Study of Using 24V to Replace 12V Intermediate Bus for VRM

Using Buck converter as the final stage for 48V-to1V conversion currently is still one of the main solutions for CPU and GPU power delivery due to its extremely mature monolithic IC industry, controller development and reliability. However, the 12V intermediate bus becomes a bottleneck for improving the efficiency of the 48 V-to-1V conversion. This paper investigates the efficiency performance by using 24V intermediate bus instead of the traditional 12V bus, due to its potential of reducing the PCB copper conduction loss on the intermediate bus. We propose to use a $48 \mathrm{~V}-\mathrm{to}-24 \mathrm{~V}$ resonant switching-capacitor converter to replace a cascaded resonant 48V-to-12V converter, and use a 24V-to-1V Buck to replace a 12V-to-1V Buck. With detailed loss analysis, this paper highlights the significant loss reduction using 48V-to-24V instead of 48V-to-12V. The 24V-to-1V Buck shows a very similar efficiency to 12V-to-1V Buck especially at heavy load. That is because 40V GaN devices significantly reduce the impact of the switching loss increase from 12V bus to 24V bus. Besides, for 1V conversion, the conduction loss dominates the total loss. Finally, the efficiency of the proposed two-stage 48V-to-1V with 24V intermediate bus is very similar and even slightly higher to the efficiency of the traditional 48V-to-1V with 12V intermediate bus, but achieves much higher density and lower cost due to the simplicity of the 48V-to-1V conversion using 24V bus. The calculation results demonstrate that the 24V intermediate bus will be a practical improvement of the current 12V intermediate bus.