Design and application of automotive grade IGBT in hybrid vehicle

In response to the demand for automotive power modules, Infineon has greatly improved the life expectancy of IGBTs by enhancing the power cycle and temperature cycling characteristics of IGBTs and increasing the structural strength of IGBTs.





Requirements for power semiconductor modules in hybrid vehicles

Poor working environment (high temperature, vibration)

The IGBT is located in the inverter and needs to supply energy to the motor of the hybrid system under high ambient temperature and mechanical shock according to the specific vehicle driving conditions.

Depending on the vehicle design, the inverter may be placed in the rear of the car, in the gearbox or under the hood close to the internal combustion engine, so the IGBT module is subject to severe temperatures (-40 ° C ~ 150 ° C) and mechanical conditions (vibration, shock The test.

The IGBT module is usually cooled by engine coolant, and the ambient temperature can reach Ta=105°C under the limit condition, which puts higher requirements on the power density and heat dissipation design of the power module.

Complex drive conditions

Unlike motor drag in industrial applications, hybrid vehicle driving conditions are more complicated. For example, corresponding to urban working conditions, it is necessary to frequently switch to various states of acceleration, deceleration, and cruise. Therefore, the current and voltage through the IGBT are not constant, but The vehicle operating conditions fluctuate repeatedly, and the IGBT module needs to operate reliably under the impact of current and voltage cycles.

High reliability requirements

Failure of the IGBT power module will cause the vehicle to lose power immediately, seriously affecting the reputation of the vehicle manufacturer and the user experience.

Automobile manufacturers need IGBT modules to be replaced in the HEV life cycle, which puts higher requirements on the durability of IGBTs (the design life of automobile vehicles is 15 years).

Cost control requirements

Large-scale production of automobiles is different from train traction applications. Under the conditions of high performance requirements, the cost cannot be exchanged for reliability, and the cost and performance need to be balanced, which puts higher requirements on product design. Therefore, for various restrictions in automotive applications, special IGBTs are required to meet demanding application requirements.

IGBT structure

Figure 3 shows the structure of a power module with a substrate. A ceramic substrate with a thin copper layer on both sides is soldered on the substrate. The IGBT chip is soldered to the designed copper layer. The surface of the chip is pressure bonded to the copper layer by a bonding wire. Most standard modules use this method of production. Currently 70% to 80% of power modules are manufactured in accordance with standard modular construction. The ceramic is generally made of Al2O3, and the substrate is made of copper. The IGBT backplane is mounted with a heat sink via thermal grease.



Infineon automotive grade IGBT reliability improvement

Reliability is the biggest challenge for IGBTs in automobiles. In addition to the design considerations of conventional parameters such as voltage and current, the main parameters related to IGBT reliability are: thermal cycling and power cycling. The lifetime of the IGBT, other parameters such as the mechanical reliability characteristics of the IGBT also require additional attention.

Power cycle

Generally, the inverter design mainly considers the limitation of IGBT Tjmax (highest junction temperature), but in hybrid vehicle applications, the inverter is less in constant working conditions, and acceleration, cruise, and deceleration will bring current and voltage changes. The resulting ΔTj (fast change in junction temperature) will affect the lifetime of the IGBT to a greater extent. When the IGBT is turned on, the bond line will also oscillate, which is more reliable for the connection between the bond wire and the IGBT chip. The large impact, repeated swings may lead to the end of life of the binding line (EOL, End of Life), such as the bond wire and IGBT chip solder off, bond line breakage, etc., directly lead to IGBT damage.



In order to simulate the operating conditions of the car, in response to the frequent acceleration, deceleration, and current shocks caused by HEV, Infineon defines "power cycling second" (current heating, external water cooling) through accelerated aging. Test, simulate the welding reliability of the bonded wire under electrical impact, Infineon automotive grade IGBT needs to withstand ΔTj=60k, maximum temperature of 150°C, 0.5s < tcycl<5s, 150kc power cycle without damage.

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Compared with traditional industrial applications, the IGBT in the hybrid vehicle (HEV) has a harsh working environment, which puts higher requirements on the reliability of long-term use of the IGBT.

Compared with traditional industrial modules, there are mainly the following improvements:

● Bound wire material improvement;

● The chip structure is strengthened;

● Binding line connection loop optimization;

● Optimized welding process.

Temperature cycle

In the HEV, the inverter is usually located in the front compartment close to the engine or in the vicinity of the transmission mechanism. The IGBT module will withstand high ambient temperature and temperature changes, which has a great influence on the internal welding layer of the IGBT module.



The IGBT module consists of multiple layers of different materials (see Figure 3). Each material has a different CTE (coefficient of thermal expansion). The difference in CTE affects the life of the power module. When the module is used, the temperature changes between different layers. The mechanical stress causes the weld to fall off. Our goal is to use the material with the smallest difference in thermal expansion coefficient for the welding combination. On the other hand, even if their coefficients of thermal expansion are very well matched, the cost of the material itself may be too high, or it may be difficult to process or the processing cost is too high during the production process. For example, AlSiC substrates in train traction applications. The coefficient of thermal expansion is almost the same as that of the substrate, so it has better thermal cycling characteristics. However, the application of hybrid vehicles is difficult to accept due to the high cost.

Infineon's improved Al2O3 ceramic substrate technology can also meet the thermal cycle requirements of hybrid vehicles without significantly increasing costs.

In general, IGBT modules accelerate the test soldering reliability through Thermal Cycling. For automotive grade IGBTs, Infineon defines a more severe thermal shock test (TST), which has a larger temperature range than the TC test. , -40 ° C ~ +125 ° C, 1000 cycles (common industrial module TST only 50 times).



According to Infineon's calculation method, the life of the automotive-grade IGBT module is 2.5 times that of the industrial grade, which is 1/4 of the traction level, which can meet the requirements of the full life of the vehicle without replacing the module, and the cost is well balanced.

Strengthening of mechanical structure

In addition to the improvement of the above IGBT internal packaging process, Infineon automotive grade IGBTs also enhance the IGBT housing and terminal blocks, including temperature characteristics and mechanical structural characteristics to cope with the harsh application environment of the car, such as the following aspects .

(1) Enhanced temperature characteristics. Compared with the usual industrial applications, automotive IGBTs need to withstand higher temperature shocks. If the IGBT housing material is not strong enough, it will break under temperature shock. Infineon automotive grade IGBTs need to be in thermal shock test -40 ° C ~ + It is intact at 125 °C for 1000 times. The improved IGBT housing reliability is greatly enhanced by plastic materials and optimized process parameters.

(2) Strengthened structural characteristics. In HEV, IGBT vibration greatly exceeds that of ordinary industrial modules, and the casing and terminals will withstand large mechanical shocks. Infineon automotive grade IGBTs can withstand mechanical vibrations exceeding 5g and mechanical shocks exceeding 30g.

Infineon Automotive Grade IGBT Products

In order to meet automotive-grade applications, Infineon has introduced two IGBT modules for HEV, including two products:

● HybridPACK1—400A/650V IGBT 6 unit for mild hybrid vehicles with motor power of 20kW~30kW;

● HybridPACK2—800A/650V IGBT 6 unit for a full hybrid vehicle with a motor power of around 80kW.

Main product features:

● 6-unit IGBT simplified inverter design;

● The working junction temperature is 150 °C, and the maximum temperature is 175 °C;

● IGBT technology;

● Improved binding line process;

● Improved ceramic substrate increases soldering reliability;

● 6 NTC;

● Improved binding line process;

● Improved ceramic substrate increases soldering reliability;

● Direct water cooling system to improve module cooling capacity.

in conclusion

As power devices are increasingly used in automobiles, higher demands are placed on reliability, such as the power cycling and temperature cycling characteristics described herein. For automotive applications, Infineon's automotive-grade IGBT modules offer high reliability, long life and moderate cost. Only in the case of hybrid automotive applications require dedicated power semiconductor modules to ensure the reliability of the core components. The success of a hybrid car.