Application of power devices in hybrid electric vehicles

The growth of the hybrid electric vehicle (HEV) market is largely based on the mile per gallon metric and the added benefit of every penny invested and the reliability of the hybrid system in the field. Consumers compare hybrid vehicles to standard vehicles and expect at least the same performance and reliability at an overall lower cost of ownership. The added cost of the hybrid vehicle must be paid back through fuel and maintenance cost savings during ownership.

Power modules and the power devices within them used in inverters and dc-dc converters in HEVs are the main performance, reliability and cost drivers. Efficiency, power density and specific power are some of the key performance indicators. The most important reliability specifications are thermal cycling and power cycling.

Classification of hybrid vehicles

In a hybrid vehicle drive system, one or more electric motors are used together with the combustion engine. Hybrid vehicles can be classified according to the degree of hybridization and system architecture. The degree of hybridization, which can be divided into micro, mild and full levels, determines the function performed by the electric motor. This classification also determines the required power level and the preferred system architecture.

Series, parallel and power split are the most commonly used architectures. The degree of hybridization and system architecture chosen for a particular vehicle depends primarily on the required functionality, vehicle size, age and set fuel economy targets. The power electronics content of each hybrid system is different, depending on the functionality, power requirements and architecture.

When only the start-stop function is required (e.g. in station wagon applications), a parallel mild hybrid approach is common, where the starter and alternator are replaced by an integrated starter/alternator system. In these systems, the voltage and power levels are relatively low and the fuel consumption improvement is around 10%.

In addition to the start-stop function, a mild hybrid system boosts/assists engine power when needed, and it also captures energy from regenerative braking, which can improve fuel consumption by around 15%. The added functionality requires higher energy consumption, so high-voltage devices (80 V to 600 V) are used.

Running the vehicle in full electronic mode requires a full hybrid system with high voltage and current capabilities. Depending on the application, a full hybrid system can have series, parallel and power split architectures, which can reduce fuel consumption by 35%.

Challenges of Power Electronics in HEV Systems

The power electronics in HEV systems need to efficiently transfer energy from dc to ac (battery to motor), from ac to dc (generator to battery), and from dc to dc (from low battery voltage to high inverter input voltage for boost converters and from high voltage battery to low voltage battery for buck converters). Because high voltages and high currents are switched in this energy conversion, power device technologies with the lowest losses are required. For lower system voltages and currents, MOSFET technology has better power density than IGBTs, and they are used in micro-hybrid applications. For mild hybrid applications, IGBTs are the device of choice when the system voltage is above 120V. For full hybrid applications, 600V to 1200V IGBTs are the only devices used.

In general, conventional NPT IGBTs have a trade-off between conduction loss and switching loss characteristics. If conduction losses are reduced, switching losses increase. Infineon's trench FieldStop IGBT and accompanying EmCon diode technology reduce conduction and switching losses while increasing chip current density compared to conventional devices. Lower losses are achieved by using a field stop layer, which reduces device thickness and reduces the voltage drop across the device. Figure 1 shows the cross-sectional layers of different IGBT technologies used for planar and trench devices. In addition, Field-Stop devices can operate continuously at a junction temperature of 150 °C (maximum 175 °C), which enhances chip current density and makes it easier to use higher cooling temperatures.

Power modules embedded in a convenient package can withstand extreme temperature environments, vibration and other harsh environmental conditions. In addition to the temperature changes caused by device operation, ambient temperature variations and vibration generated in the vehicle create reliability challenges. The expected service life of power modules in hybrid vehicle applications is 15 years/150,000 miles, so when designing the module, it is necessary to enable it to have the expected reliability. For example, in some cases, higher device performance will have an adverse effect on the stability of the module. From a device technology perspective, some power devices can operate at high junction temperatures, but this higher junction temperature will produce higher temperatures at the online bonding interface, thereby reducing the stability of the module power cycle. Therefore, a comprehensive set of device and packaging technology specifications need to be established to optimize performance, reliability and cost.

Power semiconductor modules for hybrid vehicles

应用需要功率模块具有高电流密度，这也就意味着每单位电流容量具有更小的体积。器件越小，包纳其于其内的底层也就越小，结果就得到一个模块虽小但功率密度更高的模块。图2显示的是英飞凌预期的1200V器件体积的减小情况。显然，与NPT器件相比，FieldStop器件显著缩小了体积。

The package design and interconnect technology have a great impact on the parasitic inductance of the module, and they can also be used to improve power density. In addition, the material selected will also have an impact on performance and reliability. For example, the cost of silicon nitride substrate is much higher than that of aluminum oxide substrate, but the thermal performance of the former is significantly better than that of the latter. Similarly, expensive aluminum silicon carbide substrates have much higher thermal cycling reliability than cheaper copper substrates.

The growth of the hybrid electric vehicle (HEV) market is largely based on the mile per gallon metric and the added benefit of every penny invested and the reliability of the hybrid system in the field. Consumers compare hybrid vehicles to standard vehicles and expect at least the same performance and reliability at an overall lower cost of ownership. The added cost of the hybrid vehicle must be paid back through fuel and maintenance cost savings during ownership.

Power modules and the power devices within them used in inverters and dc-dc converters in HEVs are the main performance, reliability and cost drivers. Efficiency, power density and specific power are some of the key performance indicators. The most important reliability specifications are thermal cycling and power cycling.

Classification of hybrid vehicles

In a hybrid vehicle drive system, one or more electric motors are used together with the combustion engine. Hybrid vehicles can be classified according to the degree of hybridization and system architecture. The degree of hybridization, which can be divided into micro, mild and full levels, determines the function performed by the electric motor. This classification also determines the required power level and the preferred system architecture.

Series, parallel and power split are the most commonly used architectures. The degree of hybridization and system architecture chosen for a particular vehicle depends primarily on the required functionality, vehicle size, age and set fuel economy targets. The power electronics content of each hybrid system is different, depending on the functionality, power requirements and architecture.

When only the start-stop function is required (e.g. in station wagon applications), a parallel mild hybrid approach is common, where the starter and alternator are replaced by an integrated starter/alternator system. In these systems, the voltage and power levels are relatively low and the fuel consumption improvement is around 10%.

In addition to the start-stop function, a mild hybrid system boosts/assists engine power when needed, and it also captures energy from regenerative braking, which can improve fuel consumption by around 15%. The added functionality requires higher energy consumption, so high-voltage devices (80 V to 600 V) are used.

Running the vehicle in full electronic mode requires a full hybrid system with high voltage and current capabilities. Depending on the application, a full hybrid system can have series, parallel and power split architectures, which can reduce fuel consumption by 35%.

Challenges of Power Electronics in HEV Systems

The power electronics in HEV systems need to efficiently transfer energy from dc to ac (battery to motor), from ac to dc (generator to battery), and from dc to dc (from low battery voltage to high inverter input voltage for boost converters and from high voltage battery to low voltage battery for buck converters). Because high voltages and high currents are switched in this energy conversion, power device technologies with the lowest losses are required. For lower system voltages and currents, MOSFET technology has better power density than IGBTs, and they are used in micro-hybrid applications. For mild hybrid applications, IGBTs are the device of choice when the system voltage is above 120V. For full hybrid applications, 600V to 1200V IGBTs are the only devices used.

In general, conventional NPT IGBTs have a trade-off between conduction loss and switching loss characteristics. If conduction losses are reduced, switching losses increase. Infineon's trench FieldStop IGBT and accompanying EmCon diode technology reduce conduction and switching losses while increasing chip current density compared to conventional devices. Lower losses are achieved by using a field stop layer, which reduces device thickness and reduces the voltage drop across the device. Figure 1 shows the cross-sectional layers of different IGBT technologies used for planar and trench devices. In addition, Field-Stop devices can operate continuously at a junction temperature of 150 °C (maximum 175 °C), which enhances chip current density and makes it easier to use higher cooling temperatures.

Power modules embedded in a convenient package can withstand extreme temperature environments, vibration and other harsh environmental conditions. In addition to the temperature changes caused by device operation, ambient temperature variations and vibration generated in the vehicle create reliability challenges. The expected service life of power modules in hybrid vehicle applications is 15 years/150,000 miles, so when designing the module, it is necessary to enable it to have the expected reliability. For example, in some cases, higher device performance will have an adverse effect on the stability of the module. From a device technology perspective, some power devices can operate at high junction temperatures, but this higher junction temperature will produce higher temperatures at the online bonding interface, thereby reducing the stability of the module power cycle. Therefore, a comprehensive set of device and packaging technology specifications need to be established to optimize performance, reliability and cost.

Power semiconductor modules for hybrid vehicles

Applications require power modules with high current density, which means smaller size per unit of current capacity. The smaller the device, the smaller the underlying layer that contains it, resulting in a smaller module with higher power density. Figure 2 shows Infineon’s expected 1200V device size reduction. Obviously, FieldStop devices are significantly smaller than NPT devices.

The package design and interconnect technology have a great impact on the parasitic inductance of the module, and they can also be used to improve power density. In addition, the material selected will also have an impact on performance and reliability. For example, the cost of silicon nitride substrate is much higher than that of aluminum oxide substrate, but the thermal performance of the former is significantly better than that of the latter. Similarly, expensive aluminum silicon carbide substrates have much higher thermal cycling reliability than cheaper copper substrates.

When designing a power module for an HEV, key obstacles need to be identified at the beginning of the design. Appropriate device technology, underlying layout, and packaging techniques need to be adopted to meet performance, reliability, and cost targets. Table 1 shows a comparison of the performance and reliability of three modules: a standard half-bridge 62mm module for industrial variable speed drives, a six-pack HybridPACK1 module for mild hybrids (Figure 3), and a six-pack HybridPACK2 module for full hybrids.

The same 600V Trench FieldStop device technology is used in all three modules, but the packaging technology used is different. The device current achieved in the 62mm and HybridPACK1 modules is 400A (two 200A IGBTs and two 200A diodes per switch), while the current of the HybridPACK2 module is 800A (four 200A IGBTs and four 200A diodes per switch). The packaging technology used for the power and signal thermal connections of the 62mm, HybridPACK1 and HybridPACK2 modules is: welding, wire bonding and ultrasonic welding respectively. Through layout improvements and the use of wire bonding for power and signal thermal connections, the power density of the HybridPACK1 module has been improved by 50% compared to the 62mm module. Although the parasitic inductance has increased by 50%, this is not a major issue for the 600V devices because the worst system voltage case in mild hybrid applications is below 200V.

通过创新的超声波焊接工艺和改进的布局，HybridPACK2模块的功率密度增加了120%以上。多个线连接及为了移动绑定工具分配的空间使线绑定热连接在封装内很占空间；超声波焊接则省去了该空间且速度也比线绑定工艺快。另外，线绑定的电流输送能力有限。因厚的铜终端在超声波焊接时与底层融固在一起，所以，超声波焊接的电流载运能力不受限制。更紧凑的封装还显著降低了HybridPACK2封装的自感。对全混合应用来说，因系统电压会高于400V，且大电流会产生很大的dI/dt，所以低的寄生感应很重要。

模块的热阻抗主要取决于每开关所占的芯片面积、模块的材料堆叠及底层布局。材料堆叠特性直接影响模块的热阻抗，而布局则增加了交叉传导部分。在62mm和HybridPACK1模块中，采用了平的铜基层，而HybridPACK2则采用集成的针翅管（pin-finned）铜基层。对带有平基层的模块来说，需将导热脂和散热层的热阻抗加起来以得到“从结到环境”的热阻抗。借助拿掉了导热脂层并直接将底层与针翅管基板焊接在一起，从而显著改善了HybridPACK2模块的热阻抗表现。

模块内临近材料的热扩展不匹配将使连接部位产生压力形变并最终导致故障。最大的压力产生在铜基板上为与底层焊接在一起所涂覆的焊料点上。为加强可靠性，模块制造商传统上采用氮化铝底层与铝硅碳化物基板的组合，此举显著增加了成本。为替代昂贵的铝硅碳化物，英飞凌开发出采用铜基板和改进的氧化铝底层的HybridPACK1和HybridPACK2模块。这种材料组合可满足可靠性目标要求，但成本却降低了很多。汽车的可靠性目标是从-40 °C到125 °C的1000次循环。

in conclusion

功率模块的性能、可靠性和成本是HEV市场增长的主要驱动器。为降低成本，需降低功率模块内器件的功率密度和结温度。英飞凌的沟道FieldStop IGBT和EmCon就是在增加结温度的同时可降低导通和开关损耗的这样一类器件。通过采用高效的功率器件和超声波焊接技术可显著改进模块的功率密度；同样，采用集成的针翅管基层可改进热性能。改进的氧化铝底层和铜基板方法能以低成本为HybridPACK模块提供最优异的可靠性。对全混合应用来说，HybridPACK2是一款优异的模块，它提供了高功率密度、低自感、低热阻及最佳可靠性和最低成本。

By Sayeed Ahmed, HEV Product Marketing Manager, Infineon Technologies AG

REFERENCES

1.McKinsey & Company， “Drive — The future of Automotive Power，” 2006.

2.R. Amro et al， “Power Cycling at High Temperature Swings of Modules with Low Temperature Joining Technique，” ISPSD 2006， Naples.

3.T. Laska et al， “The Field Stop IGBT （FS IGBT） — A New Power Device Concept with a Great Improvement Potential，” Proceedings of the 12th ISPSD， pp.355-358， 2000.

4.P. Kanschat et al， “600V IGBT3： A Detailed Analysis of Outstanding Static and Dynamic Properties，” Proc. PCIM Europe， pp. 436-441， 2004.

5.A. Kawahashi et al， “A New-Generation Hybrid Electric Vehicle and its Supporting Power Semiconductor Devices，” Proceedings of 16th ISPSD， pp. 23-29， 2004.