Current sensors can deliver more efficient and less thermally-challenging industrial systems by monitoring performance under all conditions in real time.
A lot of the current buzz in electronic systems development is about “smart” products, and the ability to oversee the performance and functionality of these solutions. Between competition and consumer expectations, a product on the market today must perform its functions almost perfectly, or as close as can be achieved through modern technology and processes. Short battery life, poor RF connections, bad thermal management, and other non-critical aspects of performance are also make-or-break parameters when operating in the real world.
The explosion in the growth of the electronics marketplace at every level is challenging the industry at every level. From new semiconductor materials to advanced solutions like artificial intelligence, new technologies and approaches are creating new application spaces while rejuvenating old ones. All of this is being driven by the migration to integrate data technology into every aspect of electronics.
All of this is creating a disruptive period of demanding growth, which places additional pressure on system designers to ensure the product created is safe, efficient, reliable, and cost-effective (especially that last one). That means design engineers must select the best solutions for every aspect of the systems they are designing, especially the elements involved in monitoring circuit performance. Advanced current-sensing solutions can address these needs in a comprehensive and cost-effective way.
Figure 1 Current sensors are used to regulate and manage power for a diverse range of applications. Source: Aceinna
When it comes to consumer and medical wearables, advanced personal electronics, and the internet of things, the smaller, more functional, and longer lasting, the better. Similarly, industrial and automotive applications are pushing boundaries to achieve smaller, more efficient, and less thermally-challenging. This can only be achieved by constantly monitoring system performance under all conditions in real time.
When it comes to electronic systems, it is important to distinguish between efficiency and optimum performance. Some focus on efficiency, forgetting that even though a system is very efficient, it may not be cost-effective because it doesn’t respond to the application as needed during periods of challenging operation. Only by real-time monitoring of power usage can one be confident of both during operation.
There is no precision without feedback, and it is impossible to compete today without having your product work in as precise a manner as possible. Current sensing can provide the critical performance information an embedded intelligent system needs to manage itself in a non-invasive manner, in that it doesn’t have to become a major infrastructure element in your design.
The migration from passive systems to “smart” solutions with intelligent feedback and control has delivered significant operating improvements. In general, power efficiency and motor-drive open-loop current sense accuracy has been highly beneficial to improved operation over the full temperature range. Growing and developing Industry 4.0 needs and processes have moved the goal post to improved performance at temperatures as high as 85°C or 105°C.
In the area of advanced solar inverters, systems are achieving higher levels of accuracy over the temperature range. Similarly, applications needing extremely wide dynamic range with very good accuracy and precision will need higher accuracy over temperature, and can implement a single closed-loop current sense system rather than two open-loop current sensors to track lower and higher currents.
One of the basics in electronics is that power management is thermal management. Power efficiency and thermal performance go hand in hand, as wasted energy from the system is always expressed as heat. If you can improve efficiency, you can reduce the temperature, and your electronics work better and more reliably.
Conversely, if your electronics operate poorly, there is more waste heat, and, therefore, more thermal-management, reliability, and safety issues. Optimizing both power and thermal management will significantly improve productivity, cost-effectiveness, safety, and reliability.
Inverter, motor drive, power supply, UPS, and external charging stations must be able to operate at an ambient operating temperature range from -40°C to 85°C, often up to 105°C.
Even in inverter applications, where the internal maximum temperature is kept relatively low, such power systems are typically specified for 85°C operation, at least to ensure proper operational headroom without derating. Ambient operating temperature requirements for automotive onboard chargers can go up to to 125°C, while motor drives can go up to levels from 105°C to 150°C, depending on the location.
Although many systems use fans and other temperature-regulating mechanisms to manage system thermal performance, for systems with rapidly-changing temperatures and performance dynamics, this can be difficult. In addition, external cooling mechanisms take up extra space that could be used for other aspects of the design, consume additional energy, and present their own efficient operation issues.
For systems with potentially rapid changing temperature, measuring the system current can be a faster method for predicting and managing the thermal performance of the system. The management controller that is monitoring the actual current level can determine if the current level is rapidly increasing, indicating a potential catastrophic event.
Monitoring the current in real-time while the system is operating is a leading indicator of potential out-of-range events and failure conditions, enabling the system to predict potential catastrophic events before they occur, protecting the system and critical components. No matter what the concern, system performance, system reliability, or fault identification of basic safety are situations that must be addressed as early as possible. Current sensing can detect a potential issue, minimizing system downtime and/or preventing catastrophic failure.
Timing and performance
Synchronization and regulation are important factors to consider in advanced power systems, as power-factor correction (PFC) stages are also time-oriented systems. The output ripple of the circuit must be filtered to avoid current distortion, and the loop frequency is related to the system bandwidth.
Think of the PFC stage as a system delivering power from an input voltage, managed by a control signal, so even if the system control loop bandwidth is lower, currents are measured during each power switch cycle, for cycle-by-cycle current. Under ideal conditions, there should be a high multiple of the switching frequency to have a flat gain response, and low phase margins at the switching frequency. Low frequencies can work, with some compromise on gain and phase delays at switching frequency.
Although overall control loop bandwidth may be much lower than the switching frequency, the current measurements should be taken at the switching frequency for cycle-by-cycle control. Most Totem pole PFCs are switching at ~65kHz to 150kHz, which will require bandwidth of ideally 650kHz (at least >300kHz) to 1.5MHz. This switching frequency is being pushed to 300kHz in some cases in advance designs and will require ~3-MHz bandwidth (at least 1.5-MHz bandwidth).
Power conversion with high currents of up to 1,000A will nominally switch at levels from less than 1kHz up to 20kHz, typically with IGBTs and silicon MOSFETs. Other circuits can switch up to around 40-50kHz with wideband silicon carbide (SiC)/gallium nitride (GaN) power switches, and further advances in SiC/GaN power stages may move this high-current switching up to 100kHz eventually, requiring bandwidth from 500kHz up to 1MHz.
No precision without feedback
In order to achieve those levels of performance, you must have accurate measurements using accurate and precise current monitoring, and the latest current sensing systems offer significant performance advantages over legacy solutions. An example of these new solutions is Aceinna’s latest anisotropic magneto resistive (AMR)-based isolated current sensors, which can deliver highly accurate, wide-bandwidth sensing for best-in-class performance on bandwidth, output step response, and accuracy in a single-chip solution.
Figure 2 The AMR-based isolated current sensors can deliver highly accurate, wide-bandwidth sensing. Source: Aceinna
These fully integrated, AMR-based isolated bi-directional current sensors provide much higher DC accuracy and dynamic range compared to legacy and existing solutions. For example, the ±20-A version has a typical accuracy of ±0.6% and will achieve an accuracy of ±2% (max) at 85°C. The family of products include ±5-A, ±20-A, and ±50-A parts, which come in an SOIC-16 package with a low impedance (0.9mΩ for ±50 A) current path, certified by UL/IEC/EN for isolated applications.
Such advanced current sensors can guarantee an offset of ±60 mA, or ±0.3% of FSR (max) over temperature, so high accuracy can be achieved over a roughly 10:1 range of currents, providing significant improvement in dynamic range vs. leading Hall-sensor-based devices. Features include 1.5-MHz signal bandwidth with low phase delay vs. frequency, fast output step response of 300ns, and 4.8-kV isolation, making them ideal for current sensing in fast current-control loops and protection for high-performance power supplies, inverters, and motor-control applications.
The task of developing and deploying an optimal power and motion-controlled automation solution can be daunting, and unless done well, system setup will lead to sub-optimal performance. Specifying and choosing the proper components, devices, and methodologies to address power, performance, and thermal management will go far toward making sure your final power solution will be cost-effective, productive, and deliver the required performance in an optimum manner.
This article was originally published on EDN.
Teoman Ustun is VP of Automotive Business Unit at Aceinna Inc. Before Aceinna, Ustun was the co-founder and CEO of Mulberry Sensors, a startup developing laser-based, chip-scale, high performance molecular sensors. He has also worked for 11 years at Analog Devices Inc. (ADI), where he directed advanced sensors organization, formulated and executed strategy, and prioritized organic and inorganic sensor investments.
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