Load-step testing a power supply with an electronic load

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 load's sync/trigger output (or on the voltage dip itself)
• Keep sense wiring short — measure at the supply terminals, not across long leads

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