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Optimal Design of Planar Magnetic Components for A Two-Stage Rail Grade DC-DC Converter

Image of proposed two-stage dc/dc module with planar magnetics.
Fig. 1. Proposed two-stage dc-dc module with planar magnetics.
The increased power consumption and power density demands of modern technologies have increased their demands on various power supplies. In order to achieve high efficiency and power density, a gallium-nitride (GaN)-based two-stage dc-dc module is proposed for railway applications, as shown in Fig.1. The first stage is a two-phase interleaved buck converter working under critical conduction mode (CRM). CRM is the simplest way to achieve zero voltage switching (ZVS) and can fully utilize the benefits of GaN devices. This regulation stage should convert the wide input voltage (64 V-160 V) to a constant 48 V bus voltage, and the switching frequency is above 400 kHz. The second stage is a 2 MHz unregulated 48 V/24 V LLC converter (DCX). This LLC DCX can always work at its most efficient point, at even several MHz. Since all devices can achieve ZVS and a high frequency is used, a high power density converter with excellent planar magnetic components becomes achievable.

This paper focuses on the design and optimization of planar magnetic components (i.e., the inductors and transformer in Fig. 1), with six-layer printed circuit board (PCB) windings. The benefits of negative coupled inductors for the buck converter are discussed. It is helpful to reduce the conduction losses under CRM and integrate the inductors. In order to reduce the inductor loss, the planar coupled inductors with winding interleaving are optimized based on finite element simulation. Two inductors are easy to couple through a customized EI core, and a same air gap design is proposed to improve the manufacturability. Similarly, the planar transformer of the LLC DCX is optimized with a customized EI core based on an analytical model. All the devices—magnetic components, digital controller, auxiliary power supply, sensing and communication chips—are integrated on a single quarter brick PCB in order to build a standalone dc-dc module. Finally, a standalone prototype converter is built with the proposed planar coupled inductors and transformer in a quarter brick form. The measured peak efficiency can reach 95.8 percent, with power density at 195 W/in3.

Image of prototype system.
Fig. 2. Prototype system.
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