Abstract:
A space efficient circuit arrangement for supplying power to an LED array, the power supply circuit ( 100 ) has a rectifier ( 105 ), a starting circuit coupled to the rectifier ( 105 ), a gate drive arrangement coupled to the starting circuit, and a resonant converter circuit ( 120, 125 ) coupled between the rectifier ( 105 ) and a resonant load circuit ( 135 ). The resonant load circuit includes a resonant inductance ( 150 ), a resonant capacitance ( 155 ) coupled to the resonant inductance ( 150 ), and a load connected in parallel to the resonant capacitance ( 155 ). A plurality of light emitting elements ( 170, 175 ) and a capacitor ( 160 ) define at least a portion of the load. All of the circuit components may be placed on the same circuit board as the light emitting elements ( 170, 175 ), thereby taking up less space in a traffic signal housing and making retrofitting a traditional incandescent lamp traffic signal easier.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This application relates to an electrical circuit and, in particular, to a power supply circuit for operating a light source, particularly, an array of light emitting diodes (LEDs). 
     2. Discussion of the Art 
     Incandescent lamps are used in a wide variety of environments and generally have found substantial commercial success in meeting various demands. More recently, however, industry is seeking an alternative light source that is more efficient, has an extended life, and can endure the rigors of applications that experience constant exposure to the elements and frequent on/off switching. An exemplary use of an incandescent lamp in this type of environment is in traffic signals. While incandescent lamps have traditionally been used in traffic signals, the incandescent lamp encounters constant exposure to the elements and has a relatively short life span of typically around eight thousand hours, which is further shortened if it is frequently switched on and off. Additionally, the incandescent lamp is inefficient due to the fact that much of the light energy produced by it is wasted by filtering the light. As a result, traffic signals utilizing incandescent lamps require frequent maintenance and typically must be replaced once or twice a year. 
     Light emitting diodes (LEDs) have been suggested as alternative light sources due to their robust structure that are able to withstand constant exposure to the elements and the long life associated with their efficient operation. Known advantages of using an LED array in lieu of an incandescent lamp include increased efficiency, little or no maintenance, greater resistance to the elements, and greater mechanical durability. 
     Additionally, an LED array consumes less power to produce the same light output as an incandescent lamp. Further advantages are that an LED array may function for more than twenty years before requiring replacement, an LED array does not require a light reflector, and a fault in the LED array does not necessarily mean that the entire LED array will fail. In addition to performing the same functions as an incandescent lamp, a single LED array may also be used to display different illuminated symbols such as “no left turn”, “turn only”, and “do not enter”. 
     Despite all of these advantages, however, there are still several concerns which have prevented widespread adoption of LED arrays in, for example, traffic signals. The most significant is that an LED array is not easily retrofitted. In the environment of traffic signals, incandescent lamps typically operate with a 120 volt 60 Hz AC power supply, and LEDs typically require a DC current of approximately 5 to 20 milliamps and a forward operating voltage of between 1.5 to 2.5 volts. Second, “standard” incandescent lamp traffic signal housings are designed to accept a “standard” incandescent bulb. 
     While these issues have been addressed in the prior art, the known solutions have raised other issues. For example, retrofitting an existing incandescent lamp traffic signal using only an inductor or L-C circuit connected to LED pairs connected in series presents a significant problem since the required inductor for an array of approximately twenty LEDs is 6 Henries. An inductor of this size is very heavy, making the LED assembly much heavier than a traditional incandescent lamp assembly. Thus, a pole or electrical line to which a traffic signal is mounted must be sufficient to support the increased weight for the long duration. Accommodations for an enlarged inductor must also be made in the traffic signal housing, along with attendant difficulties regarding installing, maintaining, or retrofitting the LED array in the limited space of a standard incandescent lamp traffic signal. Needs of a technician working in a bucket ladder high above the ground must also be considered in such a design. 
     Accordingly, a need exists for an alternative manner of supplying power to LED arrays, particularly those used in traffic signals. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a more cost efficient electrical circuit for supplying power to an LED array. 
     A power supply circuit for an LED array includes a rectifier for converting an AC current to DC current; a starting circuit coupled to the rectifier for providing a path for the AC current; a gate drive arrangement coupled to the starting circuit; a resonant converter circuit regeneratively controlled by the gate drive arrangement; and a resonant load circuit coupled to the resonant converter circuit for inducing the AC current in the resonant load circuit. The resonant load circuit incorporates a resonant inductance, a resonant capacitance coupled to the resonant inductance, and a load connected in parallel to the resonant capacitance. 
     A plurality of light emitting elements and a capacitor define at least a portion of the load. 
     In other preferred embodiments, the load includes at least one pair of oppositely polarized light emitting elements connected in parallel or at least one pair of oppositely polarized branches of light emitting elements. 
     This circuit has a number of advantages over the prior art. The power supply circuit uses smaller components and decreases the space requirements in the electrical compartment of a traffic signal than any known prior art circuit. 
     Another advantage resides in the use of integrated circuits which weigh less and provide for easier placement in traffic signals. 
     Still another advantage is realized since integrated circuits will fit on the same circuit board as the LED array. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an exemplary embodiment of a power supply circuit according to the present invention; 
     FIG. 2 is a schematic diagram of a second preferred embodiment of a power supply circuit according to the present invention; 
     FIG. 3 is a schematic diagram of a third preferred embodiment of a power supply circuit according to the present invention; and 
     FIG. 4 is a schematic diagram of yet another preferred embodiment of a power supply circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 depicts a power supply circuit  100  for an LED traffic signal in accordance with a first preferred embodiment of the present invention. A first rectification means or full-wave bridge rectifier  105  coupled to an AC source  110  converts an AC current to DC current. A smoothing capacitor  115 , connected in parallel to the bridge rectifier  105  maintains an average voltage level. A DC-to-AC converter, which includes first and second switches  120  and  125 , is coupled to the smoothing capacitor  115 . An electromagnetic interference (EMI) filter  130 , shown as an inductor, is coupled between the bridge rectifier  105  and the DC-to-AC converter. 
     The first and second switches  120  and  125  are respectively controlled by a gate drive circuit to convert DC current from the output of the bridge rectifier  105  to AC current received by a resonant load circuit  135 . DC bus voltage V BUS  exists between a bus conductor  140  and a reference conductor  145 , shown for convenience, as a ground. 
     The resonant load circuit  135  includes a resonant inductor  150  and a resonant capacitor  155 . The resonant load circuit  135  also includes a load. The load includes a matching capacitor  160  and at least one group  165  of LEDs  170 ,  175  connected in series. The LEDs  170 ,  175  are preferably oppositely polarized in a manner well known in the art. The LEDs  170 ,  175  are disposed in parallel so that the LEDs  170 ,  175  encounter the same electric potential, and each diode is illuminated during each half cycle. The group  165  of LEDs  170 ,  175  may be shunted across the resonant capacitor  155 . The matching capacitor  160 , which affects how the resonant inductor  150  and resonant capacitor  155  network perceives the impedance of the LEDs  170 ,  175 , is coupled between a node NI and the group  165  of LEDs  170 ,  175 . The matching capacitor  160  may limit the current through the LEDs. 
     Capacitors  180 ,  185  are standard bridge capacitors for maintaining their connection node N 1  at about one half the bus voltage V BUS . Arrangements alternative to the bridge capacitors  180 ,  185  are known in the art. Other arrangements for interconnecting the LEDs  170 ,  175  in the resonant load circuit  135  will be discussed further below. 
     In the power supply circuit  100 , the first and second switches  120  and  125  are complementary to each other. For instance, the first switch  120  may be an n-channel enhancement mode device as shown, and the second switch  125  is a p-channel enhancement mode device, also known as MOSFET switches. However, other n-channel, p-channel or bipolar junction transistor switches may be used. Each of the first and second switches  120  and  125  has a respective gate (or control terminal) G 1 , G 2 , respectively. The voltage from the gate G 1  to source (reference terminal) S 1  of the first switch  125  controls the conduction state of that switch. Similarly, the voltage from the gate G 2  to source S 2  of the second switch  125  controls the conduction state of that switch. As illustrated, the sources Si and S 2  are connected together at a common node N 2 . Drains D 1  and D 2  of the first and second switches  120  and  125  are connected to the bus conductor  140  and the reference conductor  145 , respectively. 
     The gate drive circuit is connected between the common control node N 3  and the common node N 2 . The gate drive circuit includes a driving inductor  190  which is mutually coupled to the resonant inductor  150  in such a manner that a voltage induced therein is proportional to the instantaneous rate of change of an AC load current. The driving inductor  190  is further connected at one end to the common node N 2 . The end of the resonant inductor  150  connected to the common node N 2  may be a tap from a transformer winding forming the driving inductor  190  and the resonant inductor  150 . The driving inductor  190  provides the driving energy for operation of the gate drive circuit. A second inductor  195  is serially connected to the driving inductor  190  between a blocking capacitor  200  and the driving inductor  190 . The second inductor  195  is used to adjust the phase angle of the gate-to-source voltage appearing between the common control node N 3  and the common node N 2 . 
     A bi-directional voltage clamp  205 , preferably comprised of back-to-back Zener diodes, is located between the common control node N 3  and the common node N 2 . The bi-directional voltage clamp  205  advantageously clamps positive and negative excursions of gate-to-source voltage ratings of the first and second switches  120  and  125  so that their gate-to-source maximum ratings are not exceeded. The bi-directional voltage clamp  205  may be removed from the power supply circuit  100  when the gate drive circuit is at a sufficiently low value. 
     A capacitor  210  between the control node N 3  and the common node N 2  is preferably provided to predictably limit the rate of change of gate-to-source voltage between the common control node N 3  and the common node N 2 . This beneficially assures, for instance, a dead time interval in the switching modes of the first and second switches  120  and  125 , wherein the first and second switches  120  and  125  are off between the times of either the first switch  120  or the second switch  125  being turned on. The capacitor  210  also provides a second resonant circuit consisting of the capacitor  210  and the second inductor  195 . 
     The blocking capacitor  200  and three resistors R 1 , R 2 , and R 3 , forming a starting circuit, are coupled to the gate drive circuit. The starting circuit provides a path for input from AC source  110  to start inductor action. The starting circuit operates as follows. The blocking capacitor  200  becomes initially charged upon energizing of the AC source  110  via resistors R 1 , R 2 , and R 3 . At this instant, the voltage across the blocking capacitor  200  is zero. During the starting process, the driving inductor  190  and the resonant inductor  150  act essentially as a short circuit due to the relatively long time constant for charging of the blocking capacitor  200 . Upon initial bus energizing, the voltage on the common node N 2  is approximately ⅓ of the bus voltage V BUS  with the resistors R 1 , R 2 , and R 3  being of equal value, for instance. The voltage at the common control node N 3 , between the resistors R 1 , R 2 , R 3  is ½ of the bus voltage V BUS . In this manner, the blocking capacitor  200  becomes increasingly charged, from left to right, until it reaches the threshold voltage of the gate-to-source voltage of the first switch  120  (e.g., 2-3 volts). At this point, the first switch  120  switches into its conduction mode, which then results in current being supplied by the first switch  120  to the resonant load circuit  135 . In turn, the resulting current in the resonant load circuit  135  causes regenerative control of the first and second switches  120  and  125  in the manner described above. 
     During steady state operation of the power supply circuit  100 , the voltage of common node N 2  between the first and second switches  120  and  125  becomes approximately ½ of the bus voltage V BUS . The voltage at the common control node N 3  also becomes approximately ½ of the bus voltage V BUS  so that the blocking capacitor  200  cannot again become charged and create another starting pulse for turning on the first switch  120 . The capacitive reactance of the blocking capacitor  200  is much smaller than the inductive reactance of the driving inductor  190  and the second inductor  195  so that the blocking capacitor  200  does not interfere with operation of the driving inductor  190  and the second inductor  195 . 
     Thus, the starting circuit of the power supply circuit  100  does not require a triggering device, such as a diac, which is traditionally used for starting circuits. Additionally, the resistors R 1 , R 2  and R 3  are non-critical value components, which may be 100K ohms or 1 M ohm each, for example. Preferably, the values of the resistors R 1 , R 2 , and R 3  are approximately equal. 
     An optional snubber capacitor  215  may be employed to deplete the energy in the resonant inductor  150 . The snubber capacitor  215  is coupled in parallel to the resistor R 3 . While it is shown that the resistor R 3  shunts the second switch  125 , the resistor R 3  may shunt the first switch  120 . 
     FIG. 2 depicts a power supply circuit  250  for an LED traffic signal in accordance with a second preferred embodiment of the present invention. The power supply circuit  250  is identical to the power supply circuit  100  of FIG. 1, with the exception of the arrangement of the LEDs  170 ,  175  in the resonant load circuit  100 . Thus, the power supply circuit  250  offers the same benefits and advantages as the power supply circuit  100 . 
     In the power supply circuit  250 , the resonant load circuit  255  includes the resonant inductor  150 , the resonant capacitor  155 , and the matching capacitor  160 . The LEDs  170 ,  175  are arranged such that at least one pair  260  of oppositely polarized LEDs  170 ,  175  is connected in parallel. 
     FIG. 3 depicts a power supply circuit  300  for an LED traffic signal in accordance with a third preferred embodiment of the present invention. The power supply circuit  300  is identical to the power supply circuit  100  of FIG. 1, with the exception of the arrangement of the LEDs  170 ,  175 . Thus, the power supply circuit  300  offers the same benefits and advantages as the power supply circuits  100  and  250  described above. In this arrangement, however, the power supply circuit  300  does not require an equal number of LEDs  170 ,  175 . 
     In the power supply circuit  300 , resonant load circuit  305  includes the resonant inductor  150 , the resonant capacitor  155 , and the matching capacitor  160 . The LEDs  170 ,  175  are arranged such that at least one pair  310  of branches  315 ,  320  of LEDs  170 ,  175  are oppositely polarized and connected in parallel. Each branch  315  or  320  may contain an unlimited number of LEDs  170  or  175  polarized the same way. While it is preferred that each oppositely polarized branch  315  or  320  contains an equal number of LEDs  170  or  175 , having an uneven number of LEDs  170  or  175  is acceptable as long as the voltage across each oppositely polarized branch  315  or  320  of LEDs  170  or  175  is substantially the same. The matching capacitor  160  accounts for any imbalance in the voltage between the branches  315 ,  320 . However, the uneven distribution of the LEDs  170  or  175  between the branches  315 ,  320  is limited by the reverse voltage allowed by the LEDs  170  or  175 . 
     FIG. 4 depicts another power supply circuit  400  for an LED traffic signal. The power supply circuit  400  is identical to the power supply circuit  100  of FIG. 1, with the exception of the resonant load circuit  405 . As in the resonant load circuit  135  described above, the resonant load circuit  405  comprises the resonant inductor  150 , the resonant capacitor  155 , and the matching capacitor  160 . The resonant circuit further includes at least one group  410  of LEDs  415  connected in parallel and polarized in the same direction. The groups  410  of the LEDs  415  are connected in series. Because the LEDs  415  are polarized in the same direction, the resonant load circuit  405  requires three additional components, namely a second full-wave bridge rectifier  420 , a diode  425 , and a current limiting inductor  430 . 
     The second bridge rectifier  420 , which is coupled in parallel to the resonant capacitor  155 , re-converts the AC current to DC current. The inductor  430  is coupled between the second bridge rectifier  420  and the LEDs  415 . The LEDs  415  may be shunted across the resonant capacitor  155 . The diode  425  is connected in parallel to the second bridge rectifier  420 . The diode  425  allows current to flow continuously through the current limiting inductance  430 , which limits the current supplied to the LEDs  430 . 
     In summary, the present invention provides a manner of efficiently powering LEDs from an AC source using integrated circuit components. The invention minimizes the space required to retro-fit standard incandescent lamp traffic signals to LED traffic signals. The application of the invention is associated with an array of LEDs although it will be appreciated that the number of LEDs in a particular array may vary. 
     Furthermore, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired that the present invention be limited to the exact construction and operation illustrated and described herein, and accordingly, all suitable modifications and equivalents which may be resorted to are intended to fall within the scope of the claims.