A Novel ZVZCS Full-Bridge DC/DC Converter Used ... - IEEE Xplore

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PAHLEVANINEZHAD et al.: NOVEL ZVZCS FULL-BRIDGE DC/DC CONVERTER USED FOR ELECTRIC VEHICLES 2753charger which is an isolated DC/DC converter, is used to chargethe high voltage battery and provide galvanic isolation betweenthe utility mains and the traction battery [31], [32], [35], [36].Full-bridge topology is the most popular topology used in thepower range of a few kilowatts (1–5 KW) for DC/DC converters.Since the switch ratings are optimized for the full-bridgetopology, this topology is extensively used in industrial applications.High efficiency, high power density, and high reliabilityare the prominent features of this topology [38]–[40].For applications in the range of a few kilowatts, MOSFETsare mostly used to implement the full-bridge converters. In orderto have robust and reliable operation, MOSFETs shouldbe switched under zero voltage. Operation with zero voltageswitching (ZVS) has numerous advantages including, for example,reduction of the converter switching losses and a noise-freeenvironment for the control circuit. Zero voltage switching isusually achieved by providing an inductive current flowing outof the full-bridge legs during the switch turn-ON and by placinga snubber capacitor across each switch during the switchturn-OFF. The inductive current can be produced by insertingan inductor in series with the power transformer or by insertingan inductor in parallel with the power transformer [4]–[11].In a practical full-bridge configuration, the internal drain-tosourcecapacitor of the MOSFET is usually utilized as thesnubber capacitor, the series inductor is usually the leakageinductance of the power transformer, and the parallel inductoris implemented by using the magnetizing inductance of thepower transformer. Thus, external passive components are notrequired, which makes the power circuit very simple and efficient.However, the full-bridge converter with the series inductorloses its ZVS capability at light loads, and the converter withthe parallel inductor loses its ZVS under heavy loads. Loss ofZVS implies extremely high switching losses at high switchingfrequencies and very high EMI due to the high di/dt of thesnubber discharge current. Loss of ZVS can also cause a verynoisy control circuit, which leads to shoot-through and loss ofthe semiconductor switches. The ZVS range can be extended byincreasing the series inductance. However, having a large seriesinductance limits the power transfer capability of the converterand reduces the effective duty ratio of the converter.In battery charger applications, ZVS is vitally important sincethe converter might be operating at absolutely no-load for along period of time. In this application, when the battery ischarged, the load is absolutely zero and the converter shouldbe able to safely operate under the zero load condition. SinceZVS in conventional full-bridge PWM converters is achieved byutilizing the energy stored in the leakage inductance to dischargethe output capacitance of the MOSFETs, the range of the ZVSoperation is highly dependent on the load and the transformerleakage inductance. Thus, this converter is not able to ensureZVS operation for a wide range of load variations [12].In [11], a novel approach has been adopted to extend the ZVSrange in the full-bridge converter. In this approach, an auxiliaryinductor is put at the leading leg by deriving an auxiliary windingon the main transformer and confirms ZVS for the leading leg.Although the proposed scheme can effectively extend ZVS ofthe leading leg MOSFETs, it is not able to guarantee ZVS forFig. 1.Conventional full-bridge converter.the lagging leg of the converter. Thus, when the battery is fullycharged, the lagging leg switches may not be switched underZVS. Moreover, the voltage at the secondary side still suffersfrom the spikes due to the leakage inductance and voltage-fedrectifier, commonly seen in full-bridge converters.A fundamental problem related to the conventional fullbridgephase-shift DC/DC converter is the voltage spikes acrossthe output diodes. Fig. 1 shows the schematic of the conventionalfull-bridge converter. Basically, the leakage inductanceof the transformer causes the voltage spikes across the outputdiodes. Theses spikes are intensified by increasing the switchingfrequency of the converter. Thus, the diodes should be designedoverrated to be able to withstand the voltage spikes, which leadsto higher losses due to the higher forward voltage drop of thediodes and poorer reverse recovery characteristics. In addition,the spikes significantly increase the EMI noise of the converter.This fact makes the topology not very practical for high frequency,high voltage applications. There are quite a few referencesthat proposed solutions for the voltage spikes across theoutput diodes. Some references tried to decrease the leakageinductance as much as possible though the transformer windingstructures, which effectively decreases the peak of the voltagespikes across the output diodes. However, reducing the leakageinductance decreases the ZVS operating range of the full-bridgeconverter, which results in a very narrow range of ZVS operation.In [13], an R-C-D snubber circuit is used to mitigate thevoltage spikes across the diodes. The main problem with thesnubber circuit is the amount of losses in the snubber resistor,which considerably degrades the efficiency of the converter especiallyat higher power and it can only reduce the peak valueof the voltage spikes. In [14], an active clamp circuit has beenadded to the converter to clamp the voltage across the outputdiodes. This method can effectively clamp the voltage spikes ofthe output diodes. However, the active clamp circuit increasesthe complexity of the converter and causes small losses in theclamp circuit. Several energy recovery clamp circuits (ERCCs)have been proposed in [15]–[20]. In [21], an improved ERCCmethod has been proposed to accommodate the effects of voltagespikes for a wide range of input voltage. Although the ERCCtechniques are able to reduce the voltage stress of the output
2754 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 27, NO. 6, JUNE 2012diodes, the amount of the voltage stress depends on the dutyratio and input voltage of the converter in most of the ERCCstechniques. In addition, using extra semiconductors is inevitablein all these aforementioned methods.The problem of voltage spikes is essentially related to thevoltage-driven output rectifiers. This is due to the fact thatthe full-bridge inverter produces high frequency voltage pulsesacross the output diode rectifier, which is connected to the outputinductor as shown in Fig. 1. The voltage-driven rectifierworks perfectly if there is no leakage inductance in between theoutput of the full-bridge inverter and the diode rectifier. However,the existence of the leakage inductance makes the rectifierconnect two current sources, i.e., leakage inductance and outputinductance, together. This connection creates high voltagespikes across the output diodes. In this paper, a new topology isproposed based on a current driven rectifier, which effectivelyrectifies the voltage stress problems related to the full-bridgeDC/DC converter. The proposed topology provides zero currentswitching (ZCS) for the output rectifiers, which eliminatesreverse recovery losses of the output diode rectifiers.In [37], a new modified current-fed full-bridge is presentedin order to reduce the voltage stress arising in the conventionalcurrent-fed full-bridge converter. In the current-fed full-bridgein [37], the inductor stores energy as the transformer transmitsenergy from the DC source to the load. This scheme is able to effectivelyreduce the voltage spikes at the input-side (full-bridgeMOSFETs) of the converter. In addition, the input inductor isreplaced by coupled inductors in series with two MOSFETs toreduce the size of the inductor. The fundamental difference betweenthe proposed converter and the converter in [37] is that inthe proposed converter, the full-bridge section at the transformeris the conventional voltage-fed topology, while in [37] the inputsection of the converter is a modified current-fed topology.In the battery charger applications, the converter should beable to operate from absolutely no-load to full-load condition.In order to achieve zero voltage switching for the whole loadrange, the proposed converter uses a passive circuit to providethe required inductive current for the power MOSFETs. In [22],the detailed analysis of the auxiliary circuit is given.This paper is organized as follows: In Section II, the proposedtopology is introduced and different operating modes aredescribed. The steady-state operation of the converter is analyzedin Section III. In Section IV, features of the proposedtopology are presented. Design procedure for key componentsof the converter is given in Section V. Section VI shows theexperimental results obtained from the converter prototype andfinally, Section VII, provides the conclusion.II. PROPOSED TOPOLOGYThe main problem regarding the conventional full-bridge converteris the series connection of the leakage inductor and outputinductor through the diode rectifier. In other words, the dioderectifier at the secondary side of the converter connects two currentsources together. This leads to large voltage spikes acrossthe diode rectifier and very lossy commutation of the current inthe rectifier diodes. This phenomenon is intensified in high volt-Fig. 2.Proposed converter.age applications due to the inevitable large leakage inductance.This fact forces designers to over design the diode rectifier atthe output side. For instance, designers usually use even 1200 Vdiodes for a 400 V output DC power supply. This phenomenonis usually mitigated by using snubber circuits or clamp circuits.However, the snubber circuits are usually lossy especially forhigh frequency applications. In addition, the voltage spikes arehighly intensified by increasing the load, which makes the snubbercircuit a very poor solution for high frequency applications.Basically, this problem exists with all topologies that use transformerswith voltage-driven rectifiers. Generally, in converterswith voltage-driven rectifiers, there is an inductor at the outputof the rectifier and the leakage inductance of the transformerinterferes with this inductor.Moreover, in full-bridge DC/DC converters with high outputvoltages (such as 400 V or higher), the equivalent capacitanceviewed from the transformer primary side is not negligible.This is due to the high number of turns at the secondary andto the required high voltage semiconductors with high value ofoutput capacitance. In high frequency applications, these parasiticcapacitances resonate with the leakage inductance of thetransformer, which leads to a very high resonant current in theprimary of the transformer. Therefore, this capacitor completelydeteriorates the performance of the converter in high switchingfrequency applications. The conventional full-bridge topologyexhibits very poor performance due to the secondary side componentsin high frequency high voltage applications.The topology introduced in this paper presents a novel yetsimple solution to this problem. The proposed topology is essentiallya ZVZCS type full-bridge converter with a currentdrivenrectifier. Fig. 2 shows the power circuit of the proposedtopology. In this topology, the full-bridge inverter converts theDC-bus voltage to a high frequency quasi-square wave voltage.Then there is an inductor in series with the transformer, whichacts as a current source for a current driven rectifier. The currentdriven rectifier rectifies the output current of the transformer andtransfers power to the output. Fig. 3 illustrates the key waveformsof the converter. In one switching cycle, the circuit has14 modes during steady-state operation. Due to the symmetricalstructure, the analysis is only given for half a switching cycle.In half cycle, the steady state behavior of the circuit is dividedinto the following operating modes:Mode I: (t 0 ≤ t ≤ t 1 ): At t 0 , S 2 is turned OFF. The outputcapacitor of S 1 is discharging and that of S 2 is chargingup with the reactive current provided by the auxiliary circuit.
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A Novel ZVZCS Full-Bridge DC/DC Converter Used ... - IEEE Xplore

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