Tektronix App Note: Measuring the Control Loop Response of a Power Supply Using an Oscilloscope

Most power supplies and regulators are designed to maintain a constant voltage over a specified current range. They are essentially amplifiers with a closed feedback loop. An ideal supply needs to respond quickly and maintain a constant output, but without excessive ringing or oscillation. Control loop measurements help to characterize how a power supply responds to changes in output load conditions. Although frequency response analysis may be performed using dedicated equipment, newer oscilloscopes may be used to measure the response of a power supply control loop. Using an oscilloscope, signal source and automation software, measurements can be made quickly and presented as familiar Bode plots, making it easy to evaluate margins and compare circuit performance to models. A Bode plot maps the frequency response of the system through two graphs – a magnitude plot and a phase plot (phase shift in degrees). From these plots gain margins and phase margins can be determined to gauge power supply stability.

From this application note, you will learn:

  • An overview of the basics of control loops, frequency response analysis and Bode plots

  • A review of gain and phase margins

  • How to set up control loop response analysis on an oscilloscope

  • How to interpret frequency response plots and measurements

This application note uses a 5 Series MSO oscilloscope with Option 5-PWR Advanced Power Measurement and Analysis to demonstrate the principles of Bode Plots. The 4 Series MSO and 6 Series MSO have similar power analysis options with similar capabilities. The controls are very similar between these three instruments so this application note can be used to help understand the application of any of these three oscilloscopes in frequency response analysis.

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