Abstract:
A drive circuit ( 1 ) for driving a load ( 3 ) comprises: a switched mode power supply ( 10 ) for supplying at the output ( 2   a   , 2   b ) a switched output current (IL); a controller ( 20 ) for controlling the power supply; a current sensor ( 15 ) for generating a current sense signal (Vi  5 ) representing the output current (IL); a voltage sensor ( 30 ) for generating a voltage sense signal (Sy) rep-&gt;resenting the output voltage (Vp; Vp+Vis) of the circuit. The controller receives the current sense signal, and generates a switching time control signal (Sc) for the switched mode power supply ( 10 ) on the basis of the current sense signal. The controller further receives the voltage sense signal. In response to a change in the voltage sense signal, the controller changes the switching time control signal such as to effectively compensate an effect of the output voltage change on the average value of the output current.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to a drive circuit for a load, specifically for LED applications. More particularly, the present invention relates to a drive circuit comprising a switched mode power supply. 
       BACKGROUND OF THE INVENTION 
       [0002]    LEDs are conventionally known as signaling devices. With the development of high-power LEDs, LEDs are nowadays also used for illumination applications. In such applications, it is important that the LED current is accurately kept at a certain target value, since the light output (intensity of the light) is proportional to the current. This applies especially in so-called multi-color applications, where a plurality of LEDs of different colors are used to generate a variable mixed color that depends on the respective intensities of the respective LEDs: a variation in the light intensity of one LED may result in an unwanted variation of the resulting mixed color. 
         [0003]    Driver circuits for driving an arrangement of LEDs with substantially constant current are already known. Typically, such constant current driver circuit comprises a current sensor for sensing the LED current, and a sensor signal is fed back to a controller, which controls a power source such that the sensed current is substantially constant kept at a predetermined level. 
         [0004]    Although such control system would normally function satisfactorily, a problem occurs in that the voltage developed over the LED may vary, and that as a result the power source may give an incorrect current. This problem occurs especially in case the power source is a switched mode power source. 
         [0005]    The present invention aims to provide a drive circuit where this problem is overcome or at least reduced. More particularly, the present invention aims to provide a drive circuit which is less sensitive to variations in the forward voltage of the LEDs. 
       SUMMARY OF THE INVENTION 
       [0006]    According to an important aspect of the invention, the driver circuit also comprises a voltage sensor for sensing the LED voltage, and a voltage sense signal is also fed back to the controller. In response to sensed voltage variations, the controller suitably adapts its control of the power source such that the actual LED current is maintained constant. In a particular embodiment, current control is performed by comparing the sensed current signal to a reference signal, and the reference signal is suitably amended in response to sensed voltage variations. 
         [0007]    It is noted that US-2003/0.117.087 discloses a drive circuit for LEDs, where both the LED current and the LED voltage are measured and both measuring signals are used to control the LED driver. However, in the system described in said publication, control is aiming at keeping the current sense signal and the voltage sense signal constant. In contrast, according to the invention, a variation in the voltage sense signal is accepted, and in response a corresponding variation in the current sense signal is effected, such that the actual LED current remains constant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
           [0009]      FIG. 1  is a block diagram schematically showing a driver circuit; 
           [0010]      FIG. 2  is a graph schematically illustrating a waveform of an output current provided by the driver circuit of  FIG. 1 ; 
           [0011]      FIGS. 3-6  are block diagrams schematically illustrating preferred details of a controller according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  is a block diagram schematically showing a driver circuit  1  having output terminals  2   a ,  2   b  for connection to a LED arrangement  3 . It is noted that the LED arrangement  3  may consist of only one LED, but it is also possible that the LED arrangement comprises a plurality of LEDs arranged in series and/or in parallel. The driver circuit  1  further comprises a controllable switched mode power supply  10 , and a controller  20  for controlling the power supply  10 . 
         [0013]    Switched mode power supplies are known per se, therefore the description of the exemplary switched mode power supply  10  illustrated in  FIG. 1  will be kept brief. If fed from a mains supply, the power supply  10  comprises a converter  11  for converting alternating voltage to direct voltage. A controllable switch  12 , for instance a transistor, is coupled to a first output terminal of the converter  11 . An inductor  13 , typically a coil, is coupled in series with the controllable switch  12 . At the junction of the switch  12  and the inductor  13 , a diode  14  is coupled to a second output terminal of the converter  11 , while the opposite end of the inductor  13  is coupled to a first output terminal  2   a  of the driver circuit  1 . A second output terminal  2   b  of the driver circuit  1  is coupled to the second output terminal of the converter  11 . 
         [0014]    The controller  20  has a control output  21  coupled to a control terminal of the switch  12 , providing a switching time control signal Sc determining the operative state of the switch  12 , more specifically determining the switching moments of the switch  12 . The control output signal Sc is typically a block signal that is either HIGH or LOW. One value of the control output signal Sc, for instance HIGH, results in the switch  12  being closed (i.e. conductive): current flows from the converter  11  through the inductor  13  and the LED arrangement  3  back to the converter, while the current magnitude increases with time. The inductor  13  is being charged. The other value of the control output signal Sc, for instance LOW, results in the switch  12  being open (i.e. non-conductive). The inductor  13  tries to maintain the current, which now flows in the loop defined by the inductor  13 , the LED arrangement  3  and the diode  14 , while the current magnitude decreases with time. The inductor  13  is being discharged. 
         [0015]      FIG. 2  is a graph illustrating this operation. At times t 1  and t 3 , the control output signal Sc becomes HIGH and the output current I L  through the LEDs starts to rise. At times t 2  and t 4 , the control output signal Sc becomes LOW and the output current I L  through the LEDs starts to decrease. The time interval from t 1  to t 2  will be indicated as ON-duration t ON . The time interval from t 2  to t 3  will be indicated as OFF-duration t OFF . The sum of t ON  and t OFF  is the current period T. 
         [0016]    At times t 1  and t 3 , the output current I L  has a minimum magnitude  11 , while at times t 2  and t 4 , the output current I L  has a maximum magnitude  12 . The average output current I AV  is a value between I 1  and I 2 , depending on the ratio of t ON  and t OFF , or the duty cycle Δ defined as t ON /T. Assuming that the current magnitude rises and falls linearly with time, the average output current I AV  is given by the following formula: 
         [0000]        I   AV =( I   1   +I   2 )/2  (1) 
         [0017]    In general, times when the control output signal Sc becomes HIGH, such as t 1  and t 3 , will be indicated as SWITCH_ON-times t SON , and times when the control output signal Sc becomes LOW, such as t 2  and t 4 , will be indicated as SWITCH_OFF-times t SOFF . The controller  20  determines the SWITCH_ON-times t SON  and SWITCH_OFF-times t SOFF  on the basis of the momentary value of the LED current I L . To this end, the driver circuit  1  comprises a current sensor  15 , in the exemplary embodiment of  FIG. 1  implemented as a resistor connected in series with the LED arrangement  3  between the second output terminal  2   b  and mass. The LED current I L  results in a voltage drop V 15  over the current sense resistor  15  proportional to the LED current I L . The voltage V 15  constitutes a current measuring signal, which is provided to the controller  20  at a current sense input  22 . The controller  20  further comprises a comparator  23  and a threshold voltage source  24 . The comparator  23  has a first input receiving the threshold voltage V TH  from the threshold voltage source  24 , and a second input receiving the current measuring signal V 15  from current sense input  22 . The output signal Scomp from the comparator  23  is coupled to a monopulse generator  25 , whose output, possibly after further amplification, constitutes the switch control signal Sc. 
         [0018]    There are several types of operation possible for the controller  23 . It is possible that the controller  23  makes its switch control signal Sc LOW when the current measuring signal V 15  becomes higher than the threshold voltage V TH , and that the OFF-duration t OFF  has a fixed value. In that case, the output signal of the monopulse generator  25  is normally HIGH and the monopulse generator  25 , on triggering, generates a LOW pulse with duration t OFF . It is also possible that the controller  23  makes its switch control signal Sc HIGH when the current measuring signal V 15  becomes lower than the threshold voltage V TH , and that the ON-duration t ON  has a fixed value. In that case, the output signal of the monopulse generator  25  is normally LOW and the monopulse generator  25 , on triggering, generates a HIGH pulse with duration t ON . It is further possible that the controller  23  is provided with two comparators and two threshold voltage sources of mutually different threshold voltages, one comparator comparing the current measuring signal with one threshold voltage and the other comparator comparing the current measuring signal with the other threshold voltage, wherein the controller  23  makes its switch control signal Sc HIGH when the current measuring signal V 15  becomes lower than the lowest threshold voltage and wherein the controller  23  makes its switch control signal Sc LOW when the current measuring signal V 15  becomes higher than the highest threshold voltage (hysteresis control). All of these types of operation result in a current waveform as illustrated in  FIG. 2 . 
         [0019]    When a LED is driven with a LED current I L , a voltage drop occurs over the LED, which voltage drop is indicated as forward voltage V F . The magnitude of the forward voltage V F  is a device property of the LED, and is substantially independent of the magnitude of the LED current I L . However, this device property may change over time, for instance through ageing or as a function of temperature. Also, the device property may be different in different LEDs. Further, it may be desirable to change the number of LEDs in the LED arrangement, also resulting in a change of forward voltage V F . A problem is, that the average LED current I AV  depends on the forward voltage V F , so a change in the forward voltage V F  may cause a change in the average LED current which is not noticed by the controller  20  from monitoring the current sensor  15 . This can be understood as follows for the case of a controller operating with constant tOFF duration. 
         [0020]    Switch  12  is switched OFF when the measured current signal V 15  is equal to the threshold voltage V TH , therefore 
         [0000]        I   2   =V   TH   /R sense  (2) 
         [0000]    Rsense being the resistance value of the sense resistor  15 . 
         [0021]    During an OFF-interval, the LED current is provided by the inductor  13 . The voltage over the inductor  13  will be indicated as V 13 . Ignoring the voltage drop over the diode  14 , V 13  is equal to the sum of V F  and V 15 : 
         [0000]        V   13   =V   F   +V   15   (3) 
         [0022]    The current through the inductor will decrease as a function of time in accordance with the following formula: 
         [0000]      Δ I   L   =−V   13   ·Δt/L   (4) 
         [0000]    wherein L indicates the inductance of the inductor  13 . 
         [0023]    In a first approximation, for brief t OFF , it may be assumed that V 13  is constant. Thus, the value of I 1  can be approximated according to the following formula: 
         [0000]        I   1   =I   2   +ΔI   L   =V   TH   /R sense− V   13   ·t   OFF   /L   (5) 
         [0000]    Using formulas (1) and (3), the average current I AV  can be expressed as 
         [0000]        I   AV   =V   TH   /R sense− V   TH   ·t   OFF /2 L−V   F   ·t   OFF /2 L   (6) 
         [0024]    For the case of a controller operating with constant t ON  duration, or for the case of a controller operating with two threshold voltages, similar formulas can be derived. 
         [0025]    In all cases, the relationship between the average current and the forward voltage V F  can, in first approximation, be expressed as 
         [0000]        I   AV   =I (0)+ c·V   F   (7) 
         [0026]    I(0) being a constant value not depending on V F , 
         [0027]    and c being a constant, whose value, which may be positive or negative, can be determined in advance. 
         [0028]    From formula (7), the following relationship can be derived: 
         [0000]        dI   AV   /dV   F   =c   (8) 
         [0029]    According to the invention, the driver circuit  1  is designed to compensate for the dependency of formula (8). To this end, the driver circuit  1  further comprises a voltage sensor  30  arranged for providing a measuring signal S V  representing the forward voltage V F , which measuring signal S V  is received by the controller  20  at a voltage sense input  26 . In the exemplary embodiment illustrated in  FIG. 1 , the voltage sensor  30  is implemented as a series arrangement of two resistors  31 ,  32  connected between first output terminal  2   a  and mass, the measuring signal S V  being taken from the node between said two resistors  31 ,  32 . It is noted that this measuring signal S V  actually represents V F +V 15 , but the controller  20  already knows V 15  from the signal received at its current sense input  22  so the controller can easily derive VF by performing a subtraction operation V F =S V −V 15 , illustrated by a subtractor  27  in  FIG. 3 . Alternatively, different possibilities for arranging a voltage sensor which actually measures the voltage between the output terminals  2   a ,  2   b  can easily be found, such as a sensor connected between the output terminals  2   a ,  2   b , but the embodiment shown has the advantage of simplicity. 
         [0030]    On the other hand, with reference to formula (5), it is noted that the average current I AV  can actually be expressed as 
         [0000]        I   AV   =V   TH   /R sense−( V   F   +V   15 )· t   OFF /2 L   (9) 
         [0000]      = I (0)+ c′·S   V   (10) 
         [0031]    In response to the measuring signal S V , the controller  20  is designed to adapt the timing of its control signal Sc such that the actual average current I AV  remains unaffected. For implementing this compensation action, there are several possibilities. 
         [0032]    In a possible embodiment, in a case where the OFF-duration t OFF  is constant, the controller  20  is designed to change the OFF-duration t OFF  in response to variations in the forward voltage V F . From formula (6) or (9), it can easily be seen that an increase in V F  can be counteracted by a decrease in t OFF  while a decrease in V F  can be counteracted by an increase in t OFF . Likewise, in a case where the ON-duration t ON  is constant, the controller  20  can be designed to change the ON-duration t ON  in response to variations in the forward voltage V F . These embodiments are illustrated in  FIG. 3 , where the monopulse generator  25  is shown as a controllable generator which is controlled by a timing control signal Stc derived from the voltage sense signal S V . 
         [0033]    It is also possible that the timing of the comparator output signal Scomp is changed. From the above formulas, it can easily be seen that an increase in V F  can be counteracted by an increase in I 2 , which can be effected by an added delay to the comparator output signal Scomp.  FIG. 4  is a block diagram comparable to  FIG. 3 , showing an embodiment where the controller  20  comprises a controllable delay  41  arranged between the comparator  23  output and the monopulse generator  25 , which controllable delay  41  is controlled by a delay control signal Sdc derived from the voltage sense signal S V . This approach can also be used in an embodiment comprising two threshold voltage sources and two comparators for hysteresis control. It is noted that the above applies in cases where, in formula (7) or (10), c or c′, respectively, is negative; if c or c′, respectively, is positive, an increase in V F  can be counteracted by a decrease in I 2 , which can be effected by a reduced delay in the comparator output signal Scomp. 
         [0034]    It is also possible that the timing of the comparator is changed by changing its input signals. From formula (6) or (9), it can easily be seen that an increase in V F  can be counteracted by an increase in V TH , also resulting in an increased  12 . A similar effect can be achieved by decreasing the current sense signal V 15 . It is noted that the above applies in cases where, in formula (7) or (10), c or c′, respectively, is negative; if c or c′, respectively, is positive, an increase in V F  can be counteracted by a decrease in V TH  and/or increasing the current sense signal V 15 . Possible embodiments are illustrated in the block diagrams of  FIGS. 5 and 6 . 
         [0035]      FIG. 5  shows an embodiment where the controller  20  comprises an adder  51  and a compensation block  52  receiving the voltage sense signal S V  and deriving a compensation signal S 5  from the voltage sense signal Sv, which compensation signal S 5 , being positive or negative, is supplied to one input terminal of the adder  51  while another input terminal receives the threshold voltage V TH  from the threshold voltage generator  24 . Alternatively, the threshold voltage generator  24  may be a controllable generator, controlled by the compensation signal S 5  to vary the threshold voltage V TH . 
         [0036]      FIG. 6  shows an embodiment where the controller  20  comprises a subtractor  61  and a compensation block  62  receiving the voltage sense signal Sv and deriving a compensation signal S 6  from the voltage sense signal Sv, which compensation signal S 6 , being positive or negative, is supplied to one input terminal of the subtractor  61  while another input terminal receives the current sense signal V 15  from current sense input  22 . 
         [0037]    In the above embodiments, the controller  20  controls the moments of switching the switch  12  OFF, while the OFF-duration t OFF  is constant. In embodiments where the controller  20  controls the moments of switching the switch  12  ON while the ON-duration t ON  is constant, an increasing output voltage should also be compensated by a delayed switching moment, which is now achieved by decreasing the threshold voltage or increasing the current sense signal. 
         [0038]    With reference to the above formulas, it is noted that the compensation signal S 5  or S 6 , respectively, may be considered to depend from the voltage sense signal Sv in a linear way. Even if the circuit is not completely linear, a linear compensation will usually be sufficient in practice. In case of a suitable dimensioning, the voltage sense signal Sv can be applied to adder  51  or subtractor  61  directly, and the compensation block may be omitted. 
         [0039]    It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. 
         [0040]    For instance, in the above several types of controller have been described by way of example, but the present invention can also be implemented with different types of controller; for example, the present invention can also be implemented with a peak detect PWM controller. In a general solution, compensation can take place by adding or subtracting a signal to or from the current sense signal or the reference threshold level, proportional to the load output voltage. 
         [0041]    In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.