0
\$\begingroup\$

I have asked a similar type of question previously. This one is a little different.

Assume, we have the Bode plot of a Control Loop System. In this case, DC-DC Converter.

I measure the Gain Crossover Frequency and Phase crossover Frequency. Does the difference between the gain cross over frequency and phase crossover frequency, provide any meaning or make any sense? Any insight can be drawn?

Does this difference make sense or provide any insight = Gain Crossover frequency ~ Phase crossover frequency?

And can someone give me some concept or idea about the relation between the crossover frequency and the switching frequency of the DC-DC Converter? An analogy might be helpful.

Thanks.

\$\endgroup\$
6
  • \$\begingroup\$ What precisely do you mean by "crossover frequency"? \$\endgroup\$
    – Andy aka
    Oct 1 '19 at 10:46
  • \$\begingroup\$ Does the difference between the two cross over frequency mean anything or make any sense? \$\endgroup\$
    – Newbie
    Oct 1 '19 at 14:03
  • \$\begingroup\$ Hmm...What precisely do you mean by "crossover frequency"? \$\endgroup\$
    – Andy aka
    Oct 1 '19 at 14:19
  • \$\begingroup\$ Just some variable \$\endgroup\$
    – Newbie
    Oct 1 '19 at 14:27
  • \$\begingroup\$ You are undermining your own question by not explaining properly what you mean by "cross-over frequency". \$\endgroup\$
    – Andy aka
    Oct 1 '19 at 14:30
2
\$\begingroup\$

Usually the gain crossover frequency is defined as the frequency at which the open-loop gain crosses zero dB. The phase crossover is usually the frequency where the phase crosses -180 degrees.

The difference in frequency between the two can give hints about stability (E.g. if the phase crossover is much higher than the gain crossover it's likely that the system is stable).

If the gain crossover frequency is equal to the phase crossover frequency then you have by definition zero phase margin and your system will be unstable.

However, the phase margin (Difference between the phase and -180 measured at the gain crossover frequency) and the gain margin (The difference between unity gain and the gain measured at the phase crossover) are the traditional measures of stability.

There's no predetermined relationship between the control loop crossover frequency and the converter's switching frequency. A switching converter is a sampled data system though, and the phase lag and ZOH phase response of the loop will make closing the control loop near the switching frequency impossible.

So the loop crossover is typically < 1/5 of the switching frequency, but that's a rule of thumb, not a physical relationship. Converters with a right half plane zero may need to close the loop with lower bandwidth.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.