Intrinsically safe opto-coupler circuit having an optimum data transmission rate

Abstract

An opto-coupler comprising an LED and a photo-transistor whose collector resistor is connected in parallel with a diode comprising the emitter-base junction of a second transistor. This diode prevents saturation of the photo-transistor and at the same time, the current through the emitter-base junction diode of the second transistor is amplified by the gain of the second transistor to achieve and optimum signal output level across the collector resistor of the second transistor. The emitter-base junction diode of the second transistor prevents the photo-transistor from saturating and, in so doing, improves the rise and fall times of the collector current of the photo-transistor. The operation of the photo-transistor in a nonsaturation mode and the optimization of its rise and fall times for its collector currents result in an improvement in the data rate of the input signal that can be accommodated by the opto-coupler circuit.

Description

FIELD OF THE INVENTION

This invention relates to an opto-coupler, and in particular, an opto-coupler having an increased data transmission rate as well as being suitable for use in a hazardous location. This invention further relates to an opto-coupler having an increased spacing between its LED and the base of its associated photo-transistor. This invention further relates to an opto-coupler in which the photo-transistor is operated in a nonsaturating mode in order to increase the data transmission capabilities of the photo-transistor.

PROBLEM

Opto-couplers comprising an LED and an associated photo-transistor fabricated together as a single device are known and are widely used in applications in which DC and AC isolation must be maintained between a signal source and a signal output representing an amplified or processed input signal. Opto-couplers are used in instances in which the isolation provided by capacitance coupling between an input and an output circuit is not sufficient. One such instance in which capacitance coupling is not sufficient is in applications in which the input and output circuits are operated in hazardous locations in which the safety provided by capacitance coupling alone is not sufficient. In such instances, it is well known to use opto-coupler circuitry in which an LED receives an input signal to be amplified or otherwise processed and, in so doing, generates an optical output signal representative of the received input signal. The LED is part of a device package that also includes a photo-transistor whose base receives the optical output of the LED and, in turn, controls the collector current of the photo-transistor. The photo-transistor may be advantageously operated as an amplifier whose output is developed across a collector resistor.

An opto-coupler operated in this manner provides increased isolation compared to that provided by the capacitor coupled transistor amplifier. Although any opto-coupler provides increased isolation capability, it is known to use specialized opto-couplers in extremely hazardous applications. In such instances, the spacing between the LED and the base of the photo-transistor is increased to approximately 2 mm and encapsulated in solid insulation to achieve high breakdown voltage levels. These changes in opto-coupler structure made to meet the requirements of hazardous applications exacerbate operational limitations inherent in all photo-transistor opto-couplers: First, photo-transistors typically have large collector-base junction areas and a very thick base region compared with standard switching transistors in order to maximize the sensitivity of the photo-transistor to light. This results in a larger capacitance between the collector and base of the photo-transistor. This increased capacitance is multiplied by the Miller effect or gain of the photo-transistor and introduces a time constant that limits the switching speed of the opto-coupler. A second result of the large base region is that more charge can be stored in the base during saturation (the condition of both p-n junctions in the transistor simultaneously being forward biased). Therefore the base storage time is longer because recombination of the stored charge must occur before the output can switch. The problem is further worsened if the base terminal of the photo-transistor is not made available external to the package and therefore cannot be connected to external circuitry for the purpose of removing base charge more rapidly. Opto-couplers made for hazardous applications are of this type. These factors limit the switching speed of the opto-couplers and render them less than ideal in data transmission applications in which the desired data rate of the input signal applied to the LED may be 100 khz or more. As a result, the currently available opto-couplers that are suitable for use in hazardous application, and which have an increased spacing between the LED and the base of the photo-transistor, are limited for use in applications in which the input signal data application is less than 10 khz.

The maximum data transmission rate through an opto-coupler can be characterized by the output fall time (T_phl) and the output rise time (T_plh). The sum of these two propagation delays approximately equals the period of the highest frequency signal that the opto-coupler can transmit. In traditional opto-coupler circuits like that shown in FIG. 1, the limitation to data transmission rate is dominated by the output rise time (when the LED is turned off), which may be as much as 10 times longer than the fall time. The output rise time is affected primarily by the two sources detailed above, charge stored in the base during saturation, and the time constant formed by the collector-base capacitance and the collector resistance. Minimizing the collector resistance helps decrease the rise time, but also reduces available output voltage swing. When the LED is switched on, the output signal fall time T_phl across the collector resistor is limited by the discharge time of the collector-base capacitance at the collector current at which the photo-transistor is operated (higher current means faster fall time). The value of the collector resistor determines how much available collector current the photo-transistor must expend and therefore, how much current is available to discharge the collector-base capacitance (higher resistance means faster fall time).

The above two effects result in a trade off in optimizing the drive current of the photo-transistor. As the drive current is increased to improve the output fall time, more charge is stored in the base of the photo-transistor and the output rise time increases. As the drive current is reduced, the collector to base capacitance of the photo-transistor takes longer to discharge and the fall time of the signal output increases.

Similarly, a trade off exists in choosing a value for the collector resistance. As the resistance is increased to improve output voltage swing and output fall time, the photo-transistor is forced into saturation, and base charge storage causes an increase in output rise times. Additionally, the increased time constant formed by the collector resistance and collector to base capacitance further lengthens the rise time.

The above disadvantages result from the fact that the use of a collector resistor having a value sufficiently high to generate a required signal output causes the photo-transistor to become saturated. When the photo-transistor turns on, the transistor saturates and its output voltage on its collector is equal to the collector to emitter saturation voltage of the photo-transistor which is approximately 0.2 volts for a silicon device. When the photo-transistor is turned off, the output at the collector rises to the level of the supply voltage to the collector.

SOLUTION

The above and other problems are solved and an advance in the art is achieved by the present invention in which the photo-transistor is prevented from saturating in order to minimize the dependence of the collector rise time on the base storage time. Also, sufficient external gain is provided to allow the output voltage swing at the photo-transistor collector to be small while maintaining full necessary swing at the output of the opto-coupler. This allows the use of a low value of collector resistance, and a reduction in the rise time of the output signal at the collector.

In accordance with one possible exemplary embodiment, the collector resistor of the photo-transistor is connected in parallel with the base-emitter junction of a second transistor. The emitter of the second transistor is connected to a voltage source and the base of the second transistor being connected to the collector of the photo-transistor. In this manner, the base-emitter junction of the second transistor functions as a diode connected in parallel with the collector resistor of the photo-transistor. The base-emitter junction of the second transistor further controls the collector current of the second transistor which is operated as an amplifier. The collector of the second transistor is connected in series with an output resistor to ground. In this application, wherein the second transistor is a PNP type and when the photo-transistor turns on, the emitter-base junction diode of the second transistor holds the photo-transistor's collector to within approximately 0.6 volts of the power supply voltage. This prevents the photo-transistor from saturating. As a result, the value of the collector resistor of the photo-transistor can be significantly smaller than the prior art circuit because the required voltage swing at the collector is only 0.6 volts since the output signal across the collector resistor of the photo-transistor is amplified by the second transistor. The available current through the collector of the second transistor is H_fe times current of the first photo-transistor where H_fe is the gain of the second transistor. The output current of the second transistor is limited by the value of its collector resistor which can be adjusted to cause the saturation of the second transistor and produce an acceptable output voltage signal at the collector of the second transistor.

When the photo-transistor turns off, the base of the second transistor is pulled up to the level of the power supply. This pulls the second transistor out of saturation and turns it off and its output voltage falls. Because the second transistor does not suffer from the large base storage time of the photo-transistor and because the base lead of the second transistor can remove its accumulated charge without significant dependancy on recombination, the output switching time of the circuitry comprising the photo-transistor and the second transistor is greatly increased and can handle data rates well in excess of 10 khz.

The present invention is well suited for use in hazardous applications in which the use of a photo transistor with 2 mm spacing is required. A spacing of 2 mm is required to meet the standards that are enforced for "intrinsic safety" relating to instrumentation used in hazardous areas of a process control plant. In order to be safely utilized in a hazardous area, process control instrumentation must utilize protective measures to prevent the ignition of materials in the explosive environment. Intrinsic safety is a method of protection in which the atmosphere is allowed to come in contact with the equipment only because the equipment has been designed in a way that it is incapable of causing ignition in the atmosphere, even in the presence of faults occurring within or applied to the equipment. Intrinsic safety requirements are met by limiting the amount of energy in a circuit such that sparks and heat cannot be generated at levels sufficient to ignite the atmosphere.

The amount of energy in an intrinsically safe circuit ("IS circuit") is limited both in terms of instantaneous energy and in terms of stored energy. The instantaneous energy is limited by "barrier" circuits that are located in a flameproof enclosure or a safe area. Every connection between the circuit within the flameproof enclosure and the IS circuit is through a barrier circuit that limits the maximum current and voltage available to the IS circuit. The stored energy in the IS circuit is limited by minimizing the size of energy storage devices, e.g. capacitors and inductors, in the IS circuit.

DESCRIPTION OF THE DRAWINGS

These and other advantages and features of the present invention may be better understood from a reading of the following detailed description thereof taken in conjunction of the drawings in which;

FIG. 1 illustrates a prior art opto-coupler circuit.

FIG. 2 illustrates a first possible circuit embodiment.

DETAILED DESCRIPTION

FIG. 1 discloses a prior art opto-coupler comprising an LED light emitting diode and a photo-transistor Q1. The LED and transistor Q1 are fabricated as a single semi-conductor device with the LED being separated from Q1 a predetermined spacing provided by transparent material 107. The LED is energized by a series circuit comprising voltage source of V_cc, path 101, the LED, resistor R1 and path V_i and signal source element 106. Signal source 106 supplies electrical signals to the LED which generates optical signals corresponding to the electric signals applied by signal source 106. Photo-transistor Q1 is connected in a common emitter configuration with the emitter being grounded at terminal 103 and with the collector being connected in series with resistor R2 to a voltage source V_dd. Output path 104 is the junction of the lower leg of resistor R2 and the collector of Q1 and is the signal output for the collector of Q1.

In noncritical applications where isolation between the input signal V_i and the output signal V_o is the only criterion, the spacing 107 between the LED and the base of Q1 may be made as small as possible while maintaining the desired input signal/output AC and DC signal isolation. In such applications, a plurality of opto-coupler circuits each comprising an LED and a matching photo-transistor Q1 may be mounted on a single integrated circuit package if desired. However, in hazardous applications wherein safety concerns are paramount, a separation 107 of approximately 2 mm consisting of a solid insulating material between the LED and base of Q1 is required. Although this 2 mm spacing provides an adequate level of isolation for use in hazardous applications, the offsetting disadvantage is that opto-couplers so constructed are inherently slow and limit the communication rate across the intrinsically safe barrier 107. This limitation heretofore has limited the effective data transmission rate for such devices to approximately 10 khz or less. There is however, a need in the market place for enhanced communication rates across these devices.

In the prior art circuit of FIG. 1, it is typical to operate the photo-transistor Q1 in a saturated mode to develop the largest possible output signal across collector resistor R2. This requires that resistor R2 be of a large value. As a result, when transistor Q1 turns on, it saturates and the collector voltage of 104 falls to approximately 0.2 volts which is the collector to emitter saturation voltage of Q1. When Q1 turns off, the output voltage at terminal 104 rises to a level of V_dd. Transistors of the type used for Q1, have a large collector-base junction area and a very thick base region in order to maximize the sensitivity of the base to light signals generated by the LED. This type of device construction results in a large collector to base capacitance which, in turn, is multiplied by the gain of Q1. The base storage time is also large because the base region volume is large to increase its sensitivity to optical signals.

The maximum switching speed of photo-transistors of this type is limited by the above factors. When the LED switches from an on to an off state in response to the reception of an input signal V_i, the charge stored in the base of Q1 during saturation must be depleted internally to Q1. This limits the rise time T_plh of the collector current of Q1 and the output signal V_o. When the LED switches on in response to an input signal V_i and generates a corresponding optical signal that is received by the base of Q1, the fall time of the output signal V_o is limited by the charge time of the collector to base capacitance of Q1. This charge time varies in accordance with the value of the collector current as well as by the value of the collector resistor R2. The minimum value of R2 is also limited by the required output voltage swing that must be generated across R2.

The above two effects, the output rise time and the output fall time, result in a trade off in optimizing the collector current. As the drive current to the LED is increased, it generates a more intense optical signal which is received by the base of Q1. This results in an increased charge being stored on the base of Q1 so that the output rise time of the output signal V_o increases. Conversely as the drive current to the LED is reduced, the charge received by the base of Q1 is reduced and the collector to base capacitance takes longer to charge. This increases the fall time of T_phl of the output signal V_o. As a result of these limitations and the optimized component values used in the circuit of FIG. 1, the maximum data rate of less than 10 khz can be achieved by the circuit of FIG. 1.

Description of FIG. 2

The circuit details of one possible exemplary embodiment of the invention are shown on FIG. 2. FIG. 2 is identical to that of FIG. 1 except that FIG. 2 additionally contains transistor Q2 and its interconnections with the circuitry shown on FIG. 1. Specifically, the base of transistor Q2 is connected to terminal 104 which is the junction of the lower leg of resistor R2 and the collector of Q1. The emitter of Q2 is connected via path 102 to the voltage source V_dd. The collector of Q2 is connected in series with resistor R3 to terminal 103 ground. The signal output of this two stage circuit is terminal 108.

The circuitry of FIG. 2 solves the problems associated with the circuit of FIG. 1. The photo-transistor Q1 in FIG. 1 is prevented from saturating. This minimizes the dependancy of the output signal rise time on the base storage time of transistor Q1. Also, sufficient drive current for Q1 is provided with a low impedance at the collector of Q1 to reduce the fall time T_phl of the output signal V_o. The emitter to base junction of Q2 is effectively in parallel with the collector resistor R2 of Q1. Since the emitter to base forward voltage drop is approximately 0.6 volts, the collector of Q1 is kept at a potential at least within 0.6 volts of that of a voltage source V_dd which may advantageously be 5 volts. Thus, regardless of the resistance of resistor R2, the collector of Q1 is always kept at a voltage level that is within 0.6 tenths of a volt of that of the 5 volt source V_dd. This prevents Q1 from saturating and overcomes the above discussed base storage problems that preclude the operation of the circuit at a higher data rate than 10 khz. The value of the collector resistor R2 in FIG. 2 can be significantly smaller than the value of R2 in FIG. 1. In FIG. 1, the required output voltage swing must developed across R2. This is not the case in FIG. 2. The available current through the collector of Q2 is equal to the collector current of Q1 times the gain of transistor Q2. The output signal of transistor Q2 is determined by the value of resistor R3 which can easily be selected to cause the saturation of transistor Q2 to produce an acceptable output voltage V_o of a desired amplitude.

In FIG. 2, when the photo-transistor Q1 turns off, the base of transistor Q2 rises to V_dd. This pulls transistor Q2 out of saturation and turns it off so that its output voltage across resistor R3 falls. Because the transistor Q2 has a connection to its base, it does not suffer from the large base storage problem of transistor Q1. The connection to the base of Q2 can remove the charge on the base without dependancy on the recombination of the electrons within Q2. This increases the output switching time of transistor Q2 relative to Q1. With 5 volt supplies, and a LED forward current of 5 milliamps, and a value of 820 for resistor R2 and 4.7 k for resistor R3, switching speeds for the circuitry of FIG. 2 have been achieved of at least 150 khz.

An advantage of the circuit of FIG. 2 is that the base storage time of Q1 is reduced by not allowing Q1 to saturate. This is achieved by the forward connection of the emitter-base junction of Q2 in parallel with resistor R2 so that the collector of Q1 is always within 6 tenths of a volt of the 5 volt source V_dd. When transistor Q1 is turned off by an optical signal from the LED, the voltage on its collector can go to the 5 volt V_dd potential. When transistor Q1 is turned on by an LED signal, the voltage on its collector can go no lower than 4.4 volts which is within 0.6 volts of the 5 volt V_dd source. This is achieved by the connection of the diode comprising the base-emitter of transistor Q2.

The circuitry of FIG. 2 performs a dual function in so far as concerns the emitter-base junction of Q2. Namely, Q2 prevents the saturation of Q1 and the base emitter current of Q2 also controls the collector current of Q2. This permits Q2 to operate as an amplifier in which the base current of Q2 is multiplied by the gain of Q2.

There are three advantageous to the circuitry of FIG. 2. The first is that Q1 is never saturated and because of this, the low to high transition time T_plh of the Q1 current can be optimized. A second advantage is that resistor R2 need not be selected for an optimum voltage drop to be achieved across it. Resistor R2 can be selected to optimize the value of the high to low transition time T_phl for the collector current of Q1. A third advantage to the circuit of FIG. 2 is that the base and collector of Q2 provide a voltage gain across resistor R3 so that the output signal level achieved from the circuit is done by a selection of the optimum value of the resistor R3.

By comparison, in the prior art circuit of FIG. 1, the resistor R2 of Q1 in FIG. 1 had to be large enough to provide the desired output signal. However, when R2 of FIG. 1 becomes sufficiently large, transistor Q1 saturates more easily. Once Q1 saturates, the data rate that it can accommodate decreases. In FIG. 2, photo-transistor Q1 is operated in a nonsaturating mode. This results in an increased data rate that can be handled by the circuit of FIG. 2.

It is to be expressly understood that the claimed invention is not to be limited to the description of the preferred embodiment but encompasses other modifications and alterations within the scope and spirit of the inventive concept.

Claims

1. A signal processing system that provides signal isolation between input signals and output signals of said system;

said system including means for converting electrical input signals to optical signals and further comprising means for converting said optical signals to electrical output signals comprising:

a photo-transistor having a base responsive to the receipt of optical signals for, controlling the collector current of said photo-transistor;

a first resistor connecting the collector of said photo-transistor to a voltage source;

a diode comprising an emitter-base junction of a second transistor connected in parallel with said first resistor;

said diode being effective to prevent saturation of said photo-transistor as well as to control the collector current of said second transistor;

a second resistor connecting the collector of said second transistor to ground;

the voltage developed across said second resistor in response the reception of said optical signals by said base of said photo transistor being said output signals.

2. The system of claim 1 comprising;

a source of said electrical input signals;

an LED;

means for applying said electrical input signals from said source to said LED;

said LED being responsive to the receipt of said electrical input signals for converting said electrical signals to optical signals; and

means for applying said optical signals to said base of said photo-transistor.

3. The system of claim 1 further comprising;

means having a dimension of at least 2 mm separating said LED from the base of said photo-transistor.

4. The system of claim 1 further comprising;

means including said diode for operating said photo-transistor at a collector current less than saturation to optimize the rise and fall times of the collector current of said photo-transistor.

5. The system of claim 1 wherein said system provides ac and DC Isolation between said input signals and said output signals.

6. A system for providing ac and DC signal isolation between input signals and an output signals of said system comprising:

an opto-isolator comprising means for converting electrical input signals received from a source to optical signals and further comprising means for converting said optical signals to electrical signals;

said means for converting said optical signals to electrical signals comprising:

a photo-transistor having a base responsive to the receipt of optical signals for controlling the collector current of said photo-transistor;

a first resistor connecting the collector of said photo-transistor to a voltage source;

a diode comprising an emitter-base junction of a second transistor being connected in parallel with said first resistor;

said diode being effective to prevent saturation of said photo-transistor as well as to control the collector current of said second transistor;

a second resistor connecting the collector of said second transistor to ground;

the ac voltage developed across said second resistor in response the reception of said optical signals by said base of said photo-transistor being said output signals of said system.

7. A method of operating a system providing ac and DC signal isolation between input signals and output signals of said system;

said system including means for converting electrical input signals received from a source to optical signals and further comprising means for converting said optical signals to electrical output signals;

said method comprising the steps of:

applying said optical signals to a base of a photo-transistor;

controlling the collector current of said photo-transistor in response to the receipt of said optical signals;

connecting a first resistor between the collector of said photo-transistor to a voltage source;

connecting a diode comprising an emitter-base junction of a second transistor in parallel with said first resistor;

said diode being effective to prevent saturation of said photo-transistor as well as to control the collector current of said second transistor;

connecting a second resistor between the collector of said second transistor to ground;

the voltage developed across said second resistor in response the reception of said optical signals by said base of said photo transistor being said output signals.

8. The method of claim 7 further comprising the steps of:

applying said electrical input signals from a source to an LED to convert said electrical signals to optical signals; and

applying said optical signals to said base of said photo-transistor.

9. The method of claim 7 wherein a distance of at least 2 mm separates said LED from the base of said photo-transistor.

10. The method of claim 7 further comprising the steps of:

operating said photo-transistor at a collector current less than saturation to optimize the rise and fall times of the collector current of said photo-transistor.