electronica 2012 ELECTRIC VEHICLES


Reaching the tipping point


Michael Wittmann describes how a new hybrid electric vehicle power platform, the COOliR2 Drive, provides a new power management platform approach for H(EV) vehicles


T


o achieve mass market adoption of Electric- (EV) and Hybrid Electric Vehicles (HEV), the automotive industry and governments


around the world have set aggressive goals to reduce the size, weight and system cost of electric power train components while at the same time increasing system reliability for long lifetime, low maintenance and low warranty cost. As an example, the Electrical and Electronics Technical Team (EETT) of the US Department of Energy (DOE) Vehicle Technologies Program has set goals to improve the power density of the electric traction system by 2020 by 60 percent (power per system volume in [KW/L]) and specific power (power per system weight in [KW/kg]) by 32 percent while at the same time reducing cost by 60 percent


resolve. In some cases technologies which have been known in the industry but have previously being considered too costly or too challenging for volume manufacturing are also required. In these cases, innovation in manufacturing technology and processes are required. As semiconductors, and in particular IGBTs and matching diodes, are at the core of power modules for electric power trains, the industry has focused on continuously improving semiconductor performance to achieve higher current densities at lower conduction and switching losses in order to contribute to more efficient system solutions.


Figure 1: COOLiR2Die package outlines


(cost per power in [$/KW]). Within the European Union (EU), similar targets have been derived by the automobile industry driven by EU CO2 reduction targets applied to fleet average and new car registrations. From an engineering perspective, these are very challenging improvement targets designed to bring the system cost of an Electric Vehicle (EV) drive train on-par with an internal combustion engine drive train to enable mass volume market adoption. Hybrid Electric Vehicles (HEV) will also greatly benefit from reduced cost, size and weight of the electric system making HEVs much more affordable with respect to fuel economy and performance benefits. Achieving such aggressive goals set by industry and governments results in multiple conflicting engineering challenges which require a combination of breakthrough technologies to


16 CIE electronica 2012


A new approach to power switches The (H)EV power train requires power switch devices to be robust across the entire temperature range with no de-rating of output power. Many devices available on the market are rated at less than 650V breakdown voltage and such ratings are typically given for room temperature. Furthermore, breakdown voltages typically drop significantly towards low temperatures. In the actual application this means that output power has to be de-rated significantly in “cold-start” situations to protect the power switches from failing. De-rating typically translates into a noticeable loss in power and is very undesirable for (H)EV manufacturers and their customers. IR’s new COOLiRIGBT and COOLiRDiode devices have been developed for automotive quality electric power trains. The devices provide maximum guaranteed junction temperatures of 175˚C to allow system manufacturers to increase power density and cost optimise the cooling system, and are designed to maintain high breakdown voltages across the entire operating temperature range of typically -40˚C to +175˚C. While many engineers first focus on the IGBT performance, the impact on efficiency and system robustness of the matching free-wheeling diode is often underestimated. The diode is the “partner device” for IGBTs in motor drive inverter applications. While MOSFETs come with an intrinsic reverse conducting diode, the physical structure of IGBTs does not provide for this. IGBTs need a separate “Copak” diode to allow for a reverse conduction current flow, the so called free-wheeling current in a motor drive inverter. In fact, the inverter performance largely depends on the performance and characteristics of this free-wheeling diode. Design compromises


inherent to approaches integrating the diode with the IGBT (so called reverse conducting IGBT concepts) make traditional devices unsuitable for the (H)EV market where efficiency and performance is essential. To address this, IR has developed COOLiRDiode technology that matches the extended temperature range and robust characteristics of COOLiRIGBTs. This new diode exhibits a very fast but ultra-soft recovery characteristic at high and low current levels allowing for high efficiency switching at reduced EMI levels. These silicon devices are available in various voltage ranges with optimised switching and conduction losses for motor drive inverter applications in the 10kHz frequency range (5kHz – 20kHz). Different optimisations for asymmetric Motor- and Generator Mode power specifications can be provided based on the same technology platform.


Another critical component for (H)EV drivetrains are DC-DC converters used for battery charging, voltage boost applications and to bridge the high-voltage battery with the traditional 12V and upcoming 48V boardnets. DC-DC converters typically operate at significantly higher switching frequencies. The COOLiRIGBT platform is capable of a switching speed up to 200 kHz while maintaining the platform ruggedness of a 175˚C rated device to offer a superior and lower cost DC-DC implementation compared to widely used superjunction MOSFET devices.


The critical role of packaging technology While continuously improving semiconductor performance is a key contribution to more efficient system solutions, overcoming package related limitations also plays a critical role. Bond Wire fatigue has been identified as one of the key limiting factors for long term reliability under temperature and power cycling conditions. In some cases the power density of electric drivetrain power electronics has to be reduced below nominal capabilities to meet reliability


Figure 2: FE Analysis of Die Free Package Resistance (DFPR)


requirements limited by bond wire failures. This goes against industry targets to increase power density in the electric drivetrain. The automotive industry and its suppliers are making incremental progress in increasing bond wire reliability which is a general objective beyond automotive applications. Unfortunately, some of the techniques applied to achieve higher bond wire reliability are in conflict with other requirements such as ultra-thin semiconductor dies required to maximize electrical and thermal performance in electric drivetrain components. However, complete elimination of this failure mechanism is now possible as a result of IR’s bondwireless approach that provides a step function increase in reliability. Fully utilizing the


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68