Requirement for continuous improvement in energy efficiency keeps design engineers on the edge as they work hard to come up with better solutions. Test instruments are the key to aid engineers with design-validation and performance testing. Today’s power supplies include those designed for a wide variety of uses as well as high-performance options for a variety of applications. These are used in test environments including research and development and production in embedded, semiconductor, nanotechnology and superconductor industries.
“High sourcing accuracy and built-in control functions make these ideal for Hall effect measurements, resistance measurements using delta mode, pulsed measurements and differential-conductance measurements. Programmable pulse widths limit power dissipation in delicate components and support pulsed I-V measurements,” says Naresh Narasimhan, country manager, Tektronix India.
So what are the latest improvements seen in these equipment?
FPGA-aided power supplies
A source measure unit (SMU) is a superset of a power supply module that can not only supply voltage or current but also measure and control both current and voltage. It uses closed-loop feedback control to ensure that the desired output voltage is correctly applied to the load under test. Typically, a DC power supply can only source voltage/current. An SMU, on the other hand, can source as well as sink voltage/current so that it can act as a DC supply as well as a controlled load.
In a lot of SMU or power supply applications, you need to give voltage to sensitive circuits like a micro-electro-mechanical system (MEMS). Even small voltages beyond the operating maximum voltages could potentially damage some MEMS sensors. The other side is that it is very difficult to maintain this very stringent voltage level when you are powering up a circuit.
Raviteja Chivukula, technical marketing engineer, National Instruments, explains, “Because of resistance, inductance and capacitance present in any circuit, components and wires, any normal DC power supply connected even for a small time could create a small spike. This will be difficult to remove for a normal SMU while ensuring that the rise and test times are small. In our case, online gains of the control unit on the SMU are tuneable by the customer. What happens here is that the SMU supplies voltage and then measures and controls the voltage and current experienced by the device under test (DUT). It will now use its control loop (with custom control loop gains set by the user) to make sure that the DUT does not experience overshoots. Usually this control loop is analogue so is not customisable, but newer models are customisable because the control loop runs on field-programmable gate arrays (FPGAs).”
National Instruments launched PXIe-4139 last year, featuring their SourceAdapt technology to address this issue by helping to custom-tune the SMU response to a given load. This provides an SMU response with minimal settling times (faster test) and eliminates overshoots and oscillations, protecting sensitive DUTs and also helps ensure system stability.
How does this work? Instead of the traditional analogue control loop, these are powered by digital control-loop technology. Because the control loop resides in the digital domain, this feature gives you the programmatic control of critical control-loop parameters, which, in turn, gives you the ability to train the SMU to behave in a certain way, to a specific load. You would typically find ideal control-loop settings for given types of loads or DUTs during the system development process. Once the ideal settings are identified and stored in the control programme, all you need is to know which DUT to test.
“By applying the appropriate settings for that particular DUT, you get the SMU to provide a perfect response without overshoots or oscillations (which is the primary cause of DUT damage for sensitive components and some MEMS sensors) and at the same time, without slowing down the SMU response (hence, faster test time),” adds Chivukula.
Keithley’s Model 2460 Source-Meter-branded SMU comes with a capacitive touchscreen graphical user interface (GUI). The press release states that it offers higher power sourcing (up to 105V, 7A DC/7A pulse, 100W maximum) with 0.012 per cent basic measurement accuracy and 6½-digit resolution, making it ideal for high-power, high-precision I-V characterisation of modern materials and high-power devices.
Targeting the LED lighting sector, Yokogawa Corp. of America is introducing a new multichannel SMU in addition to GS820 models, which feature isolated 2-channel source and measurement functions. These offer four-quadrant operation consisting of the current source and current sink. On the 50V model, voltage ranges will be 200mV, 2V, 20V and 50V. Current ranges are selectable from 200 nano-amps to one amp.
Photovoltaic power system simulation/emulation
Firms that work on photovoltaic cells, panels and solar arrays need to improve the micro-inverters in their solar panels. In order to test a good micro-inverter or solar inverter, you need to simulate cell behaviour, which is a really hard task. You might need to simulate sunny, cloudy, rainy or dark days and other similar weather changes.
An engineer using a normal power supply would find it difficult to transition from sunny to rainy. Why? You need a power supply with substantial power and high wattages, at the same time being able to modulate it from low to mid to high in a seamless manner. There are power supplies that are capable of doing this with effectively less noise, and Keysight’s N89xxAPV photovoltaic array simulator is one recent launch.
The simulator drops down voltage and power levels based on sunny days going to cloudy and then to rainy without the engineer having to code. “Previously you had to cascade power supplies. Doing that for a few supplies is fine but cascading many systems creates a lot of noise and makes it difficult to manage a bunch of boxes. Engineers need to ensure their solar inverters are capable of converting maximum power from the solar array to which these are connected. Developing and verifying the performance of complex inverter maximum power point tracking (MPPT) algorithms and circuits is also challenging,” says Mukul Pareek, marketing program manager – wireless and digital at Keysight Technologies.
Tektronix has recently released Keithley 2281S battery simulator and precision DC power supply. This battery simulator is claimed to offer the industry’s first emulation of battery performance from full charge to total discharge using a battery model that includes state of charge, amp-hour capacity, equivalent series resistance and open circuit voltage. “Battery simulation or power supplies with fast transient responses are designed to test battery-operated products. These can capture load-current pulses as narrow as 50µs and sleep-mode currents as low as a few µA to simulate a discharged battery and test charger control circuitry,” says Narasimhan.
Incremental specification improvements
“High-density equipment is one area where we see change. The number of channels that can be packed is significantly higher than what was previously covered, and it has now reached 70 channels in a 3U 48.3cm (19-inch) rack space. In our PXI product line, we now have a 200V, which is an increase from 60V SMU,” explains Chivukula.
SL Power Electronics announced its 425W power supply under the series TU425 family, which is claimed to be 90 per cent efficient and comes with active current share for redundant applications. It is compliant to IEC61000-4 standard and claims to help reduce the time spent on isolating the measurement part of a design from external electromagnetic influences.
HAMEG instruments, a subsidiary of Rohde & Schwarz, has come out with R&S HMC804x power supplies, which can feature up to three channels and features tracking multiple channels and enables something called FuseLink. This combines safety switch-off current limits for multiple channels. “The function is especially useful if two circuits with different voltages connected to a power supply require overload protection, while at the same time a third circuit needs to continue operating, for example, for a fan to provide interruption-free cooling,” states their press release.
N8900 is a new auto-ranging DC power supply that can handle 15kW per piece and in a cascade can provide up to 100kW. Maximum power output was 5kW in 2013. The increase from 5kW to 15kW was achieved without an increase in box size. This instrument allows you to define wattage or power, as well as maximum and minimum voltage and current.
Then, the power supply auto-ranges from minimum and maximum at full power. Auto-ranging saves money because it provides a wide range of power output and dynamic range from only one device. N7900 series from Keysight features an inbuilt dissipator unit which is in sync with the power supply. This saves the user a lot of bench space, the hazard of connections, reduction in programming requirements (you do not have to separately programme the dissipator), comes with better specs and is claimed to be the fastest bench power supply by Keysight.
NI launched PXIe-4139 that helps engineers get extended range-pulsing capability up to 500W and sensitivity down to 100fA. Additionally, NI PXIe-4139 system SMU can take measurements at 1.8MS/s, which is 100 times faster than traditional SMUs.
Teledyne Lecroy has introduced true 12-bit oscilloscopes in the market with software enhancement to 15 bits, providing the necessary accuracies that may be required for precision design. “We also introduced 8-channel scopes to address the 3-phase power design requirement and with motor-drive-analysis software, making it a versatile design, testing and analysis tool for any motor development. It provides flexibility of time-domain measurement and also specifics to power design at any stage of the circuit, whether it is analogue, digital or protocol signals, thus giving complete information of both electrical and mechanical measurements,” says Rajendran Thanu, speaking on behalf of Teledyne Lecroy. This instrument targets design engineers and manufacturers working on power supplies, including those working on 3-phase power supply designs and motor controllers or drivers.
Keithley introduced two high-voltage power supplies optimised for high-voltage devices and materials-testing and high-energy physics and materials-science research. Model 2290-5 5kV power supply and model 2290-10 10kV power supply are well-suited for high-voltage breakdown testing of power semiconductor components. This includes devices made of current- and next-generation wide-bandgap materials like silicon carbide (SiC) and gallium nitride (GaN), designed for use in green, energy-efficient power generation and transmission systems and hybrid and all-electric vehicles.
Intuitive user interfaces could be the next big thing
It was about a decade ago that user interface design began to define the success or failure of consumer products. It now seems that test instrument vendors are testing out if there could be a similar effect in the engineering tools and equipment space.
One such change is in battery simulators. In contrast to usual battery simulators targeting manufacturing applications, Keithley 2281S incorporates a thin-film transistor display that shows voltage, current, amp-hour readings, source settings and many additional settings. An icon based main menu provides access to source setup, measurement setup, display formats, trigger options and system settings.
A really interesting one is the new GUI on N89xxAPV that allows you to drag-and-drop a cloud or rainy environment, or change the time of rain. When manufacturers make these solar products, they create it based on the region they want to serve. So a US-focused product might be tuned to work without a lot of sun, but an Indian version might get a lot of sun.
The simulator allows these to test micro-inverters based on region by easily changing weather conditions, and the GUI makes it easier than a textual format. Coding using C or C# or other scripts are time consuming and difficult to get right. Previously, some original equipment manufacturers were even depending on pre-configured tests provided from other manufacturers and as a result were limited to those tests only.
Is it time to take a step back
While most specification-level feature advances are always welcomed by engineers, some online forums have often had this discussion of whether test equipment really needs touchscreens, remote control and other features inspired from consumer electronics. In fact, a recent survey done at EDN Network showed that more than half of the engineers who voted opted for knobs and buttons rather than touchscreens and tablets. Interesting, isn’t it?
