# Evaluation of AC VRM Topologies for High-Frequency Power Distribution Systems

In this paper is shown that the regulated series resonant rectifier (SRR) and series-parallel resonant rectifier (SPRR) with variable resonant capacitance are good candidates for ac voltage regulator modules (VRMs) in high-frequency power distribution systems with a sinusoidal ac bus. Control characteristics of the SRR and SPRR are derived using the sinusoidal approximation method. The effect of the transformer magnetizing inductance is also investigated. Experimental results obtained on a 5-V/8.5-A SRR with variable resonant capacitance designed for a 28-Vrms , 1-MHz sinusoidal ac bus are provided.

Today’s high-speed microprocessors represent highly dynamic power loads that necessitate the use of distributed power architectures. Typically, these distributed powerdelivery systems use a conventional multiple-output “silver box” as a front end to supply power directly to “not-sodynamic” loads (e.g., disk drives) and to generate a dc bus for distribution to the dc/dc point-of-load regulators that are used for powering the processors. Generally, these point-of-load regulators, also known as voltage regulator modules (VRMs), are located very close to the processors to minimize the interconnect inductance so that the desired transient response of the VRMs is obtained with a minimal decoupling capacitance . While the described dc-distribution power system has been proven to meet the performance requirements of today’s desktop and server power systems, a number of claims have been recently made that a high-frequency ac-distribution power system is a more cost-effective solution to powering today’s and future data processing equipment . Generally, these claims are based on the fact that in an acdistribution power system, which consists of a front-end inverter and ac/dc point-of-load converters (ac VRMs), compared to a dc-distribution power system, two conversion steps (the rectification step in the front-end converter and the inversion step in the point-of-load converter) are eliminated. Because of the reduced number of components, the cost of the ac-distribution power system is expected to be reduced. The proposals for the high-frequency ac-distribution power systems range from trapezoidal ac-bus systems to pure sinusoidal ac-bus systems with bus frequencies in the MHz range. Generally, it is recognized that the sinusoidal ac-bus system has advantages over the trapezoidal ac-bus system with respect to EMI performance. However, the sinusoidal ac-bus system requires a more complex inverter and ac VRM design . So far, only one commercial implementation of the ac-distribution power system has been reported [4], which uses 3-5-MHz sinusoidal ac bus. While the size and performance of this ac-distribution power system apparently meet the specifications, the cost-effectiveness of the system is not known. In this paper, topologies for ac VRMs for HF sinusoidal ac-bus systems are discussed. First, ac VRM topologies are derived from the conventional full-wave rectifier topologies. Next, different control methods for ac VRM output-voltage regulation are evaluated. In Sections III and IV, voltagetransfer-function and input-power-factor characteristics of the proposed ac VRMs are derived and analyzed using the sinusoidal approximation method. The effect of the transformer magnetizing inductance is also investigated. In Section V, a comparison of different ac VRM topologies is given. Experimental results are provided in Section VI.

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