Unleashing energy-saving innovations

BY TAD KEELEY,
Senior Director, Analog & Power Product Marketing,
Renesas Electronics America
http://am.renesas.com/

Electrical power conversion is integral to: improving the efficiency of existing applications, the electrification of more applications, the generation of energy through solar, wind and other alternative sources and improved energy transmission. Leading power semiconductor suppliers, like Renesas Electronics, continuously invest R&D funds to improve the performance of power MOSFETs, IGBTs and other devices. For each product category, a new generation is typically launched every 18 to 36 months, with each new generation providing substantially better performance than the previous generation. The next two figures illustrate the rate-of-progress to compare performance across different device generations.

Figure 1 shows the RSS(ON) (on-resistance) progress from generation to generation for a fixed die size for low-voltage common-drain dual MOSFETs typically used in Li+ battery packs. This is an application where the MOSFET power losses are directly proportional to RSS(ON), and switching losses are relatively inconsequential. For these low-voltage MOSFETs, the same sized production device today has an RSS(ON) of less than half that of a device from just four years ago. So, conduction losses (I2R) are effectively halved with the newest generation.

Fig. 1: Progress in Renesas’ common-drain MOSFETs

IGBTs are a very different power device technology but show a similar improvement trend. Figure 2 shows the progress (2010 to 2013) for 600V 5us tsc IGBTs with fall time, tf (tf is roughly proportional to turn-off energy losses and compares the switching performance from one device to another), on the x-axis and saturation voltage, Vce(sat) (Vce(sat) is proportional to conduction losses), on the y-axis.

Fig. 2: Progress in 600V 5us tsc IGBTs Source: Renesas Electronics

These IGBTs are popular in motor drives, where conduction losses, switching losses, and short circuit ruggedness are all important performance characteristics. Under the same saturation current, voltage, gate resistance, temperature and current density test conditions, the Vce(sat) is lower by about 10% and the tf is lower by about 60% for a new-generation IGBT compared to the previous generation IGBT. For typical IGBT applications, switching losses are a substantial contributor to total power losses, so the significant reduction in tf has a big impact on energy savings. Figures 1 and 2 are generally indicative of the progress being made with power devices across a wide spectrum of voltages, power levels and device technologies. To the circuit designer, a regular adoption of the newest generation devices provides a clear path to incorporate periodic energy savings into their products or systems.  

In my job, I hear feedback from many engineers on how their products or systems can benefit from these advances, and there are a few common themes. Even during a product refresh or redesign phase, there are some who are hesitant to change their existing but dated power devices, because upgrading may cause a ricochet of associated issues that require EMI filter, driver, or control circuit re-engineering. These engineers are leaving energy savings on the table. A second, more prevalent theme is engineers who periodically replace old power devices with new ones to capture the energy savings available from the latest generation but seek to minimize any further architectural changes to their system or product. For products where power conversion is integral to performance (including power supplies, motor drives and renewable energy, among others), this is a standard practice of product line redesigns. While the efficiency benefit from this practice is often limited to a fraction of a percent for a single conversion step, power conversion losses often cascade from one conversion step to another through a system; a system-wide power device upgrade may boost efficiencies by a percent or more.

System energy savings from upgrading power devices can be much greater when accompanied by architectural changes enabled by the latest generation of power semiconductors. A striking example is the addition of variable speed drive (VSD) motors to air conditioners (A/C) and other appliances, which reduce the power consumption by about 30% when compared to similarly rated units with a fixed-speed motor. A key enabler of VSD has been the advent of low-cost, 3-phase IGBT integrated power modules (IPMs), which make the system architecture change economically viable and ease the design transition to variable speed drive. In the US, where about 15% of electricity consumption goes toward cooling buildings, and the majority of installed A/C units use fixed-speed motors, the energy savings from the transition to VSD can be massive. Other prominent examples where power device advances contribute to re-architected power circuits that are moving toward the mainstream include:
•    Phase shedding and burst mode operation for CPU dc/dc conversion
•    Neutral point clamped PV string inverter modules replacing standard 3 phase inverters
•    The adoption of wide-band-gap based power devices such as GaN or SiC in conjunction with driver changes, higher switching frequencies and smaller filter components such as capacitors, transformers and inductors

Power semiconductors contribute to even greater energy savings when their roles in recent innovations, such as the accelerating electrification trend and the outright invention of new energy efficient systems, are considered. Electrification examples include electric motors replacing combustion engines electrical systems replacing hydraulic systems, induction cooktops replacing gas stoves in the home and renewable energy sources replacing fossil fuel-based plants. Examples of new energy saving systems, which can benefit from very high performance power semiconductors, include micro-inverters for PV systems, and the re-emergence of high-voltage DC transmission systems as a key smart grid technology. Of course, other technological advances, notably high-performance batteries to store energy, have facilitated these accelerating trends, but performance, power density, and ruggedness advances in power semiconductors have been essential ingredients.

Power semiconductor advancements have created a dynamic environment for energy saving innovations. Engineers and designers who have avoided updating power devices for fear of the myriad issues that can result should instead confidently look to their power devices as a source of innovation and opportunity. The engineer’s concern about issues arising is valid, but I hope the point that benefit of energy savings versus some re-engineering effort is a positive trade-off. For upgrading power semiconductors in a circuit plays a direct, incremental role in energy savings, and looking to fully exploit the new power devices with a wider re-architecting of a system can also be explored for even bigger energy savings.