Patent Publication Number: US-7902761-B2

Title: Dimmable LED lamp

Description:
GOVERNMENTAL RIGHTS 
     None. 
    
    
     CROSS REFERENCES 
     None. 
     BACKGROUND OF THE INVENTION 
     The invention relates to the field of light-emitting diode (“LED”) lamps, specifically LED lamps capable of being dimmed to reduce energy usage and prolong the life of the lamp. 
     Lamps of all kinds, whether incandescent, halogen, sodium vapor, or LED, produce heat as a byproduct of the electrical production of light. The hotter the lamp temperature becomes the shorter the life of the lamp. Lamps are typically designed to provide a designed life at a specified voltage and amperage. For instance, a typical incandescent bulb powered with 110 volts of AC power that draws 25 to 110 W has a typical life expectancy of 2,000 hours. If the voltage applied to an incandescent lamp&#39;s filament is reduced by 5%, the life of the bulb is doubled and the light output is reduced by approximately 20%. Thus, it is known in the art that dimming a lamp results in prolonged life. 
     Energy prices have generally risen above the rate of inflation since the 1970&#39;s, and energy efficiency has become increasingly important in the global economy. Lamps of all kinds are subject to increasing energy efficiency standards, and the United States has passed laws phasing out inefficient incandescent bulbs by 2014. 
     LED lamps represent one type of lighting technology vying to replace incandescent bulbs on the market. Typical LED lamps have several LEDs mounted in series to a circuit board. LEDs are semiconductors in which the current flows in only one direction; that is, typical LED lamps require DC current for proper operation. For years, this property led to a limitation on AC-powered LEDs in which an AC-DC converter was required as part of the LED lamp. Such a converter is inherently inefficient, as it serves as an additional source of heat and requires space not available in many lamp applications. Thus, it is an object of the invention to provide a dimmable LED lamp that utilizes AC power without requiring a separate AC-DC converter. 
     U.S. Pat. No. 7,417,259 (the “&#39;259 patent”) discloses an LED lamp that natively runs on AC power. To achieve this, the &#39;259 patent discloses two sets of LEDs wired in series, with each set having an opposite polarity. Thus, when alternating current is applied to the circuit, one set of LEDs is alternatively lit at any given time. The &#39;259 patent thus provides an LED lamp that runs on AC power without the necessity of an AC-DC converter; however, the LED lamp disclosed in the &#39;259 patent is not capable of being dimmed without flickering due to the inherent instability of AC current. That is, in virtually all applications, it is well known that AC power supplied by the public grid will fluctuate in both voltage and frequency, yet the &#39;259 patent does not account for such fluctuations. It is thus an object of the invention to provide an AC-powered LED lamp that is also capable of being dimmed by various dimming circuit means. 
     Dimming devices are well known in the art. For example, U.S. Pat. No. 794,983 (the “&#39;983 patent”) discloses a rheostat, which is a device used to vary the resistance in a circuit, thus varying the voltage available to the rest of the circuit. Rheostats are capable of dimming incandescent bulbs by reducing the voltage across the filament of the incandescent bulb. While rheostats are well suited to dimming an incandescent bulb, rheostats are not as efficient as some would believe because rheostats do not actually reduce the power used by the circuit; rather, the power is converted to heat by the rheostat. Thus, the energy is not converted to light by the incandescent bulb filament. It is an object of the invention to provide a dimmable LED lamp in which the actual circuit power consumption is reduced when the LED lamp is dimmed by a user. 
     Traditional rheostats are standard dimming devices used in the lighting industry, making them commercially viable for retrofit uses of AC-powered LED lamps. However, no prior art solves the problem of flickering when using a rheostat with an AC-powered LED lamp. To illustrate, while incandescent bulb filaments produce both heat and light proportional to the voltage applied across such filaments, LEDs require a minimum “on voltage” to be operable. The on voltage for an LED depends on the current applied and the type of semiconducting material from which the LED is made, but the on voltage is always significant. Due to this limitation, AC voltage waves for LEDs are said to be “square waveforms,” whereas AC voltage waves for incandescent bulbs are more typically sinusoidal waveforms. Thus, instead of having three points in time at which the voltage is zero as in a sine wave, a square wave for an LED has three much longer discrete blocks of time during which the voltage is zero. When a rheostat is connected to an LED lamp, the square wave defined by the on voltage creates an unwanted visible flicker. It is thus an object of the invention to provide an AC-powered LED lamp that does not have such a visible flicker. 
     Other dimming devices standard in the industry include silicon-controlled rectifiers (“SCRs”) and triodes for alternating current (“TRIACs”). When these types of semiconducting dimmers are used to dim an AC-powered LED lamp, the phase-controlled sinusoidal voltage waveform nonetheless results in a visible flicker and other unwanted harmonics. It is thus an object of the invention to provide an AC-powered LED lamp that does not have a visible flicker, regardless of the dimming circuitry used. It is a further object of the invention to provide an AC LED lamp capable of being connected to a rheostat, SCR dimmer, TRIAC dimmer, or other dimmer known in the prior art, thus achieving the goals of power savings and reduction of heat, even in retrofit applications. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention solves the problems of the prior art by providing an LED lamp that is capable of being dimmed. The LED lamp has one or more bi-directional LED chips that can be powered with AC power. Each bi-directional chip is enclosed within resistors of substantially equal resistance in order to ensure the voltage applied across each bi-directional LED chip is approximately equal. A varistor shields the bi-directional LED chips from voltage spikes that could damage the bi-directional LED chips. An LC filter removes high-frequency voltage spikes to protect the bi-directional LED chips, thus reducing flicker and increasing light output for a predetermined voltage. A dimmer is connected to the AC power source to preferentially regulate voltage available to the remainder of the circuit. The invention is compatible with dimmers of the prior art, including rheostats, SCR dimmers, DIAC and TRIAC dimmers, and other dimmers known to persons having ordinary skill in the art. 
     The advantages of the invention include providing extended life to LED lamps by reducing the junction temperature of the LEDs integrated within the bi-directional LED chips; the inventors have realized nearly double the life expectancy at half the illumination output by reducing the junction temperature by approximately twenty percent (20%). 
     These and other advantages provided by the invention will become apparent from the following detailed description which, when viewed in light of the accompanying drawings, disclose the embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing an implementation of the invention having a power source, load, rheostat, and surge protection. 
         FIG. 2  is a circuit diagram showing one implementation of a bi-directional LED chip  103 . 
         FIG. 3   a  is a circuit diagram showing the connection of multiple bi-directional LED chips  103  in parallel. 
         FIG. 3   b  is a block diagram symbol for multiple bi-directional LED chips  103  that are connected in parallel. 
         FIG. 4  is a circuit diagram showing an implementation of the invention having a power source, load, SCR dimmer, and surge protection. 
         FIG. 5   a  is one method of implementing an LC filter into the invention. 
         FIG. 5   b  is another method of implementing an LC filter into the invention. 
         FIG. 6  is a chart showing the decreased junction temperature and increased life expectancy realized by the invention. 
         FIG. 7  is a circuit diagram showing the preferred arrangement of the invention. 
     
    
    
     LISTING OF COMPONENTS 
       101 —AC-powered LED lamp 
       103 —bi-directional LED chip 
       105 —AC power source 
       107 —varistor 
       109 —resistors 
       111 —rheostat 
       113 —light-emitting diodes (“LEDs”) 
       115 —electrodes 
       117 —n-LED block 
       119 —semiconductor-controlled rectifier (“SCR”) dimmer 
       121 —SCR gate 
       123 —variable resistor 
       125 —capacitor 
       127 —Darlington transistor 
       129 —NPN transistor 
       131 —PNP transistor 
       133 —LC filter 
       135 —dimmer 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention as disclosed herein provides a virtually flicker-free, AC-powered LED lamp capable of being preferentially dimmed. As seen in  FIG. 1 , AC-powered LED lamp  101  comprises a circuit in which one or more bi-directional LED chips  103  are connected to AC power source  105 . Bi-directional LED chip  103  is a typical AC-powered LED binary chip designed to be either switched off or supplied with full power from a power grid, such as 110V AC or 220V AC. One bi-directional LED chip is manufactured by Seoul Semiconductor under the trade name Acriche. A varistor  107  is placed in parallel to bi-directional LED chip  103  in order to shunt excess current due to voltage spikes from AC power source  105 . Resistors  109  having substantially equivalent resistance are connected to either side of each bi-directional LED chip  103  in order to apply the proper predetermined amount of voltage across each bi-directional LED chip  103 . A rheostat  111  may be added in series with AC power source  105  in order to control the voltage applied to bi-directional LED chips  103 . In such configuration, rheostat  111  provides dimming functionality to AC-powered LED lamp  101 . Such configuration provides backwards compatibility with rheostats of the prior art. 
     Resistors  109  and varistor  107  provide the basic functionality to AC-powered LED lamp  101 . Without resistors  109 , voltage levels may vary across bi-directional LED chip  103 , leading to chip damage, particularly when more than one bi-directional LED chip  103  is used. Likewise, without varistor  107 , voltage levels may spike across bi-directional LED chip  103  (even when resistors  109  are present), thus damaging bi-directional LED chip  103 . 
       FIG. 2  shows a basic implementation of a bi-directional LED chip  103 . Bi-directional LED chip  103  comprises two strands of LEDs  113  that are connected in series. The two strands of LEDs  113  are connected in parallel to electrodes  115 . Because LEDs  113  are diodes, current flow is unidirectional. Thus, when an AC power source  105  is supplied to the electrodes  115  of bi-directional LED chip  113 , the supplied current alternates between flowing through each strand of LEDs  113 . That is, current only flows through one strand of LEDs  113  at a time due to the unidirectional property of LEDs  113 . Of course, persons having ordinary skill in the art will recognize that the concept of a bi-directional LED chip  103  may be implemented in any number of ways with any number of LEDs  113 ; the circuit diagram provided in  FIG. 2  is for illustrative purposes only and is not intended as a limitation to the invention described herein. 
     Turning now to  FIG. 3   a  and  3   b , multiple bi-directional LED chips  103  may be connected in parallel to provide AC-powered LED lamp  101  with additional light output. Resistors  109  (R 1 , R 2 , . . . R 2n−1 , R 2n ) are required to be connected in series with each bi-directional LED chip  103  in order to provide equal voltage across all bi-directional LED chips  103 ; resistors  109  in series with bi-directional LED chips have equal resistances. Equal voltage across bi-directional LED chips  103  provides uniform color and lumen output for each bi-directional LED chip  103 . 
       FIG. 3   b  shows the block diagram symbol for an n-LED block  117  in which n bi-directional LED chips  103  are connected in parallel. Depending on the light output desired, n may range from one (1) to ten (10) bi-directional LED chips  103  or even higher. As one example, an n-LED block  117  that uses four (4) bi-directional LED chips  103  draws 16 watts of power to provide a light output of 594 lumens. Such light output is virtually identical to the 595 lumen output of a 65-watt incandescent bulb, and the invention uses less than 25% of the power of a standard incandescent bulb. 
     As seen in  FIG. 4 , rheostat  111  may be replaced with a semiconductor-controlled rectifier (“SCR”) dimmer  119 . As compared to dimming provided by rheostat  111 , SCR dimmer  119  provides dimming capability with reduced power usage. SCR dimmer  119  is connected in series with respect to n-LED block  117 . SCR dimmer  119  comprises an SCR gate  121 , a variable resistor  123 , a capacitor  125 , a Darlington transistor  127 , and several resistors  109 . Variable resistor  123  responds to human or automated input to provide a predetermined resistance capable of reducing current across n-LED block  117 . Variable resistor  123  is connected in series to a resistor  109  (R 1 ) in order to ensure that the circuit comprising SCR dimmer  119  maintains a finite resistance. Variable resistor  123  operates to control the voltage at which capacitor  125  charges. When capacitor  125  charges, Darlington transistor  127  activates and triggers SCR gate  121 . Darlington transistor  127  is comprised of NPN transistor  129 , PNP transistor  131 , and resistor  109  (R 2 ). The voltage level at which SCR gate  121  triggers is controlled by resistors  109  (R 3 -R 5 ). Once SCR gate  121  is triggered, LED lamp  101  illuminates. 
     Another component of LED lamp  101  is an LC filter  133 , which provides reduced harmonics and flickers caused by conduction of n-LED block  117  and by changes to the conduction phase angles generated by the SCR dimmer  119 . LC filter  133  also increases the duration the voltage applied across bi-directional LED chip  103  remains above the threshold level required to illuminate bi-directional LED chip  103 . Various methods of implementing LC filter  133  are shown in  FIGS. 5   a  and  5   b.    
     The inventors have provided examples of the invention in use with various dimming devices known in the prior art, including a rheostat and an SCR dimmer. Persons having ordinary skill in the art will recognize that other dimmers, such as TRIACs, may be substituted for the dimmers disclosed herein. The inventors intend to claim all such substitutions as may fall within the scope of the invention. 
     Turning now to  FIG. 6 , the benefits of using dimmable LED lamp  101  include lower junction temperature in each LED  113  that comprises bi-directional LED chip  103 . Lower junction temperature translates into longer life for bi-directional LED chip  103 . The following table presents the data points shown in  FIG. 6 : 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Illumination Level (%) 
                 Junction Temp. (° C.) 
                 Life Expectancy (hours) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 100% 
                 65 
                 25000 
               
               
                 90% 
                 62 
                 29000 
               
               
                 80% 
                 60 
                 35000 
               
               
                 70% 
                 58 
                 42000 
               
               
                 60% 
                 54 
                 45000 
               
               
                 50% 
                 52 
                 49000 
               
               
                   
               
            
           
         
       
     
     The preceding disclosure discusses the various necessary and optional components of the invention, which the inventors have used to create several preferred embodiments. The general circuit diagram for each of the preferred embodiments is shown in  FIG. 7 . In the first preferred embodiment, AC power source  105  has a voltage of 120 VAC. Resistors  109  used in n-LED block  117  have a resistance of 470Ω. N-LED block  117  has 3 bi-directional LED chips  103  drawing 2 W, giving LED lamp  101  a power draw of 6 W. Varistor  111  has a maximum resistance of 50 kΩ. Varistor  107  is a metal oxide varistor having an operating voltage range of 200V to 460V RMS. LC filter  133  has a capacitance of 0.1 to 2.2 μF and an inductance of 1-5 mH. The first preferred embodiment utilizes a dimmer  135 , which may be a rheostat  111 , an SCR dimmer  119 , or other dimming device. 
     The difference between the first preferred embodiment and the remaining three preferred embodiments deals with the number of LEDs  113  in n-LED block  117  and the values for voltage, resistance, capacitance, inductance, and power for the elements of the circuit comprising LED lamp  101 . The table below summarizes the preferred embodiments: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                 Element 
                 Pref. Emb. 1 
                 Pref. Emb. 2 
                 Pref. Emb. 3 
                 Pref. Emb. 4 
                 Pref. Emb. 5 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 V source  (VAC) 
                 120 
                 120 
                 120 
                 12 
                 120 
               
               
                 n 
                 3 
                 5 
                 4 
                 1 
                 1 
               
               
                 R n  (Ω) 
                 470 
                 470 
                 470 
                 470 
                 470 
               
               
                 W (per chip) 
                 2 
                 2 
                 4 
                 3 
                 4 
               
               
                 W (total) 
                 6 
                 10 
                 16 
                 3 
                 4 
               
               
                 V RMS  (VAC) 
                 4-460 
                 4-460 
                 4-460 
                 4-460 
                 4-460 
               
               
                 C (μF) 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 L (mH) 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
         
       
     
     While the inventors have described above what they believe to be the preferred embodiments of the invention, persons having ordinary skill in the art will recognize that other and additional changes may be made in conformance with the spirit of the invention and the inventors intend to claim all such changes as may fall within the scope of the invention.