Abstract:
An LED array switching apparatus comprises, on a single PCB substrate: a plurality of LED arrays D 1  to Dn connected in series, each LED array having a forward voltage; an AC voltage supply coupled to the plurality of LED arrays; and a plurality of constant current sources G 1  to Gn, coupled to outputs of LED arrays D 1  to Dn, respectively, each of the constant current sources being switchable between a current regulating state and an open state such that as the voltage of the AC voltage supply increases, LED arrays are switched on and lit to form a higher forward voltage LED string, and as the voltage of the AC voltage supply decreases, LED arrays are switched off and removed from the LED string starting with the most recently lit LED array.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This utility application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/524,163, filed Aug. 16, 2011, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to improved methods and apparatuses for driving light emitting diodes (“LEDs”), such as those used in LED lamps and lighting fixtures, and improved methods and apparatus for driving that facilitate locating driving circuitry and the LEDs together on a single package. 
     Typically, the use of LEDs as an AC powered light source requires a driver to convert AC line power to a regulated DC power for maintaining constant light output. Such an LED driver is a self-contained power supply that has output matched to the electrical characteristics, such as forward voltage and driving current, of the LED. Usually, the driver is built on an individual PCB and connects to an LED PCB which contains the array of LEDs. 
       FIG. 1  shows typical circuitry of an AC-DC LED driver using the known flyback converter topology. The circuitry  10  includes an AC power source  12  with live (L) and neutral (N) terminals, and a rectifier circuit  14  that converts the AC, e.g., sinusoidal, input waveform to a DC waveform, e.g., a half wave rectified waveform. The circuit operates such that when the switch Q 1  conducts, current is directly drawn from the rectified sinusoid. Energy is stored in the magnetizing inductance of the primary winding of transformer T 1 . The rectifying Zener diode D 1  is reverse biased and the LED current is supplied by the secondary capacitor Cout. When Q 1  turns off, diode D 1  conducts and the stored energy is delivered to the secondary winding of transformer T 1  and to the output. The controller chip U 1 , which can be, for example an iW3620 Digital PWM Constant Current Controller for AC/DC LED Driver, manufactured by iWatt, Inc., regulates the LED current by comparing the information about the secondary output voltage and LED current, which information is reflected via the auxiliary winding of transformer T 1 , to a constant reference and, based on the comparison, adjusting the duty cycle of switch Q 1 . 
     AC-DC drivers such as the one shown in  FIG. 1  are complex and bulky circuits. With the goal of making a simpler circuit, LED manufacturers have developed an AC LED circuit, which is basically LED lighting circuitry that can operate on AC power without the need for the complex AC-DC driver of the type shown in  FIG. 1 . However, the AC line voltage is sinusoidal and the majority of low frequency AC LED systems operate in the rectified mode, which turns off the AC LED at a rate of double the line frequency. 
       FIG. 2  is a schematic of a conventional AC LED circuit. In such a circuit, the LEDs can be driven directly by the AC power source  102  without the use of a complex converter. In operation of the illustrated circuit, during the positive half cycle of the, e.g., sinusoidal, AC source V, LED string S 2  is reverse biased and LED string S 1  is conducting and emitting light. During the negative half cycle of AC source V, LED string S 1  is reverse biased and LED string S 2  is conducting and emitting light. The forward voltages of LED strings S 1  and S 2  are equal. The resistor R limits the current through the LED strings S 1  and S 2 . 
       FIG. 3  shows the light output (i.e., luminous flux vs. time) of an AC LED circuit. It can be seen from the figure that the off-time in such a circuit is about 40%, due to the fact that the LED current of the AC LED circuit is discontinuous at the zero crossing of the AC voltage waveform. Since the rectified AC line voltage cycles from zero to peak level and back to zero, the LED string turns off whenever the line voltage level falls below the forward voltage of the LED string. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, an LED array switching apparatus comprises, on a single PCB substrate: a plurality of LED arrays D 1  to Dn connected in series, each LED array having a forward voltage; an AC voltage supply coupled to the plurality of LED arrays; and a plurality of constant current sources G 1  to Gn, coupled to outputs of LED arrays D 1  to Dn, respectively, each of the constant current sources being switchable between a current regulating state and an open state such that as the voltage of the AC voltage supply increases, LED arrays are switched on and lit to form a higher forward voltage LED string, and as the voltage of the AC voltage supply decreases, LED arrays are switched off and removed from the LED string starting with the most recently lit LED array. 
     In another aspect, the plurality of LED arrays and the plurality of constant current sources are formed as semiconductor chips, and the semiconductor chips are affixed to the PCB substrate by direct chip attachment (DCA). 
     In another aspect, the semiconductor chips are affixed to the PCB substrate using thermal adhesive. 
     In another aspect, an electrical connection between the semiconductor chips and the PCB substrate is provided by bond wiring. 
     In another aspect, the bond wiring is by thermosonic Au ball bonding. 
     In another aspect, the bond wiring is by ultrasonic Al wedge bonding. 
     In another aspect, the LED array switching apparatus further comprises an encapsulant encapsulating at least the semiconductor chips. 
     In another aspect, LEDs of a particular array are placed next to LEDs of other arrays. 
     In another aspect, the PCB substrate area for placing semiconductor chips for the LED arrays and the PCB substrate area for placing semiconductor chips for constant current sources are set in proportion to the amount of heat dissipated by the respective types of semiconductor chips. 
     In another aspect, the PCB substrate is formed from a material in the group consisting of ceramic, glass, organic and flex substrates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures are for illustration purposes only and are not necessarily drawn to scale. The invention itself, however, may best be understood by reference to the detailed description which follows when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a diagram of conventional circuitry of an AC-DC LED driver; 
         FIG. 2  is schematic diagram of a conventional AC LED circuit; 
         FIG. 3  is light output waveform of an AC LED circuit such as the one shown in  FIG. 2 ; 
         FIG. 4  is functional block diagram of a driving circuit in accordance with an aspect of the present invention; 
         FIGS. 5A to 5F  are diagrams that show different stages of switching of the circuitry of  FIG. 4  in response to an input waveform; 
         FIG. 6  shows the LED arrays that are conducting during a half AC cycle in accordance with the stages of switching of the circuitry shown in  FIGS. 5A to 5F ; 
         FIG. 7  shows the light output waveform of the circuit in accordance with the present invention; 
         FIG. 8  is a schematic diagram of a proposed detailed implementation of the circuit in accordance with an aspect of the present invention, using packaged LEDs; 
         FIG. 9  is a diagram of a circular PCB layout of the circuit shown in  FIG. 8 ; 
         FIG. 10  is an illustration of a direct chip attachment (DCA) assembly; and 
         FIG. 11  is an example of a light engine in accordance with an aspect of the present invention using a DCA assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to retain the simple AC connection of an AC LED circuit design, while reducing light off-time associated with conventional AC LED circuits, in accordance with aspects of the present invention, an AC light engine is built on a single PCB with multiple LED arrays and switchable current sources. In contrast to conventional AC-DC LED drivers, such as the one shown in  FIG. 1 , which are complex and bulky, a driving circuit in accordance with the present invention is simple and small in size, allowing it be packed together with LEDs on a single PCB to form a light engine. The switching configuration in accordance with the present invention can be powered by AC mains directly, and is simple and small enough to pack with the LEDs to form a LED light engine/module which can be powered directly from the AC line. In accordance with the present invention, the driving circuit and the LEDs can be put together on a single PCB. 
       FIG. 4  shows the functional blocks of proposed LED driving circuitry  200  in accordance with an aspect of the present invention. The circuit  200  uses a string of LEDs and the LED string is divided into n LED arrays D 1  to Dn, where n&gt;1. Each LED array may include one or more LEDs. AC source  1  supplies an AC waveform to diode array  2 , which acts as a rectifier. G 1  to Gn are constant current sources which can be disabled (i.e., changed to an open circuit condition) by successive current sources. 
     The operation of the circuit is next shown making reference to  FIGS. 5A-5F  for the case in which the voltage of AC source voltage is ramping up from zero. When the rectified voltage is just above the forward voltage of LED array D 1 , current begins to flow through LED array D 1  and current source G 1 , as shown in  FIG. 5A . Current source G 1  regulates the current through LED array D 1  as the rectified voltage is further increased. LED array D 2  begins to conduct when the rectified voltage reaches the sum of the forward voltages of LED arrays D 1  and D 2 , as shown in  FIG. 5B . As the current through LED array D 2  and current source G 2  increases to the regulated value, the current through LED array D 1  and current source G 1  decreases to zero. The current through LED arrays D 1  and D 2  is then regulated by current source G 2 , as shown in  FIG. 5C .  FIG. 5D  shows the current path when the rectified voltage is increased to a point where current source Gn−1 regulates the current through LED arrays D 1  to Dn−1. Further increasing the rectified voltage causes LED array Dn to conduct, as shown in  FIG. 5E .  FIG. 5F  shows the current path when the current through LED array Dn and current source Gn is increased to keep current sources G 1  and Gn−1 to an open condition. 
     As would be understood by one of skill in the art, the switching sequence shown in  FIGS. 5A-5F  would be reversed if the rectified voltage is declining. In particular, the situation in which the rectified voltage is high enough to pass a regulated current through LED arrays D 1  to Dn and current source Gn is shown in  FIG. 5F . As the rectified voltage is decreased, the current through current source Gn starts to decrease and current begins to flow through current source Gn−1, as shown in  FIG. 5E . When the rectified voltage decreases to a value below the sum of forward voltage sum of LED arrays D 1  to Dn, current through LED array Dn is stopped, as shown in  FIG. 5D . The same pattern would repeat in a second half wave of the rectified signal. 
       FIG. 6  is a diagram that illustrates the forward voltage string that is created during one half cycle, in accordance with the circuit shown in  FIG. 4 . As can be seen in the figure, the forward voltage string initially includes LED array D 1  only. As the voltage of the voltage supply  1  approaches its peak, the forward voltage string includes LED arrays D 1 -Dn, and then, as the voltage of the voltage supply decreases, the length of the forward voltage string is again reduced to D 1 . 
       FIG. 7  shows the light output waveform of the proposed LED driving circuit with the LED strings divided into 5 arrays (i.e., n=5) with forward voltage of ratio of the arrays of 5:4:3:2:1. As can be seen from the waveform, the off-time is reduced to 10% using this configuration. 
       FIG. 8  shows the schematic of the proposed circuit implemented using 15 packaged LEDs divided into 5 arrays, A 1  to A 5 , in accordance with the present invention. Resistor RZ 1  and zener diode ZD 1  provide a constant reference voltage for five current sources, G 1  to G 5 . Mosfet M 1 , resistor R 1  and transistor Q 1  form the first current source G 1 , which drives LEDs  1 - 5 . Mosfet M 2 , resistor R 2  and transistor Q 2  form the second current source G 2 , which drives LEDs  1 - 9 . Mosfet M 3 , resistor R 3  and transistor Q 3  form the third current source G 3 , which drives LEDs  1 - 12 . Mosfet M 4 , resistor R 4  and transistor Q 4  form the fourth current source G 4 , which drives LEDs  1 - 14 . Mosfet M 5  and resistor R 5  form the fifth current source G 5 , which drives LEDs  1 - 15 . 
       FIG. 9  shows an example of a preferred circular PCB layout  400  of the circuit of  FIG. 8 . In this embodiment, the LEDs and components of the circuit of  FIG. 8  are placed on a single PCB  401 , and the connections to AC power are labeled as neutral (N)  402  and live (L)  403 . In accordance with the illustrated embodiment, the LEDs of a particular array are preferably placed next to LEDs of other arrays, to average the brightness of different arrays. So, for example, in the illustrated embodiment, LED 2  of array A 1 , is placed close to LED 8  of array A 2 , but A 1  is located distally from LED 4  of array A 1 . 
     To achieve optimal thermal performance, the PCB area for placing LEDs and components of current sources should preferably be in proportion with the amount of heat they dissipate. For example, if 85% of heat is dissipated in the LEDs and the remaining 15% of heat is dissipated in other components, the PCB area for LEDs should be about 85%, while PCB area for components should be about 15%. 
     Instead of undergoing traditional assembly and soldering as an individual PCB, according to one aspect of the present invention, direct chip attachment (DCA) can be used to produce the light engine. In this technique, the LEDs and other components are in the form of semiconductor chips, which are directly mounted on and electrically interconnected to its final circuit board.  FIG. 10  shows a DCA assembly example. In the example, a PCB substrate  502  is formed. Each component, whether it be an LED, or an associated component, is formed as a semiconductor chip  503  and affixed to the substrate using thermal adhesive  504 . Connections from the chip  503  to the PCB substrate  502  are provided by bond wiring  506 , which is attached to the PCB substrate  502  with bond pads  505  and attached to the chip  503  with chip bond pads  508 . Preferably, the entire circuit is encapsulated in a silicone encapsulant  510 , which, for example can function to keep moisture and dirt away from the chip and associated connections, and to make the circuitry less susceptible to mechanical and chemical damage. 
     Aside from conventional printed circuit boards, various substrates may be used in making the LED light engine in accordance with the present invention. For example, ceramic and glass ceramic substrates, which exhibit excellent dielectric and thermal properties, may be used. Another option are organic substrates, which weigh and cost less while providing a low dielectric constant. In addition, flex substrates, which, being pliable, have the ability to bend, may be employed in accordance with another advantageous aspect of the present invention. 
     By applying the die attaching adhesive  504  to the substrate  502 , a plurality of LED and component chips  503  are mounted on the substrate  502  to form a complete driving circuit. A curing process, such as exposure to heat or ultraviolet light, follows, which allows the thermal adhesive  504  to attain its final mechanical, thermal, and electrical properties, and electrical connections are made by wirebonding, in the manner shown in  FIG. 10  for a single chip. 
     The wirebonding process used according to this aspect of the present invent is similar to that used in traditional semiconductor assembly, and thermosonic Au ball bonding or ultrasonic Al wedge bonding may be employed to connect wires between dies and the substrate. Finally, as described above, the die and bond wires and the other components are encapsulated to protect them from mechanical and chemical damage. 
       FIG. 11  shows one example of DCA layout of the light engine. In the illustrated example, the LED chips placed in region  601  and are wire bonded, using bond wire  604  to form a high voltage string of LED arrays. The other components, i.e., the rectifiers and current sources, are placed on region  602  of the assembly and wire bonded to the LED arrays. In such a configuration, a simple AC power connection is all that required to power up the light engine. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This provisional application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.