Abstract:
The present invention relates to a driver device ( 60 ) for driving a load ( 12 ), in particular an LED unit comprising one or more LEDs, said driver device comprising input terminals ( 28, 30 ) for receiving an input voltage (V 12 ) from an external power supply ( 16 ), output terminals for providing an output voltage to the load ( 12 ) for driving the load ( 12 ), a converter unit ( 34 ) for converting the input voltage (VI  2 ) to a converted voltage (VI  4 ) and for providing the converted voltage (VI  4 ) to internal connection elements ( 63, 64 ) of the driver device ( 60 ), a signal control device ( 62 ) for applying an electrical signal (I) to at least one of the connection elements ( 63, 64 ), and a detection circuit for detecting a phase angle of the input voltage (VI  2 ) by measuring a voltage drop of the converted voltage (VI  4 ) caused by the electrical signal (I).

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/IB13/050468, filed on Jan. 18, 2013, which claims the benefit of U.S. Provisional Patent Application No. 61/593,378, filed on Feb. 1, 2012. These applications are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a driver device and a corresponding 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 
     In the field of LED drivers for offline applications such as retrofit lamps and new lamps or modules, solutions are demanded to cope with high efficiency, high power density and high power factor among other relevant features. While practically all existing solutions comprise one or another requirement, it is essential that the proposed driver circuit properly conditions the form of the mains energy into the form required by the LEDs while remaining in compliance with present and future power mains regulations. It is of critical importance to control the amount of power delivered to the lamps to control the brightness of the lamps, while having a high efficiency and reduced power loss in the power converter. To control the amount of power delivered to the lamps, phase cut dimming is one option having a high efficiency and a low power loss. If driver devices are used including a phase cut dimmer, the lamps derive the electrical power from the phase cut mains voltage and have to recover the phase cut position, in order to set the power level of the lamp accordingly. Trailing edge phase cut dimmers, which are preferably used, do not always provide a voltage step with a significant edge, which is easy to detect due to the filter capacitors across the lamp and across the dimmer. Therefore, the lamps are provided with a bleeder circuit having one or more bleeder resistors to drain the charged capacitor, in order to verify that the dimmer is turned off. However, the bleeding current increases the power loss of the lamps. 
     WO 2010137002 A1 discloses a phase cut dimmer device for driving an LED unit, wherein the LED unit comprises a bleeder circuit to adjust the rectified phase cut input voltage. The bleeder circuits comprise detection means to detect the voltage drop at two predefined voltage levels to activate one of the two bleeder circuits. Detection of the phase angle of the phase cut voltage in an accurate manner is not possible with this bleeder circuit. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a driver device and a corresponding method for driving a load, in particular an LED unit comprising one or more LEDs, providing a high power factor, reduced losses and low cost. Further, it is an object of the present invention to provide a corresponding light apparatus. 
     According to an aspect of the present invention, a driver device for driving a load, in particular an LED unit comprising one or more LEDs, is provided comprising:
         input terminals for receiving an input voltage from an external power supply,   output terminals for providing an output voltage to the load for driving the load,   a converter unit for converting the input voltage to a converted voltage and for providing the converted voltage to internal connection elements of the driver device,   a signal control device for applying an electrical signal to at least one of the connection elements, and   a detection circuit for detecting a phase angle of the input voltage by measuring a voltage drop of the converted voltage caused by the electrical signal.       

     According to another aspect of the present invention, a drive method for driving a load, in particular an LED unit comprising one or more LEDs, is provided, said method comprising:
         receiving an input voltage from an external power supply at input terminals,   converting the input voltage to a converted voltage and providing the converted voltage to internal connection elements,   applying an electrical signal to at least one of the connection elements by means of a signal control unit, and   detecting a phase angle of the input voltage by detecting a voltage drop of the converted voltage caused by the electrical signal.       

     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 the light assembly as provided according to the present invention. 
     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. 
     The present invention is based on the idea to detect whether the input voltage from the external power supply is applied to the input terminal by applying the electrical signal to the internal connection elements. The electrical signal creates a voltage dip in the converted voltage, wherein the dip is limited to a low peak if the input voltage is applied to the input terminal and wherein the peak is large if the input voltage is not provided to the input terminals. Therefore, if a phase cut dimmer device is connected to the external power supply and the input voltage is a phase cut input voltage, a detection circuit can precisely detect the phase angle on the basis of the peak value of the voltage drop or a voltage dip of the converted voltage, and the connected load can be controlled accordingly. Therefore, power consuming bleeding currents can be avoided to detect the phase angle of the input voltage. By virtue thereof, the total losses in the driver device due to bleeding are reduced with low technical effort and low cost. 
     In an embodiment, the electrical signal is a current drawn from or provided to the input terminal. This is an effective possibility to create a voltage dip in the converted voltage to detect the phase angle of the input voltage. 
     In an embodiment, the signal control device comprises an electrical storage element for storing electrical energy and a controllable switch for electrically connecting the electrical storage element to at least one of the connection elements. By means of the electrical storage element the electrical signal can be provided to the connection element for a short time frame with low technical effort and low power loss. 
     In a further embodiment, the signal control device comprises a charge control element connected to the electrical storage element for controlling the electrical charge stored in the electrical storage element. This is an effective and simple solution to provide a defined voltage potential for providing the electrical signal as desired. 
     According to a further embodiment, the electrical storage element is a charge capacitor. The charge capacitor can provide a defined voltage potential to the connection element and can be charged quickly to create a short voltage drop or dip in the converted voltage with low power loss. 
     According to an alternative embodiment, the signal control device comprises a current path including a resistor and a controllable switch for connecting the connection elements to each other. By connecting the connection elements to each other, a short bleeding current pulse can be provided to create a voltage dip in the converted voltage with low technical effort. 
     According to a further alternative embodiment, the signal control device comprises a controllable current source for providing the electrical signal. The advantage of the controllable current source is that the electrical signal can be set precisely to create a predefined voltage dip which can be detected easily. 
     According to a further embodiment, the converter unit comprises a rectifier unit connected to the input terminals for rectifying the input voltage to a unipolar voltage provided to the connection elements. This is a simple circuitry for deriving a unipolar voltage for driving an LED unit from an alternating bipolar voltage provided by the mains. 
     According to a preferred embodiment, the detection circuit comprises a differentiator circuit for measuring the voltage drop or dip of the converted voltage. The differentiator circuit is a simple solution for measuring a voltage drop of the converted voltage, since the change of the converted voltage is detected and since the differentiator can be implemented with reduced effort, e.g. in an integrated circuit. 
     It is preferred that the signal control device is adapted to provide the electrical signal for a time period of less than 1/10 of a half-cycle of the input voltage, in particular less than 200 μs. Since the power loss of the signal control device is dependent on the duration of the electrical signal, the power loss can be reduced by providing the electrical signal for a short time frame of less than 1/10 of the half-cycle of the input voltage. 
     According to a further preferred embodiment, the input voltage is an alternating phase cut voltage, and wherein the signal control unit is adapted to apply the electrical signal at different points in time within each half cycle of the input voltage to detect the phase angle of the input voltage. This is an effective and simple possibility to detect the phase angle of the phase cut input voltage with low power consumption. 
     According to an embodiment, the driver device is connected to a dimmer device providing the phase cut input voltage, and wherein the driver device is adapted to receive a trailing edge phase-cut voltage as the input voltage. 
     According to an embodiment of the driving method, the input voltage is an alternating phase cut voltage and the point in time at which the electrical signal is applied is varied within each half cycle of the input voltage to detect the phase angle of the input voltage. This is an effective solution to detect the phase angle of the input voltage quickly within a few half cycles of the input voltage and with low power loss. 
     According to a further embodiment of the driving method, the point in time is varied stepwise in consecutive half cycles of the input voltage to detect the phase angle of the input voltage. This reduces the control effort, since the phase angle is detected iteratively within a few half cycles of the input voltage. 
     As mentioned above, the present invention provides a solution to detect the phase angle of a phase cut input voltage with low technical effort by applying an electrical signal to one of the connection elements and by detecting the respective voltage dip created in the converted voltage. Therefore, the phase angle can be detected precisely and easily to drive the attached load accordingly with a high power factor and low loss. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 
         FIG. 1 a    shows a schematic block diagram of a dimmer and driver device for driving an LED unit, 
         FIG. 1 b    shows a rectified voltage for driving an LED unit, a corresponding mains voltage and a control signal for driving the dimmer device, 
         FIG. 2  shows a schematic block diagram of a driver device having a signal control unit for detecting a phase angle of the phase cut voltage provided by the dimmer device, 
         FIG. 3  shows a preferred embodiment of the driver device of  FIG. 2 , 
         FIG. 4  shows a timing diagram of the drive voltage for driving the load provided by the driver device of  FIGS. 2 and 3 , a corresponding rectified mains voltage and pulsed driving signal for driving the signal control unit, and 
         FIG. 5  shows a schematic block diagram illustrating a search unit for detecting the phase angle of the phase cut voltage provided by the dimmer device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of a driver device  10  for driving a load, in particular an LED unit  12 , is schematically shown in  FIG. 1 a   . The driver device  10  is connected to a dimmer device  14 , which is connected to an external voltage supply  16 , e.g. an external mains voltage supply, and adapted for providing a phase cut AC voltage V 12  from the AC supply voltage V 10 . The dimmer device  14  comprises a bi-directional switch  18  and a control unit  22  for controlling the switch  18 . The dimmer device  14  converts the AC supply voltage V 10  to a phase cut voltage V 12  by switching the switch  18  and disconnecting the connection between the external voltage supply  16  and an output terminal of the dimmer device  14 . The dimmer device  14  further comprises a capacitor  26  connected in parallel to the switch  18 . The control unit  22  controls the switch  18  by means of a control signal  24  to provide a trailing edge phase cut signal V 12 . 
     The control unit  22  comprises a timing circuit which requires a zero crossing detection for restarting a timer at every zero crossing of the mains voltage V  10  to keep the dimmer device  14  operating properly. 
     The driver device  10  comprises a first input terminal  28  and a second input terminal  30  for connecting the driver device  10  to the external voltage supply  16 . The first input terminal  28  is connected to the output terminal of the dimmer device  14  to receive the phase cut voltage V 12 . The second input terminal  30  is connected to a neutral line of the external voltage supply  16 . The driver device  10  may comprise an input impedance  32  connected to the first input terminal  28 . The input impedance  32  may be formed by a resistor, an inductor, an EMI-filter, or the like. The driver device  10  comprises a rectifier  34  for rectifying the phase cut voltage V 12  to a rectified voltage V 14 . The driver device  10  further comprises a first bleeder  36  and a second bleeder  38 . The bleeders  36 ,  38  each comprise a resistor  40 ,  42  and a controllable switch  44 ,  46 . The resistors  40 ,  42  comprise a different resistance, wherein the first bleeder  36  comprises a large resistor  40 , and wherein the second bleeder  38  comprises a small resistor  42 . The bleeders  36 ,  38  are applied to the rectified voltage V 14  by switching the switches  44 ,  46 , wherein the second bleeder  38  is applied when a zero crossing of the supply voltage V 10  is detected or the mains voltage V 10  drops below 50V and wherein the first bleeder  36  is applied when the amplitude of the mains voltage drops below 200 V to reduce the power dissipation in the resistor  42 . The bleeders  36 ,  38  connect the input terminals  28 ,  30  to each other during a certain time period of the phase cut voltage to adapt the driver device  10  to the dimmer device  14  so that the timing circuit of the dimmer device  14  operates as desired. 
     The driver device  10  further comprises a diode  48  and a capacitor  50 , wherein the capacitor  50  is connected in parallel to the LED unit  12  to provide a respective drive voltage for driving the load  12 . The load  12  comprises LEDs including either a linear or a switched DC/DC converter for matching the voltage of the LEDs to the voltage of the capacitor  50 . 
     In  FIG. 1 b    a diagram is shown illustrating the voltage waveform of the rectified voltage V 14 , the corresponding supply voltage V 10  (dashed lines) provided by the external voltage supply  16  and the control signal  24  provided by the control unit  22  for controlling the switch  18  of the dimmer device  14 . 
     The control signal  24  switches the controllable switch  18  off and disconnects the external voltage supply  16  at t1. The rectified voltage V 14  follows the supply voltage V 10  until the first bleeder  36  is activated at t2. The rectified voltage V 14  follows the supply voltage V 10 , since the input impedance of the driver device  10  is large compared to the impedance of the capacitor  26  of the dimmer device  14 . Since the capacitor  26  is discharged at t1 and the voltage V 10  is applied to the terminals  28 , 30  via the discharged capacitor  26 , it is not possible to differentiate the phase cut voltage V 12  and the supply voltage V 10  until the first bleeder  36  is activated at t2. At t3 when the voltage V 14  is decreased, e.g. below 50V, the second bleeder  38  is activated. At t4, when the zero crossing of the supply voltage V 10  is detected, the control signal  24  is applied to close the controllable switch  18  again and to provide the supply voltage V 10  to the output of the dimmer device  14 . Both bleeders  36  and  38  are turned off at t4. The minor distortion of the rectified voltage V 14  results in non-linearity and a dead zone of the dimming curve, since the phase angle of the phase cut voltage V 12  cannot be detected. Compensation of this non-linearity can be overcome by applying the weak bleeder  36  earlier, however, this would increase the power dissipation of the driver device  10 . Therefore, it is necessary to detect the phase angle of the phase-cut voltage to drive the LED accordingly. 
       FIG. 2  shows a driver device  60  including a signal control unit  62  for controlling the rectified voltage V 14 . Main elements are identical to the elements of  FIG. 1  and denoted by identical reference numerals. Here, only the differences are explained in detail. 
     The signal control unit  62  is connected in parallel to the rectifier  34 . The rectifier  34  is connected to the load  12  by means of connection elements  63 ,  64 . The signal control unit  62  is electrically connected to the connection elements  63 ,  64 . The rectifier  34  provides the rectified voltage V 14  to the load  12  for driving the load  12 . 
     The signal control unit  62  is connected to the connection elements  63 ,  64  and provided to apply an electrical signal I to the connection elements  63 ,  64 . The electrical signal I is an electrical current I drawn from the electrical element  63 . The electrical signal I provides a voltage dip to the rectified voltage V 14 , which is measured by a measuring device  65  of the signal control unit  62 , wherein the peak value of the voltage dip is dependent on the status of the dimmer device  14 . In other words, the peak value of the voltage dip is dependent on whether a controllable switch  18  is switched on and the supply voltage V 10  is provided to the rectifier  34  or the controllable switch is switched off and a capacitor  26  of the dimmer device  14  is connected to the rectifier  34 . The electrical signal I is applied for a short time frame, preferably 50-100 μs, to the connection element  63 . If the controllable switch  18  of the dimmer device  14  is switched on, the peak value of the voltage dip of the rectified voltage V 14  is small. If the controllable switch  18  of the dimmer device is switched off, the peak value of the voltage dip is large. Therefore, the signal control unit  62  can detect the status of the dimmer device  14  and, therefore, the driver device  10  can detect the phase angle of the phase cut voltage V 12  by applying the electrical signal and by measuring the peak value of the created voltage dip of the rectified voltage V 14 . 
     According to one embodiment, the signal control unit  62  comprises a current path including a low resistance to connect the connection elements  63 ,  64  to each other to provide the current I and to create the voltage dip of the rectified voltage V 14 . According to another embodiment, the signal control unit  62  comprises a controllable current source to draw the current I from the connection element  63  to the connection element  64  to create the voltage dip in the rectified voltage V 14 . According to a further embodiment, the signal control unit  62  comprises a charge capacitor to draw the current I from the connection element  63  and to provide the voltage dip in the rectified voltage V 14  as will be described in detail in the following. 
       FIG. 3  shows the driver device  60  including the signal control unit  62  for controlling the rectified voltage V 14  according to a preferred embodiment. Identical elements are denoted by identical reference numerals, and here merely the differences are explained in detail. 
     The signal control unit  62  is connected to the connection elements  63 ,  64  in parallel to the rectifier  34 . The signal control unit  62  comprises a capacitor  66 , a controllable switch  68  and a resistor  70 . The capacitor  66 , the controllable switch  68  and the resistor  70  are connected in series to each other. A controllable switch  72  is connected in parallel to the capacitor  66 . The controllable switch  72  is provided to connect terminals of the capacitor  66  to each other to discharge the capacitor  66 . The controllable switch  68  is controlled by a control signal  69 . During operation, the capacitor  66  is connected in parallel to the rectifier  34  by closing the controllable switch  68 . When the controllable switch  68  is closed, the current I charges the capacitor  66  and the voltage dip is created in the rectified voltage V 14 . If the controllable switch  18  of the dimmer device  14  is switched on and the supply voltage V 10  is provided to the rectifier  34 , the charge current I is limited by the series resistance of the input impedance  32  and the resistor  70  of the signal control unit  62 . Therefore, a limited small peak value of the voltage dip of the rectified voltage V 14  is created corresponding to the voltage drop across the input impedance  32 . If the controllable switch  18  is switched off, the voltage across the capacitor  66  is defined by the impedance ratio of the capacitor  26  of the dimmer device and the capacitor  66  of the signal control unit  62 . If the capacity of the capacitors  26 ,  66  is identical (e.g. 100 nF), the rectified voltage V 14  drops approximately to 50%. Therefore, a significant voltage dip of the rectified voltage V 14  can be provided if the dimmer device  14  is switched off. The voltage dip of the rectified voltage V 14  is measured when the controllable switch  68  is closed by means of a differentiator circuit. The differentiator circuit detects the peak value of the voltage dip and accordingly determines whether the controllable switch  18  is switched on or off. 
     The controllable switch  68  is preferably closed for a short time frame, e.g. 50 μs-100 μs. The controllable switch  68  and the controllable switch  72  are actuated in an alternating form such that one of the controllable switches  68 ,  72  is open while the other controllable switch  68 ,  72  is closed. Since the controllable switch  72  connects the connection elements of the capacitor  66  to each other, the capacitor  66  is discharged by means of the discharge current I 2  when the controllable switch  68  is open. Therefore, it is ensured that the capacitor  66  is discharged when the controllable switch  68  is closed to draw the current I from the connection element  62 . 
     To detect the phase angle of the phase cut voltage V 12 , the controllable switch  68  can be closed frequently or once per half period of the supply voltage V 10 . Since the power dissipation of the driver device  10  increases when the voltage dip is applied to the rectified voltage V 14 , the voltage dip is generated preferably only once per half period of the supply voltage V 10 . To detect the phase angle of the phase cut voltage V 12 , the point in time when the voltage dip is generated is shifted from one half period of the supply voltage V 10  to the other, as described below. 
       FIG. 4  shows a diagram illustrating the voltage waveform of the rectified voltage V 14 , the absolute value of the supply voltage V 10  and the control signal  69  for controlling the controllable switch  68 . 
     The control signal  69  for closing the controllable switch  68  is provided for several short time frames to connect the capacitor  66  to the rectifier  34  and to provide the current I. The duration of the driving pulses of the control signal  69  is less than 1/0 of the half-cycle of the input voltage V 12 , e.g. less than 200 μs. At each driving pulse of the control signal  69 , the rectified voltage V 14  shows a small voltage dip  74  during the time frame before the dimmer device  14  is switched off at t1. After the dimmer device  14  has been switched off at t1 by opening the controllable switch  18 , the peak value of the voltage dip increases such that the rectified voltage V 14  drops to approximately 50%. The large peak value of this large voltage dip  75  can be easily detected by means of the differentiator circuit. 
     Therefore, the phase angle of the phase cut voltage V 12  can be easily detected by creating the voltage dip in the rectified voltage V 14 , and the LED unit  12  can be driven accordingly. 
     The energy loss per driving pulse is determined by the electrical energy stored in the capacitor  66  and depends on the voltage across the capacitor  66 . The voltage across the capacitor  66  is limited by the time constant of the resistance of the resistor  70  and the capacitance of the capacitor  66 . To reduce the energy loss of the driver device  10 , the electrical signal I can be provided by the signal control unit  62  only once per half cycle of the supply voltage V 10 . 
       FIG. 5  shows a schematic block diagram of a search unit for detecting the phase angle of the phase cut voltage V 12 , generally denoted by  80 . The search unit  80  comprises a search algorithm device  82 , a zero crossing detector  84  and a differentiator  86 . The zero crossing detector  84  and the differentiator  86  each measure the rectified voltage V 14 . The zero crossing detector  84  detects the zero crossing of the rectified voltage V 14  and provides a corresponding signal to the search algorithm device  82 . The differentiator  86  detects any variation of the rectified voltage V 14  including the voltage dips  74 ,  75  created by the electrical signal I. The differentiator  86  provides information as to whether a large voltage dip  75  or a small voltage dip  74  is detected to the search algorithm device  82  by means of a control signal. The search algorithm device  82  provides the control signal  69  or in general a control signal  69  to control the signal control unit  62  and to provide the respective electrical signal I to the connection elements  63 ,  64 . The search algorithm device  82  provides the short drive pulses to create the voltage dip  74 ,  75  of the rectified voltage V 14 . If a large voltage dip  75 , i.e. a trailing edge of the phase cut voltage V 12 , is not detected by the differentiator  86 , the search algorithm device  82  shifts the driving pulse in the following half cycle of the rectified voltage V 14  to a later position to detect the phase angle of the phase cut voltage V 12 . If a large voltage dip  75  is detected, the search algorithm shifts the driving pulse in the following half cycle of the rectified voltage V 14  to an earlier position to determine the phase angle more precisely. Therefore, the algorithm converges within 5 to 10 half cycles (with an accuracy of 3-5°) of the rectified voltage V 14  to determine the phase angle precisely. The search unit  80  may be formed by an integrated digital circuit such as a microcontroller. 
     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. 
     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. 
     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. 
     Any reference signs in the claims should not be construed as limiting the scope.