Efficiency Is More Than a Datasheet Number
A DC power supply may deliver the correct voltage and still waste a meaningful share of its input energy as heat. That loss affects far more than an efficiency percentage. It can raise operating temperature, reduce stability, shorten service life, and drain batteries faster in remote IoT sensors, communication modules, and other long-duration systems.
That is why efficiency should be measured across the operating range of the design, not treated as a single headline value. A practical test reveals how efficiently the device under test (DUT) converts input power into usable output power as the load changes.
Start with the Basic Efficiency Equation
Power supply efficiency is the ratio of output power to input power. For a DC system, power is the product of voltage and current:
Pin = Vin x Iin | Pout = Vout x Iout
Efficiency (%) = (Pout / Pin) x 100
At each load point, measure the input voltage and current as well as the output voltage and current. Those four values are enough to calculate efficiency at that specific operating condition.
One measurement, however, only describes one point. To understand the supply's behavior, repeat the calculation at multiple current levels and plot efficiency against load. The resulting curve makes it easier to see where the design performs well and where losses become more significant.
The Minimum Test Setup
The most basic arrangement uses a DC source to power the DUT and a DC electronic load connected to the DUT output. The electronic load creates a controlled current demand so the supply can be evaluated at repeatable operating points.
For every selected load current:
- Apply power to the DUT and allow the operating point to settle.
- Set the electronic load to the required current draw.
- Measure Vin, Iin, Vout, and Iout at that load point.
- Calculate Pin, Pout, and efficiency, then record the result.
- Advance to the next current level and repeat until the required load range is covered.
A useful efficiency curve typically requires 10 or more load-current steps. The exact number should reflect how much detail is needed and how quickly the DUT's behavior changes across its operating range.
Why Built-In Measurements May Not Be Enough
Many commercial power supplies and electronic loads offer approximately 3.5-digit measurement resolution. That can be convenient for setup and monitoring, but it may not provide enough resolution or accuracy to characterize a demanding DUT with confidence, especially when the efficiency differences under investigation are small.
A higher-resolution approach uses four digital multimeters (DMMs), with one instrument assigned to each critical quantity:
- Input voltage, Vin
- Input current, Iin
- Output voltage, Vout
- Output current, Iout
This separates source and load control from precision measurement. It also creates a modular setup whose measurement capability can be selected to match the DUT and the accuracy target.
An Example Instrument Configuration
The supplied SIGLENT application note describes an experimental setup using the following instruments:
- SIGLENT SPD1168X programmable DC power supply
- SIGLENT SDL1020X-E DC electronic load
- SIGLENT SDM3055 multimeters for Iin and Iout
- SIGLENT SDM3045X multimeters for Vin and Vout
The DUT in the example is a Linear Technologies LTM4646 power module. The model list illustrates one implementation of the method; the same measurement structure can be adapted to other compatible instruments and DUTs.
Automation Turns a Repetitive Test into a Repeatable One
Manually stepping through 10 or more load levels, configuring instruments, reading four channels, and calculating each result is time-consuming. It also creates opportunities for transcription errors, inconsistent settling time, and missed data points.
A computer-controlled workflow can coordinate the source, electronic load, and DMMs. At each programmed load point, the software can set the current, wait for the system to settle, collect all four measurements, calculate efficiency, and store the result for plotting.
A robust automated sequence should include:
- A defined start, stop, and increment for load current
- A consistent delay or settling criterion before measurements
- Instrument range and configuration control
- Validation for overload, compliance, or unexpected readings
- Structured data logging for traceability and later analysis
The real value of automation is not only speed. It improves consistency, makes the test easier to repeat after a design change, and produces a clean dataset for comparing revisions or operating conditions.
Two Measurement Details That Can Distort the Result
1. Keep Load Leads Short
Current flowing through test leads creates an I x R voltage drop. As current increases, long or resistive leads can reduce the voltage available at the load and change the apparent operating point. Keeping the load connection short helps minimize this error.
2. Check DMM Burden Voltage
A DMM measuring current introduces a voltage drop on its current range, commonly called burden voltage. If that drop is too high for the expected current, the DUT may reach its output-voltage limit before the test reaches the intended maximum load. Select a DMM and current range with sufficiently low burden voltage for the measurement.
Practical check: Before trusting the final curve, confirm that the DUT remains within its intended voltage limits at the highest load and that the wiring and ammeters are not consuming an excessive share of the available voltage.
From Four Measurements to a Useful Design Decision
A credible DC power supply efficiency measurement does not require an exotic test platform. It requires the right quantities, enough load points, appropriate measurement resolution, and careful control of the setup itself.
Begin with Vin, Iin, Vout, and Iout. Calculate efficiency at each programmed load. Use precision DMMs when the built-in readings of the source and load do not meet the accuracy requirement. Then automate the sequence when repeatability, speed, or comparison across design revisions matters.
The result is more than a percentage. It is an efficiency profile that shows how the power supply behaves where the application will actually use it - and gives the engineering team a stronger basis for thermal, battery-life, reliability, and design trade-off decisions.
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