Abstract:
LED lamp circuitry that emulates an incandescent lamp&#39;s behaviour upon remote verification of the LED lamp. The LED lamp circuitry presents an input power switch circuit, a fuse blow-out circuit and a cold filament detection circuit permitting the use of LED lamps in applications, such as railway signal light applications, where there is a need for remote monitoring of the lamps, while keeping the advantageous features of lower power consumption and longer life.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 09/996,300, filed on Nov. 28, 2001, which is a continuation-in-part of U.S. application Ser. No. 09/543,240 filed on Apr. 5, 2000, now abandoned. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to the electric power supply of light-emitting loads, in particular light-emitting diode (LED) lamps that require remote monitoring.  
         BACKGROUND OF THE INVENTION  
         [0003]    Light-emitting diode (LED) lamps are becoming more and more popular in automotive traffic lights, railway signal lights and other applications. Their lower power consumption is an attractive feature, but the main reason for their popularity is their long life (100 000 hours) compared to standard incandescent lamps (5 000 hours). Manifestly, these features allow important reduction in maintenance costs.  
           [0004]    In certain applications, such as railway signal lights, these lamps may be used, as those skilled in the art would know, for main line signalling and/or grade crossing signalling. Grade crossing signals are usually situated in populated areas such-as road intersections and redundant signals are used. Remote monitoring of the LED lamps in grade crossing signals is therefore not common. Main line signals, on the other hand, can be installed in remote areas, which are not easily accessible. Remote monitoring for checking the integrity of the lamps signals is therefore common practice in order to be able to downgrade the aspect in case of a detected signal failure.  
           [0005]    For lamps equipped with standard incandescent bulb, electrical integrity can be easily verified. If the filament of the incandescent bulb is in normal condition, current flows through the bulb according to Ohm&#39;s law (I=V/R). Otherwise, if the filament is open, no current flows through the bulb and it should be replaced.  
           [0006]    For LED lamps, however, LED current is controlled by a power supply. Current characteristics are therefore not identical in a LED lamp and in an incandescent lamp. In a LED lamp, alternative current (ac) line voltage is rectified and then converted to a suitable level by a dc-dc (direct current) converter, which also regulates LED current. In case of LED failure, or failure of any other electrical component in the LED lamp, it is possible for the power supply to continue drawing current at or near the nominal current value, even if the LED&#39;s are not emitting any light. Remote monitoring systems could therefore see the LED lamp as functioning correctly when in reality it is not. This situation is not acceptable since it can lead to very hazardous train operations and cause major accidents.  
           [0007]    Another problem, related to LED lamps and their power supplies and controllers, is caused by electric components which retain residual voltage differentials after power is removed from the LED lamp. The resulting characteristic is that a LED lamp will effectively light up when the power applied to it reaches a first high level while it will be turned off only when the power reaches a second lower level. The resulting problem is that if a certain power is induced by, for example, other nearby cables, the LED lamp could remain on while in fact it should be off. This could also lead to dangerous situations.  
           [0008]    These particularities of LED lamps limit their widespread use in situations where they need to be remotely monitored such as in railway main line signalling applications.  
         OBJECTS OF THE INVENTION  
         [0009]    An object of the present invention is therefore to allow LED lamps to become compatible with remote detection systems designed for monitoring of incandescent lamps.  
           [0010]    Another object of the invention is to provide LED lamp circuitry which will emulate an incandescent lamp&#39;s behavior upon remote monitoring of the LED lamp.  
           [0011]    Yet another object of the invention is to provide a control circuit for enabling/disabling the power supply to LED lamps in relation to the level of the line voltage.  
         SUMMARY OF THE INVENTION  
         [0012]    According to the present invention, there is provided a module for powering and monitoring a light-emitting diodes load by means of a power line, the module comprising:  
           [0013]    an input power switch circuit having an input connected to the power line for receiving power from the power line and monitoring a voltage magnitude thereof to enable and disable the module according to the voltage magnitude of the power line;  
           [0014]    a fuse blowout circuit having a fuse through which the input power switch circuit monitors the voltage magnitude of the power line, said fuse blowout circuit being adapted for blowing out said fuse to disable the input power switch circuit if no current flows through the light-emitting diodes load after a pre-determined time when the input power switch circuit is activated;  
           [0015]    a cold filament test circuit having an input connected to the power line for emulating an impedance of an incandescent light during a power stage set-up time during which no current is supplied to the light-emitting diodes load;  
           [0016]    a current detector circuit for detecting a current supplied to the light-emitting diodes load and for disabling the fuse blowout circuit and the cold filament test circuit when the current of the light-emitting diodes load reaches a predetermined current level; and  
           [0017]    a boost power stage circuit having an input connected to the power line and an output connected to the light-emitting diodes load for powering the light-emitting diodes load.  
           [0018]    Preferably, the module further comprises a serpentine trace connected in series with the fuse of the fuse blow out circuit for disabling the input power switch circuit upon physical damage to the serpentine trace.  
           [0019]    Preferably, the module comprises an input filter circuit connected between the power line and the input power switch for protecting the module.  
           [0020]    Preferably, a dummy load resistor is connected across the power line after the power switch circuit to cancel out a negative slope effect on an input impedance of the module.  
           [0021]    Preferably, the module further comprises a start-up circuit having a first input connected to the input filter circuit and a second input connected to the current detection circuit, and having an output connected to the boost power stage circuit for starting up the module.  
           [0022]    Preferably, the boost power stage circuit has an output capacitor and the module further comprises a quick-bleeder circuit having an input connected to the output capacitor for forcing the output capacitor to discharge at a faster rate through a shunt resistor when the module is turned off.  
           [0023]    The embodiments described herein present the advantage that they permit the use of LED lamps in applications, such as railway signal light applications, where there is a need for remote monitoring of the lamps, while keeping the advantageous features of lower power consumption and longer life.  
           [0024]    Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a schematic block diagram showing a LED module including a functional circuitry, a light source LED array, and a safety circuitry according to a preferred embodiment of the present invention.  
         [0026]    [0026]FIG. 2 is a functional block diagram of a LED module according to a preferred embodiment of the present invention.  
         [0027]    [0027]FIG. 3 is an electrical circuit diagram of a combined protected input filter circuit and input power switch circuit according to a preferred embodiment of the present invention.  
         [0028]    [0028]FIG. 4 is an electrical circuit diagram of a LED current detection circuit shown in FIG. 2 according to a preferred embodiment of the present invention.  
         [0029]    [0029]FIG. 5 is an electrical circuit diagram of a time delay FBO (Fuse Blow Out) circuit shown in FIG. 2 according to a preferred embodiment of the present invention.  
         [0030]    [0030]FIG. 6 is an electrical circuit diagram of a cold filament test circuit shown in FIG. 2 according to a preferred embodiment of the present invention.  
         [0031]    [0031]FIG. 7 is an electrical circuit diagram of a boost converter start-up circuit shown in FIG. 2 according to a preferred embodiment of the present invention.  
         [0032]    [0032]FIG. 8 is an electrical circuit diagram of a bleeder circuit shown in FIG. 2 according to a preferred embodiment of the present invention.  
         [0033]    [0033]FIG. 9 is an electrical circuit diagram of a power stage circuit shown in FIG. 2 according to a preferred embodiment of the present invention.  
         [0034]    [0034]FIG. 10 is a top view of a power supply unit with a serpentine trace for detecting physical damage thereof. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    Referring to FIG. 1, a dc (direct current) line voltage is supplied to an LED (light-emitting diodes) module  1  via line  11 . The LED module  1  consists of a functional circuitry  10 , a PCB (printed circuit board) LED light source array  12  and a safety circuitry  14 .  
         [0036]    The functional circuitry  10  includes an input power switch circuit  22  (shown in FIG. 2) that typically converts a +10 Vdc input voltage to an 100 mA output constant current for the Red, White and Yellow LEDs, and 60 mA for the Green LEDs of the LED light source array  12 .  
         [0037]    The safety circuitry  14  includes a fuse blow out circuit  30  and a LED current detector circuit  38  (shown in FIG. 2) that monitors the LED&#39;s current and turns off permanently the input power switch circuit  22  (see FIG. 2) by blowing the FBO fuse when the LEDs current is typically below 20% of its nominal value.  
         [0038]    The PCB LED light source array  12  may be, for example, a matrix of high-brightness 5 mm LEDs configured for redundancy. As will be described further below, the current flowing in the LEDs is regulated by a PSU&#39;s (Power Supply Unit) feedback loop providing constant light flow. The LEDs preferably form a pattern made of 4 columns (one group of 4 LEDs connected in parallel) by 22 rows (22 groups connected in series) for the Red LEDs, 4×33 for the Yellow LEDs and 6×15 for the Green and White LEDs. In case of an LED failure in a group over the course of operation, the current is redistributed to the other LEDs of the same group and the signal maintains its light output. The LEDs are also more generally referred to in the present specification as light-emitting diode loads. Various embodiments of LED arrays can be used. These embodiments are well known to those of ordinary skill in the art and, accordingly, will not be further described in the present specification.  
         [0039]    Referring now to FIG. 2, the LED module  1  may be made of 3 physical parts: the PCB LED array  12 , a dummy load  16  and a PCB PSU (power supply unit)  18 .  
         [0040]    Dummy Load  
         [0041]    The input line current is monitored by the system LOD (light out detection) function that consists to check if the lamp is functional or not. In a preferred embodiment, the module  1  detects a Light Out if the input current is below a predetermined value.  
         [0042]    The PSU  18  regulates the LEDs current in order to maintain constant light intensity. The power stage circuit  20  provides output constant power and assuming that the internal losses are almost constant for different input voltage conditions, it could be assumed that the input power delivered to the PSU  18  is constant. Having a constant input power, the line current amplitude is higher at 8Vdc and lower at 16Vdc. In terms of input impedance, the PSU  18  has a negative slope resistance.  
         [0043]    A dummy load resistor  16  may be added across the input line to cancel out the negative slope effect of the PSU&#39;s input impedance. The input power switch circuit  22  isolates the dummy load when the PSU  18  is off.  
         [0044]    PSU PCB  
         [0045]    The +10Vdc input line voltage is fed to the PSU PCB  18  via the connector J 3 . The connector J 3  provides also an interface connection to feed the +10Vdc to the dummy load resistor  16  when the power switch circuit  22  turns on. The PSU&#39;s power stage circuit  20  converts the +10Vdc to a constant current that flows in the LEDs  12  via the wiring cable  24  connected to connector J 1  and the LED array PCB connector  26 .  
         [0046]    As shown in FIG. 2, the PSU  18  provides the following functions that will be described below:  
         [0047]    protected input filter circuit  28 ;  
         [0048]    input power switch circuit  22 ;  
         [0049]    fuse blow out (FBO) circuit  30 ;  
         [0050]    cold filament test (CFT) circuit  32 ;  
         [0051]    start-up circuit  34 ;  
         [0052]    power stage circuit  20 ;  
         [0053]    bleeder circuit  36 ;  
         [0054]    LED current detection circuit  38 ;  
         [0055]    Connectors  
         [0056]    The connector J 3  is a 4 circuits connector that is used to mate the +10Vdc voltage source and the dummy load wires with AWG  16  wires, as shown in FIG. 3. The connectors J 2  and J 4  that are illustrated in FIG. 3 are used only for testing the PSU  18  during the manufacturing process to verify the main functions of the PSU  18 .  
         [0057]    Protected Input Filter Circuit  
         [0058]    Referring to FIGS. 2 and 3, the protected input filter circuit  28  provides protection against the PSU&#39;s internal overload, input voltage reverse polarity and line voltage surges. The protected input filter circuit  28  filters the switching frequency of the power stage input current in order to meet FCC conducted and radiated FCC Class A EMC.  
         [0059]    Referring to FIG. 3, the fuse F 1  provides protection against overload greater than 2A. The power supply has a constant output current and that condition will occur only when a component fails short as described above.  
         [0060]    The diode D 1  provides protection against reverse polarity connection. The diode D 1  may be a MUR420 diode having a current rating of 4A and can handle the input line current that can vary between 1.2 and 2A.  
         [0061]    The PSU  18  may withstand a surge of 1000 volts 1.2/50 μs open circuit voltage and a 8/20 μs short circuit current surge having a source impedance of 2 ohms. The varistor V 1  clamps V IN  to 170V when subjected to these threats.  
         [0062]    The switching frequency of the power stage input current is filtered by L 1  and C 1 . Measurements of the conducted and radiated emission show that the EMC specifications are met.  
         [0063]    Input Power Switch Circuit  
         [0064]    Railroads safety issue requires a circuit to control the turn-on and turn-off of the LED module  1 . The implementation of the input power switch circuit  22  of the PSU  18  provides such protection against out of range low input voltage.  
         [0065]    The input power switch circuit  22  has a turn-on feature that monitors the input line voltage. The specifications typically require to turn on the light signal at 8Vdc and to turn it off at 4Vdc. The input power switch circuit  22  is therefore designed to turn on when the input line voltage exceeds 7Vdc and turns off below 5.5Vdc providing sufficient margins.  
         [0066]    Referring to FIG. 3, there is shown a combined protected input filter and input power switch circuit. The input power switch circuit  22  shown in FIG. 2 is linked to the input voltage by a 125 mA fuse F 70  that is shown in FIG. 3. The fuse F 70  blows when a FBO (fuse blow out) command is enabled at line F 2 . That way the PSU  18  will turn off and the CFT (cold filament test) circuit  32  will detect a failure by the system&#39;s controller as will be explained further below.  
         [0067]    Also, to make sure that upon physical damage of the signal (by bullet or other impact) the input switch is kept off, a serpentine trace  42  (shown in FIG. 10) is added in series with fuse F 70  all around the PSU  18 . This trace occupies a complete layer of a multi-layer PCB so that if a bullet penetrates the power supply PCB  18  or if the power supply&#39;s PCB  18  is damaged, the trace  42  opens. This is equivalent as having the fuse F 70  blown and ensures detection of a dark signal in case of physical damage.  
         [0068]    Referring to FIG. 3, the function of diode D 70  is to prevent capacitor C 70  from discharging when the FBO command is activated at line F 2 . This occurs when fuse F 70  is shorted to ground. The energy bank of capacitor C 70  keeps mosfets Q 70  and Q 71  on long enough to blow fuse F 70  when the FBO circuit  30  is activated. The resistor R 70  provides the adequate time constant with capacitor C 70  to allow the FBO circuit  30  to open fuse F 70  when required. Furthermore, the resistor R 70  limits the inrush current through fuse F 70  at turn-on.  
         [0069]    The mosfets Q 70  and Q 71  which act as a power switch provide the function of a solid state switch that isolates the power stage circuit  20  when the input voltage is below the input voltage range. The mosfets Q 70  and Q 71  turn on when the voltage at line  3  of comparator U 70 A reaches 1.225V and turns off when it is below it. Diode D 71  is a 1.225V high precision voltage reference diode that is stable under temperature variations. Resistor R 73  limits the bias current of diode D 71 . Resistors R 71  and R 72  form the voltage divider that reduces down the input voltage to be compared to the voltage reference. The comparators U 70 A and U 70 B combined with the hysteresis resistor R 74  provide noise immunity against false triggering signals. Diode D 75  forces line  1  of comparator U 70 A to LOW when comparator U 70 B reacts faster than comparator U 70 A. Line  7  of comparator U 70 B provides the interface command of the mosfets Q 70  and Q 71  acting as the power switch.  
         [0070]    Diodes D 71 , D 72 , D 73  and D 74  provide immunity against the varistor V 1  clamped voltage lightning surge. Resistor R 77  limits the current when input line voltage surge occurs.  
         [0071]    LED Current Detection Circuit  
         [0072]    Referring to FIG. 4, the LED current detection circuit  38  disables the FBO, CFT and start-up circuits  30 ,  32 ,  34  when the LED current exceeds 20% of its nominal value. If the LED current does not reaches 20% of I NOM  within 300 ms then the FBO circuit  30  blows out F 70  and the PSU  18  turns off.  
         [0073]    In the current detection circuit  38 , the voltage sense V S  (the voltage across the current sense resistor) is compared to a reference voltage. In normal operation, voltage sense V S  is regulated at 2.5V and the reference voltage is set at 17% of the nominal value. The 4.7V zener diode D 53  is biased by resistor R 57  from voltage V CC  to provide voltage V REF  and the voltage divider resistors R 58  and R 59  reduce voltage V REF  to 0.43V or 17% of nominal current I NOM  providing a margin of 3%. Voltage sense V S  is applied at line  6  of comparator U 50 B (inverted input) and the 0.45V reference voltage at line  5  of comparator U 50 B (non-inverted input). At turn-on, voltage sense V S  is 0V and the comparator output at line  7  of comparator U 50 B- 7  is floating (LM2903 is an open collector comparator) which enable the FBO, CFT and start-up circuits  30 ,  32 ,  34  to operate. Typically after 50 ms, voltage sense V S  reaches 0.43V and line  7  of comparator U 50 B is shorted to ground to disable the FBO, CFT and start-up circuits  30 ,  32 ,  34 . The time taken by voltage sense V S  to reach 0.43V depends directly to the input line voltage amplitude, the amount of LEDs in series and the forward voltage of the LEDs.  
         [0074]    Fuse Blow Out (FBO) Circuit  
         [0075]    Referring to FIG. 5, the fuse blow out (FBO) circuit  30  forces the fuse F 70  to blow out when the LED current is lower than 20% of its nominal value. If that condition occurs, the link between voltage V IN  and the input power switch circuit  22  is permanently opened, as the mosfets Q 70  and Q 71  open and the PSU  18  turns off. The LED module  1  will then be unusable anymore and the system&#39;s CFT (cold filament test) circuit  32  detects a failure.  
         [0076]    A time delay circuit  40  has been implemented in order to provide enough time to the PSU  18  to turn on (100 to 170 ms) and sufficiently short to blow the fuse F 70  in a flashing mode (330 ms). The time delay is obtained from the time constant given by resistors R 50 , R 51  and capacitor C 50 . Capacitor C 50  (1 uF) charges through resistor R 50  (523 k) up to half V REF  (2.4V) and is fed to line  3  of comparator U 50 A via resistor R 53 . At turn-off, resistor R 51  provides a path to ground to discharge capacitor C 50 . In order to minimize the offset voltage of the comparator U 50 A, the resistance value of resistor R 52  matches the input impedance at line  3  of comparator U 50 A (parallel combination of resistors R 53  and R 54 ). Resistors R 53  and R 54  provide the comparator threshold voltage, at line  2  of comparator U 50 A, which matches 63% of half V REF  (1.5V). Capacitor C 50  being 1 μF, the time delay is easily computed by dividing the value of resistor R 53  by 2 where the result is in milliseconds (1 uF×523 k/2=262 ms).  
         [0077]    At turn-on, capacitor C 50  charges only during 50 ms, typically, and is clamped by diode D 50  to ground by line  7  of comparator U 50 B when 20% of LED current I LED  is reached, as described above with regard to the Led current detection circuit  38 . The clamping voltage is about 0.5V at 25° C. and will vary at hot and cold temperature. In case of a failure occurrence, where line  7  of comparator U 50 B is floating after turn-on, then capacitor C 50  starts charging from 0.5V toward 2.4V and reaches a 1.5V comparator threshold voltage faster but this does not cause any concern. Line  1  of comparator U 50 A becomes floating when capacitor C 50  charges above 1.5V, voltage V CC  is applied to the gate of the power mosfet Q 50  via resistor R 55 , mosfet Q 50  saturates pulling to ground diode D 55 , and the +10Vdc input voltage appears across fuse F 70  and fuse F 70  blows out. In normal operation, line  7  of comparator U 50 B is shorted to ground, line  1  of comparator U 50 A maintains the mosfet&#39;s Q 50  gate to ground and the FBO command is disabled. Diode D 54  limits the gate-source voltage of mosfet Q 50  below its maximum limit of 20V. The purpose of diode D 55  is to isolate fuse F 70  from voltage V CC  when the FBO circuit  30  is enabled.  
         [0078]    Cold Filament Test (CFT) Circuit  
         [0079]    Originally, the Cold Filament Test (CFT) has been incorporated to verify if the filament of the incandescent lamp is open or not. The system controller supplies the lamp for 2 ms and checks the lamp current. Of course, 2 ms is too short for an incandescent lamp to radiate light and is sufficient to validate its status. The same test may be performed on the LED module  1  to check it.  
         [0080]    When the system controller applies the input voltage to the PSU  18 , the input power switch circuit  22  turns on and capacitor C 1  starts to charge up. The voltage across capacitor C 1 , V FL , is applied directly to the gate of mosfet Q 60  via R 60  (see FIG. 6). Typically, mosfet Q 60  starts to conduct when V FL  reaches 4.2V. V FL  rises up to the +10Vdc input line voltage. Mosfet Q 60  saturates and connects resistors R 61  and R 62  to ground providing 7.5 ohms across the +10Vdc input line voltage. The system controller starts monitoring the LED module&#39;s input current after the application of the input voltage and the current must be greater than a pre-determined value, otherwise the test fails. The load current of the CFT circuit  32  combined with the dummy load current and the inrush current of capacitor C 1  during turn-on provides the necessary current at 8Vdc. Diode D 60  limits the gate-source voltage of mosfet Q 60  below its maximum limit of 20V.  
         [0081]    In normal operation during turn-on, the CFT circuit  32  stays enabled until 20% of the LED current is reached. Then, line  7  of comparator U 50 B (see FIG. 4) goes low and the gate of mosfet Q 60  is kept below the gate threshold voltage via diode D 52  disabling the CFT circuit  32 .  
         [0082]    Start-Up Circuit  
         [0083]    Referring to FIG. 7, the start-up circuit  34  that is shown in FIG. 2 is a switch-mode boost converter that uses the voltage across capacitor C 1 , V FL , (shown in FIG. 3) to generate voltage V CC . The duty cycle is constant and set to get an output voltage of 15V for an input voltage of 7V. The Pulse Width Modulator (PWM), U 1  (shown in FIG. 9), needs 15V to start up. The start-up circuit  34  stays enabled until 20% of the LED current is reached. The start-up circuit stops feeding V CC  and lines  6  and  10  of transformer T 1  start feeding V CC  via resistor R 49  and diode D 5  (shown in FIG. 9).  
         [0084]    The boost converter is fed from V FL  and is made of inductor L 30 , mosfet Q 30 , diode D 31  and capacitor C 3 . Inductor L 30  builds energy in its core when mosfet Q 30  is ON and inductor L 30  transfers its energy to capacitor C 3  via diode D 31  when mosfet Q 30  is OFF. Mosfet Q 30  is driven at a constant rate of 50% provided by timer circuit U 30  and the voltage at capacitor C 3  is about twice V FL . Line  3  of timer circuit U 30 , SE555CN Timer, works in the a stable mode where the duty cycle is set by resistors R 33 , R 34  and capacitor C 32 . The supply voltage at line  8  of timer circuit U 30  is limited to 14V by diode D 32 . Voltage V FL  could reach 36V for 80 ms. Resistor R 31  is the bias resistor of diode D 32 . Capacitor C 31  is a high frequency bypass capacitor used to filter the control voltage at line  5  of timer circuit U 30 . The reset at line  4  of timer circuit U 30  is kept high by the pull-up resistor R 32  to ensure the operation at line  3  of timer circuit U 30 . The start-up circuit  34  stays enabled until 20% of the LED current is reached. Then, line  7  of comparator U 50 B (shown in FIG. 4) goes low pulling down to ground the reset pin at line  4  of timer circuit U 30  to disable line  3  of timer circuit U 30 .  
         [0085]    Quick-Bleeder Circuit  
         [0086]    Referring to FIG. 8, the purpose of the quick-bleeder circuit  36  (also shown in FIG. 2) is to turn off faster the LED module  1 . The bleeder circuit  36  uses a peak voltage detector to monitor the switching waveform voltage of transformer T 1 . At turn-off, the switching waveform voltage disappears and a 1 Kohm resistor R 1  is shunted across the output capacitor C 7  to force capacitor C 7  to discharge faster.  
         [0087]    The auxiliary voltage, V AUX , is a square waveform that is used to feed V CC  via diode D 5  (shown in FIG. 7). Capacitor C 6  charges up to V AUX  via resistor R 49  and diode D 8 . Diode D 8  prevents capacitor C 6  from discharging when V AUX  is 0V. Capacitor C 6  discharges slowly through resistor R 17  and transistor Q 5 , based on a time constant established by capacitor C 6  and resistor R 17 . Capacitor C 6  recharges at the beginning of each cycle of V AUX . The saturation of transistor Q 5  is maintained as long as the Voltage across capacitor C 6  is sufficient to drive the base current such as the forced hFE is greater than 15 (forced hFE=Ic/Ib). The collector of transistor Q 5  forces the gate of transistor Q 4  to ground thus keeping transistor Q 4  OFF.  
         [0088]    The LED module turn-off command occurs when the system controller removes the +10Vdc from the input voltage line. The input power switch circuit  22  turns off and the switching waveform voltage V AUX  stops when the energy of the input filter made of inductor L 1  and capacitor C 1  vanishes. Capacitor C 6  stops recharging and discharges slowly toward 0V at a time rate of 100 μs. After 500 uμs, transistor Q 5  turns off, the gate of transistor Q 4  charges up to 14V, limited by diode D 9 , via resistor R 16 . Transistor Q 4  turns on when V GS  exceeds 4.2V and resistor R 1  is pulled down to ground. Capacitor C 7  discharges through the LEDs and resistor R 1 . Without the use of the bleeder resistor R 1 , capacitor C 7  would discharge at a constant rate established by the characteristic V F −I F  of the LEDs down to V F  minimum. At V F  minimum, I F  is very small and capacitor C 7  would discharge even slower. The resultant would be that the LEDs would still emit light that would be detected by the eyes. Resistor R 1  will force capacitor C 7  discharging down to 0V in a short period of time.  
         [0089]    Boost Power Stage Circuit  
         [0090]    Referring to FIG. 9, the boost power stage circuit  20  that is shown in FIG. 2 is a switch-mode converter that transforms the +10Vdc voltage across capacitor C 1 , V FL , to a constant output DC current to feed the LEDs. That way the LEDs emit constant light. A boost converter topology is used since the resultant voltage across the LEDs is 57V for 22 Red LEDs, 75V for 33 Yellow LEDs and 52V for 15 Green LEDs.  
         [0091]    The Pulse Width Modulator, U 1 , starts up when V CC  exceeds 15V. The power stage is fed from V FL  and is made of transformer T 1  (primary winding inductance at lines  1  and  5 ), mosfet Q 1 , diode D 7 , and capacitor C 7 . Transformer T 1  (at lines  1  and  5 ) builds energy in its core when mosfet Q 1  is ON and that energy is transferred to capacitor C 7  via diode D 7  when mosfet Q 1  is OFF. Mosfet Q 1  is driven by line  7  of PWM U 1  where resistor R 8  limits the turn-on gate current.  
         [0092]    The Pulse Width Modulator, U 1 , (MC33262) does not have an oscillator but the operation frequency is determined by the power stage. The power stage is a peak detector current-mode boost converter that operates in critical conduction mode at a fixed on-time and variable off-time. The critical conduction mode is the boundary limit between the continuous and the discontinuous conduction mode of the power inductor current leading to stable current loop without the need of slope compensation. There is no switching loss at turn-on when using the critical mode.  
         [0093]    The off-time is determined when transformer T 1  is completely discharged. The voltage at transformer T 1  (lines  10  and  6 ), V AUX , is fed to line  5  of PWM U 1  via resistor R 5 . When the voltage at line  5  of PWM U 1  goes below 1.5V, PWM U 1  resets the drive output at line  7  of PWM U 1  and mosfet Q 1  turns on. The switching power stage current is sensed by the parallel combination of resistors R 7  and R 9 .  
         [0094]    The on-time ends when the boost inductor current reaches a determined peak value. The boost inductor current is sensed by resistors R 7  and R 9 . The resultant sensed voltage is filtered by resistor R 6  and capacitor C 5  and fed to line  4  of PWM U 1 . The voltage at line  4  of PWM U 1  is compared to a voltage reference established by the product combination of the voltage at lines  2  and  3  of PWM U 1 . The power mosfet Q 1  turns off when the voltage at U 1 - 4  exceeds the voltage reference. The voltage at U 1 - 3  is proportional to the input voltage V FL  determined by the voltage divider made of resistors R 2  and R 3  thus allowing feedforward compensation for the input voltage variations. The voltage across the LEDs current sense resistor is fed to line  1  of PWM U 1  and internally inverted. That feedback voltage is available at line  2  of PWM U 1  where capacitor C 4  is used to compensate the loop. The LEDs current being constant, the peak current of transitor T 1  at lines  1  and  5  is directly proportional to the input voltage and the on-time remains constant.  
         [0095]    Capacitor C 2  is a high frequency bypass capacitor used to filter the feedforward voltage at line  3  of PWM U 1 . Diode D 10  clamps the voltage at −0.2V to prevent false triggering.  
         [0096]    The power stage provides the feature to select the LEDs current using a shunt with S 1 . The current selection is: 40 mA, 60 mA, 80 mA, 100mA and 120 mA. Current sense resistors R 40 , R 41 , R 43 -R 47  are used to set the LEDs current at the predetermined value shown above. In normal operation, the voltage is regulated to 2.5V at line  1  of PWM U 1  and the current value is obtained by dividing 2.5V by the current sense resistor. Resistor R 42  and capacitor C 8  is a low pass filter to attenuate the switching ripple across capacitor C 7 .  
         [0097]    Although the present disclosure describes particular types of transistors in the different circuits shown in the Figures, it should be kept in mind that these different types of transistors can be substituted or replaced by other available types of transistors.  
         [0098]    Although preferred embodiments of the present invention have been described in detail herein and illustrated in the accompanying drawings, it is to be understood that the present invention is not limited to this precise embodiment and that various changes and modifications may be effected therein without departing from the scope or spirit of the present invention.