TPS56121 Nyquist Plot judgement

What are the axes? I think the plots may be rotated or something. It would be helpful to see the unit circle for reference. The Nyquist plot represents the loop response as a complex number, and plots "real" part on the X axis, and the "imaginary" part on the Y axis. The way to judge stability from a Nyquist plot is to see whether the curve includes (-1,0) point. If the curve encircles this point (i.e. (-1,0) is inside the curve), then the system is unstable.

To look for the gain/phase margin at crossover (like you would in a bode plot)... look at where the plot crosses the Real axis in the left-half, the distance from this point to the (-1,0) point is the gain margin. Then find the point where the curve crosses the unit circle for the first time in the left-half, the angle between that point and the origin is the phase margin. lpsa.swarthmore.edu/.../NyquistStability.html

Matt-san,

X axis is real part, and Y axis is imaginary part. However positive and negative are displayed inverted. Thus it looks as usual if the Nyquist plot is rotated 180 degree.

Do you judge the previous Nyquist plot is stable or unstable?

And also, I took TPS56121EVM Bode and Nyquist plot.
Do you judge this is stable or unstable?

7851.TPS56121EVM_Bode_Nyquist Plot.pptx

Best Regards,
Kohei Sasaki