The benefits of compact magnetic core-based sensors for high-power EV traction inverters
As the electric vehicle (EV) market grows ever more competitive, vehicle distance and speed of charging are vital. Like all EV components, high-power integrated traction inverters — which convert DC into AC and capture energy for the vehicle — need to combine superior performance with being as small and light as possible.
To meet the high demands of the e-mobility market, engineers are faced with several challenges when it comes to current sensing, as Sofiane Serbouh, Product Manager of Large Drives at LEM, explains.
“As the market for EVs continues to grow, so do the pressures for manufacturers to produce vehicles that are capable of being driven longer distances before they need to be charged again at affordable cost,” she explains. “A high-power integrated traction inverter is vital in helping designers within the e-mobility sector to achieve these goals. As such, the current sensor must offer high integration and accuracy levels while being affordable and capable of operating reliably in harsh and demanding environments.”
Magnetic core-based sensors
Coreless current sensors represent a promising solution as they can enable smaller and lower cost components to be used, however there are still many hurdles to overcome with this technology before it can be widely adopted by the market, such as strong variation of the magnetic field, inability to move them after assembly, and question marks over accuracy. Until these technical barriers can be overcome, fully calibrated current sensors with a magnetic core will remain the preferred means of achieving highly accurate current measurements in EV traction inverters.
“The benefits of these sensors include the ability to concentrate and amplify the magnetic field so that sensing can take place with less output noise and with measurements protected from external distances,” says Serbouh. “This not only ensures a high signal-to-noise ratio over a broad bandwidth but it also delivers stable and reliable measurements thanks to the magnetic core’s calibration, the busbar and the Hall-effect based ASIC.”
Introducing the HSTDR
One issue with magnetic core-based sensors is their tendency to be bulky, which presents challenges in terms of integration at inverter level. To solve this, LEM has developed a compact and affordable current sensor with a magnetic core that is designed to offer reliable current measurement over a wide range of applications. The HSTDR single-phase current sensor with integrated busbar is 75% smaller than traditional current sensors with an opening hole, with its small footprint offering greater flexibility to designers working within the constrained space availability of a traction inverter box.
“It was important that the restriction on the integrated busbar was able to ensure mechanical stability while at the same time eliminating unnecessary power loss and heat dissipation – the HSTDR design keeps this restriction to less than 20µΩ allowing to drive up to 700A continuously,” explains Serbouh. “Also, because the magnetic core sits directly behind the busbar, there had to be an isolation barrier between the electronic and the magnetic core. This was important to create sufficient clearance and creepage distance for an EV’s 800V battery system. The third design consideration was to ensure the sensor’s output was unaffected by high switching speeds (dV/dt), which was important since the sensor would be placed in noisy and harsh environments – the HSTDR is capable of recovering with 3.50s dV/dt in the range of 20kV/0s typical for SiC inverters.”
Weighing around 50% less than other sensors on the market, the HSTDR can handle vibrations of up to 10g, and in terms of accuracy provides global error over temperature and lifetime of less than 3.5% over a dynamic range up to 1500A. The sensor also helps to ensure accurate torque control by delivering consistent performance over a range of frequency levels with minimal part-to-part phase shift dispersion.
“There are also substantial benefits in terms of productivity and time-saving by having within a single package a fully calibrated sensor that includes core, busbar and sensing element,” adds Serbouh.