JPH1117215A - Semiconductor relay - Google Patents

Abstract

PROBLEM TO BE SOLVED: To definitely prevent damages to the output MOSFET. SOLUTION: This semiconductor relay comprises a light-emitting element 1 emitting light in response to an input signal, a light receiving element 2 generating a photoelectromotive force after receiving light from the light-emitting element 1, an output MOSFET 3 which changes to a low impedance state when a photoelectromotive force generated by a light receiving element 2 is applied and an electric charge is created, control means 50 controlling the charge and discharge of MOSFET 3 connected between gate and source of the output MOSFET 3, an overcurrent detecting resistor 4 connected to the source of the output MOSFET 3, and a thyristor 7 which changes to a low impedance state for discharging a charge between the gate and source of output MOSFET 3 when the voltage at both ends of an overcurrent detecting resistor 4 exceeds the predetermined voltage. In this case, according the configuration, a switching element 10 is provided which changes to a low impedance state based on the light from the lightemitting element 3 so as to permit a current flow between the anode and the cathode of the thyristof 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor relay having an overcurrent protection function.

[0002]

2. Description of the Related Art FIG. 2 shows a conventional semiconductor relay of this type. The light-emitting element A emits light in response to an input signal, the light-receiving element B receives light from the light-emitting element A to generate photovoltaic power, and the photovoltaic power generated by the light-receiving element B is applied. An output MOSFETC that changes from a high-impedance state to a low-impedance state between the drain and the source when the voltage between the gate and the source exceeds a threshold value due to the charge; and an output MOSFETC.
The charge of the charge and discharge in the photovoltaic output MOSFETC includes control transistor D ₁ together with a high impedance state in the event to a low impedance state when the disappearance of the photovoltaic by the connected light receiving element B between the gate and the source of the Control means D for controlling the output, an overcurrent detecting resistor E connected to the source of the output MOSFET C, and an output MO when the voltage across the overcurrent detecting resistor E exceeds a predetermined voltage.
A thyristor F that changes from a high-impedance state to a low-impedance state between the anode and the cathode so as to discharge charges charged between the gate and the source of the SFET.

[0003] Specifically, the control unit D is composed of control transistor D ₁ and the control resistor D ₂ which is connected between the gate and source of the control transistor D _1.

Next, the operation will be described. When the light emitting element A emits light according to the input signal, the light receiving element B receives the light of the light emitting element and generates a photovoltaic voltage. Then, MO for output
SFETC is charged by applying a photovoltaic voltage between its gate and source, and when the voltage between the gate and source exceeds the threshold, the state between the drain and source changes from a high impedance state to a low impedance state, and a drain current flows. Become like

When the drain current flows over a predetermined current, the voltage across the overcurrent detecting resistor connected to the source of the output MOSFET C increases, and when the drain current exceeds the predetermined voltage, the thyristor F is connected between its anode and cathode. Changes from the high impedance state to the low impedance state, and discharges the electric charge charged between the gate and the source of the output MOSFET C. Thus, the output MOSFET C
When the charge charged between the gate and the source of the output MOSFET C is discharged, the state between the drain and the source of the output MOSFET C changes from a low impedance state to a high impedance state, and
The OSFETC drain current stops flowing.

When the light emitting element A stops emitting light, the light receiving element B stops generating photoelectromotive force, and the output MOSF
When the charge charged between the gate and the source is discharged and the voltage between the gate and the source falls below the threshold value,
The state between the drain and source changes from the low impedance state to the high impedance state, and the drain current stops flowing.

[0007]

In the above-described conventional semiconductor relay, as described above, the output MOSFE is used.
When the drain current of TC exceeds a predetermined current, the state between the anode and the cathode of the thyristor F changes from the high impedance state to the low impedance state, and the output MOSFET
Since the charge charged between the gate and the source of C is discharged, it is possible to prevent the output MOSFET C from being damaged due to the drain current continuing to flow over a predetermined current.

However, while discharging the electric charge between the gate and the source of the output MOSFET C,
For example, if the current flowing between the anode and the cathode of the thyristor F becomes smaller than the so-called holding current required to maintain a low impedance state between the anode and the cathode due to the small photoelectromotive force based on the light receiving element B, the output Before the charge charged between the gate and the source of MOSFETC is sufficiently discharged, the state between the anode and the cathode of thyristor F returns from the low impedance state to the high impedance state, and the output MOSFETC having a predetermined current or more.
There is a possibility that the output MOSFET C may be damaged due to the continuous flow of the drain current.

The present invention has been made in view of the above points, and an object of the present invention is to provide a semiconductor relay capable of reliably preventing an output MOSFET from being damaged.

[0010]

In order to solve the above-mentioned problems, the present invention is directed to a light-emitting element which emits light in response to an input signal, and a photo-electromotive force generated by receiving light from the light-emitting element. And the drain-source changes from a high-impedance state to a low-impedance state when the voltage between the gate and the source exceeds a threshold value by applying a photoelectromotive force generated by the light-receiving element and charging the electric charge. The output MOSFET includes an output MOSFET and a control transistor connected between the gate and the source of the output MOSFET, which is in a high impedance state when photovoltaic power is generated by the light receiving element and is in a low impedance state when the photovoltaic power is lost. Control means for controlling charge and discharge of electric charge, an overcurrent detection resistor connected to the source of the output MOSFET, A thyristor that changes from a high-impedance state to a low-impedance state between an anode and a cathode so as to discharge a charge charged between a gate and a source of an output MOSFET when a voltage between both ends of a current detection resistor becomes a predetermined voltage or more. In the relay, a switching element that changes from a high impedance state to a low impedance state based on light from the light emitting element is provided so that a current can flow between the anode and the cathode of the thyristor.

According to a second aspect of the present invention, in the first aspect of the invention, the switching element is configured to directly receive light from the light emitting element and change the impedance state.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIG. This semiconductor relay includes a light emitting diode (light emitting element) 1, a photodiode array (light receiving element) 2, an output MOSFET 3, a resistor 4 for overcurrent detection,
A first resistor 5, a second resistor 6, a thyristor 7, a first diode 8, a second diode 9, a phototransistor (switching element) 10, a control MOSFET (control transistor) 11, and a control resistor 12 It is provided with.

The light emitting diode (light emitting element) 1 emits an optical signal according to an input signal input between the input terminals 20a and 20b. The photodiode array (light receiving element) 2
A plurality of photodiodes 2a are connected in series, and receive an optical signal from the light emitting diode 1 to generate a photoelectromotive force. More specifically, the number of photodiodes 2a forming the photodiode array 2 is in series with the output MOSFET.
It is set to generate a voltage higher than the threshold voltage of T3.

The output MOSFET 3 is in an N-channel enhancement mode. The gate is connected to the anode of the photodiode array 2, the drain is connected to the output terminal 20c, and the source is the overcurrent detecting resistor 4.
Is connected to the output terminal 20d via the. Note that a load 30 and a power supply 40 are connected in series between the output terminals 20c and 20.

The anode of the thyristor 7 is connected to the anode of the photodiode array 2 and the gate of the output MOSFET 3 via the first diode 8, and the second diode 9, the phototransistor 10 and the first resistor 5 are connected. Is connected to the output terminal 20c via the. More specifically, the anode of the thyristor 7 is connected to the cathodes of the first and second diodes 8, 9, respectively, and the anode of the second diode 9 is connected to the emitter of the phototransistor 10, respectively.

The cathode of the thyristor 7 is an output terminal
20d, is connected to the source of the output MOSFET 3 via the overcurrent detection resistor 4, and is further connected to the cathode of the photodiode array 2 via the control resistor 12. The gate of this thyristor is
Is connected to the source of the output MOSFET 3 via the resistor 6 of FIG.

The control MOSFET (control transistor) 11 is an N-channel type depletion mode, and together with the control resistor 12, constitutes control means 50 for controlling charging and discharging of the output MOSFET 3. In the control MOSFET 11, the gate is connected to the source via the control resistor 12, the cathode is connected to the photodiode array 2, and the source is connected to the source of the output MOSFET 3 via the overcurrent detection resistor 4. In addition, it is connected to the cathode of the photodiode array 2 via the control resistor 12, and the drain is connected to the gate of the output MOSFET 3.

Next, the operation of this device will be described. When the light emitting diode 1 emits an optical signal according to the input signal, the photodiode array 2 receives the optical signal of the light emitting diode 1 and generates a photovoltaic voltage. With this photovoltaic,
The electric charge is charged between the gate and the source of the output MOSFET 3, and the current flows between the drain and the source of the control MOSFET 11, and the current flows through the control resistor 12. Thus, when the current flows through the control resistor 12, the control resistor
A potential difference is generated at both ends of the MOSFET 12, and the potential difference causes a high impedance state between the drain and the source of the control MOSFET 11, so that electric charge is efficiently charged between the gate and the source of the output MOSFET 3, and When the source-to-source voltage exceeds the threshold voltage, the output MOS
The state between the drain and source of FET3 changes from the high impedance state to the low impedance state.

The electric charge charged between the gate and the source of the output MOSFET 3 is output even if the drain voltage of the output MOSFET 3 is reduced because the second diode 9 is in the opposite direction. It is not released to the drain side of MOSFET3.

On the other hand, the phototransistor 10 receives an optical signal based on the optical signal from the light emitting diode 1 and, more specifically, changes from a high impedance state to a low impedance state. Although current can flow, the current supplied from the power supply 40 must flow because the anode and cathode of the thyristor are in a high impedance state and the first diode 8 is in the opposite direction. There is no.

When an input signal is not input to the light emitting diode 1 and light emission stops, the photodiode array 2 stops generating photovoltaic power. Then, no potential difference occurs between both ends of the control resistor 12, and the drain-source state of the control MOSFET 11 changes from the high impedance state to the low impedance state, and the output MOSFET
The electric charge charged between the gate and the source of the FET3 is quickly discharged through the drain and the source of the control MOSFET 11, and the state between the drain and the source of the output MOSFET3 changes to a high impedance state.

On the other hand, the phototransistor 10 changes from a low impedance state to a high impedance state when the input signal is not input to the light emitting diode 1 and the light emission stops.

Further, when the state between the drain and the source of the output MOSFET 3 changes from the high impedance state to the low impedance state and a current flows between the drain and the source, an overcurrent flows. Of the thyristor changes from a high impedance state to a low impedance state, and the charge charged between the gate and source of the output MOSFET is discharged.

As described above, when the state between the anode and the cathode of the thyristor 7 changes from the high impedance state to the low impedance state, the phototransistor 10 which has already changed from the high impedance state to the low impedance state causes the power supply 40 to switch from the high impedance state to the low impedance state. After being supplied, a current set to an appropriate value by the first resistor 5 is applied to the thyristor 7.
Current flows between the anode and the cathode of the thyristor 7, and the current continues to flow between the anode and the cathode of the thyristor 7.
The charge charged between the gate and source of T3 is discharged through the thyristor. Then, the potential between the gate and the source of the output MOSFET 3 becomes lower than the threshold value, the state between the drain and the source of the output MOSFET 3 changes from the low impedance state to the high impedance state,
No current flows between the drain and source of FET3.

As described above, the light emitting diode
When the input signal is not input to 1 and no light is emitted,
The photodiode array 2 stops generating photovoltaic power, and the state between the drain and source of the output MOSFET 3 changes to a high impedance state.

On the other hand, when no input signal is input to the light emitting diode 1 and light emission stops, the photodiode array 2 stops generating photovoltaic power and the phototransistor 10 changes from a low impedance state to a high impedance state. The thyristor 7 returns from the high impedance state to the low impedance state between the anode and the cathode.

In such a semiconductor relay, as described above, the voltage across the overcurrent detecting resistor 4 connected to the source of the output MOSFET 3 becomes higher than a predetermined voltage.
Once the state between the anode and the cathode changes from the high impedance state to the low impedance state, the current between the anode and the cathode of the thyristor 7 is changed by the phototransistor 10 which changes from the high impedance state to the low impedance state based on the light from the light emitting diode 1. Swept away. Therefore, for example, even if the photovoltaic power based on the photodiode array 2 alone does not reach the so-called holding current necessary to maintain the low impedance state between the anode and the cathode, the current passed by the phototransistor 10 is added. As a result, the current between the anode and the cathode becomes higher than the holding current, the low impedance state between the anode and the cathode is maintained, and the charge charged between the gate and the source of the output MOSFET 3 is sufficiently discharged. Will not be damaged.

The phototransistor 10 directly receives light from the light emitting diode 1 and changes its impedance state. The phototransistor 10 directly receives light from the light emitting diode 1 and changes its state from a high impedance state to a low impedance state. Since the impedance state is not changed by being turned on by the current passing through the other optically coupled element, it is not necessary to separately provide another optically coupled element that directly receives the light from the light emitting diode 1.

In this embodiment, the switching element is the phototransistor 10 which directly receives light from the light emitting diode 1 and changes from a high impedance state to a low impedance state. Without receiving light, for example, the light from the light-emitting diode 1 is directly received and turned on by a current passing through another optical coupling type element that has changed from a high impedance state to a low impedance state, and changes from a high impedance state to a low impedance state Element may be used.

[0030]

According to the first aspect of the present invention, there is provided an output MOSF.
When the voltage across the overcurrent detection resistor connected to the source of the ET becomes equal to or higher than a predetermined voltage, and once the anode-cathode state changes from the high impedance state to the low impedance state, the light-emitting element A current is caused to flow by the switching element that has changed from the high impedance state to the low impedance state based on the light. Therefore, for example, only by the photovoltaic power based on the light receiving element, even when the so-called holding current necessary to maintain the low impedance state between the anode and the cathode does not reach, the current passed by the switching element is added, Since the current between the anode and the cathode becomes equal to or greater than the holding current, the low impedance state between the anode and the cathode is maintained, and the charge charged between the gate and the source of the output MOSFET is sufficiently discharged.
T is not damaged.

According to a second aspect of the present invention, in addition to the effect of the first aspect, the switching element directly receives light from the light emitting element and changes the impedance state. Since the light is not directly turned on and turned on by the current passing through another optical coupling element that has changed from the high impedance state to the low impedance state and the impedance state does not change, the light from the light emitting element is directly received. It is not necessary to provide another optical coupling element.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional example.

[Explanation of symbols]

 1 Light-emitting diode (light-emitting element) 2 Photodiode array (light-receiving element) 3 Output MOSFET 4 Overcurrent detection resistor 7 Thyristor 10 Phototransistor (switching element) 11 Control MOSFET (control transistor) 50 Control means

Claims (2)

[Claims] A light emitting element that emits light in response to an input signal;
A light-receiving element that receives light from the light-emitting element to generate photovoltaic power; and a photo-electromotive force generated by the light-receiving element is applied to charge the electric charge. Output MOSFET that changes from high impedance state to low impedance state between sources
And a control transistor connected between the gate and source of the output MOSFET, which is in a high impedance state when photovoltaic power is generated by the light receiving element and is in a low impedance state when the photovoltaic power is lost.
Control means for controlling the charge and discharge of the electric charge in
An overcurrent detection resistor connected to the source of the OSFET,
A thyristor that changes from a high-impedance state to a low-impedance state between the anode and the cathode so as to discharge the charge charged between the gate and the source of the output MOSFET when the voltage across the overcurrent detection resistor becomes equal to or higher than a predetermined voltage;
A switching element that changes from a high impedance state to a low impedance state based on light from the light emitting element so that a current can flow between the anode and the cathode of the thyristor. Semiconductor relay. 2. The semiconductor relay according to claim 1, wherein said switching element directly receives light from said light emitting element and changes its impedance state.