Characterization and performance comparison of ripple-based control methods for voltage regulator modules

Three recently developed control methods for voltage regulator modules (VRM), namely, V/sup 2/ control, enhanced V/sup 2/ control, and enhanced V/sup 2/ control without output voltage dynamic feedback, are analyzed and compared in this paper. All three methods utilize the output voltage-switching ripple for pulse-width modulation, hence are collectively referred to as ripple-based control. A general modeling method based on the KBM (Krylov-Bogoliubov-Mitropolsky) ripple estimation technique is applied to develop averaged models for single-channel as well as multichannel buck converters utilizing each of the control methods. Unlike existing models that are limited to small-signal operation, the proposed models are valid for large signal and are capable of predicting sub harmonic instability without having to include an artificial sample-and-hold block as used in existing models. The paper also shows that adding parallel, high-quality ceramic capacitors at the output, which are ignored in previous works, can lead to pulse skipping and ripple instability, and a solution based on proper selection of the ceramic capacitors and/or ramp compensation at the pulse-width modulator is presented. The models are further applied to analyze and compare the performance of the three control methods in terms of ripple stability, effective load current feedforward gain, and output impedance.