Switching Frequency Determination of DC-DC Converters With Hysteretic Control

Hysteretic control provides unique advantages in dc-dc converter applications but it is less popular than other types of control because the switching frequency depends on the operating point, the feedback ripple, and the component parasitics. Furthermore, it is difficult to predict the frequency due to the complex interaction of those factors. This paper proposes harmonic balance analysis to systematically determine the switching frequency of the buck and boost-like converters with hysteretic current control, and extends the method to buck converters with hysteretic voltage control. The analysis includes the most important second-order effects (feedback ripple, component parasitics, a second output capacitor, and switching delays). For the less complex cases the switching frequency is expressed in closed form, and for the more complex cases the paper proposes a graphical solution. The predictions are verified by simulations and, for the buck converter with hysteretic current control, by experiments. The derived results are useful for designing a converter with predictable switching frequency, and can also be employed during the design process of a converter with synchronized or stabilized frequency. Given an arbitrary control scheme or circuit variation, the switching frequency can be readily obtained by following the proposed approach.