A High Input Voltage Three-Phase ZVZCS DC-DC Converter with Vin /3 Voltage Stress on Primary Switches
Abstract
A high-voltage three-phase ZVZCS dc-dc converterwith low voltage stress is presented in this paper. The voltagestress on the primary switches is only one-third of the inputvoltage. By using the phase shift pulsewidth modulation (PWM)and secondary snubber, four of the six primary switches areoperating with zero-voltage switching (ZVS) and two of them areoperating with zero-current switching (ZCS), in order to reducethe switching loss. Moreover, the converter has wide softswitchingrange. Analysis of the converter’s operation is given. A2.2kW, 1000V/110V dc-dc converter prototype has been built andtested. Experimental results are favorably compared withtheoretical predictions.
I. INTRODUCTION
High input voltage imposes high-voltage stress across theswitching devices of DC-DC converters. Solutions of thethree-level converters (TLC)[1-2] and their variants topologies[3] can reduce the voltage stress on the switches to one half ofthe input voltage, which allows using devices of low-voltagerating in the converters. However, in high- voltageapplications, like the railway system, need to convert thesupply voltage from 1500V to 110V. Higher voltage ratingdevices are thus needed. However, the device on-resistanceincreases with voltage rating nonlinearly. This leads powerelectronics designers to explore new converter circuits that canreduce the device voltage requirement, so that devices withlower on-resistance can be used.A high-voltage converter deriving from the three-phaseconverters [4]-[5] has been proposed in [6]. It has the distinctfeature of making the voltage stress equal to one-third of theinput voltage. The converter achieves ZVS for all the primaryswitches. However, the ZVS converter presents somelimitations: limited soft switching range, circulating energyduring freewheeling stage etc. To overcome the limitation ofZVS, much research has investigated ZVZCS for the singlephasefull-bridge and three-level converters by resetting thecurrent in transformer windings during the freewheeling stage[7][8], so that the leading leg is switched with ZVS andlagging leg is switched with ZCS. However in the traditionalthree-phase converter, the duty cycle is determined by theconduction time of the switches. Each switch pair acts asleading leg for one winding and lagging leg for the other. So,the ZVZCS, which is based on the phase shift PWM, cannot bedirectly applied to the switches of the three-phase derivedconverter.An improved high-voltage converter that integrates theconcept of ZVZCS and three-phase converter in [6] isproposed in this paper. The three primary switch pairs areoperating in phase shift PWM, acting as leading and lagginglegs. Each switch is conducting the same time duration and itsduty cycle is determined by the phase shift. With thesecondary passive snubber the proposed converter offers widesoft-switching range. There is no circulating energy during thefreewheeling stage. The structure of the novel converter isdescribed in Section II. The analysis of the switchingtopologies and derivation of the main waveforms equations aregiven in Section III. The experimental results of a 2.2kWprototype will be presented in Section IV.
II. CONVERTER STRUCTURE
The proposed converter is shown in Fig. 1, and the timingdiagram is shown in Fig. 2. Three dc capacitors, C1 to C3generate by-pass voltages of Vi / 3. Each one of the threeswitches pairs, including SP1 (formed by S1 and S2), SP2(formed by S3 and S4), and SP3 (formed by S5 and S6), isconnected across a dc capacitor. The two switches in eachswitch pair are operated in anti-phase. The voltage stress onthe switches is thus equal to Vi / 3. The transformer primarywindings are connected at the mid points of the three switchpairs across dc capacitors (CX, CZ).On the secondary side, the two windings yz’ and zx’, eachwith conduction time of DTs/2, are connected together throughtwo individual rectifiers with the output inductor L1. Itsrectified output is vrec1 shown in Fig. 1. The winding xy’having the conduction time of DTS is rectified and is connectedto another output inductor L2.


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