Abstract:
Disclosed is an LED bulb having a current-compensated driving circuit. A compensation current  352  is coupled to a main driving capacitor used to power LEDs and functions to keep the load current more constant. This has dual advantages of saving power and making the light output more uniform. Saving power also means that the circuit runs cooler than without compensation. Additionally this circuit includes an inductor placed in line with a driving capacitor. The inductor functions to reduce rapid current influx to the capacitor during operation of the driving circuit, which reduces EMF and reduces component stress. One benefit of using an inductor is that most of the current absorbed by the inductor is provided back to the circuit during a later portion of the AC cycle, which also limits energy losses by the driving circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This disclosure claims benefit of U.S. provisional application 61/614,298, filed Mar. 22, 2012 entitled DRIVING CIRCUIT FOR LIGHT EMITTING DIODE APPARATUS AND METHOD OF OPERATION, and claims benefit of U.S. provisional application 61/601,941, filed Feb. 22, 2012, entitled METHOD OF PRODUCING LED LIGHTING APPARATUS AND APPARATUS PRODUCED THEREBY, the contents of both of which are incorporated by reference herein. Additionally this disclosure is related to US non-provisional patent application entitled METHOD OF PRODUCING LED LIGHTING APPARATUS AND APPARATUS PRODUCED THEREBY, filed on even date herewith Ser. No. 13/774,941, which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This disclosure is directed to lighting, and, more particularly, to a new driving circuit for a Light Emitting Diode (LED) lighting apparatus, and methods of the circuit operation. 
     BACKGROUND 
     Light Emitting Diodes (LEDs) are specialized diodes made from semiconductor material or materials. LEDs differ from standard diodes in that, when LEDs are energized by small amounts of electric current, they emit light that is visible to humans. Early generation LEDs generated primarily red, yellow, or green colored lights, but relatively recent advances in LED technology provide blue and white LED lights as well. White LEDs may be particularly bright, and, because they are made of a stable, solid state material, have a very long working lifetime. Additionally, LEDs operate at relatively low voltage, and their electrical current requirements are decreasing as LED technology matures. 
     Although LEDs operate at relatively low voltage, in general, individual LEDs do not generate a large amount of light, such as for room lighting, or even for general reading, for example. Instead, multiples of individual LEDs are often grouped together and operated in concert. When added together, light from a group of LEDs may be used to replace general room lighting. As described above, LEDs have a fantastically long operational life, and are therefore cost effective from a replacement standpoint. To gain even wider adoption, however, total cost of operation, including operating cost in the form of electric power should be reduced. 
     Embodiments of the invention address this and other limitations of the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an LED bulb including a driving circuit according to embodiments of the invention. 
         FIG. 2  is a schematic diagram illustrating general concepts of embodiments of the invention. 
         FIG. 3  is a detailed schematic diagram illustrating an example embodiment of the driving circuit according to embodiments of the invention. 
         FIG. 4  is a table illustrating various phases of operation of the circuit according to embodiments of the invention. 
         FIG. 5  is a waveform diagram illustrating many waveforms and outputs according to embodiments of the invention. 
         FIGS. 6A and 6B  are waveform diagrams illustrating power savings achieved using embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a side view of an LED bulb  10  that may be driven by a driving circuit according to embodiments of the invention. In general, a driving circuit  40  is located inside the LED bulb  10 . The driving circuit  40  connects between an energy source, such as the electrical socket served by an Edison screw  12 , and the load, which is made of a number of individual LED devices  30 , mounted in an array. In disclosed embodiments, the LED array includes twelve LED devices  30 , wired in series. Of course, countless variations are possible, such as number of LED devices and how they are connected, for example in series, parallel, or mixed series and parallel without deviating from the scope of the invention. 
     A circuit board  14 , hereinafter called the main board  14 , may be conveniently partially or fully contained within a support  16 , which runs through the middle of the bulb  10 . The main board  14  is a printed circuit board that supports the driving circuitry  40 , as described in detail below. 
     An embodiment of a driving circuit according to embodiments of the invention is illustrated in  FIG. 3 , and a timing diagram of the driving circuit is illustrated as  FIG. 5 . A simplified view of the circuit is illustrated as  FIG. 2  for discussion purposes. 
     In general, with reference to  FIG. 2 , a current compensated circuit  240  may be an example of the driving circuit  40  of  FIG. 1 . The compensated circuit  240  includes an alternating current (AC) power source  212 , which may rectified into a ½ wave that varies between 0 and 90 volts, as described below. A first capacitor  244  and a second capacitor  248  are coupled between the point of the highest voltage and a circuit ground, which is also the ground of the LED. A load current  250  represents current flowing through a group of LEDs. A compensation current  252  is coupled between nodes of the first capacitor  244 . In general, the compensation current  252  increases the stability of the load current  250 , keeping the load current more constant than if the compensation current  252  were not present. Looking at  FIG. 2 , it is recognizable that the voltage between the two capacitors  244 ,  248  have no DC path back to a supply or ground unless I compensation  252  and I load  250  are completely balanced. A controller  260  monitors the I load  250  and sends a signal to control the I compensation  252  to maintain a circuit balance, as described in more detail below. The compensation current  252  also keeps the output voltage of the circuit relatively stable, which allows the LEDs to shine relatively uniformly. 
     In operation, the AC signal charges the capacitor  244 , which is then coupled through a switch (not shown) to the capacitor  248 . Charge passes from the capacitor  244  to capacitor  248 , raising the voltage on the capacitor  248 . After the switch has coupled the capacitors  244 ,  248 , the rising AC signal charges both capacitors  244 ,  248 . After the AC signal reaches its voltage peak and starts to fall, the capacitor  248  discharges by virtue of driving the load current  258 . Eventually during the AC cycle the switch (not shown) disconnects the capacitor  244  from the capacitor  248  and the capacitor  248  continues to discharge, providing power to the LED bulbs. Then, the cycle repeats the process with the switch re-connecting the again-charged capacitor  244  to the capacitor  248  as the capacitor  248  is discharging. In practice, the capacitor  244  of the compensated circuit  240  may be formed by two physical capacitors, one of that is active for the first half of an AC signal cycle, and the other one active during the second half of the AC signal cycle. 
     With reference to  FIG. 3 , a driving circuit  300  generally includes an input portion  310 , an energy storage section  320 , and an output driving section  370 . 
     The input portion  310  includes two nodes,  312 ,  314  coupled to AC power, such as power supplied from a standard light socket. Embodiments of the invention work regardless of the input voltage and frequency, but have been optimized for 120v, 60 Hz AC power. 
     A capacitor C 6  is coupled between nodes  312 ,  314 , as is a bridge rectifying circuit  316 . As is known, the bridge rectifying circuit converts Alternating Current (AC) to Direct Current (DC), so that the voltage output of the bridge rectifying circuit  316  is always positive. In a preferred embodiment, the output of the bridge rectifying circuit  316  has a maximum voltage of approximately 90 volts. 
     The AC input nodes  312 ,  314  are also coupled to the energy storage section  320  of the circuit  300 . In general, a capacitor C 1  and C 2  are each coupled respectfully to one of the AC input nodes  312 ,  314 , and both capacitors C 1  and C 2  are coupled to a second bridge rectifier  322 . The output of the second bridge rectifier  322  is referred to as a node  324 . 
     A switch Q 2  is coupled between the node  324  and the capacitor C 1 , while switch Q 3  is coupled between the node  324  and the capacitor C 2 . The switches Q 2  and Q 3  may be implemented by transistors, as is known in the art. When the switches Q 2  and Q 3  are implemented by transistors, the gates of Q 2  and Q 3  may be connected to the AC input nodes  312 ,  314 , as illustrated in  FIG. 3 . 
     A diode D 4  prevents current from flowing out of the capacitor C 1  into the AC input  312 , while a diode D 5  prevents current flowing from the capacitor C 2  to the AC input  314 . 
     An inductor L 1 , which in implementation may be very large, such as 0.5-3 mH, is coupled to capacitor C 3 , which is the main source of stored power for driving the output driving section  370 . The inductor L 1  is coupled to the node  324 , which is the output from the second bridge rectifying circuit  322 . The capacitor C 3  is coupled between the inductor L 1  and circuit ground, illustrated with an open arrowhead, which may be a different voltage reference than earth ground. 
     An output driving portion  370  of the driving circuit  300  most importantly includes the LEDs  372 , which are the components ultimately driven by the driving circuit  300 . As mentioned above, in one embodiment the individual LED components in the LEDs  372  may generally be connected in series, which means the current flowing through each of the devices is identical or nearly identical. The LEDs  372  are coupled to ground through one of Q 7  or Q 8 . A reference diode D 9 , which may be a schottky diode, controls the amount of current that flows through the LEDs  372 . In a preferred embodiment, there are twelve LEDs in the group of LEDs  372 , each of which is series connected. Of course, other configurations are possible depending on desired parameters such as the amount of light output from the bulb  10 . 
     Another important feature of the output driving portion  370  of the driving circuit  300  centers around the operational amplifier  378 . Inputs to the op amp  378  are coupled to either side of a sensing resistor R 2 , which is used to sense the current passing through the LEDs  372 . The op amp  378  then generates its output to operate transistors as current sources so that the amount of current passing through the LEDs  372  is balanced by the current passing through the transistors Q 7  and Q 8 . This is an example of controlling the compensated current  252  referred to above with reference to  FIG. 2 . 
     In operation, and with reference to  FIGS. 3, 4 and 5 , the AC signal input generally cycles between +/−160 volts (giving an RMS (Root Mean Squared) voltage of approximately 120 volts AC). This is illustrated in the top portion of  FIG. 5 . A rectifier, such as the BG 1 , reference  316  of  FIG. 3 , modifies the input AC signal so that it is always positive. Thus, the lower portion of  FIG. 5  shows two identical phases in a full cycle in which the voltage starts at zero, rises to  160   v  and returns to zero. The phases are generally labeled phase A and phase B or cycle A and cycle B, which together make a single AC cycle. 
       FIG. 4  is a phase table that is used in conjunction with the timing diagram illustrated in  FIG. 5 , which describes an AC cycle  502  (solid line going from 0 to 160 v in the top portion of  FIG. 5 ), rectified one-half AC cycles  503 A and  503 B, illustrated by a dotted line), dashed lines  512 A and  512 B which have the same period as the rectified AC cycles  503 A and  503 B but have an amplitude limited at 90 v, and a heavy line labeled  525 , which illustrates the voltage at Node  324 , which is the output of the capacitor C 3  after it has passed through the inductor L 1 . 
     In general, capacitor C 1  is active in cycle A, while the capacitor C 2  is building and storing charge. Then, in cycle B, capacitor C 2  is active while capacitor C 1  is building and storing charge. 
     The cycle begins in  FIG. 5  partially through phase A 1  (AΦ 1 ). During this phase, the capacitor C 3  is discharging from about 85 volts to approximately 50 volts, with the energy discharging to power the LEDs  372  of  FIG. 3 . At the beginning of phase A 2  (AΦ 2  ), the switch Q 2  connects the capacitor C 1  to C 3 , and charge previously stored in the capacitor C 1  charges capacitor C 3 , which causes the voltage on C 3  to raise to approximately 70 volts, as illustrated in  FIG. 4 . Including the inductor L 1  in series with the capacitor C 3  prevents a large rush of current that would otherwise occur when the capacitor C 1  is connected to C 3  in the A 2  (AΦ 2  ) stage. Including the inductor L 1  prevents premature wear of circuit components, as well as reducing noise or other interference generated by the circuit  300 . 
     Also in stage A 2  (AΦ 2  ) is the nulling of the amplifier  378  during the time when the two capacitors, C 1  and C 3  in this phase, add their voltages. These voltages carry both the input signal and the error signal from component tolerances also get nulled. 
     In phase A 3  (AΦ 3 ), the voltage on the coupled capacitors C 1  and C 3  falls as the LEDs  372  continue to draw current from them. 
     During this time, between phases A 2  and A 3 , some current, approximately 50 uA, is dissipated across the inductor L 1  as the capacitor C 3  is rapidly charged and then discharges. The inductor is an important feature of the driving circuit  300 , though. Without the inductor L 1 , a large amount of current would surge through the switch Q 2  as it turns on, because such action effectively couples the two capacitors directly together while they may have different charges. Thus, without the inductor L 1 , a current spike would occur as capacitor C 3  was charged by C 1  during phase A 2 . If all of the components were ideal components, the current would actually be infinite. Instead, were the inductor L 1  not present, the brunt of the large current spike would be borne by the switch Q 2 . Additionally, such a current spike would cause a large amount of EMI (Electro-Magnetic Interference), which is greatly diminished by including the inductor L 1  within the driving circuit  300 . 
     Returning back to  FIGS. 4 and 5 , In phase A 4  (AΦ 4 ), the coupled capacitors C 1  and C 3  are both charged by the incoming AC voltage to approximately 90 volts. After the peak voltage is reached, the capacitors C 1  and C 3  discharge by powering the LEDs  372 . At the end of A 4  (AΦ 4 ), the capacitor Q 2  turns off, which disconnects the capacitor C 1  from the capacitor C 3 . 
     In phase B 1  (BΦ 1 ), the capacitor C 3  continues to discharge, powering the LEDs  372 , while the AC input signal drops to zero. The stored charge in capacitor C 3  continues to discharge, however, until the beginning of phase B 2  (BΦ 2 ). At that time, the beginning of phase B 2 , the switch Q 3  turns on, which connects the capacitor C 2  to the capacitor C 3 . The same benefits of having the inductor L 1  used during the A 2  (AΦ 2 ) phase are present during the (BΦ 2 ) phase. 
     From there, the remainder of the phases B 3  and B 4  continue just as the phases A 3  and A 4  previously, except that it is the capacitor C 2  connected to the capacitor C 3 , through the operation of the switch Q 3 , rather than the capacitor C 1  connected to the capacitor C 3 . 
     Overall, the operation of the driving circuit  300  compared to other driving circuits keeps the output voltage low, providing only what the LEDs  372  need. In one embodiment, the LED elements making up the group of LEDs  372  use approximately 3.5V each, for a total of 3.5V×13LEDs=42V. The AC input cycles between 0V and approximately 90V, which averages at approximately 40-45 volts. Any voltage higher than the 45 volt operating voltage wastes power in the driver circuit, which produces excess heat, while any lower voltage produces less light from the LEDs. 
       FIGS. 6A and 6B  better illustrates the power savings achieved using embodiments of the invention. Shown are two voltage graphs during times T 0  and T 1 . In  FIG. 6A , a voltage trace  603  shows a voltage rising from 0 to 160 volts. A standard AC voltage would then return to zero volts, such as illustrated in  FIG. 5 , however, the trace  604  in  FIG. 6A  includes delay from capacitors in the network illustrated in  FIG. 5 , and therefore has a slower decay. 
     Signal trace  625  in  FIG. 6B  matches the output signal  525  of  FIG. 5  that is used to drive the LEDs according to embodiments of the invention. Signal trace  625  likewise includes capacitive delay, and, for this and other reasons, does not return to zero as quickly as signal trace  612 . 
     Another significant difference between traces  604  and  625  is that signal trace  604  has a maximum voltage of 160 v, while signal trace  625  has a maximum voltage of approximately 90 v. 
     Area under a voltage curve has a direct relationship to power consumed while operating a device with the voltage. For example, In  FIG. 6A  an area bounded by times T 0  and T 1  on the left and right, 0 volts on the bottom, and the signal trace  604  on the top has a first area. The first area is the total of area portions A, B, and C. Similarly, in  FIG. 6B  an area bounded by times T 0  and T 1  on the left and right, 0 volts on the bottom, and the signal trace  625  on the top has a second area. The second area is made of area portions D and E. 
     Comparing the area portions illustrates the power savings by using a driving circuit according to embodiments of the invention. Areas A and D represent energy consumed, typically expressed as heat, while operating the circuit below a turn-on voltage of the multiple LEDs described above. Areas A and D are approximately similar. Area B represents the power used if the voltage signal that makes the signal trace  604  were used to operate LEDs. In other words, energy consumed in Area B generates heat in the driving circuit, but additionally generates light by lighting LEDs. Area E similarly is that energy consumed making heat in the driving circuit and light by lighting the LEDs. A large difference between the areas under the graphs of  FIGS. 6A and 6B  is illustrated with area C, however. Area C represents heat generated if a capacitor-modified AC signal  604  were used to drive a group of LEDs, as is the case with some driving circuits. This heat is wasted, however, because the group of LEDs does not shine any brighter if the driving voltage is operated above approximately 90 volts. Therefore the heat generated in Area C represents energy wasted in other, standard, driving circuits. 
     Comparing the first and second areas of  FIGS. 6A and 6B  shows that a standard driving circuit of  FIG. 6A  is approximately 40.5% efficient, while the driving circuit of  6 B according to embodiments of the invention is approximately 61.8% efficient. Thus, LEDs driven with a driving circuit according to embodiments of the invention is more efficient, and consumes less energy than standard driving circuits. 
     Having described and illustrated the principles of the invention with reference to illustrated embodiments, it will be recognized that the illustrated embodiments may be modified in arrangement and detail without departing from such principles, and may be combined in any desired manner. And although the foregoing discussion has focused on particular embodiments, other configurations are contemplated. 
     In particular, even though expressions such as “according to an embodiment of the invention” or the like are used herein, these phrases are meant to generally reference embodiment possibilities, and are not intended to limit the invention to particular embodiment configurations. As used herein, these terms may reference the same or different embodiments that are combinable into other embodiments. 
     Consequently, in view of the wide variety of permutations to the embodiments described herein, this detailed description and accompanying material is intended to be illustrative only, and should not be taken as limiting the scope of the invention. What is claimed as the invention, therefore, is all such modifications as may come within the scope and spirit of the following claims and equivalents thereto.