Auto Electronics

Putting the electrical power in hybrid powertrains

Performance, reliability and cost aspects of power device and power module design in hybrid vehicle applications.

The growth of the hybrid electric vehicle (HEV) market depends, to a large extent, on the performance benefits in miles per gallon per incremental cost, and on the reliability of hybrid systems in the field. Consumers compare a hybrid vehicle with a standard vehicle, and expect at least the same performance and reliability at a lower total cost of ownership. The additional cost for a hybrid has to be compensated for by the savings in fuel and maintenance costs over the ownership period.

The power modules, and the power devices in them, that are used in the inverters and dc-dc converters of an HEV are major performance, reliability and cost drivers. Some of the key performance indicators are efficiency, power density and specific power. The most important reliability specifications are thermal cycling and power cycling.

CLASSIFICATION OF HYBRID ELECTRIC VEHICLES

In the propulsion systems of hybrid vehicles, one or more electric motors are used in combination with a combustion engine. Hybrids can be classified based on the level of hybridization and the system architecture. The level of hybridization, categorized as micro, mild or full, determines the functions that the electric motor performs. This classification also determines the power level required and preferred system architecture.

Series, parallel and power-split are the commonly used architectures. The choice of the level of hybridization and system architecture for a particular vehicle mainly depends on the desired function-alities, the size of the vehicle, the drive cycle and the target fuel economy. The power electronics content in each hybrid system varies and is dependent on the functionalities, power requirements and architecture.

Parallel micro hybrids, in which the starter and the alternator are replaced by an integrated starter alternator system, are popular when only a start-stop function is desired, such as in passenger vehicles. Voltage and power level are relatively low in these systems, and fuel consumption improvement is in the 10% range.

In addition to a start-stop function, a mild hybrid system can boost/assist the engine power when needed, and also can capture energy from regenerative braking, contributing to a fuel consumption improvement in the 15% range. Higher power is needed for the added functionalities, so higher-voltage devices (80 V to 600 V) are used.

To run a vehicle in full electric mode, a full hybrid system, with high voltage and high current capabilities needs to be used. A full hybrid system can have a series, parallel or power-split architecture, depending on the application, and reduce fuel consumption by as much as 35%.

CHALLENGES FOR POWER ELECTRONICS IN HEV SYSTEMS

The power electronics in an HEV need to convert the energy efficiently from dc to ac (battery-to-motor), ac to dc (generator-to-battery) and dc to dc (low battery voltage-to-high inverter input voltage for a boost converter, and high-voltage battery-to-low-voltage battery for a buck converter). Because high voltage and high current are switched during this energy transformation, a power device technology with minimum losses needs to be used. For lower system voltages and currents, MOSFET technology has better power density than IGBTs, and is used for micro hybrid applications. For mild hybrid applications, IGBT is the device technology of choice for system voltages greater than 120 V. For full hybrids, 600 V to 1200 V IGBTs are the only devices used.

In general, traditional NPT IGBTs have a trade-off of characteristics between the on-state loss and the switching loss. If the on-state loss decreases, the switching losses increase. Infineon trench FieldStop IGBT and associated EmCon diode technologies increase chip current density while reducing the on-state and switching losses compared to traditional devices. The lower loss is achieved by introducing a fieldstop layer, which reduces the thickness of the device and lowers the voltage drop across the device. Figure 1 shows the cross-sections of different IGBT technologies for both planar and trench devices. In addition, Field-Stop devices can operate at a 150 °C junction temperature continuously, with a 175 °C maximum, which enhances the chip current density and facilitates the use of higher coolant temperatures.

The power module, which houses the power devices in a convenient package, is subjected to extreme temperatures, vibration and other harsh environmental conditions. In addition to the temperature swing due to the operation of the devices, the environmental temperature variation and vibration in a vehicle create a reliability challenge. The expected lifetime of power modules for hybrid applications is 15 years/150,000 miles, so the modules need to be designed to ensure the expected reliability. For example, high-er device performance can, in some cases, negatively affect the reliability of the module. Some power devices can run at high junction temperatures from a device technology point of view, but the higher junction temperature produces a higher temperature difference at the device-wire bond interface, which reduces the module power cycling reliability. Therefore, a comprehensive device and packaging technology portfolio needs to be established to optimize the performance, reliability and cost.

POWER SEMICONDUCTOR MODULES FOR HYBRID VEHICLES

Hybrid applications require high power density in modules, which means smaller devices per unit current capability. The smaller the device, the smaller the substrate area to accommodate it, which results in a smaller module footprint and higher power density. Figure 2 shows Infineon's projections for chip size reduction for 1200 V devices. It is evident that FieldStop devices achieve significant chip size reductions compared to NPT devices.

Packaging design and interconnect technologies have a strong influence on the parasitic inductance of the module, and can also be used to improve power density. In addition, the choice of material has performance and reliability implications. For example, the cost of silicon nitride substrate is significantly higher than the cost of aluminum oxide substrate, but the thermal performance of the former is considerably better than the latter. Also, expensive aluminum silicon carbide baseplates have substantially higher thermal cycling reliability than cheaper copper baseplates

In designing power modules for HEVs, the key hurdles need to be identified at the beginning of the design process. Appropriate device technology, substrate layout and packaging technologies need to be used to meet the performance, reliability and cost targets. Table 1 shows a comparison of performance and reliability of three modules, a standard half-bridge 62 mm module for industrial variable-speed drives, a six-pack HybridPACK1 module (Figure 3) for mild hybrids and a six-pack HybridPACK2 module (Figure 4) for full hybrids.

Table 1
Module Package Mod-
ule V
Mod-
ule A
Length mm Width mm Module Foot-
print mm^2
Power Density KVA/
mm^2
Para-
sitic Induc-
tance nH
# of Ther-
mal Shock (-40C to 125C)
62 mm, dual 600 400 106.4 75 23940 10.03 20 100
Hybrid-
PACK1
600 400 139 113 15707 15.28 30 1000
Hybrid-
PACK2
600 800 216 100 21600 22.22 14 1000

The same 600 V trench FieldStop device technology is used in all the modules, but the packaging technologies are different. The implemented device current for the 62 mm and HybridPACK1 modules is 400 A (two 200A IGBTs per switch and two 200 A diodes per switch), while the current for the HybridPACK2 module is 800 A (four 200 A IGBTs per switch and four 200 A diodes per switch). The packaging technologies for the power and signal terminal connections used in the 62 mm, HybridPACK1 and HybridPACK2 modules are solder, wire-bond and ultrasonic welding, respectively. The power density of the HybridPACK1 module has been increased by more than 50% over the 62 mm module through modification in layout and the use of wire-bonded power and signal terminal connections. Although the parasitic inductance increased by 50%, this is not a major issue for 600 V devices because the worst-case system voltage for mild hybrid applications is below 200 V.

The power density of the HybridPACK2 module was increased by more than 120% by using an innovative ultrasonic welding process and modifying the layout. A wire-bonded terminal connection takes up significant space in the package for multiple wire-connection and allocation for bonding tool movement; this space is reduced by ultrasonic welding, which is also faster than the wire-bonding process. In addition, the current-carrying capacity of wire-bonds is limited. Since thick copper terminals are joined to the substrate during the ultrasonic welding process, the current-carrying capacity is not constrained. Tighter packaging also reduces the HybridPACK2 package inductance significantly. For full hybrid applications, low parasitic inductance is essential, since the system voltage is more than 400 V and the high current will cause very high dI/dt.

The thermal resistance of the module principally depends on the chip area per switch, the material stack of the module and the substrate layout. The characteristics of the material stack directly affect the thermal resistance of the module, while the layout adds the cross-conduction part. Flat copper baseplates are used for the 62 mm and HybridPACK1 modules, while integrated pin-finned copper baseplates are used for the HybridPACK2. For modules with flat baseplates, the thermal resistance of the grease layer and the heat-sink needs to be added to get junction-to-ambient thermal resistance. The thermal resistance of the HybridPACK2 module is significantly enhanced by removing the grease layer and directly soldering the substrates to the pin-finned baseplate.

Thermal expansion mismatch between the adjoining materials of the module causes the interface to stress and eventually fail. The highest stress occurs at the substrate attach solder joint for copper baseplates. To enhance reliability, module manufacturers traditionally provide modules with aluminum nitride substrate and aluminum silicon carbide baseplates, significantly increasing cost. Instead of using expensive aluminum silicon carbide, Infineon developed HybridPACK1 and HybridPACK2 modules with copper base-plates and an improved aluminum oxide substrate. This combination of materials meets reliability goals at a significantly reduced cost. The automotive reliability target is 1000 cycles from -40 °C to 125 °C.

CONCLUSION

The performance, reliability and cost of power modules are key drivers for the growth of the HEV market. To reduce cost, the power density and junction temperature of the devices in the power modules needs to be increased. Infineon's trench FieldStop IGBT and EmCon diodes are examples of devices that reduce conduction and switching losses while increasing junction temperature The power density of the modules can be enhanced significantly by using efficient power devices and ultrasonic welding technology, and thermal performance can be improved by using an integrated pin-finned baseplate. The improved aluminum oxide substrate with copper baseplate approach provides HybridPACK modules optimum reliability at low cost. The HybridPACK2 is an excellent module for full hybrid applications, providing high power density, low inductance, low thermal resistance, and optimum reliability and cost.

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.

ABOUT THE AUTHOR

Sayeed Ahmed, Ph.D. is the marketing manager for HEV Products for Infineon Technologies North America.

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