Achieving Accurate Sensing With Integrated Current Sensors

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

 

Bodo spoke with Clément Amilien, global product line manager of IC Current Sensors & Electrical Safety at LEM, about enabling accurate sensing in applications like inverters and motor drives.

Bodo: Can you describe the evolution of integrated current sensors (ICSes) and how market trends have shifted toward miniaturization and performance improvements?

Clément: ICSes are, first of all, current sensors. The need to move from large current sensor modules to ICSes is a direct result of the increased power density in electronic systems, with higher voltages and higher currents in smaller and lighter products—examples include new industrial servo robots working at 800 V. ICS takes advantage of semiconductor packaging techniques to meet the isolation challenges arising from the increased voltage, as well as the heat produced by higher currents. Finally, ICSes allow for a more efficient manufacturing process, as they can be automatically assembled onto PCBs thanks to their SMD design.

 

Clement Amilien. Image used courtesy of Bodo’s Power Systems [PDF]

 

Bodo: Why has LEM decided to go for ICSes in-house?

Clément: LEM has been developing current sensors for more than 50 years for various industries. It was natural to continue supporting our clients in solving the design challenges of miniaturized power electronics. One of the goals of the ICS portfolio is to offer an alternative beyond the shunt market. Indeed, thanks to its integrated design with built-in signal treatment and isolation, an ICS is more compact and effective isolation than a shunt with an isolated amplifier or a digital isolator. Thanks to our system knowledge, we are well-positioned to conceive, develop, design, and sell the right integrated current sensing products and bring new value to the market with innovative packaging, improved performance, and constant quality. We have built on our many years of in-house semiconductor design, test, and manufacturing experience to produce the right setup and long-term strategy in terms of technology and product roadmaps.

 

Bodo: How has customer feedback helped develop integrated current sensors, particularly in identifying key features?

Clément: While voltage and current keep increasing, it is important to understand what the “mission profile” of the application is, that is to say, what level of performance the system is required to achieve. This will drive the isolation requirements and the current capabilities of the package. Also, a power electronics system is a mix of hardware and software requirements. For example, how the system is controlled will affect the required accuracy, sampling frequency, and the sensor’s bandwidth. Equally, standards and norms that the customer needs to follow will affect the sensor’s characteristics. Last but not least, testing the part in the customer’s system environment will confirm whether the sensor fits the task and offers insight into how the sensor should be used to achieve the expected performance.

 

LEM HMSR DA series is the first ICS with Sigma Delta Bitstream output. Image used courtesy of Bodo’s Power Systems [PDF]

 

Bodo: How do the features of the HMSR series address the needs of power electronics engineers, especially regarding high-voltage, high-current applications, and immunity to external fields?

Clément: Higher voltages are definitely a trend. This will drive isolation requirements according to the level of safety and customer protection required, either basic or reinforced. This is the case both inside the part (breakdown voltage to avoid an internal short circuit) and outside the product in terms of the creepage and clearance distances between the high-voltage primary circuit and the low-voltage secondary circuit. HMSR enjoys 8mm dCl/dCp for 800V applications. For high-current applications, HMSR integrates a large primary conductor with superior fusing capability, allowing the sensor to accept a very high current without melting. The device is rated at up to 20 kA impulse current.

Regarding immunity to external fields, ICS products use a differential measurement. Two sensing elements sense the same magnetic fields with opposite polarity, and by combining the difference of these two measured fields, unwanted external fields are canceled while relevant fields are summed. In addition, HMSR uses an internal micro-core that creates a natural barrier to external fields and a magnetic concentration of the field to be measured.

 

Bodo: The HMSR DA is marketed as the first ICS to offer sigma-delta bitstream output. What is the significance of this digital output, particularly in terms of response time, quantization noise reduction, and the benefits of using digital filters?

Clément: The beauty of the ∑∆ digital output of the HMSR DA is that it offers substantially improved performance in resolution and noise reduction versus traditional analog voltage outputs while letting the customer configure the filtering and the sampling according to the acceptable response time (delay). For example, for a high-resolution output, a high-order slower filter is used. For a faster result with less resolution, a smaller order faster filter is used. As a consequence, the signal-to-noise ratio is much higher. Also, a higher OSR (over-sampling ratio) will increase the delay but will improve the resolution. The sensor can adapt to customer’s needs as required.

 

Bodo: What advantages does the HMSR DA’s sigma-delta bitstream output offer compared to other digital output sensors, ICS, or current sensing solutions?

Clément: The ∑∆ bitstream is a digital signal that carries the information more accurately than an analog voltage output—this resolution is measured in an effective number of bits (ENOB). The HMSR DA digital output will exhibit a maximum of 13 effective bits, while a comparative ICS with an analog output is limited to nine effective bits. For every additional bit, the resolution is multiplied by two. This is the number of pieces of information in which the signal is divided to be treated - the higher the number, the more accurate the digital waveform is compared to the actual original signal without noise. Other ICSes would carry and pick up more noise. More specifically, the HMSR DA, thanks to its integrated isolation and ASIC for gain, compensation, and analog-to-digital conversion, is a cost-effective and more compact solution to replace a shunt and a digital isolator in high-voltage applications such as industrial servo robots.

 

Image used courtesy of Bodo’s Power Systems [PDF]

 

Bodo: In which applications does the HMSR DA make the most sense? How does the digital output address noise and interference challenges in these applications?

Clément: The main applications for the HMSR DA are servo drives where the motor control loop needs an input signal with a waveform as close as possible to the original current signal, even with a delay, to achieve very precise positioning—for example, for an industrial robot arm. High effective resolution and low noise, thanks to oversampling (OSR) and filtering, make this possible. Over longer distances (wires), a digital signal will not be polluted by EMIs, while an analog one will be. This also allows the drive to be sited away from the servo motors and gives some freedom in the design of the entire system, such as a robot. Also, as the subsequent delay is constant (fixed) for a given OSR and filtering order, it is possible to consider this without compromising the system’s performance.

 

Bodo: What are LEM’s future goals for integrated current sensors? How do you envision these advancements impacting sensor performance and application range?

Clément: Voltage and current keep increasing, and system efficiency targets are higher than ever. This means an ever more precise control loop enabled by better software and more responsive hardware. As a result, accuracy must be improved all along the measurement scale, from 1 A or less to hundreds of amps. For digital applications, an increased clock frequency will help with the effective resolution of the output signal and its accuracy, thanks to a higher ENOB (effective number of bits).

Also, and more generally to the whole ICS portfolio, higher efficiency can be obtained thanks to faster SiC or GaN transistors - at the current sensor level, the sampling frequency needs to match a figure up to 10 times the system switching frequency for a very accurate control loop. This has a direct effect on the required bandwidth of the sensor. Last but not least, packaging technologies keep improving to cope with higher isolation levels—1000 V applications are in sight—and better heat dissipation.

 

Bodo: How will LEM encourage engineers to adopt the HMSR technology? How does the HMSR DA fit into the broader roadmap of digital ICS development?

Clément: LEM has a comprehensive portfolio of ICSes to replace shunts along an isolated (digital) amplifier and shrink PCB space. Depending on the application, many options exist to find the right setup to integrate a sensor with ∑∆ digital output—the over-sampling ratio (OSR), the filtering order, the delay needed, and the effective resolution required.

Finding the right fit is possible while staying on top of performance. For example, going from an OSR of 256 to one of 128 will divide the delay by two, which is beneficial but will only decrease the resolution by 0.5 bits (from 13.3 to 12.8 effective bits). In the broader digital roadmap, another key performance driver is the clock frequency, which means a higher bandwidth will mean a better effective resolution. For the moment, the digital product portfolio mainly addresses servo drives with specific needs for effective resolution and delay. However, new developments will broaden the portfolio towards any application where moving a signal with very little distortion in noisy environments over longer distances is key, with tighter ENOB and signal delay requirements. Digitalization is only just beginning.

 

LEM current sensors trend toward smaller and smarter. Image used courtesy of Bodo’s Power Systems [PDF]

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine.