• The Transactions of the Korean Institute of Power Electronics
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• v.26 no.6
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• pp.437-445
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• 2021

This paper proposes an operating frequency design method that limits the voltage applied to aload-side converter during the initial operation of a loosely coupled resonant dual-active bridge (LCR-DAB) converter and an initial operating strategy that applies it. The LCR-DAB converter uses two wireless power transfer coils instead of the high-frequency transformer of the general DAB converter. The wireless power coil has a physical distance of several tens of millimeter or more between the two coils; thus, the LCR-DAB converter is a bidirectional isolated power conversion system that can easily achieve high insulation performance. However, for the initial operation of the LCR-DAB, if the power-side converter is operated at the resonance frequency while the load-side converter is not operating, then a very high voltage due to resonance is applied to the load-side converter, thereby causing damage to the converter. Therefore, a method that can stably charge the DC link voltage of the secondary-side converter during the initial operation is needed. This paper proposes a method to initially charge the secondary-side DC link by operating the primary-side converter at a frequency with limited voltage gain rather than at a steady-state operating frequency. The validity of the proposed frequency design method and initial operating sequence is verified through simulation and experimentation of the 1 KW LCR-DAB converter.

• The Transactions of the Korean Institute of Power Electronics
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• v.25 no.4
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• pp.286-292
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• 2020

This study proposes a power decoupled multi-port dual-active-bridge (DAB) DC-DC converter employing multiple transformers. Conventional multiport DAB DC-DC converters experience a power coupling issue from the use of a single transformer, which essentially requires complex power decoupling control. To solve this issue, a multiport DAB DC-DC converter employing multiple transformers is proposed to decouple output power without additional complex control algorithms. The proposed converter uses multiple transformers that can expand output ports easily. Therefore, transformers and the proposed multi-port DAB converter can be designed simply. In addition, the number of coupling inductors can be reduced in the proposed three-port DAB converter compared with that in conventional multiport DAB converters. The power decoupling characteristics and equivalent circuit of the proposed converter are analyzed using theoretical model approaches. Finally, a 3-kW laboratory prototype is developed to verify the effectiveness of the proposed converter.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.2
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• pp.102-108
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• 2017

Proper design guides are proposed for a practical dual-active bridge (DAB) converter based on the mathematical model on the steady state. The DAB converter is popular in bidirectional application due to its zero-voltage capability and easy bidirectional operation for seamless control, high efficiency, and performance. Some design considerations are taken to overcome the limitation of the DAB converter. The practical design methodology of power stage is discussed to minimize the conduction and switching losses of the DAB converter. Small-signal model and frequency response are derived and analyzed based on the generalized average method, which considers equivalent series resistance, to improve the dynamics, stability, and reliability with voltage regulation of the practical DAB converter. The design of closed-loop control is discussed by the derived small-signal model to obtain the pertinent gain and phase margin in steady-state operation. Experimental results of a 3.3 kW prototype of DAB converter demonstrate the validity and effectiveness of the proposed methods.

• The Transactions of the Korean Institute of Power Electronics
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• v.23 no.5
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• pp.366-372
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• 2018

Three-port dual-active bridge (DAB) converter in a DC microgrid was studied due to its high power density and cost-effectiveness. The other advantages of DAB include galvanic isolation and bidirectional power conversion capability using simple control modulation. The three-port DAB converter consists of a three winding transformer and three bridges. The transformer has three phases, which means that the ports are coupled. Thus, the three-port DAB converter causes unwanted power flows when the load connected to each port changes. The basic operational principles of the three-port DAB converter are presented in this study. The decoupling control strategy of the independent port power transfer is presented with a mathematical power model to overcome the unexpected power flow problem. The validity of the proposed analysis and control strategy is verified with PSIM simulation and experiments using a 1-kW prototype power converter.

• Journal of the Korean Society for Railway
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• v.19 no.6
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• pp.727-735
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• 2016

Smart transformers are effective at reducing the weight and increasing the efficiency of traction systems for railroad applications. A smart transformer generally consists of rectifier modules and the Dual-Active-Bridge (DAB) converter modules. The efficiency of the smart transformer depends on not only the electrical characteristics, but also on the control method of the converter modules. Especially, a DAB converter has a high order degree of freedom of voltage modulation to control the power transferred through the high frequency transformer, and a voltage modulation method, are very critical for the efficiency of the DAB converter. This paper proposes a new voltage modulation method for the DAB converter to increase the efficiency in the low/medium power transfer condition. The proposed modulation method controls the reactive power in the high frequency transformer, making it zero. And, the switching loss is dramatically reduced by using the received converter module as a diode rectifier. The feasibility of the proposed modulation method is verified by computer simulation of the 900Vdc DAB converter power control.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.1
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• pp.44-52
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• 2017

This paper describes a 100 kW high-efficiency isolated DC-DC converter for DC distribution system. The DC-DC converter consists of two dual-active-bridge (DAB) converters in parallel. The operating principle of the DAB converter is explained, and the algorithm for parallel operation of the DAB converters is proposed. Simulation and experiments are conducted to verify the performance of the proposed system. Experimental results demonstrate that the developed converter excellently marks 97.4 percent of peak efficiency under its normal operating condition.

• Journal of Power Electronics
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• v.18 no.2
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• pp.482-491
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• 2018

The practical design methodology of a three-phase dual active bridge (3ph-DAB) converter applied to low voltage direct current (LVDC) applications is proposed by using a mathematical model based on the steady-state operation. An analysis of the small-signal model (SSM) is important for the design of a proper controller to improve the stability and dynamics of the converter. The proposed lead-lag controller for the 3ph-DAB converter is designed with a simplified SSM analysis including an equivalent series resistor (ESR) for the output capacitor. The proposed controller can compensate the effects of the ESR zero of the output capacitor in the control-to-output voltage transfer function that can cause high-frequency noises. In addition, the performance of the power converter can be improved by using a controller designed by a SSM analysis without additional cost. The accuracy of the simplified SSM including the ESR zero of the output capacitor is verified by simulation software (PSIM). The design methodology of the 3ph-DAB converter and the performance of the proposed controller are verified by experimental results obtained with a 5-kW prototype 3ph-DAB converter.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.2
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• pp.159-165
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• 2017

Small-signal modeling and controller design methodology are proposed to improve the dynamics and stability of a DC-DC dual active bridge (DAB) converter. The state-space average method has a limitation when applied to the DAB converter because its state variables are nonlinear and have zero average values in a switching period. Therefore, the small-signal model and the frequency response of the DAB converter are derived and analyzed using a generalized average method instead of conventional modeling methods. The design methodology of a lead-lag controller instead of the conventional proportional-integral controller is also proposed using the derived small-signal model. The accuracy and performance of the proposed small-signal model and controller are verified by simulation and experimental results with a 500 W prototype DAB converter.

• The Transactions of the Korean Institute of Power Electronics
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• v.23 no.5
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• pp.352-358
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• 2018

A split-capacitor (SC) dual-active-bridge (DAB) converter is proposed in this study. The DC-link capacitors of input and output are split in the proposed converter. The primary and secondary windings of transformer are connected to the midpoints of the DC-links. Hence, the SC DAB converter can inherently prevent transformer from saturation. Although the switch current stress of the proposed converter is twice that of the conventional DAB converter, the switch voltage stress is reduced by half. Therefore, the proposed converter can reduce switching loss and achieve high efficiency in a high switching frequency. Given the SC structure, the proposed converter can readily be connected to neutral-point-clamped- or half-bridge-type converters. The topology of the proposed converter is presented and the operating principle is analyzed in detail. A 3-kW hardware prototype was built and tested to verify the performance of the proposed converter.

• The Transactions of the Korean Institute of Power Electronics
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• v.27 no.5
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• pp.391-396
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• 2022

In this study, a new voltage-balancing dual-active bridge converter that integrates a DAB converter with a voltage balancer is proposed for a bipolar DC distribution system. The proposed converter is configured to connect two loads to the transformer secondary center tap of the DAB converter, and no additional components are added. The proposed converter has the same operation as the conventional DAB converter, and it makes both output voltages similar. Moreover, the imbalanced current offset between the two loads is bypassed only on the secondary side of the transformer. Consequently, the proposed converter integrates a voltage balancer without any additional components, and no additional loss occurs in the corresponding components. Thus, high efficiency and high power density can be achieved. The feasibility of the proposed converter is verified using 3 kW prototypes under 380 V input and 190/190 V output conditions.