Design Con 2015

Testing a power supply – Noise (Part 2)

Robert Hanrahan -April 10, 2013

Read the introduction to this series in Part 1, and the conclusion in Part 3, which covers stability.

This is Part 2 of a three-part series which discusses how to properly test a DC/DC power supply to ensure it works reliably over various operating conditions. The series is intended to provide the design engineer with a sufficient understanding about some, but not necessarily all, of the testing needed to verify a reliable power supply design.

Part 1 covers the fundamentals about testing, including the necessary equipment and how to prepare a circuit for testing. Part 1 covers how to accurately measure start-up time, current limit, and power supply efficiency.

In Part 2, we discuss how to measure various sources of noise and output voltage errors found in a switch-mode power supply. We also discuss good oscilloscope probing techniques to help ensure accurate measurements.

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Video 3: Power supply noise

 

Good probing technique for measuring noise

When measuring noise with an oscilloscope, be careful to ensure what you see is actually noise from the circuit – not noise being coupled into the scope probe. For an accurate measurement, use the shortest ground stub available with your passive probe, or use an active/differential probe with probe pins. An active probe generally provides the best results, yet many have voltage limits that are below the voltage you might be measuring. Also, some active probes have limited AC coupling capability, resulting in difficulty when measuring noise. Reference the operating manual for the specific active probe before attempting to use it for power supply testing (Figure 1).

 


 

Figure 1.  When measuring noise do not use long ground wires

Noise

Power supply noise can be generated from many different sources. Like any amplifier, all power supplies generate low levels of noise, such as thermal noise, while switch-mode power supplies generate noise from their inherent switching. While linear regulators also produce noise, that topic will not be discussed here. 

For best results use an oscilloscope with appropriate bandwidth and sample rate. As discussed earlier, be sure to use a good passive or active probe for these measurements. Otherwise, ambient noise can be picked up and cause false readings. If a passive probe is used, a short ground spring or ground stub is needed. Place it directly across an output capacitor. Be careful to avoid placing the probe or probes ground wire near any inductor or transformer as this may cause magnetic coupling into the probe, again, resulting in false readings. Basic switch-mode power supply noise can be separated into two different types: ripple, and transient switch noise.

Measuring Load transient performance

Load transients are the droop and/or overshoot, which take place on the power supply output when an instantaneous change in load current takes place on its output (Figure 2).

 

 

Figure 2. Load transients are caused by a fast change in load current.

 

The transient amplitude is a function of many variables, including the regulation circuit. The actual load current change and rate of change has a significant effect on the transient’s magnitude. For proper testing, a worst case load step and slew rate must be defined.

 

  1. Before connecting the DC power supply to your power circuit, set the proper input voltage and verify correct polarity.
  2. Connect the dynamic load to the power supply output. Set electronic load to provide a switching load from 10 to 90 percent of maximum output current (or pre-determined levels determined to be realistic limits for the specific system). Set the switch duty cycle to 50 percent and frequency of switching to a few hundred Hz. Set slew rate on the electronic load as appropriate for the load being driven, or per the system specification.
  3. Connect an oscilloscope across the output capacitor using a very short ground spring or wire ground stub. Set the oscilloscope for AC coupling or DC coupling with appropriate offset. Using normal trigger mode, set bandwidth limiting on the oscilloscope to help provide a cleaner waveform, resulting in easier measurements.
  4. Connect the power supply to the input.
  5. Apply power, adjust the vertical gain on the scope to measure the positive and negative transient spikes. With normal trigger mode, set a positive slope to capture the overshoot (when the load is reduced). Set a negative slope trigger to measure undershoot and droop (when load in increased).
  6. Note the peak voltages and the time it takes to settle back to within 10 percent of the nominal output voltage. Note: Though not necessary, it may be useful to trigger the oscilloscope off the electronic load’s trigger output, if provided.

 


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