A photpcoupler includes: a light emitting element; a first photodiode array; a second photodiode array; a third photo diode array; an enhancement-mode MOSFET; a first depletion-mode MOSFET; and a second depletion mode MOSFET. The light emitting element converts the input electrical signal into the optical signal. A drain current of the enhancement-mode MOSFET is supplied to the external load when the optical signal is ON. A drain current of the first depletion-mode MOSFET is supplied to the external load when the optical signal is OFF and a voltage passing through the second depletion-mode MOSFET switched to the ON state is supplied to the gate of the first depletion-mode MOSFET. And the drain current of the first depletion-mode MOSFET is larger than a drain current of the first depletion-mode MOSFET when a gate voltage of the first depletion-mode MOSFET is zero.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-038912, filed on Feb. 28, 2013; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally a photocoupler.

BACKGROUND

By using a photocoupler, signal transmission is possible in a state in which the input terminals are electrically insulated from the output terminals.

Therefore, the photocoupler can be used to transmit signals in an electronic device that includes different power supply systems such as a DC voltage system, an AC power supply system, a telephone line system, etc.

In the case where the signal to be controlled on the secondary side is a direct-current voltage, the photocoupler may have a Form C contact operation such as that of a mechanical relay by combining a normally-off transistor and a normally-on transistor.

In such a case, it is necessary to maintain good balance of the operating currents of the two signal paths while maintaining a small chip size.

DETAILED DESCRIPTION

In general, according to one embodiment, a photpcoupler includes: a light emitting element; a first photodiode array; a second photodiode array; a third photo diode array; an enhancement-mode MOSFET; a first depletion-mode MOSFET; and a second depletion mode MOSFET. The light emitting element is configured to convert the input electrical signal into the optical signal. The first photodiode array is configured to generate a first photovoltaic power by the optical signal being incident on the first photodiode array. The second photodiode array is configured to generate a second photovoltaic power by the optical signal being incident on the second photodiode array. The third photodiode array is configured to generate a third photovoltaic power by the optical signal being incident on the third photodiode array. The enhancement-mode MOSFET is configured such that the first photovoltaic power is supplied to a gate of the enhancement-mode MOSFET. The first depletion-mode MOSFET is configured such that the second photovoltaic power is supplied to a gate of the first depletion-mode MOSFET. The second depletion-mode MOSFET is configured such that the third photovoltaic power is supplied to a gate of the second depletion-mode MOSFET. A drain current of the enhancement-mode MOSFET is supplied to the external load when the optical signal is ON. A drain current of the first depletion-mode MOSFET is supplied to the external load when the optical signal is OFF and a voltage passing through the second depletion-mode MOSFET switched to the ON state is supplied to the gate of the first depletion-mode MOSFET. And the drain current of the first depletion-mode MOSFET is larger than a drain current of the first depletion-mode MOSFET when a gate voltage of the first depletion-mode MOSFET is zero.

FIG. 1is a configuration diagram of a photocoupler according to a first embodiment.

The photocoupler includes at least a light emitting element (a Light Emitting Diode)20and a light receiving circuit10. The light receiving circuit10includes a first photodiode array30, a second photodiode array32, a third photodiode array34, an enhancement-mode MOSFET (M1), a first depletion-mode MOSFET (M2), and a second depletion-mode MOSFET (M3).

The light emitting element20converts an input electrical signal into an optical signal. The light emitting element20emits, for example, light of a wavelength of red light (650 nm) to infrared light (1000 nm).

The first photodiode array30, the second photodiode array32, and the third photodiode array34receive the light emitted from the light emitting element20and produce first photovoltaic power E1, second photovoltaic power E2, and third photovoltaic power E3. The first to third photodiode arrays30,32, and34respectively include multiple photodiodes30a,32a, and34athat are connected in series and control circuits30b,32b, and34b.

The first photovoltaic power E1 can be supplied to the gate of the enhancement-mode MOSFET (M1). The second photovoltaic power E2 can be supplied to the gate of the first depletion-mode MOSFET (M2). The third photovoltaic power E3 can be supplied to the second depletion-mode MOSFET (M3).

When the optical signal from the light emitting element20is ON, a drain current IDE of the enhancement-mode MOSFET (M1) is supplied to an external load46b. On the other hand, when the optical signal is OFF, the second depletion-mode zMOSFET (M3) is switched ON and changes the gate of the first depletion-mode MOSFET (M2); and a drain current IDD (main) of the first depletion-mode MOSFET (M2) is supplied to an external load46a.

In other words, the photocoupler according to the first embodiment converts an input electrical signal into an optical signal, converts the optical signal into an output electrical signal, and supplies the output electrical signal to the external load46. Because the electrical signal is transmitted via the optical signal, the electrical signal can be transmitted in the state in which the input terminals of the photocoupler are electrically insulated from the output terminals of the photocoupler. Therefore, the photocoupler according to the first embodiment can be used to transmit signals in an electronic device including different power supply systems such as a DC voltage system, an AC power supply system, a telephone line system, etc.

FIG. 2is an operation waveform diagram of the first embodiment.

The light receiving circuit10of the photocoupler further includes a first terminal40that is connected to a first electrode30cof the first photodiode array30and the source of the enhancement-mode MOSFET (M1), a second terminal42that is connected to the drain of the enhancement-mode MOSFET (M1), the drain of the first depletion-mode MOSFET (M2), and the drain of the second depletion-mode MOSFET (M3) to supply a direct current (DC) voltage or a pulse voltage, and a third terminal44that is connected to a first electrode32cof the second photodiode array32and the source of the first depletion-mode MOSFET (M2).

A power supply48is connected to the second terminal42; and a direct-current voltage or a pulse voltage is supplied to the second terminal42. The load46ais connected between the second terminal42and the third terminal44; and the load46bis connected between the first terminal40and the second terminal42. The connection between the load46and the power supply48is not limited to that ofFIG. 1.

InFIG. 2, the photovoltaic power is not generated in the third photodiode array34when the optical signal from the light emitting element20is OFF. Therefore, a gate voltage VG of the second depletion-mode MOSFET (M3) becomes substantially zero; and the second depletion-mode MOSFET (M3) is switched ON. In such a case, the voltage of the second terminal42that passes through the second depletion-mode MOSFET (M3) that is switched ON is supplied to the gate of the first depletion-mode MOSFET (M2) during the interval in which the direct-current voltage or the pulse voltage is applied to the second terminal42. The first depletion-mode MOSFET (M2) can supply, to the external load46, a drain current (the solid line) that is larger than the drain current (the broken line) of the first depletion-mode MOSFET (M2) when a gate voltage VG of the first depletion-mode MOSFET (M2) is zero.

The gate voltage VG of the first depletion-mode MOSFET (M2) shifts to the positive side in the case where both the first depletion-mode MOSFET (M2) and the second depletion-mode MOSFET (M3) are n-channel MOSFETs.

FIG. 3is a graph showing the dependence of the drain current (IDD) on the drain voltage (VD) of the n-channel depletion-mode MOSFET.

In the depletion-mode MOSFET, the drain current IDD flows even when the gate voltage VG is zero because the channel region is already formed in the manufacturing process. The drain current decreases when the gate voltage VG is caused to be negative; and the drain current approaches zero as the absolute value is increased. On the other hand, the drain current IDD increases as the gate voltage VG is increased on the positive side.

Thus, the second depletion-mode MOSFET (M3) can be called the sub DMOSFET because the second depletion-mode MOSFET (M3) is provided to increase the drain current IDD of the first depletion-mode MOSFET (M2). Conversely, the first depletion-mode MOSFET (M2) can be called the main DMOSFET because the first depletion-mode MOSFET (M2) supplies the current to the external load46a. The gate width of the sub MOSFET (M3) may be narrower than the gate width of the main DMOSFET.

During the interval in which the optical signal is OFF as shown inFIG. 2, the drain current IDD (VG>0) of the main DMOSFET (M2) can be supplied to the external load46aaccording to the direct-current voltage or the pulse voltage applied to the second terminal42.

When the optical signal is switched ON as shown inFIG. 2, the gate voltage VG becomes negative and the sub DMOSFET (M3) is switched OFF due to the third photovoltaic power E3 generated by the third photodiode array34. Therefore, the gate voltage VG of the main DMOSFET (M2) cannot be increased even when the voltage is applied to the second terminal42. The gate voltage VG of the main DMOSFET (M2) becomes negative due to the second photovoltaic power E2 of the second photodiode array32. Therefore, the main DMOSFET (M2) is switched OFF.

On the other hand, a gate voltage VG1 of the enhancement-mode MOSFET (M1) becomes positive and the enhancement-mode MOSFET (M1) is switched ON by the first photovoltaic power E1. During the interval in which the enhancement-mode MOSFET (M1) is switched ON, the drain current IDE is supplied to the external load46bvia the first terminal40and the second terminal42when the direct-current voltage or the pulse voltage is applied to the second terminal42. Although the drain currents IDD and IDE are shown as pulse currents inFIG. 2, these currents of course may be direct currents.

In the photocoupler according to the first embodiment, the enhancement-mode MOSFET (M1) that is normally-off operates as a Form A contact; and the depletion-mode MOSFET (M2) that is normally-on operates as a Form B contact. Therefore, the photocoupler has a 1A1B combination and operates as a Form C contact that can switch the signal path.

The photocoupler of the first embodiment can be called a photorelay.

Although the enhancement-mode MOSFET (M1), the first depletion-mode MOSFET (M2), and the second depletion-mode MOSFET (M3) are n-channel MOSFETs inFIG. 1, each of these may be a p-channel MOSFET. In the case of the p-channel MOSFET, the polarity of the power supply48and the polarities of the first, second, and third photodiode arrays30,32, and34may be reversed.

FIG. 4is a configuration diagram of a photocoupler according to a comparative example.

The photocoupler includes a light emitting element120, a first photodiode array130, a second photodiode array132, an enhancement-mode MOSFET (M11), and a depletion-mode MOSFET (M12).

The first photodiode array130and the second photodiode array132respectively generate the first and second photovoltaic powers by receiving the light emitted from the light emitting element120. The first and second photodiode arrays130and132respectively include multiple photodiodes130aand132athat are connected in series and control circuits130band132b.

The first photovoltaic power can be supplied to the gate of the enhancement-mode MOSFET (M11). The second photovoltaic power can be supplied to the gate of the depletion-mode MOSFET (M12).

It is difficult to increase the drain current in the case where the n-channel depletion-mode MOSFET having the lateral structure is used in a range where VG≦0. On the other hand, in the enhancement-mode MOSFET, it is easy to increase the drain current IDE by increasing the gate voltage VG by the photovoltaic power of the first photodiode array. Therefore, in the comparative example, it is difficult to maintain good balance between the drain current IDD of the depletion-mode MOSFET and the drain current IDE of the enhancement-mode MOSFET.

Conversely, in the first embodiment, the gate voltage VG of the main DMOSFET (M2) is shifted to the positive side by the sub DMOSFET (M3); and the drain current IDD can be increased with good space efficiency of the light receiving circuit10. As a result, the drain current IDD (VG>0) of the main DMOSFET (M2) can approach the drain current IDE of the enhancement-mode MOSFET (M1); and the balance of the operating currents can be improved.

FIG. 5Ais a schematic plan view of the light receiving circuit side of the photocoupler of the first embodiment, andFIG. 5Bis a schematic cross-sectional view of the photocoupler along line A-A.

The photocoupler of the drawing has a structure in which a light receiving circuit11made of one chip such as Si, etc., is mounted on a first leadframe, the light emitting element20is mounted on a second leadframe, the light emitting element20and the light receiving circuit11subsequently are caused to oppose each other, and the light receiving circuit11and the light emitting element20are integrally formed with a molded resin body60. The molded resin body60covers one end portion of the first terminal40, one end portion of the second terminal42, and one end portion of the third terminal44. One other end portion of the first terminal40, one other end portion of the second terminal42, and one other end portion of the third terminal44protrude toward the outside from the molded resin body60and are connected to interconnect units of a mounting substrate.

The light emitting element20is connected to terminals52and54on the input side.

The light receiving circuit11has a lateral structure in which the first to third photodiode arrays30,32, and34, the enhancement-mode MOSFET (M1), the first depletion-mode MOSFET (M2), and the second depletion-mode MOSFET (M3) are disposed in the lateral direction. The layout, the disposition of the terminals, etc., of the light receiving circuit11may be different from those ofFIGS. 5A and 5B.

The light receiving circuit11is bonded to a light receiving circuit die pad unit50that is linked to the first leadframe. The light receiving circuit11and the first to third terminals40,42, and44are connected as inFIG. 1by bonding wires.

After the molded resin body60is formed, the die pad unit50is cut to be separated from the leadframe in the state in which suspension pins50aremain. A transparent resin layer is provided in the optical path between the light emitting element20and the light receiving circuit11; and misoperations of the photocoupler due to ambient light can be suppressed by providing a light-shielding resin layer on the outer side of the transparent resin layer.

In the photocoupler, for example, an insulating substrate such as a ceramic, etc., may be used instead of the leadframe. In such a case, the light emitting element20and/or the light receiving circuit11is disposed on the insulating substrate. Also, the first to third terminals40,42, and44may be, for example, conductive units, etc., provided in an insulating substrate.

According to the embodiment, a photocoupler is provided in which it is easy to maintain good balance of the operating currents even when the signal path is switched. Such a photocoupler can be used to transmit signals in an electronic device that includes different power supply systems such as a DC voltage system, an AC power supply system, a telephone line system, etc.