Mofan Tian

A novel active gate drive for HV-IGBTs using feed-forward gate charge control strategy

Gate drives of todays high-voltage insulated gate bipolar transistors (HV-IGBTs) are in most cases voltage source based and with fixed gate resistors. Consequently, the contradiction between high-speed switching for minimized power loss and low-speed switching for low noise and low switching stress cannot be resolved simultaneously. This paper proposes a novel active gate drive (AGD) which operates basing on the feedforward gate charge control strategy. The proposed gate drive is composed of both digital and analog parts to achieve flexible control of the turn-on and turn-off operation of the IGBT. Thus, the conflicting requirements faced with the conventional gate drive (CGD) can be satisfied. The effectiveness of the feedforward gate charge control strategy has been verified by simulations, and promising results have been obtained. Besides, a chopper circuit is establishing to validate the operation of the active gate drive.

Design and implementation of a high power three-level T-type inverter for a photovoltaic system

Nowadays, the grid-connected photovoltaic systems are an important part of the renewable energy sources, and their performance is getting more and more important. Many reported researches focused on the traditional NPC topology. However, few researches were about the T-type NPC inverter. In this paper, a high power three-phase T-type neutral point clamped grid connected photovoltaic inverter prototype is designed and implemented. The design and implementation procedure of the inverter is discussed in detail. Experimental results showed that the output power of the proposed inverter could reach up to 100kW with unit power factor, while the total harmonic distortion of the grid current was less than 3%. And the total efficiency of the system is measured as 95.6%. The experimental results demonstrated the T-type NPC inverter can operate with a good performance.

Hierarchical model predictive control of modular multilevel matrix converter for low frequency AC transmission

The modular multilevel matrix converter (M3C) applied to a low frequency alternating current (LFAC) transmission system is investigated. Due to the complexity and tight coupling of the converter, a novel hierarchical model predictive control (MPC) scheme with a cascaded structure of power control, capacitor voltage averaging and balancing control for M3C is proposed. The terminal behavioral model of M3C is derived out and the fast separated space-vectors approximation method is realized. The proposed scheme clearly achieves the system-level multi-objective control without the empirical procedure of the weighting factor design and significantly reduces the computational cost. This is the first time to apply the MPC to a converter in a hierarchical structure. Simulation results of a 7-level M3C are provided to illustrate the systems performance.

An improved switching loss model for a 650V enhancement-mode GaN transistor

This paper proposes an improved switching loss model for a 650V enhancement-mode gallium nitride (GaN) transistor. The interpolation fitting method is used to fit the strong nonlinear capacitance and transconductance, and it shows a better accuracy than the given function or polynomial fitting method. Meanwhile, because the input capacitance has a strong nonlinear relationship with gate-source voltage and a weak nonlinear relationship with drain-source voltage, this paper uses Qc-Vgs curve instead of the Ciss-Vds curve in the proposed model to improve the accuracy. The parasitic inductance is also considered in the model. Then the switching processes are analyzed in detail, and they are described as a fifth-order nonlinear differential equation. Finally, the accuracy of the model is validated by experiment. The error of the predicted switching loss is within 20% when the load current changes from 3.5A to 20A.

Instability analysis of enhancement-mode GaN based half-bridge circuits

This paper analyzes the problem of instability in enhancement-mode gallium nitride (GaN) transistors based half-bridge circuits. The instability may cause sustained oscillation, resulting in overvoltage, excessive EMI, and even device breakdown. This problem does not occur in silicon or silicon carbide transistors based circuit because of their different reverse conduction characteristics from GaN devices. GaN devices operate in the saturation region when they conduct reversely during the dead time. Under the influence of parasitic parameters, the GaN-based half-bridge circuit exhibits positive feedback under certain conditions, thus resulting in sustained oscillation. A small-signal model is proposed to study this positive feedback phenomenon. Based on the model, the influence of circuit parameters on instability are investigated and guidelines to suppress the oscillation are given. Finally, the analyses are verified by experiment.

EMI modeling and experiment of a GaN based LLC half-bridge converter

This paper focuses on the electromagnetic interference (EMI) research and analysis of the MHz switching frequency GaN MOSFET based on the LLC resonant DC-DC converter. In this paper, first, the CM coupling paths are studied to get simplified models. Then the impact of the parasitic capacitors (both the capacitors to the ground and the capacitors in the devices) on the CM current are analyzed. Finally the high frequency model is derived from the work above. Moreover this paper makes the comparison between normal Si MOSFET and GaN-based MOSFET behavior in EMI. The modeling result proves that GaN MOSFET has a worse behavior on EMI test. An experiment layout which is aimed at minimizing the irrelevant factors is designed and the experiment result proves the accuracy of the modeling.

Impact of transformer stray capacitance on the conduction loss in a GaN-based LLC resonant converter

Conduction loss, which is greatly affected by transformer stray capacitance, has a huge influence on the efficiency of LLC resonant converters. The reason is that different transformer stray capacitance means different winding charge or discharge during the dead time, and then the magnetizing current and the dead time are changed accordingly. In order to investigate the impact of transformer stray capacitance on device conduction loss in LLC resonant converters, this passage is arranged as follows. Firstly, the transformer stray capacitance model is established and several methods of calculating the transformer stray capacitance are introduced. Next, the relationship between transformer stray capacitance and device conduction loss in LLC resonant converters is derived. Finally, transformers with different stray capacitances are applied in a 1.1MHz 400V–12V 200W GaN-based LLC resonant converter. The loss analysis shows that lower transformer stray capacitance contributes to smaller dead time and higher efficiency.

Switching transient analysis for normally-off GaN transistors with p-GaN gate in a phase-leg circuit

Gallium Nitride (GaN) transistors are emerging as promising candidates for making high-frequency, low-loss and small-size power converters. To realize normally-off, p-GaN gate technique is widely adopted in commercially available GaN-based power devices. However, owing to the distinctions in device structure, the intrinsic capacitances with regard to gate region vary from those of Si MOSFET. Besides, with drain-bias rising, the variation of gate regions net charge could make the threshold voltage of GaN transistor unstable. Thus, the switching transient waveforms of GaN transistor could be significantly influenced by the aforementioned factors, and the commonly used analysis method for Si MOSFET would not be sufficient. In this work, the threshold voltage instability is firstly analyzed, which is related to drain-to-gate voltage stress. Due to the difficulties in directly measuring the gate-related capacitances and their dynamic behaviors, a hybrid physical-behavioral modeling method is proposed, which is capable of extracting the relationship between the gate-related capacitances and their bias from the static measurements. The proposed analysis methods are then implemented on a GaN-based phase-leg circuit. Through the comparison with the experimental results and the simulated waveforms of the most advanced analysis, the proposed analysis approach exhibits outstanding performance.

A wire-embedded converter used for wearable devices

This paper presents an integrated solution of flexible DC-DC converter by embedding a flexible polyimide printed circuit (FPC) board and an inductor made of flexible ferrite-polymer composite in a wire. The cooper in the wire is utilized as the winding to make the embedded inductor with flexible ferrite-polymer sheets. Different wire shapes of the inductor are simulated and compared to optimize the design. A step-up prototype is fabricated and corresponding tests are executed to verify the performance. The flexible wire-embedded converter can output 5V-1W with variational input, and achieve 70% efficiency.

A wire-integrated inductor used for DC-DC converter

In this paper, a wire-embedded inductor is fabricated by flexible ferrite-polymer composite. The cooper in the wire is utilized as the winding to make the embedded inductor with flexible ferrite-polymer sheets. Different structures of the inductor are simulated and compared to optimize the design. Two 250nH inductors with 10cm length, 2mm width are made, and corresponding tests are executed on a buck demo board. The flexible inductor can output 15W with the highest efficiency achieving 70%. This type of inductor can replace traditional inductors in wearable devices and significantly increase the power density.