Design of a Power Module for Automotive and CAV Traction Drives


Andre Christmann and David Levett at Infineon Technologies North America

This article will explain the continuum of often conflicting technical and application requirements faced when designing a new IGBT based power module targeted for use in automotive and Commercial, Construction and Agricultural Vehicle (CAV) traction drives.

When the Infineon design team started working on a new IGBT based power module, the HybridPACK™ Drive, HPDrive,  for the automotive and CAV traction market it was important to focus on the needs of the end user and have a clear understanding of the application requirements. Amongst these are such diverse elements as low cost, high efficiency, power density, torque at the locked rotor, lifetime due to temperature cycling and again cost. This paper describes how all aspects of the new module design were combined with the stated goal of providing an end product that would be the best fit for all of these, often disparate, requirements and what improvements were made compared with the well-established, existing HybridPACK™ 2, HP2, module. The focus here will be on three of these technical requirements: power loss reduction, module packaging and higher stall current ratings, with cost casting its long shadow over all design decisions. The resulting module package is shown in Figure 1.

HybridPACK™ Drive, HPDrive module package

Figure 1: HybridPACK™ Drive, the HPDrive module package

Reduction of Losses

In a typical power converter, the IGBT module is the source of the majority of the power losses. If the losses can be reduced, not only will this help with the increased capability of the energy source, for example, a battery pack or generator driven by an internal combustion engine ICE, but it will reduce the requirements for the entire cooling system i.e. pumps, heat exchangers etc. All this can save cost and weight in a vehicle.  There are three main sources of losses in the module: silicon conduction losses, silicon switching losses, and copper or I2R losses. 

Conduction Losses

For the module, a new chip was developed named Electric Drive Train 2, EDT2. The new EDT2 750V IGBT chip features a similar vertical structure to the previous IGBT3 650V generation, but with an improvement using a “Micro Pattern Trench” structure based on the TRENCHSTOP™ 5 chip design, with a sub-μm mesa width see Figure 2. This construction enables the device to have a very low forward voltage drop (Vcesat), see Figure 3, while still keeping a ≈ 5µS short-circuit capability. 

Vertical structure of the IGBT3 650V and EDT2 cell geometry

Figure 2: Vertical structure of the IGBT3 650V and EDT2 cell geometry

Output characteristics of an IGBT3 and EDT2 chips of the same size

Figure 3: Output characteristics of an IGBT3 and EDT2 chips of the same size

Switching Losses

The p-emitter strength was chosen as a compromise between surge voltage capability and switching speed. Besides the maximum peak overvoltage withstand level the p-emitter also influences the Eoff vs. Vcesat trade-off point. The 100V increase in blocking voltage from 650V to 750V minimizes overvoltage as a limiting factor, resulting in less of a need to slow down the switching speed, e.g. by the use of an increased external gate resistor, to stay within the IGBT RBSOA curve. Figure 4 shows how, at a bus voltage of 400V, the turn off losses were reduced by up to 29% compared to an IGBT3 operating with an increased gate resistor value.

Comparison of turn-off losses for IGBT3 and EDT2 measured in the same package at VDC = 400V.

Figure 4: Comparison of turn-off losses for IGBT3 and EDT2 measured in the same package at VDC = 400V.

Optimized chip layout for HPDrive

Figure 5: Optimized chip layout for HPDrive

I2R Loss

Often overlooked, I2R or copper losses internal to the module can be significant, especially at high rms currents. Three main components of this loss are the main bus terminals, the top surface of the Direct Copper Bonded DCB ceramic and the bond wires connecting to the top side of the chips. By designing the IGBT chip with a gate pad located at the side of the chip, see Figure 5, rather than the center, allows a larger copper area for the main emitter current path.  Also by adjusting the shape of the chip to be more rectangular, compared to the chips used in the HP2, allows for an increased number, 10 compared to 8, and shorter, bond wires, see Figure 5, for the top side die to attach. In total, the series resistance was reduced by 20%.

HPDrive Package Design

The base HPDrive package has carried over the pin fin and internal material stack from the well proven HP2 family. However, the HPDrive comes with a 36% smaller baseplate with weight reduced by 40%, all leading to a lower cost. It also introduces a flexible control pin placement system, giving the module designer the ability to optimize the placement of the control pins sees Figure 1. This reduces the copper trace area required for control connections on the DBC. Also as the control pins are not molded into the package, future upgrades such as on-chip temperature or current sensing can be much more easily integrated into the package. For the connection of the signal pins, it was decided to use PressFit technology. This is a fast and reliable production process compared to selective soldering and the gas-tight connection is very robust against corrosive environments and vibration. For the power tabs, the DC terminals are height staggered to simplify the connections to a laminated DC bus and there is an option for elongated output terminals to fit a current sensor as shown in Fig 6. With the current sensor terminals at the same height as the module control terminals, the current sensor can be attached to the gate driver PCB directly eliminating the use of additional cables and connectors. A smaller package and optimization of the module internal layout has enabled a 40% reduction in inductance from 14nH (HP2) to 8nH. This reduces the voltage overshoot for the IGBT’s allowing faster switching speeds and/or operation at a higher DC bus voltage.

Locked Rotor/Stall Current Rating

Stall or locked rotor mode, sometimes also referred to as “hill-hold”, is often the point of highest temperature stress on the device silicon as the current can flow in a single device at the peak of the sine wave current waveform. As a result, this corner operating point can set the limit for the power module current capability. The new EDT2 chips have the capability to operate for short time periods, less than 10 sec, at 175°C to meet this stall condition requirement. Table 1 compares operation for the HPDrive and the HP2 at the stall and shows that 300mm2 of EDT2 chips can match the stall current rating of 400mm2 IGBT3 chips.

Comparison between HPDrive and HP2 modules at: 2kHz, 400Vdc bus, 500Arms, 707A peak, 80C coolant and 50% duty cycle

Table 1: Comparison between HPDrive and HP2 modules at 2kHz, 400Vdc bus, 500Arms, 707A peak, 80C coolant, and 50% duty cycle

However maximum junction temperature is not the only limiting factor but the module lifetime due to temperature cycling effects is also a key design criterion. To mitigate the effect of high ΔT events the chip solder system has been improved to provide a 40% increase in the power cycling seconds capability, for example, 60.000 cycles at a ΔT of 100K of the chip.

HPDrive with extended tabs to fit LEM current sensor example HAH3DR 800-S07

Figure 6: HPDrive with extended tabs to fit LEM current sensor example HAH3DR 800-S07

Two Different Module Design Paths

The development of a new lower loss, higher temperature rated, IGBT and Diode EDT2 chipset, offered two different module design paths, the first to make a smaller lighter module with a similar output current capability as the existing HP2 module. At the same time, the new package could incorporate some technical advantages such as lower inductance and press fit control signal pins. This was the chosen design path for the HPDrive. The second design path is to incorporate the EDT2 chips into the existing HP2 package, which allows an increase in current rating from 800A to 1100A.  So for customers already using, or considering the HP2 package, there is the option of either increasing the current and voltage rating or keeping the same performance in a smaller lower cost package. In addition, the new HPDrive module and will come in two flavors: with a pin-fin baseplate rated at 820A and with a flat baseplate rated at 660A. These new ratings will expand the HybridPACK™ family and complement the extensive Infineon range of automotive and CAV rated power modules, such as EconoDUAL™ 3 or PrimePACK™, designed for vehicle traction drives. 

Conclusion

The authors hope with this paper to illustrate the importance of a holistic design approach, where all the different component parts of a power module must work together to best meet the application requirements. For example reduced chip on state voltage, chip gate pad layout and plastic frame control pin placement all contribute to lower losses. Increased chip voltage rating and lower module inductance allow for operation at higher bus voltages. Higher chip temperature rating, improved chip bonding technology/materials with lower losses all contribute to higher stall/locked rotor current capability and with it increased motor starting torque rating. Overall a physically smaller module with reduced active chip area, lower losses and the ability to use the latest high volume assembly techniques all contribute to a lower system cost. 

More information: Infineon    Source: Bodo's Power Systems, February 2017