Abstract:
A method for transmitting a signal using an optical coupling device includes converting a first electric signal into an optical signal, converting the optical signal into a first current, and outputting a second current that corresponds to the first current as a second electric signal. The second current may be larger than the first current.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 14/016,006, filed on Aug. 30, 2013, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-059137, filed Mar. 21, 2013, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an optical coupling device. 
     BACKGROUND 
     An optical coupling device that can transmit a signal while its input terminal and output terminal are electrically insulated from each other, has become popular in electronic equipment that employs different types of power source systems, such as a DC power source system, an AC power source system, and a telephone line system. In such optical coupling devices, a transistor output photocoupler, in which a phototransistor is employed as a light receiving element, has wide versatility and wide applicability. 
     The optical coupling device is characterized by a current transmission rate (CTR), which indicates a current flowing into a light receiving element I C  as a percentage of a current flowing into a light emitting element I F . The current transmission rate of the optical coupling devices is required to have a wide range so that it can be used in different applications. Further, a switching time of the optical coupling devices is required to be small regardless of changes in the load resistance. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram of an optical coupling device according to a first embodiment. 
         FIG. 2  is a schematic circuit diagram for explaining a current transmission rate. 
         FIG. 3  is a schematic circuit diagram of an optical coupling device according to first and second comparison examples, that shows a load resistor connected to the optical coupling device. 
         FIG. 4  is a waveform chart, wherein ( a ) shows a waveform of a forward current of a light emitting element, ( b ) shows a waveform of an output voltage of the optical coupling device according to the first comparison example in which a large load resistance is connected, ( c ) shows a waveform of an output voltage of the optical coupling device according to the second comparison example in which a small load resistance is connected, and ( d ) shows a waveform of an output voltage of the optical coupling device according to the first embodiment. 
         FIG. 5  is a graph showing correlation between a switching time and load resistance values with respect to the first and second comparison examples. 
         FIG. 6  is a schematic circuit diagram of an optical coupling device according to a second embodiment. 
         FIG. 7  is a schematic circuit diagram of an optical coupling device according to a third embodiment. 
         FIG. 8  is a schematic circuit diagram of an optical coupling device according to a fourth embodiment. 
         FIG. 9  is a schematic circuit diagram of an optical coupling device according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments provide an optical coupling device which can shorten a switching time while ensuring a wide range of current transmission rate, and which can be formed in a compact size. 
     In general, according to one embodiment, an optical coupling device includes a light emitting element configured to convert an electric signal into an optical signal, a photo transistor circuit configured to convert the optical signal into a current, the photo transistor circuit including a first transistor having a collector connected to a power source and an emitter through which the current is output, and a current mirror circuit. The current mirror circuit includes a second transistor having a collector connected to the emitter of the first transistor, a base connected to the emitter of the first transistor, and an emitter connected to a ground, and a third transistor having a collector connected to an output terminal, a base connected to the base of the second transistor, and a emitter connected to the ground. 
     Hereinafter, embodiments are explained in conjunction with drawings. 
       FIG. 1  is a schematic circuit diagram of an optical coupling device according to a first embodiment. 
     An optical coupling device  5  includes: a light emitting element  10 ; a phototransistor (light receiving element)  20 ; a current mirror circuit  30 ; a power source voltage terminal  44 ; an output terminal  46 ; and an output side ground terminal  48 . 
     The optical coupling device  5  may also include an input terminal  40  and an input-side ground terminal  42 . 
     The light emitting element  10  is connected to the first input terminal  40 , and to one end portion of the input-side ground terminal  42  via a connecting wire or the like. The light emitting element  10  converts an input electric signal into an optical signal having a wavelength ranging from red light to infrared light. 
     The phototransistor  20  includes an emitter and a collector which is connected to the power source voltage terminal  44 , and converts an optical signal into an electric signal. The phototransistor  20  has an npn junction or a pnp junction, and may have a vertical structure or a lateral structure. 
     The current mirror circuit  30  includes a first transistor Q 1  and a second transistor Q 2 . In  FIG. 1 , each of the first and second transistors Q 1  and Q 2  is formed of a bipolar transistor. However, the first and second transistors Q 1 , Q 2  may be formed of a MOSFET or the like. 
     The phototransistor  20  and the current mirror circuit  30  may be formed as a single chip. In such a light receiving circuit, a substrate made of Si may be used. With the use of the single-chip light receiving circuit, the optical coupling device  5  can be formed in a compact size. Further, assembling steps of the optical coupling device can be simplified. 
     An emitter current of the phototransistor  20  is input into the first transistor Q 1  (IQ 1 ). In the first transistor Q 1 , a base and a collector are connected to each other. In addition, a planner size of the second transistor Q 2  is set to be n (n≧1) times as large as a planner size of the first transistor Q 1 . Due to such a setting of the planner sizes, the second transistor Q 2  can output a collector current IQ 2  which is n times as large as a collector current IQ 1  of the first transistor Q 1  from an output terminal  46 . In this specification, the planner size of the bipolar transistor means an area of a collector-base bonding region. 
     The optical coupling device  5  may also include a resin molded body  70 . The resin molded body  70  covers the light emitting element  10 , the phototransistor  20 , the current mirror circuit  30 , one end portion of the power source voltage terminal wiring  44 , one end portion of the output terminal wiring  46 , one end portion of the output side ground terminal wiring  48 , one end portion of the input terminal wiring  40 , and one end portion of the input-side ground terminal wiring  42 . Further, the other end portion of the power source voltage terminal wiring  44 , the other end portion of the output terminal wiring  46 , and the other end portion of the output side ground terminal wiring  48  project outwardly from the resin molded body  70  and are connected to a wiring part of a mounting printed circuit board. 
     In addition, a light transmitting resin layer may be provided to an optical path through which an optical signal from the light emitting element  10  is transferred toward the phototransistor  20 . By disposing the resin molded body  70  such that it has a light blocking function and covers the light transmitting resin layer, the optical coupling device  5  can suppress an erroneous operation caused by an external disturbance light. 
       FIG. 2  is schematic circuit diagram for explaining a definition of a current transfer ratio. 
     Assuming a forward current of the light emitting element  10  as I F  and a collector current of the phototransistor  20  as I C , the current transfer ratio CTR (%) of the optical coupling device  5  can be expressed by Equation (1).
 
CTR= I   C   /I   F ×100(%)  Equation (1)
 
       FIG. 3  is a schematic circuit diagram of an optical coupling device  105  according to a comparison example where a load resistor is connected to the optical coupling device  105 . 
     The load resistor (pull-up resistance)  150 , which has a resistance RL, is connected between a collector of the phototransistor  120  and a power source voltage terminal  144 . 
     In  FIG. 4 , ( a ) shows a waveform of a forward current of a light emitting element, ( b ) shows a waveform of an output voltage of the optical coupling device according to the comparison example with a large load resistance, for example, 10 kΩ or more, ( c ) shows a waveform of an output voltage of the optical coupling device according to the second comparison example with a small load resistance, and ( d ) shows a waveform of an output voltage of the optical coupling device according to the first embodiment. 
     It is assumed that a forward current I F  expressed by ( a ) of  FIG. 4  flows into the light emitting element  10  as an input electric signal. In the first comparison example (( b ) of  FIG. 4 ) where the load resistance RL is large, a base of the phototransistor  20  is likely to be saturated with carriers so that an output voltage Vout at a low (L) level approximates a ground level GND. 
     However, when the phototransistor  20  is switched to an OFF state from an ON state, a large amount of carriers are stored in the base in a saturated state. In this example, because the load resistor  150  is connected to a current path, OFF-time toff taken to discharge the carriers is prolonged. 
     On the other hand, in the second comparison example (( c ) of  FIG. 4 ) where a resistance RL of the load resistor  150  is small, for example, 2 kΩ, a number of the carriers stored in a base is small, and hence the OFF time toff can be shortened. However, the base of the phototransistor  20  is hardly saturated. As a result, an output voltage Vout of L level floats from a ground level GND, and hence a noise margin in the transfer of the electric signal can be lowered. 
       FIG. 5  is a graph expressing correlation between a load resistance RL and a switching time (a switching time toff, a switching time ton, and a storage time ts) with respect to the first and second comparison examples. 
     In  FIG. 5 , an axis of ordinate denotes a switching time (μs), and an axis of abscissa denotes a load resistance RL (kΩ). The switching times include the storage time ts, which is defined as a time period taken to discharge a base-emitter equivalent capacitance, the OFF time toff, and the ON time ton. 
     The ON time ton is defined as a time period between a rising edge of the input electric signal as shown in  FIG. 4 ( a )  and a timing at which an output voltage Vout is lowered to a predetermined ratio (for example, 10%) with respect to the difference A between a power source voltage Vcc and a low level L of an output voltage Vout. The OFF time toff is defined as a time period between a falling edge of the input electric signal as shown in  FIG. 4 ( a )  and a timing at which an output voltage Vout is elevated to a predetermined ratio (for example, 10%) with respect to the difference A between the power source voltage Vcc and the low level L of an output voltage Vout. 
     Although the storage time is and the OFF time toff are increased along with increase of the load resistance RL, the ON time ton does not change relative to the load resistance RL when compared to the OFF time toff. Here, it is assumed that a collector-emitter voltage V CE  of the phototransistor is 5V, and that a forward current I F  of the light emitting element is 16 mA. 
     To the contrary, in the first embodiment, as shown in ( d ) of  FIG. 4 , the load resistor is not connected to the current path of the phototransistor  20 , and instead the current mirror circuit  30  is connected to the current path of the phototransistor  20 . Accordingly, the saturation of the carriers is not generated in the phototransistor  20 , and hence the OFF time toff can be reduced. Further, the output voltage Vout at a low level can be set to approximately a ground level GND regardless of the resistance RL of the load resistance. 
     In the first embodiment, by changing a ratio n of a planner size of the second transistor Q 2  with respect to a planner size of the first transistor Q 1 , an output current of the current mirror circuit  30  can be set. For example, by setting the ratio n to 1 (n=1), the electric current IQ 1  and the electric current IQ 2  can be set approximately equal. By setting the ratio n to 2 (n=2), the electric current IQ 2  can be set approximately twice as large as the electric current IQ 1 . In this manner, by changing the ratio n, the output current I C  can be set. The current transferred ratio CTR can be set to a value ranging from 100% to 2000%, and hence the optical coupling device can be applicable in various ways. In this case, it is unnecessary to increase capacitance of the phototransistor  20 , and hence the increase of the switching time can be suppressed. 
     The size of the phototransistor can be smaller than the size of a photodiode. Further, the size of the current mirror circuit  30  can be smaller than the size of the phototransistor. Even when a current mirror circuit, which is mounted on a package with the phototransistor  20  in the above-mentioned embodiment, is externally mounted on a mounting member separately from the phototransistor  20  mounted on the mounting member, the substantially same optical characteristic can be achieved. In the first embodiment, a load resistance can be increased while maintaining the short switching time, and hence the power consumption can be reduced. 
       FIG. 6  is a schematic circuit diagram of an optical coupling device according to a second embodiment. 
     The optical coupling device  5  of the second embodiment further includes a first resistor  72 , which is connected between an emitter of a first transistor Q 1  and an output-side ground terminal  48 , and a second resistor  74 , which is connected between an emitter of a second transistor Q 2  and the output-side ground terminal  48 . By changing a resistance RQ 1  of the first resistor  72  and a resistance RQ 2  of the second resistor  74 , a collector current IQ 2  of the second transistor Q 2  can be changed. 
     For example, when the ratio n is 1 (n=1) and the resistances RQ 1 , RQ 2  of the first and the second resistors  72 ,  74  are set equal to with each other (RQ 1 =RQ 2 ), the electric current IQ 1  is approximately equal to the electric current IQ 2 . Further, when the ratio n is 1 (n=1) and the resistance RQ 2  of the second resistor  74  is set equal to a half of the resistance RQ 1  of the first resistor  72  (RQ 2 =RQ 1 /2), the electric current IQ 2  is approximately twice as large as the electric current IQ 1 . In this manner, a range of the current transfer ratio CTR can be widely set. 
       FIG. 7  is a schematic circuit diagram of an optical coupling device according to a third embodiment. 
     A load resistor  50  is mounted within a resin molded body  70  of the optical coupling device  5 . The load resistor  50  is connected between a power source voltage terminal  44  and an output voltage terminal  46 . In this case, a phototransistor  20 , a current mirror circuit  30 , and the load resistor  50  can be formed as a single chip. Accordingly, the optical coupling device  5  and electronic equipment can be easily miniaturized, the assembling step can be also simplified. 
       FIG. 8  is a schematic circuit diagram of an optical coupling device according to a fourth embodiment. 
     An emitter current of a phototransistor is input to a base of a bipolar transistor. That is, a phototransistor circuit  79  comprises a Darlington connection circuit. Accordingly, a current transfer ratio CTR can be enhanced more easily. 
       FIG. 9  is a schematic circuit diagram of an optical coupling device according to a fifth embodiment. 
     A current mirror circuit  31  may include a first transistor M 1  and a second transistor M 2 , both of which are formed of an enhancement type MOSFET. Current mirror circuits formed of bipolar transistors may exhibit a large irregularity in a current uplifting ratio h FE . Accordingly, irregularity in current transfer ratio CTR is also lager. According to an experiment carried out by inventors of the present disclosure, it is found that conformity in current transfer ratio CTR of optical coupling devices can be improved with the use of the current mirror circuit  31  formed of a MOSFET. 
     According to the optical coupling devices of the first to fifth embodiments, switching times ton, toff can be shortened while maintaining a wide range of current transfer ratio CTR, and the optical coupling device can be easily formed in a compact size. Such optical coupling devices can perform the transmission of signals in a state where an input terminal and an output terminal are electrically insulated from each other. Accordingly, the optical coupling devices can be widely used in electronic equipment having different power source systems such as a DC voltage system and an AC power source system and a telephone line system. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.