Investigation of Alternative Power Architectures for CPU Voltage Regulators
Since future microprocessors will have higher current in accordance with Moore's law, there are still challenges for voltage regulators (VRs). Firstly, high efficiency is required not only for easy thermal management, but also for saving on electricity costs for data centers, or battery life extension for laptop computers. At the same time, high power density is required due to the increased power of the microprocessors. This is especially true for data centers, since more microprocessors are required within a given space (per rack). High power density is also required for laptop computers to reduce the size and the weight.
To improve power density, a high frequency is required to shrink the size of the output inductors and output capacitors of the multi-phase buck VR. It has been demonstrated that the output bulk capacitors can be eliminated by raising the VR control bandwidth to around 350kHz. Assuming the bandwidth is one-third of the switching frequency, a VR should run at 1MHz to ensure a small size. However, the efficiency of a 12V VR is very poor at 1MHz due to high switching losses. As a result, a 12V VR can only run at 300kHz to 600kHz, and the power density is very low.
To attain high efficiency and high power density at the same time, two-stage power architecture was proposed. The concept is "Divide and Conquer". A single-stage VR is split into two stages to get better performance. The second stage has about 5V-6V input voltage; thus the duty cycle can be extended and the switching losses are greatly reduced compared with a single-stage VR. Moreover, a sub-20V MOSFET can be used to further improve the efficiency at high frequencies.
The first stage of the proposed two-stage architecture is converting 12V to 5-6V. High efficiency is required for the first stage since it is in series with the second stage. Previous first stage which is a buck converter has good efficiency but bulky size due to low frequency operation. Another problem with using a buck converter is that light-load efficiency of the first stage is poor. To solve these problems, switched-capacitor voltage dividers are proposed. Since the first stage does not require voltage regulation, the sweet point for the voltage divider can be determined and high efficiency can be achieved. At the same time, since there are no magnetic components for the switched-capacitor voltage divider, high power density can be achieved. By very careful design, a power density of more than 2000W/in3 with more than 97% efficiency can be achieved for the proposed voltage divider. The light-load efficiency of the voltage divider can be as high as 99% by reducing the switching frequency at light load.
As for the second stage, different low-voltage devices are evaluated, and the best device combinations are found for high-frequency operation. It has been demonstrated that 91% efficiency can be achieved with 600kHz frequency, and 89% efficiency can be achieved with a 1MHz frequency for the second stage. Moreover, adaptive on-time control method and a non-linear inductor structure are proposed to improve CCM and DCM efficiency for the second stage respectively.
Previously the two-stage VR was only used as a CPU VR. The two-stage concept can also be applied to other systems. In this dissertation, the two-stage power architecture is applied to two different applications: laptop computers and high-end server microprocessors. The common characteristics of the two applications are their thermal design power (TDP) requirement. Thus the first stage can be designed with much lower power than the maximum system power. It has been demonstrated that the two-stage power architecture can achieve either higher efficiency or higher power density and a lower cost when compared with the single-stage VR.
To get higher efficiency, a parallel two-stage power architecture, named sigma architecture, is proposed for VR applications. The proposed sigma VR takes advantage of the high-efficiency, fast-transient unregulated converter (DCX) and relies on this converter to deliver most of the output power, while using a low-power buck converter to achieve voltage regulation. Both the DCX converter and the buck converter can achieve around 90% efficiency when used in the sigma VR, which ensures 90% efficiency for the sigma VR. The small-signal model of the sigma VR is studied to achieve adaptive voltage positioning (AVP). The sigma power architecture can also be applied to low-power point of load (POL) applications to reduce the magnetic component size and improve the efficiency. Finally, the two-stage VR and the sigma VR are briefly compared.