Digital synthetic ripple modulator for a DC-DC converter

Voltage regulator modules (VRMs) powering the future microprocessors are required to meet the stringent specifications on the core voltage ripple and voltage regulation. These specifications are driven by the microprocessor's higher di/dt requirements and the need to operate at a lower supply voltage for reduced power consumption. The conventional VRMs resort to multi-phase pulse width modulation (PWM) control schemes to cater to the high current demands and provide balanced load current sharing. The control schemes also involve a combination of voltage-mode and current-mode or current-mode and hysteretic-mode control. Thus, these schemes add to the cost and complexity of the VRM.;Synthetic ripple modulation (SRM) involves the generation of an artificial ripple, synthesized from a converter parameter which is then bonded to the output voltage of the VRM. This artificial scheme of carrier signal generation for PWM control enables voltage-hysteretic modulation to be achieved in the low-voltage VRM modules for microprocessors. With the inherent low-voltage ripple exhibited by a low-voltage VRM that is insufficient for conventional hysteretic operation, the SRM scheme on the other hand provides sufficient ripple for the PWM carrier signal. The SRM scheme blends in the advantages of current-mode control and hysteretic control providing superior transient performance.;The introduction of digital control to the PWM control of switching power converters has gained popularity owing to its benefits of lower sensitivity to process and mismatch variations, programmability, and the reduction of passive components used in tuning. The digital control based SRM generates the duty ratio with inverse relation to a sampled converter parameter. The on-time and off-time duration of the switches forming the duty ratio is generated by successively accumulating the sampled inductor voltage. The accumulated output forming the synthetic ripple is added to the error between output voltage and reference voltage. The carrier signal thus generated is modulated between the higher and lower hysteretic thresholds, thus generating the duty ratio. A unique scaling process also allows the implementation of programmable switching frequency for converters. The major contributions of the dissertation in the field of engineering are given below. (1) Digital synthetic ripple modulator architecture for applications in power electronics. (2) Digital inverse timing generator with wide dynamic range and programmable frequency.;The digital SRM scheme is verified experimentally in the control of a synchronous buck DC-DC converter. Experimental data are provided to delineate the potential advantages such as inverse timing generation, programmable switching frequency, linear control of output variable with reference variable under open-loop conditions, natural feed-forward control, and superior transient performance.