Tale of the Bode Plot Failure

by Mike Jones, Applications Engineer, Mike’s blog, EEWeb, EEweb, Jan 29 2011

Focus:
In this blog, the author relates a power supply design experience in which his bode plots of gain and phase margin (simulated in Matlab) failed to predict an instability observed in bench measurements. Jones relates his analysis, referencing the Barkhausen criteria and conditions that led him to apply the Nyquist stability criterion. Using his Matlab model to do a Nyquist plot ultimately confirmed that the system was unstable. The author describes the circuit modification used to obtain stability and points out the underlying assumptions that apply when using phase margin to predict stability.

What you’ll learn:

• How to understand why Bode plots sometimes fail to predict instability in power supply designs
• How to understand why Nyquist plots are needed in some cases to verify stability of power supply designs

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