Abstract:
A photocoupler semiconductor device comprises a light receiving device ( 12 ) for generating electric current; a control circuit ( 16 ); and a constant current circuit ( 14 ) having a resistor ( 20 ′) for detecting the generated current to provide the control circuit ( 16 ) with a current command. The resistor ( 20 ′) allows the electric current to increase with the amount of current received.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to discharge control circuits for a metal-oxide-semiconductor field effect transistor (MOSFET) output photocoupler (hereinafter “photocoupler semiconductor device”).  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 7 shows the electrical circuit of a conventional photocoupler semiconductor device. A control circuit  16  comprises a p-n-p transistor  26 , a diode  28 , an n-p-n transistor  30 , and a photodiode  24 . The emitter electrode of the p-n-p transistor  26  is connected to the gate electrodes of MOSFETs  18 - 1  and  18 - 2 , the collector of the n-p-n transistor  30  is connected to the base of the p-n-p transistor  26 , and the cathode of the photodiode  24  is connected to both the collector of the p-n-p transistor  26  and the base of the n-p-n transistor  30 . The cathode of the diode  28  is connected to the emitter of the p-n-p transistor  26  and its anode to both the anode of the light receiving device  12  (a series connection of light receiving elements) and the base of the p-n-p transistor  26 . The collector of the n-p-n transistor  30  is connected to the base of the n-p-n transistor  26  and the its emitter to both the anode of the photodiode  24  and the sources of the MOSFETs  18 - 1  and  18 - 2 .  
           [0005]    How to drive the photocoupler semiconductor device will be described.  
           [0006]    The MOSFETs  18 - 1  and  18 - 2  are turned on as follows. The input current If to the light emitting element  10  produces light. The light receiving device  12  receives the light and supplies power produced across the anode and cathode to the gates of the MOSFETs  18 - 1  and  18 - 2 , turning on the gate-source regions of the MOSFETs  18 - 1  and  18 - 2  and conducting electric current.  
           [0007]    The photocoupler semiconductor device is turned off as follows. When the input current, If, to the light emitting element  10  is turned off, the light receiving device  12  no longer produces power. The MOSFETs  18 - 1  and  18 - 2 , however, are not turned off immediately because of the accumulated charges therein. Consequently, the potential at the gate of the MOSFETs  18 - 1  and  18 - 2  is higher than the potential at the anode of the light receiving device  12  so that electric current flows from the gates of the MOSFETs  18 - 1  and  18 - 2  to the anode of the light receiving device  12 . At this point, the diode  28  of the control circuit  16  is inversely biased so that the current does not flow to the diode  28  but the emitter of the p-n-p transistor  26 , turning on the p-n-p transistor  26 . Since the photodiode  24  is inversely biased, the current flows from the collector of the p-n-p transistor  26  to the base of the n-p-n transistor  30 , turning on the n-p-n transistor  30 . Consequently, the current flows from the gate of the MOSFET  18  through the emitter of the n-p-n transistor  30 . As a result, the connection points E and F are short-circuited to discharge electric charges of the MOSFETs  18 - 1  and  18 - 2 , which turns off the photocoupler semiconductor device.  
           [0008]    The turn-on time of the photocoupler semiconductor device of FIG. 7 is determined by the time period in which the light current from the light receiving device  12  accumulates electric charges in the gate capacity of MOSFET. Accordingly, as shown in FIG. 8, the turn-on time varies with the input current to the light emitting element  10 . FIG. 8 shows the input current vs. turn-on time characteristic for the conventional photocoupler semiconductor device, wherein the relationship shows a substantially hyperbolic curve.  
           [0009]    In order to provide a slow-on control having a long turn-on time, it has been necessary to control the magnitude of an input current precisely. To solve such a problem, it has been proposed that a constant current circuit, such as shown in FIG. 9, is added across the output terminals  2  and  2 ′ of the light receiving device  12  to control the dependency of the turn-on time. FIG. 9 shows the circuit diagram of a photocoupler semiconductor device equipped with a conventional constant current control. This constant current circuit  14  comprises a resistor  20  and an n-p-n transistor  22 . One end of the resistor  20  and the emitter electrode of the n-p-n transistor  22  are connected to the connection point  2 ′. The other end of the resistor  20  is connected to the base electrode of the n-p-n transistor  22 . The collector of the n-p-n transistor  22  is connected to the output terminal  2 .  
           [0010]    In such a structure, the electric current from the photodiode produces a voltage drop across the resistor  20 . If the voltage drop, which is a potential difference between the emitter and the base of the n-p-n transistor  22 , exceeds the threshold (0.6 V for example), the n-p-n transistor  22  is turned on, discharging the electric charges from the MOSFETs  18 . At this point, the voltage drop by the transistor  22  is below 0.6 V, turning off the transistor  22 .  
           [0011]    The off condition of the transistor  22  supplies electric charges to the MOSFETs  18 , producing the voltage drop, resulting in the on-condition again. Such a sequential operation repeats to make the electric current constant in the constant current circuit. The value of electric current is determined by the resistor  20  and the threshold between the base and the emitter of the n-p-n transistor  22 . For example, the electric current is 1 μA for a threshold of 0.6 V and a resistance of 0.6 MΩ.  
           [0012]    At this point, a loop path is formed to couple the anode of the light receiving device  12 , the gates and the sources of MOSFETs  18 - 1  and  18 - 2 , and the cathode of the light receiving device  12 . When the current value exceeds a set value of 1 μA, the constant current circuit starts to operate, turning on the MOSFETs  18 - 1  and  18 - 2 . Accordingly, it is possible to make the on-time, Ton, of the photocoupler semiconductor device circuit and the response time of the rise time, Tr, substantially constant despite variations of the input current, If, to the light emitting element  3 .  
           [0013]    Since the constant current circuit  14  keeps the light current constant, the accumulation time in the gate capacity of MOSFETs  18 - 1  and  18 - 2  is made constant, which in turn makes the turn-on time constant. The dependency on the input current, If, of the photocoupler semiconductor device with the constant current circuit  14  is shown in FIG. 10, wherein the turn-on time remains substantially constant with respect to the input current.  
           [0014]    The above photocoupler semiconductor device with the constant current circuit, however, cannot be used for controlling the turn-on time under a plurality of conditions. For example, to provide a slow-on and fast-on operation in the Office Channel Unit (OCU) switch of Integrated Services Digital Network (ISDN), it cannot be used in such a case that the required turn-on time is 10 ms for a certain timing and 1 ms for another timing.  
         SUMMARY OF THE INVENTION  
         [0015]    Accordingly, it is an object of the invention to provide a photocoupler semiconductor device with a constant current circuit enabling to change the turn-on time without difficulty.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a circuit diagram of a photocoupler semiconductor device according to the first embodiment of the invention;  
         [0017]    [0017]FIG. 2 is a plan view of a circuit pattern according to the first embodiment;  
         [0018]    [0018]FIG. 3 is a graph showing the input current vs. turn-on time characteristic of the photocoupler semiconductor device;  
         [0019]    [0019]FIG. 4 is a plan view of a circuit pattern according to the second embodiment of the invention;  
         [0020]    [0020]FIG. 5 is a graph showing the input current vs. turn-on time characteristic according to the second embodiment;  
         [0021]    [0021]FIG. 6( a ) is a plan view of a semiconductor resistor element constituting a resistor according to the invention;  
         [0022]    [0022]FIG. 6( b ) is the semiconductor resistor element with a light shielding film;  
         [0023]    [0023]FIG. 6( c ) is a sectional view taken along line A-A′ of FIG. 6( b );  
         [0024]    [0024]FIG. 7 is a circuit diagram of a conventional photocoupler semiconductor device;  
         [0025]    [0025]FIG. 8 is a graph showing the input current vs. turn-on time characteristic of the conventional photocoupler semiconductor device;  
         [0026]    [0026]FIG. 9 is a circuit diagram of a conventional photocoupler semiconductor device with a constant current control; and  
         [0027]    [0027] 10  is a graph showing the input current vs. turn-on time characteristic of the conventional photocoupler semiconductor device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    Embodiments of the invention will now be described with reference to the accompanying drawings.  
         [0029]    In FIG. 1, the constant current circuit according to the first embodiment comprises a light receiving or photovoltaic device  12 , a constant current circuit  14 , and a control circuit  16 . The resistor  20 ′ of the constant current circuit, however, is different from the conventional one in FIG. 9 as follows. That is, it has a resistance characteristic that the current value increases with the light amount received.  
         [0030]    In FIG. 2, the resistor  20 ′ has an opening  37  in the light shielding film for receiving light that strikes the photovoltaic device  12 , too. Transistor circuits and photodiodes are represented by Tr and rounded squares, respectively.  
         [0031]    The resistor  20 ′, which has such a pattern as shown in FIG. 2, is made by a process such as shown in FIGS.  6 ( a ) through ( c ).  
         [0032]    The semiconductor resistor element is provided on one of the n-type monolithic islands  34  formed on a dielectric partition board  35 . A p-type semiconductor layer  36  and an n-type semiconductor layer  33  having a density higher than that of the board are formed by diffusion process on the monolithic island  34 , respectively. Then, an insulation film  32  is coated and a light shielding film  31  is formed thereon with aluminum. The p-type semiconductor  36  constitutes the resistor  20 ′. The n-type semiconductor layer  33  is provided to not only prevent current leak through the surface of the n-type monolithic island  34  but also conduct current to the p-type semiconductor layer  36 .  
         [0033]    In this resistor  20 ′, the n-p-n junction can form a parasitic phototransistor. To prevent it, the light shielding film  31  is provided over the entire surface of the resistor  20 ′ to prevent incident light to the p-n-p junction. The p-type semiconductor layer  36  is formed in a zigzag fashion to increase the distance between the terminals and thus the resistance. According to the invention, the opening  37  is provided in the light shielding film  31  and the insulation film  32  to expose the p-type semiconductor layer  36  so that it can receive light. When light strikes the opening  37 , the junction between the p-type semiconductor layer  36  and the n-type monolithic island  34  operates as a parasitic phototransistor. This diffusion resistance allows the current flow to increase with the amount of light received in the normal range of usage.  
         [0034]    The operation of MOSFETs  18 - 1  and  18 - 2  that are connected to the semiconductor resistor element will be described below.  
         [0035]    In FIG. 1, when light strikes the light receiving device  12 , each light receiving diode  12  produces an electromotive force of approximately 0.6 V so that a voltage of about 5 V is established across the terminals  2  and  2 ′. The electric charges produced by this voltage are accumulated in the gates of the driven transistors MOSFETs  18 - 1  and  18 - 2  to turn them on. When the p-type semiconductor layer  36  receives the light, it works as not only a resistor but also a parasitic phototransistor in cooperation with the n-type semiconductors  33  and  34 .  
         [0036]    When there is no light illumination, there is no electromotive force so that the electric charges stored in the gates of MOSFETs  18 - 1  and  18 - 2  are discharged through the p-type semiconductor layer  36  to turn them off.  
         [0037]    By providing the opening  37  in the light shielding film  31  it is possible to form a parasitic phototransistor in the region of the opening  37  so that the number of parasitic phototransistors is increased so as to change the resistance characteristic.  
         [0038]    The input current vs. turn-on time characteristic of the photocoupler semiconductor device according to the invention is shown in FIG. 3. When the input current increases, the turn-on time decreases more than the conventional device that is represented by dotted line. In other words, the turn-on time can be selected in a range wider than that of the conventional one with respect to the input current.  
         [0039]    In operation, when electric current flows through the light emitting element  10 , such as a light emitting diode, the light emitting element  10  gives off light and the light receiving device  12 , such as photodiodes, generates an electromotive force to apply a voltage to the gates of MOSFETs  18 - 1  and  18 - 2  to turn them on. The light from the light emitting element  10  strikes the resistor  20 ′, too. When the light is weak, the resistance value of the resistor  20 ′ is little changed but as the light increases, the parasitic phototransistor operates to reduce the resistance value.  
         [0040]    The constant current of the constant current circuit  14  is determined by this resistor value so that the larger the received light amount, the higher the electric current. Consequently, at a time of small light amounts, the resistance is high to provide a low level of current, thus lengthening the turn-on time. As the received light amount increases, the current value increases to shorten the turn-on time. The turn-on time dependency on the input power is shown in FIG. 3 in comparison with the conventional one.  
         [0041]    As shown in FIG. 3, the turn-on time varies widely with the input current so that it is possible to control the turn-on time to be approximately 10 ms at approximately 1 mA of the LED current and below 2 ms at 10 mA or more of the LED current regardless of the LED power.  
         [0042]    According to the second embodiment, two or more openings are provided so as to increase the LED current dependency or the received light amount. The circuit pattern according to the second embodiment is shown in FIG. 4, wherein two openings  37  are provided in the light shielding film  31  that covers the p-type semiconductor layer  36 . Alternatively, three or more openings  37  may be provided.  
         [0043]    In operation, electric current makes the LED  10  emit light, the photodiodes  12  generate the output corresponding to the received light amount and the resistor  20 ′ has the resistant value corresponding to the light amount to the LED  10 . As described above, the constant current circuit  14  with the resistor  20 ′ supplies a constant current to MOSFETs  18 - 1  and  18 - 2  to turn them on. Since there are two openings  37  in the light shielding film  31 , such parasitic phototransistor effects as described above are produced at two places to quickly shorten the turn-on time with the increased light amount received.  
         [0044]    The input current vs. turn-on time characteristic of the second embodiment is shown in FIG. 5. As the input current increases, the turn-on time decreases to a large extent. The turn-on time is approx. 10 ms at 1 mA of the LED current and 1 ms or lower at 10 mA or more of the LED current.  
         [0045]    Alternatively, the opening  37  may take any shape as well as square. The constant current circuit  14  may be any circuit capable of providing a constant current by means of the resistor that its resistant value increases with the received light amount.  
         [0046]    As has been described above, according to the invention, it is possible to shorten the turn-on time of a photocoupler semiconductor device to a large extent as the input current increases. Thus, it is possible to provide the characteristic that the turn-on time varies widely with the input current.