CA1275708C - Semiconductor switching circuit - Google Patents
Semiconductor switching circuitInfo
- Publication number
- CA1275708C CA1275708C CA 543599 CA543599A CA1275708C CA 1275708 C CA1275708 C CA 1275708C CA 543599 CA543599 CA 543599 CA 543599 A CA543599 A CA 543599A CA 1275708 C CA1275708 C CA 1275708C
- Authority
- CA
- Canada
- Prior art keywords
- fet
- gate
- output
- source
- resistor
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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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
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/04—Modifications for accelerating switching
- H03K17/041—Modifications for accelerating switching without feedback from the output circuit to the control circuit
- H03K17/0412—Modifications for accelerating switching without feedback from the output circuit to the control circuit by measures taken in the control circuit
- H03K17/04123—Modifications for accelerating switching without feedback from the output circuit to the control circuit by measures taken in the control circuit in field-effect transistor switches
Abstract
Abstract of the Disclosure A semiconductor switching circuit comprises an output FET receiving a photovoltaic output generated by a diode array responsive to a light signal from a light emitting element, a depression mode driving FET connected at the drain and source to the gate and source of the output FET, and a constantvoltage conduction element connected in parallel with a resistor connected across the gate and source of the driving FET. The sensitivity of the circuit is elevated by setting the value of this resistor relatively high, whereas the high speed operation can be assured by having discharge current of an accumulated charge across the drain and gate of the output FET
bypassed through the resistor.
bypassed through the resistor.
Description
~;~'7~708 SEMICONDUCTOR ~WITCHING CIRCUIT
Field of the Invention This invention relates to semiconductor switching circuits and, more particularly, to a semiconductor switching circuit which converts a light signal into an electrical signal by means of a photovoltaic diode array optically coupled to a light emitting element, such as a light emitting diode, which converts an input signal into a light signal; the converted electrical signal being utilized for driving an output device such as a field effect transistor (''FET'I), preferably a metal-oxide-semiconductor field effect transistor ("MOSFET").
Bac~round of the Invention There has been proposed in U.S. Patent No. 4,227,098 to Dale M. Brown et al a circuit in which a photovoltaic diode array is optically coupled to a light emitting diode, electrical-ly connected in parallel across a resistive impedance, and further electrically connected in series with the gate and source electrodes of a MOSFET. When current flows across the input terminals of the light emitting diode, an output voltage is generated across both terminals of the photovoltaic diode array, and is applied across the gate and source of the MOSFET.
Consequently, the impedance of the MOSFET changes to a different value. In the case of an enhancement mode MOSFET, the MOSFET
switches from the OFF state to the ON state. The resistive impedance then discharges electrostatic charge accumulated across the gate and source electrodes of the MOSFET. If no such resistive impedance is present, the MOSFET will not return to the OFF state, even when the input current to the light emitting diode is interrupted.
On the one hand, it is desirable to increase the value of the resistive impedance in order to reduce the required minimum input current for switching the MOSFET to the ON state.
~r . ~ .
7(~3 On the other hand, the resistive impedance is preferably minimized in order to reduce the time required to turn the MOSFET
OFF after interruption of the input current. These conflicting goals are difficult to achieve simultaneously. When the input current is in the minimum current range, the voltage across the gate and source of the MOSFET varies proportionally to the input current, so that the output impedance of the MOSFET may take an intermediate value, causing the MOSFET to assume an intermediate state between the ON and OFF states, which may cause a large power loss and consequential damage to the circuit elements.
U.S. Patent No. 4,390,790 to Edward T. Rodriguez discloses a semiconductor switching circuit in which a photo-voltaic diode array optically coupled to a light emitting diode is connected in series with a MOSFET. A normally on type junction FET is connected to the gate and source of the MOSFET.
An additional photovoltaic diode array is connected across the gate and source of this junction FET, through a resistor.
According to Rodriguez, the intermediate state of the MOSFET can be prevented by the provision of an additional diode array for driving the junction FET. However, the circuit arrangement required to combina the junction FET with the additional diode array causes another problem, in that a larger integrated circuit chip is required for the diode array and the optical conduction coefficient accordingly deteriorates, resulting in lower photosensitivity and switching capability of the circuit.
U.S. Patent No. 4,492,883 to William J. Janutka, discloses a circuit to turn OFF a MOSFET, in which a P-channel type FET is connected between the gate and the source of the MOSFET; a Zener diode is inserted between the gate and the source of the P-channel FET, (i.e., between the gate of the MOSFET and the gate of the P-channel FET); and a resistor is connected between the gate and the drain of the P-channel FET. This circuit arrangement may also inhibit the intermediate state of the MOSFET. However, in order to achieve high speed switching operation, it becomes necessary to use a high-voltage, high-1~7570~3 current power source, which leads to further problems when thecircuit is coupled to a photovoltaic diode array.
One approach to the problem would be to connect a photovoltaic diode array in series with a hiqh impedance element between the gate and the source of a ~OS~ET. An ON type driving transistor could then be connected with its drain connected between the gate of the MOSFET and the photovoltaic diode array, its gate connected between the photovoltaic diode array and the impedance element, and its source connected between the impedance element and the source of the MOSFET. The voltage developed across the impedance element due to the current supplied by the photovoltaic diode array would bias the driving transistor into a high impedance state. This arrangement prevents the MOSFET
from entering an intermediate state between ON and OFF while input current is near the minimum value required to switch the MOSFET ON. This arrangement also facilitates higher switching speeds than the circuits described in the aforementioned United States patents. However, this arrangement still suffers a number of shortcomings. For example, it is desirable to improve the input sensitivity of the circuit and to further shorten its response time to achieve higher switching speeds. The sensitiv-ity of the circuit may be increased by selecting a large value for the impedance element, thereby reducing the minimum current required to turn the MOSFET ON. Unfortunately, a large impedance value increases the time required to switch the MOSFET OFF.
Summary of_the Invention The object of the present invention is to provide a semiconductor switching circuit with improved sensitivity and increased operating speed wherein the state of the output transistor is prevented from being intermediate between the ON
and OFF states.
According to the present invention, the above object may be attained by a semiconductor switching circuit in which a . ..
S~()8 photovoltaic diode array is connected in series with a resistor;
both the array and the resistor being further connected between the gate and source of an output FET. A light-emitting element which generates a light signal in response to an input current is optically coupled to the photovoltaic diode array. A
constant-voltage conduction element is connected in parallel across the resistor. The drain of a depletion mode driving FET
is connected between the photovoltaic diode array and the gate of the output FET. The source of the driving FET is between the source of the output FET and the resistor. The gate of the driving FET is connected between the photovoltaic diode array and the resistor.
Upon the application of a voltage greater than its threshold volta~e, the constant-voltage conduction element becomes conductive. In this improved circuit, the threshold voltage of the constant-voltage conduction element is selected so that it is higher than the threshold voltage required to switch the driving FET from its normal ON state to its high impedance OFF state. The driving FET can therefore be biased into the OFF state by a voltage developed across the resistor and the constant-voltage conduction element due to the flow of current generated when the photovoltaic diode array receives a light signal.
The sensitivity of the improved semiconductor switching circuit described above can be increased by selecting a high value for the resistor connected between the gate and the source of the driving FET. When this is done, even a slight input current will result in a voltage drop across the resistor large enough to switch the driving FET OFF. With the driving FET
switched OFF, the gate potential of the output FET can be elevated sufficiently to switch the output FET ON. During the transient period of this switching operation, the potential at the drain of the output FET drops from source voltage level to near zero. This causes a change in the potential difference between the drain and the gate of the output FET and results in ~.~7~7~3 charge flowing from the gate of the output FET through the photovoltaic array and through the resistor. The increased current through the resistor results in a voltage drop across the resistor and constant-voltage conduction element greater than the threshold voltage of the constant-voltage conduction element.
The constant-voltage conduction element then begins to conduct and acts as a bypass to the resistor, maintaining the discharge current at a high level which depends upon the drain-gate capacitance of the output FET. Fast switching of the output FET
from OFF to ON is thereby assured.
Other objects and advantages of the present invention shall be made clear in the following disclosure and accompanying drawings.
Brief Description of the Drawings Figure 1 is a circuit diagram of a semiconductor switching circuit according to the present invention wherein a Zener diode is employed as the constant-voltage conduction element;
Figure 2 is a graph showing the relationship between the value of the resistor and the sensitivity of the circuit (the minimum input current needed to switch the output MOSFET from OFF
to ON);
Figure 3 is a graph showing the relationship between the switching speed of a semiconductor switching circuit, with and without a constant-voltage conduction element, and the input current;
Figure 4 is a circuit diagram of a semiconductor switching circuit according to the present invention wherein an enhancement mode FET is employed as the constant-voltage conduction element.
~57~
While the invention shall be explained with reference to the specific embodiments shown in the accompanying drawings, it should be appreciated that the intention is to cover all design modifications which come within the scope of the appended claims rather th~n any specific embodiments.
Description of the_Preferred Embodiment Figure 1 shows an embodiment of the semiconductor switching circuit of the present invention. The circuit 10 includes an output FET 12 which is preferably a MOSFET, and most preferably an n-channel enhancement mode MOSFET. The drain and source of output FET 12 are connected through output terminals 14 and 14a in series with a power source 15 and a load 16.
Current can flow from the power source and through the load when MOSFET 12 is switched from its high-impedance OFF state to its low-impedance ON state. For proper operation of the circuit, power source 15 is connected so as to make the drain of MOSFET
12 positive with respect to the source.
Circuit 10 further comprises a diode array 11, and a resistor 13 which are connected in series between the gate and source of output FET 12. Diode array 11 is optically coupled to a light-emitting element (not shown), such as a light-emitting diode, which generates a light signal in response to an input current signal. Resistor 13 has a relatively high value. On receipt of a light signal from the light emitting element, diode array 11 generates a photovoltaic output. The current flowing through load 16 can be controlled by applying the photovoltaic output of diode array 11 between the gate and source of MOSFET
~2. This causes MOSFET 12 to switch from a first impedance state to a second impedance state.
The final elements of the circuit are driving FET 17 and Zener diode 18~ Depletion mode driving FET 17 has its drain connected to the gate of output FET 12, its source connected to the source of output FET 12 and its gate connected between 1~
~X~757(~8 photovoltaic diode array 11 and resistor 13. Driving FET 17 is preferably a junction FET (JFET) which can be biased into its OFF
state by a voltage developed across resistor 13.
Zener diode 18 is connected in parallel with resistor 13 and constitutes the constant-voltage conduction element which is unique to the present invention. 2ener diode 18 is selected to have a breakdown vol~age higher than the threshold voltage of driving FET 17. Furthermore, Zener diode 18 is inserted in circuit 10 with polarity such that when a voltage higher than the breakdown voltage of Zener diode 18 is applied across Zen~r diode 18, it conducts and the resulting current flow establishes a charge across the source and gate of output MOSFET 12.
The operation of the circuit shown in Figure 1 will now be explained. To aid in understanding the present invention, the operation of a circuit lacking Zener diode 18 will be described first. A light signal received at diode array 11 causes a photovoltaic voltage to be generated by diode array 11. This voltage, in turn, causes a current to flow around circuit 10.
The current flows through driving FET 17, which is normally ON, through resistor 13, and back to diode array 11. When the voltage drop across resistor 13 exceeds the threshold voltage of driving FET 17, the latter switches OFF. Consequently, the current generated by photovoltaic diode array 11 flows to the gate of output MOSFET 12 and elevates the potential of the gate of MOSFET 12 relative to the source. This causes MOSFET 12 to switch from OFF to ON. During the transient period of this switching operation, the potential at the drain of MOSFET 12 drops from the level of power source 15 to near zero. The potential difference between the drain and gate of MOSFET 12 therefore varies greatly, with the result that some of the charge accumulated at the gate of MOSF~T 12 discharges through diode array 1~ and resistor 13. The time required for this discharge is a large fraction of the operation time, or, in other words, the response time, of circuit 10.
~.~757~)~
In circuit 10, the minimum input current, or working current, required to turn output MOSFET 12 ON gradually decreases as the value of resistor 13 is increased. This is shown in Figure 2. Curves k, 1, and m, trace the combinations of working current and resistance which result in gate-source voltages at the driving FET of -0.3V, -0.5V and -0.9V respectively. As can be seen, the minimum input current to the li~ht-emitting element which will result in MOSFET 12 turning ON, Ion, decreases as the resistance of resistor 13 is increased. The working curr~nt is about 2 mA when the resistance and the required gate-source voltage are: 1.0 Mn and -0.3V; 1.7Mn and -0.5V; or 3.0Mn and -O.9V.
As shown in Figure 3, when resistor 13 has a given value, the response time, Ton, of circuit 10 cannot be decreased merely by increasing the input current, I~, to the light emitting element and, consequently, increasing the intensity of the light shining on diode array 11. Curves n, o and p show the relation-ship between Ton and If where the value of resistor 13 is 0.3Mn, 1.0Mn and 3.0Mn respectively. It is clear from Figure 3 that high speed operation of this circuit is very difficult to realize. For example, if the value of resistor 13 is greater than 3.0Mn, the response time Ton will exceed 600 ~sec./ even when the input current to the light emitting element, If, is greater than 50mA. The slow response is due to the time required to discharge the charge accumulated on the gate of MOSFET 12, which discharge is impeded by resistor 13.
In the embodiment of the invention shown in Figure 1, Zener diode 18 is connected in parallel with resistor 13. If, while charge at the gate of MOSFET 12 is being discharged, the voltage across the parallel circuit of resistor 13 and Zener diode 18 exceeds the breakdown voltage of Zener diode 18, then Zener diode 18 becomes conductive and the discharge current flows mainly through Zener diode 18, bypassing resistor 13. With Zener diode 18 included in the circuit aforesaid, the time for the charge at the gate of output MOSFET 12 to be discharged can be _ 9 _ ,..~, ~
~'~'757~)~
made small, even when resistor 13 has a large value. Curve Q in Figure 3 shows that, with Z,ener diode 18 in the circuit, the relationship between If and Ton is linear. MOSFET 12 can there-fore be switched at high-speed from its OFF state to its ON state even when resistor 13 has a large value and the working current is correspondingly small.
When MOSFET 12 reaches its ON state, the discharge from the gate of MOSFET 12 terminates, the voltage across the parallel combination of resistor 13 and Zener diode 18 drops, and the Zener diode ceases to conduct current. A slight current continues to flow between the drain and source of driving FET 17 and, due to the presence of resistor 13, driving FET 17 is maintained in its high impedance state.
When the input current to the light emitting element is cut off, photovoltaic diode array 11 ceases to receive a light signal, the photo~oltaic output of diode array 11 is interrupted, and the voltage between the gate and ~ource of driving FET 17 drops. Driving FET 17 then returns to its ON state, allowing the charge which has accumulated at the gate of MOSFET ~2 to be quickly discharged through driving FET 17. MOSFET 12 is thereby switched into its OFF state and a high impedance results between terminals 14 and 14a. While the charge accumulated at the gate of driving FET 17 must be discharged through resistor 13 before driving FET 17 can return to its ON state, the time required is short because the capacity of the gate of driving FET 17 is much smaller than that of output MOSFET 12. Therefore, the time taken to discharge driving FET 17 does not substantially impair the high-speed operation of circuit 10.
Figure 4 shows an alternative semiconductor switching circuit according to the present invention. Each circuit element which appears in both Figures 1 and 4 is denoted in Figure 4 by a numher which is 100 larger than the corresponding label in Figure 1. The circuit of Figure 4 differs from that of Figure 1 only in that the constant-voltage conduction element in 7~
parallel with resistor 113 is an enhancement mode FET 118 with its gate and drain short-circuited. The threshold voltage of F~T
118 is selected so as to be higher than the threshold voltage of driving FET 117. FET 11~ is connected in parallel across resistor 113 and between the gate and source Gf driving FET 117 with polarity such that when FET 118 conducts current, a charge is developed across the gate and source of MOSFET 112.
As the voltage across the parallel combination of resistor 113 and enhancement mode FET 118 rises past a predeter-mined threshold voltage, the path between the drain and source of FET 118 begins to conduct~ Because of the short circuit between the gate and the drain of enhancement mode FET 118, FET
118 performs substantially the same function in the circuit of Figure 4 as does Zener diode 18 in the circuit of Fig. 1. A
particular advantage of the embodiment of the invention shown in Fiure ~ is that enhancement mode FET 118 is similar in structure to MOSFET 112 and driving FET 117 This simplifies the process of fabricating circuit 110 on a single chip and makes it easier to mass produce circuit 110.
Other aspects of the operation and arrangement of semiconductor switching circuit 110 shown in Figure 4 are substantially the same as the corresponding aspects of circuit 10 shown in Figure l.
In the present invention, a variety of design modifica-tions may be made to the circuits within the scope of the appended claims. For example, while the circuits discussed above use a MOSFET for the output transistor, a static induction type transistor (SIT) may be used instead. The junction FET (JFET) used as the depletion mode driving FET 17 or 117 may be replaced by a depletion mode MOSFET or a static induction type transistor (SIT). Further, it may be possible to use a P-channel or depletion mode MOSFET in place of t.he N-channel enhancement mode MOSFET 12 or 112.
According to the present invention, a semiconductor switching circuit which is both very sensitive and which is capable of high speed switching operation can be constructed as described above. The output FET can be switched from OFF to ON
with a very small input current and yet the eharge at the gate of the output FET can be quickly discharged, redueing the transient period required for the output FET to switch into the ON state. The prior art semiconduetor switching circuits discussed above have not been able to simultaneously offer these two antinomic capabilities.
Field of the Invention This invention relates to semiconductor switching circuits and, more particularly, to a semiconductor switching circuit which converts a light signal into an electrical signal by means of a photovoltaic diode array optically coupled to a light emitting element, such as a light emitting diode, which converts an input signal into a light signal; the converted electrical signal being utilized for driving an output device such as a field effect transistor (''FET'I), preferably a metal-oxide-semiconductor field effect transistor ("MOSFET").
Bac~round of the Invention There has been proposed in U.S. Patent No. 4,227,098 to Dale M. Brown et al a circuit in which a photovoltaic diode array is optically coupled to a light emitting diode, electrical-ly connected in parallel across a resistive impedance, and further electrically connected in series with the gate and source electrodes of a MOSFET. When current flows across the input terminals of the light emitting diode, an output voltage is generated across both terminals of the photovoltaic diode array, and is applied across the gate and source of the MOSFET.
Consequently, the impedance of the MOSFET changes to a different value. In the case of an enhancement mode MOSFET, the MOSFET
switches from the OFF state to the ON state. The resistive impedance then discharges electrostatic charge accumulated across the gate and source electrodes of the MOSFET. If no such resistive impedance is present, the MOSFET will not return to the OFF state, even when the input current to the light emitting diode is interrupted.
On the one hand, it is desirable to increase the value of the resistive impedance in order to reduce the required minimum input current for switching the MOSFET to the ON state.
~r . ~ .
7(~3 On the other hand, the resistive impedance is preferably minimized in order to reduce the time required to turn the MOSFET
OFF after interruption of the input current. These conflicting goals are difficult to achieve simultaneously. When the input current is in the minimum current range, the voltage across the gate and source of the MOSFET varies proportionally to the input current, so that the output impedance of the MOSFET may take an intermediate value, causing the MOSFET to assume an intermediate state between the ON and OFF states, which may cause a large power loss and consequential damage to the circuit elements.
U.S. Patent No. 4,390,790 to Edward T. Rodriguez discloses a semiconductor switching circuit in which a photo-voltaic diode array optically coupled to a light emitting diode is connected in series with a MOSFET. A normally on type junction FET is connected to the gate and source of the MOSFET.
An additional photovoltaic diode array is connected across the gate and source of this junction FET, through a resistor.
According to Rodriguez, the intermediate state of the MOSFET can be prevented by the provision of an additional diode array for driving the junction FET. However, the circuit arrangement required to combina the junction FET with the additional diode array causes another problem, in that a larger integrated circuit chip is required for the diode array and the optical conduction coefficient accordingly deteriorates, resulting in lower photosensitivity and switching capability of the circuit.
U.S. Patent No. 4,492,883 to William J. Janutka, discloses a circuit to turn OFF a MOSFET, in which a P-channel type FET is connected between the gate and the source of the MOSFET; a Zener diode is inserted between the gate and the source of the P-channel FET, (i.e., between the gate of the MOSFET and the gate of the P-channel FET); and a resistor is connected between the gate and the drain of the P-channel FET. This circuit arrangement may also inhibit the intermediate state of the MOSFET. However, in order to achieve high speed switching operation, it becomes necessary to use a high-voltage, high-1~7570~3 current power source, which leads to further problems when thecircuit is coupled to a photovoltaic diode array.
One approach to the problem would be to connect a photovoltaic diode array in series with a hiqh impedance element between the gate and the source of a ~OS~ET. An ON type driving transistor could then be connected with its drain connected between the gate of the MOSFET and the photovoltaic diode array, its gate connected between the photovoltaic diode array and the impedance element, and its source connected between the impedance element and the source of the MOSFET. The voltage developed across the impedance element due to the current supplied by the photovoltaic diode array would bias the driving transistor into a high impedance state. This arrangement prevents the MOSFET
from entering an intermediate state between ON and OFF while input current is near the minimum value required to switch the MOSFET ON. This arrangement also facilitates higher switching speeds than the circuits described in the aforementioned United States patents. However, this arrangement still suffers a number of shortcomings. For example, it is desirable to improve the input sensitivity of the circuit and to further shorten its response time to achieve higher switching speeds. The sensitiv-ity of the circuit may be increased by selecting a large value for the impedance element, thereby reducing the minimum current required to turn the MOSFET ON. Unfortunately, a large impedance value increases the time required to switch the MOSFET OFF.
Summary of_the Invention The object of the present invention is to provide a semiconductor switching circuit with improved sensitivity and increased operating speed wherein the state of the output transistor is prevented from being intermediate between the ON
and OFF states.
According to the present invention, the above object may be attained by a semiconductor switching circuit in which a . ..
S~()8 photovoltaic diode array is connected in series with a resistor;
both the array and the resistor being further connected between the gate and source of an output FET. A light-emitting element which generates a light signal in response to an input current is optically coupled to the photovoltaic diode array. A
constant-voltage conduction element is connected in parallel across the resistor. The drain of a depletion mode driving FET
is connected between the photovoltaic diode array and the gate of the output FET. The source of the driving FET is between the source of the output FET and the resistor. The gate of the driving FET is connected between the photovoltaic diode array and the resistor.
Upon the application of a voltage greater than its threshold volta~e, the constant-voltage conduction element becomes conductive. In this improved circuit, the threshold voltage of the constant-voltage conduction element is selected so that it is higher than the threshold voltage required to switch the driving FET from its normal ON state to its high impedance OFF state. The driving FET can therefore be biased into the OFF state by a voltage developed across the resistor and the constant-voltage conduction element due to the flow of current generated when the photovoltaic diode array receives a light signal.
The sensitivity of the improved semiconductor switching circuit described above can be increased by selecting a high value for the resistor connected between the gate and the source of the driving FET. When this is done, even a slight input current will result in a voltage drop across the resistor large enough to switch the driving FET OFF. With the driving FET
switched OFF, the gate potential of the output FET can be elevated sufficiently to switch the output FET ON. During the transient period of this switching operation, the potential at the drain of the output FET drops from source voltage level to near zero. This causes a change in the potential difference between the drain and the gate of the output FET and results in ~.~7~7~3 charge flowing from the gate of the output FET through the photovoltaic array and through the resistor. The increased current through the resistor results in a voltage drop across the resistor and constant-voltage conduction element greater than the threshold voltage of the constant-voltage conduction element.
The constant-voltage conduction element then begins to conduct and acts as a bypass to the resistor, maintaining the discharge current at a high level which depends upon the drain-gate capacitance of the output FET. Fast switching of the output FET
from OFF to ON is thereby assured.
Other objects and advantages of the present invention shall be made clear in the following disclosure and accompanying drawings.
Brief Description of the Drawings Figure 1 is a circuit diagram of a semiconductor switching circuit according to the present invention wherein a Zener diode is employed as the constant-voltage conduction element;
Figure 2 is a graph showing the relationship between the value of the resistor and the sensitivity of the circuit (the minimum input current needed to switch the output MOSFET from OFF
to ON);
Figure 3 is a graph showing the relationship between the switching speed of a semiconductor switching circuit, with and without a constant-voltage conduction element, and the input current;
Figure 4 is a circuit diagram of a semiconductor switching circuit according to the present invention wherein an enhancement mode FET is employed as the constant-voltage conduction element.
~57~
While the invention shall be explained with reference to the specific embodiments shown in the accompanying drawings, it should be appreciated that the intention is to cover all design modifications which come within the scope of the appended claims rather th~n any specific embodiments.
Description of the_Preferred Embodiment Figure 1 shows an embodiment of the semiconductor switching circuit of the present invention. The circuit 10 includes an output FET 12 which is preferably a MOSFET, and most preferably an n-channel enhancement mode MOSFET. The drain and source of output FET 12 are connected through output terminals 14 and 14a in series with a power source 15 and a load 16.
Current can flow from the power source and through the load when MOSFET 12 is switched from its high-impedance OFF state to its low-impedance ON state. For proper operation of the circuit, power source 15 is connected so as to make the drain of MOSFET
12 positive with respect to the source.
Circuit 10 further comprises a diode array 11, and a resistor 13 which are connected in series between the gate and source of output FET 12. Diode array 11 is optically coupled to a light-emitting element (not shown), such as a light-emitting diode, which generates a light signal in response to an input current signal. Resistor 13 has a relatively high value. On receipt of a light signal from the light emitting element, diode array 11 generates a photovoltaic output. The current flowing through load 16 can be controlled by applying the photovoltaic output of diode array 11 between the gate and source of MOSFET
~2. This causes MOSFET 12 to switch from a first impedance state to a second impedance state.
The final elements of the circuit are driving FET 17 and Zener diode 18~ Depletion mode driving FET 17 has its drain connected to the gate of output FET 12, its source connected to the source of output FET 12 and its gate connected between 1~
~X~757(~8 photovoltaic diode array 11 and resistor 13. Driving FET 17 is preferably a junction FET (JFET) which can be biased into its OFF
state by a voltage developed across resistor 13.
Zener diode 18 is connected in parallel with resistor 13 and constitutes the constant-voltage conduction element which is unique to the present invention. 2ener diode 18 is selected to have a breakdown vol~age higher than the threshold voltage of driving FET 17. Furthermore, Zener diode 18 is inserted in circuit 10 with polarity such that when a voltage higher than the breakdown voltage of Zener diode 18 is applied across Zen~r diode 18, it conducts and the resulting current flow establishes a charge across the source and gate of output MOSFET 12.
The operation of the circuit shown in Figure 1 will now be explained. To aid in understanding the present invention, the operation of a circuit lacking Zener diode 18 will be described first. A light signal received at diode array 11 causes a photovoltaic voltage to be generated by diode array 11. This voltage, in turn, causes a current to flow around circuit 10.
The current flows through driving FET 17, which is normally ON, through resistor 13, and back to diode array 11. When the voltage drop across resistor 13 exceeds the threshold voltage of driving FET 17, the latter switches OFF. Consequently, the current generated by photovoltaic diode array 11 flows to the gate of output MOSFET 12 and elevates the potential of the gate of MOSFET 12 relative to the source. This causes MOSFET 12 to switch from OFF to ON. During the transient period of this switching operation, the potential at the drain of MOSFET 12 drops from the level of power source 15 to near zero. The potential difference between the drain and gate of MOSFET 12 therefore varies greatly, with the result that some of the charge accumulated at the gate of MOSF~T 12 discharges through diode array 1~ and resistor 13. The time required for this discharge is a large fraction of the operation time, or, in other words, the response time, of circuit 10.
~.~757~)~
In circuit 10, the minimum input current, or working current, required to turn output MOSFET 12 ON gradually decreases as the value of resistor 13 is increased. This is shown in Figure 2. Curves k, 1, and m, trace the combinations of working current and resistance which result in gate-source voltages at the driving FET of -0.3V, -0.5V and -0.9V respectively. As can be seen, the minimum input current to the li~ht-emitting element which will result in MOSFET 12 turning ON, Ion, decreases as the resistance of resistor 13 is increased. The working curr~nt is about 2 mA when the resistance and the required gate-source voltage are: 1.0 Mn and -0.3V; 1.7Mn and -0.5V; or 3.0Mn and -O.9V.
As shown in Figure 3, when resistor 13 has a given value, the response time, Ton, of circuit 10 cannot be decreased merely by increasing the input current, I~, to the light emitting element and, consequently, increasing the intensity of the light shining on diode array 11. Curves n, o and p show the relation-ship between Ton and If where the value of resistor 13 is 0.3Mn, 1.0Mn and 3.0Mn respectively. It is clear from Figure 3 that high speed operation of this circuit is very difficult to realize. For example, if the value of resistor 13 is greater than 3.0Mn, the response time Ton will exceed 600 ~sec./ even when the input current to the light emitting element, If, is greater than 50mA. The slow response is due to the time required to discharge the charge accumulated on the gate of MOSFET 12, which discharge is impeded by resistor 13.
In the embodiment of the invention shown in Figure 1, Zener diode 18 is connected in parallel with resistor 13. If, while charge at the gate of MOSFET 12 is being discharged, the voltage across the parallel circuit of resistor 13 and Zener diode 18 exceeds the breakdown voltage of Zener diode 18, then Zener diode 18 becomes conductive and the discharge current flows mainly through Zener diode 18, bypassing resistor 13. With Zener diode 18 included in the circuit aforesaid, the time for the charge at the gate of output MOSFET 12 to be discharged can be _ 9 _ ,..~, ~
~'~'757~)~
made small, even when resistor 13 has a large value. Curve Q in Figure 3 shows that, with Z,ener diode 18 in the circuit, the relationship between If and Ton is linear. MOSFET 12 can there-fore be switched at high-speed from its OFF state to its ON state even when resistor 13 has a large value and the working current is correspondingly small.
When MOSFET 12 reaches its ON state, the discharge from the gate of MOSFET 12 terminates, the voltage across the parallel combination of resistor 13 and Zener diode 18 drops, and the Zener diode ceases to conduct current. A slight current continues to flow between the drain and source of driving FET 17 and, due to the presence of resistor 13, driving FET 17 is maintained in its high impedance state.
When the input current to the light emitting element is cut off, photovoltaic diode array 11 ceases to receive a light signal, the photo~oltaic output of diode array 11 is interrupted, and the voltage between the gate and ~ource of driving FET 17 drops. Driving FET 17 then returns to its ON state, allowing the charge which has accumulated at the gate of MOSFET ~2 to be quickly discharged through driving FET 17. MOSFET 12 is thereby switched into its OFF state and a high impedance results between terminals 14 and 14a. While the charge accumulated at the gate of driving FET 17 must be discharged through resistor 13 before driving FET 17 can return to its ON state, the time required is short because the capacity of the gate of driving FET 17 is much smaller than that of output MOSFET 12. Therefore, the time taken to discharge driving FET 17 does not substantially impair the high-speed operation of circuit 10.
Figure 4 shows an alternative semiconductor switching circuit according to the present invention. Each circuit element which appears in both Figures 1 and 4 is denoted in Figure 4 by a numher which is 100 larger than the corresponding label in Figure 1. The circuit of Figure 4 differs from that of Figure 1 only in that the constant-voltage conduction element in 7~
parallel with resistor 113 is an enhancement mode FET 118 with its gate and drain short-circuited. The threshold voltage of F~T
118 is selected so as to be higher than the threshold voltage of driving FET 117. FET 11~ is connected in parallel across resistor 113 and between the gate and source Gf driving FET 117 with polarity such that when FET 118 conducts current, a charge is developed across the gate and source of MOSFET 112.
As the voltage across the parallel combination of resistor 113 and enhancement mode FET 118 rises past a predeter-mined threshold voltage, the path between the drain and source of FET 118 begins to conduct~ Because of the short circuit between the gate and the drain of enhancement mode FET 118, FET
118 performs substantially the same function in the circuit of Figure 4 as does Zener diode 18 in the circuit of Fig. 1. A
particular advantage of the embodiment of the invention shown in Fiure ~ is that enhancement mode FET 118 is similar in structure to MOSFET 112 and driving FET 117 This simplifies the process of fabricating circuit 110 on a single chip and makes it easier to mass produce circuit 110.
Other aspects of the operation and arrangement of semiconductor switching circuit 110 shown in Figure 4 are substantially the same as the corresponding aspects of circuit 10 shown in Figure l.
In the present invention, a variety of design modifica-tions may be made to the circuits within the scope of the appended claims. For example, while the circuits discussed above use a MOSFET for the output transistor, a static induction type transistor (SIT) may be used instead. The junction FET (JFET) used as the depletion mode driving FET 17 or 117 may be replaced by a depletion mode MOSFET or a static induction type transistor (SIT). Further, it may be possible to use a P-channel or depletion mode MOSFET in place of t.he N-channel enhancement mode MOSFET 12 or 112.
According to the present invention, a semiconductor switching circuit which is both very sensitive and which is capable of high speed switching operation can be constructed as described above. The output FET can be switched from OFF to ON
with a very small input current and yet the eharge at the gate of the output FET can be quickly discharged, redueing the transient period required for the output FET to switch into the ON state. The prior art semiconduetor switching circuits discussed above have not been able to simultaneously offer these two antinomic capabilities.
Claims (8)
1. A semiconductor switching circuit, comprising:
(a) a diode array optically couplable to light emitting means for producing a light signal to cause said array to generate a photovoltaic output;
(b) a resistor connected in series with said photovoltaic diode array;
(c) an output FET having a gate, a source and a drain, said output FET being connected to said photovoltaic diode array for switching of said output FET from a first impedance state to a second impedance state upon application of said photovoltaic output to said gate and source;
(d) a depletion-mode driving FET having a gate, a source and a drain, said driving FET drain being connected to said output FET gate, said driving FET source being connected to said output FET source, and said driving FET gate and source being connected in parallel across said resistor; and, (e) a constant-voltage conduction element connected in parallel across said resistor, said element having a threshold voltage exceeding the threshold voltage of said driving FET.
(a) a diode array optically couplable to light emitting means for producing a light signal to cause said array to generate a photovoltaic output;
(b) a resistor connected in series with said photovoltaic diode array;
(c) an output FET having a gate, a source and a drain, said output FET being connected to said photovoltaic diode array for switching of said output FET from a first impedance state to a second impedance state upon application of said photovoltaic output to said gate and source;
(d) a depletion-mode driving FET having a gate, a source and a drain, said driving FET drain being connected to said output FET gate, said driving FET source being connected to said output FET source, and said driving FET gate and source being connected in parallel across said resistor; and, (e) a constant-voltage conduction element connected in parallel across said resistor, said element having a threshold voltage exceeding the threshold voltage of said driving FET.
2. A semiconductor switching circuit, comprising:
(a) a diode array optically couplable to light emitting means for producing a light signal to cause said array to generate a photovoltaic output;
(b) a resistor connected in series with said photovoltaic diode array;
(c) an output FET having a gate, a source and a drain, said output FET being connected to said photovoltaic diode array for switching of said output FET from a first impedance state to a second impedance state upon application of said photovoltaic output to said gate and source;
(d) a depletion-mode driving FET having a gate, a source and a drain, said driving FET drain being connected to said output FET gate, said driving FET source being connected to said output FET source, and said driving FET gate and source being connected in parallel across said resistor, whereby a voltage developed across said resistor, upon generation of said photovoltaic output by said photovoltaic diode array, biases said driving FET into the OFF state; and, (e) a constant-voltage conduction element connected in parallel across said resistor, whereby said element becomes conductive upon application to said element of a voltage exceeding the threshold voltage of said driving FET, thereby discharging the internal capaci-tance existing between said output FET drain and said output FET gate.
(a) a diode array optically couplable to light emitting means for producing a light signal to cause said array to generate a photovoltaic output;
(b) a resistor connected in series with said photovoltaic diode array;
(c) an output FET having a gate, a source and a drain, said output FET being connected to said photovoltaic diode array for switching of said output FET from a first impedance state to a second impedance state upon application of said photovoltaic output to said gate and source;
(d) a depletion-mode driving FET having a gate, a source and a drain, said driving FET drain being connected to said output FET gate, said driving FET source being connected to said output FET source, and said driving FET gate and source being connected in parallel across said resistor, whereby a voltage developed across said resistor, upon generation of said photovoltaic output by said photovoltaic diode array, biases said driving FET into the OFF state; and, (e) a constant-voltage conduction element connected in parallel across said resistor, whereby said element becomes conductive upon application to said element of a voltage exceeding the threshold voltage of said driving FET, thereby discharging the internal capaci-tance existing between said output FET drain and said output FET gate.
3. A semiconductor switching circuit according to claim 1 or 2, wherein said constant-voltage conduction element is a Zener diode having a breakdown voltage exceeding the threshold voltage of said driving FET.
4. A semiconductor switching circuit according to claim 1 or 2, wherein said constant-voltage conduction element is an enhancement mode FET having a short-circuited gate and drain, and having a threshold voltage exceeding the threshold voltage of said driving FET.
5. A semiconductor switching circuit according to claim 1 or 2, wherein said output FET is a MOSFET.
6. A semiconductor switching circuit according to claim 1 or 2, wherein said output FET is a static induction transistor.
7. A semiconductor switching circuit according to claim 1 or 2, wherein said driving FET is a depletion mode MOSFET.
8. A semiconductor switching circuit according to claim 1 or 2, wherein said driving FET is a static induction transis-tor.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP18827286 | 1986-08-11 | ||
| JP188272/1986 | 1986-08-11 | ||
| JP255023/1986 | 1986-10-27 | ||
| JP61255023A JPS63153916A (en) | 1986-08-11 | 1986-10-27 | Semiconductor switching circuit |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1275708C true CA1275708C (en) | 1990-10-30 |
Family
ID=26504827
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 543599 Expired - Lifetime CA1275708C (en) | 1986-08-11 | 1987-07-31 | Semiconductor switching circuit |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4801822A (en) |
| CA (1) | CA1275708C (en) |
| DE (1) | DE3726682A1 (en) |
| FR (1) | FR2602620B1 (en) |
| GB (1) | GB2194699B (en) |
| IT (1) | IT1211712B (en) |
| SE (1) | SE500062C2 (en) |
Families Citing this family (25)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4859875A (en) * | 1987-08-28 | 1989-08-22 | Siemens Aktiengesellschaft | Optocoupler for power FET |
| JPS6481522A (en) * | 1987-09-24 | 1989-03-27 | Agency Ind Science Techn | Optical control circuit and semiconductor device constituting said circuit |
| EP0369048A1 (en) * | 1988-11-15 | 1990-05-23 | Siemens Aktiengesellschaft | Circuit arrangement for regulating the load current in a power MOSFET |
| GB2230395B (en) * | 1989-03-15 | 1992-09-30 | Matsushita Electric Works Ltd | Semiconductor relay circuit |
| JPH0758804B2 (en) * | 1989-05-17 | 1995-06-21 | 株式会社東芝 | Photo coupler device |
| US5055722A (en) * | 1989-12-20 | 1991-10-08 | Sundstrand Corporation | Gate drive for insulated gate device |
| US5151602A (en) * | 1990-02-15 | 1992-09-29 | Matsushita Electric Works, Ltd. | Semiconductor relay circuit using photovoltaic diodes |
| FR2670902B1 (en) * | 1990-12-21 | 1993-04-23 | Siemens Automotive Sa | DEVICE FOR DETECTION AND DISCRIMINATION OF FAULTS IN A LOAD ELECTRICAL SUPPLY CIRCUIT. |
| US5278422A (en) * | 1991-09-02 | 1994-01-11 | Matsushita Electric Works, Ltd. | Normally open solid state relay with minimized response time of relay action upon being turned off |
| DE4206393C2 (en) * | 1992-02-29 | 1995-05-18 | Smi Syst Microelect Innovat | Solid state relay and method for its manufacture |
| US5221847A (en) * | 1992-06-26 | 1993-06-22 | At&T Bell Laboratories | Break-before-make control for form C solid-state relays with current limiter bypass |
| JPH06177418A (en) * | 1992-12-04 | 1994-06-24 | Toshiba Corp | Photocoupler device |
| JP2801825B2 (en) * | 1993-01-14 | 1998-09-21 | 株式会社東芝 | Photo coupler device |
| US5969581A (en) * | 1998-05-28 | 1999-10-19 | The United States Of America As Represented By The Secretary Of The Navy | Opto-electronically controlled RF waveguide |
| US6153948A (en) * | 1998-08-13 | 2000-11-28 | Cogan; Adrian I. | Electronic circuits with wide dynamic range of on/off delay time |
| US6555935B1 (en) * | 2000-05-18 | 2003-04-29 | Rockwell Automation Technologies, Inc. | Apparatus and method for fast FET switching in a digital output device |
| US20050133081A1 (en) * | 2003-11-25 | 2005-06-23 | Ixys Corporation | Photo voltaic solar cells integrated with mosfet |
| ITMI20041128A1 (en) * | 2004-06-04 | 2004-09-04 | Cesi Ct Elettrotecnico Sperime | PHOTOVOLTAIC CELL WITH PROTECTION DEVICE |
| JP2006186344A (en) * | 2004-12-02 | 2006-07-13 | Toshiba Corp | Optical coupling device |
| JP4528321B2 (en) * | 2007-09-26 | 2010-08-18 | シャープ株式会社 | Switching circuit, circuit, and circuit including switching circuit and drive pulse generation circuit |
| ES2469315B1 (en) * | 2012-12-14 | 2015-04-06 | Bsh Electrodomésticos España, S.A. | Connection component, in particular, connection component of household appliance |
| US9713437B2 (en) * | 2013-01-26 | 2017-07-25 | Cianna Medical, Inc. | Microwave antenna apparatus, systems, and methods for localizing markers or tissue structures within a body |
| US10720913B1 (en) | 2019-05-28 | 2020-07-21 | Infineon Technologies Austria Ag | Integrated failsafe pulldown circuit for GaN switch |
| US10958268B1 (en) | 2019-09-04 | 2021-03-23 | Infineon Technologies Austria Ag | Transformer-based driver for power switches |
| US10979032B1 (en) | 2020-01-08 | 2021-04-13 | Infineon Technologies Austria Ag | Time-programmable failsafe pulldown circuit for GaN switch |
Family Cites Families (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3299297A (en) * | 1965-12-17 | 1967-01-17 | Westinghouse Electric Corp | Semiconductor switching circuitry |
| US4227098A (en) * | 1979-02-21 | 1980-10-07 | General Electric Company | Solid state relay |
| US4303831A (en) * | 1979-07-30 | 1981-12-01 | Bell Telephone Laboratories, Incorporated | Optically triggered linear bilateral switch |
| US4390790A (en) * | 1979-08-09 | 1983-06-28 | Theta-J Corporation | Solid state optically coupled electrical power switch |
| US4419586A (en) * | 1981-08-27 | 1983-12-06 | Motorola, Inc. | Solid-state relay and regulator |
| US4492883A (en) * | 1982-06-21 | 1985-01-08 | Eaton Corporation | Unpowered fast gate turn-off FET |
| JPS60119126A (en) * | 1983-11-30 | 1985-06-26 | Matsushita Electric Works Ltd | Driving circuit of field effect transistor |
| JPS6194413A (en) * | 1984-10-15 | 1986-05-13 | Matsushita Electric Works Ltd | Semiconductor device |
| US4665316A (en) * | 1984-11-21 | 1987-05-12 | Telmos Incorporated | Photovoltaic relay switch |
| US4804866A (en) * | 1986-03-24 | 1989-02-14 | Matsushita Electric Works, Ltd. | Solid state relay |
-
1987
- 1987-07-28 US US07/078,791 patent/US4801822A/en not_active Expired - Lifetime
- 1987-07-31 CA CA 543599 patent/CA1275708C/en not_active Expired - Lifetime
- 1987-08-10 FR FR8711362A patent/FR2602620B1/en not_active Expired - Lifetime
- 1987-08-10 SE SE8703111A patent/SE500062C2/en unknown
- 1987-08-10 GB GB8718919A patent/GB2194699B/en not_active Expired - Lifetime
- 1987-08-11 IT IT4829587A patent/IT1211712B/en active
- 1987-08-11 DE DE19873726682 patent/DE3726682A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| GB2194699A (en) | 1988-03-09 |
| SE8703111D0 (en) | 1987-08-10 |
| FR2602620B1 (en) | 1993-04-16 |
| IT1211712B (en) | 1989-11-03 |
| US4801822A (en) | 1989-01-31 |
| DE3726682C2 (en) | 1989-10-26 |
| GB8718919D0 (en) | 1987-09-16 |
| IT8748295A0 (en) | 1987-08-11 |
| SE8703111L (en) | 1988-02-12 |
| GB2194699B (en) | 1990-04-18 |
| DE3726682A1 (en) | 1988-02-25 |
| FR2602620A1 (en) | 1988-02-12 |
| SE500062C2 (en) | 1994-03-28 |
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