Light responsive semiconductor switch with shorted load protection

• US 6,339,236 B1
• Filed: 09/27/2000
• Issued: 01/15/2002
• Est. Priority Date: 09/27/1999
• Status: Expired due to Fees

• Current Assignee: Panasonic Electric Works Company Limited (Panasonic Holdings Corporation)
• Original Assignee: Matsushita Electric Works Ltd (Panasonic Holdings Corporation)
• Inventors: Hagihara, Yosuke, Nagahama, Hideo, Tomii, Kazushi
• Primary Examiner(s): Abraham, Fetsum
• Application Number:
  US09/671,434
• Time in Patent Office: 475 Days
• Field of Search: 257/228, 257/227, 257/223, 257/290, 257/363, 257/355, 257/360
• US Class Current: 257/228
• CPC Class Codes: H03K 17/0822 in field-effect transistor switches
• CPC Class Codes: H03K 17/785 controlling field-effect transistor switches
• CPC Class Codes: H03K 3/356 Bistable circuits

Abstract

An improved light responsive semiconductor switch with shorted load protection capable of successfully interrupting a load overcurrent. The switch is includes an output transistor which is triggered by a photovoltaic element to connect a load to a power source thereof, and an overcurrent sensor which provides an overcurrent signal upon seeing an overcurrent condition in the load. 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 transistor. A latch circuit is included to be energized by the photovoltaic element and to provide an interruption signal once the overcurrent signal is received and hold the interruption signal. The interruption signal turns on the shunt transistor so as to flow the current from the photovoltaic element through the shunt path, thereby turning off the output transistor for interruption of the overcurrent. The current limiting resistive element is connected in series with the shunt transistor to limit the current from the photovoltaic element when the shunt transistor is turned on, thereby providing a supply voltage from the photovoltaic element to the latch circuit. Thus, the latch circuit is enabled to keep providing the interruption signal for reliable interruption of the overcurrent.

Claims

1. A light responsive semiconductor switch with shorted load protection, said switch comprising:

an output switching transistor connected between a pair of output terminals which are adapted to be connected to a load circuit composed of a load and a power source energizing said load, said output switching transistor having a control electrode with a threshold voltage at which said output switching transistor conducts to connect said load to said power source;

a photovoltaic element generating an electric power upon absorption of light from a light source across positive and negative electrodes of said photovoltaic element, said electric power providing an operating voltage decreasing with an increasing current flowing from said photovoltaic element;

an overcurrent sensor coupled to said load circuit to provide an overcurrent signal when said load circuit sees an overcurrent flowing through said load from said power source;

a shunt transistor connected in series with a current limiting resistive element across said photovoltaic element to define a shunt path of flowing the current from said photovoltaic element through said current limiting resistive element away from said output switching transistor; and

a latch circuit connected to said overcurrent sensor and said shunt transistor, said latch circuit being energized by said photovoltaic element and providing an interruption signal once said overcurrent signal is received and hold said interruption signal after the removal of said overcurrent signal, said interruption signal causing said shunt transistor to become conductive to flow the current from said photovoltaic element through said shunt path, lowering said operating voltage being applied to said control electrode of said output switching transistor below said threshold voltage so as to turn off said output switching transistor for disconnection of said load from said power source;

wherein said shunt transistor and said current limiting resistive element are formed separately from said latch circuit; and

that said current limiting resistive element is connected between said control electrode and the positive electrode of said photovoltaic element to limit the current from said photovoltaic element, when said shunt transistor is conductive, to such an extent as to lower the operating voltage being applied to said control electrode of said output switching transistor below said threshold voltage, while allowing the photovoltaic element to give a supply voltage to the latch circuit for holding said interruption signal.

2. The semiconductor switch as set forth in claim 1, wherein

said overcurrent sensor comprises a current sensing resistor inserted in said load circuit, and a transistor switch which is connected to receive a voltage developed across said current sensing resistor to provide said overcurrent signal to said latch circuit when said voltage exceeds a predetermined level.

3. The semiconductor switch as set forth in claim 2, further including:

a second shunt transistor in the form of a metal oxide semiconductor field-effect transistor (MOSFET) connected across said shunt transistor with a drain of said second shunt transistor being connected to a point between the control electrode of said output switching transistor and said current limiting resistive element and with a source of said second shunt transistor being connected to the source of said shunt transistor, said second shunt transistor having a gate which is connected to receive the voltage developed across said current sensing resistor such that, in response to the voltage of said current sensing resistor exceeding the predetermined level, said second shunt transistor becomes conductive to flow the current from said photovoltaic element through said current limiting resistive element and through said second shunt transistor away from said output switching transistor prior to said latch circuit responding to provide said interruption signal of turning on said shunt transistor.

4. The semiconductor switch as set forth in claim 2, wherein

said overcurrent sensor further include a low-pass filter which negates a high frequency voltage appearing across said current sensing resistor so that said overcurrent sensor provides said overcurrent signal to said latch circuit only when the voltage across the current sensing resistor exceeds said predetermined level and lasts over a certain time period.

5. The semiconductor switch as set forth in claim 2, further including

a delay timer which delays providing said overcurrent signal from said overcurrent sensor to said latch circuit for a short time period immediately upon said photovoltaic element generating the electric power, thereby negating a transient voltage appearing across said current sensing resistor immediately after the activation of said photovoltaic element.

6. The semiconductor switch as set forth in claim 1, wherein

said overcurrent sensor comprises a current sensing resistor connected in series with a bypass switching transistor across said output terminals in parallel with said output switching transistor, and a transistor switch which is connected to receive a voltage developed across said current sensing resistor to provide said overcurrent signal to said latch circuit when said voltage exceeds a predetermined level.

7. The semiconductor switch as set forth in claim 1, wherein

said output switching transistor comprises a single metal oxide semiconductor field-effect transistor (MOSFET) whose gate-source is connected across said photovoltaic element, and whose drain-source is connected between said output terminals.

8. The semiconductor switch as set forth in claim 1, wherein

a pair of output switching transistors each in the form of a metal oxide semiconductor field-effect transistor (MOSFET) are connected in series between said output terminals with sources of the individual MOSFETs being connected to each other and with gates of the individual MOSFETs being commonly connected to receive said operating voltage from said photovoltaic element.

9. The semiconductor switch as set forth in claim 1, wherein

said latch circuit is realized by a flip flop having a set input, a reset input, and an output, said set input being connected to receive said overcurrent signal, said reset input being connected to receive said operating voltage from said photovoltaic element, and said output being connected to turn on and off said shunt transistor.

10. The semiconductor switch as set forth in claim 9, wherein

said shunt transistor is realized by a metal oxide semiconductor field-effect transistor (MOSFET) whose drain-source is connected in series with said current limiting resistive element across said photovoltaic element, and wherein said flip-flop comprises:

a first resistive element and a first metal oxide semiconductor field-effect transistor (MOSFET) whose drain-source is connected in series with said first resistive element across said photovoltaic element, and

a second resistive element and a second metal oxide semiconductor field-effect transistor (MOSFET) whose drain-source is connected in series with said second resistive element across said photovoltaic element,

said first MOSFET having a gate connected to a point between said second resistive element and a drain of said second MOSFET,

said second MOSFET having a gate connected to a point between said first resistive element and a drain of said first MOSFET,

said point between the second resistive element and the drain of said second MOSFET being connected to the gate of said shunt transistor (MOSFET),

said second MOSFET receiving at its gate the operating voltage from said photovoltaic element through said first resistive element so as to become conductive upon receiving said operating voltage, thereby lowering said operating voltage applied through said second resistive element to the gate of said first MOSFET and to the gate of said shunt transistor (MOSFET) to make said first MOSFET and said shunt transistor nonconductive,

said second MOSFET also receiving at its gate said overcurrent signal which makes said second MOSFET nonconductive, thereby raising the voltage applied to the gates of said first MOSFET and said shunt transistor (MOSFET) so as to make said first MOSFET and said shunt transistor (MOSFET) conductive, which keeps said second MOSFET non-conductive for continued conduction of said shunt transistor (MOSFET) for keeping the interruption of said output switching transistor until removal of the operating voltage from said photovoltaic element.

11. The semiconductor switch as set forth in claim 10, wherein

each of said current limiting resistive element, said first resistive element and said second resistive element is in the form of a punch-through space charge resistor, said punch-through space charge resistor comprising:

a semiconductor substrate having a conductive type which is one of n-type and p-type;

a well diffused in the surface of said substrate, said well being of a conductive type opposite of said substrate;

a pair of regions diffused in the surface of said well in a spaced relation with each other, said regions being of the same conductive type as said substrate; and

a pair of electrodes respectively formed on said regions to apply said operating voltage between said regions partly through said well, said regions being cooperative to form therebetween a depletion layer responsible for carrying a minute current and therefore defining resistance for each of said current limiting resistive element, said first resistive element and said second resistive element.

12. The semiconductor switch as set forth in claim 10, wherein

said current limiting resistive element, said first resistive element and said second resistive element are realized by diodes, respectively.

13. The semiconductor switch as set forth in claim 10, further including

an additional photovoltaic element which provides an offset voltage upon absorption of the light,

said overcurrent sensor comprising a current sensing resistor inserted in said load circuit to develop a detection voltage thereacross, and a third metal oxide semiconductor field-effect transistor (MOSFET) which provides said overcurrent signal to said latch circuit upon being turned on,

said additional photovoltaic element being connected to said third MOSFET such that said offset voltage is added to said detection voltage and is applied to a gate of said third MOSFET for turning on said third MOSFET when said detected voltage plus said offset voltage exceed a predetermined level.

14. The semiconductor switch as set forth in claim 10, further including

a biasing means for supplying a bias current from said photovoltaic element to the gate of said shunt transistor when said latch circuit provides the interruption signal in response to said overcurrent signal.

15. The semiconductor switch as set forth in claim 10, further including

a block means for blocking the current of said photovoltaic element from flowing to the gate of the output switching transistor when said latch circuit provides the interruption signal in response to said overcurrent signal.

16. The semiconductor switch as set forth in claim 1, wherein

said output switching transistor is in the form of a metal oxide semiconductor field-effect transistor (MOSFET) having a gate defining said control electrode, a zener diode being connected across gate-source of said output switching transistor in parallel with said photovoltaic element, a cathode of said zener diode being connected to the gate of said output switching transistor, said zener diode having a breakdown voltage higher than an open-circuit voltage of said photovoltaic element.

17. The semiconductor switch as set forth in claim 1, wherein

said output switching transistor is in the form of a metal oxide semiconductor field-effect transistor (MOSFET) having a gate defining said control electrode, a diode being connected across said current limiting resistive element with an anode of said diode connected to the gate of said output switching transistor.

18. The semiconductor switch as set forth in claim 1, wherein

said output switching transistor is in the form of a metal oxide semiconductor field-effect transistor (MOSFET) having a gate defining said control electrode, a diode being connected in series with a resistor across said current limiting resistive element with an anode of said diode connected to the gate of said output switching transistor.

19. The semiconductor switch as set forth in claim 1, wherein

said output switching transistor is in the form of a metal oxide semiconductor field-effect transistor (MOSFET) having a gate defining said control electrode, a discharging metal oxide semiconductor field-effect transistor (MOSFET) being connected across said current limiting resistive element with a source of said discharging MOSFET being coupled to a connection between said current limiting resistive element and the positive electrode of said photovoltaic element and with drain and gate of said discharging MOSFET commonly connected to the gate of said output switching transistor.