Photocoupler

Abstract: It is an object of the present invention to provide a photocoupler which maintains a high electric isolation performance and has an improved optical coupling efficiency without any variation of a conventional structure and a conventional production process. The photocoupler includes a light emitting device 1 and a photo detector 2 disposed in a facing relationship to each other, a silicon resin 4 covering over the light emitting device 1, a light transmitting resin 5 molding the silicon resin 4 which covers over the light emitting device 1 and the photo detector 2, and a light shielding resin 6 molding the light transmitting resin 5. In the photocoupler, a light scattering material is mixed in the silicon resin 4 which covers over the light emitting device 1. The amount of light scattering material mixed in the silicon resin 4 can be selected from 0.1 to 5.0 percentage by weight of the amount of the silicon resin 4. Further, the silicon resin 4 can be shaped so that the face facing the photo detector 2 is concave.

Description: The present invention relates to a photocoupler, and more particularly to a photocoupler wherein a light emitting device and a photo detector are disposed in a facing relationship to each other. Fig. 1 is a cross-sectional view of a photocoupler in a first example of the prior art. As shown in Fig. 1, a light emitting device 1 and a photo detector 2 are mounted on a single lead frame 3, and a light transmitting siliconeresin 7 covers the light emitting device 1 and the photo detector 2. Then, in order to avoid influence of disturbance light, a light shielding resin 6 encloses (hereinafter referred to as molds) the light emitting device 1, the photo detector 2, the lead frame 3 and the silicone resin 7. For the light shielding resin 6, a resin of a non-black color (cream color or the like) is in most cases employed so as to reflect much of the light emitted from the light emitting device 1. In the first example of the prior art, since the lead frame 3 is constituted by a single frame and the light emitting device 1 and the photo detector 2 are disposed on the same plane, the photocoupler can be produced readily. Since the structure of the photocoupler makes use of reflected light, it is advantageous in that it has a high optical coupling efficiency. Fig. 2 is a cross-sectional view of a photocoupler in a second example of the prior art. As shown in Fig. 2, a light emitting device 1 is mounted on a lead frame 3a, and a photo detector 2 is mounted on another lead frame 3b. The lead frames 3a and 3b are disposed such that the light emitting device 1 and the photo detector 2 are faced each other. A light transmitting resin 5 molds the light emitting device 1, the photo detector 2, and the lead frames 3a and 3b. A light shielding resin 6 molds the light transmitting resin 5. Also, in order to moderate the stress coming from the light transmitting resin 5, a transparent silicone resin 7 covers the light emitting device 1. In the second example of the prior art, epoxy resins are generally employed for the light transmitting resin 5 and the light shielding resin 6. By adding filler into the epoxy resins, the thermal expansion coefficients of the light transmitting resin 5 and the light shielding resin 6 can be made equal. Accordingly, even if thermal hysteresis is applied to the product in the production process or upon mounting, the light transmitting resin 5 and the light shielding resin 6 do not exfoliate from each other along their interface, and therefore, the photocoupler of the second example of the prior art is advantageous in that it can resist high voltage for isolation and accordingly it is a high performance electrical isolator. Fig. 3 is a cross-sectional view of a photocoupler disclosed in JP-U-27067/88. The structure shown in Fig. 3 is substantially the same as that shown in Fig. 1 except for the light transmitting silicone resin 7. Since corresponding devices in Figs. 1 and 3 are designated by identical reference numerals, overlapping description is omitted herein. Referring to Fig. 3, in place of the light transmitting silicone resin 7 shown in Fig. 1, a silicone resin 4, in which a light scattering material is contained, covers a light emitting device 1 and a photo detector 2. Therefore, the structure shown in Fig. 3 has an improved optical coupling efficiency compared with the structure shown in Fig. 1. Fig. 4 is a cross-sectional view of a photocoupler disclosed in JP-U-78056/89. The structure shown in Fig. 4 is substantially the same as that shown in Fig. 2 except for the silicone resin 7. Since corresponding devices in Figs. 2 and 4 are designated by identical reference numerals, overlapping description is omitted herein. In Fig. 4, differing from Fig. 2, a light emitting device 1 is not covered with the silicone resin 7. Then, a cylindrical lens 8 is disposed between the light emitting device 1 and a photo detector 2, which are disposed in a facing relationship to each other, so that a great amount of light emitted from the light emitting device 1 may be condensed into the photo detector 2. Therefore, the structure shown in Fig. 4 has an improved optical coupling efficiency compared with the structure shown in Fig. 2. Fig. 5 is a cross-sectional view of a photocoupler disclosed in JP-A-4034985. In Fig. 5, a light emitting device 1 and a photo detector 2 are disposed in a horizontally facing relationship to each other, and the light emitting device 1 and the photo detector 2 are each covered with a silicone resin 7. A light transmitting resin 5 covers the area between and around the silicone resin 7 which covers the light emitting device 1 and the photo detector 2. Then, a light shielding resin 6 molds the light emitting device 1, the photo detector 2, lead frames 3a and 3b, the silicone resin 7 and the light transmitting resin 5. In Fig. 5, the silicon resin 7 which covers the light emitting device 1 and the silicone resin 7 which covers the photo detector 2 serve as convex lenses respectively. Therefore, the structure shown in Fig. 5 has an improved optical coupling efficiency compared with the structure shown in Fig. 2. With regard to the structures shown in Figs. 1 and 3, although the optical coupling efficiency is high, since the thermal expansion coefficient is different between the silicone resin 7 or the silicone resin 4 and the light shielding resin 6, the adhesion along the phase boundary between them is weak, so that a dielectric breakdown is very likely to occur there. Therefore, the structures shown in Figs. 1 and 3 are disadvantageous in that they resist low voltage for isolation and exhibit an insufficient electric isolation performance. On the other hand, with the structure shown in Fig. 2, although the electric isolation voltage is high, since only the direct light of the light emitted from the light emitting device 1 reaches the photo detector 2, the structure shown in Fig. 2 is disadvantageous in that it decreases optical coupling efficiency. The structure shown in Fig. 4, directed to solution of the problem described above, has another problem, however, in that it is difficult to attach the cylindrical lens 8 inside the light transmitting resin 5. Also, the structure shown in Fig. 5 has a further problem in that it is difficult to cover the light emitting device 1 and the photo detector 2 with the silicone resin 7 in the form of lenses. Both of the structures shown in Figs. 4 and 5 have problems in that the production process is complicated. JP-A-59-098565 discloses a photocoupler using a light transmitting resin containing up to 3 % of TiO 2 . In view of these facts, an object of the present invention is to provide a photocoupler which maintains a high electric isolation performance and has an improved optical coupling efficiency. This object is achieved by the features of claim 1. Since the light emitting device of the photocoupler of the present invention, which has the structurally high electric isolation performance, is covered with a transparent silicone resin, in which the light scattering material is mixed, light emitted laterally from side faces and the like of a light emitting device, which light does not reach a photo detector in conventional photocouplers, can be received as scattered light by the photo detector. Further, by selecting the amount of light scattering material to be mixed from 0.1 to 5.0 percentage by weight of the amount of the transparent silicone resin, the total amount of scattered light and direct light coming from the light emitting device to the photo detector can be increased compared with the amount of light reaching conventional photocoupler. Therefore, the optical coupling efficiency can be increased by approximately 1.2 times that of conventional photocoupler without varying the structure and the production process of the conventional photocoupler. Furthermore, by shaping the silicone resin in which the light scattering material is mixed so that the face facing the photo detector is concave, the ratio of the scattered light is increased, and light emitted from the light emitting device can be received in a greater amount by the photo detector. Therefore, the optical coupling efficiency can be increased by approximately 1.5 times that of the conventional photocoupler. As described above, the optical coupling efficiency can be improved while maintaining the electric isolation performance without varying the structure and the production process of the conventional photocoupler. Therefore, the photocoupler of the present invention can drive a current similar to that of the conventional photocoupler with an amount of emitted light smaller than that of the conventional photocoupler, and higher optical transmission than ever can be achieved with an amount of emitted light similar to that of the conventional photocoupler. The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention. Fig. 1 is a cross-sectional view of a photocoupler in a first prior art example; Fig. 2 is a cross-sectional view of a photocoupler in a second prior art example; Fig. 3 is a cross-sectional view of a photocoupler disclosed in JP-U-27067/88; Fig. 4 is a cross-sectional view of a photocoupler disclosed in JP-U-78056/89; Fig. 5 is a cross-sectional view of a photocoupler disclosed in JP-A-4034985; Fig. 6 is a cross-sectional view of a photocoupler in a first embodiment of the present invention; Fig. 7 is a diagram illustrating the relative value of the amount of received light to the amount of light scattering material in the first embodiment; and Fig. 8 is a cross-sectional view of a photocoupler in a second embodiment of the present invention. The two embodiments of the present invention will be described with reference to the accompanying drawings. Fig. 6 is a cross-sectional view of a photocoupler in a first embodiment of the present invention. As shown in Fig. 6, a light emitting device 1 is mounted on a lead frame 3a, and a photo detector 2 is mounted on a lead frame 3b. Thereafter, although not shown in Fig. 6, necessary wire bonding is carried out on the lead frames 3a and 3b. Then, the light emitting device 1 is covered with a silicone resin 4 in which a light scattering material such as TiO 2 or SiO 2 or the like is mixed from 0.1 to 5.0 percentage by weight. Thereafter, a light transmitting resin 5 molds so as to cover the silicon resin 4 which covers the light emitting device 1, the photo detector 2 and the lead frames 3a and 3b. Further, in order to prevent malfunctions by disturbance light, a light shielding resin 6 molds the light transmitting resin 5. According to the structure shown in Fig. 6, since light emitted from the light emitting device 1 is scattered in the silicone resin 4, part of light emitted in lateral directions from side faces and the like of the light emitting device 1 makes scattered light, and this scattered light reaches the photo detector 2. While direct light emitted from the light emitting device 1 and reaching directly to the photo detector 2 decreases compared with that in cases where no light scattering material is mixed in the siliconeresin, the total amount of received light, which is the sum total of the direct light and the scattered light, increases. Fig. 7 is a diagram showing the relative value of the amount of received light to the amount of light scattering material in the first embodiment and illustrates a light receiving state of the photocoupler shown in Fig. 6. Referring to Fig. 7, the lateral axis represents the amount of light scattering material mixed in the silicone resin 4, and the vertical axis represents the relative value of the amount of received light. While the amount of received light (1) of the direct light decreases as a result of addition of the light scattering material, since the amount of received light (2) of the scattered light increases, the total amount of received light (3), which is the sum of (1) and (2), increases. As shown in Fig. 7, the optical coupling efficiency increases by selecting the amount of light scattering material to be mixed from 0.1 to 5.0 percentage by weight of the silicone resin 4. Particularly, when the amount of light scattering material is selected at the optimum amount in the proximity of 1.0 percentage by weight of the silicone resin 4, an optical coupling efficiency of approximately 1.2 times that compared with the silicone resin in which no light scattering material is added is obtained. Fig. 8 is a cross-sectional view of a photocoupler in a second embodiment of the present invention. The structure shown in Fig. 8 is substantially the same as that shown in Fig. 6 except for the shape of silicone resin 4. Since corresponding devices of Figs. 6 and 8 are designated by identical reference numerals, overlapping description is omitted herein. In the second embodiment shown in Fig. 8, in order to improve further the effect of the first embodiment shown in Fig. 6, the shape of silicone resin 4 selected is concave so as to increase its light scattering effect. Therefore, in the second embodiment, although not shown in Fig. 8, an optical coupling efficiency of approximately 1.5 times that of the silicone resin in which no light scattering material is added is obtained. It is to be understood that although the characteristics and advantages of the present invention have been set forth in the foregoing description, the disclosure is illustrative only, and changes may be made in the shape and arrangement of the parts within the scope of the appended claims.

Claims: A photocoupler having a light emitting device (1) and a photo detector (2) disposed in a facing relationship to each other; a transparent silicone resin (4) which covers said light emitting device (1); a light transmitting resin (5) which molds said transparent silicone resin (4) and said photo detector (2); and a light shielding resin (6) which molds said light transmitting resin (5); characterized in that 0.1 to 5 % by weight of a light scattering material is mixed in said transparent silicone resin (4). A photocoupler according to claim 1, characterized in that said transparent silicone resin is shaped so that the face facing said photo detector (2) is concave.

Inventors: NAGANO HIROKI, NAGANO, HIROKI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: H01L31
IPC codes: H01L31