Patent Publication Number: US-2012025228-A1

Title: Light-emitting device with temperature compensation

Description:
TECHNICAL FIELD 
     The application relates to a light-emitting device, and more particularly, to a light-emitting device with temperature compensation. 
     REFERENCE TO RELATED APPLICATION 
     This application claims the right of priority based on Taiwan application Serial No. 099125241, filed on Jul. 28, 2010, and the content of which is hereby incorporated by reference. 
     DESCRIPTION OF BACKGROUND ART 
     The light-emitting principle of light-emitting diode (LED) is to use the energy difference of the electrons moving between n-type semiconductor and p-type semiconductor, and the energy is released in the form of the light. This is different from the light-emitting principle of incandescent lamp, so LED is called the cold light source. 
     Furthermore, LED has the advantages of high durability, long life, light weight, and low power consumption. Today, LED is highly appreciated in lighting market and is regarded as a new generation of lighting tools, so it has gradually replaced traditional lightings, and is used in various fields such as traffic signal, backlight module, street lighting, and medical equipment. 
     In the application of lighting field, the near sunlight (white color light) spectrum emitted from LED is required to match human&#39;s visual habits. The white color light described above can be generated by mixing the three primary colors of red, blue, and green emitted from LED in different ratios through the deployment of operating current by the circuit design. Because the cost of circuit module is high, the method is not widespread. Another method uses ultraviolet spectrum light-emitting diode (UV-LED) to excite red, blue, and green phosphors capable of absorbing a part of light emitted by UV-LED and emitting the red color light, the blue color light, and the green color light. The red color light, the blue color light, and the green color light are mixed to generate the white color light. But the luminous efficiency of UV-LED still needs to be improved, the application of the product is not widespread. 
     Nevertheless, when the electric current is driven into the LED, in addition to the electric energy-photo energy conversion mechanism, part of the electric energy is transformed into the thermal energy, thus causing changes in the photoelectric characteristics. When the junction temperature (T j ) of the LED is increased from 20° C. to 80° C., the curve of the photoelectric characteristics of blue light LED and red light LED is illustrated in  FIG. 1 . As shown in  FIG. 1 , the vertical axis represents the relative value of the photoelectric characteristic value at different junction temperatures compared with that at 20° C. junction temperature of the light emitting device, such as light output power (P 0 ; rhombus symbol), wavelength shift (W d ; triangle symbol), and forward voltage (V f ; square symbol). The solid line shown in  FIG. 1  represents the characteristic curve of the blue light LED, and the dotted line shown in  FIG. 1  represents the characteristic curve of the red light LED. When the junction temperature is increased from 20° C. to 80° C., the light output power of the blue light LED drops about 12% and the hot/cold factor is about 0.88; the light output power of the red light LED drops about 37% and the hot/cold factor is about 0.63. Furthermore, in terms of the wavelength shift, there is no big difference between the blue light LED and the red light LED but is only slightly changed with the difference of T j . In terms of the forward voltage changes, when the junction temperature is increased from 20° C. to 80° C., the decline of the blue light LED and the red light LED is respectively about 7˜0.8%. Namely, the equivalent resistances of the blue light LED and the red light LED decline about 7˜8% under the operation of constant current. As mentioned above, because the temperature dependences of the blue light LED and the red light LED photoelectric characteristics are different, the undesirable phenomenon of the unstable red/blue light output power ratio happens during the period from the initial operation to the steady state. When the warm white light-emitting device comprising the red light LED and the blue light LED is used in the lighting field, the light color instability during the initial state and the steady state owing to the different hot/cold factors of the blue light LED and the red light LED causes the inconvenient when using the lighting. 
     SUMMARY OF THE APPLICATION 
     The present application provides a light-emitting device which comprises a light-emitting diode group comprising a plurality of light-emitting diode units electrically connected to one another; a temperature compensation element electrically connected to the light-emitting diode group described above. When a junction temperature of the light-emitting diode group is increased from a first temperature to a second temperature during operation, the current flowing through the light-emitting diode group at the second temperature is larger than the current flowing through the light-emitting diode group at the first temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the relationship curve between the junction temperature and the photoelectric characteristics of the light-emitting device; 
         FIG. 2  is a diagram of the light-emitting device of the first embodiment according to the present application; 
         FIG. 3  is a diagram of the light-emitting device of the second embodiment according to the present application; 
         FIG. 4  is a diagram of the light-emitting device of the third embodiment according to the present application; 
         FIG. 5  is a diagram of the light-emitting device of the fourth embodiment according to the present application; 
         FIG. 6  is a diagram of the light-emitting device of the fifth embodiment according to the present application; 
         FIG. 7  is a structure diagram of the light-emitting device of a light-emitting diode group according to the above-described embodiments the present application; and 
         FIG. 8  is a structure diagram of the light-emitting device according to the fourth embodiment or the fifth embodiment of the present application. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The embodiments of the present application are illustrated in detail, and are plotted in the drawings. The same or the similar part is illustrated in the drawings and the specification with the same number. 
       FIG. 2  illustrates an electric circuit diagram of the light-emitting device of the first embodiment according to the present application. The light-emitting device  200  comprises a first light-emitting diode group  202 , a second light-emitting diode group  204 , and a thermal resistor  206  with positive temperature coefficient. The first light-emitting diode group  202  comprises a first quantity of light-emitting diode units  208  connected to one another in series, the second light-emitting diode group  204  comprises a second quantity of light-emitting diode units  208  connected to one another in series, and the first light-emitting diode group  202  is electrically connected to the second light-emitting diode group  204  in series. The light-emitting diode unit  208  comprises the hot/cold factor no more than 0.9, preferably no more than 0.85, and further preferably no more than 0.8, and comprises a light-emitting diode capable of emitting visible or invisible wavelength, such as red, blue or ultraviolet wavelength light-emitting diodes, or formed by AlGaInP-based material, or GaN-based material. The hot/cold factor means the ratio of the light output power of the light-emitting diode at T j =80° C. and the light output power of the light-emitting diode at T j =20° C. when the junction temperature of the light-emitting diode in increased from 20° C. to 80° C. 
     In the embodiment, the second light-emitting diode group  204  is electrically connected to the thermal resistor  206  in parallel. The first light-emitting diode group  202  has an equivalent internal resistance R 1 , the second light-emitting diode group  204  has an equivalent internal resistance R 2 , and the thermal resistor  206  has a resistance R PTC , wherein R 1  and R 2  decrease when the junction temperature is increased. As shown in  FIG. 1 , when the light-emitting diode unit  208  is the red light or the blue light light-emitting diode, and T j  is increased from 20° C. to 80° C., R 1  and R 2  respectively decreases about 7˜8%. The resistance R PTC  of the thermal resistor  206  with positive temperature coefficient increases in the correlation when the temperature is increased, such as R PTC  increases in the linear or the non-linear correlation when the temperature is increased. During the operation of the light-emitting device  200 , an electric current I 1  such as 20˜1000 mA flowing through the first light-emitting diode group  202  is divided into I 2  flowing through the second light-emitting diode group  204  and I 3  flowing through the thermal resistor  206  when I 2  flows through the second light-emitting diode group  204  and the thermal resistor  206 , wherein I 1 =I 2 +I 3 . In addition, the potential difference of the two terminals of the second light-emitting diode group  204  is equal to the potential difference of the two terminals of the thermal resistor  206 . Namely, I 3 *R PTC =I 2 *R 2 . From the above two relationships, the electric current I 2  flowing through the second light-emitting diode group  204  is positive-correlated to R PTC /(R 2 +R PTC ). Namely, I 2  is respectively positive-correlated to R PTC  and negative-correlated to R 2 . In the embodiment, the junction temperature of the light-emitting device  200  is increased during operation. For example, the resistance R PTC  of the thermal resistor  206  is increased due to the increase of the junction temperature, and the resistance R 2  of the second light-emitting diode group  204  is decreased due to the increase of the junction temperature when the junction temperature is increased from the initial operation first temperature 20° C. to the steady state second temperature 80° C. Therefore, under the constant electric current I 1 , the electric current I 2  flowing through the second light-emitting diode group  204  is increased, and the light output power of the second light-emitting diode group  204  is increased due to the increase of I 2 . In other words, the light output power of the second light-emitting diode group  204  can be controlled by R PTC  to reduce the decline of the light output power of the second light-emitting diode group  204  caused by hot/cold factor when the junction temperature is increased, and the function of the temperature compensation is achieved. In addition, the decline of the light output power of the light-emitting device caused by hot/cold factor during the increase of the junction temperature can be offset or controlled by adjusting the quantity of the light-emitting diode units of the first light-emitting diode group and the second light-emitting diode group, or selecting the thermal resistor with suitable temperature coefficient. As shown in  FIG. 3 , the thermal resistor  206  of the embodiment can be electrically connected to the first light-emitting diode group  202  and the second light-emitting diode group  204  in parallel at the same time. Thus, the electric current flowing through the first light-emitting diode group  202  and the second light-emitting diode group  204  is increased compared with that at the initial temperature when the junction temperature of the light-emitting device is increased. 
       FIG. 4  is an electric circuit diagram of the light-emitting device of the third embodiment according to the present application. The light-emitting device  400  comprises a light-emitting diode group  402  and a thermal resistor  405  with negative temperature coefficient. The light-emitting diode group  402  comprises a plurality of light-emitting diode units  408  connected to one another in series. The light-emitting diode group  402  comprises the light-emitting diode capable of emitting visible or invisible wavelength, such as red, blue or ultraviolet wavelength light-emitting diodes, or formed by AlGaInP-based material, or GaN-based material. 
     In the embodiment, the light-emitting diode group  402  and the thermal resistor  405  are electrically connected in series. The light-emitting diode group  402  has an equivalent internal resistance R 1 , and the thermal resistor  405  has a resistance R NTC , wherein R 1  decreases when the junction temperature is increased. As shown in  FIG. 1 , when the light-emitting diode unit  408  is the red light or the blue light light-emitting diode, and T j  is increased from 20° C. to 80° C., R 1  decreases about 7˜8%. The resistance R NTC  of the thermal resistor  405  with negative temperature coefficient decreases in a correlation when the temperature is increased, such as R NTC  decreases in the linear or the non-linear relationship when the temperature is increased. When the light-emitting device  400  is operated under the constant electric voltage, the electric current I 1  flowing through the light-emitting diode group  402  is about 20˜1000 mA under the input V in  of constant electric voltage. According to Ohm&#39;s law, the electric current I 1  is inversely proportional to the total resistance of the light-emitting device  400  and the input voltage V in , that is, I 1 =V in /(R 1 =R NTC ). In other words, the electric current I 1  flowing through the light-emitting diode group  402  is negative-correlated to R NTC  and R 1 . In the embodiment, the junction temperature of the light-emitting device  400  is increased during operation. For example, the resistance R NTC  of the thermal resistor  405  and the resistance R 1  of the light-emitting diode group  402  are decreased due to the increase of the junction temperature when the junction temperature is increased from the initial operation first temperature 20° C. to the steady state second temperature 80° C. Thus, I 1  is increased, and the light output power of the light-emitting diode group  402  is increased due to the increase of I 1 . In other words, the light output power of the light-emitting diode group  402  can be controlled by the R PTC  to reduce the decline of the light output power of the light-emitting diode group  402  caused by hot/cold factor when the junction temperature is increased, and the function of the temperature compensation is achieved. In addition, the decline of the light output power of the light-emitting device caused by hot/cold factor during the increase of the junction temperature can be reduced by adjusting the quantity of the light-emitting diode units of the light-emitting diode group  402 , and/or selecting the thermal resistor with suitable temperature coefficient. 
       FIG. 5  is an electric circuit diagram of the light-emitting device of the fourth embodiment according to the present application. The light-emitting device  500  comprises a first light-emitting module  510 , a second light-emitting module  520  connected to the first light-emitting module  510  in parallel, and a thermal resistor  506  with positive temperature coefficient electrically connected to the second light-emitting module  520 . The first light-emitting module  510  comprises a first light-emitting diode group  502 , and the second light-emitting module  520  comprises a second light-emitting diode group  503  and a third light-emitting diode group  504 . The first light-emitting diode group  502  comprises a first quantity of the first light-emitting diode units  507  connected to one another in series, the second light-emitting diode group  503  comprises a second quantity of the second light-emitting diode units  508  connected to one another in series, and the third light-emitting diode group  504  comprises a third quantity of the second light-emitting diode units  508  connected to one another in series. The thermal resistor  506  is electrically connected to the third light-emitting diode group  504  in parallel, and electrically connected to the second light-emitting diode group  503  in series. The first light-emitting module  510  or the first light-emitting diode unit  507  has the hot/cold factor more than 0.85; the second light-emitting module  520  or the second light-emitting diode unit  508  has the hot/cold factor less than that of the first light-emitting module  510  or the first light-emitting diode unit  507 , for example less than 0.85, or preferably less than 0.8. In the embodiment, the first light-emitting diode unit comprises the blue light light-emitting diode with the hot/cold factor about 0.88, and the second light-emitting diode unit comprises the red light light-emitting diode with the hot/cold factor about 0.63. Other visible or invisible wavelength light-emitting diode can also be included, such as green, yellow or ultraviolet wavelength light-emitting diodes, or formed by AlGaInP-based material, or GaN-based material. 
     In the embodiment, the third light-emitting diode group  504  is electrically connected to the thermal resistor  506  in parallel. The second light-emitting diode group  503  has an equivalent internal resistance R 1 , the third light-emitting diode group  504  has an equivalent internal resistance R 2 , and the thermal resistor  506  has a resistance R PTC , wherein R 1  and R 2  decrease when the junction temperature is increased. As shown in  FIG. 1 , when the second light-emitting diode unit is the red light or the blue light light-emitting diode, R 1  and R 2  respectively decreases about 7˜8%. The resistance R PTC  of the thermal resistor  506  with positive temperature coefficient increases in the correlation when the temperature is increased, such as R PTC  increases in the linear or the non-linear correlation when the temperature is increased. During the operation of the light-emitting device  500 , an electric current I 0  is divided into I 1  flowing through the first light-emitting module  510  and I 2  flowing through the second light-emitting module  520 . The electric current I 2  flowing through the third light-emitting diode group  504  and the thermal resistor  506  of the second light-emitting module  520  is divided into I 3  flowing through the third light-emitting diode group  504  and I 4  flowing through the thermal resistor  506 , wherein I 2 =I 3 +I 4 . In addition, the potential difference of the two terminals of the third light-emitting diode group  504  is equal to the potential difference of the two terminals of the thermal resistor  506 . Namely, I 4 *R PTC =I 3 *R 2 . From the above two relationships, the electric current I 3  flowing through the third light-emitting diode group  504  is positive-correlated to R PTC /(R 2 +R PTC ). Namely, I 3  is positive-correlated to R PTC  and negative-correlated to R 2 . In the embodiment, the junction temperature of the light-emitting device  500  is increased during operation. For example, the resistance R PTC  of the thermal resistor  506  is increased due to the increase of the junction temperature, and the resistance R 2  of the third light-emitting diode group  504  is decreased due to the increase of the junction temperature when the junction temperature is increased from the initial operation first temperature 20° C. to the steady state second temperature 80° C. Therefore, I 3  increases due to the increase of the junction temperature and the light output power of the third light-emitting diode group  504  also increases due to the increase of I 3 . In the embodiment, the hot/cold factor of the first light-emitting module  510  is larger than that of the second light-emitting module  520 , so the decline of the light output power of the second light-emitting module  520  is larger than that of the first light-emitting module  510  when the junction temperature is increased. Thus, the light color mixed by the light emitted from the first light-emitting module  510  and the light emitted from the second light-emitting module  520  shifts to the light color emitted from the first light-emitting module  510  when the junction temperature is increased. But the decline of the light output power of the second light-emitting module  520  caused by hot/cold factor can be reduced when the junction temperature is increased by controlling the R PTC  of the thermal resistor  506 , and the function of the temperature compensation can be achieved. In addition, the decline of the light output power of the second light-emitting module caused by hot/cold factor during the increase of the junction temperature can be offset or controlled by adjusting the quantity of the light-emitting diode units of the second light-emitting diode group and the third light-emitting diode group, or selecting the thermal resistor with suitable temperature coefficient. Furthermore, the thermal resistor  506  of the embodiment can be electrically connected to the second light-emitting diode group  503  and the third light-emitting diode group  504  in parallel at the same time. Thus, the electric current flowing through the second light-emitting diode group  503  and the third light-emitting diode group  504  is increased compared with that at the initial temperature when the junction temperature of the light-emitting device is increased. 
     The fifth embodiment of the present application is illustrated in  FIG. 6 . The difference between the fifth and the fourth embodiments is that the second light-emitting module  520  is connected to the thermal resistor  605  with negative temperature coefficient in series. Based on the related description similar to the third embodiment and the fourth embodiment, the function of temperature compensation of the present application is achieved. In addition, the first light-emitting module and the second light-emitting module of the above-described fourth and fifth embodiments are not limited to be connected in parallel, and each of them also can be connected to an independent control current source or voltage source. 
       FIG. 7  is a structure diagram of a light-emitting diode group according to the above-described embodiments of the present application. A light-emitting diode group  700  comprises a substrate  700 , and a plurality of light-emitting diode units formed or attached to the substrate  700  in an array type, and is divided by a trench  711 . Each of the plurality of light-emitting diode units comprises an n-type contact layer  720  formed on the substrate  710 , an n-type cladding layer  730  formed on the contact layer  720 , an active layer  740  formed on the n-type cladding layer  730 , a p-type cladding layer  750  formed on the active layer  740 , a p-type contact layer  760  formed on the p-type cladding layer  750 , a connecting wire  770  electrically connected to the n-type contact layer  720  of the light-emitting diode unit and the p-type contact layer  760  of another light-emitting diode unit in series, and an insulation layer  780  formed between the trench  711  and the connecting wire  770  to avoid the short circuit path. In the embodiment of the present application, the light-emitting diode group  700  comprises a high voltage array-type single chip including the plurality of light-emitting diode units collectively formed on the single substrate, such as the blue light high voltage array-type single chip or the red light high voltage array-type single chip, and the operation voltage depends on the quantity of the light-emitting diode units connected in series. The material of the above-described n-type or p-type contact layer, the n-type or the p-type cladding layer, or the active layer comprises the III-V group compound such as Al x In y Ga (1-x-y) N or Al x In y Ga (1-x-y) P, wherein 0≦x, y≦1; (x+y)≦1. 
       FIG. 8  is a structure diagram of the light-emitting device according to the fourth embodiment or the fifth embodiment of the present application. The first light-emitting module  510  of the light-emitting device  600  comprises the blue light high voltage array-type single chip illustrated in  FIG. 7 , and the second light-emitting module  520  comprising the red light high voltage array-type single chip illustrated in  FIG. 7  is electrically connected to a thermal resistor  605 ; two electrodes  509  are electrically connected to the first light-emitting module  510  and the second light-emitting module  520  to receive a power signal; the first light-emitting module  510 , the second light-emitting module  520 , the thermal resistor  605  and the electrode  509  are collectively formed on a board  501 . 
     The principle and the efficiency of the present application illustrated by the embodiments above are not the limitation of the present application. Any person having ordinary skill in the art can modify or change the aforementioned embodiments. Therefore, the protection range of the rights in the present application will be listed as the following claims.