Abstract
For dual active bridge (DAB) converters, integrating the phase-shifting inductance (PSI) in the medium-frequency transformer (MFT) is an effective way to improve the overall power density. Different from the existing leakage-inductance-integrated (LII) structure, a concentric-winding (CW) enhanced leakage-inductance-integrated (ELII) structure, which includes an additional core, is proposed in this paper. In order to explain the operating mode of CW ELII MFT, a magnetic circuit model is established, and the analysis is carried out under the typical DAB excitation. The total leakage inductance of CW ELII MFT is divided into the winding leakage inductance and the additional leakage inductance for calculation. The integrated structure makes the heat dissipation of the MFT challenging. Therefore, the air–water combined cooling method is adopted in the design. A thermal resistance model is built for the winding air channel under forced convection. On this basis, MFT designs with different integration structures for different leakage inductance requirements are compared. Finally, a 200 kW/4 kHz/200 μH MFT prototype was designed and manufactured, which achieved the power density of 5.16 kW/dm3 and the efficiency of 99.30%. The prototype was tested in a DAB converter, which is a module of a 2 MW modular multilevel converter-bidirectional DC–DC converter (MMC-BDC).
Highlights
In recent years, benefiting from advances in switching devices, magnetic materials, and control methods, power conversion systems based on medium-frequency transformer (MFT) have gradually increased
A typical application occasion is vessel integrated power system (IPS) [4], and a modular multi-level converter-bidirectional DC/DC converter (MMC-BDC) for IPS is shown in Figure 1 [5]
The CW LII MFT is combined air–water cooling, the difference is that the CW LII structure has no additional core, and the target leakage inductance is achieved by adjusting the width of the main insulation layer
Summary
In recent years, benefiting from advances in switching devices, magnetic materials, and control methods, power conversion systems based on MFTs have gradually increased. With the increase of operating frequency, the volume and weight of the transformer can be reduced, improving the power density of the converter. The converter adopts multiple DAB converters to realize DC/DC energy conversion and bidirectional flow, which can serve DC loads or energy storage elements. The phase-shifting inductance (PSI) is determined by the power capacity of the DAB converter and provides the instantaneous energy storage in the conversion process [6]. The volume and weight of the auxiliary inductor may even be close to the MFT [3]. In [7], the integrated/non-integrated MFT designs are compared. The integrated design has obvious advantages in power density, the non-integrated design is chosen for the sake of heat dissipation and installation flexibility.
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