Abstract:
A LED lamp which could directly replace an ordinary tungsten, halogen, or electricity-saving light bulb includes a filament, a lamp base and a thermally conductive electric insulator. The filament includes at least one AC LED device, and the thermally conductive electric insulator is filled in a cavity of the lamp base to mechanically contact with the filament and an electrode of the lamp base. When the AC LED device is powered on, the thermally conductive electric insulator provides a thermal channel to transfer heat from the filament to the electrode for heat dissipation enhancement. The LED lamp can be directly inserted into an ordinary bulb socket that is generally used in lighting fixtures, without having to modify the system of the lighting fixtures or use an additional adapter.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related generally to electric lamps and, more particularly, to a LED lamp which could directly replace an ordinary tungsten, halogen, or electricity-saving light bulb. 
       BACKGROUND OF THE INVENTION 
       [0002]    A light emitting diode (LED) lamp using a direct current (DC) LED device as the filament must be equipped with a power converter for converting the alternating current (AC) power voltage into a DC input voltage for the DC LED device. The power converter not only requires additional component cost for the LED lamp, but also cannot fit entirely into the standard lamp bases of ordinary light bulbs. For a LED lamp to be equipped with a power converter, it is necessary to develop special molds to produce containers and corresponding mechanism different from those of ordinary light bulbs to fit the power converter therewithin, which nevertheless increases the cost and volume of the LED lamp. On the other hand, a DC LED device generates heat when it is powered on and therefore, an additional heat dissipation mechanism is required to handle the heat. If the heat is not effectively dissipated, the resulting high temperature will reduce the emissive efficiency and service life of the DC LED device and produce other adverse effects such as wavelength shift. Moreover, the power converter, particularly the inductor and integrated circuit therein, also generates heat during power conversion, and the consequent high temperature may damage the inductor and integrated circuit and cause failure of the LED lamp accordingly. The problems caused by insufficient heat dissipation are aggravated especially in high power applications, such as in lighting fixtures for illumination purposes, where the DC LED device generates relatively more heat. To adapt to the relatively small space within ordinary lamp bases, some LED lamps use a plurality of low power lamp type LED devices in conjunction with a simple bridge rectifier circuit. However, low power LED devices are poorly accepted in the market due to their generally low brightness, and these LED lamps tend to have serious light attenuation problems as a result of poor heat dissipation. 
         [0003]    In recent years, AC LED devices are maturing technically, have improved in brightness, and therefore have had commercial value. An AC LED device includes a plurality of serially and/or parallel connected LED electronic elements manufactured on an epitaxial chip. The epitaxial chip is packaged and then connected in series with a resistor having a particular resistance so as to withstand high voltage, e.g., 110 V or 220 V, mains electricity, thus dispensing with the power converter or rectifier circuit required for a DC LED device. In consequence, the cost of an AC LED lamp is lowered in comparison with its DC counterpart, and the circuit related quality issues reduced. An AC LED device, though conveniently applicable in small spaces, still demands heat dissipation. This is especially true in high power applications, such as lighting fixtures for illumination purposes, where the AC LED device generates relatively more heat. If a heat dissipating device is added, the resultant LED lamp will be bulky and costly. However, if no additional assistance is provided to enhance heat dissipation from the AC LED device, the emissive efficiency and service life of the AC LED device will be reduced, wavelength shift is likely to happen, and even worse, the LED epitaxial chip may be burned out. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the present invention is to provide a LED lamp which enhances the heat dissipation of the AC LED device in the LED lamp. 
         [0005]    Another object of the present invention is to provide a LED lamp which could directly replace an ordinary tungsten, halogen, or electricity-saving light bulb. 
         [0006]    A LED lamp according to the present invention comprises a filament, a lamp base and a thermally conductive electric insulator. The filament includes at least one AC LED device, and the thermally conductive electric insulator is filled in a cavity of the lamp base to mechanically contact with the filament and an electrode of the lamp base. When the AC LED device is powered on, the thermally conductive electric insulator provides a thermal channel to transfer heat from the filament to the electrode for heat dissipation enhancement. 
         [0007]    Standard lamp bases for ordinary light bulbs can be selected for the lamp base of a LED lamp according to the present invention, and thus the LED lamp could be inserted into the ordinary bulb sockets that generally used in lighting fixtures, without having to modify the system of the lighting fixtures or use an additional adapter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
           [0009]      FIG. 1  is a cross-sectional view of a LED lamp in a first embodiment according the present invention; 
           [0010]      FIG. 2  shows an equivalent circuit of the LED lamp depicted in  FIG. 1 ; 
           [0011]      FIG. 3  provides three AC LED epitaxial chips; 
           [0012]      FIG. 4  is a top view of a filament using multiple LED epitaxial chips; 
           [0013]      FIG. 5  is a cross-sectional view of a LED lamp in a second embodiment according the present invention; and 
           [0014]      FIG. 6  is a cross-sectional view of a LED lamp in a third embodiment according the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  provides a first embodiment according to the present invention, in which a standard lamp base  10  for use with a small light bulb is used to accentuate the features of the present invention. The lamp base  10  has two electrodes  12  and  14  for receiving an AC power source. As would be understood by a person of ordinary skill in the art, the electrode  12  is a metal housing having a spiral-threaded configuration  16  and a cavity  18  therein. In this embodiment, an AC LED device  20  is used as the filament of the LED lamp, which includes an AC LED epitaxial chip  22  bounded on a leadframe  24  and covered with an encapsulant  26 . As the LED packaging is a well-known technique, the package structure of the AC LED device  20  is not detailed in the drawing for the sake of simplicity. A resistor  30  has one end soldered to the electrode  14  and an opposite end connected to a wire  32  that is soldered to the AC LED device  20 . Another wire  34  has its two ends soldered to the electrode  12  and the AC LED device  20 , respectively. This LED lamp has the equivalent circuit shown in  FIG. 2 , in which the AC LED epitaxial chip  22  and the resistor  30  are connected in series between the electrodes  12  and  14 . As would be understood by a person of ordinary skill in the art, a so-called AC LED epitaxial chip includes LED electronic elements oriented in two opposite directions and connected in parallel between two pins, with at least one LED electronic element in each direction. The LED electronic elements oriented in the two opposite directions are lit during the positive and negative half cycles of the AC power source, respectively. The resistor  30  has a resistance R chosen according to the current intensity required by design. The resistor  30  also serves to protect the AC LED epitaxial chip  22 . More specifically, when a surge occurs in the AC power source connected to the electrodes  12  and  14 , the resistor  30  will absorb most of the surge voltage. Referring back to  FIG. 1 , a major feature of the present invention is to fill the cavity  18  with a thermally conductive electric insulator  36  such that the thermally conductive electric insulator  36  is in mechanical contact with the electrode  12  and the filament, i.e. the leadframe  24  in this case, to provide a thermal channel to transfer the heat generated by the AC LED epitaxial chip  22  to the electrode  12  when the AC LED epitaxial chip  22  is powered on to emit light, thereby enhancing the heat dissipation therefrom. As would be understood by a person of ordinary skill in the art, the leadframe  24  typically includes a metal plate for facilitating heat dissipation from the AC LED epitaxial chip  22 . Therefore, by attaching the leadframe  24  to the thermally conductive electric insulator  36 , good thermal conduction effect can be achieved. In addition to enhance the heat dissipation from the AC LED epitaxial chip  22 , the thermally conductive electric insulator  36  also assists in heat dissipation from the resistor  30  because the resistor  30  is buried therein. 
         [0016]    For the thermally conductive electric insulator  36 , it may select epoxy resin, or thermal conductor powder such as aluminum oxide, aluminum nitride, boron nitride, or any other thermally conductive materials in powder form, or a mixture thereof. Table 1 shows experiment results of using three different thermally conductive materials in the LED lamp of  FIG. 1 . 
         [0000]                                    TABLE 1               Thermally       Power       Condition after       conductive   Voltage of   consumed by   Output   being lit       electric   AC power   AC LED   brightness   continuously for       insulator 36   source   device 20   (lm)   1000 hours                   Epoxy resin   110 V   1   65   No abnormality                       detected, except                       for relatively high                       temperature       Epoxy resin +   110 V   1   68   No abnormality       aluminum               detected       oxide powder       Aluminum   110 V   1   68   No abnormality       oxide powder               detected                    
As shown in Table 1, when epoxy resin, which has a lower thermal conductivity, was used as the thermally conductive electric insulator  36 , a higher temperature was detected after the LED lamp was powered on. On the other hand, the mixture of epoxy resin and thermal conductor powder has a higher thermal conductivity, and therefore no abnormality was found during the lighting test. Good thermal conduction effect was also obtained by directly using thermal conductor powder, filled into the cavity  18  and compacted, as the thermally conductive electric insulator  36 . In general, the LED lamp under test had satisfactory output brightness, and substantially no abnormality was detected after the LED lamp was lit continuously for 1000 hours. Other materials may also be used as the thermally conductive electric insulator  36 , which preferably has a thermal conductivity ranging from 0.25 to 30 W/mK.
 
         [0017]    As shown in  FIG. 1 , the LED lamp according to the present invention has approximately the same size as the lamp base  10 , possesses good heat dissipation ability, and is capable of high power applications that are unachievable by the prior art devices. Ordinary light bulbs are equipped with standard lamp bases. For example, lamp bases under the standards E12, E14, E17, E26 and E27 are for the ordinary tungsten light bulbs, and MR16 and GU10 lamp bases are for the ordinary halogen light bulbs. The lamp base of an ordinary halogen light bulb has an electrode formed as a columnar metal housing and separated from the other electrode by an electric insulator. Some other standard lamp bases use two needle-like electrodes that are insulated from each other. The lamp base for a LED lamp according to the present invention can be one of ordinary tungsten or halogen light bulbs or other standard lamp bases where there is always a cavity to be filled with the thermally conductive electric insulator  36 , and in consequence at least one electrode serves to facilitate heat dissipation from the filament of the LED lamp. As the electrodes of standard lamp bases are exposed outside, fair heat dissipation effect is attainable. 
         [0018]    Referring to  FIG. 1 , a lamp cover  40  may be further added to the LED lamp, depending on demands. The lamp cover  40  can be a glass cap, a plastic cap, an epoxy resin cap, or a silicone cap. If a glass cap or a plastic cap is selected, it is bounded to an end of the lamp base  10  by a mechanical means such as gluing, mortise-and-tenon engagement, or screw thread engagement. If an epoxy resin cap or a silicone cap is selected, it is dispensed over the filament in an amount sufficient to completely cover the filament, and the epoxy resin or silicone is heated and cured if necessary. The lamp cover  40  functions as a protective shell for preventing moisture, dust, or external force from affecting internal components of the LED lamp. Besides, the lamp cover  40  also serves as an optical component. More specifically, the lamp cover  40  may be frosted or configured with geometric patterns so as to produce the desired optical effects. The frosted structure of the lamp cover  40  can be formed by sand blasting, etching, electrostatic powder coating, coating with silicone, spraying with paint, or injection molding. 
         [0019]    Alternatively, the filament may include a circuit board to be bounded with the AC LED epitaxial chip  22  thereon. In this case, the circuit board is attached on the thermally conductive electric insulator  36 , and the AC LED epitaxial chip  22  may be a surface mounting device (SMD) or have a chip on board (COB) package structure, in addition to the lamp type LED device  22  shown in  FIG. 1 . 
         [0020]    An AC LED epitaxial chip including more than two LED electronic elements may be used for the AC LED epitaxial chip  22  to provide brighter illumination.  FIG. 3  provides three such AC LED epitaxial chips  22 . The first one in the left includes two LED strings parallel connected in opposite directions between two pins of the AC LED epitaxial chips  22 , each LED string having two or more LED electronic elements. The second case in the middle includes two or more pairs of LED electronic elements serially connected between two pins of the AC LED epitaxial chips  22 , each pair of LED electronic elements parallel connected in opposite directions to each other. The last case in the right includes five or more LED electronic elements having a bridge configuration between two pins of the AC LED epitaxial chips  22 . There have been commercial products can be selected for these cases. 
         [0021]    If it is desired to increase the brightness of a LED lamp, more AC LED devices  20  can be connected in series, in parallel, or in series and parallel in the filament. For example, as shown in  FIG. 4 , a filament includes nine AC LED devices  20  bounded on a circuit board  28  in such a manner that three rows of AC LED devices  20  are connected in parallel between solder pads  52  and  54  on the circuit board  28 , and each row includes three AC LED devices  20 . If each of the AC LED devices  20  operates at a power of 1 W, the filament shown in  FIG. 4  can operate at a power as high as 9 W. 
         [0022]      FIG. 5  provides a second embodiment according to the present invention, in which a circuit board  28  has a through hole  60 , a thermally conductive member  50  passes through the through hole  60  and has a first end above the circuit board  28  and a second end buried in a thermally conductive electric insulator  36 , and an AC LED device  20  having a plastic leaded chip carrier (PLCC) package structure is bounded to the first end of the thermally conductive member  50 . The thermally conductive member  50  has two strips  56  and two flanges  58 . Each of the strips  56  has an axial length ranging from 0.1 to 10 cm, preferably ranging from 0.5 to 3.0 cm. The flanges  58  are sandwiched between the AC LED device  20  and the circuit board  28 . The circuit board  28  has through holes  62  to be soldered with the pins of the AC LED device  20  by means of solder  68 , and through holes  64  to be soldered to an electrode  12  by means of solder  70 . The through holes  62  and  64  may be replaced by blind holes or other structures, as is well known in the art of circuit board. A resistor  30  is soldered between an electrode  14  and the circuit board  28  such that the resistor  30  and the AC LED device  20  are connected in series between the electrodes  12  and  14 . The circuit board  28  has a glass fiber reinforced substrate or a metal substrate. Preferably, the circuit board  28  is also in mechanical contact with the thermally conductive electric insulator  36 . Alternatively, the resistor  30  is bounded on the circuit board  28 . In some other embodiments, a second resistor is bounded on the circuit board  28  and connected with the first resistor  30  in series. In these two cases, the resistor bounded on the circuit board  28  may be a variable resistor. If necessary, the LED lamp in this embodiment is provided with a lamp cover  40 , as in the previous embodiment. 
         [0023]      FIG. 6  provides a third embodiment according to the present invention, in which an AC LED device  20  is bounded to a circuit board  28  with a COB package structure, and the circuit board  28  is attached on a thermally conductive electric insulator  36 . The circuit board  28  has an aluminum metal layer  72 , a copper metal layer  76 , and a thermally conductive layer  74  sandwiched therebetween, and this structure exhibits better heat dissipation capability than a glass fiber reinforced substrate. The circuit board  28  is soldered to an electrode  12  by solder  70 , and a resistor  30  is soldered between an electrode  14  and the circuit board  28 , such that the resistor  30  and the AC LED device  20  are connected in series between the electrodes  12  and  14 . Alternatively, the resistor  30  is bounded on the circuit board  28 . In some other embodiments, a second resistor is bounded on the circuit board  28  and serially connected to the first resistor  30 . In these two cases, the resistor bounded on the circuit board  28  may be a variable resistor. If necessary, the LED lamp is provided with a lamp cover, as in the previous embodiment. In other embodiments, the AC LED device  20  may be a SMD that is bounded on the circuit board  28  by surface mounting technology (SMT). 
         [0024]    Depending on practice applications, it is selected the AC LED device  20  having a rated power ranging from 0.3 to 5 W, preferably from 1 to 3 W, and the resistor  30  preferably having a resistance ranging from 50 to 50,000Ω. In addition, it is selected the AC LED device  20  having a rated input voltage ranging from 12 to 240 V. For a LED lamp using a single AC LED device  20 , the rated input voltage of the AC LED device  20  is selected to be 110 or 220 V, depending on the power lines in its application. For a LED lamp using serially connected AC LED devices  20 , the rated input voltage of each AC LED device  20  is selected to be smaller, for example 12 V. 
         [0025]    While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.