GaN-based Inverter for Photovoltaic Applications
Photovoltaic (PV) panels convert solar power into electrical power, generating clean renewable energy with no carbon emissions or fossil fuel consumption. In recent years, the worldwide solar energy production has skyrocketed as the cost of PV panels has dropped. In 2015, over 35 GWh of solar photovoltaic energy was produced in the United States, which is 35 times the PV output in the U.S. four years earlier .
The output voltage of a PV panel is dc, instead of the 60 Hz ac voltage required by the grid and most electrical loads. This dc voltage varies with the solar irradiance throughout the day and year. Power electronics are required to convert the varying dc voltage to stable 60 Hz ac voltage, whether for a grid-tied system or for an islanded system with typical ac loads. The power electronics make up a significant portion of the overall solar power system cost, so PV inverter cost reduction is a key research area.
Typical inverter architecture for single-phase photovoltaic applications
PV inverters are a hot topic in power electronics research today, focusing on cost and size reduction, efficiency and reliability improvements, and added features like reactive power support. This figure shows a typical single-phase PV inverter architecture, including the transistors (Q1-Q4), gate drivers, and filter. The filter is usually composed of inductors and capacitors, which can make up a large portion of the inverter size and cost. In addition to the components shown here, auxiliary electronics are required to provide regulated voltage levels, control signals, and protection. The transistors and passive components also require a heatsink or cooling system to keep them within their rated operating temperature range.
One way to improve an inverter is to select a new topology, as opposed to the conventional hard-switching full-bridge converter shown here. But even with the same topology, the size and cost of filters can be greatly reduced by increasing the switching frequency of the transistors. However, higher frequency causes higher switching losses, which reduces the inverter efficiency and requires a larger heatsink to keep the transistors from overheating. This tradeoff is at the heart of PV inverter research.
This projects aims to design a lower cost PV inverter using GaN-based power electronics, which maintaining the same capabilities, efficiency, size, and design margins for reliability as in todayâ€™s commercially available products. In order to achieve this goal, the key research areas include design of the gate drivers, heatsink, control, and filter, as well as the topology and layout of the inverter.
How WBG Can Help
Currently available PV inverters are based on Silicon power devices, such as metal-oxide-semiconductor field effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs). More recently, Silicon Carbide (SiC) Schottky diodes have been added to commercial PV inverters to reduce switching losses. This project aims to use Gallium Nitride heterojunction field effect transistors (GaN HFETs) to radically shift the design tradeoff toward a higher switching frequency, with improvements in system efficiency, filter size, and heatsink requirements.
GaN is a good fit for this application for a number of reasons. First, a GaN FET can achieve much lower switching losses than a Si MOSFET or IGBT with the same current capability. Therefore, it can be operated at a higher switching frequency with the same overall power loss. And second, the lateral GaN HFET lacks a traditional body diode, and instead experiences a "diode-like behavior" that enables reverse conduction with no reverse recovery. This means that a GaN-based inverter can eliminate antiparallel diodes, while simultaneously reducing switching loss. In simple terms, using GaN HFETs instead of conventional Si devices enables lower cost and higher efficiency.
Designing with GaN presents new challenges. The ultra-fast switching transients cause excessive dv/dt and di/dt (voltage and current slew rates), amplifying the effects of parasitics inductance and capacitance in the circuit board and auxiliary components. Gate drivers and isolation devices are not typically designed to withstand these slew rates, so these must be reviewed and potentially redesigned. The printed circuit board (PCB) must be designed densely, with very little space between components to minimize parasitics. The control and protection become more challenging as the speed of switching transients and frequency of switching events increase. And lastly, the near-chipscale surface-mount package and small die size of the GaN devices make them especially difficult to keep cool, and traditional heatsink design practices are no longer sufficient. This project combines elements of each of these design challenges, and the solutions may prove useful for PV applications as well as other WBG-based power electronics.
 U.S. Energy Information Administration, "Electricity Data Browser," 2016. Available: http://www.eia.gov/