JP2008091624A - Optical coupler - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical coupler which has an improved switching time. <P>SOLUTION: The optical coupler comprises a light emitting element, a light receiving element, an output stage MOSFET, and a discharge circuit. The discharge circuit includes a discharging MOSFET and a first rectifier element. When the coupler is put in a turned-ON state, electric charges are accumulated in the gate of the output stage MOSFET from a first terminal. When the output stage MOSFET is put in a conducted state and the coupler transits to its turned-OFF state, the charges accumulated in the gate of the output stage MOSFET are passed through the forward junction of the first rectifier element and accumulated in the gate of the discharging MOSFET to put the discharging MOSFET in its conducted state. Charges accumulated in the gate of the output stage MOSFET are discharged through the discharging MOSFET in the conducted state to put the output stage MOSFET in its nonconducted state. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical coupling device.

In an optical coupling device such as a photorelay using a semiconductor, when an LED having an emission wavelength in the infrared band emits light, the photodiode array is optically coupled to generate photovoltaic power. As a result, the gate voltage becomes equal to or higher than the threshold voltage, and the output stage MOSFET becomes conductive. When the LED is turned off, the photovoltaic power becomes zero. As a result, the gate voltage becomes lower than the threshold voltage, and the output stage MOSFET becomes nonconductive. In this way, signal transmission is possible with the power supply system insulated between the input and output. By the way, when the optocoupler is turned on by switching the gate of the MOSFET from non-conduction to conduction, if the resistance value of the discharge circuit is increased in order to reduce the photocurrent, the discharge time becomes long when turning off. There is. There is a technical disclosure example regarding an optically coupled field effect transistor switch in which a discharge circuit is composed of a bipolar transistor and a phototransistor inserted between a base and an emitter, and discharges charges accumulated in an output field effect transistor (Patent Document) 1).
JP-A-2-309811

  The present invention provides an optical coupling device with improved switching time.

  According to one embodiment of the present invention, a light emitting element, a light receiving element that receives light from the light emitting element and generates a photovoltaic force between the first and second terminals, and a gate connected to the first terminal A discharge circuit connected between the output stage MOSFET, the light receiving element, and the output stage MOSFET, and discharging the charge accumulated in the gate of the output stage MOSFET, the discharge circuit comprising: , Including a discharging MOSFET and a first rectifying element, and in the turn-on state, the charge from the first terminal is accumulated in the gate of the output stage MOSFET, and the output stage MOSFET is turned on and transitioned to the turn-off state Then, the electric charge accumulated in the gate of the output stage MOSFET moves through the forward junction of the first rectifying element and accumulates in the gate of the discharge MOSFET. The discharge MOSFET is turned on, and the charge accumulated in the gate of the output stage MOSFET is discharged through the discharge MOSFET that is turned on, thereby making the output stage MOSFET non-conductive. An optical coupling device is provided.

  The present invention provides an optical coupling device with improved switching time.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram showing an optical coupling device according to a first specific example of the present invention.
A light emitting diode (LED) 20 is disposed between the input first terminal 10 and the input second terminal 11 of the optical coupling element. The emitted light of the LED 20 is incident on a photodiode array 22 (light receiving element) in which a plurality of photodiodes are connected in series to generate a photovoltaic force. That is, a photoelectromotive force is generated between the light receiving first terminal 12 (anode in FIG. 1) and the light receiving second terminal 13 (cathode in FIG. 1) by the emitted light, and a photocurrent I flows.

  Since the photodiodes generate a photocurrent and are clamped at about 0.6V per one, the N series connected photodiode arrays 22 are clamped at a voltage in the vicinity of N × 0.6V. The LED 20 is made of a compound semiconductor material such as InGaAlP, GaAlAs, or GaN, and has a light emission wavelength range from visible light to infrared light. The photodiode array 22 is made of silicon having light receiving sensitivity within this wavelength range.

  The output stage of the optical coupling device is constituted by two MOSFETs 40 and 42, and the drain of the MOSFET 40 is connected to the output first terminal 14 and the drain of the MOSFET 42 is connected to the output second terminal 15. The sources of the MOSFET 40 and the MOSFET 42 are connected in common, and further connected to the light receiving second terminal 13. Furthermore, the gates of the MOSFETs 40 and 42 are connected in common and connected to the light receiving first terminal 12.

Here, the action of turn-on and turn-off of the optical coupling device will be described.
To turn on the optical coupling device, the LED 20 is turned on. Electric charges are accumulated in the gates of the MOSFETs 40 and 42 by the photocurrent I generated in the photodiode array 22 by the emitted light. When the gate voltage of the MOSFET becomes equal to or higher than the threshold value (Vth), the MOSFETs 40 and 42 are turned on, and the optical coupling device is turned on.

  On the other hand, the LED 20 is turned off to turn off the optical coupling device. As a result, the photocurrent I is not supplied to the gate of the output stage MOSFET, so that the charge accumulated in the gate is discharged. When the gate voltage becomes lower than Vth, the MOSFETs 40 and 42 become non-conductive, and the photocoupler is turned off. Become. When the MOSFET has an enhancement type and silicon N channel structure, Vth is 2 to 3V.

  As a result, the power supply system is separated from the input side constituted by the input first terminal 10 and the input second terminal 11 and the output side constituted by the output first terminal 14 and the output second terminal 15. In this state, signal transmission is possible.

  In this optical coupling device, a discharge circuit is required to discharge the electric charge accumulated in the gate in the turn-off state. In the first specific example shown in FIG. 1, the discharge circuit 18 is inserted between the photodiode array 22 and the MOSFETs 40 and 42. The discharge circuit 18 includes a discharge MOSFET 30 and an inverting circuit. The inverting circuit includes a phototransistor 32 connected between the gate of the discharge MOSFET 30 and the light receiving second terminal 13, a resistor 34 connected between the gate of the discharge MOSFET 30 and the light receiving first terminal 12, and a diode ( (Rectifier element) 36 and a series connection circuit. The discharge MOSFET 30 has a drain connected to the first light receiving terminal 12 and a source connected to the second light receiving terminal 13. In FIG. 1, the discharge MOSFET 30 is also an enhancement type N-channel MOSFET.

  Further, a resistor 38 is connected between the base and the emitter of the phototransistor 32 so that charges accumulated in the base can be released. The phototransistor 32 is turned on by the emitted light from the LED 20, and the voltage between the collector and the emitter becomes almost zero. As a result, since the discharge MOSFET 30 becomes non-conductive, the impedance between the drain and the source of the discharge MOSFET 30 becomes high, and high-speed turn-on is possible while keeping the loss of the photocurrent I in the discharge circuit 18 low.

  On the other hand, at the time of turn-off, the phototransistor 32 becomes non-conductive, and the charge accumulated in the gates of the MOSFETs 40 and 42 moves to the gate of the discharge MOSFET 30 through the forward junction of the resistor 34 and the diode 36 and is accumulated. The MOSFET 30 becomes conductive. As a result, the gate-source of the discharging MOSFET 30 is reduced to the on-resistance Ron, the charges accumulated in the gates of the MOSFETs 40 and 42 are rapidly discharged, the MOSFETs 40 and 42 are turned off, and the optical coupling device is turned off.

  In this case, even if the discharge proceeds and the gate voltages of the MOSFETs 40 and 42 become lower than the gate voltage of the discharging MOSFET 30, the charge of the gate of the discharging MOSFET 30 is held because of the diode 36. As a result, the discharging MOSFET 30 does not become non-conductive, and the gate-source voltages of the output stage MOSFETs 40 and 42 can be reduced to almost zero. Further, the charge remaining at the gate of the discharging MOSFET 30 is rapidly discharged when the phototransistor 32 is turned on at the time of turn-on, and the discharging MOSFET 30 is rapidly turned off.

  If a bipolar transistor is used instead of the discharging MOSFET 30, a potential difference of about 0.6 V remains between the collector and the emitter at the time of turn-off, and a voltage remains between the gate and the source of the output stage MOSFET.

  Thus, in this specific example, when the discharge circuit 18 is turned on, the gate of the discharge MOSFET 30 and the second terminal 13 of the light receiving element are substantially at the same potential, and when turned off, the gate of the discharge MOSFET 30 and the first of the light receiving element are first. An inverting circuit having substantially the same potential as that of the terminal 12 is included.

    In the optical coupling device according to the comparative example, no diode is provided between the gate of the discharging MOSFET and the light receiving first terminal, and only a resistor is provided.

Here, changes in the gate-source voltages of the MOSFETs 40 and 42 and the discharging MOSFET 30 will be described in more detail with reference to FIGS. Figure 2 is a graph showing the simulation results of the gate-source voltage V _G of the MOSFET in the first embodiment and the comparative example, FIG. (A) the specific example, FIG. (B) in Comparative Example is there. FIG. 3 is a graph showing the gate-source voltage dependence of the on-resistance Ron of the MOSFET.

In the case of the first specific example in FIG. 2A, the vertical axis represents the gate-source voltage V _G (V), and the horizontal axis represents time (μsec). When the LED is turned off, the supply of charge to the gates of the MOSFETs 40 and 42 is stopped, and the accumulated charge moves to the gate of the discharging MOSFET 30 via the resistor 34 and the first diode 36 and begins to be accumulated. As a result, the voltage _{V G} between the gate and source of the discharge MOSFET rapidly discharge MOSFET30 conducts so raised, the charge accumulated in the gate of the MOSFET40 and 42 via a low on-resistance Ron is discharged.

In this way, the gate voltage V _G of the discharging MOSFET 30 starts to decrease gradually after reaching a peak after about 0.2 μsec. On the other hand, the gate-source voltage V _G of the MOSFETs 40 and 42 starts to rapidly decrease from 20 V as the electric charge is discharged. In this specific example, the charge supplied to the gate of the discharge MOSFET 30 is the charge remaining at the gates of the MOSFETs 40 and 42. The gate-source voltage _{V G} of the discharge proceeds MOSFET40 and 42, supply of the threshold voltage than the lower charge of the gate of the discharge MOSFET30 is decreased, the electric charge to the first diode 36 is can be trapped, it can maintain the voltage V _G between the gate and source of about 10V. As a result, as illustrated in Figure 3, while suppressing the on-resistance and 5Ω or less discharge is continued, the gate-source voltage V _G is lower than the threshold value Vth MOSFET 40 and 42 become non-conductive. In this case, it a rapid zero voltage V _G between the gate and the source.

On the other hand, in the comparative example of FIG. 2 (b), the discharge of the charge progresses and the gate-source voltage _{V G} of the MOSFET is supplied charge is decreased lower than the gate-source voltage _{V G} of the discharge MOSFET Since there is no diode, the charge accumulated in the gate of the discharging MOSFET passes through the resistor and is discharged via Ron or the like. As a result, the gate-source voltage V _G is rapidly reduced than the embodiment illustrated in Figure 2 (a).

In this comparative example, the gate-source voltage V _G is decreased to 5V, the on-resistance Ron than 3 becomes higher 20 [Omega, further increases in the below. Because of this high on-resistance, a decrease in the gate-source voltage V _G of the MOSFET is discharged charge suppression becomes gentle with respect to time. When the gate-source voltage _{V G} of the discharge MOSFET by the discharge is lower than the threshold value Vth, the discharging MOSFET becomes non-conductive, the drain-source becomes high impedance. As a result, the discharge of charge is suppressed, and the charge remains in the gate of the MOSFET.

In this way, the time to drop below the threshold voltage becomes longer because of the gradual reduction of the gate-source voltage V _G of the MOSFET. That is, in the comparative example, the time during which the MOSFET is turned off is about 0.9 μsec, which is three times as long as about 0.3 μsec of the first specific example. In the simulation of FIG. 2, the time constant varies depending on the circuit constant, so the absolute value of the time represented on the horizontal axis is not limited.

FIG. 4 is a circuit diagram showing an optical coupling device according to a second specific example of the present invention.
In the second specific example, the discharge circuit 18 is inserted between the photodiode array 22 and the MOSFETs 40 and 42. The discharge circuit 18 includes a discharge MOSFET 30 and an inverting circuit. The inverting circuit includes a series connection circuit of a second resistor 33 and a first diode (rectifier element) 36 inserted between the gate of the discharging MOSFET 30 and the light receiving first terminal 12, and the light receiving second terminal 13. And a second diode 37 inserted between the source of the discharge MOSFET 30.

  At the time of turn-on, the discharge MOSFET 30 becomes non-conductive, so that the impedance between the drain and source of the discharge MOSFET 30 becomes high, and the photocurrent I can sufficiently reduce the loss in the discharge circuit 18 and can be turned on at high speed.

  On the other hand, at the time of turn-off, the electric charge accumulated in the gates of the MOSFETs 40 and 42 is supplied to the gate of the discharging MOSFET 30 through the second resistor 33 and the first diode 36, and the discharging MOSFET 30 becomes conductive. As a result, the impedance between the gate and the source of the MOSFETs 40 and 42 is reduced to the on-resistance, and the charge accumulated in the gate is rapidly discharged.

In this case, even if the discharge proceeds and the gate-source voltage of the MOSFETs 40 and 42 becomes lower than the gate-source voltage of the discharge MOSFET 30, the charge of the gate of the discharge MOSFET 30 is due to the first diode 36. Is retained. As a result, discharge MOSFET30 does not become non-conductive, can be a zero voltage _{V G} between the gate and the source of the MOSFET40 and 42. Further, the charge remaining at the gate of the discharge MOSFET 30 is discharged by the photocurrent I generated by the photodiode array 22 at the time of turn-on, and the discharge MOSFET 30 becomes non-conductive. As described above, the MOSFETs 40 and 42 can be turned off at high speed.

  Also in the second specific example, when the discharge circuit 18 is turned on, the gate of the discharge MOSFET 30 and the light receiving second terminal 13 have substantially the same potential, and when turned off, the gate of the discharge MOSFET 30 and the light receiving first terminal 12 have substantially the same potential. Inverting circuit is included.

FIG. 5 is a circuit diagram showing an optical coupling device according to a third specific example of the present invention.
In the third specific example, the discharge circuit 18 is inserted between the photodiode array 22 and the MOSFETs 40 and 42. The discharge circuit 18 includes a discharge MOSFET 30 and an inverting circuit. The inverting circuit includes a CMOS inverter 50 and a series connection circuit of a second phototransistor 56 and a third resistor 58 inserted between the light receiving first terminal 12 and the light receiving second terminal 13. In this case, the emitter of the second phototransistor 56 is connected to the third resistor 58 and also connected to the input terminal of the CMOS inverter 50. The output terminal of the CMOS inverter 50 is connected to the gate of the discharging MOSFET 30.

  At turn-on, the phototransistor 56 becomes conductive, and the photovoltaic power generated by the photodiode array 22 is input to the CMOS inverter 50 and becomes H level. Therefore, the output of the CMOS inverter 50 becomes L level, the discharge MOSFET 30 becomes non-conductive, charges are accumulated in the gates of the MOSFETs 40 and 42, and high-speed turn-on is possible.

On the other hand, at the time of turn-off, the second phototransistor 56 becomes non-conductive, the charge accumulated in the gate of the CMOS inverter 50 is discharged through the third resistor 58, the input of the CMOS inverter 50 becomes L level, and the output becomes H Become a level. As a result, the gate charges of the MOSFETs 40 and 42 are rapidly discharged. Also in this case, since the first diode (rectifier element) 36 is provided, the gate-source voltage V _{G of the} discharge MOSFET 30 is maintained even if the gate-source voltage of the MOSFETs 40 and 42 decreases. Therefore, the discharging MOSFET 30 is kept in a conductive state, the gate voltages of the MOSFETs 40 and 42 can be rapidly reduced to zero, and high-speed turn-off can be performed.

  Also in the third specific example, when the discharge circuit 18 is turned on, the gate of the discharge MOSFET 30 and the light receiving second terminal 13 have substantially the same potential, and when turned off, the gate of the discharge MOSFET 30 and the light receiving first terminal 12 have substantially the same potential. Inverting circuit is included.

  In each of these first to third specific examples, the discharge MOSFET 30 is non-conductive at turn-on, so that the impedance is high, and charge can be accumulated in the gate of the output stage MOSFET while suppressing loss of photocurrent, thereby enabling rapid turn-on. And On the other hand, at the time of turn-off, the charge accumulated in the gate of the discharge MOSFET can be confined by the diode and the conduction state can be continued, so that the charge accumulated in the gate of the output stage MOSFET can be discharged to almost zero. As a result, the output stage MOSFET can be made non-conductive and turned off rapidly. As described above, the first to third specific examples provide the optical coupling device in which the switching time is shortened.

  Unlike the above specific example, a method in which the discharge circuit is configured by a depletion type FET is also conceivable. In this case, at the time of turn-on, the photocurrent is passed through a resistor, thereby setting the gate to a lower potential than the source and making the FET nonconductive. As a result, the gate-source impedance of the output stage MOSFET can be increased, and the charge can be rapidly accumulated in the gate while reducing the loss of photocurrent. Further, at the time of turn-off, charges move through the resistor, the gate-source voltage of the MOSFET is made equal, and the MOSFET is made conductive. As a result, the gate-source impedance of the output stage MOSFET can be lowered, and the charge accumulated in the gate of the output stage MOSFET can be rapidly discharged.

  The output stage MOSFET is often an enhancement type. For this reason, when integrating the optical coupling device excluding the LED, if a process for manufacturing both enhancement type and depletion type MOSFETs is provided, the process becomes complicated. On the other hand, in this specific example, the process can be simplified because, for example, only the manufacturing process of the N-channel MOSFET can be used to make an integrated circuit except for the LED.

  FIG. 6 is a schematic diagram showing a specific example of the layout of the integrated one chip according to the embodiment of the present invention. A photodiode array 22 is disposed between the MOSFETs 40 and 42. The photodiodes constituting the photodiode array 22 are connected in series by a wiring layer 74 made of a metal material. In the region of the discharge circuit 18, a discharge MOSFET 18, a diode, a photodiode, a resistor, and the like are formed. The drains D of the MOSFETs 40 and 42 are connected to the bonding pads 60 and 62 by the wiring layer 70, respectively, and are further connected to the output first terminal 14 and the output second terminal 15 by the bonding wire 78. The regions of the MOSFETs 40 and 42, the photodiode array 22, and the discharge circuit 18 are separated by a trench 76. By using a one-chip IC integrated except for LEDs, the optical coupling device can be miniaturized.

Next, a modification of the diode structure for confining charges in the gate of the discharging MOSFET 18 in the integrated circuit will be described.
FIG. 7 is a diagram illustrating a configuration of a modified example of a diode (rectifying element) that can be used in each of the specific examples described above. In FIG. 7A, the base (B) and collector (C) of the NPN transistor are connected, and a pn junction diode between the emitter (E) is used. In FIG. 7B, the base and emitter of the PNP transistor are connected, and a pn junction between the collector is used. In FIG. 7C, the drain and gate of the N-channel MOSFET are connected, and a pn junction between the back gate and the source is used. In FIG. 7D, the anode and gate of the thyristor are connected, and a pn junction between the cathode and the cathode is used.

  In the above specific example, the output stage MOSFET is composed of two pieces. If it does in this way, it can apply to the use which needs an alternating current load and a bi-directional pressure | voltage resistance. On the other hand, if the output stage MOSFET is one stage, it is mainly used for direct current, but the configuration can be simplified.

  In the above specific examples, the output stage and the discharging MOSFET have an N channel structure, but may have a P channel structure. In this case, the conductivity of the photodiode array 22, the first diode 36, the second diode, the first photodiode 36, the second photodiode, etc. may be reversed.

  The embodiments of the present invention have been described above with reference to the drawings. However, the present invention is not limited to these specific examples. Even if a person skilled in the art changes the design regarding the size, shape, arrangement, etc. of the MOSFET, light receiving element, LED, photodiode, diode, resistor, etc. constituting the optical coupling device, it does not depart from the gist of the present invention. As long as it is within the scope of the present invention.

It is a circuit diagram showing the optical coupling device concerning the 1st example of the present invention. It is a graph of the simulation result of the gate voltage of MOSFET, The figure (a) represents this example, and the figure (b) represents the comparative example, respectively. It is a graph showing the gate-source voltage dependence of the on-resistance of MOSFET. It is a circuit diagram showing the optical coupling device concerning the 2nd specific example of this invention. It is a circuit diagram showing the optical coupling device concerning the 3rd example of the present invention. It is a schematic diagram showing the layout of the semiconductor integrated circuit concerning the example of this invention. It is a figure showing the structure of the diode used for this example.

Explanation of symbols

12 light receiving first terminal, 13 light receiving second terminal, 14 output first terminal, 15 output second terminal 18 discharge circuit, 20 LED, 22 photodiode array, 30 discharge MOSFET, 32 first phototransistor, 33 second Resistor, 34 first resistor, 36 first diode, 37 second diode, 40 and 42 MOSFET, 50 CMOS inverter, 56 second phototransistor, 58 third resistor,

Claims (5)

A light emitting element;
A light receiving element that receives light from the light emitting element and generates a photovoltaic force between the first and second terminals;
An output stage MOSFET having a gate connected to the first terminal;
A discharge circuit connected between the light receiving element and the output stage MOSFET and discharging the charge accumulated in the gate of the output stage MOSFET;
With
The discharge circuit includes a discharge MOSFET and a first rectifying element,
In the turn-on state, the charge from the first terminal is accumulated in the gate of the output stage MOSFET, and the output stage MOSFET is turned on.
When transitioning to the turn-off state, the electric charge accumulated in the gate of the output stage MOSFET moves through the forward junction of the first rectifying element and accumulates in the gate of the discharging MOSFET to make the discharging MOSFET conductive. The optical coupling device is characterized in that the charge accumulated in the gate of the output stage MOSFET is discharged through the discharge MOSFET which is in a conductive state, and the output stage MOSFET is made non-conductive. The discharge circuit is:
A first phototransistor having a collector connected to the gate of the discharging MOSFET and an emitter connected to the second terminal;
A series connection circuit of the first rectifier and the first resistor connected between the gate of the discharging MOSFET and the first terminal;
Including
The source of the output stage MOSFET and the source of the discharging MOSFET are connected to the second terminal,
The optical coupling device according to claim 1, wherein a drain of the discharging MOSFET is connected to the first terminal. The discharge circuit is:
A series connection circuit of the first rectifier element and a second resistor connected between the gate of the discharging MOSFET and the first terminal;
A second rectifying element connected between a connection point of the source of the discharge MOSFET and the source of the output stage MOSFET and the second terminal;
Including
The drain of the discharging MOSFET is connected to the first terminal,
The gate of the discharging MOSFET is connected to the second terminal;
2. The optical coupling device according to claim 1, wherein the second rectifying element is connected to form a reverse junction with respect to a voltage between a gate and a source of the discharging MOSFET in a conductive state. The discharge circuit includes a CMOS inverter, a second phototransistor, and a third resistor,
A collector of the second phototransistor is connected to the first terminal;
The third resistor is connected between an emitter of the second phototransistor and the second terminal;
An emitter of the second phototransistor is connected to an input terminal of the CMOS inverter;
The output terminal of the CMOS inverter is connected to the gate of the discharging MOSFET,
The first rectifying element is connected between the first terminal and a power supply terminal of the CMOS inverter,
The drain of the discharging MOSFET is connected to the first terminal,
2. The optical coupling device according to claim 1, wherein a ground terminal of the CMOS inverter, a source of the discharge MOSFET, and a source of the output stage MOSFET are connected to the second terminal. 5. The optical coupling device according to claim 1, wherein the discharge MOSFET is an enhancement type N-channel MOSFET.

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