Testing a power supply 每 Noise (Part 2)

By authorswitching power supply,Switching Power Supply manufacturer,,AC DC power adapter,Cable,Power Bank,LED Power Supply,China 2013/9/11 23:22:04

Measuring line transient performance

Line transients are the droop and/or overshoot that takes place on the output of a power supply when an instantaneous change in input voltage takes. The transient amplitude is a function of many variables including the regulation circuit. The actual line voltage change and the change rate have a significant effect on the magnitude of the transient. For proper testing the worst case line voltage-step and slew rate must be defined.  

  1. Connect a dynamic load to output wires. Set the electronic load to the maximum output current expected.
  2. Set the DC power supply to switch between the minimum and maximum input voltage expected (manual or automatically, if the power supply is capable).
  3. Connect the power supply to the input.
  4. Connect the oscilloscope across the output capacitor using a very short ground spring or wire stub. Set oscilloscope for AC coupling (or DC coupling with appropriate offset voltage) with normal trigger. Bandwidth limiting may be enabled on the oscilloscope to help provide a cleaner waveform resulting in easier measurements.
  5. Apply power, adjust the vertical gain on the scope and measure the positive and negative transient spikes. Set the scope to normal trigger mode with positive slope for to capture the overshoot, and set to negative slope to measure undershoot and droop.


Note the peak voltages and the time it takes to settle back to within 10 percent of the nominal output voltage.

Switching ripple noise

Switching ripple noise is created when a switch-mode power supply and associated load charges and discharges the output capacitor, respectively, during every cycle of the pulse-width modulator (PWM) engine. The frequency will be that of the PWM oscillator and often looks like a triangle wave (Figure 3).




Figure 3. Switching power noise shown without bandwidth limiting.




Figure 4. Use bandwidth limiting to make it easier to measure switching ripple noise


  1. Before connecting the DC power supply to your power circuit, set the proper input voltage and verify correct polarity.
  2. Connect the DC power supply to the input.
  3. Set the electronic load to the maximum load expected and connect it to the output.
  4. Set the oscilloscope to bandwidth limit to avoid measuring the transient noise (Figure 4).
  5. Oscilloscope may be set to AC or DC coupling with an appropriate offset voltage as needed.
  6. Turn on the input power from the DC power supply.
  7. Connect an oscilloscope probe with a short ground stub directly across the output capacitor and set the scope to normal trigger mode with the trigger level at midpoint of the waveform.
  8. Measure and log the peak-to-peak value of the waveform disregarding any high frequency spikes (they will be measured in the next section).
  9. Log the frequency of the waveform.