High-bandgap semiconductors based on as GaN and SiC (silicon carbide) technologies are now a mainstay for power system designers. Among the benefits of these new components are high speeds at which they can switch high current.
This translates into smaller filtering components, greater power efficiency as well as smaller, lighter finished products.
These benefits come at the cost of a need for being able to measure that switched current in real time and to communicate the measurement to the rest of the system. Devices based on anisotropic magneto resistance (AMR) are one way to do it.
How Do AMR Current Sensors Work?
According to the Biot-Savart Law, whenever current passes through a wire a proportionately strong magnetic field is generated. Exploiting this fact of nature to build an IC that measures current requires three steps:
A “wire” carrying the current to be measured is built into the IC, or positioned directly below it. That generates a magnetic field directly proportional to the current.
On another level of the IC, there must be a means to convert that magnetic field into voltage. Anisotropic magneto resistive (AMR) technology makes use of permalloy, an alloy of nickel and iron whose resistance changes proportionally when presented with a magnetic field. Via analog circuitry, the resistance is then converted to a voltage.
The voltage generated is directly proportional to the original current. It then needs to be converted into a useable format.
The only contact the sensor has with circuit whose current it measures is though magnetism. Thus, in the same manner as a transformer, the chip is electrically isolated.
Semiconductor manufacturers have risen to the challenge, and multiple manufacturers now offer components to take advantage of this technology.
MCx1101 from ACEINNA
Members of ACEINNA's MCx1101 family are touted for applications such as server farms, the IoT, telecom power supplies, EV charging, inverters and motor control and more.
Image from ACEINNEA
They are available in 3.3-volt and 5.0-volt versions with current carrying capacities ranging from 5 amps to 50 amps, AC or DC. And operate at a 1.5 MHz bandwidth. Members of this family of current sensors can all detect an overcurrent condition in 200 nanoseconds.
The units have a primary current path resistance of 0.9 milliohms, 4.8 kV isolation, and typically consumes 4.5 mA. They come in a SOIC-16 package and can operate in a temperature range of -40 to +105°C.
According to ACEINNA, "These devices are factory-calibrated to achieve low offset error and provide a precise analog voltage output that is linearly proportional to the conduction current... The AMR sensor device structure is designed to eliminate sensitivity to stray and common mode magnetic fields. Due to the inherently low output noise of ACEINNA’s sensor technology, additional filtering is not required to reduce noise that reduces accuracy at low-level currents in systems with dynamic load profiles."
Image from ACEINNA
ACEINNA provides the EB0013 evaluation board to help designers get up and running with this family AMR current sensors.
The CFS1000 from Sensitec
This CFS1000 can handle up to 1000 amps of DC, pulsating DC or AC up to 500 kHz. There is no current path through the device. Rather, it employs an external busbar. Below is a conceptualization of the device measuring power (a function of current) as it flows into an EV.
Image from a Sensitec video
Of course, as the busbar is not, in this case, matched to the AMS element, doing so is part of the OEM’s design process. Sensitec offers it’s own free simulation tool—“Calc-U-Bar”—to simplify the process.
This CFS1000 includes fast overcurrent detection with a tunable threshold. It is an AEQ-Q100 qualified device.
IST8110 from Isentek
The IST8110 series from Isentek are another AMR-based family of current sensors. They can measure DC and AC signals up to 500 kHz. The units communicate via I2C digital output. The chip itself is a 7.0 x 7.0 x 1.0 mm 8-pin LGA package.
The series is composed of three modular packages, the ICSM1015, ICSM1030, and ICSM1050 whose primary current measurement range is ±15A、±30A、and ±50A, respectively. Accuracy is within 2% between -40°C and 125°C.
AMR Technology in Flowmeters
AMR has wide application in liquid flow meters. The principal is illustrated in the picture below, taken from a Murata video.
A rendering of an AMR-based current sensor. Altered screengrab from Murata
The pinwheel structure rotates at a rate in proportion to the amount of water that passes. The red and blue structure, attached to the flywheel, is magnetic. As it passes under the two AMR-based chips illustrated, the chips register a given amount of water flow. If the direction of flow changes, external circuitry will register a changing pulse pattern.
These types of devices are electrical in nature, and are far more robust than equivalent mechanical units. They are also more accurate. And, because the output measurement is an electrical signal, it can more easily connect to an IoT node.
Other Ways to Measure Current
Other forms of measurement are tunnel magnetoresistance (TMR) and giant magnetoresistance (GMR). Kate Smith describes the physics behind them and provides descriptions of commercial products based on that science in “Magnetoresistance in Magnetic Field Sensors: Applications for TMR Sensors.”
The Hall Effect
The Hall Effect comes into play when a magnetic field is applied perpendicular to a flow of electricity. The result is a charge separation, with a measurable “Hall Voltage” generated as shown below.
The Hall Effect. Image from the National Programme on Technology Enhanced Learning (NPTEL)
The Hall effect can be used to produce ICs that measure current. An example is the MLX91207 from Melexis. Note that the bandwidth of this device is 70 KHz. Hall effect current sensors, typically, support lower bandwidths than AMR-based devices.
A resistor of tiny value is inserted in series with the line whose current is to be measured. As I = E/R, the current can be measured with the addition of simple components. The disadvantage here is the inherent lack of isolation between current be measured and the devices doing the measurement.
What current sensors do you reach for in your designs? Share your expertise in the comments below.