JP2927245B2 - MOSFET output type photocoupler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a MOSFET output type photocoupler, and more particularly to an improvement in noise immunity.

[0002]

2. Description of the Related Art FIG. 6 is a circuit diagram of a MOSFET output type photocoupler described in Japanese Patent Application Laid-Open No. 60-119124. In this circuit, when a forward input current flows through the light emitting element 1, the light emitting element 1 emits light, and this light generates a photovoltaic voltage at both ends of the photodiode array 2. This electromotive voltage is applied to the gate of MOSFET3, and MOSFE
T3 enters the operating state. At this time, a forward current flows through the diode 16, and the voltage drop of the diode 16 reverse-biases the base and the emitter of the transistor 17. Therefore, the transistor 17 maintains the non-conductive state, so that the photovoltaic current generated from the photodiode array 2 flows only into the MOSFET 3. Next, when the forward current stops flowing through the light emitting element 1, the electric charge accumulated in the gate of the MOSFET 3 starts discharging. The discharge current immediately after the start of the discharge flows through the path of the gate of the MOSFET 3, the photodiode array 2, the base of the transistor 17, and the emitter of the transistor 17, so that the transistor 17 is turned on. At the next moment, the discharge current is M
Discharge occurs from the gate of the OSFET 3 through the collector and the emitter of the transistor 17.

The diode 16 and the transistor 1
Without the circuit consisting of 7 (called the discharge time shortening circuit),
The discharge time takes tens to hundreds of ms, which is not practical. By the way, since the light emitting element 1 and the photodiode array 2 are arranged in horizontal or opposed positions, an electric capacitance (referred to as a light emitting side-light receiving side coupling capacitance) exists between them. Therefore, the MO shown in FIG.
In the SFET output type photocoupler, when common mode noise enters between the light emitting side and the light receiving side when the light emitting element 1 is not emitting light, a transient current flows from the light emitting side to the light receiving side due to the light emitting side-light receiving side coupling capacitance. And this transient current is M
The current flows into the gate of the OSFET 3 and the MOSFET 3 malfunctions. To solve this problem, Japanese Patent Application Laid-Open
In the example of Japanese Patent No. 99616, an n-channel normally-on type static induction transistor (SIT) 19 is used as shown in FIG. 7 instead of the transistor 17 of FIG.

In the circuit shown in FIG. 7, when a forward current flows through the light emitting element 1, the light emitting element 1 emits light, and this light generates a photovoltaic voltage in the photodiode array 2. This electromotive voltage is applied to the gate of MOSFET 3 and MOSF
ET3 enters the operating state. At this time, the current is MOSFE
Flows through T3 and SIT19. The current flowing through these two paths flows to the diode 16 and the diode 16
SIT19 becomes non-conductive when the voltage drop of SIT19 reverse biases the gate and emitter of SIT19. Thereafter, the current does not flow through the SIT 19 but flows only into the MOSFET 3. By the way, according to such a circuit configuration, even if common mode noise enters when the light emitting element 1 does not emit light, the transient current flows into the SIT 19 having a lower impedance than the MOSFET 3, so that the MOSFET 3 does not malfunction. . The circuits having the function of preventing the malfunction of the MOSFET 3 as described above are collectively called a malfunction prevention circuit. However, even when the light emitting element 1 emits light and a photovoltaic voltage is generated in the photodiode array 2, for the same reason,
At first, most of the photovoltaic current flows into the SIT 19. Thereafter, the SIT 19 becomes non-conductive, and most of the photovoltaic current flows into the gate of the MOSFET 3 and
It takes a certain amount of time until the SFET 3 enters the operating state, and this causes a problem that the on-time is delayed.

On the other hand, in the example of JP-A-6-29813, another photocoupler including a light emitting element 21 and a phototransistor 22 as shown in FIG. 8 is connected. In this circuit, when a forward current flows into the light emitting element 1, the light emitting element 1 emits light, and this light generates a photovoltaic voltage in the photodiode array 2. This electromotive voltage is M
The voltage is applied to the gate of the OSFET 3, and the MOSFET 3 is activated. When a forward current flows through the light emitting element 1,
The output of inverter 20 is low and the input of inverter 20 is high. Therefore, at this time, the light emitting element 21
, No current flows, so that the phototransistor 22 maintains the non-conductive state, and does not affect the operation of the MOSFET 3. Conversely, when no forward current flows through the light emitting element 1, a forward current flows through the light emitting element 21 and the phototransistor 22 is turned on. At this time, even if common mode noise enters, the transient current flows into the phototransistor 22 in a conductive state, so that the MOSFET 3 does not malfunction. Further, although the on-time is not delayed, another photocoupler (the light emitting element 21 and the phototransistor 22) and the inverter 20 are required, so that the cost is increased and an extra space is required.

[0006]

As described above, the MOSF
Common mode noise immunity (CMRR: Common Mode) so that the ET output type photocoupler does not malfunction.
In order to improve the rejection ratio, the conventional technology has a problem that an on-time delay occurs, or the cost increases and the device becomes larger. The present invention has been made to solve the above problems, and an object of the present invention is to provide a MOSFET output type photocoupler capable of improving CMRR at low cost while maintaining characteristics such as on-time and a package size. Is to do.

[0007]

According to the present invention, there is provided a light emitting device which generates an optical signal in response to an input signal, and is electrically insulated from the light emitting device. An optically coupled light receiving element for receiving a light signal of the light emitting element and generating a photoelectromotive force is provided, and an output MOSFE operated by the photoelectromotive force of the light receiving element.
The input terminal of T is directly connected to the output terminal of the light-receiving element, and the light-shielded phototransistor which is shielded from the light signal of the light-emitting element, conducts when noise flows in and short-circuits the output of the light-receiving element is connected to the light-receiving element. Connect to the output terminal and
The MOSFET output type photocoupler is arranged near the element.

With this configuration, even if common mode noise is generated when the light emitting element does not emit light, the light shielding type phototransistor is arranged near the light receiving element. The phototransistor is turned on. Therefore, the transient current flowing into the circuit flows through the light-shielded phototransistor, and the
Since it does not flow into the gate of the SFET, malfunction of the MOSFET can be prevented. On the other hand, since the light-blocking phototransistor is shielded from light, the light-blocking phototransistor maintains a non-conductive state even when the light-emitting element emits light. Therefore, the current due to the photoelectromotive force of the light receiving element does not flow to the light-shielding phototransistor but flows to the gate of the MOSFET. Therefore, there is no time loss until the MOSFET enters the operating state, and no delay in the on-time occurs.

In addition, since the light-shielding type phototransistor can be formed on one chip together with the light receiving element, it is not necessary to increase the package size. Further, M
A diode is connected to the OSFET output type photocoupler in the forward direction through a path through which the current due to the photovoltaic power of the light receiving element flows through the MOSFET. When the current flows through the diode, the diode is turned off, and the current stops flowing. When this occurs, a transistor is brought into a conductive state and short-circuits the input circuit of the MOSFET. In this circuit, when no current flows due to the photoelectromotive force of the light receiving element, no current flows to the diode, and the transistor is turned on. Therefore, when the electric charge accumulated in the gate of the MOSFET starts discharging, the discharging current flows through the transistor, so that the discharging time can be shortened.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. (Embodiment) FIG. 1 shows a high CMRR MO according to the present invention.
FIG. 2 is a circuit diagram showing an embodiment of an SFET output type photocoupler. The circuit of the present embodiment, as shown in FIG.
A light emitting element 1 that generates an optical signal in response to an input signal, a photodiode array 2 as a light receiving element that receives the optical signal and generates a photovoltaic power, and operates by the photovoltaic power of the photodiode array 2 Output MOSFET
3 and a base light-shielded phototransistor 4 that conducts when common mode noise flows in and short-circuits the output of the photodiode array 2.

The light emitting element 1 and the photodiode array 2 are electrically insulated, but optically coupled. Then, the anode of the photodiode array 2
(The output terminal) , the gate (input terminal) of the MOSFET 3 and the collector of the base light-shielding phototransistor 4 are directly connected, and the cathode of the photodiode array 2, the source of the MOSFET 3 and the emitter of the base light-shielding phototransistor 4 And are connected.

Next, FIG. 2 is a sectional view of a product according to the present embodiment. The photodiode array 2 and the light-shielded base phototransistor 4 are formed in the same chip, and are called a light receiving device 2 '. The size of this chip is about 1 mm square. As shown in FIG. 2, the light emitting element 1 and the light receiving device 2 ′ are arranged facing each other at an interval of about 0.5 to 1.0 mm, and the optical coupling path is formed of the silicon resin 6. The light emitting element 1 is bonded to a lead frame 5,
The light receiving device 2 'and the MOSFET 3 are bonded to a lead frame 5'. The entire structure is molded with a black epoxy resin 7.

Next, a sectional view of the light receiving device 2 'is shown in FIG. The substrate uses a dielectric isolation method in which each element is separated by a dielectric 8. The base region of the base light-shielding phototransistor 4 is shielded from light by an aluminum film 14 ′ having a thickness of several μm via an insulating film such as an oxide film 13. This aluminum film 14 'is in a floating state and does not make contact with any wiring. Further, the entire chip is covered with a protective film such as a nitride film 15. In FIG. 3, 9 is an n ⁺ region, 10 is a p ⁺ region, 11 is a contact n ⁺ region, 12 is an emitter n ⁺ region, and 14 is an aluminum wiring. The number of photodiode arrays 2 is usually about 12 to 20, and the photovoltaic voltage is 6
８8V, and the threshold voltage of the MOSFET 3 is about 3-5V.

Next, the operation of the circuit of this embodiment will be described with reference to FIG. When a forward input current flows through the light emitting element 1, the light emitting element 1 emits light, and this light generates a photoelectromotive voltage across the photodiode array 2.
This photovoltaic voltage is applied to the gate of MOSFET3,
OSFET3 is activated. On the other hand, the base light-shielding type phototransistor 4 has the aluminum film 1 as described above.
Since the light is shielded by 4 ', no base current flows even when the light emitting element 1 emits light, and the non-conductive state is maintained. Therefore, the photovoltaic current flows into the MOSFET 3 without flowing through the base light-shielded phototransistor 4, so that the delay of the on-time does not occur.

Next, when common mode noise enters between the light emitting side and the light receiving side when the light emitting element 1 is not emitting light, a transient current flows from the light emitting side to the light receiving side due to the light emitting side / light receiving side coupling capacitance. Since the base light-shielding type phototransistor 4 is arranged near the photodiode array 2, this transient current also flows through the base of the base light-shielding type phototransistor 4, and the base light-shielding type phototransistor 4 becomes conductive. For this reason, in the conventional technology, MOSFET
The transient current flowing into the transistor 3 flows into the base light-shielding type phototransistor 4, so that the malfunction of the MOSFET 3 can be prevented.

FIG. 4 is a circuit diagram showing another embodiment of a high CMRR MOSFET output type photocoupler according to the present invention. The difference from the embodiment of FIG. 1 is that a discharge time reducing circuit including a diode 16 and a transistor 17 as shown in FIG. 4 is added to this circuit. That is, the diode 16 is connected so that a forward current flows from the source of the MOSFET 3 to the emitter of the base light-shielding phototransistor 4. Then, the collector of the transistor 17 is connected to the gate of the MOSFET 3, and the emitter of the transistor 17 is connected to the MOSF.
ET3 is connected to the source, and the base of the transistor 17 is connected to the emitter of the base light-shielded phototransistor 4.

Next, the operation of the circuit of this embodiment will be described. When the light emitting element 1 emits light and this light generates a photovoltaic voltage in the photodiode array 2, the photovoltaic voltage is applied to the gate of the MOSFET 3 and
3 is in operation. At this time, a forward current flows through the diode 16, and the voltage drop of the diode 16 reverse-biases the base and the emitter of the transistor 17.
Therefore, the transistor 17 maintains the non-conductive state, so that the photovoltaic current generated from the photodiode array 2 does not flow through the transistor 17. Further, as described above, at this time, the base light-shielding type phototransistor 4 is also in a non-conductive state, so that the photovoltaic current flows only into the MOSFET 3.

Next, when no photovoltaic voltage is generated in the photodiode array 2, the electric charge accumulated in the gate of the MOSFET 3 starts discharging. The discharge current immediately after the start of the discharge flows through the path of the gate of the MOSFET 3, the photodiode array 2, the base of the transistor 17, the emitter of the transistor 17, and the transistor 17 is turned on. At the next moment, the discharge current is discharged from the gate of the MOSFET 3 to the collector and the emitter of the transistor 17. As described above, by short-circuiting the input circuit of the MOSFET 3, the discharge time can be reduced. The reason why the discharge time shortening circuit can be added in addition to the malfunction prevention circuit is that the malfunction prevention circuit in the present embodiment is constituted only by the base light-shielding type phototransistor 4, so that a large space is required for the malfunction prevention circuit. This is because they do not.

Next, a plan view of a chip including the photodiode array 2 used in the present embodiment is shown in FIG. The chip size is about 1 mm square, and the photodiode array 2
In addition, a light shielding base phototransistor 4, a diode 16, a transistor 17, an anode-side bonding pad 18, and a cathode-side bonding pad 18 'are provided. As shown in FIG. 5, the entire surface of the base light-shielded phototransistor 4, diode 16 and transistor 17 is covered with an aluminum film 14 'having a thickness of several μm and is shielded from light. This aluminum film 14 'is in a floating state and does not make contact with any wiring.

[0020]

As described above, according to the present invention, even if common mode noise is generated when the light emitting element is not emitting light, the transient current caused by the light is blocked by using the light shielding type phototransistor. And does not flow into the gate of the MOSFET. Therefore, malfunction of the MOSFET can be prevented, and C
The MRR can be improved about twice. On the other hand, even if the light emitting element emits light and a photoelectromotive current is generated from the light receiving element, it does not flow to the light-shielded phototransistor but flows to the MOSFET. Therefore, there is no time loss until the MOSFET is brought into the operating state, so that the on-time is not delayed.

Further, since the light-shielding type phototransistor can be formed on one chip together with the light receiving element, the CMR can be performed at a low cost without increasing the package size.
R can be improved. Furthermore, if a diode and a transistor are provided in a chip having the above-mentioned light-shielding phototransistor and light-receiving element, the charge accumulated in the gate of the MOSFET can be quickly discharged when the photoelectromotive force of the light-receiving element is lost. it can.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a high CMRR MOSFET output type photocoupler according to the present invention.

FIG. 2 is a cross-sectional view of a product according to an embodiment of a high CMRR MOSFET output type photocoupler according to the present invention.

FIG. 3 is a cross-sectional view of a chip including a light receiving element in one embodiment of a high CMRR MOSFET output type photocoupler according to the present invention.

FIG. 4 is a circuit diagram showing another embodiment of a high CMRR MOSFET output type photocoupler according to the present invention.

FIG. 5 is a plan view of a chip including a light receiving element in another embodiment of a high CMRR MOSFET output type photocoupler according to the present invention.

FIG. 6 is a circuit diagram of a conventional MOSFET output type photocoupler.

FIG. 7 is a circuit diagram of a conventional MOSFET output type photocoupler.

FIG. 8 is a circuit diagram of a conventional MOSFET output type photocoupler.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light emitting element, 2 ... Photodiode array, 2 '... Light receiving element, 3 ... MOSFET, 4 ... Base light-shielding phototransistor, 5 and 5' ... Lead frame, 6 ... Silicon resin, 7 ... Epoxy resin, 8 ... Dielectric, 13 oxide film, 14 aluminum wiring, 14 'aluminum film, 15 nitride film, 16 diode, 17 transistor.

Claims (4)

(57) [Claims] 1. A light emitting element in response to an input signal to generate an optical signal, the light emitting element and is electrically be insulated Rutotomoni optically engaged binding <br/> and by receiving the optical signal A light-receiving element for generating photovoltaic power, and an output MOSF whose input terminal is directly connected to an output terminal of the light- receiving element and operates by the photovoltaic force of the light-receiving element
And ET, to short-circuit the output of said light receiving element is rendered conductive during which noise flows is shielded against arranged near and optical signal of the light emitting element of the light receiving element is connected to an output terminal of said light receiving element A MOSFET output type photocoupler comprising a light shielding type phototransistor. 2. The MOSFET output type photocoupler according to claim 1, wherein a light shielding film is formed in a base light receiving region of the light shielding type phototransistor. 3. The MOSFET according to claim 1, wherein the current generated by the photovoltaic force of the light receiving element is the MOSFET.
And a diode connected in the forward direction to a path flowing through the diode, and becomes non-conductive when current flows through the diode, and becomes conductive when current stops flowing to short-circuit the input circuit of the MOSFET. A MOSFET output type photocoupler having a transistor. 4. The MOSFET output type photocoupler according to claim 3, wherein a light shielding film is formed on the whole of the light shielding type phototransistor, the diode and the transistor.