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