1203608

Design of a High Efficiency 30 kW Boost Composite Converter

Hyeokjin Kim, Hua Chen, Dragan MaksimoviC and Robert Erickson Department of Electrical, Computer and Energy Engineering

University of Colorado Boulder Boulder, Colorado, 80309

AbstractAn experimental 30 kW boost composite converter is described in this paper. The composite converter architecture, which consists of a buck module, a boost module, and a dual active bridge module that operates as a DC transformer (DCX), leads to substantial reductions in losses at partial power points, and to significant improvements in weighted efficiency in applications that require wide variations in power and conversion ratio. A comprehensive loss model is developed, accounting for semiconductor conduction and switching losses, capacitor losses, as well as dc and ac losses in magnetic components. Based on the developed loss model, the module and system designs are optimized to maximize efficiency at a 50% power point. Experimental results for the 30 kW prototype demonstrate 98.5% peak efficiency, very high efficiency over wide ranges of power and voltage conversion ratios, as well as excellent agreements between model predictions and measured efficiency curves.

BoostMboost(D)

I. Introduction

Dc-dc converters in applications such as maximum power point tracking in photovoltaic power systems, or drive trains in electric or hybrid vehicles (EV, HEV), are required to operate efficiently over wide ranges of power and voltage conversion ratios. System performance depends on improvements in weighted converter efficiency. In EV or HEV applications, such system efficiency improvements translate directly into increased equivalent miles-per-gallon (MPGe) performance[1] . Even more importantly, loss reductions translate into reduced size and cost of the cooling system.

The traditional boost converter employed in EVs or HEVs[2] has low efficiency at light load power. Also, power density is low due to the low switching frequency and high power losses. To increase average efficiency and power density, a modified dual active bridge has been proposed [3], [4], with the number of switching devices reduced by half compared to the conventional dual active bridge(DAB). However, the switch voltage stresses are equal to the output voltage or the input voltage. For the case when the voltage rating is higher than 600 V, silicon IGBTs are commonly employed, as high voltage silicon MOSFETs are not competitive, while emerging wide bandgap devices are more expensive. The switching frequency with silicon IGBTs is limited to around 10 kHz. Also, switching loss with conventional DAB control (phase shift between primary and secondary) increases under light load conditions. Several previous studies have been focused on reducing voltage stresses over switching devices while

Fig. 1. Composite converter architecture consisting of buck, boost, and DCX (DC transformer) modules [8]

maintaining high peak efficiency. In [5], a soft-switched continuous current mode(CCM) boost converter was introduced to achieve a higher voltage conversion ratio and to extend zero-voltage switching(ZVS) range by auxiliary circuits. Also, voltage stresses and current stresses imposed on the discrete components are reduced. However, low efficiency is observed under light load conditions and accurate control is necessary to regulate the output voltage. In [6], a multiphase bidirectional flyback converter was introduced to reduce number of discrete components and capacitor RMS current. However, this flyback topology has large voltage stress over switching devices and is not suitable for wide voltage conversion range. In [7], a four level flying-capacitor dc-dc converter (3X dc- dc converter) was described. This topology greatly improves efficiency over wide power range and also reduces voltage stresses over switching devices. However, the flying capacitors of this topology have not only large RMS current but also large energy rating (0.5CV2aring) due to the required capacitance and voltage ratings. Also the output voltage regulation is restricted to discrete values, , 2VIN, and 3V1N.

To reduce the voltage stresses over switching devices, maintain high efficiency over wide operating range and reduce the energy rating of capacitors, a composite converter architecture has been proposed [8], as shown in Fig. 1 with

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