Patent Publication Number: US-7710050-B2

Title: Series connected power supply for semiconductor-based vehicle lighting systems

Description:
FIELD OF THE INVENTION 
     The present invention relates to power supplies for semiconductor-based vehicle lighting systems. More specifically, the present invention relates to a power supply for powering series connected semiconductor-based lighting systems. 
     BACKGROUND OF THE INVENTION 
     Automotive lighting systems are increasingly making use of semiconductor light sources, such as light emitting diodes (LEDs), due to their reliability, power efficiency and the reduced amount of waste heat they produce, compared to incandescent light sources. With improvements in semiconductor devices, it has recently become possible to construct high output lighting systems, such as vehicle headlamp systems, using LED light sources. 
     However, while semiconductor light sources do offer advantages over other light sources, such as incandescent or gas discharge sources, they also have some weaknesses. In particular, LEDs are susceptible to over-voltages, wherein too much voltage is applied to their semiconductor junctions, resulting in too much current flowing through the semiconductor junctions, damaging the LED and shortening its life. Also, if too little current is supplied, LEDs produce less light (fewer lumens) and the lighting system may not output sufficient lumens to meet safety and/or regulatory requirements. 
     As automotive electrical systems typically experience relatively wide voltage swings and as automotive lighting systems typically must operate over wide temperature ranges and conditions, it has been difficult to provide appropriate electrical power to semiconductor light sources at a reasonable cost. 
     In addition to controlling the electrical power supplied to the LEDs, it can also be desirable to turn some LEDs on and some off. For example, a headlamp may have LEDs which are only illuminated when the headlamp is forming a high beam pattern. In prior art systems, a power supply would be provided for each set or group of LEDs to be separately illuminated and, while such a design could provide the desired flexibility, it was also quite expensive. 
     Also, as the characteristics of the semiconductor junctions in each LED vary, it is difficult to connect LEDs in parallel to the power supply as the parallel connected LED with the lowest junction resistance would receive too much current while the parallel connected LED with the highest junction resistance would receive too little current. Thus parallel connected semiconductor lighting systems are generally avoided. However, series connected semiconductor light sources also suffer from disadvantages in that the failure of a single semiconductor light source (which generally fail as open circuits) results in the failure of the entire series connected string of semiconductor light sources. Further, such series connected power supplies have been unable to provide for the dimming of some LED light sources in a lighting system. Any dimming of an LED in the series would result in every other LED also being dimmed. 
     It is desired to have a power supply for semiconductor-based automotive lighting systems, particularly high output lighting systems such as headlight systems, which is not subject to these problems. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a novel power supply for semiconductor light sources which obviates or mitigates at least one disadvantage of the prior art. 
     According to a first aspect of the present invention, there is provided a power supply for series-connected semiconductor light sources, comprising: a constant current source to supply a pre-selected level of electrical current to the series connected semiconductor light sources; and a bypass switch across each respective one of the semiconductor light sources, each bypass switch operating when closed to provide a current path around a respective semiconductor light source. 
     Preferably, the constant current source is a buck boost converter. Also preferably, the bypass switches are semiconductor switches and the power supply further includes a controller to operate the semiconductor switches. Also preferably, the controller is operable to pulse width modulate the operation of at least one bypass switch to dim the corresponding semiconductor light source and is further operable to detect failures of semiconductor light sources. 
     The present invention provides a novel and advantageous power supply for lighting systems employing semiconductor light sources. The semiconductor light sources are connected in series to a constant current source and a bypass switch is provided around each semiconductor light source, or each sub-string of series connected semiconductor light sources. By opening or closing respective bypass switches, individual semiconductor light sources or sub-strings of semiconductor light sources can be illuminated or extinguished as desired. If the bypass switches are electrically controllable, such as semiconductor switches or relays, failures of one or more semiconductor light sources can be determined by the power supply and failed light sources can be bypassed and/or redundant semiconductor elements illuminated to replace failed light sources. Further, if the bypass switches are semiconductor switches, the power supply can employ pulse width modulation techniques to dim one or more semiconductor light sources as desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
         FIG. 1  shows a schematic of a first embodiment of the present invention; 
         FIG. 2  shows a schematic of a second embodiment of the present invention; and 
         FIG. 3  shows a schematic of another configuration of the embodiment of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A series switching power supply for powering semiconductor light sources in accordance with the present invention is indicated generally at  20  in  FIG. 1 . Power supply  20  includes a constant current source  24  which delivers a pre-selected current independent (within its supported current and voltage ranges) of the load of the devices connected between its output terminals. Such constant current sources are well known and a presently preferred example of such a constant current source is a “buck boost” converter. Buck boost converters are well known and are commonly used for DC to DC power conversion and can easily be configured to act as a constant current source. Many other designs can be employed for constant current source  24 , including Single Element Primary Inductor Circuit (SEPIC) types. 
     As illustrated, the output of constant current source  24  is supplied to a series-connected set of semiconductor light sources, in this embodiment LEDs  26 . While the illustration shows four LEDs  26  connected in series, as will be apparent to those of skill in the art the present invention is not so limited and more or fewer LEDs  26  can be connected, as desired. 
     Power supply  20  further includes a bypass switch  28  for each LED  26 . When a bypass switch  28  is closed, the current supplied from constant current source  24  bypasses the respective LED  26  to prevent that LED  26  from being illuminated while allowing the other LEDs  26 , whose respective bypass switches  28  are open, to still be illuminated. 
     As will be apparent, LEDs  26  need not be identical devices but should have similar forward current operating levels. In such a case, an appropriate current level is selected to be supplied by constant current source  24  and the selected current level will be provided to each operating LED  26  independent of the number of LEDs  26  which are operating. As bypass switches  28  are switched between open and closed positions, their respective LEDs  26  will correspondingly be illuminated or extinguished and yet each operating LED  26  will always be provided with the selected current level. 
     If an LED  26  should fail as an open circuit, which is the most common failure mode of an LED, its respective bypass switch  28  can be closed so that the current from constant current source  24  will still be provided to LEDs  26  whose bypass switches are open. Similarly, if it is desired to illuminate some of LEDs  26  and not others of LEDs  26 , the respective bypass switches  28  of the LEDs  26  which are to not be illuminated are closed, bypassing those non-illuminated LEDs  26 . 
     The design and/or selection of bypass switches  28  is not particularly limited and can comprise mechanical switches, relays and/or semiconductor switching devices. 
       FIG. 2  shows another embodiment of a power supply  60  in accordance with the present invention, wherein like components to those in  FIG. 1  are indicated with like reference numerals. In this embodiment, power supply  60  is equipped with bypass switches  64  which are electrically controllable, in this specific implementation MOSFET devices, that are controlled by a controller  68 , such as a microprocessor or microcontroller. 
     Controller  68  can operate bypass switches  64  to bypass one or more LEDs  26  to illuminate or extinguish LEDs  26  as desired. However, in addition to operating bypass switches  64  to bypass LEDs  26 , controller  68  can also perform a variety of other control functions on LEDs  26 . For example, controller  68  can use pulse width modulation (PWM) on the gate of one or more bypass switches  64  to control the light emitted by the respective LEDs  26 , thus dimming one or more of LEDs  26  as desired. 
     Further, controller  68  can verify correct operation of LEDs  26 . If an LED  26  has failed in an open circuit mode, as indicated by no current flow from current source  24 , then controller  68  can close each bypass switch  64 , in turn, until current flow occurs and the bypass switch  68  whose closing initiated the current flow will correspond to the failed LED  26 . Controller  68  can also turn off, or otherwise control, constant current source  24 . For example, controller  68  can turn off constant current source  24  when all of bypass switches  28  are closed to save energy. 
     If an LED  26  has failed in a short circuit mode, which is an uncommon failure mode for LEDs, controller  68  will monitor the change in the voltage across current source  24  as each bypass switch  64  is opened and closed in turn. As an LED  26  will have an expected voltage drop across it, controller  68  can detect an LED  26  which has suffered a short circuit failure by comparing the voltage across current source  24  when the respective bypass switch  64  is open to the voltage across current source  24  when the respective bypass switch  64  is closed. If the voltage does not change by a value approximately equal to the expected voltage drop across LED  26 , then controller  68  will determine that the respective LED  26  has failed. 
     When an open circuit or short circuit failure has been detected, controller  68  can output an appropriate signal  72 , indicating that one or more LEDs  26  has failed. Signal  72  can merely indicate that a failure has been determined, or it can indicate which respective LED  26 , or LEDs  26 , has failed. Signal  72  can be used in a variety of manners, as will be apparent to those of skill in the art, to provide a warning indicator to the operator of a vehicle that the lighting system may not be meeting regulatory requirements or indicating that the lighting system requires servicing and/or signal  72  can be provided to other devices such as other lighting systems which may then operate in another mode to compensate for the failure of the one or more LEDs  26 , etc. As will be apparent to those of skill in the art, the make up of signal  72  is not particularly limited and signal  72  can be an analog signal, a digital signal and/or a digital signal compatible with a communication bus used in a vehicle. In this later case, signal  72  can provide comprehensive information onto the bus, including which LED or LEDs  26  have failed, the amount of current being supplied by, and/or the voltage across, constant current source  24 , etc. 
     As is well known to those of skill in the art, the lifetime of a semiconductor light source, such as an LED  26 , is dependent upon the temperature of the semiconductor junction with higher temperatures resulting in decreased expected lifetimes. Accordingly, power supply  20  can be further equipped with one or more temperature sensors  76  which operate to provide an input to controller  68  indicating the temperature adjacent at least one LED  26 . Controller  68  can respond to the signals from sensors  76  to reduce the current supplied to LEDs  26  to inhibit or reduce damage to the semiconductor junction when high temperatures are detected. 
     Specifically, controller  68  can be responsive to a sensor  76  to reduce the current supplied from constant current source  24  to all LEDs  26 . Alternatively, if two or more sensors  76  are employed with power supply  20 , controller  68  can respond to each respective sensor  76  to pulse width modulate the respective bypass switch  64  to the respective LEDs  26  whose temperature is indicated by each respective sensor  76  to independently vary the average current supplied to the respective LEDs  26 . 
     As a power supply in accordance with the present invention can illuminate or extinguish individual LEDs  26  as desired, and as a power supply in accordance with the present invention can detect failures of LEDs  26 , another contemplated advantage of the present invention is that redundant LEDs  26  can be provided in a lighting system. These redundant LEDs  26  would not normally be illuminated but would be illuminated by the power supply if a failure of another LED  26  was detected. 
     It is contemplated that the present invention provides numerous other advantages. Power supplies in accordance with the present invention are generally easier to design than prior art LED power supplies and generally occupy less volume than comparable prior art power supplies, allowing the power supply to be located with the LEDs  26  and other lighting system components in a common housing. By locating the power supply in a common housing with LEDs  26 , the length of electrical leads from the power supply to the LEDs  26  is also generally reduced, reducing line losses in those leads and increasing the efficiency of the lighting system. 
     When used in vehicle lighting systems, such as vehicle headlamp systems, the cost and volumetric size advantages of the present invention are believed to be particularly desirable and the ability to easily detect failed semiconductor light sources and/or to illuminate redundant semiconductor light sources are particularly advantageous, as is the ability to dim semiconductor light sources by pulse width modulating the respective bypass switches. 
     While the description above only discusses having a bypass switch  64  for each LED  26 , it is contemplated that in some circumstances two or more series connected LEDs  26   a ,  26   b  can be provided as a sub-string with a single bypass switch  64 , as shown in  FIG. 3 . In such a case each series connected sub-string of LEDs  26  is treated logically as a single LED  26  by controller  68 , thus LEDs  26   a ,  26   b  are illuminated or extinguished as a set and a failure of either of LED  26   a  or LED  26   b  is treated as a failure of both LEDs  26   a ,  26   b  by power supply  60 . 
     To detect short circuit failures of one or more of LEDs  26   a ,  26   b  in a sub-string, controller  68  is programmed as to which bypass switches  64  are associated with sub-strings LEDs  26  as the expected voltage drop across a sub-string will generally be larger than the expected voltage drop across a single LED  26 . Then, when the above-described voltage drop test is performed, controller  68  monitors for an appropriate voltage level change for single LEDs  26  and an appropriate voltage level change for sub-strings of LEDs (e.g. LED  26   a  and  26   b ). 
     The present invention provides a novel and advantageous power supply for lighting systems employing semiconductor light sources. The semiconductor light sources are connected in series to a constant current source and a bypass switch is provided around each semiconductor light source, or each sub-string of series connected semiconductor light sources. By opening or closing respective bypass switches, individual semiconductor light sources or sub-strings of semiconductor light sources can be illuminated or extinguished as desired. If the bypass switches are electrically controllable, such as semiconductor switches or relays, failures of one or more semiconductor light sources can be determined by the power supply and failed light sources can be bypassed and/or redundant semiconductor elements illuminated to replace failed light sources. Further, if the bypass switches are semiconductor switches, the power supply can employ pulse width modulation techniques to dim one or more semiconductor light sources as desired. 
     The above-described embodiments of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art, without departing from the scope of the invention which is defined solely by the claims appended hereto.