  A general and exact critical condition of saddle-node bifurcation is derived
in closed form for the buck converter. The critical condition is helpful for
the converter designers to predict or prevent some jump instabilities or
coexistence of multiple solutions associated with the saddle-node bifurcation.
Some previously known critical conditions become special cases in this
generalized framework. Given an arbitrary control scheme, a systematic
procedure is proposed to derive the critical condition for that control scheme.
