All semiconductor devices need power to operate and they do so using DC voltage and current. In computing applications, the AC power drawn from the electrical grid needs to be converted to DC power using multiple stages of DC/DC conversion as shown in Figure 1 for a data center application. Each DC/DC conversion has a power efficiency which reduces significantly in the last two stages (48V-12V-1V) as we approach the integrated circuits. The overall efficiency is the product of the individual efficiencies and therefore for the last two stages, the efficiency becomes 55%. Hence, for every 2W drawn from the grid, in just the last two stages 1W of power is lost before it even gets to the transistors. The power lost is dissipated as heat. With data centers consuming 70 billion KWh today in USA, a 20% improvement in efficiency can translate to 20% reduction in energy consumption. A similar trend is expected for AI applications as well.
Figure 1: Data Centers, DC/DC Converters and Power Efficiencies
The 2019 Heterogeneous Integration Roadmap has identified the following specific needs and potential solutions as: i) tight integration with in-package integrators, ii) reduction of ohmic losses in power delivery network for 250-800W per package, iii) 48V/1V converters with distributed DC/DC converters in package, iv) advanced GaN based power devices and v) coordination with active package cooling system, to name a few. Our focus at PRC is therefore on SoP with Integrated Voltage Regulators (IVR), as shown in Figure 2 with the key metrics as defined in Table 1. The goal of IVR is to bring the converter (power source) in close proximity to the SOC which significantly reduces Cu losses due to shorter current paths in the printed circuit board (PCB) and support high efficiency, is integrated, highly miniaturized, and supports high conversion ratio single stage converters.
Table 1: Key metrics for Power Delivery & IVR
The PRC technology focus is shown in Figure 2 consisting of inductors and capacitors embedded in the substrate with Si and GaN devices assembled on top along with thermal management solutions from both sides. The high voltage (48V or 12V) at low current enters from the bottom of the substrate, is down converted to 1V high current and fed to an SOC on the same substrate, or to an SOC on an interposer. The PRC research focus areas are i) design to determine the optimum topologies and inductances/capacitances required, ii) modeling based on machine learning (ML) for optimization and predictions, iii) characterization and integration of magnetic core inductors (>50nH/mm3, >2A/mm2 saturation current and 10mohm resistance), iv) embedded capacitors (>0.1mF/mm2 @ 10MHz) and v) thermal management to account for joule heating of the embedded inductors.