Design and implementation of a variable-frequency multiphase VRM with optimized phase-reduction control

This paper proposes a control method to improve the efficiency in voltage regulator module (VRM) at light loads which is an important application on motherboard in personal computer (PC). The proposed control strategy is a combination of variable-frequency control method and phase-reduction control method. The proposed variable-frequency control method is used to modulate the switching frequency high as the load becomes low or light to achieve zero voltage switching (ZVS) on both high-side and low-side switches and to improve the light load efficiency significantly. The phase-reduction control method associates with the proposed variable-frequency control to enhance the efficiency improvement further as the load becomes light gradually. In addition, compared to conventional control methods, one advantage of the proposed variable-frequency control method can keep the ripple low to meet the desired design specification even at light load. To verify the proposed control strategy, an eight-phase VRM is implemented and the design specifications are output current 81 A, output voltage 1.3 V, the maximum power about 10W and the ripple below 1% for full-range load, in which the switching is modulated from 240 kHz to 550 kHz as the load varies. The experimental results shows comparisons the proposed control method and other conventional constant frequency techniques and efficiency can be increased up to 30%.