Method and apparatus for current sensing and measurement

A method and apparatus for current sensing and measurement employs two cascaded MOSFET current mirrors, wherein the mirrored current leaving the first current mirror is fed to the input of the second current mirror. Each current mirror contains a high current MOSFET and a low current MOSFET, connected source-to-source and gate-to-gate. The MOSFETs are matched so that drain-to-source current flowing in the high current MOSFET is proportional to the drain-to-source current flowing in the low current MOSFET. The ratio of high current to low current for each current mirror is M, where M is 100 or less. Voltage biasing networks are employed to maintain constant drain-to-source voltages for both MOSFETs in each current mirror.

BACKGROUND

Electronic current sensing and measurement is utilized in a wide variety of electronic devices. Current sensing and measurement methods and devices can be divided into two basic modes, voltage-based (indirect) and current-based (direct).

FIG. 1is a simplified schematic of a typical voltage-based current sensing and measurement circuit100. A current measurement resistor Rm102is placed in series with a load (not shown) in which the current is to be measured. A differential amplifier104is utilized to measure the voltage drop across Rmand the current is computed from the measured voltage drop and the known value of Rm.

This technique described above with respect to voltage-based current sensing and measurement circuit100has a number of drawbacks for measurement of large currents or current ranges having a large dynamic range (e.g., the range of the smallest current to be measured to the largest current to be measured). For large currents (e.g. on the order of several amperes) Rmneeds to be as small as possible to minimize parasitic voltage drop and dissipated power. Manufacturing very low resistance values accurately is very difficult and expensive, however, particularly if it must be integrated on a monolithic integrated circuit. For very small currents, the limit of measurement will be determined by the D.C. parameters of differential amplifier104, particularly the amplifier's offset voltage and, to a lesser extent, the input offset currents. As a result, the dynamic range will be limited to three or four orders of magnitude.

Current-based current sensing and measurement apparatus typically rely on what is commonly known as a “current” mirror.FIG. 2is a simplified schematic200of a bipolar current mirror200that uses matched transistors to create an “image” current, or scaled replica, of the current to be measured, Iin. With bipolar current mirror200, the current to be measured flows through “n” matched transistors204, all having common emitter, base, and collector connections. Although three transistors204are illustrated inFIG. 2, n can be any number. Matching of the emitter base voltage characteristics assures that the current to be measured is equally shared by all the transistors204. With the bipolar current mirror200, an additional matched transistor202sharing the emitter and base connections of transistors204is employed to create the mirrored current Im, which is approximately 1/n of the current Iin.

One major drawback of bipolar current mirroring is excessive power dissipation at high current values. Since the typical emitter-base voltages for bipolar transistors are on the order of 0.6 to 0.7 volts, a current level of, for example, 1 ampere will result in a power dissipation of 600 to 700 mW. This high power dissipation creates difficulties for monolithic circuitry (e.g. integrated circuits), requiring expensive packaging, large dies sizes, and perhaps external heat sinking. As a result, current limits for bipolar current mirrors are typically no more than about 10 mA.

U.S. Pat. No. 6,888,401, incorporated herein by reference, describes a MOSFET-technology current mirror.FIG. 3is a schematic of a MOSFET current mirror300utilizing matched MOSFETs (metal oxide semiconductor field effect transistors)302and304as described therein. The current Iinto be measured flows through MOSFET302, which is designed to handle M times the current of MOSFET304, at the same gate-to-source voltage. Thus, a current Iinflowing through MOSFET302induces a mirror current Im=Iin/M in MOSFET304.

InFIG. 3, operational amplifier308, in conjunction with bias control block312, sets the output voltage of the MOSFET current mirror300by biasing the gate voltage of MOSFETs302and304such that the drain-to-source voltage of MOSFET302remains constant. This assures minimum power dissipation while keeping MOSFET302in linear operation. Operational amplifier (“op-amp”)310and MOSFET306keep the drain-to-source voltages of MOSFETs302and304equal to each other, within the error of the input offset voltage of op-amp310. This assures tight matching of MOSFETs and reduces errors that may result if the drain-to-source voltages are allowed to vary.

While the performance of the circuit ofFIG. 3is an improvement over the voltage-based version ofFIG. 1and the bipolar current mirror ofFIG. 2, it still exhibits a degree of inaccuracy at large M values of about 1000 or greater. For these large M values, accuracy is typically limited to 8-10%, primarily due to monolithic circuit layout issues which affect the matching of MOSFETs302and304, proximity effects, interconnect resistance, and package stress.

Improvements in the accuracy of the MOSFET current mirror shown inFIG. 3can be made if the gain M can be reduced. At reduced gains, the matching of MOSFETs302and304can be more precise, and the current measurement accuracy can be significantly improved. However, reducing M can create other problems, particularly when trying to measure currents on the order of a few amperes.

These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.

SUMMARY

In an embodiment, set forth by way of example and not limitation, an electrical current sensing and measurement apparatus includes a first current mirror having an input terminal, a high current output terminal, and a mirrored current output terminal, and a second current mirror having an input terminal, a high current output terminal, and a mirrored current output terminal, wherein the mirrored current output terminal of the first current mirror is connected to the input terminal of the second current mirror.

In another embodiment, set forth by way of example and not limitation, an electrical current sensing and measurement apparatus includes a first current mirror having an input terminal, a high current output terminal, and a mirrored current output terminal, a second current mirror having an input terminal, a high current output terminal, and a mirrored current output terminal, and a third current mirror having an input terminal, a high current output terminal, and a mirrored current output terminal, wherein the mirrored current output terminal of the first current mirror is connected to the input terminal of the second current mirror, and the mirrored current output terminal of the second current mirror is connected to the input terminal of the third current mirror.

In another embodiment, set forth by way of example and not limitation, a method for measuring electrical current includes providing a first current mirror having an input terminal, a high current output terminal, and a mirrored current output terminal, providing a second current mirror having an input terminal, a high current output terminal, and a mirrored current output terminal, and feeding an electrical current leaving the mirrored current output terminal of the first current mirror to the input terminal of the second current mirror.

These and other embodiments, features and advantages will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIGS. 1-3were discussed with reference to the prior art.FIG. 4is a schematic diagram of a current sensing and measurement apparatus400incorporating two cascaded MOSFET current mirrors402and404.

Current mirror402includes a MOSFET403aand a MOSFET403b, connected gate-to-gate and source-to-source. Current mirror402input terminal is connected to the two source terminals of MOSFET403aand MOSFET403b. Current mirror402has a high current output terminal connected to the drain connection of high current MOSFET403b, and a mirrored current output terminal connected to the drain of low current MOSFET403a. The gain factor M1is the ratio of the high current output at the drain of MOSFET403bto the mirrored current output at the drain of MOSFET403a. A voltage bias or control may be applied to the common gate connection of MOSFET403aand MOSFET403b.

Current mirror404includes a MOSFET405aand a MOSFET405b. Current mirror404input terminal is connected to the source terminals of MOSFET405aand MOSFET405b. Current mirror404has a high current output terminal connected to the drain connection of high current MOSFET405b, and a mirrored current output terminal connected to the drain of low current MOSFET405a. The gain factor M2is the ratio of the high current output at the drain of MOSFET405bto the mirrored current output at the drain of MOSFET405a. A voltage bias or control may be applied to the common gate connection of MOSFET405aand MOSFET405b.

Since the mirrored current leaving current mirror402is fed to second current mirror404, the mirrored current Imleaving current mirror404will be reduced by approximately the product of both current mirror gains, or about M1×M2, compared to the current to be measured (Iin). The gain factor is given by:
Im=Iin/(M1*M2+M1+M2); typicallyM1*M2>>M1+M2

Since both M1and M2are much less than M for the single stage current mirror ofFIG. 3(for an equivalent overall gain), the accuracies of M1and M2are significantly better than that of M. As an example, a gain value M of 1000 may typically have a precision of 8-10%. For an overall gain of 1000, the dual stage cascaded current mirrors would require M1=M2=√M˜32. At lower M1and M2values the current measurement accuracy can attain precisions of 1-2% or better.

Returning toFIG. 4, MOSFET406provides a current path for the measured current leaving the first current mirror402. It also maintains a voltage drop across the drain-to-source for MOSFET405aand MOSFET405bin the second current mirror404. MOSFET408is part of a regulator that maintains the drain-to-source voltages for MOSFET405aand MOSFET405bat the same level and serves a similar function as MOSFET306ofFIG. 3.

The principles illustrated by the dual stage cascaded current mirrors ofFIG. 4can be extended to any number of additional stages. The actual number of stages one may wish to employ will be determined more by practical matters such as circuit complexity, voltage drop, power dissipation, and diminishing returns with respect to overall current measurement accuracy. However, it may be evident that two stages provide sufficient accuracy for many applications, since there is a significant improvement in measurement accuracy between M factors of 1000 or greater and M factors on the order of 50-100.

FIG. 5is a schematic diagram of a current sensing and measurement apparatus500incorporating “n” cascaded MOSFET current mirrors. A first stage current mirror502, having a MOSFET510aand a MOSFET510band a current gain factor of M1, provides its mirror current to second stage current mirror504. Second stage current mirror504, incorporating a MOSFET512aand a MOSFET512b, has a gain factor of M2and provides its mirror current to third stage current mirror506. Third stage current mirror506is composed of MOSFET514aand MOSFET514b, having a current gain factor of M3. The pattern continues to the nthstage current mirror508, composed of MOSFET516aand MOSFET516b, with a gain factor of Mn. MOSFETs518,520, and522provide current return paths for measurement currents leaving each of the stages, while also providing voltage regulation to keep the appropriate current mirror drain-to-source voltages at a suitable level. MOSFET506serves a similar purpose as MOSFET408ofFIG. 4. To a first order approximation, the gain factor of all n stages will be approximately the product of all the individual stage gains, so that each stage can provide a relatively low gain for improved overall system accuracy.

FIG. 6is a schematic diagram of a current sensing and measurement apparatus600incorporating two cascaded MOSFET current mirrors and which operates in a similar manner to current sensing and measurement apparatus400ofFIG. 4. Components of current sensing and measurement apparatus600that are analogous to components of current sensing and measurement apparatus400will use the same reference numbers and will not be discussed again in detail for the sake of brevity.

Bias regulation of a current mirror402ofFIG. 6is provided by an operational amplifier (“op-amp”)602and a bias control regulator610(which serves as a voltage source). Assuming op-amp602is ideal, and Vos1is zero, op-amp602will provide a gate voltage to MOSFET403aand MOSFET403bsuch that the drain voltage of MOSFET403b(the high current output of current mirror402) is equal to the bias control voltage from regulator610at the inverting input of op-amp602. For real op-amps, Vos1will be finite but in the order of a millivolt or less for high quality amplifiers.

Bias regulation of a current mirror404ofFIG. 6is provided by an op-amp606and a bias control regulator612. By adjustment of the voltage drop across a MOSFET406ofFIG. 6, an op-amp606maintains the voltage at its non inverting terminal, which is also the drain voltage of a MOSFET405bin current mirror404(the high current output of current mirror404), equal to the voltage at its inverting terminal (within any error offset voltage Vos3). A voltage at the inverting terminal of op-amp606is also the output voltage of bias regulator612.

In the current sensing and measurement apparatus ofFIG. 6, the drain-to-source voltage of the MOSFETs in current mirror402is determined by difference between Vccand the bias voltage from regulator610, and the drain-to-source voltage of the MOSFETs in current mirror404is determined by the difference between the bias voltage from regulator610and the bias voltage from regulator612.

To maintain high current measurement accuracy, it is important to keep the drain-to-source voltages for both MOSFETs in the current mirror at the same potential. The biasing circuits above keep the drain-to-source voltages of the high current MOSFET in each pair fixed (e.g. MOSFET403band MOSFET405b). The remaining two amplifiers keep the drain voltages of both MOSFETs in each mirror at the same potential. The net effect of these two regulation systems is to keep the drain-to-source voltages of each MOSFET in a current mirror regulated at a constant voltage. Op-amp604keeps the potential at its inverting input, connected to the drain of MOSFET403a, equal to (within the error of bias offset voltage Vos2) the voltage of its non-inverting input, connected to the drain of MOSFET403b. It does so by altering the voltage applied to the gates of the MOSFET405aand MOSFET405bin current mirror404. In like manner, op-amp608controls the drain voltages of MOSFET405aand MOSFET405bin current mirror404. The drain of MOSFET405bis connected to the non-inverting input of op-amp608, the drain of MOSFET405abeing connected to the inverting input. Op-amp608alters the gate voltage of MOSFET408, which directly impacts the drain voltage of MOSFET405auntil it is equal to the drain voltage of MOSFET405b. In an example embodiment of the circuitry shown inFIG. 6, M1=40; M2=100; total gain is 1/4140; current accuracy is 1% over a dynamic range of 5 decades, from 40 micro-amps to 4 amperes.

Although various embodiments have been described using specific terms and devices, such description is for illustrative purposes only. The words used are words of description rather than of limitation. For example, it is to be understood that the term “MOSFET” is use generically herein to include various types of field effect transistors (FETs), e.g. IGFETs and MISFETs and equivalents thereof. It is to be understood that changes and variations may be made by those of ordinary skill in the art without departing from the spirit or the scope of various inventions supported by the written disclosure and the drawings. In addition, it should be understood that aspects of various other embodiments may be interchanged either in whole or in part. It is therefore intended that the claims be interpreted in accordance with the true spirit and scope of the invention without limitation or estoppel.