To measure a switch-mode power supply (SMPS) accurately, match the probe to the circuit node, observe the probe’s voltage and safety ratings, and set the oscilloscope to capture the signal of interest without obscuring it with offset, bandwidth, or timing errors. A scope can characterize input, switching, and output behavior; control-loop and power-supply rejection ratio (PSRR) frequency sweeps are generally better suited to a vector network analyzer (VNA) or frequency-response analyzer (FRA).
The right setup depends on the question: output ripple, a switching transition, input inrush, and loop response are different measurements—not one universal scope test. Tektronix’s power-supply measurement guide and Keysight’s SMPS measurement note describe these measurement categories and the setup considerations involved.
What can you measure with an oscilloscope?
An oscilloscope displays voltage over time and, with a suitable current probe, can capture current alongside voltage. That makes it useful for examining short switching events and time-varying output behavior. It does not mean every power-supply measurement uses the same probe or that a scope alone is the right tool for every test.
| Measurement group | Examples | Key setup consideration |
|---|---|---|
| Input | Input power quality, current harmonics, and inrush | Use input voltage and current measurement equipment rated for the circuit, and configure acquisition to capture the event or waveform of interest. See Keysight’s measurement note. |
| Switching | Conduction behavior such as RDS(on) or VCE(sat), switching loss, slew rate, and modulation | Depending on the node and measurement, use differential voltage and current probes; control offset and channel timing. See the Tektronix switching-loss note and its power-supply measurement guide. |
| Output | Ripple, turn-on and turn-off behavior, transient response, and efficiency | Probe connection, noise, bandwidth, and operating conditions affect what the scope displays. Follow the supply specification or applicable measurement standard when comparing results. See Tektronix’s voltage-probing guide. |
| Frequency response | PSRR and control-loop gain and phase | These frequency sweeps typically require a VNA or FRA and an appropriate injection and measurement setup rather than only a basic time-domain scope capture. See Keysight’s SMPS characterization overview. |
The vendor application notes describe instrument techniques and features; they do not set pass/fail limits for a particular supply. Use the supply’s own specification and any applicable standards to decide whether a measured result is acceptable.
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How should you plan the measurement?
- State the question. Decide whether you need ripple, transient response, inrush or input behavior, switch conduction, switching loss, or frequency response. This determines which nodes, probes, and acquisition settings matter.
- Identify the nodes and hazards. Use the schematic and component documentation to determine whether a node is ground-referenced, floating, high-side, or otherwise hazardous. Check the voltage, differential, common-mode, and safety ratings of the scope and every probe before connecting them.
- Choose the probes and scope configuration. Match probe type, attenuation, bandwidth, and range to the expected signal. Set sample and acquisition modes, bandwidth limits, and record length to preserve the event or signal detail you need while controlling noise.
- Prepare the probes. Null differential-probe offset and degauss a current probe when its instructions call for it. If you will compare voltage and current in switching-loss analysis, deskew the channels before capturing the transition.
- Capture and document the result. Record the operating conditions and instrument setup along with the waveform. Repeat under relevant input and load conditions when the design question calls for it.
A probe’s attenuation ratio alone does not establish that a connection is safe. Follow the probe and scope manuals for connection instructions and ratings. Tektronix’s probing guide and switching-loss note discuss probe choice and measurement setup.
What probe do you need for a switch node?
Voltage on a floating or high-side node
A switch node may move rapidly and sit at a voltage that is not referenced to scope ground. A differential probe is often needed to measure voltage across a switching device or between two non-ground-referenced points. Select it for both the expected differential voltage and common-mode voltage, as well as sufficient bandwidth, attenuation, and safety rating for the actual circuit. A probe that meets one rating may still be unsuitable if another limit is exceeded.
Do not treat a switch node as an ordinary ground-referenced signal by default. Confirm the circuit reference and the probe’s connection method and limits in the relevant manuals before attaching it. Tektronix’s guide to probing power converters covers differential probing and voltage-measurement considerations.
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Current through a switching device
For switching-loss work, use a current probe suited to the expected current range and waveform. Check its bandwidth and setup requirements, and degauss it where applicable. The current probe must capture the transition with useful signal range; a trace that is clipped, offset, or dominated by noise cannot support a reliable loss calculation.
What to compare when choosing equipment
- Voltage and common-mode ratings for voltage probes, and current range for current probes.
- Bandwidth and attenuation or noise characteristics appropriate to the signal.
- Enough oscilloscope channels to capture the required signals simultaneously.
- Probe delay and the scope’s deskew support for timing-sensitive voltage/current measurements.
- Compatibility with the intended scope, its software, and any deskew fixture.
A deskew fixture can help align probe timing for switching-loss measurements. Power-analysis software may automate supported measurements on compatible instruments, but it does not remove the need for correct probing, ratings, and setup. Keysight’s characterization overview describes scope and probe roles, including deskew; Tektronix’s switching-loss note covers manual and automated approaches.
How do you measure output ripple?
Ripple is the AC variation superimposed on the supply’s DC output. It may be reported as a peak-to-peak voltage or as a percentage of nominal output voltage. State which form you report: for percentage ripple, calculate (measured peak-to-peak ripple ÷ nominal output voltage) × 100%.
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- Check the required method. Consult the supply specification or applicable standard for the measurement point, connection method, and bandwidth. Different methods can produce different observed ripple.
- Connect a suitable voltage probe. Use the prescribed measurement point and connection method, and keep within the probe’s ratings. The probe setup can affect noise and the waveform the scope sees.
- Set acquisition and bandwidth deliberately. Apply only a bandwidth limit consistent with the required method; a limit can reduce displayed noise but can also remove signal content. Select acquisition settings that capture the ripple rather than an unrelated transient or an unrepresentative interval.
- Report the conditions with the value. Identify whether the result is peak-to-peak or percentage of nominal output, and include the bandwidth and connection method. Note the input and load conditions if they are relevant to the specification or comparison.
Tektronix’s probing guide discusses ripple and voltage-probe setup; its power-supply guide covers scope settings and measurement considerations.
How do you measure MOSFET switching loss?
Switching loss is associated with the interval in which voltage across the device and current through it overlap during a transition. Capture both waveforms at the same time, then align their timing. Even a small timing error can change the apparent overlap around a brief turn-on or turn-off event.
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- Prepare the probes and scope. Null differential-probe offset, degauss the current probe when applicable, and deskew the voltage and current channels. Make sure both waveforms fit the available range and preserve the relevant switching detail.
- Capture a simultaneous transition. Use acquisition settings that resolve the turn-on or turn-off event. Apply a bandwidth limit only when it suits the measurement purpose, and state the settings used.
- Calculate power and energy over a defined interval. Compute instantaneous device power as p(t) = v(t) × i(t). Integrate p(t) over the selected turn-on or turn-off interval to obtain energy for that transition. State the interval and acquisition conditions; changing the interval can change the reported result.
- Relate event energy to operation only when appropriate. If you convert transition energy to an average switching-loss contribution, identify the operating conditions and switching frequency used. Do not present a single capture as a universal loss value for the supply.
Probe offset, current-probe degaussing, bandwidth, dynamic range, and voltage/current channel skew can all affect the result. Tektronix’s switching-loss application note explains manual and automated measurement approaches, and its power-supply guide covers related setup issues.
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How do you capture other important SMPS behavior?
Input quality and inrush
Input power quality, current harmonics, and inrush call for appropriate voltage and current measurement equipment and acquisition settings for the waveform or event. Confirm that the instruments and probes are rated for the input circuit. A scope can capture time-varying behavior, but the relevant input test method and acceptance limits come from the supply specification or applicable standards.
Switch conduction, slew rate, and modulation
Conduction measurements such as RDS(on) or VCE(sat), along with slew rate and modulation, require probing the relevant node or device terminals without exceeding probe limits. For fast events, offset, bandwidth, signal range, and connection method affect interpretation. Keysight’s measurement note and Tektronix’s guide describe these categories and related setup considerations.
Transient response and efficiency
For output transients, capture the output during the relevant change in load or input and preserve the event in the acquisition record. For efficiency, measure input and output power using a method appropriate to the supply and its operating conditions; a scope may contribute to that characterization, but the necessary probes, measurements, and analysis depend on the test method. Repeat under the input and load conditions relevant to the design question rather than treating one operating point as representative of all conditions.
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When is an oscilloscope not enough?
A time-domain capture is not the same as a frequency-response measurement. PSRR and control-loop gain-and-phase characterization commonly use a VNA or FRA with a suitable injection and measurement setup. Choose that equipment and method when the question is how the supply responds across frequency, rather than what a voltage or current waveform does over time. Keysight’s SMPS test and characterization overview describes the distinction.
What should you record for repeatable results?
Record enough information for someone to understand how the waveform was obtained and reproduce the operating point. Include:
- Oscilloscope and probe models, probe attenuation, and the channels used.
- Probe range and relevant voltage, common-mode, or current ratings.
- Bandwidth limits, sample and acquisition mode, and record length.
- Probe offset and current-probe degaussing steps, where applicable.
- Voltage/current deskew method or settings when measuring switching loss.
- Input voltage, load, and other operating conditions relevant to the result.
- For ripple, the reported measure, bandwidth, and connection method; for switching loss, the transition interval and calculation method.
Compare the result with the supply’s specification and any applicable measurement standard, using the same relevant method and conditions. Instrument-vendor application notes explain technique; they do not establish a particular supply’s acceptance limits.
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