# Some tips needed to calculate phase margin.

Discussion in 'General Electronics Chat' started by supermankid, Sep 13, 2013.

1. ### supermankid Thread Starter Member

May 26, 2013
40
1
I recently was reading stuffs on control engineering and learned about the phase margin. I thougt I understood untill I saw this picture. Normally they consider phase margin as the difference between the angle at 0 db to 180 degree.

so if I have phase shift of -140 degrees(at 0 dB) then, I have phase margin of (-140-(-180))=40 degrees

ok

Now I saw a picture of a power supply bode plot and saw that at 0dB, the phase shift was about 63 degree. and i thougt
the phase margin was
60-(-180)=220 huhhhh!!

but actually the phase margin was just 63....

I thougt we have phase margin for -180 phase shift only cause the negative feedback will become positive after that...
so we want to avoid this situation....but i didnot understand how to calculate phase margin on the second case...

some help needed!!

case 1

case 2

2. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
508
Makes no sense. If the phase shift is 63, the phase margin is 117.

To understand it simply: the phase shifts as it goes through the feedback loop. If it gets to -180 degree shift, it is in phase with the source and becomes positive feedback (unstable). The phase margin is how far away you are from the -180 point. So if you have shifted 63, you have 117 left before you hit 180. Don't get hung up on the negative sign. Feedback in a control loop has to be negative to be stable, that's why when you get to 180 it goes unstable: it becomes positive feedback.