RELATED APPLICATIONS

[0001] This application claims the benefit of priority to, U.S. Provisional Patent Application No. 61 /896,475 filed October 28, 2013, and entitled "VOLTAGE CLAMP DEVICE FOR INTRINSIC SAFETY," which is hereby incorporated by reference herein, in its entirety, for all purposes.

FIELD OF THE DISCLOSURE

[0002] The present disclosure is directed to an intrinsically safe device and, more particularly, to an intrinsically safe voltage clamping device with thermal and/or power limiting.

BACKGROUND

[0003] Some industrial processes, such as those in the petroleum industry, require devices to operate in hazardous atmospheres or other dangerous conditions. Such devices are often governed by an "intrinsically safe" standard, such as the ISA- 60079-1 1 standard, specifying certain conditions for devices in hazardous

atmospheres. The conditions seek to either limit the amount of energy stored in device circuitry (e.g., by limiting voltages to capacitors or currents to inductors) or limit the discharge of accumulated energy (e.g., by restricting the spacing of components) such that a discharge of energy will not cause an ignition. Further, in the case of the ISA-60079-1 1 standard, a circuit must remain safe during normal operation of the circuit even with a certain number of faults.

[0004] In some standards, an individual apparatus (or device) is treated differently than an assembly of components that is part of a larger apparatus. In the ISA- 60079-1 1 standard, for example, a shunt safety assembly manufactured as an individual apparatus must adhere to a different section of the standard, as compared with a shunt safety assembly that is part of a larger apparatus. The differing requirements for an individual apparatus can allow manufacturers to produce devices with advantageous properties, such as physically compact packages. However, such self-contained voltage clamping devices can also suffer from failures related to overheating, making them impractical in many applications.

SUMMARY

[0005] An intrinsically safe voltage clamping device comprises a regulated rail, a ground rail, and a shunt regulator assembly. The shunt regulator assembly is coupled to both the regulated rail and the ground rail and includes one or more regulating components. Also, the shunt regulator assembly is configured to clamp a voltage applied across the regulated rail and the ground rail to a safety clamp voltage value. The intrinsically safe voltage clamping device also includes a thermally activated component configured to, when the temperature of at least one of the regulating components exceeds a threshold value, cause one or more limiting components to reduce a power dissipated in the at least one of the regulating components.

[0006] In another embodiment, a process control device comprises a device component having first and second terminals, the device component storing energy when a voltage is applied across the first and second terminals of the device component. The process control device also includes two or more voltage clamping devices electrically coupled to the device component. Each voltage clamping device is disposed in parallel with the other of the voltage clamping devices, and each voltage clamping device is configured to clamp the voltage applied across the device component to a safety clamp voltage. Further, each voltage clamping device comprises a shunt regulator assembly including one or more regulating components, and a thermally activated component configured to, when the temperature of at least one of the regulating components exceeds a threshold value, cause one or more limiting components to reduce a power dissipated in the at least one of the regulating components.

[0007] In another embodiment an intrinsically safe voltage clamping device comprises a regulated rail, a ground rail, and a shunt regulator assembly. The shunt regulator assembly is coupled to both the regulated rail and the ground rail and includes one or more regulating components. Also, the shunt regulator assembly is configured to clamp a voltage applied across the regulated rail and the ground rail to a safety clamp voltage value. The intrinsically safe voltage clamping device also includes a current-sensing resistor configured to, when the current through the resistor exceeds a threshold value, cause one or more limiting components to reduce a power dissipated in the at least one of the regulating components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 is a block diagram of an example system in which primary and redundant voltage clamping devices are implemented.

[0009] Fig. 2A illustrates an example circuit which may be packaged as an intrinsically safe voltage clamping device and implemented as one of the voltage clamping devices illustrated in Fig. 1 .

[0010] Fig. 2B illustrates another example circuit which may be packaged as an intrinsically safe voltage clamping device and implemented as one of the voltage clamping devices illustrated in Fig. 1 .

[0011] Fig. 3A - 3C are block diagrams of example device packages in which intrinsically safe voltage clamping devices, such as the circuit illustrated in Fig. 2, can be packaged.

DETAILED DESCRIPTION

[0012] The present disclosure is directed to an intrinsically safe voltage clamping device and, specifically, to preventing thermal damage to voltage regulating device components in a self-contained device package via thermal- and/or power-limiting components. In particular, a voltage clamping device according to the present disclosure meets the functional requirements of a shunt voltage regulator while, at the same time, allowing for a physically compact and intrinsically safe packaging. In embodiments, thermally limiting components are utilized to automatically lower the clamp voltage of the device and reduce power dissipation in one or more regulating components. As a result, the limiting components may protect the voltage regulating components of the device against damage related to overheating while maintaining the safety-critical maximum clamp voltage. [0013] Referring now to Fig. 1 , an example system 100 constructed in accordance with one embodiment of the present disclosure includes a process control device 106 and a power supply 108. Primary and redundant voltage clamping devices 102a and 102b may limit the voltage applied across a device component 1 10 to a respective safety clamp voltage, in an implementation. By including two voltage clamping devices 102a and 102b, the system 100 may adhere to an intrinsically safe (IS) standard requiring continued voltage clamping upon failure of one of the voltage clamping devices 102a and 102b. An example voltage clamping device is discussed in more detail with reference to Figs. 2A and 2B.

[0014] While depicted in Fig. 1 as being inside the process control device 106, the primary and redundant voltage clamping devices 102a and 102b may be internal or external to the process control device. The voltage clamping devices 102a and 102b may be modular devices which can be removably coupled to the process control device 106, or the voltage clamping devices 102a and 102b may be stand-alone devices which may be electrically coupled to the process control device via any combination of electrical leads. In general, the voltage clamping devices 102a and 102b may be coupled to or part of the process control device 106 via any suitable internal or external electrical connections, terminals, etc.

[0015] The process control device 106 may include a switch, transmitter, thermocouple, solenoid valve, etc., and, in particular, the process control device 106 may include the device component 1 10. The device component 1 10 may be any type of circuit component or assembly of components that stores energy when a voltage is applied across the device component 1 10. For example, the device component 1 10 may include one or more capacitors or inductors. Although only one device component 1 10 is illustrated in Fig. 1 , it is clear that a process control device may have any number of components capable of storing energy.

[0016] The process control device 106 may be part of a manufacturing plant, oil or gas extraction structure, refinery, HVAC (heating, ventilation, and air conditioning) system, etc. in which the process control device 102a is exposed to a hazardous environment, such as an environment with hazardous gases, chemicals, vapors, dusts, fibers, etc. As such, the process control device 106 may be an intrinsically safe process control device 106, or the process control device 106 may be made intrinsically safe in combination with the voltage clamping devices 102a and 102b.

[0017] The example power supply 108 may power the process control device and may be operatively connected to both the process control device 106 and the voltage clamping devices 102a and 102b, where the process control device 106 and the voltage clamping devices 102a and 102b are connected in parallel (e.g., via two terminals of the process control device 106 and two leads of the voltage clamping devices 102a and 102b). The power supply 108 provides power to various components and may, in some cases, provide operating voltages for other circuits or components. For example, the power source 108 may provide output positive and negative voltages that are, in turn, applied to the rails of the voltage clamping devices 102a and 102b and the device component 1 10. The positive and negative voltages are denoted in Fig. 1 as +V and -V, respectively, but it is understood that a positive voltage may be applied to one of the voltage clamping devices 102a and 102b and the device component 1 10 and the other of the rails may be grounded.

[0018] The power supply 108 may be coupled to a mains power source, for example, or the power supply 108 may be coupled to battery power source. Also, the power supply 108 may, in some cases, transform a power signal (e.g., 24V) to particular voltages (e.g., ±3.3V, ±10V), where the particular voltages can be applied across rails of the voltage clamping devices 102a and 102b and the device component 1 10.

[0019] In some implementations, the voltage clamping devices 102a and 102b include: (i) regulating components 1 12a and 1 12b, respectively, such as components that are part of a shunt voltage regulating assembly (transistors, amplifiers, voltage references, etc.); (ii) power-sensing components 1 14a and 1 14b, respectively, such as thermistors, other temperature sensors, current sensors, etc. ; and (iii) one or more limiting components 1 16a and 1 16b, respectively, such as diodes, transistors, etc., configured to selectively reduce power dissipation in the regulating components 1 12a and 1 12b based on activation of the power-sensing components 1 14a and 1 14b. Additionally, though described here as "power-sensing," the components 1 14a and 1 14b may, instead, control the limiting components 1 16a and 1 16b according to another parameter value, such as a parameter value related to or relatable to the temperature of the regulating components 1 12a and 1 12b, as will be described below with respect to Figs. 2A and 2B.

[0020] The regulating components 1 12a and 1 12b may include any components of respective shunt voltage regulator assemblies in the voltage clamping devices 102a and 102b. The regulating component 1 12a may include, for example, a transistor, amplifier, and/or voltage reference device disposed between a regulated rail and ground rail of the voltage clamping device 1 02a. In some cases, a failure (e.g., due to overheating) of the regulating component 1 12a may cause the voltage clamping device 102a to lose the ability to clamp a voltage (e.g., applied across the device component 1 10) to a safety clamp voltage.

[0021 ] To prevent failure of the regulating components 1 12a and 1 12b due to overheating, the regulating components 1 12a and 1 12b may be coupled to the power-sensing components 1 14a and 1 14b (as in Fig. 2A). The coupling between one of the regulating components 1 12a and 1 12b and the respective power-sensing component 1 14a or 1 14b may include thermal bonding between the components. For example, the thermal bonding may include a thermal pad or thermal transfer grease to assist in the transfer of heat from the one of the regulating components 1 12a and 1 12b to the respective power-sensing component 1 14a or 1 14b.

[0022] By allowing heat transfer from one of the regulating components 1 12a and 1 12b to a respective power-sensing component 1 14a and 1 14b, the power-sensing component 1 14a or 1 14b may be activated (e.g., at a certain threshold temperature). When activated, the power-sensing components 1 14a and 1 14b may activate respective limiting components 1 16a and 1 16b. The limiting components 1 16a and 1 16b may include any suitable circuit components (e.g., diodes, transistors, and resistors) electrically coupled to shunt voltage regulating assemblies of the voltage clamping devices 102a and 102b, such that the limiting components 1 16a and 1 16b reduce power dissipation in the regulating components 1 12a and 1 12b. By reducing the power dissipated in the regulating components 1 12a and 1 12b, the limiting components 1 16a and 1 16b may reduce the temperature of the regulating

components 1 12a and 1 12b and prevent overheating. [0023] In addition to being activated by the power-sensing components 1 14a and 1 14b, the limiting components 1 16a and 1 16b may be de-activated when the power- sensing components 1 14a and 1 14b detect a sufficient decrease in the temperature of the regulating components 1 12a and 1 12b. That is, after a temperature of the regulating components 1 12a and 1 12b is reduced (e.g., below a threshold), the corresponding power-sensing component 1 14a and 1 14b and limiting components 1 16a and 1 16b may be de-activated.

[0024] In other embodiments, the components 1 14a and 1 14b are not thermally coupled to the regulating components 1 12a and 1 12b, and instead regulate the temperature of the regulating components 1 12a and 1 12b according to the power dissipated in the components 1 12a and 1 12b. In one embodiment, for example, the components 1 14a and 1 14b may sense a transistor junction voltage of a transistor in the regulating components 1 12a and 1 12b. When the components 1 14a and 1 14b detect a decrease in the transistor junction voltage, the components 1 14a and 1 14b can cause the limiting components 1 16a and 1 16b to decrease the power dissipated by the regulating components 1 12a and 1 12b.

[0025] In another embodiment, the components 1 14a and 1 14b are configured to sense voltage across and current through the regulating components 1 12a and 1 12b, respectively, and to calculate the power being dissipated in the regulating components 1 12a and 1 12b. When the power exceeds a threshold value, the components 1 14a and 1 14b may cause the limiting components 1 16a and 1 16b, respectively, to limit the voltage (e.g., by decreasing the clamp voltage) and/or current, thereby decreasing the power dissipated in the components 1 12a and 1 12b and, accordingly, the temperature increase associated with that power dissipation.

[0026] In any event, the thermal limiting of the regulating components 1 12a and 1 12b may facilitate the components of the voltage clamping devices 102a and 102b being packaged in a compact and self-contained device package. This compact device package may allow a use of the voltage clamping devices 102a and 102b in applications where physical space utilized by the devices 102a and 102b is a concern. Moreover, the voltage clamping devices 102a and 102b may comply with individual apparatus requirements of an intrinsically safe standard, which requirements allow for less redundancy in an implementation of the voltage clamping devices 102a and 102b than would otherwise be required in a more complex apparatus.

[0027] Fig. 2A illustrates an example circuit 200 which may be packaged as an intrinsically safe voltage clamping device, such as one of the voltage clamping devices 102a and 102b. Although, the example circuit 200 is described below with reference to certain components, it is clear that any suitable values and types of components may be utilized to provide the voltage clamping and thermal limiting functionality of the circuit 200.

[0028] The example circuit 200 includes a shunt regulator assembly 201 configured to clamp a voltage applied across a regulated rail 202 and a ground rail 204 to a safety clamp voltage. The shunt regulator assembly may include one or more regulating components such as one or more resistors 206, an amplifier 208, a reference voltage 210 coupled to a non-inverting terminal of the amplifier 208, and a transistor 212 coupled to a voltage output terminal of the amplifier 208. In the example circuit 200, the amplifier 208 drives the transistor 212 according to the difference between the reference voltage 210 and the feedback voltage from the shunted regulated rail 202. As such, most of the power dissipated in the example circuit 200 will be dissipated in the transistor 212, and the transistor 212 may, in some cases, increase in temperature.

[0029] Such an increase in the temperature of the transistor 21 2, or in one or more other regulating components of the shunt regulator assembly 201 , may activate the thermally activated component 214 which is thermally coupled to the transistor 212 (as indicated by the box 216). Activation of the thermally activated component 214 may correspond to a variety of changes in the thermally activated component 214. For example, the thermally activated component 214 may be an NTC thermistor which drops in resistance upon an increase in the temperature of the transistor 212. It is understood, however, that the thermally activated component 214 may include any suitable temperature sensor, such as a PTC thermistor, integrated circuit, etc. Alternatively, the circuit 200 may utilize an amplifier to compare an actual temperature of the transistor 212 to a targeted maximum temperature or threshold.

[0030] Upon activation, the thermally activated component 214 causes one or more limiting components, such as a transistor 218, to reduce the power dissipated in regulating components, such as the transistor 212. In one example scenario in which an NTC thermistor is implemented as the thermally activated component 214, the thermistor 214, acting as part of a temperature-dependent voltage divider between the regulated rail and ground rail, varies the voltage at the base of the transistor 218, and may decrease in resistance (i.e., increase the voltage at the base of the transistor 218) enough to "turn on" the transistor 218. That is, the voltage drop across the thermally activated component 214 may decrease so as to increase the voltage difference between the base and the emitter of the transistor 218.

[0031] Such an increase may cause the transistor 212 to be turned on "harder" (e.g., the transistor current will increase) than dictated by the amplifier 208. As a result, the clamped voltage between the regulated rail 202 and the ground rail 204 will drop allowing a reduction in both the power dissipated in the transistor 212 and the temperature of the transistor 212.

[0032] The example circuit 200 may further include various other components (e.g., resistors and diodes) to adjust current, voltage, etc. values or ratings. In one case, the circuit 200 may be configured for applications in which 4-20mA intrinsically safe circuits are used. As such, the current typically flowing in the example circuit 200 may be less than 25mA, even though the circuit 200 may be rated for a maximum current of 130mA. During normal operation, the circuit 200 may be able to operate with up to 25mA being shunted. However, the circuit 200 may reduce a shunt voltage (e.g., via limiting components) to protect regulating components in the abnormal condition of up to 130mA of current. Alternatively, the circuit 200 may be rated for up to 195mA to meet overrating safety factors required by IS standards.

[0033] While depicted in Fig. 2A as bipolar junction transistors, the transistors 212 and 218 need not be BJTs and, instead, the circuit 200 may be designed with metal- oxide-semiconductor field effect transistor (MOSFET) technology, as will be readily appreciated. [0034] In one scenario, a 6V shunt regulator shunting 25mA would dissipate 150mW. As such, the example circuit 200 may be able to dissipate this amount of power without going into a thermally limited mode (e.g., activating the thermally activated component 214). If the maximum temperature of the die of the transistor 212 is 150 °C and the device is operating in an 85 °C environment, a thermal resistance from die to ambient of 233 °C/W would be adequate to provide a 30 °C operating margin. Further, if the circuit 200 is to be rated for 195mA, the circuit 200 need only reduce the safety clamp voltage (e.g., via limiting components, such as a transistor 216) below 1 .43V to keep the die temperature below 150°C in such abnormal operating conditions.

[0035] Fig. 2B depicts another embodiment in which current sensing is used to control the thermal load of the regulating component. In Fig. 2B, an example circuit 220 functions much the same as the circuit in Fig. 2A. That is, during normal operation of the circuit 220, the amplifier 208 similarly drives a regulating transistor 222 according to the difference between the reference voltage and the feedback voltage from the regulated rail 202. Most of the power dissipated in the circuit 220 will be dissipated in the transistor 222. If the current being shunted by the transistor 222 is low enough (e.g., below 25 mA), a transistor 224 will be in cut-off. If the current being hunted by the transistor 224 through a resistor 226 is sufficient to drop enough voltage across the resistor 226 to turn on the transistor 224, then the transistor 224 will turn on the transistor 222, decreasing the clamp voltage of the circuit 220. The current-driven voltage limiting action of the transistor 224 will override the normal shunt voltage control (provided by the combination of the amplifier 208 and the transistor 222) until the current being shunted falls below the level where the transistor 224 is being turned on. For example, the circuit 220 is capable of shunting 20 mA at the designed clamp voltage of 6 V. If the current supplied to the circuit 220 were 200 mA, however, the voltage of the regulated rail 202 would fall below 1 .4 V. The effect of the current-driven clamp voltage reduction is to limit the power dissipated in the transistor 222.

[0036] In other embodiments, the voltage across the resistor 226 may be amplified to more precisely control the current at which the clamp voltage is pulled down. In still other embodiments, a multiplier circuit can be implemented to pull down the voltage only as much as necessary to limit the power. However, these improvements are not strictly necessary to accomplish the intended thermal limitation function.

[0037] Figs. 3A-3C illustrate example configurations of device packages in which intrinsically safe voltage clamping devices may be packaged. For example, the voltage clamping devices 102a and/or 102b or the circuits 200 or 220 may be packaged in a manner similar to that illustrated in Figs. 3A-3C.

[0038] Fig. 3A is a block diagram of one example device package 300 in which an intrinsically safe voltage clamping device may be packaged. The device package 300 includes a set 302 of components configured to provide voltage clamping functionality and thermal limiting functionality, such as the sets of components (e.g., the circuit 200) illustrated in Figs. 2A or 2B. A power supply, such as the power supply 108, may apply a voltage to the components 302 via two leads 310 and 312, and the two leads 310 and 312 may, in this example package 300, be the only two leads external to the device package 300. The device package 300 may, in some cases, be manufactured or assembled in a manner that adheres to intrinsically safe standards. The device package 300 may be, for example, dust-tight and/or may meet clearance distance and mechanical requirements for a specific application. In addition, the materials used to construct the device package 300 may include materials selected based on electrical characteristics. Specifically, the materials of the device package 300 may fulfill certain electrostatic conditions to prevent an accumulation of static charge.

[0039] A two-lead device package, such as the device package 300, may also be held to different intrinsic safety requirements as compared with a voltage clamping circuit that is part of a larger device. Further, to adhere to an intrinsic safety standard, the device package 300 may be connected in parallel with a redundant voltage clamping device via the two leads 310 and 312. The redundant voltage clamping device may include similar components to the device package 300, but, in general, may include any suitable assembly of components, leads, and connections. [0040] Fig. 3B is a block diagram of another example device package 320 in which an intrinsically safe voltage clamping device may be packaged. As with the device package 300, the device package 320 includes a set 322 of components configured to provide voltage clamping functionality and thermal limiting functionality. In this case, however, a power supply may apply a voltage to the components 322 via three or more leads 324 and 326 (three leads, four leads, five leads, etc.).

[0041] The three or more leads 324 and 326 of the device package 320 may be shorted together or otherwise combined or connected such that only two leads 328 and 330 are connected to the components 322 of the device package 320. Although Fig. 3B illustrated the leads 324 and 326 being connected inside of the device package 320, it is understood that the three or more leads 324 and 326 may be combined or connected external or internal to the device package 320.

[0042] A device package, such as the device package 320, may be subject to specific sections of an intrinsic safety standard due to the inclusion of more than two electronic leads. As such, the leads 324 and 326 may be selectively shorted together internally or externally without interference with the voltage clamping functionality of the device package 320, thereby maintaining adherence to a standard. Further, the device package 320 may be redundantly combined with other voltage clamping devices to maintain intrinsic safety.

[0043] Fig. 3C is a block diagram of yet another example device package 360 in which an intrinsically safe voltage clamping device may be packaged. The device package 360 also includes a set 362 of components configured to provide voltage clamping functionality and thermal limiting functionality. However, in addition to two leads 364 and 366, the device package 360 includes one or more additional leads 368. The one or more additional leads 368 may include unused pins of an integrated circuit (IC), leads of a power transistor package, etc.

[0044] Although the device package 360 includes the additional leads 368, the additional leads 368 may not be electrically connected to any other circuit

component, (as indicated by an "X" in Fig. 3D). In this way, the device package 360 may ensure that the additional leads 368 may not interfere with voltage clamping functionality of the device package 360 and, in some cases, ensure that the device package 360 adheres to certain intrinsically safe standards. For example, a certain intrinsically safe standard may require an integrated circuit to remain safe with any combination of its leads electrically shorted.

[0045] Although Figs. 3A-3C illustrate a certain number of distinct components, leads, and connection types, it is clear that a device package may include any number and combination of components, leads, and connection types. For example, a device package may include two, three, five, etc. leads coupled to both a regulated and ground rail, or a device package may include zero, one, two, etc. electronic leads in addition to two primary electronic leads (e.g., coupled to a power supply).