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Posted on 23 July 2019

The Technical Benefits of PrimePACK™ Modules in CAV Applications

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Module baseplate is a key design element for robustness and extended system lifetime

The increase in energy consumption and crude oil demand over the last decade has created increasing pollution and global warming. Electrification of propulsion drives in vehicles improves the transmission efficiency and allows significant savings in fuel costs and CO2 emissions. Based on PrimePACK™, Infineon offers the first high reliability copper baseplate IGBT power modules dedicated for applications in Commercial, Construction and Agriculture Vehicles.

By Piotr Luniewski, Michael Sleven, Krzysztof Mainka & Dave Levett, Infineon Technologies AG

 

CAV is an abbreviation for Commercial, Construction and Agriculture Vehicles which is a catchall term for several different types of machines. Commercial vehicles include city buses and trash trucks. Construction vehicles include excavators, off-highway trucks and front end loaders used in the mining industry. Agricultural vehicles include combine harvesters and tractors.

Despite the differences in power and typical working cycles they all share a common feature: they all require an energy source. Traditionally the energy used is in the form of a fossil fuel which is converted into a wheel motion by an internal combustion engine and gearboxes or belts. This chain of energy conversion, especially when the combustion engine is not operating at its most efficient operating point, makes the whole power train inefficient and results in high fuel consumption. The goals of CO2 reduction and reduced operating costs can be achieved only if the efficiency of the whole power train from fuel to motion is improved.

Hybrid power train

The overall power train efficiency can be improved by running the internal combustion engine at its optimum operating range in terms of efficiency. One method of achieving this is by supplementing the combustion engine with another mechanical energy source e.g. an electric motor, which can supply energy at the points in the load cycle where the internal combustion engine operates at its lowest efficiency. Figure 1 shows the general block diagram of a hybrid propulsion drive system where several different topologies, for example, serial, parallel, dual-mode and others can be implemented. The block diagram shows one Mechanical Energy Source (MES) supplied from several possible fuels and two Electrical Energy Sources (EES). The first EES has the energy held in a battery. The second EES is in the form of a fuel cell which produces electrical energy again from a fuel, typically hydrogen.

General block diagram of a hybrid propulsion drive system

Irrespective of the type of power train topology a hybrid propulsion drive system requires at least one electric machine referred to as an e-machine in Figure 1. Depending on topology, the e-machine can work as motor during acceleration or in any situation where the combustion engine power has to be supplemented. It can also act as a generator and can convert the kinetic energy of a vehicle back into electrical energy and this energy can be stored for example in batteries or a super-capacitor. Selection of which type of e-motor is very dependent on the exact system design constraints for example, size, cost, weight, efficiency and on the application. The most common types of e-motors are: three-phase induction machines (IM), permanent magnet machines (PM) and switched reluctance machines (SR). Figure 2 shows a typical converter topology for IM and PM machines where six IGBT switches are used to control three motor phases. These six switches can be built into one single physical module, typically at lower powers, or at higher powers two switches are built into a single physical module.

Block diagram of a three-phase inverter with an additional brake chopper switch

In an application where the energy source cannot absorb the regenerative energy at a fast enough rate an additional chopper module and external braking resistor is often used to provide a load to dissipate excess energy.

Figure 3 depicts a three-phase inverter topology typical for switched reluctance machines. Here each motor phase consists of one highside and one low-side IGBT chopper module. In order to achieve the best inverter efficiency, the silicon with the lowest saturation voltage is commonly selected. The forward conducting diodes (FWD) can see higher currents than the matched IGBT due to the commutation switching pattern of the windings.

Block diagram of a three-phase inverter typical to switched reluctance machines

Both e-motor configurations shown in Figures 2 and 3 can operate in full four-quadrant mode and Infineon Technologies AG IGBT modules are designed to be able to drive both types of machines. Table 1 presents PrimePACKTM 1200V voltage rated modules designed for CAV applications.

List of PrimePACK™ modules optimised to CAV applications

Why design CAV modules with a baseplate?

The design lifetime of CAV vehicles and the power electronics used to drive them is typically 15 years. Sometimes the design life is specified in hours and this is typically 15,000 hours or more. This makes the long term reliability of the power modules critical. There are several papers describing and comparing different design approaches to modules [1], [2]. Regardless of module construction the key issues are the management of the temperature variations of both the silicon die and the module case. Both these temperature variations are a result of the mission profile or load cycle and the thermal path between the power semiconductor die and the coolant. This thermal path resistance is made up of several layers between the silicon die and the coolant. These layers of different materials offer both a thermal resistance and a thermal spreading of the heat horizontally through the material [3]. Good design with special attention paid to optimizing this thermal path can significantly enlarge the conductive heat area and reduce the steady state and transient thermal impedance between the heat source (silicon die) and the coolant. To demonstrate how the introduction of a copper baseplate improves thermal impedance between the silicon die and the coolant by horizontal thermal spreading, two finite element model (FEM) simulations were performed between two identical models whose only difference is the absence (Figure 4) or presence (Figure 5) of a 3mm-thick copper baseplate.

FEM simulation for a module without copper baseplate

FEM simulation for a module with a 3mm-thick copper baseplate

By comparing the two simulations it can be seen that:
• The heat conductive area for the module with a baseplate (active area 2) is larger than the area for the module without a baseplate (active area 1).
• The module with a baseplate has the same junction temperature (Tvjop=125°C) as the module without a baseplate but with 45% more power being dissipated by the IGBT silicon die. This allows for an increased inverter output power using the same silicon die and the same cooling system.

The FEM simulations presented above show results for steady state conditions. However in a vehicle, the load on the power electronics is typically cyclical and the transient thermal impedance, Zth j-a, between the silicon die and coolant is very critical. Figure 6 compares the transient thermal impedance for both types of modules.

Comparison of thermal impedances Zth j-a

The blue and red curves in Figure 6 show respectively the transient thermal impedance of a module with a copper baseplate and a module without baseplate in K/W (left Y scale). The green line shows the ratio of the two transient thermal impedances (right Y scale in percent). For pulses shorter than approximately 0.01 s, no real differences are observed. However as the pulse width of the power loss increases, the module with a baseplate shows a distinct improvement in thermal impedance due to the heat spreading in the horizontal axis by the copper baseplate. At pulse widths of around a second (typically experienced when operating a motor at low speeds), the solution with a baseplate has 70% lower thermal impedance. At steady state i.e. with pulse widths longer than 50 s the thermal impedance is 43% lower with the module using a baseplate.

Die junction temperature is the result of power loss and transient thermal impedance over any given time period. In CAV applications the mission profile or load cycle is very dynamic and the load can exhibit frequent large changes. Figure 7 depicts simulated junction temperature for both module solutions (with and without a baseplate) based on typical mission profile. The use of a baseplate reduces the junction temperature in this particular case by 19 °C compared to the module without baseplate. Not only does this reduction provide more design margin for the silicon die maximum operating temperature but significantly reduces the delta T of the die and wire bond system which in this example can increase the number of design life cycles by 19 million cycles [4] giving longer life and higher reliability of the module.

Simulated IGBT junction temperature

Module lifetime can be modelled by Coffin- Manson law and number of cycles to failure, Nf, is assumed to be proportional to DTJ-a and depends on the average junction temperature – TJ. The DTJ stands for junction temperature swing per power cycle where α is a constant value [5].

Reliability of PrimePACK™ module

Every power inverter consists of many active and passive components, each of which contributes to required design lifetime. The three main areas affecting the design life for power semiconductor modules are:
1) Delamination of materials with different coefficient of thermal expansion (CTE) caused by thermal cycling.
2) Bond wire detachment and cracking caused by power cycling.
3) Mechanical fatigue and damage caused by long term vibration.

Modules with and without baseplates are both prone to these effects as described in [6].

The PrimePACK™ modules listed in Table 1 are specifically designed for use in power inverters with power ratings of 60 kW and up. They have also been designed with numerous features that make them an excellent choice for CAV applications. They are built with the latest IGBT silicon which has both a 150 °C operating temperature and superior power cycling capability due to improved bond wire attachment technology [4]. For all power connections to the direct copper bonded substrate (DCB) ultra sonic welding is used to create a copper to copper joint which does not suffer long term degradation when subjected to thermal cycling as there is no CTE mismatch. Figure 8 shows the data for the module thermal cycling capability where Tcase refers to temperature measured bellow active die. The blue curve represents industrial modules which have a design life of approx. 3.000 cycles at ΔT=80K. The black line shows actual capabilities of the PrimePACK™ range of modules with a design life of 15.000 cycles at ΔT=80K. In this case the PrimePACK™ modules have 5 times the thermal cycling capability of traditional industrial modules. The red curve shows the thermal cycling numbers of special traction style modules used in the most demanding applications with AlSiC baseplates and substrates made of AlN. The special design of these modules enables them to achieve 30.000 cycles at same ΔT=80K temperature swing. The traction version of PrimePACK™ modules, which, using the latest Infineon bonding technology and new substrate materials, are the first IGBT modules on the market with copper baseplates targeting same number of cycles as traction modules. This is ten times the value of traditional industrial modules. The technological barrier that has been overcome to achieve this breakthrough is the effect of solder degradation as described in [1] and [2]. A design utilising PrimePACK™ modules can now meet the stringent reliability requirements of traction applications but benefit from the lower costs associated with construction using a copper baseplate.

Thermal cycling curve for PrimePACK™ module family

Conclusion

The reliability of power semiconductors used in CAV applications must meet the stringent requirements of the traction market but at the lower costs and high power density associated with the automotive market. Infineon manufactures modules with and without baseplates. Modules without baseplates have a lower material cost, fewer material layers between the die and the coolant and no solder layer between the DCB and baseplate. However a baseplate not only can provide a higher level of mechanical stiffness to the module but as is shown in this paper the effects of thermal cycling can be mitigated by employing the latest bonding technology. A properly attached baseplate provides an improvement in the thermal performance and longer life. This makes this form of construction a logical choice for CAV applications.

The rugged construction of the PrimePACK™ modules shown in figure 9 and its high level of thermal and power cycling capability make it especially suited to heavy and dynamic mission or load cycles. Simulation results presented here show the superiority of modules with a baseplate over those without a baseplate, the result of optimized heat spreading. Research and further improvements in module reliability and optimization of the thermal interface between the copper baseplate and the DCB are continuing and are described in [7].

PrimePACK™ 2 and PrimePACK™ 3 IGBT modules with NTC sensor

 

References:

1) U. Scheuermann, P. Beckedahl, The Road to the Next Generation Power Module – 100% Solder Free Design, CIPS2008.
2) A. Wintrich, Power Modules for Electric and Hybrid Vehicles, Bodo’s Power Systems, February 2009.
3) R. Ehler, Ernö Temesi, Zsolt Gyimothy, Influence of Thermal Cross Coupling at Power Modules, Vincotech.
4) M. Sleven, Technology for wind turbine inverter with increased reliability, Bodo’s Power Systems, January 2009.
5) R. Bayerer, at all, Model for Power Cycling lifetime of IGBT Modules – various factors influencing lifetime,CIPS2008.
6) J. Luzt, T. Hermann, M. Feller, R. Bayerer, Power cycling induced failure mechanisms in the viewpoint of rough temperature environment, CIPS2008.
7) R. Bayerer, Higher temperature in Power Modules – a demand from hybrid cars, a potential for the next step increase in power density for various Variable speed Drives, PCIM2008.

 

 

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