Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Charge transfer device
Reexamination Certificate
2000-09-27
2002-01-15
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Charge transfer device
C257S227000, C257S363000, C257S223000, C257S290000, C257S355000, C257S360000
Reexamination Certificate
active
06339236
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light responsive semiconductor switch with shorted load protection for use in an optical relay.
2. Description of the Prior Art
Japanese Patent Publication No. 11-163706 discloses a light responsive semiconductor switch for use in an optical relay. The switch includes a photovoltaic element which provides an operating voltage upon absorption of light from a light source, and an output transistor which is triggered by the operating voltage to become conductive for connecting a load to a power source. In order to protect the output transistor from an overcurrent due to an accidental short-circuiting of the load, the switch includes an overcurrent sensor for detection of the overcurrent condition, and a shunt transistor which, in response to the overcurrent condition, becomes conductive to flow the current from the photovoltaic element away from the output transistor to turn if off for interruption of the overcurrent. Further, the switch includes a latch circuit which, in response to the overcurrent condition, provides and holds an interruption signal fed to a control electrode of the output transistor to keep turning off the output transistor for continued interruption of the overcurrent. In this prior art, the shunt transistor is included in the latch circuit to be responsible also for the latching operation. Therefore, the shunt transistor has to satisfy two different requirements, one for the turning off of the output transistor, and the other for holding the interruption signal applied to the control electrode of the output transistor in association with a resistor in the latch circuit. With this restriction to the shunt transistor common to the latch circuit, it is rather difficult to combine the two requirements against the use of the photovoltaic element of varying current generating capacity. For example, when the photovoltaic element having a large current generating capacity is used to apply a correspondingly high voltage to the control electrode of the output transistor for rapidly turning it on, the conduction of the shunt transistor made for the latching operation may not be enough to lower the voltage applied to the control electrode of the output transistor below a threshold voltage thereof, failing to turn off the output transistor even when the shunt transistor is made conductive to draw the current from the photovoltaic element. Accordingly, the prior switch poses limitations to a circuit design and is not satisfactory for complete interruption of the overcurrent irrespective of the current generating capacity of the photovoltaic element.
SUMMARY OF THE INVENTION
In view of the above insufficiency in the prior art, the present invention has been achieved to provide an improved light responsive semiconductor switch with shorted load protection which is capable of successfully interrupting a load overcurrent. The semiconductor switch in accordance with the present invention comprises an output switching transistor connected between a pair of output terminals which are adapted for connection to a load circuit composed of a load and a power source energizing the load. The output switching transistor has a control electrode with a threshold voltage at which the output switching transistor conducts to connect the load to the power source. A photovoltaic element is included in the switch to generate an electric power upon absorption of light from a light source. The electric power provides an operating voltage decreasing with an increasing current flowing from the photovoltaic element. An overcurrent sensor is coupled to the load circuit to provide an overcurrent signal when the load circuit sees an overcurrent flowing through the load from the power source. A shunt transistor is connected in series with a current limiting resistive element across the photovoltaic element to define a shunt path of flowing the current from the photovoltaic element through the current limiting resistive element away from the output switching transistor. Also included in the switch is a latch circuit which is connected to the overcurrent sensor and the shunt transistor. The latch circuit is energized by the photovoltaic element and provides an interruption signal once the overcurrent signal is received and holds the interruption signal after the removal of the overcurrent signal. The interruption signal causes the shunt transistor to become conductive to flow the current from the photovoltaic element through the shunt path, lowering the operating voltage being applied to the control electrode of the output switching transistor below the threshold voltage so as to turn off the output switching transistor for disconnection of the load from the power source.
The characterizing feature of the present invention resides in that the shunt transistor and the current limiting resistive element are formed separately from the latch circuit, and that the current limiting resistive element is connected between the control electrode of the output switching transistor and the positive electrode of the photovoltaic element so as to limit the current from the photovoltaic element, when said shunt transistor is conductive, to such an extent as to lower the operating voltage being applied to the control electrode of the output switching transistor below the threshold voltage, while allowing the photovoltaic element to provide a supply voltage to the latch circuit for holding the interruption signal. Thus, the series combination of the current limiting resistive element and the shunt transistor which are separately formed from the latch circuit can assure to provide the supply voltage to the latch circuit and at the same time to limit the operative voltage being applied to the control electrode of the output switching transistor, so as to keep the interruption signal from the latch circuit on one hand, and to turn off the output switching transistor without fail in response to the interruption signal on the other hand, enabling successful and reliable interruption of the overcurrent. Also, since the current limiting resistive element is separately formed from the latch circuit, it is readily possible to assure the above interruption of the overcurrent irrespective of varying current generating capacity of the photovoltaic element, simply by selecting the impedance of the current limiting resistive element. With this result, the output transistor can be protected completely from the overcurrent in the load circuit.
In one version of the present invention, the overcurrent sensor is realized by a current sensing resistor inserted in the load circuit, and a transistor switch which is disposed to receive a voltage developed across the current sensing resistor to provide the overcurrent signal to the latch circuit when the voltage exceeds a predetermined level.
In another version of the present invention, the overcurrent sensor is realized by a current sensing resistor connected in series with a bypass switching transistor between the output terminals and in parallel with the output switching transistor, and a transistor switch which is disposed to receive a voltage developed across the current sensing resistor to provide the overcurrent signal to the latch circuit when the voltage exceeds a predetermined level.
For driving the load energized the DC power supply, the output switching transistor is preferably defined by a single metal oxide semiconductor field-effect transistor (MOSFET) whose gate-source is connected across the photovoltaic element, and whose drain-source is connected between the output terminals.
For driving the load energized by the AC power supply, the switch is preferred to include a pair of output switching transistors each in the form of a metal oxide semiconductor field-effect transistor (MOSFET). The two output switching transistors are connected in series between the output terminals with sources of the individual MOSFETs being connected to each other and with gates of the individual MOSFETs being co
Hagihara Yosuke
Nagahama Hideo
Tomii Kazushi
Abraham Fetsum
Matsushita Electric & Works Ltd.
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