Magnetic field pulse current sensing for timing-sensitive circuits

Abstract

A current measurement circuit for determining a start time t.sub.START, an end time t.sub.END, and/or a peak time t.sub.MAX for a current pulse passing through a current conductor. The current measurement circuit includes a pickup coil and a threshold crossing detector. The pickup coil generates a voltage V.sub.SENSE proportional to a magnetic field around the conductor, which is proportional to a change in current over time. The threshold crossing detector compares V.sub.SENSE and a threshold voltage and generates an output signal indicative of a transition time and whether a slope of V.sub.SENSE is positive or negative. The current measurement circuit can also include an integrator and a sample and hold circuit. The integrator integrates V.sub.SENSE over time and generates an integrated signal V.sub.SENSE. The sample and hold circuit compares V.sub.SENSE to t.sub.MAX and generates a second output signal which can be used to measure the pulse current.

Claims

1. A current measurement circuit for measuring characteristics of a current pulse passing through a current conductor, comprising: a pickup coil for generating a voltage V.sub.SENSE proportional to a magnetic field around the current conductor, wherein the magnetic field is proportional to a change in current through the current conductor over time; an amplifier connected to the pickup coil for amplifying V.sub.SENSE to generate an amplified V.sub.SENSE; and at least one threshold crossing detector for comparing the amplified V.sub.SENSE to at least one threshold voltage and generating an output signal based on the comparison, wherein the output signal is indicative of a transition time for the current pulse, and whether a slope of V.sub.SENSE is positive or negative.

2. The current measurement circuit of claim 1, wherein the at least one threshold voltage is ground.

3. The current measurement circuit of claim 2, wherein the output signal is indicative of a start time of the current pulse when the output signal is indicative of a first transition time for the current pulse and the slope of V.sub.SENSE changes from positive to negative in a first case or from negative to positive in a second case.

4. The current measurement circuit of claim 3, wherein the output signal is indicative of a peak amplitude time of the current pulse when the output signal is indicative of a second transition time after the first transition time for the current pulse and the slope of V.sub.SENSE remains positive or remains negative.

5. The current measurement circuit of claim 3, wherein the output signal is indicative of an end time of the current pulse when the output signal is indicative of a third transition time after the first transition time for the current pulse and the slope of V.sub.SENSE changes from negative to positive in the first case or from positive to negative in the second case.

6. The current measurement circuit of claim 1, wherein the pickup coil comprises a single loop configured to be arranged near the current conductor.

7. The current measurement circuit of claim 1, wherein the pickup coil is configured to not encircle the current conductor.

8. The current measurement circuit of claim 1, wherein the pickup coil comprises a Rogowski-type coil.

9. The current measurement circuit of claim 1, wherein the current measurement circuit is integrated on a single semiconductor chip.

10. The current measurement circuit of claim 9, wherein the single semiconductor chip further comprises the current conductor.

11. A current measurement circuit for measuring characteristics of a current pulse passing through a current conductor, comprising: a pickup coil for generating a voltage V.sub.SENSE proportional to a magnetic field around the current conductor, wherein the magnetic field is proportional to a change in current through the current conductor over time; at least one threshold crossing detector for comparing V.sub.SENSE to at least one threshold voltage and generating an output signal based on the comparison, wherein the output signal is indicative of a transition time for the current pulse, and whether a slope of V.sub.SENSE is positive or negative; an integrator for integrating V.sub.SENSE over time and generating an integrated signal V.sub.SENSE based on the integrated voltage; and a sample and hold circuit for comparing V.sub.SENSE to a peak amplitude time of the current pulse and generating a second output signal which can be used to measure the pulse current.

12. A current measurement circuit for measuring characteristics of a current pulse passing through a current conductor, comprising: a pickup coil for generating a voltage V.sub.SENSE proportional to a magnetic field around the current conductor, wherein the magnetic field is proportional to a change in current through the current conductor over time; and at least one threshold crossing detector for comparing V.sub.SENSE to at least one threshold voltage and generating an output signal based on the comparison, wherein the output signal is indicative of a transition time for the current pulse, and whether a slope of V.sub.SENSE is positive or negative; wherein the pickup coil comprises one or more turns, and wherein the one or more turns are arranged to increase a total magnetic flux resulting from the current pulse and reduce a total magnetic flux resulting from other sources.

13. The current measurement circuit of claim 12, further comprising an amplifier for amplifying V.sub.SENSE and connected to the pickup coil and to the at least one threshold crossing detector, wherein the at least one threshold crossing detector compares the amplified V.sub.SENSE.

14. A current measurement circuit for measuring characteristics of a current pulse passing through a current conductor, comprising: a pickup coil for generating a voltage V.sub.SENSE proportional to a magnetic field around the current conductor, wherein the magnetic field is proportional to a change in current through the current conductor over time; and at least one threshold crossing detector for comparing V.sub.SENSE to at least one threshold voltage and generating an output signal based on the comparison, wherein the output signal is indicative of a transition time for the current pulse, and whether a slope of V.sub.SENSE is positive or negative; wherein the at least one threshold crossing detector and a first portion of the pickup coil are integrated on a semiconductor chip, and wherein a second portion of the pickup coil is formed by at least two die terminals and an external conductor.

15. The current measurement circuit of claim 14, wherein the external conductor forms part of a mounting substrate.

16. The current measurement circuit of claim 15, wherein the mounting substrate further comprises a portion of the current conductor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features, objects, and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

(2) FIG. 1 illustrates a schematic of a conventional current measurement circuit with a shunt resistor.

(3) FIGS. 2A-B illustrate an arrangement of a Hall sensor relative to a current conductor, and a schematic of a conventional current measurement circuit with a Hall sensor.

(4) FIG. 3 illustrates a schematic of a conventional current measurement circuit with a Rogowski coil.

(5) FIGS. 4A-B show a graph of a voltage output from a conventional current measurement circuit such as the circuits shown in FIGS. 1-3 resulting from a current pulse of the same approximate shape, and a graph of the derivative of the aforementioned voltage output.

(6) FIG. 5 illustrates a current measurement circuit according to an embodiment of the present invention.

(7) FIG. 6 illustrates a current measurement circuit according to an embodiment of the present invention that is partially integrated with the remainder of the pickup coil formed by die terminals and an external conductor that may form part of a mounting substrate such as a printed circuit board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) In the following detailed description, reference is made to certain embodiments. These embodiments are described with sufficient detail to enable those skilled in the art to practice them. It is to be understood that other embodiments may be employed and that various structural, logical, and electrical changes may be made. The combinations of features disclosed in the following detailed description may not be necessary to practice the teachings in the broadest sense, and are instead taught merely to describe particularly representative examples of the present teachings.

(9) FIG. 4A shows a graph of a voltage output from a conventional current measurement circuit such as the circuits shown in FIGS. 1-3, and FIG. 4B shows the derivative of the voltage output from the conventional current measurement circuit. Graph 400 shows the output voltage V.sub.SENSE from a conventional current measurement circuit over time, where V.sub.SENSE is substantially proportional to the current pulse I.sub.PULSE being sensed. To determine the start time t.sub.START and end time t.sub.END of the current pulse I.sub.PULSE, some systems compare V.sub.SENSE to a predetermined threshold V.sub.REF and determine the start time t.sub.START* in response to V.sub.SENSE increasing above V.sub.REF and the end time t.sub.END* in response to V.sub.SENSE decreasing below V.sub.REF. As illustrated in graph 400, the half-sinusoid V.sub.SENSE has a more gradual slope than a square wave and may not change above or below V.sub.REF until some period of time after t.sub.START or before t.sub.END.

(10) Thus, the inaccurate t.sub.START* and t.sub.END* introduce error into calculations using them, for example distance calculations based on the time delay between transmission of a laser pulse and detection of its reflection from the environment. Further, the accuracy of t.sub.START* and t.sub.END* is dependent on the slope and the peak amplitude of I.sub.PULSE, which then necessitates additional calculations to compensate for changes in the peak amplitude of I.sub.PULSE. While this method for determining t.sub.START and t.sub.END may be acceptably accurate when V.sub.SENSE is a rectangular pulse, it may not meet the one ns or less accuracy required when V.sub.SENSE is a non-rectangular pulse such as a half-sinusoid.

(11) To more accurately and consistently determine t.sub.START or t.sub.END of I.sub.PULSE, some systems calculate the derivative with respect to time V.sub.SENSE of V.sub.SENSE, shown in graph 450, which experiences sharp increases from and to zero at t.sub.START and t.sub.END of I.sub.PULSE due to the change in slope of V.sub.SENSE. The systems then compare the derivative V.sub.SENSE to a different predetermined threshold V.sub.REF2. The dramatic change in the value of V.sub.SENSE at t.sub.START and t.sub.END of I.sub.PULSE reduces error in the determined start and end times t.sub.START* and t.sub.END* and removes the dependency on the peak amplitude of I.sub.PULSE and the chosen value of V.sub.REF2. Some systems set V.sub.REF2 to zero, which allows them to also determine the time t.sub.MAX at which I.sub.PULSE reaches its peak amplitude. However, calculation of the derivative V.sub.SENSE emphasizes noise relative to V.sub.SENSE and often necessitates filtering to reduce the noise. The introduced noise and additional filtering steps further complicate the system and degrade system performance and accuracy. The process of determining when a threshold is crossed and the sign of the slope of the crossing is known by various terms such as edge detection, zero-crossing detection, and threshold crossing detection. It is generally accomplished with a combination of comparators and digital logic. Such circuits and methods are generally recognized by those skilled in the art.

(12) FIG. 5 shows a current measurement circuit 500 according to an embodiment of the present invention. A pickup coil 520 is placed near conductor 510 carrying the current pulse I.sub.PULSE such that pickup coil 520 detects the time-varying magnetic field induced by I.sub.PULSE. The leads 524 and 528 are loaded with an impedance much larger than the coil impedance within the frequency band of interest, causing the voltage at the leads 524 and 528 to be proportional to the derivative of the magnetic field strength, which is itself proportional to I.sub.PULSE. The voltage at the leads 524 and 528 can be amplified by an optional amplifier 530 or output directly as the derivative V.sub.SENSE, illustrated in graph 450 in FIG. 4B, which is proportional to the derivative of I.sub.PULSE:

(13) $V_{\mathrm{SENSE}}^{}=K\frac{d{I}_{\mathrm{PULSE}}}{dt}$
where K is a constant dependent on characteristics of pickup coil 520 such as the area of the loop, the number of turns, or the position of the turns with respect to the conductor 510 carrying I.sub.PULSE.

(14) V.sub.SENSE, either from leads 524 and 528 or from the output of the optional amplifier 530, is provided to a threshold crossing detector 540 which also receives the reference voltage V.sub.REF2 and generates an output signal 550. By appropriately setting V.sub.REF2, output signal 550 can be indicative of t.sub.START, t.sub.END, or t.sub.MAX without use of a circuit or method to calculate a derivative and the resulting noise amplification. Current measurement circuit 500 also avoids the area, power consumption, and component cost associated with a high-speed analog-to-digital converter for sampling V.sub.SENSE in order to digitally calculate t.sub.START and t.sub.END.

(15) In some embodiments of the present invention, an integrator 560 can be used to integrate V.sub.SENSE to obtain V.sub.SENSE for other calculations:

(16) $V_{\mathrm{SENSE}}=K\frac{d{I}_{\mathrm{PULSE}}}{dt}={\mathrm{KI}}_{\mathrm{PULSE}}$

(17) In this example, V.sub.SENSE is provided to a sampler or sample and hold circuit 570 which also receives t.sub.MAX (from output 550) and generates an output signal 580 which can be used to measure the pulse current.

(18) The pickup coil 520 does not include a magnetic core and may be a Rogowski coil. For very large amplitude I.sub.PULSE, the resulting magnetic field is very large, and fewer turns and a smaller loop area, even a single loop in proximity to the conductor 510 may be sufficient to detect the magnetic field. The magnetic field for a very large I.sub.PULSE is much larger than the magnetic fields for currents other than I.sub.PULSE or ambient magnetic fields, such that the error introduced by the other magnetic fields can be ignored and the pickup coil 520 need not encircle conductor 510. The turn or turns of the pickup coil may be arranged to maximize the total magnetic flux resulting from the current in conductor 510 to be measured through the entirety of the coil while minimizing the total flux from other sources. Because the pickup coil 520 does not include a magnetic core, the pickup coil 520 does not saturate at high currents or experience bandwidth limitations based on the permittivity of a magnetic core. Pickup coil 520 also avoids the thermal drift associated with semiconductor components such as Hall sensors and shunt resistors.

(19) The pickup coil 520 can be monolithically integrated into a semiconductor die because it may have a smaller loop area and fewer turns than a Rogowski coil and need not encircle the conductor 510. Similarly, the pickup coil 520 can be placed in strategic locations near loads or bus capacitors. In addition, the current measurement circuit 500 is isolated from the main driver signal chain and adds negligible impedance to the main driver circuit such that current measurement circuit 500 does not substantially affect operation of the main driver circuit. The isolation between the current measurement circuit 500 and the main driver circuit allows the current measurement circuit 500 to sense both positive and negative currents even in circuits with a zero volt minimum supply voltage, which simplifies zero crossing detection for V.sub.SENSE.

(20) FIG. 6 illustrates a portion 600 of a current measurement circuit similar to current measurement circuit 500 according to an embodiment of the present invention. The integrated circuit die 640 connects to other substrates such as a mounting substrate with terminals such as terminal 650. The integrated circuit die 640 includes the remainder of the current measurement circuit, such as the integrator, threshold crossing detector, and sample and hold circuit. The portion 600 includes part of the pickup coil 620 formed by die terminals 624 and 628 and an external conductor 626 that may form part of a mounting substrate. The mounting substrate that includes conductor 626 may also include conductor 610 carrying the current pulse I.sub.PULSE. The terminals of pickup coil 620 may be in the integrated circuit die 640 itself and may be connected to the remainder of the current measurement circuit.

(21) The above description and drawings are only to be considered illustrative of specific embodiments, which achieve the features and advantages described herein. Modifications and substitutions to specific process conditions can be made. Accordingly, the embodiments of the invention are not considered as being limited by the foregoing description and drawings.