Part I: How current sensing impacts EVs

The need to transition from gasoline and diesel (fossil fuels) powered vehicles to electric vehicles (EVs) to reduce carbon-dioxide levels has revealed many controversial issues. Common issues include implementation timing, required infrastructure (including fast-charging multi-vehicle systems), legislated rather than consumer-based adoption, ongoing system enhancements, and sources of new critical raw materials.

One of the more straightforward yet open technical issues for design engineers to resolve is the current sensing technology. Safely and effectively switching the current levels involved in EV propulsion (motor control), voltage conversion, battery management, and charging systems (including the onboard charger, home charging, and the infrastructure) all require current sensors.

Batteries are the sole power source for EVs and have a limited lifetime. To extend their life, a battery-monitoring system estimates the state of health (SOH), state of charge (SOC), and state of function (SOF), which all use current and temperature sensors — often called IVT (current, voltage, and temperature) sensors.

According to Research Reports World, the EV battery current sensor market alone has a market size valued at USD 1310.0 million in 2021, increasing at a compound annual growth rate (CAGR) of 12.17% during the forecast period, and reaching USD 2610.0 million by 2027.

The good news is system designers have choices that do not require governments to dictate how they sense current or involve exotic materials.

Among these choices are:

Resistive (current) shunts
Hall-effect sensor designs
Current transformers
New technologies, such as magnetoresistance and diamond quantum sensors

System design issues for current sensors include high versus low-side current sensing, interface circuitry, bandwidth, response time, shielding, flux concentrator design, signal-to-noise ratio (SNR), crosstalk, programmability, and more.

The current shunt
A standard current shunt is the simplest solution to sense current. It’s a high precision, low value, high-power resistor. Current routed through the shunt results in a voltage drop proportional to the current (E=I*R).

Figure 1. A current shunt is one of the easiest solutions to sense current.

However, the voltage across the shunt needs to be amplified, isolated, and measured to determine the current level. Since the shunt resistor is in series with the load, the voltage drop results in a power loss, so increasingly smaller resistors values are used. This means that the analog front end (AFE) for amplifying and compensating the voltage drop must be extremely accurate to measure such small values. The AFE can also provide isolation for improved safety.

Hall-effect sensors
Hall-effect current sensors measure the magnitude of the magnetic field around a current-carrying conductor. Unlike shunts that are directly connected to the high current, Hall-effect devices are isolated from the load. With their inherent galvanic insulation, Hall-effect current sensors can measure both dc and ac currents, have low power loss, and are thermally decoupled from the power electronics.

Figure 2. Hall-effect current sensors can measure dc and ac currents with low power loss.

In a Hall-effect current sensor, the Hall element is mounted in the gap of a ferrite magnetic core (flux ring or collector) placed around the current conductor. The AFEs in commercially available devices include amplifiers to boost and convert the Hall voltage to a usable level and other circuitry. Differential current sensor measurements using two Hall cells provide high accuracy even in a noisy environment where crosstalk can occur from adjacent current lines or magnetic stray fields.

Part 2 will discuss other technologies for current sensing in EVs.

References

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