SMUs: the multi-function power tool for your bench
01 April 2022
One major restriction facing design engineers is the power usage limits of their devices. Whether the product is an IoT device that needs to work for many years on one battery, or an electric vehicle that must achieve maximum range, analysis of power usage is critical.
This article was originally featured in the April 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
Another challenge for many systems is accurate characterisation and testing of semiconductors and other non-linear devices, with voltage and current characteristics that can span both positive and negative values. A good example is the need to characterise LEDs to ensure the driver circuits are optimised. Here, Cliff Ortmeyer, Global Head of Technical Marketing at electronic component distributor, Farnell explains how, for requirements such as these, engineers are increasingly turning to SMUs (source measure units), an electronic instrument that can source and measure at the same time…
Evolved from parameter analysers, the first standalone SMU – the Keithley 236 – was launched in 1989. Today, an SMU incorporates a highly stable DC power source as a constant current source, or as a constant voltage source, as well as a high precision multimeter. As a four-quadrant device, the SMU simultaneously sources (positive) or sinks (negative) power to a pair of terminals. At the same time, it can also measure the current or voltage across those terminals.
Users can set a precise current or voltage limit and get an indication that they’ve reached that limit without losing source power. According to T&M firm, Keithley Instruments, a Tektronix company, the range of SMUs available today can cover a very broad range of current (100 fA to 50 A) and voltage (100 nV to 3 kV) source requirements, with measurement resolution of up to 6.5 digits.
The SMU contrasts with a standard power supply, which offers merely positive voltages and positive current. The so-called two-quadrant power supplies expand on this by also incorporating a load. Some high-end lab power supplies offer four-quadrant operation, yet many still primarily focus on providing power to an application, with measurement capabilities being an afterthought.
Dr Philip Weigel, Director Product Management, Power Products, Meters, Sources & Audio Analysers at electronics & wireless T&M expert, Rohde & Schwarz (R&S), explains: “For test engineers looking into the I-V curve of a diode, you need to go from negative voltages and currents, crossing the zero, going up to positive voltages and currents to analyse the I-V curve.” This ability for I and V sweeping allows testing of devices under varied conditions with a range of source, delay and measurement characteristics.
The first integrated test instrument
Combining the roles of a digital multimeter (DMM), power supply, true current source, electronic load and pulse generator in one instrument can save up to 44% of the cost of buying separate test instruments. In addition to this, a single SMU can save up to 75% of workspace – and reduce the number of cables and test leads by up to 60%, according to Keithley Instruments.
An SMU can be used in a more advanced environment and ensures that all measurements are correlated. Bradley Odhner, Technical Marketing Manager at Tektronix & Keithley Instruments, commented: “People are trying to get the most bang for their buck from equipment. SMUs are great for companies that want one instrument to do a lot of very complex things. This is even more critical as devices become increasingly miniaturised.”
Today, SMUs continue to evolve, with some models adding basic oscilloscope functions. This type of innovative approach hasn’t always been easy though, with test instrument manufacturers often restricted by the demand for producing more traditional product lines. However, companies are adapting their approach to create highly integrated test solutions, according to Mike Hoffman, Product Manager at electronics T&M expert, Keysight Technologies. “We previously had an oscilloscopes division, a power supply division and a network analyser division, all of which worked independently. Although this fostered innovation, it wasn’t conducive to improving integration between different pieces of equipment,” he said. “We’re now much better placed to develop integrated equipment that reflects the integrated work environment from which they come”.
Energy efficiency: driving a bigger role for SMUs
SMUs can be used across a wide variety of applications. Although often seen as tools to characterise components, today’s focus on improving energy efficiency and battery technology has elevated the role of SMUs. They are now commonly used in battery testing, quite often for IoT (internet of things) devices.
The push towards greater energy efficiency is driving an even greater need for precision measurement capabilities. Measuring power consumption precisely is a major challenge for engineers, especially when measuring very low values. This can be the case for IoT devices, which can switch between states frequently. Ensuring batteries achieve their maximum life is particularly challenging in these applications because IoT devices have transmit, idle, sleep and deep sleep states. This is a real challenge in research & development because developers want a device to be transmitting and reacting to the customer on demand. However, they also want the device to go into deep sleep state as fast as possible to provide longer battery life.
Optimising battery life requires instruments with a current measurement range from hundreds of nanoamps to amps, the ability to capture current pulses just a few microseconds wide, and a large memory to store the prototype device’s current profile. SMUs can effectively deliver the precise measurements that allow battery life to be optimised. Another priority is to create a model for the battery that powers the IoT device. A battery model-generating script can operate the SMU as a controlled current load and derive the model parameters.
“An SMU can also simulate different battery types. For example, a developer may have a new design of an IoT device and want to decide if nickel-cadmium batteries are suitable. They may also be thinking about use cases. For example, what if the consumer is using the device while skiing, how would the device behave in very cold conditions? With many SMUs, you can load a battery model and the SMU will simulate the voltage and impedance of the battery over its different charge states,” said Dr Weigell.
An example of this test application is the 2450 SourceMeter SMU from Keithley Instruments, which can be programmed to discharge a battery and create a model of the battery for use in the company’s 2281S-20-6 battery simulator. The model consists of both open circuit voltage and internal resistance as a function of the battery’s state of charge. The simulator emulates the real battery using the battery model. It can then be used to test a product under realistic, repeatable conditions, determining how the product performs when powered by a battery under various discharge conditions.
The precision offered by SMUs also opens up other applications. Dr Weigell commented: “Although semiconductors is the most natural application area for SMUs, two other major areas are precision electronics and research & education.” For example, an SMU is an invaluable measurement tool for researchers looking at quantum technologies or new materials research, as it can measure very small currents at very high accuracy.
LED & battery testing
Characterising components is another very common use of SMUs. Today, SMUs are widely used to ensure that LEDs are driven optimally. One priority is the I-V curve. As LEDs need to go from negative voltages and currents to positive, four quadrants are needed, which demands an SMU. Another area for investigation is when the LED starts and stops emitting light. An instrument will measure the light emittance of the diode and be correlated with the SMU’s own measurements.
Another important aspect is current overshoots. A normal power supply will regulate on the voltage. If set to five volts, it will try not to overshoot the voltage, but does not pay too much attention to what happens in the current. Many SMUs have a current priority mode, where the power supply or the SMU will regulate the current and avoid possible damage to the LED.
SMUs for all applications
Most of the major test instrument vendors now offer SMUs to fit different requirements and budgets. The state-of-the-art NGU401 from R&S offers six measurement ranges for current and a resolution of up to 6.5 digits when measuring voltage, current and power. The SMU is designed for characterising devices that work from extremely low power consumption to high currents in the ampere range.
Keithley Instruments’ 2634b series of SMUs features built-in plug-and-play Java-based I-V characterisation and test software, while the 2461 SMU is optimised for characterising and testing high power materials, devices and modules such as SiC (Silicon Carbide), GaN (Gallium Nitride), DC-DC converters, power MOSFETs and solar cells.
The B2900B & B2900BL series of precision SMUs from Keysight have a voltage maximum of ±210 V, a current maximum ± 3 A DC, and ±10.5 A pulsed sourcing capabilities. With a precision minimum of 10 fA / 100 nV sourcing and measuring resolution, the B2900B/BL series can make low-level measurements that were previously only possible using a more expensive semiconductor device analyser. The user-friendly front panel colour LCD graphical user interface (GUI) features several task-based viewing modes, enabling users to quickly perform measurements and display data. The B2900B/BL Series SMU also supports conventional SMU SCPI (Standard Commands for Programmable Instruments) commands for easy test code migration and can take measurements remotely using PC-based control software. These features improve efficiency and lower the cost of ownership when integrating SMUs into systems for production test.
With the development of a device that combines the best of a DMM and a power supply, SMUs provide developers of electronic components and products with a powerful tool that is not only cost-effective, but is much more useful and compact than the two separate instruments.
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