Fengfeng Tao

A High-Power-Density DC–DC Converter for Distributed PV Architectures

In order to maximize the solar energy harvesting capabilities, power converters for photovoltaic (PV) systems have to be designed for high efficiency, accurate maximum power point tracking (MPPT) and voltage/current performance. When many converters are used in distributed PV systems, power density also becomes an important factor since it allows for simpler system integration. In this paper, a high power density string level MPPT DC-DC converter suitable for distributed medium to large scale PV installations is presented. A simple partial power processing topology implemented exclusively with silicon carbide devices provides high efficiency and high power density. A 3.5kW, 100 kHz converter is designed and tested to verify the proposed methods.

Overview of 1.2kV – 2.2kV SiC MOSFETs targeted for industrial power conversion applications

This paper presents the latest 1.2kV-2.2kV SiC MOSFETs designed to maximize SiC device benefits for high-power, medium voltage power conversion applications. 1.2kV, 1.7kV and 2.2kV devices with die size of 4.5mm × 4.5mm were fabricated, exhibiting room temperature on-resistances of 34mOhm, 39mOhm and 41mOhm, respectively. The ability to safely withstand single-pulse avalanche energies of over 17J/cm2 is demonstrated. Next, the 1.7kV SiC MOSFETs were used to fabricate half-bridge power modules. The module typical onresistance was 7mOhm at Tj=25°C and 11mOhm at 150°C. The module exhibits 9mJ turn-on and 14mJ turn-off losses at Vds=900V, Id=400A. Validation of GEs SiC MOSFET performance advantages was done through continuous buck-boost operation with three 1.7kV modules per phase leg exhibiting 99.4% efficiency. Device ruggedness and tolerance to terrestrial cosmic radiation was evaluated. Experimental results show that higher voltage devices (2.2kV and 3.3kV) are more susceptible to cosmic radiation, requiring up to 45% derating in order to achieve module failure rate of 100 FIT, while 1.2kV MOSFETs require only 25% derating to deliver similar FIT rate. Finally, the feasibility of medium voltage power conversion based on series connected 1.2kV SiC MOSFETs with body diode is demonstrated.

High performance SiC MOSFET module for industrial applications

A novel 1.7kV, 500A low inductance half-bridge module has been developed for fast-switching SiC devices. The module has a maximum temperature rating of 175°C. There are 12 GE SiC MOSFET chips per switch and the MOSFETs body diode is utilized as the freewheeling diode. The modules typical on-resistance is 3.8mOhms at 25°C and 5.8mOhms at 175°C. Internal loop inductance measured from DC input terminals is 4.5nH, approximately 75% lower than that of a standard IGBT module. When connected to a low inductance busbars, the module can be switched in 50ns without excessive voltage and current overshoots. Double pulse inductive switching losses at VDS=1000V, Id=450A and TJ=150°C are: EON=21.5mJ, EOFF=16.5mJ and EREC=6mJ. The losses are at least ten times lower when compared to a similarly rated IGBT module, highlighting the SiC advantage for higher switching frequency applications. Short circuit testing was performed, demonstrating good ruggedness albeit the need for a fast protection circuit.

Magnetic Resonance Imaging Power: High-Performance MVA Gradient Drivers

In high-performance magnetic resonance imaging systems, the gradient driver is required to supply the gradient coil with a large current (>600 A) and a high voltage (>2000 V) to achieve a strong gradient field and a fast slew rate. In addition, extremely high fidelity for reproducing the current command from the central system is very critical to imaging quality. This paper presents the solutions for the different elements of the driver: 1) high-bandwidth (BW) power inverter; 2) ripple cancelation filter; 3) multioutput power supply (PS); and 4) digital control. A high-BW power inverter requires a stacked-bridges structure to achieve a high output ripple frequency with the existing commercial power semiconductor modules. The high voltage and the high frequency for large power modules can be obtained easier and with lower loss using the new silicon carbide devices. The control needs a digital implementation and a very fast processor. Digital control provides compensation and feedforward to improve the response. A capability improvement is obtained by reducing the switching frequency when large currents with a very low-frequency variation are needed. The control can handle it very well, but the filter has to be designed to eliminate more than one ripple frequency. Finally, many PS solutions have been used for multiple isolated outputs, but digital control compensation permits the use of much simpler unregulated PS and keeps the performance. A 2 MVA, 900 A/1200 V, platform has been built and fully tested. The experimental results proved the validity of the proposed structure and the modulation technique.

A high efficiency PV micro-inverter with grid support functions

This paper presents a new photovoltaic (PV) micro-inverter topology. The topology is based on a partial power processing resonant front end dc-dc stage, followed by an interleaved inverter stage. The input stage provides high efficiency, and flexibility of design for wide input voltage range and the output stage provides an effective switching ripple of twice the PWM frequency, which reduces the output filter requirement. The designed micro-inverter can also provide grid Volt/VAR support functions according to commands received by the grid. Circuit topology and operation are presented as well as experimental results for the micro-inverter. The overall micro-inverter efficiency is 96%.

High performance two H-bridge in cascaded gradient driver design with SiC power MOSFET

In this paper, a detailed high efficiency two H-bridge in cascaded gradient driver design with 1700V SiC MOSFET is presented. Both module and system level stray inductance are minimized to better utilize the SiC high switching speed capability. The amplifier loss is calculated in simulation with device loss model and also verified in hardware experiment. The efficiency of the amplifier is higher than 99%. Higher output ripple frequency (up to 125 kHz) provides the opportunity to design a high density output filter without magnetic components. A novel ripple current cancellation circuit (RCCC) with embedded coolant pipe is also presented and demonstrated.

A multi-objective study for down selection of a micro-inverter topology for residential applications

Micro-inverter based photovoltaic (PV) systems now represent about 8% of the U.S. residential market, and offer many advantages including safety, performance, and simplified installation. This paper presents a detailed trade-off study of micro-inverter circuit topologies. In addition to standard criteria of efficiency, reliability and cost, this study takes into account grid Volt/VAR support functions of each topology. The trade-off study concludes in the identification of a new micro-inverter architecture with high efficiency and Volt/VAR support capability suitable for application over a wide range of PV module technologies.

H-bridge building block with SiC power MOSFETs for pulsed power applications

This paper presents a H-bridge building block with 1700V SiC MOSFET for a pulsed power application to achieve both high voltage, high current and high switching frequency capability. Both power module level and system level stray inductance is minimized to better utilize the SiC high switching speed capability. Meanwhile, since the large pulsed energy is required to drive the pulsed load, voltage sensing for active DC link voltage compensation is proposed and designed.