Boston Consulting Group predicts that EVs will be more than half of all light vehicles sold by 2026. Market competition and increasingly stricter environmental regulations mean these vehicles have to become increasingly efficient. And that means efficient powertrains.
Power train designs have a direct influence on EV ranges and driving performance. According to Tesla, if you improve motor efficiency by 8 to 10 percent, range will improve by 15 to 18 percent. The more efficient a motor, the more time an EV will stay on the road.
Efficiency is the name of game
EVs are driving technology innovations across the board. The combination of accurate current sensors and smart MCUs with real-time control reduce latency and improve the accuracy of the motor-control loop, enabling smooth speed and torque transitions. With reduced harmonics distortion, the electrical efficiency and range improve. So do motor vibrations and torque ripple, which help prevent an uncomfortable drive. Traction inverter power density and efficiency allow the integration of various powertrain functions and ultimately increased range per charge.
It is critical to monitor and control the EV Powertrain operation and these devices ensure consistent, improved performance with lower costs. As SkyQuest notes, “The electric vehicle industry has become a significant consumer of current sensors, greatly aided by favorable governmental laws and more remarkable technological improvements.”
These sensors ensure the consistent performance of traction inverters, a cornerstone in the EV powertrain. Any talk of efficiency starts here. Since inverters convert battery DC to AC for the electric motor, the more efficient the inverter, the more range the battery has. These traction inverters – specifically high power inverters 100A up to 1000A – are essential for EV’s acceleration and consistent speeds.
In inverters, the 3-phase, full bridge driver converts DC battery voltage to the 3-phase AC voltage. This inverter “control loop” requires high bandwidth current sensors to improve accuracy, and to maximize motor torque and overall motor efficiency. High-side current sensors with fast response times also enable overcurrent protection during a short circuit condition from a motor phase to the system ground node. The requirement is to meet the voltage isolation, > 200 ampere (A) load current, and high bandwidth demands of HEV inverter applications.
BEV, PHEV and HEV
Electric vehicles come in multiple types:
- Hybrid electric vehicles (HEVs) employ an engine and an electric motor with a small/medium battery. The engine takes over when the car reaches a designated speed. A version of these, Mild hybrid electric vehicles, use electric power only for braking and stopping.
- Plug-in hybrid electric vehicles (PHEVs) have a traditional engine and a rechargeable battery. Their larger batteries take a couple of hours to recharge and afford longer travel than HEVs on electric power alone.
- Battery electric vehicles (BEVs) are completely electric and feature a large battery.
The biggest architectural change comes when moving from HEV to PHEV. That is the point where outside power — from the grid, possibly from an outlet at the owner’s house — now enters the vehicle. Receiving alternating current (AC) power from the outlet means the vehicle has to have an inlet, leading to an onboard charger (OBC) that converts the power to direct current (DC) to charge the battery. As the battery powers devices in the vehicle, the power stays DC but requires an inverter to change to AC to power the electric motors.
Closed Loop Sensors
As mentioned, inverter control loops requires high bandwidth current sensors to improve accuracy, and to maximize motor torque and overall motor efficiency. To achieve this, closed-loop sensors have bypassed open-loop, since they add secondary winding to the output. While closed-loop Hall-sensor provide many technical advantages, the complexity and cost makes this solution unattractive for EV inverters.
Hall-effect sensors have traditionally been used in EV inverters employing a flux ring or collector surrounding the contactor’s bus bar or output feeder. These sensors feature isolation between primary and secondary circuits, but suffer from higher hysteresis and temperature drift – not ideal for EV applications.
In addition, to maximize bandwidth and response time, Hall sensors cannot apply the compensation mechanisms for offset and temperature drift. This yields sensors that have limited performance under temperature variation.
This leads to multiple weaknesses: low bandwidth, hysteresis, output drift, and the non-linearity over temperature. As the system degrades, it begins added calibrations to maintain performance. Additionally, the core contributes to a comparatively large size and weight.
The requirements of today’s EV powertrains mean the days of Hall’s ring-flux concentrator are coming to an end, as simpler, coreless, contactless current sensors become the norm for the next generation inverters. To achieve suitable performance, these sensors requires the performance of tunnel magnetoresistance (TMR.)
TMR
TMR is the latest magnetic sensor technology with inherent advantages of being less susceptible to temperature change, while offering both extremely high magnetic sensitivity and high SNR.
Other advantages of this new go-to solution include high Signal to Noise Ratio (SNR), low power consumption, programmable overcurrent detection and fault pin, bidirectional sensing, and high-voltage isolation to ensure safety.
Versatile TMR technology solutions provide high-end performance for demanding applications such as EV Motor Control and can be incorporated into existing designs with better performance while lowering overall total solution cost. Finally, TMR is dramatically simpler than traditional solutions, with its reduced component count, delivering less reliance on the supply chain.
While many legacy hall-sensor companies worldwide are starting to feature TMR-based products in their portfolio due to their higher level performance benefits, all solutions are not created equal. Crocus Technology’s pure-play focus on TMR technology has yielded sensors with premium performance, based on XtremeSense® TMR which provides the highest accuracy, low power, high bandwidth, high sensitivity, temperature stability, low noise, and the smallest size by comparison to other magnetic technologies.
Shields
Contactless current sensors will detect magnetic fields from nearby current carrying conductors, making them susceptible to stray magnetic fields. These fields can be the nemesis of Hall-effect sensors slowing down response time and adding non-linearity error into the system.
A popular solution is to shield the sensor with expensive metal alloys having a high magnetic permeability. These costly and bulky alloy shields don’t only minimize or eliminate the external magnetic field but also redirect the useful magnetic field on the sensor. If not properly sized or spaced, this shielding can equally affect the magnetic field generated by the current conductor and can distort its measurement.
Crocus Technology XtremeSense® TMR sensors provide excellent immunity to stray magnetic fields without the need of a shield required by competing solutions. This TMR sensor has a fixed magnetic reference layer, an insulator and a magnetic sensing layer that follows the external field. The orientation of the ferromagnetic layers’ magnetization is important so electrons can tunnel across the insulator causing a resistance imbalance in the Wheatstone bridge. The result is a sensor with inherently good signal-to-noise-ratio (SNR) and stability across temperature.
To optimize performance, the TMR sensor is placed on a busbar and its current sensing range is adjusted using the busbar’s cross-sectional dimensions of Length, Width, and Thickness, as well as the air gap distance between the sensor and busbar. The spacing distance between the TMR sensor and the busbar is defined as the ‘air gap’, minimizing the air gap will increase the sensing range but will also minimize the voltage isolation.
Cross-Talk
Automotive traction inverters are getting more and more compact, and more and more powerful. Limited space, cross talk from busbars getting closer, EMI noise caused by high voltage fast switching are challenges to address.
Crocus TMR also excels over Hall in Cross-Talk. It’s U-shape (with differential sensing) and Straight Busbar (with Uniform field sensing) solutions were studied alongside a typical Notch Busbar (differential sensing) Hall solution.
This cross-talk study of Crocus CT452, 450 and Hall effect has 3 busbars and 3 sensors with only one busbar carrying current. Tthe Bx,By, and Bz magnetic field components are measured by a primary sensor, right on top of the current carrying busbar, and an adjacent sensor, right next to the current carrying busbar.
The differential Hall-effect sensor studied:
- Signal range of 2.34mT
- Cross-talk: 0.35mT
Showed a Signal-to- Cross-talk Ratio (SCR) of 14.95%
The non-differential CT450 sensor
- Signal range: 26.49mT
- Cross-talk: 0.69mT
Showed a Signal-to- Cross-talk Ratio (SCR) of 2.60%
The differential CT452
- Signal range of: 6.9T
- Cross-talk: 0.005mT
Showed a Signal-to- Cross-talk Ratio (SCR) of 0.07%
In summary, The Crocus TMR has an advantage for cross-talk applications like 3-phase traction inverter in EVs.
Crocus TMR was designed to have low cross-axis sensitivity and has an inherent advantage because of the planar axis of sensitivity. (figure below)
And CT452 with differential sensing means it is designed with > 50dB of common-mode field rejection, and with a U-shaped busbar design that minimizes cross-talk on top of its already built-in low cross-talk axis sensitivity.
Also the Crocus CT452 and CT450 have options for contactless current sensing. The CT452 requires a U-shape busbar with no software compensation needed. The CT450 requires a simple straight busbar and software compensation
Contactless current sensors provide a compact, robust, flexible and economical solution for motor control inverters for HEV/BEV/PHEV applications and are designed to offer current monitoring from 5 to 100 A on PCBs and up to 1000 A on bus bars.
Conclusion: By the Numbers
EV battery power density continues to increase to accommodate a longer driving ranges. As current grows, precise measurements require a new sensor technology. Traditional current sensors can’t meet the needs for accurate current measurement, high sensitivity, high CMFR, low power and compact design.
TMR sensors – with higher SNR, better temperature stability, high linearity, low power consumption – can be packaged in a smaller area since no shielding is required. The sensed current range can be adjusted by busbar dimensions making it ideal for a wide range of EV BMS current requirements.
Crocus TMR CT452 and CT453 sensors offers contactless 0.7% accuracy, 1 MHz bandwidth, and better than -50 dB immunity to external magnetic fields without additional mechanical components. This performance and accuracy also reduces product size and weight, enabling customers to replace large and costly current sense modules.
The CT45x family’s XtremeSense TMR technology delivers a high SNR, contactless approach which scales with the system requirements. This allows for a no-compromise, simple solution, using a single CT45x sensor. For example, the CT45x is able to measure down to 500 mA resolution and up to 1,800 A while maintaining better than 0.7% accuracy.
The CT45x offers groundbreaking noise performance, as low as 0.55 mVRMS, to enable applications to sense small current levels and small changes or variations in current through a busbar. (This performance is almost 10 times better than existing Hall effect solutions.) This results in a SNR as high as 77 dB for the CT45x measurement, which allows the system to process higher resolution data with higher accuracy.
Crocus Technology is the leading supplier of disruptive TMR XtremeSense® sensors. Their CT45x family of contactless isolated current sensors is ideal for stringent EV 3-phase motor demands, delivering high precision and high current measurements in a compact, simple solution.
