Design and Application of Automotive Grade IGBT in Hybrid Electric Vehicle

2020-06-03 08:10:08

In response to the needs of automotive power modules, Infineon has greatly enhanced the life expectancy of IGBT by enhancing the power cycle and temperature cycle characteristics of IGBT and increasing the structural strength of IGBT.

Requirements for power semiconductor modules in hybrid vehicles

Poor working environment (high temperature, vibration)

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

Depending on the vehicle design, the inverter may be placed in the car's tail box, gearbox, or under the hood near the internal combustion engine, so the IGBT module is subject to severe temperatures (-40 Â° C to 150 Â° C) and mechanical conditions (vibration, shock ) Test.

IGBT modules are usually cooled by engine coolant, and the ambient temperature can reach Ta = 105 â„ƒ under extreme conditions, which puts forward higher requirements on the power density and heat dissipation design of power modules.

Complex driving conditions

Unlike the motor drive in industrial applications, the driving conditions of hybrid vehicles are more complicated. For example, corresponding to urban conditions, it is necessary to frequently switch between acceleration, deceleration, and cruising. Therefore, the current and voltage through the IGBT are not constant, but The vehicle operating conditions repeatedly cycle and fluctuate, 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 immediately lose power, seriously affecting the credibility of the vehicle manufacturer and the user experience.

Automobile manufacturers require that IGBT modules do not need to be replaced during the entire life cycle of the HEV, which puts forward higher requirements on the durability of the IGBT (the design life of the entire vehicle is 15 years).

Cost control requirements

Mass-produced cars are different from train traction applications. Under the condition of high performance requirements, the cost cannot be exchanged for reliability, and the cost and performance need to be balanced, which places higher requirements on product design. Therefore, in view of various constraints in automotive applications, dedicated IGBTs are required to meet demanding application requirements.

IGBT structure

Figure 3 shows the structure of a power module with a substrate. Ceramic substrates with thin copper layers on both sides are soldered to the substrate. The IGBT chip is soldered on the designed copper layer. The surface of the chip is bonded to the copper layer by bonding wires. Most standard modules use this manufacturing method. At present, 70% to 80% of power modules are manufactured according to the standard module structure. Ceramics generally use Al2O3, and the substrate uses copper as the material. The heat sink is installed on the IGBT base plate through thermal grease.

Infineon automotive 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 the reliability of IGBTs are: thermal cycling and power cycling, which determines IGBT service life, other parameters such as IGBT mechanical reliability characteristics also require additional attention.

Power cycling

Generally, the inverter design mainly considers the limitation of IGBT Tjmax (maximum junction temperature), but in hybrid vehicle applications, the inverter is less in constant operating conditions, and acceleration, cruising, and deceleration will bring changes in current and voltage. The resulting Î”Tj (rapid change in junction temperature) will affect the life of the IGBT to a greater extent. When the IGBT conduction current fluctuates, the bonding wire will also swing, which has a greater reliability for the connection between the bonding wire and the IGBT chip. Large impact, repeated swings may lead to the end of life of the bonding wire (EOL, End of Life), such as bonding wire and IGBT chip welding off, bonding wire breakage, etc., directly lead to IGBT damage.

In order to simulate the operating conditions of automobiles, Infineon has defined the "second cycling power test" (power cycling second, current heating, external water cooling) to accelerate the aging due to the current shock caused by frequent acceleration, deceleration and cruise of HEV. The test simulates the welding reliability of the bonding wire under electrical shock. Infineon automotive-grade IGBTs need to withstand Î”Tj = 60k, maximum temperature saving 150 â„ƒ, 0.5s <tcycl <5s, 150kc power cycles without damage.

Compared with traditional industrial applications, the working environment of IGBTs in hybrid electric vehicles (HEVs) is harsh, which puts higher requirements on the reliability of IGBTs for long-term use.

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

â— Improvement of binding wire materials;

â— The chip structure is strengthened;

â— Optimization of binding circuit connection loop;

â— Optimized welding process.

Temperature cycling

In the HEV, the inverter is usually located in the front cabin near the engine or near the transmission mechanism. The IGBT module will withstand higher ambient temperature and temperature changes, which will have a greater impact on the welding layer inside the IGBT module.

The IGBT module is composed of multiple layers of different materials (see Figure 3). Each material has a different CTE (coefficient of thermal expansion). The difference in CTE will affect the service life of the power module. When the module is used, the temperature change will be between different layers The mechanical stress causes the welding to fall off. Our goal is to select the material with the smallest difference in thermal expansion coefficient for the welding combination. But on the other hand, even if their coefficients of thermal expansion are well matched, the cost of the material itself may be too high, or it may be difficult to process or the processing cost may be too high in 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 be accepted because of the high cost.

Infineon's improved Al2O3 ceramic substrate technology can also meet the requirements of the number of thermal cycles in hybrid vehicles without significantly increasing the cost.

Generally, IGBT modules accelerate the testing of welding reliability through passive temperature cycling (Thermal Cycling). For automotive IGBTs, Infineon defines a more severe thermal shock test (TST, Thermal Shock Test), which has a larger temperature change range than the TC test. , -40 â„ƒ ï½ž + 125 â„ƒ, 1000 cycles (only 50 times for ordinary industrial module TST).

According to Infineon's calculation method, the life of automotive-grade IGBT modules is 2.5 times that of industrial grade and 1/4 of the traction grade, which can meet the requirements of the entire life of the car without replacing the module, and it balances the cost well.

Strengthening of mechanical structure

In addition to the improvement of the above IGBT internal packaging process, Infineon automotive-grade IGBTs have also enhanced the IGBT housing and terminal blocks, including the enhancement of temperature characteristics and mechanical structure characteristics, in order to cope with the harsh application environment of automobiles, such as the following aspects .

(1) Enhanced temperature characteristics. Compared with normal industrial applications, IGBTs in automobiles need to withstand higher temperature shocks. If the IGBT shell material is not strong enough, it will break and damage under temperature shocks. Infineon automotive IGBTs need to undergo a thermal shock test of -40 â„ƒ ï½ž + It is intact at 125 â„ƒ for 1000 times. Through plastic materials and optimized process parameters, the reliability of the improved IGBT housing is greatly enhanced.

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

Infineon automotive IGBT products

In order to meet automotive-grade applications, Infineon has launched HEV-specific IGBT modules, including 2 products:

â— HybridPACK1â€”400A / 650V IGBT 6 units, aimed at mild hybrid vehicles with motor power of about 20kW ~ 30kW;

â— HybridPACK2â€”800A / 650V IGBT 6 unit, aimed at a full hybrid vehicle with a motor power of about 80kW.

Main product features:

â— 6 unit IGBT simplifies inverter design;

â— The working junction temperature is 150 â„ƒ, the maximum temperature is 175 â„ƒ;

â— IGBT technology;

â— The improved binding process;

â— The improved ceramic substrate increases welding reliability;

â— 6 NTC;

â— The improved binding process;

â— The improved ceramic substrate increases welding reliability;

â— Direct water cooling system to improve the cooling capacity of the module.

in conclusion

As power devices are increasingly used in automobiles, higher requirements are placed on reliability, such as the power cycle and temperature cycle characteristics described in this article. For automotive applications, Infineon â€™s automotive-grade IGBT modules have the characteristics of high reliability, long life, and moderate cost. Only in the case of hybrid vehicle applications, special power semiconductor modules are required to ensure the reliability of core components. The success of hybrid cars.

references:

[1] Thoben M, Mainka K, Bayerer R, et al. From vehicle drive cycle to reliability tesTIng of power modules for hybrid vehicle inverter

[2] Graf I, MÃ¼nzer M N. Semiconductors in hybrid drives applicaTIons-A survey lecture

[3] Jadhav V, Volke A. New design consider a TIon of power semiconductors in hybrid electric vehicle

[4] SchÃ¼tze T. Power and thermal cycling capability

[5] SchÃ¼tze T.Thermal equivalent circuit models

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