DC-DC Converter Control Loop Stability
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 &lt; 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.