CA1123064A - Optically coupled field effect transistor switch - Google Patents
Optically coupled field effect transistor switchInfo
- Publication number
- CA1123064A CA1123064A CA322,413A CA322413A CA1123064A CA 1123064 A CA1123064 A CA 1123064A CA 322413 A CA322413 A CA 322413A CA 1123064 A CA1123064 A CA 1123064A
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- Prior art keywords
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- gate
- fet
- array
- optically sensitive
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Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/51—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used
- H03K17/78—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used using opto-electronic devices, i.e. light-emitting and photoelectric devices electrically- or optically-coupled
- H03K17/785—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used using opto-electronic devices, i.e. light-emitting and photoelectric devices electrically- or optically-coupled controlling field-effect transistor switches
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- Electronic Switches (AREA)
- Solid State Image Pick-Up Elements (AREA)
Abstract
Abstract of the Disclosure An optically coupled switch suited for use in opto-isolators is described. A field effect transistor (FET) that need itself not be photosensitive is used and the control voltage, i.e., the gain-source voltage, which controls the amount of current flowing through the FET, is developed by a series connected photodiode array having at least one photodiode and which is connected between the gate and source of the FET controls the current through the FET. A symmetric optically coupled bilateral switch is obtained by using two series connected photodiode arrays.
The first array is connected between the gate and source and the second is connected between the gate and drain of the FET, respectively. The operation of the bilateral switch is independent of the polarity of the voltage across the FET and does not require an external voltage source to establish a bias between gate and source or gate and drain.
The first array is connected between the gate and source and the second is connected between the gate and drain of the FET, respectively. The operation of the bilateral switch is independent of the polarity of the voltage across the FET and does not require an external voltage source to establish a bias between gate and source or gate and drain.
Description
.2~4 W. C. King 2 OPTICALLY COUPLED FIELD EFFECT
TRANSISTOR SWITC~
Background o~ the Invention -1. Field of the In~ention This invention relates generally to opto-isolators and particularly to opto-isolators using an array of one or more series-connected photodiodes to provide photovoltaic control of affield effect transistor which need not itself be photosensitlve.
TRANSISTOR SWITC~
Background o~ the Invention -1. Field of the In~ention This invention relates generally to opto-isolators and particularly to opto-isolators using an array of one or more series-connected photodiodes to provide photovoltaic control of affield effect transistor which need not itself be photosensitlve.
2. Description of the Prior Art Devices that transmit signals from an input circuit to an output circuit electrically isolated from the input circuit are of considerable commercial importance.
For many purposes, adequate electrical isolation is easily ob~ained with electromechanical relays or isolation transformers. These devices do, however, suffer the drawbacks of being large and incompatible with much solid state circuitry.
For these and other reasons, devices, commonly called opto-isolators or opto-couplers, have been developed that use optical rather than electrical coupling to link two electrical circuits. These devices use a light source, commonly a light emitting diodt, ~LED), located in the input ` circuit and a photodetector, located in the output circuit and optically coupled to the light source, to couple the two circuits. Current passing through the LED causes it to emit light of which some is transmitted to the photo-detector ~here it causes an output current to be generated.
The photodetector is typically a photodiode, a ^ 30 phototransistor or a photo SCR. Another type of photo-detector which might be used is the photo-FET.
Although it has features, such as easily adjustable optical sensitivity, a linear current-voltage characteristic passing through the origin, and thermal stability, that are 35 desirable for the photodetectors used in opto-isolators, it has not been used in opto-isolators.
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, - 2 - W C. King 2 The reasons for the lack of use of photo-FET
detectors will be better understood if the operation of optically sensitive FETs is briefly described. Optically sensitive FETs typically obtain their optical sensitivity Sby making the depletion region, established by reverse biasing the gate--source junction, optically accessible, i.e., by absorbing photons in the depletion region. The reverse bias exceeds the pinch-off voltage and current eannot flow through the ~FET. The photoinduced current 10generated by the separation of the constituents of electron-hole pairs produced in the absorption process then passes through an external resistance in the gate-source circuit and changes the gate-source bias. The resulting gate-source bias is less than the pinch-off voltage and the ' 15FET is turned on.
Several drawbacks of opto-isolators which might use optically sensitive FETs are thus illustrated~ First, commercially available photo~FETs are depletion mode FE~s and a separate voltage source is required to reverse bias ~; 20the gate-source junction and thus turn the FET off.
Second, photo-FETs do not lend themselves to the construction of normally~on opto-isolators. Third, the photo-FETs do not lend themselves ~o easy construction of bilateral opto-isolators which are desirable in many 25 applications since they ean be operated without regard to the polarity of the applied voltage~
BilateraI opto-isolators~ e.g~, photo SCR~
connected anti parallel, which do not use FETs are commercially available but have drawbacks. They have 30 nonlinear characteristics through the origin of their ~ output current-voltage curves and are thus unsuitable for `, use as low level analog switches. l~dditionally, they are latching devices and an additional voltage signal is therefore required to return the switch to its initial 35 state.
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For many purposes, adequate electrical isolation is easily ob~ained with electromechanical relays or isolation transformers. These devices do, however, suffer the drawbacks of being large and incompatible with much solid state circuitry.
For these and other reasons, devices, commonly called opto-isolators or opto-couplers, have been developed that use optical rather than electrical coupling to link two electrical circuits. These devices use a light source, commonly a light emitting diodt, ~LED), located in the input ` circuit and a photodetector, located in the output circuit and optically coupled to the light source, to couple the two circuits. Current passing through the LED causes it to emit light of which some is transmitted to the photo-detector ~here it causes an output current to be generated.
The photodetector is typically a photodiode, a ^ 30 phototransistor or a photo SCR. Another type of photo-detector which might be used is the photo-FET.
Although it has features, such as easily adjustable optical sensitivity, a linear current-voltage characteristic passing through the origin, and thermal stability, that are 35 desirable for the photodetectors used in opto-isolators, it has not been used in opto-isolators.
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, - 2 - W C. King 2 The reasons for the lack of use of photo-FET
detectors will be better understood if the operation of optically sensitive FETs is briefly described. Optically sensitive FETs typically obtain their optical sensitivity Sby making the depletion region, established by reverse biasing the gate--source junction, optically accessible, i.e., by absorbing photons in the depletion region. The reverse bias exceeds the pinch-off voltage and current eannot flow through the ~FET. The photoinduced current 10generated by the separation of the constituents of electron-hole pairs produced in the absorption process then passes through an external resistance in the gate-source circuit and changes the gate-source bias. The resulting gate-source bias is less than the pinch-off voltage and the ' 15FET is turned on.
Several drawbacks of opto-isolators which might use optically sensitive FETs are thus illustrated~ First, commercially available photo~FETs are depletion mode FE~s and a separate voltage source is required to reverse bias ~; 20the gate-source junction and thus turn the FET off.
Second, photo-FETs do not lend themselves to the construction of normally~on opto-isolators. Third, the photo-FETs do not lend themselves ~o easy construction of bilateral opto-isolators which are desirable in many 25 applications since they ean be operated without regard to the polarity of the applied voltage~
BilateraI opto-isolators~ e.g~, photo SCR~
connected anti parallel, which do not use FETs are commercially available but have drawbacks. They have 30 nonlinear characteristics through the origin of their ~ output current-voltage curves and are thus unsuitable for `, use as low level analog switches. l~dditionally, they are latching devices and an additional voltage signal is therefore required to return the switch to its initial 35 state.
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3~64 - 2a -Summary of the Invention In accordance with an aspect of the invention there is provided an optically sensitive switch comprising:
a field effect transistor, said transistor having source, gate and drain electrodes, said source and drain electrodes S being connected to an electrical output circuit; a first light source, said first light source being connected to an electrical input circuit; and means for controlling the current through said field effect transistor, said means being optically coupled to said first light source;
characterized in that said means produces a voltage when illuminated that controls said current through said field effect transistor, said means comprises a first series connected photodiode array, said array being connected between said gate and source electrodes, and a resistance connected between said gate and source electrodes, said : resistance being less than that of said photodiode array and large enough to not appreciably load said photodiode array when said photodiodes are illuminated.
An optically sensitive switch using an FET and which is useful in opto-isolators is obtained, in accord~nce with this invention, by using the ;~
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Soperation. The number of series connected photodiodes in the photodiode array is sufficient to produce a voltage, when the array is illuminated, that exceeds the pinch-off voltage and turns the FE'r off. If enhancement mode FETs are used, the gate-threshol~ voltage is exceeded. when the lOdrain and source of the FET are connected to an electrical circuit and the pho~odiode array is illuminated by a light source, such as an LED connecteâ to another electrical circuit, a normally-on opto-isolator results.
An additional embodiment is a symmetric bilateral 15switch using at least two photodiode arrays and a depletion mode FET. One photodiode array is connected between the gate and the "source" and a second photodiode array is connected between the gate and "drain" of the FET. Two reverse polarity blocking diodes prevent the positive 20 "source" or "drain" voltages from coupling to the gate through the forward biased photodiodes. The two photodiode arrays may be illuminated by the same light source. The bias established by either array will be sufficient to turn off the FET.
Another embodiment achieves two levels of optical control by using the optically sensitive F~T switch described with an optically sensitive FET. One level of ` optical control is obtained as previously described and the second level is obtained by using a second LED to 30 illuminate th~ optically sensitive portion of the FET and control the current through the FET.
Brief Desrrl ~ n of_the Drawlngs FIG. l i5 a schematic representation of a conventional photosensitive FET;
FIG. 2 i5 a schematic representation of an optically sensitive FET switch with optical sensitivity provided by a series connected photodiode array, which is illuminated by an LED light source, and ~s con~ected betwèen the ' '.
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a field effect transistor, said transistor having source, gate and drain electrodes, said source and drain electrodes S being connected to an electrical output circuit; a first light source, said first light source being connected to an electrical input circuit; and means for controlling the current through said field effect transistor, said means being optically coupled to said first light source;
characterized in that said means produces a voltage when illuminated that controls said current through said field effect transistor, said means comprises a first series connected photodiode array, said array being connected between said gate and source electrodes, and a resistance connected between said gate and source electrodes, said : resistance being less than that of said photodiode array and large enough to not appreciably load said photodiode array when said photodiodes are illuminated.
An optically sensitive switch using an FET and which is useful in opto-isolators is obtained, in accord~nce with this invention, by using the ;~
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.' , ~ , ' : , : , , - 3 - W. C. King 2 voltage developed by at least one series connected photodiode array consisting of at least one illuminated photodiode, connected between the gate and source of the normally on depletion mode FET, to control the FET
Soperation. The number of series connected photodiodes in the photodiode array is sufficient to produce a voltage, when the array is illuminated, that exceeds the pinch-off voltage and turns the FE'r off. If enhancement mode FETs are used, the gate-threshol~ voltage is exceeded. when the lOdrain and source of the FET are connected to an electrical circuit and the pho~odiode array is illuminated by a light source, such as an LED connecteâ to another electrical circuit, a normally-on opto-isolator results.
An additional embodiment is a symmetric bilateral 15switch using at least two photodiode arrays and a depletion mode FET. One photodiode array is connected between the gate and the "source" and a second photodiode array is connected between the gate and "drain" of the FET. Two reverse polarity blocking diodes prevent the positive 20 "source" or "drain" voltages from coupling to the gate through the forward biased photodiodes. The two photodiode arrays may be illuminated by the same light source. The bias established by either array will be sufficient to turn off the FET.
Another embodiment achieves two levels of optical control by using the optically sensitive F~T switch described with an optically sensitive FET. One level of ` optical control is obtained as previously described and the second level is obtained by using a second LED to 30 illuminate th~ optically sensitive portion of the FET and control the current through the FET.
Brief Desrrl ~ n of_the Drawlngs FIG. l i5 a schematic representation of a conventional photosensitive FET;
FIG. 2 i5 a schematic representation of an optically sensitive FET switch with optical sensitivity provided by a series connected photodiode array, which is illuminated by an LED light source, and ~s con~ected betwèen the ' '.
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- 4 - W C. King 2 FET gate and source;
FIG. 3 is a schematic representation of the optically coupled symmetric bilateral switch; and FIG. 4 is a schematic representation of the 5optically sensitive FET switch with two levels of optical ;` control.
Detailed Uescri~tion _______________ ___._ A conventional prior art n-channel depletion mode photosensitive FET 1 is depicted in FIG. 1. The negative lObias, indicated by the minus sign, is applied to the gate G
through resistance Rl and establishes a depletion reyion.
If the bias is sufficiently large, the normally conducting drain-source channel vanishes and the FET no longer conduc~s. If light, from a source not depicted but ~; 15represented by hv, now illuminates the depletion règion, electron-hole pairs are generated as photons are absorbed.
The electric field in the depletion region separates the electrons and holes and a current flows in the external gate-source circuit. The resulting current flows through ~; 20 resistance Rl and establishes a bias, partially offsetting the applied gate-source bias, and reducing the size of the ; depletion region. If the offsetting bias is sufficiently large, current flows through the channel from the drain to the source and the F~T is turned on.
The optically sensitive F~T switch of this invention is schematically represented in FIGo 2. The drain and source electrodes of the normally on n-channel depletion mode FET 3 are connected to an electrical output circuit (not shown). The gate bias, controlling the size 30 of the depletion region and the current through the FET, is provided by a series connected photodiode array 7 having at least one photodiode connected between the gate and source of the FET. Photodiode, as used here, means any light sensitive semiconductor device. The number of photodiodes 35 is determined by the requirement that when the photodiodes are illuminated/ ~he voltage developed by the pho~odiodes is at least equal to the pinch-off voltage. As no external gate-source bias o~her than that produced by the ' , ~
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Some of the photons emitted by the LED are sabsorbed by the photodiodes and if the voltage developed between the gate and source exceeds the pinch-off voltage, the FET turns off. As the FET is connected to an electrical output circuit, the entire system operates as an ; optically coupled electrical switch.
The switch described has limited switching speed characteristics. Before the FET can be turned off, its ; input capacitance must be charged by the current produced by the photodiodes. The time required to charge the input capacitance will depend upon the intensity of the ~- 1sillumination of the photodiodes and their efficiency. When the illumination of the photodiodes is interrupted, the FET
input capacitance must discharge before the FET can turn on. The available paths, the reverse biased gate-channel junction and the photodiode array, both constitute high 20impedance paths. The result is a relatively long time constant which can be reduced by shunting the photodiodes ~ with resistance R2 which is connected in parallel with the `~ photodiode array between the gate and source. The value of R2 must be large enough to not appreciably load the 25 photodiodes when the photodiodes are illuminated and small enough to have an impedance which is small when compared to that of the photodiode array or the reverse biased gate-source junction.
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~n additional embodiment of the optically 30 sensitive switch is the optically coupled symmetric bilateral switch schematically depicted in FIG. 3. There are two photodiode arrays which control the FETo Array 15 has one or more series connected photodiodes and is l connected between the gate and source of n channel -;~ 35 depletion mode FET 9 through series connected rèverse ~, polarity blocking diode D2 and array 13 has one or more ~ series connected photodiodes and is connected between the i gate and drain through series connected reverse polarity . . ~
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- 6 - W. C King 2 blocking diode Dl. Large value, typically .~egohm, resistances R3 and ~ may be connected between the gate and drain and gate and source, respectively, of FET 9 and serve ; the same function as does resistance R2. The drain and 5source of FET 9 are connected to an electrical output circuit and light source 11 is connected to an electrical input circuit. Considerations similar to those discussed with respect to the embodiment shown in FIG. 2 determine the number of photodiodes in each array, i.e., the voltage ; lOproduced must at least equal the pinch-off voltage when the arrays are illuminated. The arrangement shown is symmetric with respect to the drain and source of the FET and consequently the switch can be operated with either polarity of voltage applied between the drain and source.
15Either resistance ~3 or R4 may be omitted, i.e., a single resistance connected between the gate electrode and either the source or drain electrode may be used, while retaining the voltage polarity symmetry if symmetric switching times are not required.
The depletion mode FET depicted is normally on regardless of the drain-source polarity. The operation of the switch, which may be used as an opto-isolator, is similar to that of the switch in FIG. 2. When LED 11 is turned on by current flowing in the electrical input 25 circuit~ light is emitted and falls on both photodiode arrays. If the "drain" is positive with respect to the "source", array 15 establishes a negative gate bias, with respect to the source, that exceeds the pinch-off voltage and FET 9 is tuened off. Divde Dl prevents the positive 30 drain voltage from coupling to the gate through the forward biased photodiode array 13 and negating the negative bias established at the gate by array 15~ When LÉD 11 is on and the drain-source polarity is the reverse of that described, array 13 establishes a negative ga~e bias that exceeds the 35 pinch-off voltage and FET 9 is turned off. Diode D2 prevents the positive source voltage from coupling to the gate through forward biased array 15 and thereby negating the negative bias established at the gate by array 13. The .. ..
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- 7 - W. C. King 2 entire system operates as an optically coupled electrical switch that is symmetric with respect to the voltage applied between the drain and source. Resistances R3 and R4 shunt the photodiode arrays to discharge the FET
scapacitance and decrease the switching time. The magnitudes of R3 and R4 are comparable to that of R2.
R3 need not equal R~ if symmetric switching times are not required.
An additional embodiment achieves two levels of 1optical control through the arrangement schematically shown in FIG. 4. The n-channel depletion mode FET 17, photodiode array 23 and LED 21 function as the optically sensitive -~ switch previously described and shown in FIG. 2. LRD 19 and E`ET 17, which is now optically sensitive, function as a ; 15conventional photosensitive FET switch shown in FIG. l.
The optically sensitive FET is on when LED 21 is off. ~hen LED 21 is on, the FET is off when LED 19 is off and is on ~ when LE~ l9 is on. LED l9 provides control in this - embodiment in a manner similar to the light~sourc~J e.g. LED in the 20conventional photosensitive ~ET switch. A bias is developed across the gate resistor R5 by the current flowing in the external circuit which is generated by photons absorbed in the depletion region.
The design parameters that must be considered are 25well known. For example, photodiode collection area, efficiency, illumination intensity and number of the photodiodes are related to the time required to turn the FET off. Additionally, after the LED is turned off, the time constant associated with the dischar~e of the ~ate-30 source capacitance through the shunt resistance (if used~and the large impedance of the pho~o~iode array and reverse gate-source junction are related to the time required for ~ the FET to ~urn on. In general, minimum switching times ,~ are obtained in saturated switching with large LED
35 currents, i.e., high photodiode array currents, low ; ~unction capacitances and low shunt resistances.
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- 8 - W. C. King 2 photodiode voltage, FET pinch-off voltage and switcning speeds are well-known to persons working in the field and need not be described in more detail.
As an example of switching speeds and other sparameters to be expected in practicer the response of the switch of FIG. 2 having a transconductance of approximately ; 32~00 umhos was measured. The pinch-off voltage of the FET
was 2.5 volts and the self pinch-off current (VGs = 0) was llO mA. R2 was 470,000 ohms, the supply voltage was 1010 volts and the load resistance was lO00 ohms. With a GaAlAs LED current of lO mAdc, the saturated switching response of an array having three series connected GaAlAs ; LEDs operated as photodetectors was observed to have a total turn~off time oE approximately 50 microseconds, and 15total turn-on time of approximately 50 microseconds.
Although the invention has been described with embodiments using n-channel depletion mode FETs which result in normally on switches and opto-isolators, the invention may also be used with p-channel depletion mode 20FETs if the polarities of the photodiode arrays and the blocking diodes are reversed. n-channel or p-channel enhancement mode FETs may also be used. With enhancement mode FETs, the photodiode array must produce a voltage that exceeds the gate-threshold voltage. These will result in 25 normally off switches and opto-isolators and the polarity of the diode array will be the reverse of that described for the n-channel depletion mode FETS. The symmetric switch using enhancement mode FETs is useful only for small values of drain-source voltage. The resistances act as a 30 voltage divider and the gate-source or gate-drain voltage ` produced by the voltage divider must be less than the gate-threshold voltage of the FET.
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FIG. 3 is a schematic representation of the optically coupled symmetric bilateral switch; and FIG. 4 is a schematic representation of the 5optically sensitive FET switch with two levels of optical ;` control.
Detailed Uescri~tion _______________ ___._ A conventional prior art n-channel depletion mode photosensitive FET 1 is depicted in FIG. 1. The negative lObias, indicated by the minus sign, is applied to the gate G
through resistance Rl and establishes a depletion reyion.
If the bias is sufficiently large, the normally conducting drain-source channel vanishes and the FET no longer conduc~s. If light, from a source not depicted but ~; 15represented by hv, now illuminates the depletion règion, electron-hole pairs are generated as photons are absorbed.
The electric field in the depletion region separates the electrons and holes and a current flows in the external gate-source circuit. The resulting current flows through ~; 20 resistance Rl and establishes a bias, partially offsetting the applied gate-source bias, and reducing the size of the ; depletion region. If the offsetting bias is sufficiently large, current flows through the channel from the drain to the source and the F~T is turned on.
The optically sensitive F~T switch of this invention is schematically represented in FIGo 2. The drain and source electrodes of the normally on n-channel depletion mode FET 3 are connected to an electrical output circuit (not shown). The gate bias, controlling the size 30 of the depletion region and the current through the FET, is provided by a series connected photodiode array 7 having at least one photodiode connected between the gate and source of the FET. Photodiode, as used here, means any light sensitive semiconductor device. The number of photodiodes 35 is determined by the requirement that when the photodiodes are illuminated/ ~he voltage developed by the pho~odiodes is at least equal to the pinch-off voltage. As no external gate-source bias o~her than that produced by the ' , ~
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Some of the photons emitted by the LED are sabsorbed by the photodiodes and if the voltage developed between the gate and source exceeds the pinch-off voltage, the FET turns off. As the FET is connected to an electrical output circuit, the entire system operates as an ; optically coupled electrical switch.
The switch described has limited switching speed characteristics. Before the FET can be turned off, its ; input capacitance must be charged by the current produced by the photodiodes. The time required to charge the input capacitance will depend upon the intensity of the ~- 1sillumination of the photodiodes and their efficiency. When the illumination of the photodiodes is interrupted, the FET
input capacitance must discharge before the FET can turn on. The available paths, the reverse biased gate-channel junction and the photodiode array, both constitute high 20impedance paths. The result is a relatively long time constant which can be reduced by shunting the photodiodes ~ with resistance R2 which is connected in parallel with the `~ photodiode array between the gate and source. The value of R2 must be large enough to not appreciably load the 25 photodiodes when the photodiodes are illuminated and small enough to have an impedance which is small when compared to that of the photodiode array or the reverse biased gate-source junction.
. . .
~n additional embodiment of the optically 30 sensitive switch is the optically coupled symmetric bilateral switch schematically depicted in FIG. 3. There are two photodiode arrays which control the FETo Array 15 has one or more series connected photodiodes and is l connected between the gate and source of n channel -;~ 35 depletion mode FET 9 through series connected rèverse ~, polarity blocking diode D2 and array 13 has one or more ~ series connected photodiodes and is connected between the i gate and drain through series connected reverse polarity . . ~
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- 6 - W. C King 2 blocking diode Dl. Large value, typically .~egohm, resistances R3 and ~ may be connected between the gate and drain and gate and source, respectively, of FET 9 and serve ; the same function as does resistance R2. The drain and 5source of FET 9 are connected to an electrical output circuit and light source 11 is connected to an electrical input circuit. Considerations similar to those discussed with respect to the embodiment shown in FIG. 2 determine the number of photodiodes in each array, i.e., the voltage ; lOproduced must at least equal the pinch-off voltage when the arrays are illuminated. The arrangement shown is symmetric with respect to the drain and source of the FET and consequently the switch can be operated with either polarity of voltage applied between the drain and source.
15Either resistance ~3 or R4 may be omitted, i.e., a single resistance connected between the gate electrode and either the source or drain electrode may be used, while retaining the voltage polarity symmetry if symmetric switching times are not required.
The depletion mode FET depicted is normally on regardless of the drain-source polarity. The operation of the switch, which may be used as an opto-isolator, is similar to that of the switch in FIG. 2. When LED 11 is turned on by current flowing in the electrical input 25 circuit~ light is emitted and falls on both photodiode arrays. If the "drain" is positive with respect to the "source", array 15 establishes a negative gate bias, with respect to the source, that exceeds the pinch-off voltage and FET 9 is tuened off. Divde Dl prevents the positive 30 drain voltage from coupling to the gate through the forward biased photodiode array 13 and negating the negative bias established at the gate by array 15~ When LÉD 11 is on and the drain-source polarity is the reverse of that described, array 13 establishes a negative ga~e bias that exceeds the 35 pinch-off voltage and FET 9 is turned off. Diode D2 prevents the positive source voltage from coupling to the gate through forward biased array 15 and thereby negating the negative bias established at the gate by array 13. The .. ..
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- 7 - W. C. King 2 entire system operates as an optically coupled electrical switch that is symmetric with respect to the voltage applied between the drain and source. Resistances R3 and R4 shunt the photodiode arrays to discharge the FET
scapacitance and decrease the switching time. The magnitudes of R3 and R4 are comparable to that of R2.
R3 need not equal R~ if symmetric switching times are not required.
An additional embodiment achieves two levels of 1optical control through the arrangement schematically shown in FIG. 4. The n-channel depletion mode FET 17, photodiode array 23 and LED 21 function as the optically sensitive -~ switch previously described and shown in FIG. 2. LRD 19 and E`ET 17, which is now optically sensitive, function as a ; 15conventional photosensitive FET switch shown in FIG. l.
The optically sensitive FET is on when LED 21 is off. ~hen LED 21 is on, the FET is off when LED 19 is off and is on ~ when LE~ l9 is on. LED l9 provides control in this - embodiment in a manner similar to the light~sourc~J e.g. LED in the 20conventional photosensitive ~ET switch. A bias is developed across the gate resistor R5 by the current flowing in the external circuit which is generated by photons absorbed in the depletion region.
The design parameters that must be considered are 25well known. For example, photodiode collection area, efficiency, illumination intensity and number of the photodiodes are related to the time required to turn the FET off. Additionally, after the LED is turned off, the time constant associated with the dischar~e of the ~ate-30 source capacitance through the shunt resistance (if used~and the large impedance of the pho~o~iode array and reverse gate-source junction are related to the time required for ~ the FET to ~urn on. In general, minimum switching times ,~ are obtained in saturated switching with large LED
35 currents, i.e., high photodiode array currents, low ; ~unction capacitances and low shunt resistances.
:~ff Variations in and relationships between shunt resistances, , .
load re~istance, photodiode curren~, number of photodiodes, ''', , .
.
Y . ' . .
.
3~36~
- 8 - W. C. King 2 photodiode voltage, FET pinch-off voltage and switcning speeds are well-known to persons working in the field and need not be described in more detail.
As an example of switching speeds and other sparameters to be expected in practicer the response of the switch of FIG. 2 having a transconductance of approximately ; 32~00 umhos was measured. The pinch-off voltage of the FET
was 2.5 volts and the self pinch-off current (VGs = 0) was llO mA. R2 was 470,000 ohms, the supply voltage was 1010 volts and the load resistance was lO00 ohms. With a GaAlAs LED current of lO mAdc, the saturated switching response of an array having three series connected GaAlAs ; LEDs operated as photodetectors was observed to have a total turn~off time oE approximately 50 microseconds, and 15total turn-on time of approximately 50 microseconds.
Although the invention has been described with embodiments using n-channel depletion mode FETs which result in normally on switches and opto-isolators, the invention may also be used with p-channel depletion mode 20FETs if the polarities of the photodiode arrays and the blocking diodes are reversed. n-channel or p-channel enhancement mode FETs may also be used. With enhancement mode FETs, the photodiode array must produce a voltage that exceeds the gate-threshold voltage. These will result in 25 normally off switches and opto-isolators and the polarity of the diode array will be the reverse of that described for the n-channel depletion mode FETS. The symmetric switch using enhancement mode FETs is useful only for small values of drain-source voltage. The resistances act as a 30 voltage divider and the gate-source or gate-drain voltage ` produced by the voltage divider must be less than the gate-threshold voltage of the FET.
~' :, ~' ';
. ~
:
' .
Claims (7)
1. An optically sensitive switch comprising:
a field effect transistor, said transistor having source, gate and drain electrodes, said source and drain electrodes being connected to an electrical output circuit;
a first light source, said first light source being connected to an electrical input circuit; and means for controlling the current through said field effect transistor, said means being optically coupled to said first light source;
characterized in that said means produces a voltage when illuminated that controls said current through said field effect transistor, said means comprises a first series connnected photodiode array, said array being connected between said gate and source electrodes, and a resistance connected between said gate and source electrodes, said resistance being less than that of said photodiode array and large enough to not appreciably load said photodiode array when said photodiodes are illuminated.
a field effect transistor, said transistor having source, gate and drain electrodes, said source and drain electrodes being connected to an electrical output circuit;
a first light source, said first light source being connected to an electrical input circuit; and means for controlling the current through said field effect transistor, said means being optically coupled to said first light source;
characterized in that said means produces a voltage when illuminated that controls said current through said field effect transistor, said means comprises a first series connnected photodiode array, said array being connected between said gate and source electrodes, and a resistance connected between said gate and source electrodes, said resistance being less than that of said photodiode array and large enough to not appreciably load said photodiode array when said photodiodes are illuminated.
2. An optically sensitive switch as recited in claim 1 in which said FET is a depletion mode FET.
3. An optically sensitive switch as recited in claim 2 in which said means further comprises a first reverse polarity blocking diode, said photodiode array and said first diode being series connected between said gate and said source; a second photodiode array, said second array having at least one photodiode; and a second reverse polarity blocking diode, said array and said second diode being series connected between said gate and said drain.
4. An optically sensitive switch as recited in claim 3 further comprising a first resistance, said first resistance being connected between said gate electrode and a first electrode selected from the group consisting of said source electrode and said drain electrode.
5. An optically sensitive switch as recited in claim 4 further comprising a second shunt resistance, said second resistance being connected between said gate electrode and a second electrode selected from the group consisting of said source electrode and said drain electrode.
6. An optically sensitive switch as recited in claim 1 in which said transistor has an optically sensitive region and further comprising: a second light source, said second source being connected to a second electrical input circuit, said second source being optically coupled to said optically sensitive region.
7. An optically sensitive switch as recited in claim 1 in which said FET is an enhancement mode FET.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US88926178A | 1978-03-23 | 1978-03-23 | |
| US889,261 | 1978-03-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1123064A true CA1123064A (en) | 1982-05-04 |
Family
ID=25394814
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA322,413A Expired CA1123064A (en) | 1978-03-23 | 1979-02-27 | Optically coupled field effect transistor switch |
Country Status (11)
| Country | Link |
|---|---|
| JP (1) | JPS5530292A (en) |
| BE (1) | BE874946A (en) |
| CA (1) | CA1123064A (en) |
| CH (1) | CH644472A5 (en) |
| DE (1) | DE2910748A1 (en) |
| ES (1) | ES478910A1 (en) |
| FR (1) | FR2420883A1 (en) |
| GB (1) | GB2017297B (en) |
| IT (1) | IT1112401B (en) |
| NL (1) | NL7902277A (en) |
| SE (1) | SE7902344L (en) |
Families Citing this family (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| USRE35836E (en) * | 1979-08-09 | 1998-07-07 | C. P. Clare Corporation | Solid state optically coupled electrical power switch |
| US4390790A (en) * | 1979-08-09 | 1983-06-28 | Theta-J Corporation | Solid state optically coupled electrical power switch |
| JPS57125814A (en) * | 1981-01-30 | 1982-08-05 | Tokyo Optical Co Ltd | Measuring machine |
| CH644244B (en) * | 1981-04-22 | Asulab Sa | ELECTRONIC WATCH WITH FIXED CONTROL BODY. | |
| DE3118364A1 (en) * | 1981-05-08 | 1982-11-25 | Siemens AG, 1000 Berlin und 8000 München | LIGHT INITIAL THYRISTOR WITH OPTOELECTRONICALLY CONTROLLED EMITTER SHORT CIRCUITS AND METHOD FOR ITS OPERATION |
| JPS57192129A (en) * | 1981-05-21 | 1982-11-26 | Omron Tateisi Electronics Co | Semiconductor relay |
| US4419586A (en) * | 1981-08-27 | 1983-12-06 | Motorola, Inc. | Solid-state relay and regulator |
| JPS62289013A (en) * | 1986-06-06 | 1987-12-15 | Agency Of Ind Science & Technol | Switching device |
| JPH01158821A (en) * | 1987-12-15 | 1989-06-21 | Matsushita Electric Works Ltd | Latching type semiconductor relay |
| EP0392373A3 (en) * | 1989-04-14 | 1992-10-21 | Siemens Aktiengesellschaft | Circuit device for potentialfree driving of a field effect transistor |
| JPH0758804B2 (en) * | 1989-05-17 | 1995-06-21 | 株式会社東芝 | Photo coupler device |
| FR2725084A1 (en) * | 1994-09-26 | 1996-03-29 | Sextant Avionique | Autonomous electric supply for electronic labels |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3770968A (en) * | 1972-02-24 | 1973-11-06 | Ibm | Field effect transistor detector amplifier cell and circuit for low level light signals |
| JPS4921049A (en) * | 1972-04-17 | 1974-02-25 | ||
| DE2511478A1 (en) * | 1975-03-15 | 1976-09-23 | Rollei Werke Franke Heidecke | CIRCUIT ARRANGEMENT FOR AMPLIFICATION OF PHOTO FLOW |
-
1979
- 1979-02-27 CA CA322,413A patent/CA1123064A/en not_active Expired
- 1979-03-13 GB GB7908885A patent/GB2017297B/en not_active Expired
- 1979-03-15 SE SE7902344A patent/SE7902344L/en not_active Application Discontinuation
- 1979-03-19 BE BE0/194098A patent/BE874946A/en not_active IP Right Cessation
- 1979-03-19 DE DE19792910748 patent/DE2910748A1/en not_active Withdrawn
- 1979-03-20 FR FR7906977A patent/FR2420883A1/en active Granted
- 1979-03-21 IT IT21188/79A patent/IT1112401B/en active
- 1979-03-22 CH CH270879A patent/CH644472A5/en not_active IP Right Cessation
- 1979-03-22 NL NL7902277A patent/NL7902277A/en not_active Application Discontinuation
- 1979-03-23 ES ES478910A patent/ES478910A1/en not_active Expired
- 1979-03-23 JP JP3340779A patent/JPS5530292A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| ES478910A1 (en) | 1979-07-01 |
| CH644472A5 (en) | 1984-07-31 |
| FR2420883A1 (en) | 1979-10-19 |
| JPS5530292A (en) | 1980-03-04 |
| FR2420883B1 (en) | 1982-10-22 |
| DE2910748A1 (en) | 1979-09-27 |
| NL7902277A (en) | 1979-09-25 |
| GB2017297A (en) | 1979-10-03 |
| BE874946A (en) | 1979-07-16 |
| SE7902344L (en) | 1979-09-24 |
| GB2017297B (en) | 1982-10-27 |
| IT1112401B (en) | 1986-01-13 |
| IT7921188A0 (en) | 1979-03-21 |
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