Signal Chain Basics #134: Methods to precisely measure high currents

As vehicles become more electrified, it becomes more critical to accurately monitor the current in many subsystems to ensure performance as well as long-term reliability. Accurate current sensing is critical for essential operations such as motor control, DC/DC converters, battery monitoring, onboard chargers or traction inverters. In many cases, the current being measured is hundreds of amperes; in many of the newer electric vehicles, the voltage bus monitored can reach as high as 800V.

The performance of any current-sensing solution depends on measuring system specifications such as accuracy, bandwidth, temperature drift and linearity. Designing a system that satisfies all of the required specifications is a challenging task. For more on this topic, I recommend reviewing the “Getting Started with Current Sense Amplifiers” video series from Texas Instruments.

Shunt-based measurements

The limitation of 100A is really about the power dissipation of the shunt resistor itself. As defined by Ohm’s law, Equation 1 calculates the power dissipated by a resistor:

As the current increases, the power dissipated increases exponentially, which means that you need a smaller-value shunt resistor. This presents challenges, however. First, as the value of the resistor decreases, the physical size of the shunt resistor increases, presenting a problem for space-limited applications. Typically, the lower the value of the shunt resistor, the more it will cost. Finally, as the shunt resistor value decreases to accommodate the high end of the current range, there is a corresponding decrease in accuracy at the low end of the range due to the offset error of the amplifier, which sessions 1.4 and 2.3 of the aforementioned TI video series address.

Most current-shunt amplifiers have a maximum common-mode voltage range of 100V or less. For applications within this voltage range, there are ways to enable shunt-based measurements at these high currents to accommodate the high-power dissipation. The simplest method is to parallel multiple higher-value shunt resistors.

Equation 2 calculates the equivalent resistance, where Rn is the value of each of the individual resistances:

Using parallel resistors divides the power dissipation across the resistances, lowering the requirements for any one resistance by a 1/n factor. The primary drawback of this method is a reduction in accuracy, especially over temperature, due to the accumulation of resistor tolerances as well as the mismatch of the drift performance.

A second method is to use a portion of the bus bar as the shunt. Low-drift alloy materials, such as Manganin, enable a stable temperature measurement using a high-performance current-sense amplifier. Texas Instruments’ Automotive, mA-to-kA Range, Current Shunt Sensor Reference Design features the INA240, a high-precision current-sense amplifier in combination with a high-performance analog-to-digital converter, the ADS1259, to provide a solution capable of three decades of current range with accuracies better than 0.3%.

High-voltage systems

As mentioned, many of these systems involve voltage rails well over 100V. Difference amplifiers offer common-mode voltage ranges in excess of 100V; however, they often are unity gain amplifiers, which presents issues when trying to measure a wide current range.

In addition to supporting high voltages, many of these systems should isolate the monitoring system from the system being monitored. There are two methods to accomplish this, depending on the current range. When the current range is relatively low (<100A), you could implement an isolated amplifier, dedicated to shunt-based current sensing such as the Texas Instruments AMC1301 (shown in Figure 1).

Figure 1

Block diagram for the TI AMC1301 isolated current-sense amplifier

This solution allows for high-precision measurement while supporting common-mode voltage ranges up to thousands of volts.

When the combination of space, power and accuracy are such that a shunt-based solution will not work, then a magnetic solution is the alternative. Magnetic solutions work based on Ampere’s law or Faraday’s law, which state that a current flowing through a conductor will produce a magnetic field proportional to the current level. By measuring the magnetic field strength and knowing the mechanical arrangement, you can then determine the current level. There are three primary magnetic technologies used for current measurements.

The first is a current transformer (CT) (Figure 2), typically used to measure AC currents. With a CT, two coils are wrapped around a magnetic core. The primary current of interest is wrapped around one side of the core, while the secondary coil is wrapped around the other side. A current-sense amplifier used on the secondary side measures the current flowing.

The ratio of the windings will determine the multiplication factor that you must apply to the measured secondary current to determine the primary current. A CT is best when your accuracy requirements are not strenuous, especially over temperature, as the magnetic core and copper windings are the primary sources of error and they all drift fairly significantly over temperature.

Figure 2

Example of a current transformer showing the ratio that steps down the current in the primary coil to the secondary coil for measurement

The second magnetic methodology is Hall effect sensors. Hall sensors produce a voltage proportional to the magnetic flux density, which is perpendicular to the face of the device. Typically, a magnetic core is placed around the current-carrying conductor and the Hall sensor is placed in a gap in the core. The core acts as a magnetic concentrator or magnifier to produce a “signal” field that is much greater in amplitude than any nearby “noise” sources to maximize the signal-to-noise ratio. In many high-current cases, the conductor in these implementations is a bus bar (Figure 3), around which the magnetic core is positioned.

Figure 3

Examples of bus bars for conducting high currents

Fluxgate technology is the third magnetics method. Fluxgate and the Hall effect are similar in that they produce a voltage proportional to the current flowing through the conductor. The primary difference is the typical sensitivity range of the devices. A Hall sensor has sensitivities from low milli-Teslas up to a Tesla, while Fluxgate sensors range from nano-Teslas up to milli-Teslas. This results in Fluxgate technology offering a more accurate solution over a wider current range.

The Texas Instruments DRV425 fully-integrated Fluxgate sensor enables an alternative arrangement for measuring current flowing through a bus bar, as shown in the Texas Instruments +/-100A Bus Bar Current Sensor Using Open-Loop Fluxgate Sensors Reference Design (Figure 4).

Figure 4

The Texas Instruments +/-100A Bus Bar Current Sensor Using Open-Loop Fluxgate Sensors Reference Design

Summary

The electrification of vehicles is creating demand for precision current sensing. Such systems in a vehicle include battery-management systems, motor currents and so on. Matching the performance requirements to the application needs can be challenging. Choosing one of these methods should provide a cost-effective solution.

References

1. Automotive Shunt-Based ±500A Precision Current Sensing Reference Design, TI Design (TIDA-03040), July 2017.

2 comments on “Signal Chain Basics #134: Methods to precisely measure high currents”

1. Pesky Varmint
March 20, 2018

Dan,

There is one other useful item for a high current sensing tookit, particularly suited for high gauge cables. The concept is really targeted at OEM's that could procure a purpose-built cable:

https://www.edn.com/design/automotive/4458898/Use-copper-to-temperature-compensate-high-current-measurements

Jerry Steele

Applications Manager, ON Semiconductor

2. RituGupta
October 11, 2018

I've actually read a little bit about how some electric vehicles catch fire too easily because of the overload to the systems. If you think about it on a smaller scale and talk about rechargeable batteries, not very many people even know how to properly care for those and the batteries are overrun and overworked in no time. What more when it comes to such a glorifies appliance? I don't have high hopes for the future of electric cars unless we figure out how to better maintain its batteries!

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