closed and opened in an alternating manner by a control unit or respectively a control-regulation unit SR. The control unit SR can preferably be embodied as an integrated circuit, more particularly an ASIC or microcontroller or a hybrid version of these. 
     In the closed condition of the switch S 1 , a current I L  flows through the LED system, the inductor L Buck  and the switch S 1 , so that the inductor L Buck  is charged. In the deactivated condition of the switch S 1 , this energy stored in the magnetic field of the inductor L Buck  is discharged in the form of a current I L  via the diode D 2  and the LED system. 
     A transistor in the form of a field-effect transistor or also a bipolar transistor is preferably used as the switch S 1 . The switch S 1  is switched by the control unit SR with high-frequency, typically within a frequency range above 10 kHz. 
     For regulation of the power supplied to the LED system or respectively for regulation of the current supplied to the LED system, the control unit SR specifies the clocking of the switch S 1 . In order to specify the standardised deactivation time of the switch S 1 , the control unit SR uses, for example, a sensor unit in the form of a measuring resistor R SHUNT , which is connected in series to the switch S 1 , preferably between the switch and the ground. The voltage picked up from the measuring resistor R SHUNT  serves to monitor the current flow through the switch S 1 . Correspondingly, the control unit SR can deactivate the switch S 1  when the current flow through the switch S 1  reaches or exceeds a given maximal value. 
     In order to specify the deactivation duration of the switch S 1  or respectively to specify the time of reactivation of the switch S 1 , a further sensor unit SE is required within the current branch through which current flows during the freewheeling phase. 
     According to the exemplary embodiment of  FIG. 3 , such a sensor unit SE comprises a secondary winding L 2  which is coupled to the inductor L Buck . More particularly, the inductor L Buck  can form the primary winding of a transformer T 1 , which, in turn, comprises the secondary winding L 2  on the secondary side. Through this secondary winding L 2 , the magnetisation condition of the inductor L Buck  can be detected or respectively, taking into consideration the transformer ratio r of the transformer T 1 , the voltage in the inductor L Buck  can be detected. This can serve for indirect detection of the voltage V LED  across the LED system. 
     In turn, in a known manner, monitoring the time-voltage characteristic in the inductor L Buck  provides information regarding the advantageous reactivation time of the switch S 1 . 
     The circuit arrangement shown in  FIG. 3  is based upon a step-down converter, also referred to as a buck converter. As an alternative, other circuit topologies can be used, wherein, more particularly, an inductor is used as an energy transferring component, for example, in the case of a step-up converter or boost converter, in the case of an inverse converter or buck-boost converter, or in the case of a flow-through converter or forward converter. The inductor used in these alternative topologies and acting as an energy-transferring component corresponds to the inductor L Buck  shown in  FIG. 3  and is coupled, in turn, to the secondary-side inductor L 2  shown in  FIG. 3 . 
     The switch S 1  can be controlled in such a manner that the control unit SR determines the duration between a deactivation and a subsequent activation of the switch S 1  dependent upon the voltage VL Buck  across the inductor L Buck . In this context, the control unit SR will preferably determine the voltage across the inductor L Buck  by means of the secondary winding L 2  coupled inductively or respectively by transformer to the inductor L Buck , because VL Buck =V′ LED ·r. 
     As described in the introduction, the following equations are preferably obtained for the voltage V′ LED  across the secondary-side inductor L 2 :
 
 V′   LED =−( VIN−V   LED )/ r ,with closed switch S 1, and
 
 V′   LED   =V   LED   /r , with open switch  S 1.
 
     In a corresponding manner, the sensor unit SE can be used to control the switch S 1  during the activation duration and during the deactivation duration of the switch S 1 . 
     During the deactivation phase of the switch S 1 , the diode D 2  is connected through, that is, in a conducting condition, so that only a negligible voltage of approximately 0.7 V is released across it. The voltage across the inductor L Buck  differs from the voltage V LED  across the LED system only through this voltage released across the diode D 2 . Accordingly, it is possible to infer the voltage across the LED system either ignoring or considering this voltage released across the diode D 2 . 
     The sensor unit SE which is connected at the output end to a Pin or respectively input  33  of the control unit SR, comprises further components in addition to the secondary winding L 2 , and in fact, preferably a resistor R 70 , an input-end capacitor C 71 , an envelope-curve demodulator  30 , a compensation circuit  31 , a level-matching circuit  32  and an output-end capacitor C 75 . 
     Initially, a resistor R 70  is provided, which is arranged in series to the secondary winding L 2 , wherein the other connection of the secondary winding L 2  is connected to ground. The output connection of the resistor R 70  is connected to the envelope-curve demodulator  30 , namely to the anode of a diode D 70  of the envelope-curve demodulator  30 . 
     The envelope-curve demodulator  30  is formed by the diode D 70 , a capacitor C 72  and a resistor R 72 . The cathode of the diode D 70  is connected respectively to a connection of the capacitor C 72  and of the resistor R 72 , wherein the capacitor C 72  and the resistor R 72  are arranged in parallel to the cathode of the diode D 70  and ground. 
     The diode D 70  is preferably a rectifier diode. Accordingly, it only allows a polarity of the preferably high-frequency voltage V′ LED  to pass. The diode D 70  is suitable for converting the voltage V′ LED  into a DC voltage. The arrangement of the capacitor C 72  and of the resistor R 72  ensures that the output voltage V 30  of the envelope-curve demodulator  30  follows the envelope curve of the voltage rectified by the diode D 70 . The combination of capacitor C 72  and resistor R 72  forms a low-pass filter. 
     A capacitor C 71  is connected to the input of the envelope-curve demodulator  30 . This capacitor C 71  is connected, at one end, to ground and, at the other hand, to the connecting point between the resistor R 70  and the envelope-curve demodulator  30 . 
     The use of the further capacitor C 71  reduces an oscillation of the detected LED current which could occur as a result of a resonance circuit comprising the secondary winding L 2 . Together with the secondary winding L 2  and the resistor R 70 , the capacitor C 71  forms an oscillation circuit in the form of a series resonance circuit, wherein the capacitor C 71  preferably adjusts this oscillation circuit to a critical oscillation absorption in such a manner that harmonics or respectively electromagnetic disturbances in the LED current are avoided. By preference, the capacitor C 71  attenuates harmonics which are generated by the resonant circuit comprising the secondary winding L 2  and the capacitor C 72 . The risk of an oscillation in the secondary winding L 2  is preferably attenuated by the resistor R 70 . 
     Through its forward voltage or respectively flow voltage, the diode D 70  applies an offset voltage and accordingly influences the detection of the envelope curve by the envelope-curve demodulator  30 . This offset voltage can be dependent upon different parameters, such as the temperature or the forward current or respectively operating point of the diode. 
     As shown in  FIG. 3 , the output voltage V 30  of the envelope-curve demodulator  30  is supplied to the compensation circuit  31 . This compensation circuit  31  comprises an operational amplifier OV, a resistor R 73  and a diode D 71 . The signal V 30  generated by the envelope-curve demodulator  30  is supplied to the non-inverting or respectively positive input of the operational amplifier OV. The resistor R 73  is arranged between the inverting or respectively negative input of the operational amplifier and ground. The diode is connected at the cathode end to the negative input of the operational amplifier OV and at the anode end to its output. The output of the operational amplifier OV corresponds at the same time to the output voltage V 31  of the compensation circuit  31 . The component LM258 from Texas Instruments can be used, for example, as the operational amplifier OV. 
     The voltage V 30  reproducing the envelope curve is amplified by the operational amplifier OV so that the diode D 71  becomes conducting. The current then flows through the resistor R 73 , so that, because of the feedback, the voltage V R73  present in the resistor R 73  corresponds to the voltage V 30 . The arrangement of the diode D 71  between the resistor R 73  and the output of the compensation circuit  31  means that the output voltage V 31  can compensate the offset voltage introduced by the diode D 70  as mentioned above. 
     In order to guarantee an optimal compensation of the offset voltage, the diodes D 70 , D 71  used are preferably of the same type or respectively of identical construction. For example, both diodes are Schottky diodes, preferably with relatively lower forward voltage. Through the use of Schottky diodes, the efficiency of the compensation circuit  31  can be improved. Alternatively, the two diodes D 70 , D 71  can be silicon diodes. The diodes D 70 , D 71  are based on the same diode technology. 
     By preference, the diodes D 70 , D 71  comprise the same or similar current-voltage characteristic in the pass band. By preference their current-voltage characteristics extend parallel or respectively substantially parallel to one another, at least in the pass band. These characteristics preferably comprise an identical or approximately identical gradient characteristic. 
     The temperature dependence of the forward voltage of both diodes D 70 , D 71  is preferably identical or similar or respectively comparable or at least substantially comparable. In order to improve the compensation of the offset voltage, the diodes D 70 , D 71  should preferably be closely thermally coupled. By preference, the difference between the forward voltages of both diodes D 70 , D 71  is independent of temperature, wherein the resulting measurement error can be taken into consideration by the control unit SR in this case. 
     Through the configuration of the two diodes D 70 , D 71 , which are preferably identical in construction, different parameters, for example, the temperature, forward current etc. are compensated, which would otherwise falsify the measurement or respectively the detection of the envelope curve. In particular, the temperature-dependent and/or operating-point-dependent voltage error caused by the envelope-curve demodulator  30  is counteracted by means of the compensation circuit  31 . 
     As a further measure for improving the compensation of the offset voltage, the resistors R 72 , R 73  should comprise the same or similar resistance value. 
     The output V 31  of the operational amplifier OV is then supplied to the input  33  of the control unit SR, which is preferably embodied as a microcontroller, preferably subject to level matching by a voltage splitter R 74 , R 75 . With this embodiment, the requirements for the detection range of the control unit SR are therefore reduced. The output-end capacitor C 75  additionally serves for filtering.