Variable frequency digital PWM controller for low-power buck converters

This Paper describes a novel digital pulse width modulation (DPWM) controller for low-power buck converters that can adapt its switching frequency according to the actual load current during runtime. This new strategy overcomes the drawback of high output voltage ripples at light loads in pulse frequency modulation (PFM) mode and nevertheless reduces switching losses. The variable frequency DPWM in combination with the model-based controller guarantees excellent voltage regulation and high efficiency. The variable frequency DPWM, based on an enhanced counter-comparator PWM and a delay-line, provides a resolution greater than 10 bits. The resolution of the DPWM increases at small load currents, which is beneficial due to the converter nonlinearity in discontinuous current mode (DCM). The control rules for changing the switching frequency ensure that a comparable performance to a constant frequency DPWM is achieved with regard to output voltage regulation and load step response. The performance of the novel DPWM is measured by simulation and it is integrated in a FPGA-based experimental environment. Moreover, attention is turned to a resource saving implementation of the DPWM.