Current fed dual active bridge converters for photovoltaic generation may typically require a given leakage or extra inductance in order to provide proper control of the currents. Therefore, the many researches have been focused on the leakage inductance control of high frequency transformer to integrate an extra inductor. In this paper, an asymmetric winding arrangement to get the controlled leakage inductance for the high frequency transformer is proposed to improve the efficiency of the current fed dual active bridge converter. In order to accurate analysis, a coupled electromagnetic analysis model of transformer connected with high frequency switching circuit is used. A design optimization procedure for high efficiency is also presented using design analysis model, and it is verified by the experimental result.
I. INTRODUCTION
The current fed dual active bridge (CF-DAB) converter is a kind of high step-up converter for photovoltaic generation.1–3 CF-DAB converter may typically require a given leakage or extra inductance in order to provide proper control of the currents.
Adding an inductor in series with the transformer to achieve the desired inductance increases overall size, cost and losses of the system. A more proper alternative is to modify the transformer to integrate the desired leakage inductance.4
In previous studies, the leakage inductance of transformer is controlled by adding a new flux path for leakage flux generation or increasing the distance between each winding.4–7 However, size and copper loss of transformer is increased as the extended winding length. In addition, complexity of structure is also increased.
In this paper, the winding arrangement and core structure of high frequency transformer with controlled leakage inductance is proposed to improve efficiency of CF-DAB converter. The proposed design method controls leakage inductance through an effective cross-sectional area of core, number of turns and winding arrangement.
By using this analysis method, design optimization for compact size and high efficiency is accomplished in this paper. In addition, the design results are verified by an experiment using the prototype of the transformer.
II. LEAKAGE INDUCTANCE CONTROL
In previous studies, leakage inductance of transformer is controlled by the air-gap length and effective cross-sectional area of the leakage flux path as shown in Fig. 1(a). In Fig. 1, the self-inductance and the leakage inductance of each winding are controlled by the length of Gap A and B independently. Due to the core structure for controlled leakage inductance, the size and copper loss of transformer are increased.
In this paper, asymmetric winding arrangement is proposed to secure controlled leakage inductance and to improve efficiency with simple and compact core structure as shown in Fig. 1(b).
The proposed transformer has advantage on the simpler construction and smaller size than previously studied conventional transformer with equal specification.
The proposed transformer can be designed to control the leakage inductance through number of turns in each winding, winding arrangement, air-gap length and area.
The leakage inductance can be controlled independently according to the asymmetric ratio of the secondary winding without self-inductance variation as shown in Fig. 2. In Fig. 2, self-inductance seems to vary according to the asymmetric ratio, but this is a finite element calculation error smaller than 0.01% of self-inductance.
III. DESIGN OPTIMIZATION
In order to optimize the proposed transformer, design optimization procedure is consisted core and winding structure design stage, self and leakage inductance calculation stage and efficiency optimization stage as shown in Fig. 3.
Due to the other design parameters such as the number of turns, air-gap length and area have same effect on leakage and self-inductance, these parameters should be determined earlier than asymmetric ratio of secondary winding.
As aforementioned, the asymmetric winding arrangement has an effect on the leakage inductance only. Due to this, when the winding arrangement is changed in design optimization procedure, the other parameters are kept to secure self-inductance.
Due to secure leakage inductance and improve an efficiency of transformer, design parameters are optimized through the proposed optimization procedure and coupled analysis model as summarized in Fig. 4 and Table I.
Comparison of analysis results.
| Parameter [Unit] | Initial | Optimized |
|---|---|---|
| Core volume [mm3] | 36,730 | 33,791 |
| Cross-sectional area of core [mm2] | 257.3 | 223.1 |
| Mean length per turn (Pri) [mm] | 134.6 | 102.5 |
| Mean length per turn (Sec) [mm] | 68.4 | 102.5 |
| Turn ratio (Pri / Sec) | 2 / 12 | 4 / 24 |
| Self-inductance (Pri) [uH] | 90.4 | 91.1 |
| Self-inductance (Sec) [mH] | 3.25 | 3.28 |
| Leakage inductance (Pri) [uH] | 3.06 | 3.32 |
| Leakage inductance (Sec) [uH] | 110.2 | 119.6 |
| Input power [W] | 543.93 | 543.44 |
| Output power [W] | 535.77 | 540.18 |
| Efficiency [%] | 98.5 | 99.4 |
| Parameter [Unit] | Initial | Optimized |
|---|---|---|
| Core volume [mm3] | 36,730 | 33,791 |
| Cross-sectional area of core [mm2] | 257.3 | 223.1 |
| Mean length per turn (Pri) [mm] | 134.6 | 102.5 |
| Mean length per turn (Sec) [mm] | 68.4 | 102.5 |
| Turn ratio (Pri / Sec) | 2 / 12 | 4 / 24 |
| Self-inductance (Pri) [uH] | 90.4 | 91.1 |
| Self-inductance (Sec) [mH] | 3.25 | 3.28 |
| Leakage inductance (Pri) [uH] | 3.06 | 3.32 |
| Leakage inductance (Sec) [uH] | 110.2 | 119.6 |
| Input power [W] | 543.93 | 543.44 |
| Output power [W] | 535.77 | 540.18 |
| Efficiency [%] | 98.5 | 99.4 |
Although the mean length per turn of secondary winding is increased, due to the expanded winding area, the efficiency of transformer is improved by about 0.9[%] at maximum output power condition. In addition, core volume is reduced by about 8[%].
IV. EXPERIMENT RESULT
Fig. 5 shows the prototype of CF-DAB converter having the proposed transformer used for the experiment. The voltage and current waveforms at the maximum output power condition is presented in Fig. 6.
Similar to analysis results, the efficiency of proposed transformer is improved from 95.7[%] to 97.1[%] at maximum output power condition.
V. CONCLUSION
The design optimization of a high frequency switching transformer with controlled leakage inductance using a coupled 3D finite element model is achieved with the realized high frequency switching of a CF-DAB converter.
According to the proposed design optimization procedure, the core structure and the winding structure are optimized. Although the core volume is reduced by about 8[%], the efficiency is improved from 95.7[%] to 97.1[%].
ACKNOWLEDGMENTS
This work was supported by the Kyungnam University Foundation Grant, 2017.