R&S®Essentials | DC Power supplies fundamentals
Author: James Lewis l Test & Measurement expert and blogger
Source measurement units (SMU) serve as versatile instruments for electronics testing and measurement. They are adept at accurately measurement voltage and current and can function as power supplies and electronic loads. Their flexibility makes them especially valuable for engineers working with IoT and wireless devices.
A source measurement unit (SMU) is a test instrument that combines a power supply, an electronic load and a high-resolution digital multimeter into a single unit. SMUs can be used to characterize semiconductors, monitor a microcontroller’s power consumption or simulate a battery for an IoT device.
The main difference between an SMU and a bench power supply is the SMU’s ability act as both a source and a sink current as well as its high readback/programming resolution.
SMUs can track dynamic changes. For example, a Bluetooth® low energy device may draw current in the single-digit nanoamp range while in sleep mode but use 100s of milliamps while reading a sensor and transmitting the results. An SMU can measure both of those current ranges, all while supplying power to the device under test (DUT).
Another SMU measurement is characterizing semiconductor devices. A four-quadrant SMU's output terminals act as a source or sink, regardless of the polarity. This makes it possible to characterize the forward and reverse characteristics of semiconductor devices, such as diodes, with a single instrument.
Read on to learn about five SMU applications and how they can help you characterize a DUT.
Since an SMU is essentially the integration of three different instruments, it has many use cases. For starters, an SMU can be used as a bench power supply with safety features such as over-voltage protection and current limiting. In addition, the built-in 6.5-digit multimeter can measure voltage, current and power consumption while sourcing power for the DUT.
Unlike a standalone bench power supply, a two-quadrant SMU can be a source or sink. The SMU performs similarly to an electronic load when acting as a sink. An electronic load is like a programmable resistor. You can put the load into a mode that drops a constant voltage, draws a constant current, or presents a constant resistance to the device under test.
Source measurement units, such as the R&S(R)NGU201 series, can automatically switch between source and load. This flexibility means you can simulate charging and discharging battery behaviors with your device.
An SMU’s built-in digital multimeter can monitor voltage, current and power consumption, whether the tool is acting as a source or load. It is helpful to see the instantaneous values on the front panel or via remote programming commands. However, seeing how those values change over time can be important when characterizing a device.
An SMU can sample those measurements as fast as 500,000 times per second (500k samples/s). This allows the SMU to act as a data-logging instrument. For example, you can save the sampled data to a CSV file on a USB drive for detailed analysis. Alternatively, you could display the trends directly on the front panel display.
In addition to capturing this logged information, you can improve the accuracy using an SMU's "sense inputs." Since there are losses in the cables between the SMU and the DUT, the voltage at the front panel connectors is higher than the voltage presented to the DUT. Dedicated "sense" inputs with separate cables connected at the DUT input allow the SMU to compensate for those losses and provide more accurate measurements.
Another improvement is specific to the R&S®NGU201. It offers an optional digital voltmeter (DVM) that you can place anywhere in a circuit. For example, you could monitor the state of a battery or the output of a point-of-load converter. This DVM operates parallel to the voltage measurement monitoring the front panel and is galvanically isolated from the channel.
Devices that implement low-power sleep states and have wireless transmitters have dynamic current demands ranging from nanoamps to amps. An SMU can accurately measure these wide current ranges while supplying power to the DUT.
When the DUT changes operational states, there is a sudden change in current consumption. A bench power supply takes some time to respond to a load change. Traditional stand-alone power supplies may respond poorly to these transitions. Instruments like the R&S®NGU series, on the other hand, can react to a load response change in less than 30 microseconds.
When characterizing a semiconductor device, you typically need to sink and source current with forward and reverse polarities. For example, consider a diode.
To draw a diode’s current-voltage (IV) curve, you must measure the current drawn from a negative voltage until you reach a positive forward voltage. While a traditional power supply, or a two-quadrant SMU, can provide a negative voltage, switching between negative and positive requires manual intervention, i.e., you must physically change the probes. A four-quadrant SMU, however, can seamlessly switch from a negative output to a positive output as it sweeps through a voltage range.
There are multiple challenges with the operational life of battery-powered devices. One challenge is that batteries store energy chemically. As a result, their power delivery varies with chemistry, temperature and load. Another challenge is that the efficiency of a DC-DC converter changes the input voltage and output load.
Testing all possible variations with physical batteries is not feasible, especially if you want to try different chemistries like lithium-ion or lithium polymer. Fortunately, SMUs like the R&S®NGU can simulate a battery. Additionally, you can program them with user-defined profiles to better match your environmental conditions.
An SMU can characterize how long your device will run under varying conditions - without any additional instrument. This is the most significant advantage of using an SMU to simulate a battery.
Another point of evaluation is a device's ability to recharge a battery. Using an SMU as an electric load means that you can simulate situations such as no cell attached, over-voltage, under-voltage or battery shorting.
Are you looking for an SMU? Our experts can help.
