Digital multimode buck converter control with loss-minimizing synchronous rectifier adaptation

This paper develops a multimode control strategy which allows for efficient operation of the buck converter over a wide load range. A method for control of synchronous rectifiers as a direct function of the load current is introduced [1]. The function relating the synchronous-rectifier timing to the load current is optimized on-line with a gradient power-loss-minimizing algorithm. Only low-bandwidth measurements of the load current and a power-loss-related quantity are required, making the technique suitable for digital controller implementations. Compared to alternative loss-minimizing approaches, this method has superior adjustment speed and robustness to disturbances, and can simultaneously optimize multiple parameters. The proposed synchronous-rectifier control also accomplishes an automatic, optimal transition to discontinuous-conduction mode at light load. Further, by imposing a minimum duty-ratio, the converter automatically enters pulse-skipping mode at very light load. Thus, the same controller structure can be used in both fixed-frequency pulsewidth modulation and variable-frequency pulse-skipping modes. These techniques are demonstrated on a digitally-controlled 100-W buck converter. Index Terms--Adaptive control, dead-time, digital control, gradient methods, multimode control, optimization methods, pulse skipping, pulsewidth modulated (PWM) power converters, pulsewidth modulation (PWM), synchronous rectifier (SR), variable frequency control.