Some aspects of magnetic designs in modern power electronics applications

Abstract

Magnetic design is an important part in modern power electronics. This thesis studies two aspects of magnetic designs: one is the optimal design of integrated magnetics for differential rectifiers and inverters, the other is the slim 3-D receiver coil structures for omnidirectional wireless power transfer applications. For the first aspect, differential rectifiers/inverters are widely applied in power electronics applications, but they require two bulky low-frequency inductors. In this thesis, a low-frequency integrated inductor structure used for differential rectifiers/inverters is proposed. By eliminating the air gap in the central leg of the magnetic core, while still keeping the air gaps in the two outer legs, the two inductors integrated on the same core are magnetically decoupled, and can work the same as two discrete inductors. Besides, a novel numerical volume optimization method considering all the dimensions of the magnetic core to achieve a minimum core volume is proposed. Theoretical analysis shows that up to 54.0% of core volume reduction might be achieved with the proposed integrated inductor. In this thesis, an integrated inductor with 34.9% volume reduction is designed and verified in the finite element analysis software Maxwell. Several integrated inductor prototypes with up to 30.9% volume reduction are also fabricated and examined experimentally in a differential rectifier circuit. Their performances including coupling coefficient, circuit efficiency, maximum switching frequency and steady-state temperature show that the optimized integrated inductor can operate normally in the differential rectifier, like two discrete inductors, while the power density of the system is significantly improved. For the second aspect, omnidirectional wireless power transfer (WPT) becomes a popular technology in power electronics nowadays. In this thesis, two novel slim 3-D receiver coil structures for omnidirectional wireless power transfer applications are proposed. These two 3-D receivers are compatible with conventional 1-D planar transmitter coil, and their “slim” structures can be accommodated in many portable electronic products. Besides, as the two proposed receivers have multiple coils, two circuit topologies of the WPT system with a single-coil transmitter and a multi-coil receiver are proposed. One topology is that the outputs of the receiver coils are connected in parallel. The other is that the outputs are connected in series. Some theoretical analyses are conducted to compare the efficiencies of the two circuits. In this thesis, the two slim 3-D receivers and a planar 1-D receiver used for reference are designed in the software Maxwell. The two circuit topologies are designed in the software Psim. The receiver prototypes and circuits are also constructed. The simulations and measurements of the coupling coefficients and circuit efficiencies under several positions and angular orientations are carried out. The results show that both two proposed 3-D receivers can achieve omnidirectional wireless power transfer with a 1-D planar transmitter as they have reasonably high coupling coefficients and circuit efficiencies. One particular design under investigation has a better performance as it is less sensitive to angular orientations and planar positioning, and uses less ferrite material. The parallel-outputs circuit topology is better as it has a higher averaged circuit efficiency.