Geometries in ASICs, processors, FPGAs and other digital controllers are shrinking to ever-lower voltage levels. Next-generation networking ASICs and processors require multiple lower voltages, higher currents, faster dynamic response, greater efficiency and power management solutions that reside close to the load. To meet these challenges, power supply companies have made significant advances over the past decade in increasing power density. Continuing these increases in power density can only be achieved with higher efficiencies and higher operating frequencies.
A novel power architecture, multiphasing isolated topologies, is emerging to contend with tomorrow's power requirements. Leading companies are multiphasing isolated forward, fly-back, single-ended soft-switching, push-pull and half-bridge converter topologies. In the last few years, low-voltage MOSFETs have seen much improvement in switching speeds. These faster-switching FETs offer lower switching losses. Combined with emerging multiphasing PWM controllers, they make megahertz switching and high-efficiency isolated converters a reality.
High-density applications with lower power levels are usually managed with 2-phase solutions, whereas higher power levels can require up to 4-phase solutions. Ultrahigh efficiency, even in lower-power applications, calls for multiphasing dual topologies, such as push-pull or half-bridge. In terms of overall performance, dual topologies are better than single-ended topologies. For example, they offer the best use of the magnetic core, as low-leakage inductance allows for high-frequency operation.
Multiphasing isolated topologies address five key parameters in power conversion: higher efficiency, lower input output ripple, faster dynamic response, ease of manufacturability and better thermal management. Each parameter is a factor in achieving high power density.
Efficiency. The best power efficiency is achieved by converting a voltage in a single stage, such as direct conversion of 48 V to 1.2 V, rather than double conversion.
High-performance applications will dictate an isolated point-of-load converter with power densities achieved only by multiphasing. For example, assume you want to convert 48 V to 1.2 V at 100 W using a 2-phase forward converter. In a 2-phase conversion, current is split equally in the two phases. The FET on-losses are I2 × R, which equates to a 50% reduction in on-losses. Lower peak currents provide lower turn-on and turn-off losses, resulting in lower switching losses. Lower turn-on and switching losses provide overall greater efficiency.
Input/output ripple reduction. Multiphasing PWM controllers increases switching frequency. The resulting frequency is equivalent to the PWM clock frequency times the number of phases. Higher operating frequency equates to less input/output capacitance and smaller input/output inductors.
Fast dynamic response. Improved dynamic response is the result of smaller output inductors allowing for fast response to current changes combined with higher operating frequency, equal to clock frequency times the number of phases, which allows for higher crossover. You could argue that crossing the isolation barrier may still affect the dynamic response. However, emerging secondary-side PWM controllers with multiphase ability make it possible to achieve any dynamic response you desire.
Ease of manufacturability. Next-generation designs demand smaller form factors and automated assembly, eschewing hand soldering of large transformers, inductors and capacitors. The multiphasing isolated topologies involve much smaller components and help move the designs away from dual manufacturing processes.
Better thermal management. Thermal management is critical at these new power densities. The challenge is even higher with the emergence of modules operating in extended temperature range. With multiphasing techniques, you spread the heat evenly over the whole converter, avoiding hot spots and improving converter reliability. A few years ago, multiphasing buck controllers became the de facto solution for high-density nonisolated buck applications. Soon, multiphasing isolated topologies will be the primary architecture in high-power density applications.
George Georgalis has been involved with the majority of topologies in power management for more than 20 years. He has held several positions in design, applications, business unit management and field applications. He holds BSEE and MSEE degrees from Cleveland State University.