How High-Voltage Current Sensing in HEV/EVs Can Solve the Challenge

Electrification has created a new paradigm for automotive powertrains—whether the design is a hybrid electric vehicle (HEV) or an electric vehicle (EV), there are always new design challenges to solve. In this technical article, I want to highlight some of the key challenges of high voltage current sensing and share additional resources to assist and simplify your design process.

High Voltage, High Current: (>200 A or more commonly 1,000 A) High voltage (≥400 V) all-electric systems are designed to reduce the current draw of the traction system that drives the vehicle. This requires an isolation solution so that the "hot" high voltage side can provide current measurement to the "cold" side (connected to a low voltage ≤5-V microcontroller or other circuit). High currents are problematic when measured with a shunt resistor due to I2R power dissipation.

To use a shunt in these situations means you must choose a sub-100-?Ω shunt resistor, but these resistors tend to be larger and more expensive than the more common milliohm resistors. Another option is to use a magnetic solution, but these magnetic solutions are less accurate and have higher temperature drift than shunt-based solutions. Overcoming these performance drawbacks greatly increases the cost and complexity of the magnetic solution.

Learn more with these design resources:

Design considerations for dual DRV425 busbar applications.”

· Busbar operation principle. ”

High voltage, low current (>400 V and <500 A)

Additionally, high voltages require an isolation solution. From a current perspective, anything below 100 A is essentially a shunt-based solution. Between 100 A and 500 A, the choice between a shunt or magnetic solution is a trade-off between cost, performance, and solution size. The white paper describes:

Precision measurements on 48-V rail, low currents (<100 A)

The main design challenge for a 48-V rail is meeting the survivability voltage required for your requirements, which may be as high as 120 V. In some 48-V motor systems, high-precision current measurement is required to achieve peak motor efficiency. These motor systems may include traction inverters, electric power steering systems, or belt starter generators. In-line measurement can show the most accurate actual motor current, but it is also very challenging due to the presence of high-speed pulse width modulation (PWM) signals, as described below:

Low-drift, high-accuracy, in-line motor current measurement with enhanced PWM rejection.”

For non-motor 48-V systems, such as DC/DC converters or battery management systems (BMS), achieving bidirectional DC current measurement is more critical than achieving switching performance, as described below:

High-side bidirectional current sensing circuit with transient protection.”

High common-mode voltage requirement to eliminate low-side induction

Low-side current sensing relaxes some amplifier requirements: the input does not need to see high voltage because the common mode for low-side sensing is ground - 0 V.

The amplifier’s common-mode voltage range must include 0 V to measure on the low side. If the application is motor low-side phase current measurement, the amplifier must have a high slew rate to adjust for the opening and closing of the switch, as described below:

Measuring multiple currents in BMS

High-precision, multi-segment current measurement (from mA to 1 kA) is a significant challenge to be solved in a single solution. Magnetic solutions are not good at measuring low currents because of their high offset levels and significant drift. Due to the very low differential input voltage levels, shunt-based measurements require very low offsets to be able to measure low currents across sub-100-μΩ sub-shunt resistors.

For example, a BMS may want to measure ±1,500 A. For a bidirectional measurement with a 0-A output voltage and a ±2.5-V output swing with a gain of 20, the maximum input voltage is ±125 mV. This results in a shunt resistor value of ≤ 83 Ω. This shunt will drop only 8.3 Ω at 100 mA, which means you need an amplifier system with very low offset to measure this level. If the system has an offset of 1 Ω, the error at this level is ~16%.

Current sensing in solenoid valves allows for smoother actuation

Many automotive applications use proportional solenoids, but when it comes to high-voltage current sensing, they are primarily used in automatic transmissions. Proportional solenoids provide a smooth ride when shifting gears or running hydraulic pumps. The actuation capabilities of a solenoid are primarily determined by two factors: solenoid actuation and solenoid position sensing.

High-precision current measurement enables precise closed-loop control of the electromagnetic plunger position.

Current sensors in solenoid valve applications follow the shunt principle. A pulse width modulated signal can flow across the solenoid valve via a milliohm shunt. This milliohm shunt is either integrated inside or outside the current sense amplifier, depending on the current range.