Load-step testing a power supply with an electronic load
How well does your supply survive a sudden load change? Use the SDL1000X transient mode to slam the current between two levels and read droop and recovery on the scope.
Static regulation tells you little about how a power supply behaves in a real system, where load current jumps — a motor starts, a radio transmits, a CPU wakes. A load step quantifies it: switch the output current between two levels fast, and watch how far the voltage droops and how quickly it recovers. Together with the Bode plot, it is the standard bench check of control-loop health.
Test setup
- SDL1000X electronic load on the supply output, in transient (dynamic) mode: level A / level B current, dwell times and slew rate — e.g. 0.5 A ↔ 2.5 A at 1 A/µs
- Oscilloscope AC-coupled on the output voltage, DC-coupled reference on a second channel if needed
- Trigger the scope on the voltage dip itself, or on the load current — see the note below on where that signal comes from
- Keep sense wiring short — measure at the supply terminals, not across long leads
Where the trigger actually comes from. The SDL1000X rear panel has an external trigger input (Ext Trig) — it accepts a trigger, it does not emit one. There is no sync or trigger output to drive the oscilloscope from. Two things that do work: trigger on the voltage dip on the output rail, which is the event you care about anyway; or use the load's current and voltage monitor outputs, a 0–10 V analogue representation of load current and voltage on rear-panel BNCs, brought to a second scope channel. The monitor output gives you a clean current trace to trigger on and to overlay on the voltage response, which is usually the more informative picture.
What to read from the waveform
- Droop / overshoot: peak deviation when the step hits — compare against your rail tolerance (e.g. ±5 %)
- Recovery time: time back into the tolerance band
- Ringing: damped oscillation is fine; sustained ringing means marginal phase margin — confirm with a Bode Plot II measurement
- Repeat across the input-voltage range and at temperature extremes for worst case
Slew rate matters: a step that rises in 10 µs exercises the output capacitors, not the control loop. Use the load's fastest sensible slew for loop testing, and the realistic application slew for system sign-off.
A supply that stays inside tolerance with clean, quickly damped recovery at the worst-case corner is ready for the real world — and the same setup doubles as your production go/no-go test.
Instruments used in this note
Transient mode with programmable slew
Catch droop & recovery — any SDS series
Siglent SPD/SPS bench supplies
Related application notes
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