Characterizing fast-scale instability in a buck-based switching amplifier for wideband tracking

This paper provides a first exploration of fast-scale stability borders for switching power converters operating as wideband switching amplifiers. Such operation is required in audio amplifiers, envelope trackers in polar RF transmitters and line drivers for Power Line Communications. A buck converter with output voltage proportional feedback in a signal tracking configuration with a sinusoidal reference is considered (Figure 1). The work first characterizes the effect of the classical set of parameters used in regulation stability analysis (output resistance, reactive components, switching frequency, feedback gain), and considers the output voltage to cover the complete dynamic range and hence disappear as a variable in the parametric space. Complementarily, the effect of the ratio of the tracking or baseband frequency to the switching frequency is added as a new variable in the design space indicative of wideband tracking operation. Characterization tools encompass families of time domain simulations as dynamic bifurcation diagrams for the time-varying reference. The paper concludes by exploring the effect of momentarily losing fast-scale stability upon the output signal spectrum, since applications such as adaptive supply of RF power amplifier are subject to strict spectral masks.