This study presents a novel transformer isolated parallel connected quasi Z-source (qZ-source) full-bridge DC-DC converter that uses a coupled inductor in both the qZ-source network and output filter inductor. Unlike traditional voltage-fed or current-fed converters, the proposed converter can be open- and short-circuited without damaging switching devices. Therefore, the desired buck and boost functions can be achieved and converter reliability can be significantly improved. All the bulky inductors in the qZ-source network and output filter can also be minimized with the proposed inductor structures. A 4 kW prototype DC-DC converter is built and tested to verify the performance of the proposed converter. Two types of full-bridge (FB) transformer-isolated DC-DC converters generally exist: a voltage-fed (V-fed) FB converter and a current-fed (C-fed) FB converter. Fig. 1 shows two traditional transformer-isolated DC-DC power conversion circuits. The V-fed FB converter [ Fig. 1 (a)] always has a buck (step-down) function, and the upper and lower switches that consist of a phase leg cannot be short-circuited [1] . Similarly, the C-fed FB converter [ Fig. 1 (b)] always has a boost (step-up) function, and the upper and lower switches that consist of a phase leg cannot be open-circuited. Thus, the two traditional DC-DC converters shown in Fig. 1 can only produce an output voltage that is smaller than or larger than the input voltage. Furthermore, they cannot be either short- or open-circuited. Consequently, traditional V-fed and C-fed converters are susceptible to electromagnetic interference (EMI) noise, which considerably impairs their system reliability Quasi Z-source (qZ-source) or Z-source converters or inverters are introduced to overcome the aforementioned problems [1] - [3] . Fig. 2 shows an example of a transformer-isolated qZ-source FB DC-DC converter. By using a qZ-source network that consists of a diode, two inductors, and two capacitors, the main switches ( S ₁ to S ₄ ) can be short- and open-circuited without damaging the switching devices. Therefore, the desired buck and boost functions can be achieved, which is appropriate for converters with a having wide input voltage range. Parallel-connected power converters operate via a general method that easily increases power level in the power electronics area. Through parallel operation, low current rating semiconductor devices can be used without over-sizing a converter module [4] - [6] . When multiple converters are connected in parallel, an interleaved pulse-width modulation (PWM) method is widely used to decrease input and output current ripples. Consequently, this method can reduce input and output capacitor volumes and increase converter efficiency. Although a qZ-source converter is resistant to EMI noise because of the presence of a qZ-source network, the two inductors and two capacitors in the qZ-source network are heavy and bulky even if the two inductors L ₁ and L ₂ are coupled. The switching frequency of the converter should be increased to decrease the size of passive components. Increasing switching frequency, however, reduces converter efficiency. This study presents a novel transformer-isolated and parallel-connected FB DC-DC converter a qZ-source network is used to achieve the desired buck and boost functions and increase converter reliability. A specially designed coupled inductor is also used in both the qZ-source network and output filter inductor to minimize inductor current ripple and volume significantly. With this structure, the proposed DC-DC converter exhibits the following advantages over conventional converters. This paper is organized as follows. The principal operation of the proposed converter is reviewed in Section II. A study on the comparative analysis of current ripples on the non-coupled inductor and coupled inductor is presented in Section III. Experiment results to verify are illustrated in Section IV. Lastly some conclusions and application of the proposed converter are given in Section V. Fig. 3 shows the overall circuit configuration of the proposed DC-DC converter. Two identical transformer-isolated qZ-source FB DC-DC converters are connected in parallel to increase the power rating of the converter. Conventional two-phase interleaved PWM switching is applied. Therefore, the top and bottom converters are switched at 90° apart, which results in a 180° phase shift at the top and bottom qZ-source networks and output filter inductors because of the FB structure. As mentioned in Section I, the inductors in the qZ-source network are heavy and take a significant portion of the entire converter. All the inductors ( L ₁ to L ₄ ) at the top and bottom qZ-source networks are coupled to decrease inductor size. The two output filter inductors ( L _o1 and L _o2 ) are also coupled by following a similar method. When V_i > V_o , the proposed converter operates in buck mode. No shoot-through interval occurs in buck mode, and thus, diodes D _z1 and D _z2 in the qZ-source networks are always conducting. No energy is stored in the qZ-source inductors; hence, Consequently, the proposed converter operates as the conventional parallel-connected V-fed FB DC-DC converter that uses PWM control. Defining D as the effective duty ratio in the output filter side, the voltage gain of the proposed converter in this mode is expressed as follows: where n is the transformer turns ratio.