Very High Frequency Integrated POL for CPUs

Point-of-load (POL) converters are used extensively in IT products. Every piece of the integrated circuit (IC) is powered by a point-of-load (POL) converter, where the proximity of the power supply to the load is very critical in terms of transient performance and efficiency. A compact POL converter with high power density is desired because of current trends toward reducing the size and increasing functionalities of all forms of IT products and portable electronics. To improve the power density, a 3D integrated POL module has been successfully demonstrated at the Center for Power Electronic Systems (CPES) at Virginia Tech. While some challenges still need to be addressed, this research begins by improving the 3D integrated POL module with a reduced DCR for higher efficiency, the vertical module design for a smaller footprint occupation, and the hybrid core structure for non-linear inductance control.

Moreover, as an important category of the POL converter, the voltage regulator (VR) serves an important role in powering processors in today's electronics. The multi-core processors are widely used in almost all kinds of CPUs, ranging from the big servers in data centers to the small smartphones in almost everyone's pocket. When powering multiple processor cores, the energy consumption can be reduced dramatically if the supply voltage can be modulated rapidly based on the power demand of each core by dynamic voltage and frequency scaling (DVFS). However, traditional discrete voltage regulators (VRs) are not able to realize the full potential of DVFS since they are not able to modulate the supply voltage fast enough due to their relatively low switching frequency and the high parasitic interconnect impedance between the VRs and the processors. With these discrete VRs, DVFS has only been applied at a coarse timescale, which can scale voltage levels only in tens of microseconds (which is normally called a coarse-grained DVFS). In order to get the full benefit of DVFS, a concept of an integrated voltage regulator (IVR) is proposed to allow fine-grained DVFS to scale voltage levels in less than a microsecond. Significant interest from both academia and industry has been drawn to IVR research.

Recently, Intel has implemented two generations of very high frequency IVR. The first generation is implemented in Haswell processors, where air core inductors are integrated in the processor's packaging substrate and placed very closely to the processor die. The air core inductors have very limited ability in confining the high frequency magnetic flux noise generated by the very high switching frequency of 140MHz. In the second generation IVR in Broadwell processors, the inductors are moved away from the processor substrate to the 3DL PCB modules in the motherboard level under the die.

Besides computers, small portable electronics such as smartphones are another application that can be greatly helped by IVRs. The smartphone market size is now larger than 400 billion US dollars, and its power consumption is becoming higher and higher as the functionality of smartphones continuously advances. Today's multi-phase VR for smartphone processors is built with a power management integrated circuit (PMIC) with discrete inductors. Today's smartphone VRs operate at 2-8MHz, but the discrete inductor is still bulky, and the VR is not close enough to the processor to support fine-grained DVFS. If the IVR solution can be extended to the smartphone platform, not only can the battery life be greatly improved, but the total power consumption of the smartphone (and associated charging time and charging safety issues) can also be significantly reduced. Intel's IVR may be a viable solution for computing applications, but the air core inductor with un-confined high-frequency magnetic flux would cause very severe problems for smartphones, which have even less of a space budget. This work proposes a three-dimensional (3D) integrated voltage regulator (IVR) structure for smartphone platforms. The proposed 3D IVR will operate with a frequency of tens of MHz. Instead of using an air core, a high-frequency magnetic core without an air gap is applied to confine the very high frequency flux. The inductor is designed with an ultra-low profile and a small footprint to fit the stringent space requirement of smartphones.

A major challenge in the development of the very high frequency IVR inductor is to accurately characterize and compare magnetic materials in the tens of MHz frequency range. Despite the many existing works in this area, the reported measured properties of the magnetics are still very limited and indirect. In regards to permeability, although its value at different frequencies is often reported, its saturation property in real DC-biased working conditions still lacks investigation. In terms of loss property, the previous works usually show the equivalent resistance value only, which is usually measured with small-signal excitation from an impedance/network analyzer and is not able to represent the real magnetic core loss under large-signal excitation in working conditions. The lack of magnetic properties in real working conditions in previous works is due to the significant challenges in the magnetic characterization technique at very high frequencies, and it is a major obstacle to accurately designing and testing the IVR inductors. In this research, an advanced core loss measurement method is proposed for very high frequency (tens of MHz) magnetic characterization for the IVR inductor design. The issues of and solutions for the permeability and loss measurement are demonstrated. The LTCC and NEC flake materials are characterized and compared up to 40MHz for IVR application.

Based on the characterized material properties, both single-phase and multi-phase integrated inductor are designed, fabricated and experimentally tested in 20MHz buck converters, featuring a simple single-via winding structure, small size, ultra-low profile, ultra-low DCR, high current-handling ability, air-gap-free magnetics, multi-phase integration within one magnetic core, and lateral non-uniform flux distribution. It is found that the magnetic core operates at unusually high core loss density, while it is thermally manageable. The PCB copper can effectively dissipate inductor heat with 3D integration. In addition, new GaN device drivers and magnetic materials are evaluated and demonstrated with the ability to increase the IVR frequency to 30MHz and realize a higher density with a smaller loss.

In summary, this research starts with improving the 3D integrated POL module, and then explores the use of the 3D integration technique along with the very high frequency IVR concept to power the smartphone processor. The challenges in a very high frequency magnetic characterization are addressed with a novel core loss measurement method capable of 40MHz loss characterization. The very high frequency multi-phase inductor integrated within one magnetic component is designed and demonstrated for the first time. A 20MHz IVR platform is built and the feasibility of the concept is experimentally verified. Finally, new GaN device drivers and magnetic materials are evaluated and demonstrated with the ability to increase the IVR frequency to 30MHz and realize higher density with smaller loss.