Investigations into design and control of power electronic systems for future microprocessor power supplies

Abstract

Voltage Regulator Modules (VRMs) are used to provide power to the microprocessors. These modules are expected to deliver high currents upto 200A at low output voltages of around 1.2V. In order to reduce losses, microprocessors use dynamic voltage scaling, whereby the supply voltage to the microprocessor is adjusted with the computation load. To this end, the processor sends a 7-bit Voltage Identification (VID) code to the VRM, that dictates its output voltage.

Since the digital interface to the microprocessor is available to the VRM, the digital control is well suited for this purpose. However, the digital controllers have the drawbacks of reduction in phase margin due to presence of Zero Order Hold (ZOH) in Digital Pulse-Width Modulators (DPWM) and the limited resolution of the DPWM output. The digital controllers designed in this work take into account the reduction in phase margin due to presence of DPWM based ZOH. The effect of quantization of filter coefficients is also analyzed and a minimum word length filter structure is proposed for such controllers. In addition, a DPWM architecture is proposed to improve the time resolution of the DPWM. The proposed scheme is fabricated in the form of an Application Specific Integrated Circuit (ASIC) and is verified using experimental results.

The VRM control requires the inductor currents to be sensed. Thus, a current sensing method is described which is based on Giant Magneto Resistive (GMR) effect. It is based on sensing the magnetic field generated by the flow of current. Using fundamental equations of the field distribution, it is shown how the sensor can be used for sensing the inductor current. Simulation and test results are provided to assist the analysis.

Due to high currents, it becomes essential to have multiphase topology, where the synchronous buck converters are connected in parallel such that each phase leg carries only a fraction of the total output current. However, the current control of such a topology will require N-current sensors. Thus, a sensing and sharing algorithm is proposed which uses only one current sensor.

The control of a VRM ensures the voltage regulation during steady state operation. However, the transient response of a DC-DC converter still gets governed by the fundamental equation of rate of change of inductor current. It is proportional to the voltage across the inductor and inversely proportional to the inductance. Two new circuit topologies are proposed which increases the slew rate of inductor current during transient and thus improve the transient response of the system. The performance of these topologies are verified with simulation and experimental results. These schemes give another design freedom to optimally design the converters, resulting in lower inductor current ripple and requiring smaller output capacitor as compared to the conventional schemes.

In all, this dissertation focuses on the design development and control of Voltage Regulator Modules for low voltage and high current applications. Theoretical developments have been appropriately supported with analytical and experimental results.