Challenges and solutions for power electronics in hybrid vehicle systems

The growth of the hybrid electric vehicle (HEV) market depends largely 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 a lower overall cost of ownership. The added cost of the hybrid vehicle must be paid back through fuel and maintenance 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 power consumption, so high-voltage devices (80V to 600V) 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 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 a smaller volume per unit current capacity. The smaller the device, the smaller the substrate that houses it, resulting in a smaller module with higher power density. Figure 2 shows the expected size reduction of the 1200V device from Infineon. Clearly, the FieldStop device is significantly smaller than the NPT device.

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 the 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 parasitic inductance has increased by 50%, this is not a major issue for 600V devices because the worst system voltage case in mild hybrid applications is below 200V.

The power density of the HybridPACK2 module has been increased by more than 120% through an innovative ultrasonic welding process and improved layout. The wire-bonded thermal connection takes up a lot of space in the package due to multiple wire connections and the space allocated for moving the bonding tool; ultrasonic welding eliminates this space and is faster than the wire bonding process. In addition, wire bonding has limited current carrying capacity. Because the thick copper terminal is fused to the substrate during ultrasonic welding, the current carrying capacity of ultrasonic welding is not limited. The more compact package also significantly reduces the self-inductance of the HybridPACK2 package. For full hybrid applications, low parasitic inductance is important because the system voltage can be higher than 400V and the high current will generate large dI/dt.

The thermal impedance of the module is mainly determined by the chip area occupied by each switch, the material stack of the module and the bottom layer layout. The material stack characteristics directly affect the thermal impedance of the module, while the layout increases the cross-conduction part. In the 62mm and HybridPACK1 modules, a flat copper base layer is used, while the HybridPACK2 uses an integrated pin - finned copper base layer. For modules with a flat base layer, the thermal impedance of the thermal grease and the heat sink layer need to be added to obtain the thermal impedance "from junction to ambient". The thermal impedance performance of the HybridPACK2 module is significantly improved by removing the thermal grease layer and directly soldering the bottom layer to the pin-finned base.

The mismatch in thermal expansion of adjacent materials within the module will cause stress deformation at the connection and eventually lead to failure. The greatest stress is generated at the solder points on the copper substrate that are applied to the bottom layer for soldering. To enhance reliability, module manufacturers traditionally use a combination of aluminum nitride bottom layer and aluminum silicon carbide substrate, which significantly increases costs. To replace the expensive aluminum silicon carbide, Infineon has developed HybridPACK1 and HybridPACK2 modules with copper substrate and improved aluminum oxide bottom layer. This material combination meets the reliability targets, but at a much lower cost. The reliability target for automobiles is 1000 cycles from -40 °C to 125 °C.