HOW-TO GUIDE
How to Measure Power Supply Startup Time With an Oscilloscope
By the OhmSavvy Research Team ·
Safety: grounds, mains and stored energy
- A scope probe ground clip is connected to mains earth through the scope. On the primary side of an off-line supply it can create a short circuit. Measure the isolated output, or use a rated differential probe.
- Large input and output capacitors can stay charged after power is removed. Discharge them before moving probes.
What power supply startup time means
Startup time is measured from a defined start event to a defined end point. The start is usually the input voltage crossing a threshold, or the enable signal going high. The end is the output reaching 90% of its nominal value, or settling within its regulation tolerance, which is a stricter test. Datasheets don't all use the same definition, so write down which one you used next to the number.
| Term | Usually measured from | To |
|---|---|---|
| Turn-on (start-up) delay | Input or enable applied | Output starts to rise, often 10% |
| Output rise time | Output at 10% | Output at 90% |
| Startup time | Input or enable applied | Output at 90% or within regulation |
| Power-good delay | Output in regulation | Power-good pin asserted |
1Connect two channels: input on CH1, output on CH2
Clip CH1 to the converter input or its enable pin and CH2 to the output, as close to the converter as you can. Both ground clips go to the converter's ground, not to a distant point. Use DC coupling on both channels: AC coupling would hide the very step you are trying to time. Set each channel so its full swing uses most of the screen height without clipping.
2Single trigger on CH1's rising edge with a slow timebase
Set the trigger source to CH1, rising edge, at about half the input's final voltage, and choose Single mode so the scope captures one turn-on and stops. Move the trigger point one or two divisions from the left of the screen so the rest of the display shows what follows. Pick a timebase that spans a few times the expected startup time; if the datasheet says 5 ms, start at 2 ms/div. Arm the scope, then switch the supply on.
If nothing captures, the edge may not reach the trigger level, or the input may already be present from a previous attempt. Our guide on how to trigger an oscilloscope explains Single and Normal modes.
3Read the delay with cursors or a delay measurement
Place vertical (time) cursors at the CH1 trigger edge and at the point where CH2 reaches 90% of its final value. The delta is the startup time. Many scopes also have an automatic delay measurement between two channels; by default it usually measures between 50% points, so change the reference levels to 90% for CH2 if your scope allows it, or rely on cursors.
The same two-channel timing method is used for logic in how to measure propagation delay, on a much faster timescale.
4Start-up delay vs rise time
Splitting the result into two parts tells you more. The delay from enable to the first 10% of output is the converter's internal wake-up: reference settling, undervoltage lockout and control circuits. The 10% to 90% portion is the output rise time, mostly set by soft-start and the output capacitance. Measure the second part with horizontal cursors at the 10% and 90% levels, exactly as in measuring rise time with oscilloscope cursors, just on a slower timebase.
5Check soft-start, monotonic rise and overshoot
The shape of the ramp matters as much as its timing. Soft-start deliberately slows the rise to limit inrush and avoid overshoot, giving a smooth straight or gently curved ramp. Processors, FPGAs and memory often require a monotonic rail: one that only ever rises during start-up. A dip or plateau partway up, often caused by a load switching on at a threshold, can upset their reset circuits.
Measure overshoot separately
Inrush current at start-up
Charging the input capacitors draws a surge of current when power is first applied. To see it, add a current probe on the input lead, or a small, low-value shunt resistor in the input return with the scope measuring across it; keep the shunt on the ground side so the probe ground clip stays at circuit ground. Trigger on the same CH1 edge and the current pulse appears alongside the voltage. A peak far larger than expected can trip upstream protection or a bench supply's current limit, which in turn stretches the startup time.
Power sequencing of multiple rails
Boards with several rails often specify an order, for example core voltage before I/O, and sometimes a minimum gap between them. Put the input or enable on CH1 and each rail on its own channel, then trigger on CH1 as before. Read each rail's 90% point with cursors and check the order and the spacing against the requirement. With two channels you can still do it pairwise, one rail at a time against the same trigger. For what happens after start-up, see how to measure a switching power supply waveform.
Scopes in our reviews, by channel count
4 channels, 50MHz, 1 GSa/s: input plus three rails in one capture.
4 channels, 100MHz, 1 GSa/s, with a well-regarded user interface.
2 channels, 100MHz, 1 GSa/s, plus a built-in waveform generator. Enough for one rail at a time.
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Compare all four scopes we cover in the best beginner oscilloscopes roundup.
Frequently asked questions
What is a typical power supply startup time?
It varies widely with the design. Small DC-DC converters with internal soft-start often reach regulation within a few milliseconds, while larger supplies with big output capacitors, or adjustable soft-start set by an external capacitor, can take tens of milliseconds or more. The datasheet usually gives a typical figure and the test conditions, which you should copy when measuring.
Should I trigger on the input voltage or the enable pin?
It depends on what you need to know. Trigger on the enable pin when the input is already present and you care how fast the converter responds to a turn-on command. Trigger on the input when the supply starts as soon as power arrives. Datasheets often specify both, so match the reference your requirement or datasheet uses.
Why does startup time change with load?
The converter has to charge the output capacitance and supply the load at the same time, and soft-start usually limits how fast the output may rise. A heavier load takes some of the available current, so the ramp can slow. Some converters also enter current limit during start-up with large capacitive loads. Measure at the load your circuit really presents.
How do I see overshoot at start-up if the timebase is slow?
At slow timebases the scope may lower its sample rate, and a brief overshoot can fall between samples. Turn on peak detect if available, or take a second capture at a faster timebase with the trigger delayed to the moment the output reaches its final level. Then measure the peak above nominal with horizontal cursors.
Can I measure startup time on a mains-powered supply?
Yes, but do not clip a standard probe ground to the primary side. Measure the isolated low-voltage output, and time it from a secondary-side signal such as a power-good pin, or put a rated differential probe on the AC input. Switching the mains on gives a random point in the AC cycle, so repeat the capture several times.
