© Analog Devices Inc.
Application Notes | July 23, 2021
Inductor current measurement in switched power supplies
Question: How do you measure inductor current?
Answer: Switched-mode power supplies commonly use inductors for temporarily storing energy. In the evaluation of these power supplies, it is often useful to measure the inductor current to gain a complete picture of the voltage conversion circuit. But what is the best way to measure the inductor currents?
Figure 1. Schematic illustrating measurement of the inductor current in a switched mode power supply.
Figure 1 shows a suggested setup for such a measurement using the example of a typical step-down converter (buck topology). A small auxiliary cable is inserted in series with the inductor. It is used to attach a current probe and display the inductor current with an oscilloscope. It is recommended that measurements be made on the side of the inductor where the voltage is stable. Most switching regulator topologies use the inductor in such a way that the voltage on one side is switched between two extreme values but remains relatively stable on the other side.
With the buck converter shown in Figure 1, the voltage at the switching node—that is, the left side of inductor L—switches between the input voltage and the ground voltage at the speed of the switching edges. On the right side of the inductor is the output voltage, which is usually relatively steady. To reduce the interference due to capacitive coupling (electric field coupling), the current measurement loop should be placed on the quiet side of the inductor, as shown in Figure 1.
Figure 2. Practical setup for inductor current measurement.
Figure 2 shows the practical setup for this measurement. The inductor is lifted and obliquely soldered back with one of the two terminals on the board. The alternate terminal is connected to the board with the auxiliary wire. This conversion can be accomplished quite easily. Desoldering with hot air is a proven method for detaching the inductor. Many SMD rework stations offer hot air with an adjustable temperature.
Current probes are offered by manufacturers of oscilloscopes. Unfortunately, they are usually quite expensive—thus the question continually arises as to whether the inductor current could also be measured by means of a shunt resistor. This is possible in principle. However, this measurement has the disadvantage that the switching noise generated in a switched-mode power supply can easily couple into a voltage measurement via a shunt resistor. Thus, especially at the points of interest, when the inductor current changes direction, the measurement results would not be a true representation of the behavior of the inductor current.
Figure 3. Inductor current measurement shown in blue and behavior of a saturated inductor added in purple.
Figure 3 shows the measurement of the inductor current (in blue) for a switched-mode power supply sensed with a current probe compatible with the oscilloscope used. In addition to the measurement shown in blue, a purple marking was added, indicating how the current flow through the inductor would look when the inductor starts to saturate excessively toward the peak currents.
This happens when an inductor is selected that does not have enough current rating for a given application. One of the main reasons for measuring inductor current in a switched-mode power supply is so that one can recognize whether the inductor was properly selected or whether inductor saturation will occur in operation or during a fault condition.
Measurement with a shunt resistor instead of a current clamp would show strongly coupled noise, especially at the peak currents, making it difficult for inductor saturation to be detected.
Sensing of the coil current is very useful in the evaluation of a power supply and can easily be accomplished with suitable equipment.
About the Author: Frederik Dostal studied microelectronics at the University of Erlangen in Germany. Starting work in the power management business in 2001, he has been active in various applications positions including four years in Phoenix, Arizona, working on switch-mode power supplies. He joined © Analog Devices in 2009 and works as a field applications engineer for power management at Analog Devices in München. He can be reached at frederik.dostal@analog.com.
Figure 1. Schematic illustrating measurement of the inductor current in a switched mode power supply.
Figure 1 shows a suggested setup for such a measurement using the example of a typical step-down converter (buck topology). A small auxiliary cable is inserted in series with the inductor. It is used to attach a current probe and display the inductor current with an oscilloscope. It is recommended that measurements be made on the side of the inductor where the voltage is stable. Most switching regulator topologies use the inductor in such a way that the voltage on one side is switched between two extreme values but remains relatively stable on the other side.
With the buck converter shown in Figure 1, the voltage at the switching node—that is, the left side of inductor L—switches between the input voltage and the ground voltage at the speed of the switching edges. On the right side of the inductor is the output voltage, which is usually relatively steady. To reduce the interference due to capacitive coupling (electric field coupling), the current measurement loop should be placed on the quiet side of the inductor, as shown in Figure 1.
Figure 2. Practical setup for inductor current measurement.
Figure 2 shows the practical setup for this measurement. The inductor is lifted and obliquely soldered back with one of the two terminals on the board. The alternate terminal is connected to the board with the auxiliary wire. This conversion can be accomplished quite easily. Desoldering with hot air is a proven method for detaching the inductor. Many SMD rework stations offer hot air with an adjustable temperature.
Current probes are offered by manufacturers of oscilloscopes. Unfortunately, they are usually quite expensive—thus the question continually arises as to whether the inductor current could also be measured by means of a shunt resistor. This is possible in principle. However, this measurement has the disadvantage that the switching noise generated in a switched-mode power supply can easily couple into a voltage measurement via a shunt resistor. Thus, especially at the points of interest, when the inductor current changes direction, the measurement results would not be a true representation of the behavior of the inductor current.
Figure 3. Inductor current measurement shown in blue and behavior of a saturated inductor added in purple.
Figure 3 shows the measurement of the inductor current (in blue) for a switched-mode power supply sensed with a current probe compatible with the oscilloscope used. In addition to the measurement shown in blue, a purple marking was added, indicating how the current flow through the inductor would look when the inductor starts to saturate excessively toward the peak currents.
This happens when an inductor is selected that does not have enough current rating for a given application. One of the main reasons for measuring inductor current in a switched-mode power supply is so that one can recognize whether the inductor was properly selected or whether inductor saturation will occur in operation or during a fault condition.
Measurement with a shunt resistor instead of a current clamp would show strongly coupled noise, especially at the peak currents, making it difficult for inductor saturation to be detected.
Sensing of the coil current is very useful in the evaluation of a power supply and can easily be accomplished with suitable equipment.About the Author: Frederik Dostal studied microelectronics at the University of Erlangen in Germany. Starting work in the power management business in 2001, he has been active in various applications positions including four years in Phoenix, Arizona, working on switch-mode power supplies. He joined © Analog Devices in 2009 and works as a field applications engineer for power management at Analog Devices in München. He can be reached at frederik.dostal@analog.com.
Nexperia uses Aixtron equipment as it enters the SiC market
Semiconductor manufacturer Nexperia, is using Aixtron's production technology to enter the high performance SiC device market.
Micron and UMC team up to strengthen supply chains
Micron says it is expanding its business relationship with United Microelectronics Corporation (UMC), which will provide "Micron opportunities to secure supply for automotive, mobile and critical customers into the future."
Global Semiconductor equipment billings jump 38% YoY in Q3
Global semiconductor equipment billings increased a robust 38% year-over-year to US D26.8 billion in the third quarter of 2021, an 8% rise from the prior quarter to register their fifth consecutive quarter-over-quarter record high, SEMI reports.
Littelfuse completes its acquisition of Carling
Littelfuse has completed its previously announced acquisition of Carling Technologies.
Amphenol acquires Halo Technology
Amphenol Corporation announces that the company has acquired Halo Technology Limited for approximately USD 715 million. Separately, Amphenol also announced the closing of the sale of the MTS Test & Simulation business to Illinois Tool Works Inc.
Jenoptik completes the acquisition of BG Medical and SwissOptic
With these acquisitions, Jenoptik continues to focus on its core competency in photonics while speeding up profitable and sustainable growth.
Sponsored content by Electrolube
Are Encapsulation Resins Suitable for EV Applications?
Beth Turner from Electrolube (MacDermid Alpha Electronic Solutions) discusses how and where encapsulation resins can be incorporated into electric vehicle design to protect vital electronics and improve overall efficiency.
TouchNetix gains automotive qualification for family of touchscreen controller ICs
UK manufacturer of high-performance touchscreen controller ICs, TouchNetix, announces that its aXiom AX112 user interface chip has gained qualification to the Automotive Electronics Council’s AEC-Q100-6 standard, validating the device’s use in applications such as the central information display in passenger cars and other types of vehicles.
Mercury Systems acquires RF module manufacturer
Mercury Systems announces that the company has acquired Norcross, Georgia based RF module manufacturer, Atlanta Micro.
Syntronixs Asia becomes part of Infineon
German semiconductor manufacturer, Infineon, has via its subsidiary Infineon Technologies (Malaysia) Sdn. Bhd. acquired Syntronixs Asia Sdn. Bhd., a Melaka-based electroplating company.
Sponsored content by Shenzen Kinwong Electronic
The development trend of printed circuit board products and Kinwong's solution
With the rapid development of electronic technology in recent years, printed circuit board (PCB) products are pursuing higher heat dissipation capabilities, with high voltage and high current characteristics, and are developing towards high-density interconnection technology (HDI).
Intel’s EU investment: Details to be expected soon
Earlier this year US chip manufacturer Intel launched a USD 20 billion expansion plan which would see the construction of two new chip plants in Arizona – as well as an expansion in Europe.
X-FAB releases statement following explosion at Texas fab
On November 12, 2021, an explosion occurred at X-FAB’s site in Lubbock, Texas, during maintenance work done at the deionized water system. One employee lost his life, another employee was taken to hospital with non-life-threatening injuries and was later released.
AKM provides an update on its fire damaged fab
In late October last year a fire broke out at Asahi Kasei Microsystem’s (AKM) semiconductor factory in Nobeoka, Miyazaki prefecture. The fire completely ravished the fab and took a total of three days to completely extinguish. But how does the company fare today?
Micron and UMC reaches a global settlement
According to the global settlement agreement, both companies will globally withdraw their complaints against the other party, and UMC will make a one-time payment of an undisclosed amount to Micron.
Sponsored content by OMRON Europe
Invest in advanced solutions from OMRON, Industry-leader.
Achieving flawless operation takes ongoing investment in technologies such as 3D AOI. The Omron VT-S1080 3D AOI delivers value through advanced, stable, and accurate 3D capabilities. The AI-assisted and IPC-correlated Solder Joint Inspection is ideal for today’s more dynamic assemblies.
A*STAR and STMicro team up on SiC R&D for the EV market
The Institute of Microelectronics (IME) at the Agency for Science, Technology and Research (A*STAR) and STMicroelectronics are launching an R&D collaboration in the field of silicon carbide for power-electronics applications in the automotive and industrial markets.
Jenoptik signs agreement to sell Vincorion
Jenoptik AG has signed an agreement to sell the Vincorion division. It will be acquired by a fund managed by private equity firm STAR Capital Partnership LLP.
Sponsored content by Siemens EDA
Digital transformation: How Siemens EDA helps you engineer a smarter future faster
We are living in an age of ever-accelerating digital innovation, where worldwide knowledge, commerce and communication are broadly accessible and literally at our fingertips. Over the past six decades, thousands of companies in the tech sector have worked diligently to bring new, ever-more sophisticated electronic innovations to market daily, culminating in today’s age of digitalization, which is rapidly changing how we live, travel, conduct business and communicate. This pace of digital transformation will accelerate even more rapidly as more companies begin to incorporate artificial intelligence (AI) and machine learning (ML) into their systems to leverage and even monetize the exponentially increasing amount of data produced by seemingly “everything digital.”
NAND Flash revenue rises by 15% QoQ for 3Q21
The growth of the NAND Flash market in 3Q21 was primarily driven by strong demand from the data center and smartphone industries, according to TrendForce’s latest investigations.
Qualcomm acquires Clay AIR
Clay AIR, an AI powered hand tracking and gesture recognition technology company, has been acquired by Qualcomm.
AmpliTech to acquire Spectrum Semiconductor Materials
AmpliTech Group, a designer, developer and manufacturer of signal-processing components, has executed a definitive agreement to acquire the assets and operations of San Jose-based Spectrum Semiconductor Materials Inc., a specialty distributor of semiconductor components.
Veeco ships first system from new manufacturing facility
Veeco Instruments says that the company has shipped the first LSA101 Laser Spike Annealing System from its new San Jose, California facility to an unnamed semiconductor manufacturer.
Smith to divest its Its solar division
The distributor of electronic components says that it has signed an agreement to sell its solar sales and services division, known as ONTILITY Powered by Smith, to BBB Industries, LLC under its new division, TerrePower.
TTI to acquire SMD Inc.
Specialty distributor of electronic components, TTI, Inc.,has entered into a purchase agreement to acquire SMD Inc., a privately held electronic component distributor headquartered in Irvine, California.
Former GlobalFoundries executive to lead IQE
Following an extensive and rigorous search process, the board of the UK supplier of compound semiconductor wafer products announces that it has appointed Americo Lemos as Chief Executive Officer, starting on 10 January 2022.
Load more news
