This paper presents comparison between phaseshift fullbridge converters with noncoupled and coupled currentdoubler rectifier. In high current capability and high stepdown voltage conversion, a phaseshift fullbridge converter with a conventional currentdoubler rectifier has the common limitations of extremely low duty ratio and high component stresses. To overcome these limitations, a phaseshift fullbridge converter with a noncoupled currentdoubler rectifier (NCDR) or a coupled currentdoubler rectifier (CCDR) is, respectively, proposed and implemented. In this study, performance analysis and efficiency obtained from a 500 W phaseshift fullbridge converter with two improved currentdoubler rectifiers are presented and compared. From their prototypes, experimental results have verified that the phaseshift fullbridge converter with NCDR has optimal duty ratio, lower component stresses, and output current ripple. In component count and efficiency comparison, CCDR has fewer components and higher efficiency at full load condition. For small size and high efficiency requirements, CCDR is relatively suitable for high stepdown voltage and high efficiency applications.
In a decentralized power system, the front end ac/dc converter is generally composed of two stages, in which one is a power factor correction (PFC) and the other is an intermediate dc/dc converter, as shown in Figure
Twostage structure of preregulator.
To achieve low output voltage, high output current, and high efficiency, a phaseshift fullbridge converter with conventional currentdoubler rectifier is widely used in mediumhigh power condition, as shown in Figure
The phaseshift fullbridge converter with conventional currentdoubler rectifier.
To solve the abovementioned problem, the phaseshift fullbridge converter with NCDR or CCDR is proposed, as shown in Figures
The proposed phaseshift fullbridge converter with NCDR.
The proposed phaseshift fullbridge converter with CCDR.
With duality method, NCDR can be derived from a voltagequadrupler circuit, and coupleddoubler can be derived from a voltagedoubler circuits. In the following, derivations of both improved currentdoubler rectifiers are described in details.
Derivation of NCDR is based on a conventional voltagequadrupler circuit, as shown in Figure
Derivation of NCDR from a voltagequadrupler based on duality principle: (a) voltagequadrupler and (b) NCDR.
Similarly, derivation of CCDR is based on a conventional voltagedoubler circuit, as shown in Figure
Derivation of CCDR from a voltagedoubler based on duality principle: (a) voltagedoubler, (b) conventional currentdoubler rectifier, and (c) CCDR.
For NCDR and CCDR, each of which has its own merits and demerits. To have an objective judgment, operational principles of NCDR and CCDR are briefly described as follows.
In Figure
Key waveforms of the proposed phaseshift fullbridge converter with NCDR.
Operational modes of the proposed fullbridge phaseshift converter with NCDR: (a) mode 1, (b) mode 2, (c) mode 3, and (d) mode 4.
At time
In Figure
Each coupled inductor individually functions as a transformer for CCDR.
Key waveforms of phaseshift fullbridge converter with CCDR.
Equivalent circuit modes of the CCDR operating over a half switching cycle.
When currents
This section will compare both the features and characteristics of NCDR as well as CCDR, which include secondary winding peak current of transformer, voltage gain, and output current ripple.
From Figures
From Figures
To objectively judge the merits and demerits of NCDR and CCDR, their performances are compared as summarized in Table
Comparison between NCDR and CCDR.
NCDR  CCDR  

Voltage gain 


 
Output current ripple 


 
Diode voltage stress 


 
Peak current of transformer secondary winding 


Performance comparison between NCDR and CCDR: (a) duty ratio, (b) output current ripple, and (c) secondary peak current of the transformer.
To verity the performance of NCDR and CCDR, two sets of 500 W prototypes with phaseshift fullbridge converters were built (see Figures
input voltage
output current
output voltage
output power
switching frequency
Experimental circuit of the phaseshift fullbridge converter with NCDR.
Experimental circuit of the proposed phaseshift fullbridge converter with CCDR.
Figure
Measured waveforms of the secondary voltage and current of the transformer: (a) NCDR and (b) CCDR.
Measured waveforms of output filter inductor current
Measured output current
Efficiency comparison between NCDR and CCDR associated with phaseshift fullbridge converters.
In this paper, the proposed phaseshift fullbridge converter with NCDR and CCDR under 500 W has been implemented. The NCDR has the merits of extended duty ratio, lower output current ripple, and lower rectifier diodes voltage stresses, which can reduce the peak current through the isolation transformer and switches. However, in comparison between efficiency of NCDR and CCDR, the NCDR has lower efficiency at full load condition. The reason behind is that NCDR is used with four inductors resulting in low conversion efficiency. For small size and high efficiency requirements, CCDR is relatively suitable for high stepdown voltage and high power conversion applications.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Science Council, Taiwan, under Grant no. NSC1022221E167024.