Design Considerations for GaN-based Photovoltaic Inverter and Solid-State Circuit Breaker

Featuring low specific on-state resistance, high switching speed, and zero reverse recovery current, Gallium Nitride (GaN) transistor is becoming one of the most promising devices for power electronic applications. This dissertation presents some design considerations for photovoltaic (PV) inverters and solid-state circuit breakers (SSCBs) using GaN devices. Several related challenges and solutions will be discussed in this dissertation.

First, the filter design for full bridge PV inverter is discussed, which includes the design and optimization of individual inductor and overall power filter. The inductor design takes the fringing effect into account, and achieves optimal design with simple algorithm. Then, the design method is applied to the power filter which consists of both coupled and non-coupled inductors. The modeling of the power filter and the optimal ratio between coupled and non-coupled inductors are also presented.

Second, the inductive coupling between components, and its impact on EMI performance is studied. An improved design procedure which identifies and models the critical couplings is proposed. The design procedure models the impact of inductive coupling on the performance of the EMI filter and provides guidelines to achieve sufficient EMI noise suppression.

Third, some often-neglected factors on loss modeling of the converter are quantified, including parasitic capacitance across the devices, on-state resistance (R_dson) variation against drain current (I_d) and junction temperature (T_j), dynamics of T_j under time-varying power dissipation (P_loss), thermal coupling among nearby GaN devices, and detailed considerations of the passive components. A systematic approach to implement a detailed loss model is also presented.

Fourth, GaN-based and Silicon (Si)-based PV inverters are designed and compared based on the same specification derived from a commercial product. Compared to the Si-based inverter, the GaN-based version can achieve lower cost by using higher switching frequency, which shrinks the filtering components. The result justifies using GaN devices in PV inverters.

Finally, dc overcurrent capability of GaN devices is evaluated, which is a critical characteristic in the design of a dc circuit breaker. The test setup and procedures are discussed, which can evaluate the dc overcurrent capability under different junction temperature, different current levels, and time durations.