Optimized Power Devices Enable Energy Efficient Solar Inverters and Micro-Converters
- Created: Tuesday, 20 July 2010
- Alberto Guerra, International Rectifier, El Segundo, CA -
As the demand for greener appliances and consumer products soars across the globe, and energy savings becomes a global phenomenon (and necessity) in residential and commercial environments, solar and wind power technologies have begun to proliferate in cities and states around the world.
Concurrently, smart grid and smart meter deployments are also on the rise, enabling users to monitor in real time energy consumption and costs, connect/disconnect loads to/from the mains, and utilize the benefits of solar (or wind) power when the energy demand rises and re-route excess power to the grid. In fact, this renewable energy growth is further fueled by the recent economic stimulus package passed by the present administration, which calls for spending $45 billion on alternative energy including some $4.3 billion on smart electric grids. Now, in this transformation, energy efficient power converters and power management devices are playing a critical role.
Since its inception some 60+ years ago, International Rectifier’s mission has been to minimize power losses and to save power via efficient power conversion solutions. With that goal in mind, IR has pioneered many innovations over the last six decades, including commercial introduction of solar cells in the late fifties. These cells were used to power the initial fleet of American satellites, enabling for the first time weather monitoring and real time television global broadcasting.
Today, IR offers a large portfolio of high-performance power management products for the renewable energy industry. From string inverters for residential, commercial, and utility level applications to micro-inverters and DC-DC converters, IR has generated leading edge power semiconductor devices that are positively changing the photovoltaic (PV) inverter landscape. For instance, IR's state-of-the-art MOSFETs and insulated gate bipolar transistors (IGBTs) are dramatically lowering power losses while improving efficiencies in PV inverters. In addition, the company has developed application specific solutions for inverter topologies, such as H-bridge and 3-level converter, that are further augmenting the energy saving solutions.
In the IGBT sector, for instance, IR offers a broad portfolio with voltage ranges spanning from 600 V to 1200 V, supporting the power needs of high-performance inverters from 5 kW to 30 kW and above. While a number of process technologies have been developed to manufacture IGBTs, and each can be selectively optimized for specific PV inverter topology, the rapid adoption of field-stop (FS) trench technology is gaining momentum. Unlike the standard IGBT wafers which are 300 µm thick, the FS IGBTs are thinned down to a minimum thickness of 70 µm. As a result, it improves switching performance, and reduces the conduction path to improve collector-to-emitter voltage (VCE(on)) characteristics. Furthermore, a thinner IGBT die also has a positive impact on the thermal performance of the transistor.
IR thin IGBTs technology is housed in industry standard packages like TO-220/D2PAK and TO-247 for use in applications up to 5kW in order to lower system costs. For higher power PV inverters (such as 3-phase grid with power output >10 kW) IR IGBT chips are used in power modules, a more rational and efficient solution than discrete components at this power level. Incidentally, power modules using IGBTs have already been field-proven in terms of reliability and performance in applications like variable frequency motor drives, high power UPS, and welding machines where IR chips are adopted and used by many module manufacturers. That enables the growing PV inverter industry to quickly capitalize on the infrastructure and quality and reliability standards already in place for these products.
Concurrently, high-power module manufacturers are also seeking enhanced thermal performance for higher current density, better power cycling, and complete elimination of wire bonds for further improvement in overall reliability. To address these growing demands, the company has recently developed a solderable front metal technology for specific die sizes. In essence, IR continues to expand its IGBT portfolio to meet the current and future challenges of PV inverter designers
Optimized Power Devices
Meanwhile, for emerging power electronics such as PV micro-inverters and micro-converters (DC-DC) optimized for single PV panels, IR has developed a family of advanced trench MOSFETs in the medium voltage range (40 V to 150 V) with benchmark performance in conduction losses. Thermally optimized, these MOSFETs are tailored for DirectFET, a revolutionary packaging technology pioneered by IR.
Another product in the company’s catalog is the intelligent power module, labeled IRAM. Capable of incorporating high voltage drivers, IRAM offers higher integration for a variety of PV inverter topologies. This lowers external component count, and simplifies assembly processes to lower system cost and achieve faster time to market. Additionally, fewer components and simpler assembly translates into better overall product reliability.
Interestingly, the emphasis on the reliability of components is a characteristic of the solar industry, where the life expectancy is very high. In fact, 20-25 years is the norm for the regular warranty. Although this level of reliability is not intrinsically a problem for mature power semiconductor devices, it could certainly raise concerns for passive components, especially capacitors. By adopting circuit topologies, such as soft switching versus hard switching, that reduce stress over these components, designers can address the reliability requirements of solar applications. That requires using the right combination of IGBT characteristics, such as breakdown voltage, VCE(on), turn-off time, switching speed, and conduction loss. The wide array of IGBTs and power MOSFETs available in IR’s portfolio gives designers ample choice to address this challenge.
Market analysts predict that the market for solar power installation will grow by about 20-30% annually for the next several years. With tariffs declining and PV panel cost falling, the PV inverter cost is also expected to drop accordingly. However, the PV inverter cost is approximately 15-20% of the total system cost. Hence, the pressure to cut the cost of semiconductor content is contained and is expected to be in line with the traditional decline in pricing but can be offset by incremental performance improvements.
There is global clamor for clean, low-cost sources of energy. The recent oil spill in the Gulf of Mexico is a grim reminder that fossil fuels are a grave threat to our environment. Renewable energy is the answer. Energy policies of industrialized and emerging countries have become more favorable toward renewable energy, further driving its growth and adoption worldwide. Although the direct impact on solar industry will be partially modulated by different levels of subsidies and tariffs, growth is imminent. And with that the demand for power semiconductor devices used in PV inverters will surge.