Patent Publication Number: US-8525425-B1

Title: LED lighting system

Description:
This application claims priority to U.S. provisional application 61/425,492, filed Dec. 21, 2010, the contents of which are hereby incorporated by reference. 
    
    
     FIELD 
     The present invention relates generally to lighting systems, in particular to lighting systems utilizing light emitting diodes. 
     BACKGROUND 
     Although light emitting diodes (LEDs) promise long operating life, their static-sensitive nature makes them susceptible to lightning-induced failures. This becomes a significant reliability issue for LEDs used in obstruction warning lights, which may be struck by lightning up to ten or more times a year. 
     Another reliability concern with respect to LEDs arises when they are electrically connected together in a series network. From an engineering standpoint electrically connecting LEDs in a series network string is desirable since all of the LEDs in the network have the same operating current, thus providing relatively uniform brightness throughout the string of LEDs. One disadvantage, however, is that if just one LED fails open-circuit due to lightning damage, a broken bond wire, a cold solder joint or a bad connection, for example, all of the remaining LEDs in the string will turn off even if they are in operable condition. To overcome this drawback, a “bypass shunt” device such as a zener diode, silicon-controlled rectifier (SCR) or “anti-fuse” is sometimes used in parallel with each LED. Accordingly, if an LED fails open-circuit, a resultant rise in voltage across electrical terminals of the failed LED turns on the bypass shunt device, thereby routing electrical current around the open circuit so that the remaining LEDs in the string that are in operable condition will illuminate. 
     An example bypass shunt arrangement is shown in  FIG. 1 . A string or network  10  comprises a plurality of LEDs  12  that are electrically connected in series and are powered by an electrical power supply  14  connected in parallel with the network. Each LED  12  includes a zener diode  16  bypass shunt connected in parallel therewith, the zener diode being reverse-biased with respect to power supply  14 . Zener diodes  16  are each configured to have a reverse breakdown voltage that is slightly greater than the forward voltage of a corresponding LED  12 , so the zener diode normally remains in a non-conducting or “off” state. However, if an LED  12  fails in an open-circuit state a voltage greater than the reverse breakdown voltage rating of the associated parallel-connected zener diode  16  is present at the terminals of the zener diode, causing it to begin conducting (i.e., switch to an “on” state) so that current supplied by the power supply is maintained in LED series network  10 . 
     A drawback of this arrangement is that, in its conducting state, the electrical power dissipated by zener diode  16  is higher than that of the operational, unshunted LEDs  12 . Consequently, heat dissipation considerations must be made for an electronic circuit assembly containing zener diodes  16 , such as a printed wiring board assembly, taking into account the potential for a plurality of zener diodes being in a conducting state and dissipating heat at any given time. In addition, zener diodes  16  are physically relatively large devices and thus typically require a significant amount of space on the aforementioned electronic assembly. 
     With reference to  FIG. 2 , a more complex string or network  20  comprises a plurality of LEDs  12  electrically connected in series and powered by power supply  14 , which is connected in parallel with the network. Each LED  12  includes a silicon controlled rectifier (SCR)  22  bypass shunt connected in parallel therewith. A voltage-sense circuit such as a trigger zener diode  24  or, alternatively, a resistive divider network (not shown) is configured to sense a voltage increase at the electrical terminals of an associated LED  12  when the LED fails to an open-circuit condition and trigger the corresponding SCR to a latched, conducting state. When the triggered SCR  22  is thus latched in its conducting state the voltage drop across its electrical terminals is much lower than the voltage drop of zener diode  16  of  FIG. 1  (typically on the order of about 0.8-1.0 Volts) so there is relatively little heat dissipation, even for conditions where somewhat high currents are present in LED string  20 . To unlatch the triggered SCR  22  and return it to its non-conducting state the current of LED string  20  must be reduced to a level below the rated holding current for the SCR. Alternatively, the power supplied to LED string  20  by power supply  14  may be momentarily interrupted to return SCR  22  to its non-conducting state. Unlatching a triggered SCR  22  may desirable for situations where an associated LED  12  autonomously resolves its fault, thereby allowing the LED to illuminate. 
     With reference to  FIG. 3  a string or network  30  comprises a plurality of LEDs  12  electrically connected in series and powered by power supply  14 , which is connected in parallel with the network. Each LED  12  includes an “anti-fuse” device  32  connected in parallel therewith. Anti-fuse  32  is available from, for example, Murata Electronics North America of Smyrna, Ga. Anti-fuse  32  changes from an off-state resistance of several megohms to an “on” or conducting state having a resistance of a few ohms when the voltage at terminals of the anti-fuse exceeds a predetermined level. Like SCR  22  of  FIG. 2 , anti-fuse  32  dissipates relatively little power when in a conducting state. However, its relatively small size limits its current-handling capability. In addition, unlike zener diode  16  of  FIG. 1  and SCR  22  of  FIG. 2 , the resistance change of anti-fuse  32  is permanent once placed into a conducting state. Thus, even if a failed LED  12  autonomously resolves its fault the LED will remain bypassed by the associated anti-fuse  32 . Furthermore, if an open-circuit failed LED  12  must be replaced, the corresponding anti-fuse  32  shunt must also be replaced, increasing maintenance labor and component expense. 
     As can be appreciated from the foregoing, although the present art has made some advances in the protection of LEDs in order to increase the overall reliability of LED lighting systems in which they are installed, there remains a need to better protect LEDs that are subject to high voltages due to electrostatic discharge and lightning strikes. This need is particularly great for LEDs that are remotely located or are otherwise relatively inaccessible, such as LEDs used in obstruction lighting. 
     SUMMARY 
     The present invention provides a means for protecting a plurality of LEDs while also improving LED lighting system fault tolerance. In addition, accurate and robust monitoring circuitry may be provided to detect and identify failed LEDs. 
     In one embodiment of the present invention a simplified interconnection of LEDs results in a reduction in the complexity of associated monitoring circuitry. LEDs are also protected from damage by their arrangement with respect to each other. In addition, shunt bypass devices may be employed to provide active protection during fast-rising lightning pulses, even with power removed from the LEDs. 
     An object of the present invention is an LED lighting system. The lighting system has an LED circuit that includes a first LED having an anode and a cathode, and a second LED having an anode and a cathode. The anode of the second LED is electrically coupled to the cathode of the first LED, and the cathode of the second LED is electrically coupled to the anode of the first LED. The first and second LEDs are in an inverse-parallel arrangement, the first LED acting as a reverse-voltage clamp for the second LED and the second LED acting as a reverse-voltage clamp for the first LED. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features of the inventive embodiments will become apparent to those skilled in the art to which the embodiments relate from reading the specification and claims with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a prior art LED fault-bypass shunt circuit utilizing zener diodes; 
         FIG. 2  is a schematic diagram of a prior art LED fault-bypass shunt circuit utilizing silicon controlled rectifiers; 
         FIG. 3  is a schematic diagram of a prior art LED fault-bypass shunt circuit utilizing anti-fuses; 
         FIG. 4  is a schematic diagram of an LED circuit utilizing LEDs in an inverse-parallel arrangement according to an embodiment of the present invention; 
         FIG. 5  is a schematic diagram of an LED circuit utilizing zenered LEDs according to another embodiment of the present invention; 
         FIG. 6  is a schematic diagram of an LED circuit utilizing the arrangement of  FIG. 4  in combination with a triac LED fault-shunt bypass circuit according to yet another embodiment of the present invention; 
         FIG. 7  is a schematic diagram of two selectable LED banks in a prior art LED system utilizing silicon controlled rectifiers; 
         FIG. 8  is a schematic diagram of two selectable LED banks in an LED lighting system utilizing shared triacs and connections according to another embodiment of the present invention; 
         FIG. 9  is a schematic diagram of two selectable LED banks with monitoring capability in a prior art LED lighting system; and 
         FIG. 10  is a schematic diagram of two selectable LED banks with monitoring capability in an LED lighting system according to still another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the discussion that follows, like reference numerals are used to refer to like elements and structures in the various figures. 
     Referring now to  FIG. 4 , the general arrangement of an LED network  100  is shown according to a preferred embodiment of the present invention. Network  100  comprises an LED circuit  102  that includes a paired arrangement of two LEDs  104 . A first and a second LED  104 - 1 ,  104 - 2  respectively, each have an anode and a cathode. The anode of the second LED  104 - 2  is electrically coupled to the cathode of the first LED  104 - 1 , the cathode of the second LED being electrically coupled to the anode of the first LED. The first and second LEDs  104 - 1 ,  104 - 2  respectively are in an inverse-parallel arrangement, and may be wired in a series-network arrangement with additional LED circuits  102  as shown in  FIG. 4 . 
     In the arrangement of  FIG. 4  the forward-voltage drop (typically on the order of about 1.8-3.8 volts) of each LED  104  is less than its reverse-voltage rating (typically on the order of about 5 volts). Accordingly, in LED circuit  102  each LED  104  of the pair acts as a reverse-voltage clamp for the other, regardless of the polarity of the voltage applied to the LED circuit by power supply  14 , since one of the pair of LEDs will always be forward-biased. This reduces the risk of damage to the LEDs  104  due to exposure to electrostatic discharge (ESD) and lightning strikes. 
     Some LEDs do not have a reverse voltage rating, but may include a zener diode internal or external to the package of the LED and electrically coupled in parallel with the LED. Although typically not capable of open-circuit shunting, an internal zener diode does provide some ESD protection. The general arrangement of an LED network  200  having LED packages  202 - 1  and  202 - 2  with internal zener diodes is shown in  FIG. 5 . Each LED package  202  includes an LED  204  and a zener diode  206 . 
     With continued reference to  FIG. 5 , an LED circuit  201  includes a first zener diode  206 - 1  that is electrically coupled to a first LED  204 - 1 , the anode of the first zener diode being electrically coupled to the cathode of the first LED and the cathode of the first zener diode being electrically coupled to the anode of the first LED. Similarly, a second zener diode,  206 - 2 , is electrically coupled to a second LED,  204 - 2 , the anode of the second zener diode being electrically coupled to the cathode of the second LED and the cathode of the second zener diode being electrically coupled to the anode of the second LED. A cathode of a first blocking diode  208 - 1  is electrically coupled to the anode of the first LED  204 . An anode of a second blocking diode  208 - 2  is electrically coupled to the cathode of the second LED  204 - 2 , while the cathode of the second blocking diode is electrically coupled to the anode of the first blocking diode  208 . LED circuit  201  may be wired in a series-network arrangement with additional LED circuits  201 , as shown in  FIG. 5 . 
     In the network  200  of  FIG. 5  blocking diode  208  is added to each LED  204  to prevent current from flowing though the zener diode  206  of the other LED in the inverse parallel arrangement of network  200 . Without blocking diodes  208 , neither of the LEDs  204  in the inverse parallel arrangement of network  200  would light because the forward voltage of zener diodes  206  is much lower than that of the LEDs&#39; forward voltage. Other arrangements are possible within the scope of the invention such as, but not limited to, using integrated multi-diode packages to conserve space. 
     By combining the inverse-parallel LED arrangement of  FIG. 4  with a bidirectional shunt device, the number of shunt devices may be effectively halved.  FIG. 6  shows an LED network  300  utilizing a triac  302  and a low-voltage transient voltage suppressor (TVS) diode  304  connected across each LED circuit  102 . 
     An LED circuit  301  includes an LED circuit  102  having a first and a second LED  104  wired in an inverse-parallel arrangement in the manner previously described in  FIG. 4 . A triac  302  has a first triac anode, a second triac anode, and a gate, the first triac anode being electrically coupled to the cathode of first LED  104 - 1  and the second triac anode being electrically coupled to the anode of the first LED. A transient voltage suppressor  304  has a first suppressor anode and a second suppressor anode, the first suppressor anode being electrically coupled to the first triac anode of triac  302  and the second suppressor anode being electrically coupled to the gate of the triac. Triac  302  is latched to a conducting state in the event first LED  104 - 1  fails to an open electrical circuit while in a conducting (i.e., operating) state. Likewise, triac  302  is latched to a conducting state in the event second LED  104 - 2  fails to an open electrical circuit while in a conducting (i.e., operating) state. LED circuit  301  may be wired in a series-network arrangement with additional LED circuits, as shown in  FIG. 6 . 
     As also shown in  FIG. 6 , the addition of an optional gate resistor  306  may be desirable for LED circuits  300  having higher LED currents, although gate dissipation is eliminated by the turn on of the triac  302 . Alternatively, a resistive divider  308  comprising a pair of resistors  310 ,  312  ( FIG. 6 ) may be used in place of TVS diode  304 , with or without gate resistor  306 , if voltage threshold variability is not a concern. 
     In operation of LED circuit  300 , in the event that an operating LED  104  fails and develops an open electrical circuit, a rising voltage across the electrical terminals of the failed LED is coupled to the gate of an associated triac  302 , latching the triac from a non-conducting state to a conducting state, typically within about 1 μs. In the conducting state triac  302  heat dissipation is relatively low due to its inherently low voltage drop. Accordingly, a triac  302  having a relatively low power rating and smaller physical size may be selected. Note that reversal of the voltage/current provided by power supply  14  to LED network  300  will cause the conducting triac  302  to switch to a non-conducting state, allowing the LED  104  in inverse-parallel with the failed LED to illuminate. In one embodiment of the present invention the drive current supplied to LED network  300  may be periodically interrupted to unlatch any conducting triacs  302 , thereby allowing any associated LEDs  104  that have autonomously cleared an internal fault an opportunity to light again. This interruption is inherent with flashing beacons but may also be automatically or manually incorporated into steady-burning lights to affirmatively clear such faults. 
     Under some conditions triacs  302  of  FIG. 6  may be placed into a conducting state spontaneously (even with power to LED network  300  removed) as a result of relatively high dV/dt electrical pulses  314  induced during lightning strikes  316 . In this case, the triac  302  in a conducting state acts to shunt potentially damaging energy away from the LEDs  104  until the drive voltage/current is momentarily interrupted in the manner previously described. 
     In some embodiments it may be desirable to select between a plurality of shunted LED circuit strings.  FIG. 7  shows the basic circuitry for a prior art LED network  400 . A first LED string  402 - 1  and a second LED string  402 - 2  are each isolated from a power supply  14  by first and second corresponding switches  408 - 1 ,  408 - 2  respectively. A binary control signal, labeled SEL, is coupled to first switch  408 - 1 . Control signal SEL is logically inverted by an inverter  410  to produce an inverse-logic control signal NSEL that is coupled to second switch  408 - 2 . In operation, only one of LED strings  402 - 1 ,  402 - 2  is in an on-state (i.e., illuminated) at any given time, first string  402 - 1  being in an on-state when SEL is a logical “1” and second string  402 - 2  being in an on-state when SEL is a logical “0.” LED strings  402 - 1 ,  402 - 2  may each be configured as any of the LED circuits, networks and shunt bypass circuits shown in  FIGS. 1 through 6  and discussed above, within the scope of the invention. 
       FIG. 8  shows an LED network  500  that achieves similar functionality to circuit  400  of  FIG. 7 . A string  502  of LEDs  104  are connected in the inverse-parallel LED circuit  102  arrangement of  FIG. 4  with the addition of triac shunts, as in  FIG. 6 . An H-bridge  504  controls the polarity of power supplied to LED pairs  102  by power supply  14 , the polarity in turn being controlled by a binary control signal SEL, a pair of switches  408  and an inverter  410  in the manner described above. H-bridge circuit  504  has a first output  508  and a second output  510 , the first and second outputs each being selectably configurable as a current source and a current sink. First output  504  is a current source when second output  510  is configured as a current sink. Likewise, second output  510  is configured as a current source when first output  508  is configured as a current sink. Of course, other functionally similar H-bridge configurations are possible within the scope of the present invention. 
     It can be seen from  FIG. 8  that, by reversing the polarity applied to the LED string  502 , either of the LEDs  104  in LED circuit  102  may be selectively energized. LED circuits  102  may find application in beacons that flash white during the day and red at night, LED  104 - 1  being a white LED and LED- 2  being a red LED (or vice versa). In other embodiments LED  104 - 2  may serve as a back-up for LED  104 - 1  (or vice versa). Another practical application includes, but is not limited to, selectably energizing LEDs in different areas of a reflector or lens of an optic portion of an LED lighting system to achieve a flood or spotlight beam pattern. 
     By grouping the LEDs inverse parallel arrangements  102  described above the number of interconnections are reduced. This is important in applications, for example, where the LED light panels are socketed. Should a socket contact fail, for example, the shunt device will bypass the open circuit, and may automatically recover if power is removed or the other LED string is selected. It will be further appreciated by one skilled in the art that any of the LED circuits, networks and shunt bypass circuits of  FIGS. 1 through 6  may be adapted for use with LED network  500  of  FIG. 8 , within the scope of the invention. 
     Safety-related applications may be subject to governmental regulations and require the monitoring of LEDs, and to issue a remote alert signal for a service call, or take other predetermined remedial action if a certain percentage or pattern of LEDs fail. One failure detection scheme is to divide the LEDs into small groups, each with its own linear current regulator, and monitor each group&#39;s voltage. A drawback of such arrangements is relatively low efficiency, coupled with relatively complex circuitry. In addition, current matching between LED groups can be difficult to achieve. 
     In a prior art LED circuit  600  shown in  FIG. 9 , LEDs  104  are connected in series, forming LED strings  602  and  604 . The LED  104  voltages are scanned and compared to a reference voltage as follows. Voltage dividers comprising resistors  606  and  608  are used to scale the LED  104  voltages with respect to electrical ground. A first analog multiplexer  610  is used to select an LED&#39;s “high” side and a second multiplexer  612  to select that LED&#39;s “low” side. The two signals are then applied to a differential amplifier circuit  614 ,  616  with a gain equal to the resistive dividers&#39; scaling factor (for example), the resulting voltage being equivalent to the forward voltage of the select LED  104 . Other gain/ratios may be utilized to accommodate variables such as circuitry having limited voltage range, for example. This signal may then be examined by a not-shown window comparator or a not-shown microcontroller analog-to-digital (A/D) input to monitor the forward voltage of the LEDs and determine if a selected LED&#39;s voltage is too high or too low, indicating an open-circuit or short-circuit respectively in the select LED. It should be noted that the thresholds may need to be adjusted if the LEDs in each string are different. 
     An LED circuit  700  is shown in  FIG. 10  according to an embodiment of the present invention. As previously discussed, the inverse-parallel arrangement of LED pairs  102  halves the number of bypass devices needed for open-circuit protection. Similarly, the number of voltage dividers and multiplexers for LED pair  102  monitoring is halved in comparison to the prior art circuit of  FIG. 9 . Additional simplification is realized by time-sharing an analog multiplexer such that the two differential voltages are derived from a sample-hold signal from the previous LED&#39;s voltage and the next LED&#39;s voltage obtained in real-time. It should be noted that the sample-hold and differential function are optional, since associated monitoring circuitry may be configured to evaluate the voltages directly by storing the first reading and subtracting the second, assuming quantization error and RF susceptibility are not a concern. In this manner the requisite number of multiplexers is halved yet again. As can be seen from comparing  FIGS. 9  and  10 , LED circuit  700  of the present invention is much less complex, thereby reducing cost while increasing reliability. In some embodiments a microprocessor may be utilized to evaluate the LEDs by going from a higher voltage to a lower voltage, providing a “sample” command at an appropriate time after currents have settled, and realizing that reversing the string voltage also requires reversing the direction of the scan sequence. Scanning from a lower to a higher voltage may also be accomplished using an analog sample gate, provided the differential amplifier inputs are reversed. 
     With continued reference to  FIG. 10 , a single analog multiplexer  702  has a plurality of input channels and an output channel, the input channels being electrically coupled to the LED circuits  102  and configured to receive at a select input channel a voltage representing the voltage between the anode and the cathode of the LEDs of the LED circuit. A sample-and-hold circuit  704  is coupled to the output channel of multiplexer  702 , the sample-and-hold circuit selectably storing a voltage measured at the select input channel for a select LED circuit  102 . Sample-and-hold circuit  704  may be configured to store one voltage point measurement and compare it to the next to determine the voltage drop for each LED. It should also be readily apparent to one skilled in the art that an open-circuited LED  104  will appear shorted while its associated shunt device is in a conducting state; thus, it is still considered a failed LED. 
     It should be noted that LED circuits  102  are shown in  FIG. 10  as examples, and that any of the LED circuits, networks and shunt bypass circuits of  FIGS. 1 through 6  may be adapted for use with the system of  FIG. 10 , within the scope of the invention. In addition, one skilled in the art will appreciate that the system of  FIG. 10  may be configured and replicated to monitor any desired number of LED strings in an LED lighting system, within the scope of the invention. 
     The system of  FIG. 10  may also be configured to further include the previously-described H-bridge  504 , as shown. 
     With regard to the LED lighting system of  FIG. 10  a microcontroller (not shown) may be advantageous for individual LED monitoring. In this instance the number of LEDs can be measured and action taken when a certain percentage of LEDs have failed or perhaps a certain group; i.e., all or most LEDs oriented in a single direction. 
     While this invention has been shown and described with respect to a detailed embodiment thereof, it will be understood by those skilled in the art that changes in form and detail thereof may be made without departing from the scope of the claims of the invention.