High-voltage low-distortion input protection current limiter

[0001] 1. Field of the Invention

[0002] The present invention relates to a current limiter circuit for protecting input circuits. More particularly, the present invention relates to a current limiter circuit for protecting input circuits from excessive over-voltage conditions and excessive input currents, while providing low distortion for small-signal input voltages.

[0003] 2. Description of the Related Art

[0004] Input circuits appear in a wide variety of applications, including instrumentation devices such as Digital Multimeters (DMMs), oscilloscopes, spectrum analyzers and general purpose data acquisition equipment. Typically, input protection is required for preventing input circuits from destruction caused by over-voltage conditions.

[0005] In many cases, a simple arrangement of diode clamps are utilized for limiting the input voltage to the internal power supplies of the circuit. Such an arrangement, however, creates a condition in which excessive current may be injected externally through the clamp diodes.

[0006] FIG. 1 shows a schematic block diagram of a typical input circuit 100 having a conventional current limiter 101 for limiting excessive current. Input circuit 100 includes an input resistor R1, a current limiter device 101, two clamp diodes D1 and D2, and an amplifier A1. An input signal input at IN is applied to resistor R1. The input signal is coupled through current limiter device 101 to the input of amplifier A1. Current limiter device 101 is depicted in FIG. 1 as a resistor. The anode of clamp diode D1 is coupled to the input of amplifier A1. The cathode of clamp diode D1 is coupled to supply voltage V_CC. The cathode of clamp diode D2 is coupled to the input of amplifier A1. The anode of clamp diode D2 is coupled to supply voltage V_EE. Clamp diodes D1 and D2 limit the input voltage that can be applied to the input to amplifier A1 to about supply voltages V_CC and V_EE. Current input device 101 limits that amount of current that can be supplied externally to clamp diodes D1 and D2 and to the input of amplifier A1 when the externally applied input voltage exceeds the clamping voltages of V_CC and V_EE.

[0007] FIGS. 2A–2C depict circuit components that are conventionally used as current limiting devices. For example, FIG. 2A depicts a resistor 201. FIG. 2B depicts a Positive Temperature Coefficient (PTC) thermistor 202. FIG. 2C depicts a light bulb 203.

[0008] Another example of a conventional input protection circuit is disclosed by U.S. Pat. No. 5,742,463 to Harris. According to Harris, such an input protection circuit includes at least two depletion-mode field effect transistors, and can provide unipolar or bipolar operation, thereby protecting an input circuit from both positive-going and negative-going voltage transients.

[0009] The goals of an ideal current limiter include the capability to prevent destruction of input components, including the limiter itself. Input current for such an ideal current limiter should be limited based on the maximum expected input voltage. The ideal current limiter should also provide a low-noise, highly-linear, low-value impedance for normal, small-voltage operating conditions, while providing a high impedance for large voltages. Thus, the impedance state of an ideal current limiter must change based on the applied voltage. Additionally, both the inrush transient current and static power dissipation of an ideal current limiter should be minimized for preventing failure of any components.

[0010] What is needed is yet a better technique for limiting overload current for preventing destruction of clamp diodes and input circuitry.

[0011] The present invention provides a current limiter circuit for limiting overload current and thereby preventing destruction of the input components of an input circuit, including the current limiter circuit itself The current limiter of the present invention is characterized by three regions of operation and provides a low-noise, highly-linear, low-value input impedance for normal, small-voltage operating conditions, while providing a protective high impedance for large voltages. The current limiter circuit is symmetrical and floats on the input signal without connection to ground or power supplies. Further, the inrush transient current and the static power dissipation of a current limiter according to the present invention are minimized.

[0012] The advantages of the present invention are provided by a current limiter device that has a signal path and a control path that are both coupled between an input terminal and an output terminal. The output terminal can be coupled to an input circuit of, for example, an instrumentation device, a digital multimeter, an oscilloscope, a spectrum analyzer or a general-purpose data-acquisition device. According to the invention, the signal path of the current limiter device has a low impedance that passes small differential signals from the input terminal to the output terminal for voltages that are typically less than about one volt across the limiter. The control path is responsive to larger bipolar signals applied across the limiter by outputting a substantially constant current that is considerably less than what would be present in the low impedance path. The substantially constant current controls the impedance of the signal path to be a large impedance, thereby blocking the large bipolar input signal from the output terminal.

[0013] An alternative embodiment of the present invention provides a current limiter circuit having a signal path and a control path that are each coupled between an input terminal and an output terminal. The output terminal can be coupled to an input circuit of, for example, an instrumentation device, a digital multimeter, an oscilloscope, a spectrum analyzer or a general-purpose data-acquisition device. The signal path includes at least one depletion-mode device, such as an N-channel depletion-mode MOSFET, and a variable-impedance device, such as a P-Channel JFET, and passes small differential signals from the input terminal to the output terminal for differential signals that are typically less than one volt. Additionally, the signal path has a low impedance for these small differential signals across the limiter. The control path includes at least one depletion-mode device, such as an N-channel depletion-mode MOSFET and outputs at least one substantially constant current in response to larger bipolar input signals applied across the limiter. Each substantially constant current is considerably less than would be present in the low impedance path and controls at least one depletion-mode device of the signal path to be a high-impedance device and the variable-impedance device to be a high-impedance device so that the large bipolar input signal is blocked from the output terminal.

[0014] Yet another alternative embodiment of the present invention provides a current limiter device having a first terminal, a second terminal, and a current limiter circuit that is coupled between the first terminal and the second terminal. The current limiter circuit has a substantially constant-resistance operating mode when the magnitude of a voltage differential between a voltage at the first terminal and a voltage at the second terminal is less than or equal to a first predetermined voltage differential. The current limiter circuit also has a substantially constant-current operating mode when the magnitude of the voltage differential between the voltage at the first terminal and the voltage at the second terminal is greater than or equal to a second predetermined voltage differential. Lastly, the current limiter circuit has a transition operating mode when the magnitude of the voltage differential between the voltage at the first terminal and the voltage at the second terminal is between the first and second predetermined voltage differentials.

[0027] The present invention provides a current limiting circuit that protects input circuits from excessive current. One exemplary embodiment of a current limiting circuit of the present invention provides a bipolar floating limiter having four depletion-mode N-channel MOSFET transistors. The bipolar floating limiter is characterized by three regions of operation and provides a linear low-impedance input for normal-level small signals and a constant current source for overload signals. The four depletion mode N-channel MOSFET transistors provide high voltage overload capability. A single P-Channel JFET provides foldback current limiting during overload conditions, thereby providing low power dissipation. Four resistors are used for configuring the limiter characteristics. Under normal small-signal operation, the current limiter circuit of the present invention is inherently linear because only resistors and FETs are used.

[0028] FIG. 3 shows a schematic diagram of an exemplary embodiment of a current limiter circuit 300 according to the present invention. Current limiter circuit 300 includes four depletion-mode N-channel MOSFET transistors Q1–Q4, a P-channel JFET transistor Q5 and four resistors R1–R4, which together form two circuit paths. The first circuit path is formed by input terminal T1 being coupled to the drain of transistor Q1. The substrate of transistor Q1 is connected to the source of transistor Q1, and the source of transistor Q1 is coupled to the drain of transistor Q5. The gate of transistor Q1 is coupled to the source of transistor Q5, and to the source and substrate of transistor Q2. The gate of transistor Q2 is coupled to the drain of Q5, and to the source and substrate of transistor Q1. The drain of transistor Q2 is coupled to output terminal T2. Output terminal T2 is typically coupled to an input circuit of an instrumentation device, such as a Digital Multimeter (DMM), an oscilloscope, a spectrum analyzer or a general-purpose data-acquisition equipment.

[0029] The second circuit path, a control path, is formed by input terminal T1 being coupled to the drain of transistor Q3. The substrate of transistor Q3 is connected to the source of transistor Q3 and to one terminal of resistor R3. The gate of transistor Q3 is coupled to the other terminal of resistor R3 and to one terminal of resistor R1. The other terminal of resistor R1 is coupled to the gate of transistor Q5 and to one terminal of resistor R2. The other terminal of resistor R2 is coupled to the gate of transistor Q4 and to one terminal of resistor R4. The other terminal of resistor R4 is coupled to the source and the substrate of transistor Q4. The drain of transistor Q4 is coupled to output terminal T2.

[0030] Current limiter circuit 300 provides current limiting in a floating symmetrical bipolar fashion. Consequently, small signal operation of current limiter circuit 300 can be described by reference to the equivalent circuit models shown in FIGS. 4A–4D. FIG. 4A shows a schematic diagram for current limiter circuit 400 for small bipolar limiter voltage V_L conditions, such that |V_L|<<1 V. Under normal small-signal conditions, there is insufficient voltage between terminals T1 and T2 for producing the VgsOff voltage of transistors Q3 and Q4. As such, resistors R1–R4 hold the gate of transistor Q5 near the mid-voltage of the terminal potentials, and the R_ds value of transistor Q5 is simply its full conduction R_ds value. Under normal small-signal condition between terminals T1 and T2, the R_ds of transistors Q1 and Q2 are also at full conduction. FIG. 4B shows a schematic diagram for an equivalent circuit model 401 showing that for |V_L|<<1 V, all circuit components can be represented by resistances. Resistance values for resistors R1–R4 are each typically greater than 10 kΩ, while the R_ds values for each transistor is typically less than 100 Ω. Thus, the normal-state resistance between terminals T1 and T2 for |V_L|<<1 V is approximately R_ds(Q1)+R_ds(Q5)+R_ds(Q2), as represented by equivalent circuit 402 in FIG. 4C. Accordingly, a simple equivalent resistance of R_ds(Q1, Q5, Q2) is shown by equivalent circuit 403 in FIG. 4D.

[0031] Assume now that input terminals T1 and T2 are connected to a large positive overvoltage. FIG. 5A shows a schematic diagram for current limiter circuit 500 for large positive limiter voltage V_L conditions, such that V_L>>+1 V. Because transistors Q2 and Q4 are of the depletion-mode MOSFET type, transistors Q2 and Q4 are in their full ON state, thereby having a low resistance between their drain and source terminals. Consequently, transistors Q2 and Q4 can be replaced by equivalent low-value R_ds resistors. FIG. 5B shows a schematic diagram for an equivalent circuit 501 having transistors Q2 and Q4 replaced by low-value R_ds resistors. Transistor Q3 and resistor R3 form a current source I1 that outputs a current determined by the VgsOff voltage of Q3 and R3. FIG. 5C shows a schematic diagram for an equivalent circuit 502 having transistor Q3 and resistor R3 replaced by source I1. Resistor R1 is in series with current source I1 and, therefore, can be eliminated from the equivalent circuit. R_ds of transistor Q4 can be approximated by a wire because resistors R2 and R4 are each greater than 10 kΩ and R_ds of transistor Q4 is <100 Ω. A bias voltage for the gate of Q5 is then developed across R2+R4 in conjunction with the current source I1, as shown by equivalent circuit 503 in FIG. 5D. Transistor Q1 forms a current source that outputs a current that is determined by its VgsOff voltage and the R_ds resistance of transistor Q5. The R_ds resistance value of transistor Q5 is then defined by its gate voltage which is approximately I1*(R2+R4). This voltage is designed to be greater than the VgsOff voltage of Q5, and therefore turns off transistor Q5 and along with it the current flow through transistor Q1, as shown by equivalent circuit 504 in FIG. 5E. Resistors R2 and R4 are in series with current source I1 and, therefore, can be eliminated. Thus, the only active current path between terminals T1 and T2 is the current source I1 for large positive limiter voltage V_L conditions, as shown by equivalent circuit 505 in FIG. 5F.

[0032] The opposite overload condition of a large negative voltage is shown in the equivalent models of FIGS. 6A–6F. Operation proceeds as similarly described for the positive overload case, but with the actions of the symmetrical devices reversed. Specifically, FIG. 6A shows a schematic diagram for current limiter circuit 600 for large negative limiter voltage V_L conditions, such that V_L<<−1 V. Because transistors Q1 and Q3 are of the depletion-mode MOSFET type, transistors Q1 and Q3 are in their full ON state, thereby having a low resistance between their drain and source terminals. Consequently, transistors Q1 and Q3 can be replaced by equivalent low-value R_ds resistors. FIG. 6B shows a schematic diagram for an equivalent circuit 601 having transistors Q1 and Q3 replaced by low-value R_ds resistors. Transistor Q4 and resistor R4 form a current source I2 that outputs a current determined by the VgsOff voltage of Q4 and R4. FIG. 6C shows a schematic diagram for an equivalent circuit 602 having transistor Q4 and resistor R4 replaced by source I2. Resistor R2 is in series with current source I2 and, therefore, can be eliminated from the equivalent circuit. R_ds of transistor Q3 can be approximated by a wire because resistors R1 and R3 are each greater than 10 kΩ and R_ds of transistor Q3 is <100 Ω. A bias voltage for the gate of Q5 is then developed across R1+R3 in conjunction with the current source I2, as shown by equivalent circuit 603 in FIG. 6D. Transistor Q2 forms a current source that outputs a current that is determined by its VgsOff voltage and the R_ds resistance of transistor Q5. The R_ds resistance value of transistor Q5 is then defined by its gate voltage which is approximately I2*(R1+R3). This voltage is designed to be greater than the VgsOff voltage of transistor Q5, and therefore turns off transistor Q5 and along with it the current flow through transistor Q2, as shown by equivalent circuit 604 in FIG. 6E. Resistors R1 and R3 are in series with current source I2 and, therefore, can be eliminated. Thus, the only active current path between terminals T1 and T2 is the current source I2 for large negative limiter voltage V_L conditions, as shown by equivalent circuit 605 in FIG. 6F.

[0033] Transistors Q1–Q4 are high voltage N-channel depletion-mode MOSFETs. Transistors Q1–Q4 provide blocking capability of many hundreds of volts, and can easily be cascaded for blocking thousands of volts. Transistor Q5 is a low-voltage P-channel JFET that operates as a variable resistor. Because the VgsOff of transistor Q5 may be greater than the VgsOff of transistors Q3 and Q4, resistors R1 and R2 are needed for producing the required gate voltage for transistor Q5. The values of resistors R1–R4 are selected for controlling the operating characteristics of current limiter circuit 300.

[0034] FIG. 7 shows an exemplary graph illustrating the three operating regions of current limiter circuit 300. The first operating region is a constant resistance region in which current limiter circuit 300 can be characterized by a constant resistance. When operating in the constant resistance region, current through current limiter circuit 300 increases proportionally with increasing voltage in the same manner as a constant resistance. The second operating region is a transition region in which the operating characteristics of current limiter circuit 300 transitions from a constant resistance region to a constant current region. The third operating region is a constant current region in which current through current limiter circuit 300 remains substantially constant for increasing voltage across the limiter.

[0035] FIG. 8 is a graph illustrating current as a function of voltage across current limiter circuit 300 with respect to typical input characteristics for other conventional current limiting devices. The current vs. voltage characteristics of current limiter circuit 300 are shown by curve 801. At low voltage, current limiter circuit 300 exhibits a linear resistance of about 33 Ω having a thermal noise of about 0.75 nV/RtHz. For voltages greater than about 25 V, the current is limited to a constant 200 μA. A maximum current of about 50 mA flows at about 2 V. Between about 2 V and about 25 V, current limiter circuit 300 transitions between a constant resistance region and a constant current source region.

[0036] The current vs. voltage characteristics for the conventional input protection circuit of U.S. Pat. No. 5,742,463 to Harris are shown by curve 802. The Harris input protection circuit exhibits a breakdown voltage of about 30 V because the entire differential limiter voltage appears on the gates of the transistors. In contrast, current limiter circuit 300 operates easily to the full source-drain breakdown voltage of the transistors, extending to many hundreds of volts. Moreover, the voltage blocking capability of the present invention can be increased into the thousands of volts by cascading transistors.

[0037] Other curves representing current vs. voltage characteristics that are shown in FIG. 8 include curve 803 for a PTC thermistor having a resistance of 18 Ω and a thermal noise of 0.55 nV/RtHz; curve 804 for a light bulb having a resistance of 560 Ω and a thermal noise of 3.1 nV/RtHz; and curve 805 for a 1 kΩ resistor having a thermal noise of 4.1 nV/RtHz.

[0038] FIG. 9 is a graph illustrating power dissipation as a function of voltage across current limiter circuit 300 with respect to typical input characteristics for other conventional current limiting devices. Curve 901 shows that the power dissipation of current limiter circuit 300 is below 250 mW at any input voltage up to about 1000 V. In contrast, the power dissipation the conventional input protection circuit of U.S. Pat. No. 5,742,463 to Harris is shown by curve 902. FIG. 9 also shows other curves representing typical power dissipation as a function of limiter voltage. Curve 903 is the typical power dissipation for a PTC thermistor. Curve 904 is the typical power dissipation for a light bulb having a resistance of 560 Ω. Lastly, curve 905 is the typical power dissipation for a 1 kΩ resistor.

[0039] FIG. 10 is a graph illustrating nonlinearity characteristics as a function of voltage across current limiter circuit 300 with respect to the conventional input protection circuit of U.S. Pat. No. 5,742,463 to Harris. Curve 1001 shows the nonlinearity characteristics of current limiter circuit 300, and curve 1002 shows the nonlinearity characteristics of the conventional input protection circuit of U.S. Pat. No. 5,742,463 to Harris. Current limiter circuit 300 provides a lower distortion in the normal operating range in comparison to the conventional Harris input protection circuit. At some voltages, the distortion exhibited by current limiter circuit 300 is better than the distortion exhibited by the Harris input protection circuit by an order of magnitude.

[0040] FIG. 11 is a graph illustrating inrush current characteristics as a function of a 100 Volt step across current limiter circuit 300 with respect to typical input characteristics for other conventional current limiting devices. Curve 1101 shows the inrush current characteristics for current limiter circuit 300. Current limiter circuit 300 has about a 50 mA narrow transient (about 100 nS in duration) and then holds a constant current of about 200 μA thereafter. Accordingly, the requirements and stress placed on any clamp diodes coupled to output terminal T2 are significantly reduced. Current limiter circuit 300 has about two orders of magnitude less inrush current than a PTC thermistor, as shown by curve 1103. Curve 1104 shows the inrush current characteristics for a light bulb having a resistance of 560 Ω. Curve 1105 shows the inrush current characteristics for a 1 kΩ resistor.

[0041] Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced that are within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.