Abstract:
The present invention relates to a driver device ( 60 ) for driving a load ( 12 ), in particular an LED unit ( 84 ) having one or more LEDs, comprising: input terminals for receiving an input voltage (V 10 ) from an external power source ( 16 ) for powering the load ( 12 ), connection means ( 64 ) for connecting the input terminals to each other and for providing a current path ( 66, 68 ) dependent on the polarity of the input voltage (VI  0 ), wherein the connection means ( 64 ) comprise a first current path ( 66 ) for connecting the input terminals in a first current direction and a second current path ( 68 ) for connecting the input terminals in a second current direction opposite to the first current direction, wherein the first and the second current path ( 66, 68 ) each comprise a current control unit ( 88, 90 ) for controlling a bleeding current ( 13, 14 ) in the respective current path ( 66, 68 ), and wherein the first and the second current path ( 66, 68 ) each comprise decoupling means ( 92, 94 ) for blocking the bleeding current ( 13, 14 ) in the respective current path ( 66, 68 ) in a reverse direction opposite to the respective current direction.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a driver device and a corresponding driving method for driving a load, in particular an LED unit comprising one or more LEDs. Further, the present invention relates to a light apparatus. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the field of LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with high efficiency, high power density, long lifetime, high power factor and low cost, among other relevant features. While practically all existing solutions compromise one or the other requirement, it is essential that the proposed driver circuits properly condition the form of the mains energy into the form required by the LEDs while maintaining compliance with present and future power mains regulations. In addition, it is required that the driver circuits comply with existing power adjusting means, e.g. dimmers or the like, so that the drivers can be used universally as a retrofit driver device including the LED units. 
         [0003]    The driver circuits should comply with all kinds of dimmers and especially the drivers should comply with phase-cut dimmers, which are preferably used to regulate the mains power with low power loss. The dimmers which are generally used to regulate the mains energy provided to a filament lamp need a low load impedance path for a timing circuit operation current to adjust the phase-cut timing. Alternatively, when this path is provided continuously, making and breaking that path for certain parts of the mains voltage cycle can also result in stable operation. The provision of this low impedance path has to be adjusted with respect to the zero crossing of the mains voltage. To achieve timely provision of this low impedance path, the zero crossing is usually detected by the driver circuit of the lamps while it is in a high impedance state. Such a zero-crossing detection is complicated and involves a high technical effort, and if a large number of LED units are connected to one dimmer circuit, the technical effort increases due to the required increase of the impedance of each individual LED unit. 
         [0004]    WO 2009/121956 A1 discloses a lighting apparatus comprising an LED assembly and a rectifier unit to connect the LED unit to a dimmer circuit. The LED unit comprises a bleeder connected in parallel to the LED unit to provide a bleeding current. The bleeder unit is controlled by a control unit connected to the LEDs to provide the bleeding current at certain point in time of the rectifier AC voltage. This control unit is complicated and the power factor of the whole lighting apparatus is reduced due to the bleeding current. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the present invention to provide a driver device and a corresponding driving method for driving a load, in particular an LED unit comprising one or more LEDs, providing compatibility to different dimmer devices, in particular to phase-cut dimmers, with low technical effort. Further, it is an object of the present invention to provide a corresponding light apparatus. 
         [0006]    According to one aspect of the present invention, a driver device is provided comprising:
   input terminals for receiving an input voltage from an external power source for powering the load,   connection means for connecting the input terminals to each other and for providing a current path dependent on the polarity of the input voltage, wherein the connection means comprise a first current path for connecting the input terminals in a first current direction and a second current path for connecting the input terminals in a second current direction opposite to the first current direction, wherein the first and the second current path each comprise a current control unit for controlling a bleeding current in the respective current path and wherein the first and the second current path each comprise decoupling means for blocking the bleeding current in the respective current path in a reverse direction opposite to the respective current direction.   
 
         [0009]    According to another aspect of the present invention, a driving method for driving a load, in particular an LED unit comprising one or more LEDs, is provided, wherein the driving method comprises the steps of:
   receiving an input voltage from an external power supply at input terminals,   connecting the input terminals to each other by means of connection means,   providing a current path for a bleeding current in a forward direction from a first of the input terminals to a second of the input terminals or from the second to the first input terminal dependent on the polarity of the input voltage, and   blocking the bleeding current in a reverse direction of the current path opposite to the forward direction.   
 
         [0014]    According to still another aspect of the present invention, a light apparatus is provided comprising a light assembly comprising one or more light units, in particular an LED unit comprising one or more LEDs, and a driver device for driving said light assembly as provided according to the present invention. 
         [0015]    Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method has similar and/or identical preferred embodiments as the claimed device and as defined in the dependent claims. 
         [0016]    The present invention is based on the idea to provide a driver device having a high impedance and a low impedance path, wherein switching from the high impedance path to the low impedance path is synchronized with the cycle of the attached power supply, in particular mains voltage. The low impedance path is provided after zero crossing of the mains voltage. The zero crossing is not detected actively, but the low impedance path is prepared during one half cycle of the mains voltage and activated at the zero crossing due to the polarity reversal of the voltage automatically. The respective low impedance path is activated by means of the current control unit and the decoupling means block the path during the first half cycle and activate the respective path automatically after the polarity reversal at the zero crossing. Hence, no detection of the zero crossing is required and there is no voltage measurement required during the high impedance mode of the driver device. Thus, the driver device according to the present invention is compatible with different dimmer devices, in particular phase-cut dimmers, and can be provided with low technical effort. Preferably, the current control units comprise a control switch, in particular a transistor like a bipolar or a MOS transistor to control the current in the respective current path. 
         [0017]    In a preferred embodiment, a control unit is provided for controlling the current control units. This is a simple solution to activate and deactivate the respective path at certain points in time or based on certain events. 
         [0018]    In a further embodiment, at least one of the input terminals is connected to a voltage converter unit which is connected to the external power source, wherein the voltage converter is a phase-cutting device provided for cutting a phase of the input voltage and for providing a phase-cut AC voltage to the driver device. This embodiment provides a high power factor and low power loss due to the phase cutting of the mains voltage. 
         [0019]    In a preferred embodiment, the decoupling means comprise a diode in the current path for blocking the bleeding current in the reverse direction and for passing the bleeding current in the forward direction. This provides a simple, cheap and effective device for decoupling the respective path in the reverse direction and providing a polarity-dependent current path. 
         [0020]    In a preferred embodiment, the connection means comprise current limiting means for limiting the bleeding current. This provides a simple and effective solution to limit the bleeding current to avoid early wearout failures due to large bleeding currents. 
         [0021]    In a further embodiment, the current measuring means are provided for measuring a load current provided to the load, and wherein the control unit is adapted to control the current control units on the basis of the measured load current. This provides an effective solution to detect a certain event or point in time to activate or deactivate the current control unit and to optimize the timing of the current control unit. In particular, the current control unit is activated when the load current is reduced to approximately zero. This optimizes the efficiency of the driving device and increases the power factor. 
         [0022]    In an embodiment, current measurement means are provided for measuring the bleeding current, wherein the control unit is adapted to control the current control units on the basis of the measured bleeding current. This provides a simple solution to control the current control unit and to adjust the timing of the current control unit. In particular, the respective current path is deactivated when the bleeding current is increased or reaches a predefined level. Hence, the efficiency of the driver device and the power factor can be increased. 
         [0023]    In an embodiment, the control unit is adapted to control the current control units on the basis of a phase angle of the input voltage detected by phase angle detection means. This provides a simple solution to deactivate the respective current path, when the phase of the input voltage is detected and the dimmer device provides the input voltage to the mains voltage and optimizes the timing of the current paths. 
         [0024]    In an embodiment, the control unit is adapted to activate one of the control units during a first half cycle of the input voltage and to deactivate the current control unit when the phase angle is detected during a second half cycle of the input voltage. This provides an optimized solution to provide a low impedance current path when the dimmer device disconnects the input voltage from the mains voltage, e.g. between the zero crossing and the set firing angle, in the case of a leading edge dimmer. According to this embodiment, an optimized timing for providing a low impedance path can be achieved. 
         [0025]    In this embodiment, the control unit is adapted to activate the current control units of the first and the second current path in an alternating manner depending on the polarity of the input voltage. This is a simple solution to provide polarity-dependent current paths for each of the respective half cycles of the input voltage with low technical effort. 
         [0026]    According to a preferred embodiment, the control unit comprises at least one signal storage element to generate an activation and/or deactivation signal for the current control unit on the basis of the detected time that the load current reaches or exceeds a predefined level. In this case, the control unit needs to be synchronized initially, wherein any subsequent pulses are forwarded to the respective other switch. This provides a simple possibility to synchronize the whole driver device to the phase of the input voltage. Preferably, the signal storage element comprises a flip-flop unit. This provides a simple and robust possibility for synchronizing the driver device. 
         [0027]    In a preferred embodiment, the control unit comprises at least one signal storage element to generate an activation and/or deactivation signal from the control unit on the basis of a detected time that the bleeder current reaches or exceeds a predefined level. This provides a further simple solution to synchronize the respective current path with the input voltage and to provide a high power factor of the driver device. 
         [0028]    As mentioned above, the present invention provides a low impedance current path dependent on the polarity of the input voltage by simple technical means and provides a solution to provide a driver device which is compatible with a phase-cut dimmer for a retrofit LED lamp. By switching the current control units of the respective path on and off in an alternating manner depending on the polarity of the input voltage, each path is prepared while the decoupling element, in particular the diode, still blocks the respective path and activates the path after zero crossing and the respective polarity change of the input voltage. Hence, a low impedance path can be provided, with low technical effort, during a period of time in one half cycle of the input voltage starting precisely with a zero crossing of the input voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings, 
           [0030]      FIG. 1  shows a schematic block diagram of a known driver device for connecting an LED unit to a phase-cut dimmer including zero-crossing detection, 
           [0031]      FIG. 2   a  shows a schematic block diagram of a first embodiment of a polarity-dependent bleeder, 
           [0032]      FIG. 2   b  shows a schematic block diagram of a second embodiment of a polarity-dependent bleeder, 
           [0033]      FIG. 3   a  shows a detailed schematic block diagram of a driver device including two polarity-dependent bleeder current paths, 
           [0034]      FIG. 3   b  shows a detailed schematic block diagram of an alternative embodiment of  FIG. 3   a,    
           [0035]      FIG. 4  shows a detailed schematic block diagram of an embodiment of the driver device according to  FIG. 3   a,    
           [0036]      FIG. 5  shows a detailed schematic block diagram of the driver device according to  FIG. 3  including current sensing circuitries, 
           [0037]      FIG. 6  shows a schematic block diagram of a control unit to control the polarity-dependent bleeder device, and 
           [0038]      FIG. 7  shows a diagram illustrating waveforms of currents and voltages of the driver devices shown in  FIGS. 3   a  and  3   b.    
           [0039]      FIG. 8  shows a flow diagram illustrating the steps of the present invention 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]      FIG. 1  shows an embodiment of a known driver device  10  for driving an LED unit  12  and for connecting the LED unit  12  via a dimmer device  14  to an external power supply  16  such as electric mains. The external power supply  16  provides an alternating voltage V 10  (e.g. mains voltage) to the dimmer device  14 . The dimmer device  14  is a phase-cut dimmer comprising a capacitor  18  and an adjustable resistor  22  for determining a point in time where the dimmer device  14  connects its output to the mains voltage V 10 . Resistor  22  can be adjusted to set the phase angle provided by the dimmer device  14 . The RC circuit formed of the capacitor  18  and the resistors  20  is connected to a first switching device  24  such as a DIAC, which is connected to a second switching device  26  such as a TRIAC. The second switching device  26  is connected to the external power supply  16  and connects the voltage V 10  to the output of the dimmer device  14 . When the voltage across a capacitor  18  reaches a certain value, the first switching device  24  provides a current pulse to the second switching device  26  which connects the external power supply  16  with the output of the dimmer device and provides the voltage V 10  to the driver device  10 . Hence, the dimmer device  14  cuts the phase of the voltage V 10  and provides a phase-cut voltage at its output terminal  28 , which serves as an input voltage V 12  for the driver device  10 . 
         [0041]    The driver device  10  comprises a rectifier unit  30  for rectifying the input voltage V 12  to a unit polar voltage V 14 . The driver device  10  further comprises a voltage measurement unit  32  connected to an input terminal  34  of the driver device  10  for detecting a zero crossing of the input voltage V 12 . The driver device  10  further comprises a bleeder device  36  including a controllable switch  38  and a resistor  40 . The bleeder device  36  provides a current path for the rectifier unit  30  by switching the controllable switch  38 , wherein the bleeder device  36  is activated by means of the voltage measurement unit  32 , which controls the controllable switch  38  via a control signal. Hence, the bleeder device  36  can be activated or deactivated for certain periods of time by means of the voltage measurement unit  32 . 
         [0042]    Consequently, the driver device  10  detects the zero crossing of the input voltage V 12  and activates the bleeder device  36  by means of the controllable switch  38  to provide a bleeding current and a continuous current path to the dimmer device  14 . 
         [0043]    Generally, the driver device  10  matches with the dimmer device  14  by providing a partially time-continuous current path through the driver device  10  to the dimmer device  14 , however, the zero crossing of the voltage V 12  has to be measured by means of the voltage measurement unit  32 , which limits the realizable impedance in the high impedance state. Particularly, if a plurality of LED units are connected to the driver device  10  as the load  12 , each of the voltage measurement units  32  in each LED loads the dimmer and hence reduces the impedance in an unwanted way. To compensate for this, each voltage measurement unit  32  has to be provided with a very large input impedance. Hence, this known driver device  10  is technically complex and expensive to produce in a retrofit LED lamp. 
         [0044]      FIG. 2   a  shows a schematic block diagram of a first embodiment of the present invention. Identical elements are denoted by identical reference numerals, and only the differences with respect to the diagram shown in  FIG. 1  are explained in detail. 
         [0045]    A polarity-dependent bleeder unit  50   a  is connected to the output terminal  28  of the dimmer device  14 , to a neutral potential  52  and to an input terminal  54  of the load  12 . A load current I 1  is provided from the dimmer device  14  via the polarity dependent bleeder  50   a  to the load  12 . The polarity-dependent bleeder  50   a  provides a bleeding current I 2  dependent on the polarity of the input voltage V 12 . The polarity-dependent bleeder  50   a  is connected to the output terminal  28 , and to the input terminal  54  of the load  12  and provided with measuring means to measure the load current I 1 . The polarity-dependent bleeding current I 2  is controlled by means of the polarity-dependent bleeder  50   a  on the basis of the measured load current I 1 . Hence, the bleeding current I 2  can be provided dependent on the load current I 1  and the polarity of the input voltage V 12 . Alternatively, the current in the neutral wire  52  from and to the load  12  may be fed through the polarity-dependent bleeder  50   a,  instead of or in addition to the current I 1 . 
         [0046]      FIG. 2   b  shows a schematic block diagram of a second embodiment of the present invention. Identical elements are denoted by identical reference numerals, wherein here only the differences are described in detail. The output terminal  28  of the dimmer device  14  is connected to the input terminal  54  of the load  12 . A polarity-dependent bleeder  50   b  is connected to the output terminal  28  of the dimmer device  14  and to neutral  52 . Since the polarity-dependent bleeder  50   b  cannot measure the load current I 1 , a separate signal line  56  is provided from the load  12  to the polarity-dependent bleeder  50   b  to provide the necessary information about the load current I 1  to provide or adjust the bleeding current I 2  in the embodiment where this is dependent on the load current I 1 . Detection of polarity may be in the polarity-dependent bleeder  50   b,  or shared with the load  12  and communicated with signal  56  or further signals in either direction, from the polarity-dependent bleeder  50   b  to the load  12  or from the load  12  to the polarity-dependent bleeder  50   b.    
         [0047]    Therefore, the polarity-dependent bleeder  50   b  provides the bleeding current I 2  dependent on the polarity of the input voltage V 12  and the information provided about a load current I 1 . 
         [0048]    Hence, different possibilities are provided by the present invention to provide the polarity-dependent bleeding current I 2  on the basis of the load current I 1  and on the basis of the polarity of input voltage V 12 . 
         [0049]      FIG. 3   a  shows a detailed block diagram of a driver device  60  for powering the load  12 . The driver device  60  comprises a rectifier unit  62  and a polarity-dependent bleeder  64  including a first current path  66  and a second current path  68 . 
         [0050]    The rectifier unit  62  comprises four diodes  70 ,  72 ,  74 ,  76  for rectifying the alternating input voltage V 12  to the rectifier voltage V 14  to power the load  12 . This input rectification is found in many LED drivers. The polarity-dependent bleeder  64  is here quite closely combined with the load driving part, such that parts of the functionality, i.e. the rectification diodes  74  and  76 , are used for both carrying the load current and carrying the bleeder current. Alternatively, the polarity-dependent bleeder  64  may be equipped with fully independent circuitry. 
         [0051]    The load  12  comprises a diode  78 , a first charge capacitor  80 , a second charge capacitor  82  in parallel to an LED unit  84  and an inductive element  86  connecting the diode  78  to the LED unit  84 . 
         [0052]    The first current path  66  and the second current path  68  each comprise a controllable switch  88 ,  90 , a diode  92 ,  94  and current limiting means, depicted as a resistor  96 ,  98 . The first current path  66  is connected in parallel to the diode  70  of the rectifier unit  60 . The diode  92  of the first current path  66  is connected in an opposite direction to the diode  70  of the rectifier unit  60 . 
         [0053]    The current path  68  is connected in parallel to the diode  72  of the rectifier unit  60 . The diode  94  of the second current path  68  is connected in the opposite direction of the diode  72  of the rectifier unit  60 . 
         [0054]    The controllable switches  88 ,  90  of the current paths  66 ,  68  are controlled by a control unit (not shown). 
         [0055]    The first and the second current path  66 ,  68  are switched on and off via the controllable switches  88 ,  90  on the basis of the load current I 1  and other input signals, as will be explained later. The load current I 1  is measured e.g. by measuring a voltage across the diode  78 . When the load current I 1  is reduced to a predefined level preferably close to zero after charging the capacitors  80 ,  82  and the polarity of the input voltage V 12  is positive, the controllable switch  88  of the first current path  66  is closed. In this state, the diode  92  blocks a bleeding current during this half cycle of the input voltage V 12 . After the zero crossing of the voltage V 10 , the input voltage V 12  will change its polarity and the diode  92  becomes conductive and a first bleeder current I 3  is provided. Hence, the first current path  66  carries a timing circuit current I 3 , which allows proper operation of the dimmer device  14 . The first bleeding current I 3  is directed in an opposite direction with respect to the diode  70  and in an opposite direction with respect to the load current I 1 . 
         [0056]    At a point in time during the second half cycle of the voltage V 10 , the dimmer device  14  applies the voltage V 10  to the driver device  60 . This input voltage V 12  results in a charging current through the diodes  72  and  74  having an opposite polarity. At this time, the controllable switch  88  is turned off and the first current path  66  is deactivated. After the load current I 1  is reduced to the predefined level, e.g. close to zero, the controllable switch  90  of the second current path  68  is closed. During this second half cycle of the input voltage V 12 , no bleeding current will flow through this second current path  68 . After the zero crossing of the input voltage V 12 , the input voltage will change its polarity and the diode  94  becomes conductive. Hence, a second bleeding current I 4  is provided in opposite direction to the load current I 1  and carries the time circuit current I 4  to allow proper operation of the dimmer device  14 . 
         [0057]    In other words, the polarity-dependent bleeder  64  is split into two paths  66 ,  68 , one for each polarity of the input voltage V 12 . At a given polarity and at a certain point in time when the load current I 1  is reduced, one of the current paths  66 ,  68  is prepared for the opposite polarity of the next half cycle. At this time the respective first bleeding current I 3 , I 4  is blocked by the respective diode  92 ,  94 . After the zero crossing of the input voltage V 12 , the respective path  66 ,  68  is automatically activated due to the changed polarity and the respective diode  92 ,  94 . When the dimmer device  14  provides the mains voltage V 10  to the driver device  60 , the activated current path  66 ,  68  is deactivated by turning off the respective controllable switch  88 ,  90 . After the load current I 1  is reduced to the predefined level, the respective other current path  66 ,  68  is prepared by closing the respective controllable switch  88 ,  90 . 
         [0058]    The diodes  92 ,  94  may be formed of pn-diodes, high voltage diode stacks, high voltage psn-diodes, silicon carbide diodes or a body diode of a MOSFET, and are preferably chosen in dependence on the desired impedance and in dependence on the application and the expected operation temperature. 
         [0059]      FIG. 3   b  is an alternative embodiment of the polarity-dependent bleeder  64 , wherein identical elements are denoted by identical reference numerals, and here only the differences are explained. In this embodiment, both current paths  66 ,  68  are related to the same potential of the load voltage. To achieve this, the current path  66  is now connected to the other input terminal  99 . The benefit of this embodiment is that the switches  88 ,  90  and measurement signals are related to the same reference potential, namely the negative supply rail. 
         [0060]      FIG. 4  shows a detailed block diagram of an embodiment of the polarity-dependent bleeder  64  shown in  FIG. 3 . Identical elements are denoted by identical reference numerals, wherein here just the differences are explained in detail. 
         [0061]    The first current path  66  comprises a p-type MOS transistor  100  and the diode  92 . The second current path  68  comprises an NPN bipolar transistor  102  and the diode  94 . The first current path  66  and the second current path  68  are connected to each other and are jointly connected to a resistor  104  connected to an input of the driver device  60 . It should be understood that also other semiconductor switches may be used for the two current paths  66 ,  68 . The first and the second current paths  66 ,  68  both use the resistor  104  as a current limiting element. Hence, the technical effort and the costs are reduced. Since the first current path  66  and the second current path  68  are directly connected to each other, the switching of the switches  100 ,  102  has to be synchronized and an overlap of the conducting period of the switches  100 ,  102  should be avoided. In other words, a short circuit should be avoided. 
         [0062]      FIG. 5  shows a detailed block diagram of the driver device  60  including current measurement means  106  for measuring currents in the driver device  60  and for timing the controllable switch  88 ,  90 . Identical elements are denoted by identical reference numerals, and here just the differences are explained in detail. The current measurement means  106  is connected to a control unit  107  to process the measured currents and to calculate the timing of the controllable switches  88 ,  90 . 
         [0063]    The current measurement means  106  comprises a first current measurement unit  108  for measuring a load current I 5  in the diode  72 , which is the load current during the negative half cycle of the input voltage V 12 . The first current measurement unit  108  comprises a Zener diode  110 , an auxiliary voltage source  112 , a capacitor  114 , a diode  116  and a resistor  118 . The current I 5  is measured by measuring the voltage drop across the resistor  118 . The voltage drop in resistor  118  can be limited to a voltage V 16  provided by the auxiliary voltage source  112  plus the voltage drop in the diode  116 . Effectively, a small amplitude current I 5  will flow through the resistor  118 , while the voltage V 16  is decoupled via diode  116 . When the current I 5  is high enough to cause a high voltage drop in resistor  118 , part of the current will flow via diode  116  and charge the capacitor  114 , and support the voltage source  112 , wherein V 16  may be clamped by the Zener diode  110 . In total, this structure may be used as a combination of current measurement (at low current levels) and auxiliary supply (at high current levels). With a proper design, i.e. of the current consumed from the voltage V 16  being quite low, no extra source  112  is required. V 16  may be used to power the control unit  107  and further control units in the system. 
         [0064]    The first current measurement unit  108  is connected to the control unit  107  to process the value of the load current I 5 . 
         [0065]    The current measurement means  106  further comprises a current measurement unit  120  for measuring the bleeding current I 4  of the second current path  68 . The current measurement unit  120  comprises a Zener diode  122  and a resistor  124  and measures the voltage drop across the resistor  124  to measure the bleeding current I 4 . The current measurement unit  120  is connected to the control unit  107  to process the measured bleeding current I 4 . A high resistor value may be selected for resistor  124 , assuring high sensitivity to low current levels. Voltage drop across resistor  124  is limited by the Zener diode  122 . 
         [0066]    The current measurement means  106  further comprises a current measurement unit  126 , which comprises a resistor  128  and which is connected to a current mirror  130  connected to the first current path  66  to measure the bleeding current I 3  in the first current path  66 . The current mirror  130  provides a current identical with or corresponding to the bleeding current I 3  to the resistor  128 . The current measurement unit  126  measures the voltage drop across the resistor  128 . The current measurement unit  126  is connected to the control unit  107  to process the value of the bleeding current I 3 . 
         [0067]    The current measurement means  106  further comprises a current measurement unit  132  for measuring a load current I 6  in the diode  76  of the rectifier unit  62 . The load current I 6  is the load current during the positive half cycle of the input voltage V 12 . The current measurement unit  132  comprises two diodes  134 ,  136  and a resistor  138 . The current measurement unit  132  measures the voltage drop across the resistor  138 . Again, at high currents, voltage drop and hence losses are limited by the diodes  134 ,  136 . The current control unit  132  is connected to the control unit  107  for processing the value of the load current I 6 . 
         [0000]      FIG. 5  shows different sensing circuits to measure the current, flowing in the circuit, both in the polarity-dependent bleeder paths  66 ,  68  as well as in the load path. Except for the measurement of I 3 , the other currents are measured in a non-linear way, i.e. there is a region where the readout signal (i.e. voltage drop) does not increase proportionally with the measured current. For the purpose here, this results in high sensitivity at low current levels while limiting the losses at high current. 
         [0068]    The control unit  107  is preferably formed of a microcontroller and measures the mains frequency and calculates the time between a rise or start of the bleeding currents I 3 , I 4  and the start or variation of the load current I 5 , I 6  and reassembles the phase angle of the input voltage V 12 . The control unit  107  calculates the current consumption and also derives control information for the LED driver. Alternatively, the control unit  107  may be formed without a microcontroller. 
         [0069]      FIG. 6  shows a detailed block diagram of a control unit  140  for controlling the controllable switches  88 ,  90 . The control unit  140  comprises two flip flops  142 ,  144 . The first flip flop  142  is provided for storing the polarity information of the input voltage V 12  and the second flip flop  144  is provided for turning the control switches  88 ,  90  on and off. 
         [0070]    The first flip flop  142  is connected to the control unit  107  and provided with a signal indicating the beginning and the end of the load current I 1 . The second flip flop  144  is connected to the output of the first flip flop  142  and receives signals for polarity synchronization via synchronization lines  146 ,  148 . The output of the first flip flop  142  and the output of the second flip flop  144  are connected to a first AND gate  150  and a second AND gate  152 . The first AND gate  150  and the second AND gate  152  are provided for switching the control switches  88 ,  90 . 
         [0071]    The first flip flop  142  deactivates the switches  88 ,  90  when the load current I 1  is provided and the dimmer device  14  provides the mains voltage V 10  to the input of the driver device  60 . The first flip flop  142  is connected to the control unit  107  and receives a signal indicating the end of the load current I 1  via a first input line  154  and a second signal indicating the beginning of the load current I 1  via a second input line  156 . 
         [0072]    The second flip flop  144  activates one of the control switches  88 ,  90  at a predefined point in time. Driver devices for driving the control switches  88 ,  90  may be connected to the AND gates  150 ,  152  (not shown). In a simple case, this control unit  140  is initially synchronized with the polarity of the input voltage V 12  and any following pulses are provided to the respective other control switch  88 ,  90 . To avoid disturbance of the synchronization, a continuous synchronization of the polarity via the input lines  154 ,  156  is preferred. 
         [0073]    In  FIG. 7 , a diagram is provided showing a) the voltage across the capacitor  18 , b) the bleeding current I 3 , c) the control signal for controlling the controllable switch  88 , d) the load current I 1 , e) the mains voltage V 10  and f) the input voltage V 12 . In  FIG. 7 , a first half cycle ΔT1 and a second half cycle ΔT2 is shown. 
         [0074]    The controllable switch  88  is closed at t1 during the first half cycle ΔT1 when the load current I 1  is reduced to zero as shown in  FIG. 7   c . Due to the blocking diode  92  the bleeding current I 3  remains zero until the zero crossing of the mains voltage is reached at t2 and the second half cycle ΔT2 starts. At t3, the dimmer device  14  provides the mains voltage V 10  to the driver device  60  and the input voltage V 12  rises. At this point the controllable switch  88  is switched off as shown in  FIG. 7   c  and the bleeding current I 3  is reduced to zero as shown in  FIG. 7   b . Hence, at t2 the high impedance path of the driver device  60  is replaced by the low impedance path  66 . In the time frame from t2 to t3 the low impedance path  66  is provided and the timing circuit of the dimmer device  14  can operate as designed. Hence, the driver device  60  is compatible with any dimmer device for a retrofit LED unit. 
         [0075]    In  FIG. 8 , a flow diagram  160  is provided showing the steps of the present invention. 
         [0076]    First, the load current I 1  is measured and the control switch  90  of current path  68  is closed when the load current is decreased to a predefined level as shown by step  162 . Then, the bleeding current I 4  is measured and the point in time that the bleeding current I 4  starts is detected as shown at step  164 . The load current I 1  is measured at step  166  and the control switch  90  is opened when the phase angle of the input voltage V 12  is detected as shown by step  168 . When the load current I 1  is decreased to a predefined level as shown by step  170 , the control switch  88  of the current path  66  is closed at step  172 . Then, the bleeding current I 3  is measured and the point in time that the bleeding current I 3  starts is detected as shown at step  174 . At step  176 , the start of the load current I 1  is detected and at step  178  the control switch  88  is deactivated to stop the bleeding current I 3 . After step  178 , the flow starts again with measuring the input current I 1  at step  162  and with preparing the current path  68  by closing the control switch  90 . 
         [0077]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
         [0078]    In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
         [0079]    A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. 
         [0080]    Any reference signs in the claims should not be construed as limiting the scope.