Smart thinking in factories and EVs

From new software-oriented solutions like artificial intelligence, to new hardware topologies, to new semiconductor materials, we are in the middle of a disruptive period of demanding growth.

Much of these advances are related to user functionality, like cloud-based IoT which relies on next-generation RF technologies. Other rapidly emerging current sensing applications include electric vehicles (EVs) and advanced driver assistance systems (ADAS) and autonomous driving, to wide-bandgap power switches based on silicon carbide (SiC) and/or gallium nitride (GaN) semiconductors. Some of the most important advances in these spaces have been in performance and efficiency.

These trends put a lot of pressure on the designers of embedded systems. For consumer and medical wearables, advanced personal electronics and the IoT, the smaller, more functional and longer lasting, the better. Similarly, industrial and automotive applications are pushing boundaries to achieve smaller, more efficient, reliable, and robust solutions. Significant improvement in all of these include reducing parts count, simplifying circuitry, and increasing operational efficiency.

Current sensing technologies

Current sensing technologies are key to creating the small precision control and protection electronic circuits needed to make devices serve applications in an efficient and cost-effective manner. There is no precision without feedback and current sensing can provide the critical performance information an embedded intelligent system needs to manage itself. The size, accuracy and speed of current sensing solution will directly impact all of these aspects.

A single-chip, isolated current sensor, based on Anisotropic Magnetoresistive (AMR) technology does not require additional components other than a decoupling capacitor. In comparison, the problem with using a shunt resistor is that it is inherently not isolated and a current transformer is bulkier than an AMR-based current sensor and it only works with AC. Compared to using a Hall-effect sensor, AMR technology offers a bandwidth of 1.5MHz and has a lower offset and noise.

Delivering better performance than a shunt register or transformer, AMR technology can respond to both DC and AC bi-directional current, with a bandwidth of 1.5MHz and a lower offset and less noise than Hall-effect-based solutions.  Offering better accuracy, higher bandwidth with lower phase shift and a very fast output step response, an AMR- based current sensor is accurate and compact for critical measurements to protect and control power systems.

Figure 1: Current sensing technologies are available for a range of applications from EV cars and ADAS systems to appliances, telecommunications and server farms

Within the sensor, the current flows through a U-bend in the lead frame, where it generates a forward or reverse field measured by two current sensors in the device. By measuring the field from both current directions, the device cancels out the external fields and magnetic anomalies which might be present. This allows a horizontally-sensing AMR chip to ignore external fields generated from other nearby components on the board.

Automotive

Much of the focus for EVs is on improving the efficiency of the powertrain, motors and on-board/off-board charging systems as well as the performance of the battery pack. The proper application of current-sensing technology in these application areas can deliver significant advantages.

As power is spent in the motor, any improvement there will cascade benefits throughout the system; increasing range or reducing thermal management needs. When driving motors, the switching frequencies and control mechanisms are critical.

Effective motor control requires accurate performance measurement, achieved with effective current sensors. For condition monitoring of motors for predictive maintenance, fast current sensors help to measure and monitor motor ripple currents to determine lifetime and performance parameters. On the protection side, the current sensor helps support safety by improving the control, accuracy, and reliability of a motor drive.

Many EV power electronics and charging systems (both on-board and off board systems), are migrating to advanced wide-bandgap semiconductors like SiC and GaN, as the benefits include higher efficiency and the ability to increase the switching frequency. A significant benefit of faster switching is the ability to shrink the size of the passives and magnetics in a circuit, with direct size and weight benefits.

When a circuit is switching faster, however, the ability to measure the performance parameters must be able to keep up, demanding real-time information from a fast and accurate current sensor. Monitoring the circuit in real-time enables advanced functionality like dynamic control of the power switching and motor drive frequency, as well as reliable and fast fault detection.

Electric trains, industrial machines, traction and robotics are starting to use reluctance motors. These are a winding-free design that generates torque through magnetic reluctance. Available in synchronous, variable, switched and variable-stepping configurations, reluctance motors can deliver high power density at low cost.

Figure 2: Advantages of AMR-based current sensing versus other current sensing technologies

Problems with reluctance motors include high torque ripple at low speed, and noise. In addition, because of the extremely high temperatures involved, reluctance motors are usually deployed with a separate harness and control system.  Advanced solutions using wide-bandgap SiC semiconductors and high bandwidth AMR sensors can take more heat, enabling size, weight, and complexity reductions of the overall system, providing cascading benefits. 

Constructed without copper coils in the rotor, reluctance motors can be lighter than comparable electric motors.  However, the required control system is very complex, because if current is not accurately controlled, the result is torque ripple which generates noise. Advanced fast current sensing improves control of the ripple current, which provides lower noise and a more reliable solution.

In high-power systems, the engineers might want to switch the whole power stage off in 1.5µs. Shutdown time means that the step response needs to be less than 500ns, and that is going to become more stringent as systems migrate into higher power and frequency levels.

Power factor correction

Power factor correction (PFC) is used to reduce the lagging power factor in inductive loads. It compensates for the phase difference between voltage and current; when the power factor drops, the system becomes less efficient.

To get 1kW of real power at a 0.2 power factor, 5kVA of apparent power needs to be transferred (1kW ÷ 0.2 = 5kVA).  This can severely impact the performance in the case of inductive loads, for example motors, refrigerators and HVAC systems, inverters and uninterruptible power supplies (UPS).

Fast turn-on and turn-off time, fast reverse recovery and lower on resistance of wide bandgap SiC and GaN-based power switches allow effective use of totem pole architecture to improve efficiency of PFC and reduce the number of components used. These benefits help power systems to achieve higher efficiency 80+ Gold and Titanium certifications.

For example, when it comes to ripple currents in the PFC in a totem pole, to measure current cycle-by-cycle to calculate the pulse-width modulation (PWM) duty ratio, the bandwidth needs to have the ability to match the circuit’s switching frequency. For example, if the PFC switching frequency is being pushed to 65, 140, 200, 300kHz, the desired bandwidth is 10 times that of the switching frequency for the current sensor.

Smart manufacturing

The smart factory and smart manufacturing are focused on automation and data exchange. In a system where powered devices are connected to an intelligent infrastructure and the internet, power conversion is also needed. Power monitoring and management are critical to the optimal operation of a smart assembly process, with everything being measured in real-time.

Figure 3: Aceinna current sensors use a U-bend with two AMR sensors to cancel out external fields

An AMR current sensor can provide accuracy, bandwidth and step response at various locations in an automated system. An accurate sensor can optimise a process and increase efficiency and productivity.

This performance advantage can be further leveraged by using AMR current sensing to determine how much the processor is being used, especially for applications involving AI, the cloud, and data storage. The sensors can also enable the use of power tracking for performance monitoring’s sake, optimisation of processor loading and thermal management.

Looking forward

Whether it is for advanced EVs, entire smart factories, UPS, inverters or motor drives, efficient and cost-effective power management is key to optimal performance. Whether driving motors or powering 5G telecomms, operation must be fast and effective. Advanced current sensing enables a higher level of control, with a higher efficiency, at higher frequencies and temperatures.

There is no precision without feedback. Precise fast current feedback enables the highest levels of efficient and safe operation in an advanced powered circuit.

The next generation of embedded devices must be able to serve the latest application spaces in the most efficient and cost-effective manner. AMR-based current sensing can ensure that the electronics are performing at their best, with cascading benefits throughout the entire system.

About the author: Khagendra Thapa, is vice president of Business Development in Aceinna’s current sensing business.