HOW-TO GUIDE
How to Measure a Switching Power Supply Waveform With an Oscilloscope
By the OhmSavvy Research Team ·
Start on low-voltage converters only
- The method here is for DC-DC converters powered from a bench supply, battery or USB, such as buck modules and regulator boards.
- Inside a mains-powered SMPS, everything on the primary side (before the transformer) is at mains potential and the bulk capacitor can stay charged after unplugging. Do not clip an earthed scope probe there.
- Never remove the scope's earth to float it.
Where to probe a DC-DC converter
A buck converter chops the input into pulses and smooths them with an inductor and capacitor. Three points tell you most of what is going on:
| Probe point | What you see | What it tells you |
|---|---|---|
| 1 Gate drive | Pulse train at the controller's drive voltage | Whether the controller is switching at all |
| 2 Switch node (SW) | Rectangular pulses from about 0 V to V in | Frequency, duty cycle, edge quality |
| 3 Output | Steady DC with small ripple | Regulation and ripple (separate guide) |
1Connect with a short ground
Power the converter off. Set the probe to 10x and fit a ground spring if you have one, then find a ground pad or the output capacitor's negative terminal right beside the switch. Hold or tack the probe so the tip sits on the SW pin or the inductor terminal it connects to. Switch nodes have fast edges, and the long ground clip lead turns those into ringing that is not really there. Then power up.
2Set up the scope
Use DC coupling so the 0 V level stays on screen, and choose a V/div that fits the input voltage into about six divisions: 5 V/div for a 24 V input. Trigger on the rising edge at roughly half the input voltage. Converters typically switch somewhere between a few hundred kilohertz and a few megahertz, so start at 1 µs/div and adjust until two or three cycles fill the screen.
01
10x + spring
Short ground at the switch
02
DC coupling
V/div to fit V in
03
Rising edge
Trigger at half V in
04
Timebase
2 to 3 cycles on screen
3Read frequency and duty cycle
Count divisions for one full period and for the high part of the pulse, or add frequency and positive duty cycle in the measure menu. The worked example below is a converter stepping 24 V down to 5 V.
The same on-time and period method is covered in more depth in how to measure PWM duty cycle.
Expected duty cycle for a buck converter
In an ideal buck converter running continuously, duty cycle equals V out ÷ V in. Real converters need slightly more to cover switch and inductor losses, so a measured figure a few percent above the ideal is normal.
Boost converters run the other way
What the waveform shape tells you
| What you see | Likely cause |
|---|---|
| Clean pulses, steady frequency | Normal regulation |
| Ringing that shrinks with a ground spring | Probe ground lead, not the circuit |
| Groups of pulses with gaps at light load | Burst or pulse-skipping power-save mode |
| Duty cycle wobbling, edges jittering | Possible instability; check input, load and feedback |
| No pulses, gate drive missing | Controller not starting: enable pin, input voltage, fault |
| Switch node stuck high or low | Failed switch or controller in a protection state |
To see the edges properly you need enough bandwidth; our guide to oscilloscope bandwidth for switching supplies works that out from rise time. For the output side, follow how to measure ripple voltage.
Mains-input supplies
Chargers, laptop bricks and PC supplies are switching supplies too, but their primary switch sits at rectified mains voltage, well over 300 V in the UK. Probing it needs a differential probe rated for that voltage, or an isolation arrangement set up by someone trained to use it. On the low-voltage secondary side, the output and any secondary-side regulators can be probed with the method above.
Which scope to use
Frequency and duty cycle on a few-hundred-kilohertz converter are within reach of any bench scope. For a clear view of edges and ringing, more bandwidth helps, and a second channel lets you show gate drive and switch node together. Compare models in our best beginner oscilloscopes roundup.
Scopes in our reviews
100MHz, 4 channels: bandwidth headroom for switching edges.
50MHz, 4 channels, strong community documentation.
100MHz, 2 channels and a built-in waveform generator.
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Frequently asked questions
What should a switch node waveform look like?
On a buck converter it is a rectangular pulse train: high at roughly the input voltage while the top switch is on, then near 0 V (or slightly below, by a diode drop) while it is off. Expect some overshoot and a short burst of ringing on each edge. A clean, regular train at a fixed frequency means the converter is regulating normally.
Why does my switching regulator's waveform keep skipping pulses?
Many regulators enter a power-saving mode at light load, often called burst, pulse-skipping or PFM mode, where they switch in short groups and then pause. Add a load resistor or connect the real load and the pulses usually become continuous. Irregular skipping under full load is more suspicious and can point to instability.
Is the ringing on the switch node a fault?
Some ringing is normal: the switch's capacitance and the board's stray inductance form a resonant circuit. First swap the probe's ground clip for a ground spring, because a long ground lead adds ringing of its own. If large ringing remains with a short ground, layout or snubbing may need attention.
How do I calculate duty cycle from the scope?
Duty cycle is on-time divided by period, times 100%. Measure the time the pulse is high and the time for one full cycle, using divisions, cursors or the measure menu. For example, 0.5 µs high in a 2.5 µs period is 20%.
Can I probe the primary side of a mains-powered SMPS?
Not with a standard earthed scope and passive probe. The primary side is at mains potential and the scope ground is earthed, so the ground clip would short it. Use a high-voltage differential probe rated for the job, or measure only the low-voltage secondary side.
