Abstract:
A power supply device includes: a regulator transformer; a primary switch for selectively supplying a current to the regulator transformer; a control circuit for reducing to 0, following each election made at the primary switch, the minimum value of a current output by the secondary side of the regulator transformer; and a coupling transformer for magnetically coupling routes along which a plurality of loads are connected in parallel to the secondary side of the regulator translator in a direction in which magnetic flux along each of the routes is offset by a current change. In this case, the control circuit increases the maximum value of the output current on the secondary side larger than twice of the target value of the current supplied to the loads.

Description:
[0001]     The present application claims foreign priority based on Japanese Patent Application No. 2004-169166, filed Jun. 7, 2004, the contents of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to a power supply device and a vehicle lamp.  
         [0004]     2. Related Art  
         [0005]     Conventionally, a vehicle lamp employing a light-emitting diode device is well known (see, for example, JP-A-2002-231013). When the vehicle lamp is turned on, the light-emitting diode element generates a forward voltage based on a predetermined threshold voltage at both ends.  
         [0006]     A wide discrepancy appears in the forward voltage generated by individual light-emitting diode devices. Therefore, to cope with the discrepancy in the forward voltage, the vehicle lamp should be turned on by controlling the current for the light-emitting diode device. However, there is a case wherein, because of light distribution design, a vehicle lamp employs a plurality of light-emitting diode devices connected in parallel. In this case, wherein a separate circuit must be designated for supplying a current to each row, the circuit size would be increased, and accordingly, the cost of the vehicle lamp would be increased.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, one or more embodiments of the present invention provide a power supply device and a vehicle lamp that employ a set of the features described in the independent claims of the present invention. The dependent claims of the invention specifically define additional effective examples for the present invention.  
         [0008]     According to a first aspect of the invention, a power supply device comprises:  
         [0009]     a regulator transformer;  
         [0010]     a primary switch, for selectively supplying a current to the regulator transformer;  
         [0011]     a control circuit for reducing to 0, following each election made at the primary switch, the minimum value of a current output by the secondary side of the regulator transformer; and  
         [0012]     a coupling transformer for magnetically coupling routes along which a plurality of loads are connected in parallel to the secondary side of the regulator translator in a direction |in which magnetic flux along each of the routes is offset by a current change. Since each time a selection is made at the primary switch the control circuit reduces to 0 the minimum value of the current output by the secondary side of the regulator transformer, currents can be supplied at desired rates for a plurality of loads.  
         [0013]     Further, the control circuit increases a maximum value for the current output by the secondary side until larger than twice the target value of the currents to be supplied to the loads. Thus, when the minimum value of the current on the secondary side is 0, the average value of the output current can easily approach the target value. In addition, since the control circuit changes switching frequencies in accordance with a voltage supplied by the primary side, the average current on the secondary side is maintained, regardless of the voltage supplied by the primary side. Thus, an average value for the current on the secondary side can be maintained, without the maximum value of the current on the secondary side being changed at the time an election is made using the primary switch. Accordingly, the power lost by the switching regulator can be minimized.  
         [0014]     Furthermore, when a target value for a current to be supplied for the loads connected in parallel to the secondary side of the regulator transformer is increased, the control circuit reduces a switching frequency for the primary switch to increase the average current on the secondary side. Thus, on the secondary side, the average value of the current can be increased without the range of the increase in the current being changed at the time the primary switch is used to make an election.  
         [0015]     In this case, regardless of the target value of the current to be supplied for the loads, or the supply voltage on the primary side, the control circuit is maintained substantially constant for a period wherein the current output by the secondary side is 0 during a switching cycle time. Thus, when the target value for the current is small, or when the supply voltage is high, the power loss can be reduced. Accordingly, for the power supply device, a temperature rise can be suppressed, a service life reduction can be prevented, and reliability can be improved.  
         [0016]     According to a second aspect of the invention, a vehicle lamp comprises:  
         [0017]     a regulator transformer;  
         [0018]     a primary switch for selectively supplying a current to the regulator transformer;  
         [0019]     a plurality of semiconductor light-emitting devices, connected in parallel to a secondary side of the regulator transformer;  
         [0020]     a control circuit for reducing to 0, each time a selection is made using the primary switch, the minimum value of a current output by the secondary side of the regulator transformer; and  
         [0021]     a coupling transformer for magnetically coupling routes for the individual semiconductor light-emitting devices in a direction in which magnetic flux is offset by a current change.  
         [0022]     In this case, regardless of the target value of the current to be supplied for the semiconductor light-emitting devices, or the supply voltage on the primary side, the control circuit is maintained substantially constant for a period wherein the current output by the secondary side is 0 during a switching cycle time.  
         [0023]     The summary above does not include descriptions of all the features or of all the sub-combinations of features that can be included without departing from the spirit of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a diagram showing the structure of a vehicle lamp, together with a reference voltage source, according to one embodiment of the present invention.  
         [0025]      FIGS. 2A and 2B  are diagrams for explaining one example operation for a power supply device.  
         [0026]      FIG. 3  is a diagram showing another example for a power supply transformer.  
         [0027]      FIGS. 4A  to  4 C are diagrams for explaining example relationships between a gate voltage at a switching device and a current flowing through a secondary coil.  
         [0028]      FIGS. 5A  to  5 C are diagrams for explaining example relationships between a gate voltage at the switching device and a current flowing through the secondary coil.  
         [0029]      FIGS. 6A and 6B  are diagrams for explaining example relationships between a gate voltage at the switching device and a current flowing through the secondary coil.  
         [0030]      FIG. 7  is a diagram showing an example structure for a voltage rise detector.  
         [0031]      FIG. 8  is a diagram showing an example structure for a current detector, together with a plurality of series resistors.  
         [0032]      FIG. 9  is a diagram showing another example for the structures of an output current supply unit and an inductance current leakage supply unit.  
         [0033]      FIG. 10  is a diagram showing another example for the structure of a voltage output unit.  
         [0034]      FIG. 11  is a diagram showing another example for the structure of the vehicle lamp.  
         [0035]      FIG. 12  is a diagram showing an additional example for the structure of the vehicle lamp. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     Embodiments of the present invention will now be described. Note, however, that the present invention is not limited to these embodiments, and not all the feature sets described in these embodiments are always required by the present invention.  
         [0037]      FIG. 1  is a diagram showing the configuration, according to one embodiment of the present invention, of a vehicle lamp  10  and a reference voltage power source  50 . The reference power source  50 , for example, is a vehicular-mounted battery that supplies a predetermined direct-current voltage to a power supply device  102 . In this embodiment, the vehicle lamp  10  includes a plurality of light sources  104   a  and  104   b  and the power supply device  102 . The embodiment provides a power supply device  102  that can supply a current, at a desired ratio, to the light sources  104   a  and  104   b.    
         [0038]     The light sources  104   a  and  104   b  are example loads, connected to the power supply device  102 , that are connected in parallel and include one or more light-emitting diode devices  12 . In one embodiment of the invention, the light-emitting diode devices  12  are example semiconductor light-emitting devices that generate light in accordance with power received from the power supply device  102 .  
         [0039]     The light sources  104   a  and  104   b  may have a different number of light-emitting diode devices  12 , and may have a plurality of light source arrays connected in series. The light source arrays are, for example, one or more serially connected arrays of the light-emitting diode devices  12 .  
         [0040]     The power supply device  102  includes: a voltage output unit  202 ; a plurality of output current supply units  210   a  and  210   b ; a current ratio setup unit  204 ; a voltage rise detector  208 ; and an output controller  206 . The voltage output unit  202  includes: a coil  308 ; a plurality of capacitors  310   a  and  310   b ; a switching device  312 ; and a power supply transformer  306 .  
         [0041]     The coil  308 , connected in series to a primary coil  402  of the power supply transformer  306 , supplies the output voltage of the reference voltage power source  50  to the power supply transformer  306 . The capacitors  310   a  and  310   b  smooth voltages at both ends of the coil  308 . The switching device  312 , which is an example primary switch for one embodiment of the invention, is connected in series to the primary coil  402  of the power supply transformer  306 , such that rendering the output of the switching device  312  on or off by the output controller  206  selects whether or not a current is supplied to the power supply transformer  306 .  
         [0042]     The power supply transformer  306 , which is an example regulator transformer for one embodiment of the invention, includes the primary coil  402  and a plurality of secondary coils  404   a  and  404   b . When the switching device  312  is rendered on, the primary coil  402  transmits, via the coil  308 , a current received from the reference voltage power source  50 . The secondary coils  404   a  and  404   b  that are provided correspond to the light sources  104   a  and  104   b , and transmit to the corresponding light sources  104   a  and  104   b , via the output current supply unit  210  and the current ratio setup unit  204 , a voltage or a current that are consonant with the current that flows across the primary coil  402  and the voltage applied at both ends of the primary coil  402 . As a result, the voltage output unit  202  supplies the voltage and the current to the light sources  104   a  and  104   b . It should be noted that the secondary coils  404   a  and  404   b  may have the same number of turns, but consonant with the number of turns, may output different voltages.  
         [0043]     The current output supply units  210   a  and  210   b  are diodes provided in consonance with the secondary coils  404   a  and  404   b , and are connected in the forward direction between the secondary coils  404   a  and  404   b . With this structure, the output current supply unit  210   a  and  210   b  can supply to the light source  104   a  and  104   b , via the current ratio setup unit  204 , voltages and currents output by the corresponding secondary coils  404   a  and  404   b.    
         [0044]     The current ratio setup unit  204  includes: a plurality of capacitors  310   a  and  310   b ; a plurality of series resistances  320   a  and  320   b ; an output transformer  314 ; a plurality of inductance current leakage supply units  316   a  and  316   b ; and a plurality of coils  322   a  and  322   b . The capacitors  318   a  and  318   b  and the series resistors  320   a  and  320   b  are provided, in correspondence with the light sources  104   a  and  104   b , and the capacitors  318   a  and  318   b  smooth a current flowing across the corresponding light sources  104   a  and  104   b . The series resistors  320   a  and  320   b  are serially connected to the corresponding light sources  104   a  and  104   b , and at both ends, generate voltages in consonance with a current flowing through the corresponding light sources  104   a  and  104   b.    
         [0045]     The output transformer  314 , which is an example coupling transformer for one embodiment of the invention, includes a plurality of output coils  406   a  and  406   b . The output coils  406   a  and  406   b  are provided in correspondence with the light sources  104   a  and  104   b ; and the output coil  406   a  is connected via the coil  322   a  to the corresponding light source  104   a , while the output coil  406   b  is connected via the coil  322   b  to the corresponding light source  104   b . The output coils  406   a  and  406   b  transmit, to the corresponding light sources  104   a  and  104   b , a current supplied by the voltage output unit  202 . It should be noted that the light emitting diodes  12  in the light source  104   a  or  104   b  are connected in series to the corresponding coil  406   a  or  406   b  via the coil  322   a  or  322 .  
         [0046]     In this embodiment, the output coils  406   a  and  406   b  are wound in opposite directions. Therefore, in accordance with the current supplied to the light sources  104   a  and  104   b  by the voltage output unit  202 , the output coils  406   a  and  406   b  generate magnetic fluxes in a direction in which they cancel each other. Further, since the output coils  406   a  and  406   b  in a transformer are coupled, the ratio at which a current flows through the output coil  406   a  and the output coil  406   b  is the opposite of that of the turn ratio. Thus, the coils  322   a  and  322   b  may represent a flux leakage by the output transformer  314 . In this case, the inductances of coils  322   a  and  322   b  are proportional to the squares of the turn ratios of the corresponding output coils  406   a  and  406   b.    
         [0047]     The leakage inductance current supply units  316   a  and  316   b  are diodes provided in correspondence with the output coils  406   a  and  406   b . The leakage inductance current supply units  316   a  and  316   b  are connected in opposite directions between the cathodes of diodes that constitute the output current supply units  210   a  and  210   b  and the low potential output terminals of the secondary coils  404   a  and  404   b  to which the anodes of these diodes are connected. In this case, the inductance current leakage supply units  316   a  and  316   b  discharge to the capacitors  318   a  and  318   b , via the corresponding output coils  406   a  and  406   b , energy accumulated by the corresponding coils  322   a  and  322   b . Thus, when currents supplied by the voltage output units  202   a  and  202   b  to the light source units  104   a  and  104   b  are reduced, the inductance current leakage supply units  316   a  and  316   b  supply to the light sources  104   a  and  104   b  currents in amounts consonant with the corresponding coils  322 .  
         [0048]     In one embodiment, the inductance current leakage supply units  316   a  and  316   b  constitute a forward converter, in addition to the power supply transformers  306   a  and  306   b , the switching device  312 , the output current supply units  210   a  and  210   b , the output coils  406   a  and  406   b  and the coils  322   a  and  322   b.    
         [0049]     During the period the switching device  312  is OFF, the inductance current leakage supply units  316   a  and  316   b  discharge, to the capacitors  318   a  and  318   b , energy accumulated by the coils  322   a  and  322   b  during the period of the switching device  312  was ON.  
         [0050]     When, for example, the inductance current leakage supply units  316   a  and  316   b  are not employed, energy accumulated by the coils  322   a  and  322   b  would be a loss during the period the switching device  312  is OFF. However, according to this embodiment, the energy accumulated by the coils  322   a  and  322   b  can be efficiently provided for the light sources  104   a  and  104   b.    
         [0051]     The voltage rise detector  208  detects the elevation of a voltage applied to each of the light sources  104   a  and  104   b.    
         [0052]     This is a voltage supplied to a node a and a node b, which are located between the light sources  104   a  and  104   b  and the corresponding coils  322   a  and  322   b , and is, for example, an absolute value for a difference between the potentials of the nodes  212  and a ground potential. The voltage rise detector  208  detects, relative to the light sources  104   a  and  104   b , that the voltages at the nodes  212  exceed a predesignated value.  
         [0053]     Or, the voltage rise detector  208  may detect an elevation of the absolute values of the potentials at the nodes  212 .  
         [0054]     The output controller  206 , which is an example control circuit of one embodiment of the invention, includes a current detector  304  and a switch controller  302 . The current detector  304  detects voltages at both ends of each of the series resistors  320   a  and  320   b , and detects currents flowing through the light source  104   a  or  104   b  that correspond to the series resistor  320   a  or  320   b . The switch controller  302  performs, for example, the well known PWM control or PFM control in accordance with the current detected by the current detector  304 , and controls the ON or OFF time of the switching device  312 . In this manner, the switch controller  302  controls the switching device  312 , so that a constant current value is detected by the current detector  304 . In one embodiment, the values of the currents flowing through both the light sources  104   a  and  104   b  are detected; however, since the current ratios are designated in advance by the output transformer  314 , only the current flowing through one of the light sources  104  may be detected.  
         [0055]     When the voltage rise detector  208  detects at the nodes  212   a  and  212   b  the elevation of the voltage for either light source  104   a  or  104   b , the switch controller  302  maintains the OFF condition of the switching device  320  and halts the output of the voltage by the voltage output unit  202 . Thus, the output controller  206  provides a failsafe function for halting the power supply device  102  upon the occurrence of an abnormality, and provides improved safety for the power supply device  102 .  
         [0056]     In another example, the switch controller  302  may selectively halt the output by the voltage output unit  202  of the voltage to the light source  104 , for which the voltage elevation at the node  212  is detected. In this case, a light source unaffected by the abnormality can be continuously on. As a result, a vehicle lamp  10  can be provided that has a high redundancy relative to failures.  
         [0057]     Because, for example, of the light distribution design of the vehicle lamp  10 , light sources  104   a  and  104   b , for which required voltage values and current values differ, may be employed. In this case, when a power supply device  102  is provided for each of the light sources  104 , costs would be increased. However, according to embodiments of the invention, in the single power supply device  102 , the secondary coils  404   a  and  404   b  are individually provided for the light sources  104   a  and  104   b , so that an appropriate voltage can be applied for each of the individual light sources  104   a  and  104   b . Further, since the output transformer  314  is employed for which the output coils  406   a  and  406   b  are provided, an appropriate current ratio can be designated for the supply of a current to the light sources  104   a  and  104   b . Thus, according to embodiments of the invention, the cost of properly turning on the light sources  104   a  and  104   b  can be low, and a vehicle lamp  10  can be provided at a low cost.  
         [0058]     As another example, the output coils  406   a  and  406   b  of the output transformer  314  may be wound in the same direction. In this case, the output coils  406   a  and  406   b  both generate magnetic fluxes in a direction in which each magnetic flux is increased by the other, and accordingly, voltages are generated at their ends in consonance with the ratio of the number of turns. Therefore, in this case, it is preferable that the number of turns for the coils  406   a  and  406   b  be consonant with the voltages to be applied to their corresponding light sources  104   a  and  104   b.    
         [0059]      FIGS. 2A and 2B  are diagrams for explaining an example operation performed by the power supply device  102 . In  FIGS. 2A and 2B , only portions required for the explanation are extracted from the power supply device  102 . In  FIG. 2A , the power supply device  102  shown is one for which normal light sources  104   a  and  104   b  are provided. In  FIG. 2B , the power supply device  102  shown is one when only the light source  104   a  is open. The open state represents a condition wherein the section between the node  212  and the ground potential terminal is in a high impedance state, resulting, for example, from the disconnection of the light source  104 .  
         [0060]     In one embodiment, the number of turns for the primary coil  402  is N p , the number of turns for both the secondary coils  404   a  and  404   b  are N n1  and N n2 , and the number of turns for both the output coils  406   a  and  406   b  are N o1  and N o2 . The secondary coils  404   a  and  404   b  are connected in series to the corresponding light sources  104   a  and  104   b  and the output coils  406   a  and  406   b  and the coils  322   a  and  322   b , which correspond to the light sources  104   a  and  104   b.    
         [0061]     The primary coil  402  receives a predetermined supply voltage V in  from the reference voltage power source (see  FIG. 1 ) via the coil  308 . In this case, the secondary coil  404   a  outputs a terminal voltage V a , denoting V oa =V in ·N s1 /N p , while the secondary coil  404   b  outputs a terminal voltage V b , denoting V ob =V in ·N a2 /N p .  
         [0062]     As is shown in  FIG. 2A , when the light sources  104   a  and  104   b  are normal, the output coils  406   a  and  406   b  transmit currents I o1  and I o2 , for which I o1 /I o2 =N o2 /N o1  is established. Thus, the current ratio setup unit  204  (see  FIG. 1 ) designates a ratio for the currents flowing through the light sources  104   a  and  104   b.    
         [0063]     Then, voltages V o1  and V o2  are applied at the nodes  212   a  and  212   b , wherein V o1 =V a −V t1 −V L1  and V o2 =V b −V t2 −V L2 . V t1  denotes a voltage generated at the output coil  406   a ; V t2  denotes a voltage generated at the output coil  406   b ; V L1  denotes a voltage generated at the coil  322   a  and represents the magnetic flux leakage at the output coil  406   a ; and V L2  denotes a voltage generated at the coil  322   b  and represents the magnetic flux leakage at the output coil  406   b.    
         [0064]     Since the output coils  406   a  and  406   b  are wound in a direction that permits the magnetic fluxes to cancel each other, the inductances at the output coils  406   a  and  406   b  are nearly zero. Further, the output coils  406   a  and  406   b  may be wound near each other, like sandwiches, to reduce the magnetic flux leakage, and special coils  322   a  and  322   b  may be separately provided for the magnetic flux leakages. Either this, or the size of the windings of the output coils  406   a  and  406   b  maybe intentionally enlarged to increase the magnetic flux leakage, and magnetic flux leakages  322   a  and  322   b  may result. Thus, the inductances L 1  and L 2  of the coils  322   a  and  322   b , which represent the magnetic flux leakages, limit the currents and determine the inclinations of the rise and the fall of the current. Therefore, when the light sources  104   a  and  104   b  are normal, the only inductance elements present between the power supply transformer  306  and the light sources  104  are L 1  and L 2 .  
         [0065]     When only the light source  104   a  is open, as is shown in  FIG. 2B , the terminal voltages V a  and V b  of the secondary coils  404   a  and  404   b  are unchanged because these voltages are determined in accordance with V in  and the turn ratio of the power supply transformer  306 . However, the output coil  406   a , which corresponds to the light source  104   a , accumulates energy in consonance with a current that flows across the output coil  406   b . At this time, a voltage V t1 , for which V t1 =V t2 ·N o1 /N o2  is established, is applied at both ends of the output coil  406   a . Further, since the light source  104   a  is open, no current flows through the coil  322   a  and V L1  is zero. As a result, the output coil  406   a  outputs to the node  212   a  a voltage V o1  for which V o1 =V a +V t1 =V a +V t2 ·N o1 /N o2  is established. Therefore, the voltage at the node  212   a , which corresponds to the light source  104   a  in the open state, is increased, compared with when the light source  104   a  is normal. Further, the inductance element for the light source  104   b  is the sum of those for the output coil  406   b  and the coil  322   b  (L 2 ), and is larger than the inductance element in the normal state.  
         [0066]     Since the terminal voltages V a  and V b  for the secondary coils  404   a  and  404   b  are unchanged when the light source  104   a  is open, to provide notification, by detecting these terminal voltages, that the open state exists is difficult. However, in this embodiment, since the voltage rise detector  208  (see  FIG. 1 ) detects an increase in the voltage V o1  or V o2  at the node  212   a  or  212   b , and the switch controller  302  (see  FIG. 1 ) halts the power supply device  102 , the open state of the light source  104  can be appropriately detected. Further, with this arrangement, the failsafe control for the open state of the light source  104 , and/or the control of a multiple light source  104  redundancy, can be appropriately performed. That is, only the light source  104   b  can be turned on or off, and at this time, the switch controller functions as a simple one-output forward converter having a comparatively large inductance element.  
         [0067]      FIG. 3  is a diagram showing another example for the power supply transformer  306 . Since the components denoted in  FIG. 3  by the same reference numerals as those used in  FIG. 1  have the same or corresponding functions, no further explanation for them will be given. The power supply transformer  306  includes the primary coil  402  and the secondary coil  404 . The secondary coil  404  generates a voltage in accordance with a current that flows via the primary coil  402  and the turn ratio, relative to the primary coil  402 . One end of the secondary coil  404  is connected to the anodes of the output current supply units  210   a  and  210   b ; the other end is grounded.  
         [0068]     In this example, a single power supply device  102  must be employed only to apply an appropriate voltage to the individual light sources  104 . Further, since the power supply transformer  306  having one output coil  406  can be employed to supply a voltage to the light sources  104 , the number of devices required can be reduced, compared with when the power supply transformer  306  has a plurality of secondary coils  404 . Therefore, both the size and the cost of the power supply device  102  can be reduced.  
         [0069]      FIGS. 4A  to  4 C are diagrams for explaining a relationship between the gate voltage for the switching device and the current flowing through the secondary coil,  404 . In  FIG. 4A  is shown an example relationship between the gate voltage for the switching device  312  and the current transmitted via the secondary coil  404 . In  FIG. 4B  is shown an example relationship between the gate voltage for the switching device  312  and the current transmitted via the secondary coil  404  when the voltage supplied to the power supply transformer  306  is lower than that in  FIG. 4A . In  FIG. 4C  is shown an example relationship between the gate voltage for the switching device and the current across the secondary coil  404  when a voltage is to be supplied that is higher than that in  FIG. 4A .  
         [0070]     In one embodiment, during a predesignated period, the output controller  206  performs the well known PWM control, and applies a High voltage and a Low voltage to the gate terminal of the switching device  312 . In  FIGS. 4A  to  4 C, T ON  represents a time in one period during which the switching device  312  receives at the gate terminal the High voltage output by the output controller  206 ; and T OFF  represents a time in one period during which the switching device  312  receives a Low voltage from the output controller  206  at the gate terminal. The switching device  312  is turned on in the T ON  period, and transmits a current to the primary coil  402 , while the switching device  312  is turned off in the T OFF  period, and halts the transmission of a current to the primary coil  402 .  
         [0071]     In the case shown in  FIG. 4A , during the T ON  period, the switching device  312  continues to supply a current to the primary coil  402 , so that the current flowing through the secondary coil  404  is increased until the switching device  312  is turned off. During this period, the current is transmitted via the secondary coil  404 , the output current supply unit  210 , the output coil  406 , the coil  322  and the capacitor  318 . Further, since the rate at which to increase the current flowing through the secondary coil  404  depends on the supply voltage V in , when the supply voltage V in  is high, the current flowing across the secondary coil  404  is sharply increased and ΔT 1  is shortened. Whereas when the supply voltage V i  is low, the current flowing across the secondary coil  404  is moderately increased, and ΔT 1  is extended.  
         [0072]     When the switching device  312  is turned off by the output controller  206 , a current is supplied via the inductance current leakage supply unit  316 , the output coil  406 , the coil  311  and the capacitor  318 , so that the strength of the current flowing through the output coil  406  is reduced. The rate at which to reduce the current in the output coil  406  does not depend on the supply voltage V in , and is determined by a circuit constant. An average current I ave  is supplied by the capacitor  318  to the light source  104  and the series resistor  320 .  
         [0073]     As is described above, during the T ON  period, the output controller  206  supplies a current to the primary coil  402 , and during the T OFF  period, halts the current flowing through the primary coil  402 , so as to supply, to the secondary coil  404 , a current that is increased during a period ΔT 1  or reduced during a period ΔT 2 . Furthermore, the output controller  206  controls the duty ratio of the pulse so that the T OFF  period is longer than the ΔT 2  period. Thus, the current flowing through the secondary coil  404  is adjusted to zero during a period represented by ΔT 3 . As is described above, under the control exercised by the switching controller  302 , the switching device  312  is repetitively turned on or off, and the output coil  406  transmits a saw-wave shaped current, as is shown in  FIG. 4A , that includes the period wherein no current was flowing. A current flowing through the output coil  406  is smoothed by the coil  322  and the capacitor  318 , and the resultant current is supplied to the light source  104 . When the maximum value of the current flowing through the output coil  406  is defined as I max , and the average current smoothed and supplied to the light source  104  is I ave , the output controller  206  controls the T ON  time so that I max  is greater than twice of I ave .  
         [0074]     The relationship between the voltages and the current at the individual sections will now be described in detail while referring to  FIG. 2A . Assuming that V aon , V bon , V con  and V don  denote voltages of V a , V b , V c  and V d  when the switching device  312  is on, the following relation is established. 
 
 V   aon   =V   in ( N   S1   /N   P )− V   f    Ex. 1 
 
 V   bon   =V   in ( N   S2   /N   P )− V   f    Ex. 2 
 
 N   o1   /N   o2 =( V   con   −V   aon )/( V   bon   −V   don )   Ex. 3 
 
 N   o1   /N   o2 =(( V   don   −V   o2 )/ L   2 )/(( V   con   −V   o1 )/ L   1 )   Ex. 4 
 
         [0075]     Assuming that V aoff , V boff , V coff  and V doff  denote voltages of V a , V b , V c  and V d  when the switching device  312  is off, the following relation is established. 
 
 V   aoff   =V   boff   =−V   f    Ex. 5 
 
 N   o1   /N   o2 =( V   aoff   −V   coff )/( V   doff   −V   boff )   Ex. 6 
 
 N   o1   /N   o2 =(( V   o2   −V   doff )/ L   2 )/(( V   o1   −V   coff )/ L   1 )   Ex. 7 
 
         [0076]     In this case, V f  denotes a voltage drop at the diode provided for the output current supply unit and the inductance current leakage supply unit.  
         [0077]     Further, in expressions 1 to 4 and expressions 5 to 7, the ratio of V aon  to V bon  completely equals to the ratio of V o1  to V o2 , the same amount of energy that the output coil  406   b  provided for the output coil  406   a  during the ON period for the switching device  312  was returned by the output coil  406   a  to the output coil  406   b  during the OFF period for the switching device  312 . However, a wide discrepancy appears in the forward voltage for the individual light-emitting diode devices  12  included in the light sources  104  and the forward voltage for the light-emitting diode device  12  is changed in accordance with the temperature, and also, a variance appears in the voltage change for the individual light-emitting diode devices. Therefore, it is difficult for the ratio V o1  to V o2  to match the ratio V aon  to V bon . Therefore, when the ratio V aon  to V bon  differs from the ratio of V o1  to V o2 , the amount of energy that differs from the amount of energy that the output coil  406   a  provided for the output coil  406   b  during the ON period of the switching device  312  is returned by the output coil  406   a  to the output coil  406   b  during the OFF period for the switching device  312 . Accordingly, an energy deviation occurs between the output coils  406   a  and  406   b , and the output transformer  314  is unevenly magnetized.  
         [0078]     When the output transformer  314  is unevenly magnetized, a direct current would be retained in one of the output coils  406   a  or  406   b . Then, the current consumed by the power supply device  102  would be increased, and the power supply device  102  would be damaged by the heat that it generates. Further, when uneven magnetization is accumulated, magnetic fluxes at the cores of the power supply transformer  306  and the output transformer  314  would be saturated, so that either the amount of current supplied to the light sources  104  is reduced or the light sources  104  are not appropriately turned on. Further, since the output controller  206  controls the switching device  312  to maintain a desired value for a current to be supplied to the light sources  104 , the switching device  312  would be damaged by generated heat.  
         [0079]     However, in one embodiment, for each switch process at the switching device  312 , the output controller  206  extends the T OFF  until it is longer than ΔT 2 , and reduces, to zero, the minimum value of the output current at the secondary coil  404 . Thus, there is a moment whereat the amount of current present in the output transformer  314  is zero. Therefore, uneven magnetization does not occur on the output transformer  314 , and a direct current is not retained in the output transformer  314 . Thus, heat generation by the power supply device  102  can be prevented, and current can be supplied to multiple light sources  104  at a desired ratio. It should be noted, however, that the amount of energy exchanged by the output coils  406   a  and  406   b  should match, to the extent possible, to prevent uneven magnetization, and that the ratio V aon  to V bon  and the ratio V o1  to V o2  should be so designated that they are as equal as possible in order to reduce a loss due to uneven magnetization.  
         [0080]     When ΔI 1  and ΔI 2  denote changes in the amount of the currents flowing through the output coils  406   a  and  406   b , L 1  and L 2  denote inductances for the coils  322   a  and  322   b , T on  denotes the period wherein the switching device  312  is on, and T off  denotes the period wherein the switching device  312  is off, the following relationship is established. 
 
Δ I   1 =(( V   con   −V   o1 )/ L   1 ) T   on =(( V   o1   −V   coff )/ L   1 )  T   off    Ex. 8 
 
Δ I   2 =(( V   don   −V   o2 )/ L   2 ) T   on =(( V   o2   −V   doff )/ L   2 )  T   off    Ex. 9 
 
         [0081]     The output controller  206  controls the T ON  period so that I max , which is the maximum value of the current for the secondary coil  404 , is twice as large as I ave , which is the target value for a current to be supplied to the light sources  104 . Through the provision of this control, when the minimum value of the current flowing through the secondary coil  404  is zero, the average value of the current supplied to the light sources  104  can easily be near the target value.  
         [0082]     Furthermore, in one embodiment, when the voltage (V in ) supplied to the power supply transformer  306  is reduced, as is shown in  FIG. 4B , the output controller  206  extends the T ON  period and maintains a constant average current for supply to the light sources  104 . Even in this case, the T OFF  period is adjusted so it is longer than the period ΔT 2 , which is a period required for the reduction of the current flowing through the secondary coil  404 . With this arrangement, the current can be supplied to the multiple light sources  104  at a desired ratio, and when the voltage (V in ) supplied to the power supply transformer  306  is reduced, the supply of a constant average current to the light sources  104  can be maintained.  
         [0083]     In addition, in one embodiment, when the voltage (V in ) supplied to the power supply transformer  306  is increased, as is shown in  FIG. 4C , the output controller  206  reduces the T ON  period and maintains the constant average current that is to be supplied to the light sources  104 . In this case, the T OFF  period is much longer than the period ΔT 2 , and uneven magnetization at the output transformer  314  does not occur.  
         [0084]     Therefore, a current can be supplied to the multiple light sources  104  at a desired ratio, and when the voltage (V in ) supplied to the power supply transformer  306  is changed, the supply to the light sources  104  of a constant average current can be maintained.  
         [0085]      FIGS. 5A  to  5 C are diagrams for explaining another example OF the relationship between the gate voltage of the switching device  312  and the current in the secondary coil  404 . In  FIG. 5A  is shown a relationship between the gate voltage at the switching device  312  and the current in the secondary coil  404 . In  FIG. 5B  is shown a relationship between the gate voltage at the switching device and the current in the secondary coil  404  when the voltage supplied to the power supply transformer  306  is higher than in  FIG. 5A . In  FIG. 5C  is shown a relationship between the gate voltage at the switching device  312  and the current in the secondary coil  404  when the voltage supplied is lower than in  FIG. 5A .  
         [0086]     In this example, the output controller  206  performs the well known PEM control during which the T OFF  period for outputting a Low voltage is constant, and applies a High voltage and a Low voltage to the gate terminal of the switching device  312 . In this example, regardless of the voltage supplied to the power supply transformer  306  and the current supplied to the light sources  104 , the T off  period is designated substantially equal to the time ΔT 2 , during which the current reaches zero in the OFF time for the switching device  312 . Therefore, as is shown in  FIG. 5A , the time during which current flows through the secondary coil  404  is 0 is short. To obtain this setup, the T OFF  time need only be determined based on the values of V o1 , V o2 , L 1  and L 2 , i.e., based on expressions 8 and 9.  
         [0087]     Assuming that the time at which the current flowing through the secondary coil  404  is zero is long, the maximum value I max  of the current that flows through the secondary coil  404  during the ON period of the switching device  312  must be increased in order to supply a desired average current to the light sources  104 . When the maximum value I max  of the current flowing through the secondary coil  404  is large, the power conversion efficiency of the power supply transformer  306  would be reduced. However, in this example, since the output controller  206  transmits, to the gate signal of the switching device  312 , a PFM signal that designates a reduction in the time whereat the current flowing through the secondary coil  404  is zero, deterioration of the power conversion efficiency of the power supply transformer  306  can be prevented. Accordingly, a rise in the temperature of the power supply device  102 , and a reduction in the service life of the power supply device  102  can be suppressed, and there liability of the power supply device  102  can be improved.  
         [0088]     When the voltage supplied to the power supply device  102  is increased, and when the switching device  312  is turned on, the amount of current flowing through the secondary coil  404  is more sharply increased than in  FIG. 5A . On the other hand, when the switching device  312  is turned off, the current flowing through the secondary coil  404  reaches zero at the time ΔT 2 , as in  FIG. 5A . In this example, as is shown in  FIG. 5   b,  when the voltage supplied to the power supply transformer  306  is raised, the output controller  206  maintains the length of the period T OFF  so it is substantially equal to the period ΔT 2 , and increases the frequency at which the switching device  312  is to be turned on or off. Through this process, even when the voltage supplied to the power supply transformer  306  is raised, the supply of a constant amount of current to the light sources  104  can be maintained.  
         [0089]     When the voltage supplied to the power supply transformer  306  is dropped, and when the switching device  312  is turned on, the current flowing through the secondary coil  404  is more moderately increased than in  FIG. 5A . On the other hand, when the switching device  312  is turned off, the current flowing through the secondary coil  404  reaches zero at the time ΔT 2 , as in  FIG. 5A . In this example, when the voltage supplied to the power supply transformer  306 , shown in  FIG. 5C , the output controller  206  maintains the length of the period T OFF  so it is substantially equal to the period ΔT 2 , and reduces the switching frequency for the switching device  312  so as to maintain the supply of a constant current to the light sources  104 . Through this process, the average current I ave  supplied to the light sources  104  can be maintained, without changing the maximum value I max  of the current that flows through the secondary coil  404  during the switching period for the switching device  312 . As a result, power loss at the power supply transformer  306  can be minimized.  
         [0090]      FIGS. 6A and 6B  are diagrams for explaining an additional example for a relationship between the gate voltage at the switching device and the current flowing through the secondary coil  404 . In  FIG. 6A  is shown the relationship between the gate voltage at the switching device  312  and the current flowing through the secondary coil  404 . And in  FIG. 6B  is shown the relationship between the gate voltage at the switching device  312  and the current flowing through the secondary coil  404  when the average current to be supplied to the light sources  104  is raised more than in  FIG. 6A .  
         [0091]     In this example, the output controller  206  performs the well known PFM control wherein the period T OFF  is constant, and applies a High voltage and a Low voltage to the gate terminal of the switching device  312 . Furthermore, in this embodiment, regardless of the voltage supplied to the power supply transformer  306  and the current supplied to the light sources  104 , the period T OFF  is designated so it is substantially equal in the length of the period ΔT 2 . In this example, the voltage V in  supplied to the power supply transformer  306  is substantially constant.  
         [0092]     As is shown in  FIG. 6B , when the target value of the current supplied to the light sources  104  is increased from I ave1  to I ave2 , the output controller  206  maintains the length of the period T OFF    50  it is substantially equal to the period ΔT 2 , and reduces the switching frequency for the switching device  312 , so that the average current supplied to the light sources  104  is increased. Through this process, the average value for the current flowing through the secondary coil  404  can be increased, without changing the rate for the increase in the current that flows through the secondary coil  404  at the switching time for the switching device  312 . As is apparent from expressions 8 and 9, the period T OFF  need only be extended by a value equivalent to an I ave  increase, i.e., an increase of ΔI.  
         [0093]      FIG. 7  is a diagram showing an example structure for the voltage rise detector  208 . In this example, the voltage rise detector  208  includes: a plurality of Zener diodes  508   a  and  508   b , a comparator  506 , a resistor  512 , a constant voltage source  510 , a counter  504  and a latch  502 . The Zener diodes  508   a  and  508   b  provided correspond to the light sources  104   a  and  104   b  (see  FIG. 1 ), and the cathodes of the Zener diodes  508   a  and  508   b  are connected to the corresponding light sources  104   a  and  104   b  while the anodes are connected to one of the input terminals of the comparator  506 . The other input terminal of the comparator  506  is grounded through the resistor  512 . And when the voltage of the corresponding node  212  is higher than the Zener voltage, the Zener diode  508  provides the voltage at the node  212  to the comparator  506 .  
         [0094]     At the input terminal, the comparator  506  receives a predetermined voltage via the constant voltage source  510 . Since the constant voltage source  510  provides for the comparator  506  a voltage lower than the Zener voltage at the Zener diode  508 , the comparator  506  inverts the output when the voltage of either node  212  is higher than the Zener voltage at the Zener diode  508 . Thus, an increase in the voltage at the node  212  that exceeds a predesignated value can be properly detected.  
         [0095]     The counter  504  delays the output of the comparator  506 , and supplies the output to the latch  502 . The latch  502  latches the output of the counter  504 , and transmits the obtained value to the switch controller  302 . Thus, an abnormality, such as an open state of the light source  104 , can be distinguished from a rise in the voltage due to a temporary voltage change caused by noise. Therefore, in this example, an increase in the voltage at the node  212  can be appropriately detected, and the open state of the light source  104 , for example, can be properly detected.  
         [0096]     In another example, the voltage rise detector  208  may include a plurality of resistors, instead of the multiple Zener diodes  508   a  and  508   b . These resistors can be located between the node  212  and the comparator  506 , instead of the Zener diodes  508 . In this example, a rise in the voltage at the node  212  can also be appropriately detected.  
         [0097]      FIG. 8  is a diagram showing an example structure of the current detector  304 , as well as a plurality of series resistors  320   a  and  320   b . In this example, the current detector  304  includes a plurality of disconnection detectors  602   a  and  602   b  and a plurality of resistors  604   a  and  604   b , which correspond to the light sources  104   a  and  104   b.    
         [0098]     The disconnection detector  602  includes a PNP transistor  606 , an NPN transistor  608  and a plurality of resistors. The base terminal of the PNP transistor  606  is connected to the emitter terminal via the resistor, and the emitter terminal is connected to a node located between the corresponding light source  104  and the series resistor  320 . The collector terminal is connected to the corresponding resistor  604 . The base terminal of the NPN transistor  608  is connected, via the resistor, to a node located between the corresponding light source  104  and the series resistor  320 , and the collector terminal is connected, via the resistor, to the base terminal of the PNP transistor  606 . The emitter terminal of the NPN transistor  608  is grounded. The resistor  604  connects the switch controller  302  and the collector terminal of the PNP transistor  606  of the corresponding disconnection detector  602 .  
         [0099]     When a corresponding light source  104  is not open, the potential at the node located between this light source  104  and the series resistor  320  is a product of the value of the current that flows through the light source  104  and across the resistance of the series resistor  320 . In this case, the NPN transistor  608  and the PNP transistor  606  are rendered on, and the resistor  604  receives, from the disconnection detector  602 , the voltage generated at both ends of the series resistor  320 .  
         [0100]     Furthermore, when the corresponding light source  104  is open because of a disconnection, a current does not flow through the series resistor  320 , so that the potential at the node between the light source  104  and the series resistor  320  is a ground potential. In this case, the NPN transistor  608  and the PNP transistor  606  are rendered off, and the resistor  604  receives a high impedance from the disconnection detector  602 .  
         [0101]     When the light sources  104   a  and  104   b  are not open, the current detector  304  supplies to the switch controller  302 , as a detected current value, the average value of the voltages generated at both ends of each of the series resistors  320   a  and  320   b . When either light source  104   a  or  104   b  is open, the current detector  304  supplies to the switch controller  302 , as a detected current value, the average value of the voltages generated at both ends at the series resistors  320   a  and  320   b  that are not open. Then, the switching controller  302  controls the switching device  312  (see  FIG. 1 ), so that the voltage received from the current detector  304  is constant.  
         [0102]     The series resistors  320  are connected in series to the light sources  104  and the output coils  406  (see  FIG. 1 ) corresponding to the light sources  104 . Therefore, when the corresponding light sources  104  are not open, a current flows across the series resistors  320   a  and  320   b  at a current ratio that is designated by the output coils  406   a  and  406   b.    
         [0103]     In this example, the series resistors  320  have resistances for which the ratio is the opposite of the ratio for the current flowing through the corresponding light sources  104 . Therefore, in this example, the series resistors  320  generate substantially equal voltages in accordance with the currents flowing through the corresponding light sources  104 . Therefore, according to this example, when the average value of the voltages generated at the ends of the individual series resistors are adjusted so they equal the setup voltage defined in common for a plurality of series resistors  320 , the current flowing through the light sources  104   a  and  104   b  can be appropriately controlled. The output controller  206  (see  FIG. 1 ) need only control the voltage output by the voltage output unit  202 , for the voltages generated at the ends of the individual series resistors  320  to equal the setup voltage.  
         [0104]     The vehicle lamp  10  (see  FIG. 1 ) may have three or more light sources  104 , and when one of the light sources  104  is open, the current detector  304  may supply to the switch controller  302  the average value of the voltages generated at the ends of the series resistors  320  that are not open. In another example, the current detector  304  may supply to the switch controller  302  the sum of the voltages generated at the ends of the individual series resistors  320 .  
         [0105]     In an additional example, a plurality of light sources  104  may be turned on by controlling a voltage to be applied to these light sources. However, in this case, the control process would be complicated because of a variance in the forward voltage of the light-emitting diode devices  12  (see  FIG. 1 ). However, according to the embodiment, since a current flowing through the individual light sources  104  is controlled, the multiple light sources  104  can be appropriately turned on.  
         [0106]      FIG. 9  is a diagram showing another example structure for the output current supply unit  210  and the inductance current supply unit  316 . In this example, the output current supply unit  210  includes a diode  802  and an NMOS transistor  804 , and the leakage inductance current supply unit  316  includes a diode  808  and an NMOS transistor  806 .  
         [0107]     The diodes  802  and  808  have the same functions as the output current supply unit  210  and the inductance current leakage supply unit  316  in  FIG. 1 . The NMOS transistor  804  and the NMOS transistor  806  are rendered on or off, by the switching controller  302 , in synchronization with the switching device  312  (see  FIG. 1 ). In this example, during a period wherein the switching device  312  is on, the NMOS transistor  804  is rendered on, and with the diode  802 , supplies a current to the output coil  406 . During the period wherein the switching device  312  is off, the NMOS transistor  806  is rendered off, and with the diode  808 , supplies a current to the output coil  406 . In this manner, the NMOS transistors  804  and  806  perform synchronous rectification with the diodes  802  and  808 . As a result, compared with rectification that uses only the diodes  802  and  804 , the power loss can be reduced. The diodes  802  and  804  may be parasitic diodes for NMOS transistors.  
         [0108]      FIG. 10  is a diagram showing an additional example for the structure of the voltage output unit  202 . In this example, the voltage output unit  202  includes a plurality of switches  702   a  and  702   b , provided in correspondence with the light sources  104   a  and  104   b  (see  FIG. 1 ). The switches  702  are used to connect the corresponding coils  406  for the reference voltage power source  50  in accordance with an instruction issued by the switch controller  302 . In this case, the switch controller  302  turns on or off the switches  702   a  and  702   b  synchronously and simultaneously. The output coils receive, from the corresponding switches  702 , rectangular waves consonant with the control by the switch controller  302 . In this example, the ratio of the currents flowing through the output coils  406   a  and  406   b  can also be appropriately designated by using the output coils.  
         [0109]      FIG. 11  is a diagram showing an additional example for the structure of the vehicle lamp  10 . Since the components in  FIG. 11  denoted by the same reference numerals as used in  FIG. 1  have the same or corresponding functions, no further explanation for them will be given, except for the following components. The vehicle lamp  10  includes a plurality of light sources  104   a  to  104   c.  Corresponding to the light sources  104   a  to  104   c,  the power supply transformer  306  includes a plurality of secondary coils  404   a  to  404   c,  a plurality of output current supply units  210   a  to  210   c,  a plurality of leakage inductance current supply units  316   a  to  316   c,  a plurality of capacitors  318   a  to  318   c  and a plurality of series resistors  320   a  to  320   c.    
         [0110]     In this example, the voltage rise detector  208  detects not only voltages at nodes  212   a  and  212   b , but also a voltage at a node  212   c  located between the light source  104   c  and a coil  322   c  corresponding to the light source  104   c.    
         [0111]     The current ratio setup unit  204  includes output transformers  314   a  and  314   b , the number of which is smaller by one than the number of light sources  104 . The output transformer  314   a  includes a plurality of output coils  406   a ,  406   b  and  406   c , and the output transformer  314   b  includes a plurality of output coils  408   b  and  408   c . The output coil  406   a  that is provided, and which corresponds to the light source  104   a , is connected in series to the light source  104   a  via the coil  322   a . The output coils  406   b  and the output coil  408   b  that are provided, and which correspond to the light source  104   b , and are connected in series to the light source  104   b  through the coil  322   b . And the output coil  406   c  and the output coil  408   c  that are provided, and which correspond to the light source  104   c , are connected in series to the light source  104   c  through the coil  322   c.    
         [0112]     The output transformer  314   a  and  314   b  will now be described in more detail. In the output transformer  314   a , the output coils  406   b  and  406   c  are wound in the same direction, in the opposite direction to the output coil  406   a . Therefore, in accordance with a current that the voltage output unit  202  supplies to the corresponding light sources  104 , the output coil  406   a  and the output coils  406   b  and  406   c  generate magnetic fluxes in a direction in which the magnetic fluxes cancel each other. In this case, the output coil  406   a  determines the ratio of the current flowing through the light source  104   a  to the current flowing through the light sources  104   b  and  104   c . Furthermore, the output transformer  314   a  determines the rate, of the total current output by the power supply transformer  306 , of the current to be supplied to the light source  104   a.    
         [0113]     When the numbers of turns of the output coils  406   a ,  406   b  and  406   c  are defined as N o1 , N o2  and N o3 , and when the currents flowing through the light sources  104   a ,  104   b  and  104   c  are defined as I o1 , I o2  and I o3 , the relation I o1 =(N o2 ·I o2 +N o3 ·I o3 )/N o1  is established. The ratio of I o2  to I o3  is determined by the output transformer  314   b.    
         [0114]     In the output transformer  314   b , the output coil  408   b  and the output coil  408   c  are wound in opposite directions. Therefore, in the current that the voltage output unit  202  supplies to the corresponding light sources  104 , the output coils  408   b  and the output coils  408   c  generate magnetic fluxes in directions in which the magnetic fluxes cancel each other. Thus, the output transformer  314   b  determines the ratio of the current flowing through the light source  104   b  to the current flowing through the light source  104   c . Further, other than the light source  104   a , the output transformer  314   b  also determines the rate of the current, of the total current output by the power supply transformer  306 , supplied to the light sources  104   b  and  104   c . As a result, according to this example, even when the vehicle lamp  10  has three or more light sources  104 , the current flowing through the light sources  104  can be appropriately designated.  
         [0115]     As another example, for the vehicle lamp  10 , first to N light sources  104  (N is an integer of two or greater) may be provided. In this case, the voltage output unit  202  applies a voltage to the N light sources  104  connected in parallel. For the power supply device  102 , (N−1), first to (H−1)th, output transformers  314  are located between the voltage output unit  202  and the light sources  104 .  
         [0116]     The k-th (k is an integer satisfying 1≦k≦N−1) output transformer  314  includes: output coils  406  connected in series to the k-th light source  104 , and (N−k) output coils  406 , which are connected in series to the (k+1) th to the Nth light sources  104 . In accordance with a current received from the voltage output unit  202 , the (N−k) output coils  406  generate magnetic fluxes in a direction in which the magnetic fluxes generated by the output coils connected in series to the k-th light source  104  are canceled. With this arrangement, the ratio of the current flowing through the N light sources  104  can be appropriately designated.  
         [0117]      FIG. 12  is a diagram showing an additional example for the structure of the vehicle lamp  10 . Since the components in  FIG. 12  denoted by the same reference numerals as are used in  FIG. 1  or  11  have the same or corresponding functions, no further explanation for them will be given. In this example, the output coils  406  and  408  are provided downstream of the corresponding light sources  104 , and the output coils are located downstream of corresponding series resistors  320 . Further, the downstream ends of the series resistors are grounded. In this case, the ratio of the current flowing through the light sources  104  can also be appropriately designated.  
         [0118]     As a further example, the cathode of the output current supply unit  210  may be grounded. In this example, the power supply transformer  306  outputs a negative voltage at the low potential output terminal of the secondary coil  404 . In this case, the ratio of the current flowing through the light sources  104  can also be appropriately designated.  
         [0119]     As is apparent from the above description, according to one embodiment of the invention, at each switch time for the switching device  312 , the output controller  206  reduces to zero the minimum value of the current that flows through the secondary coil  404 , so that the current can be supplied to the light sources  104  at a desired ratio. Further, since the output controller  206  increases, to more than twice the target value of the output current, the maximum value of the current that flows through the secondary coil  404 , even when the minimum value of the current flowing through the secondary coil  404  is zero, the average value of the current supplied to the light sources  104  can easily be moved near the target value.  
         [0120]     Furthermore, since the output controller  206  changes the switching frequency in accordance with the voltage supplied to the power supply transformer  306  and maintains the constant average current for the secondary coil  404 , the average value of the current for the secondary coil  404  can be maintained without changing the maximum value of the current flowing through the secondary coil  404  at the time the switching device  312  is switched. In addition, when the target current supplied to the light source  104  is increased, the output controller  206  reduces the switching frequency for the switching device  312  and increases the average current for the secondary coil  404 . Thus, the average value of the current for the secondary coil can be increased without changing the rate for increasing the current flowing through the secondary coil  404  at the time the switching device  312  is switched.  
         [0121]     The invention has been described by exemplary embodiments; however, the technical scope of the invention is not limited to these embodiments. It will be obvious for one having ordinary skill in the art that these embodiments can be variously modified or improved, and that such modifications or improvements are also included in the spirit of the invention Accordingly, the invention is limited only by the attached claims.