Power modules for electric and hybrid vehicles

1 Introduction

The urgent call to reduce CO2 emissions has led major automakers to develop new solutions for electric and hybrid drive vehicles. In order to develop power semiconductor modules for these applications, new module integration and packaging solutions are needed. However, the conflicting requirements of maximum power density, efficiency and reliability at low cost can only be achieved by selecting the right components, developing innovative solutions and technologies, and optimizing thermal and electrical characteristics.

The SKiM® power module series (Semikron integrated module) is the latest generation of ultra-small modules with substrate-free press-contacts from Semikron. The ceramic substrate DCB used for insulation is not soldered to the copper substrate, but connected to the heat sink by pressure, ensuring excellent thermal cycling capabilities and low thermal resistance. The pressure point is next to each chip, ensuring that the DCB is evenly connected. No substrate Figure 1 shows the cross section of the module housing, the pressure contact system and the spring contacts for connecting the gate.

Figure 1 SKiM® modules meet the needs of inverters for electric and hybrid vehicles

The circuit is a 3-phase inverter circuit. Each half-bridge has its own DC connection and temperature sensor. The gate connection of the IGBT uses spring contacts. The gate driver printed circuit board is not soldered to the module, but screwed to the module. The spring contacts ensure a reliable connection even under strong thermal cycles and vibrations.

The module is designed for inverters with output power between 30kW and 150kW, depending on operating and cooling conditions. Table 1 lists the module parameters and typical power inverter output current.

Table 1: Main module parameters

[page]2 Busbar design

A good and reliable module solution depends on the internal load connection design (Figure 2). The load connections perform various tasks within the module and are optimized according to the requirements of different tasks:

(1) Solder-free, low-inductance connection between the main terminals and the chip;
(2) High current carrying capacity and low loss characteristics suitable for large inverter currents;
(3) Symmetrical current paths that provide good current distribution between parallel chips;
(4) Pressure points close to the chip, resulting in low thermal resistance.

Figure 2 Main terminal with sandwich design and a large number of contact pins - FEM simulation of terminal temperature at an output current of 600Arms

The sandwich structure with parallel current paths to each chip ensures extremely low internal inductance. The inductance LCE generated by the screws between the DC connection and the AC connection is less than 10nH, and the total inductance between the positive and negative terminals is less than 20nH.

Finite element analysis showed that most of the inductance is caused by the end parts of the +/-DC connection. With the help of FEM simulation, the design can be optimized and the inductance can be reduced by 30% (-10nH). Further improvement is not possible because a sandwich structure cannot be used here to provide the mandatory clearance and creepage distances. The only way to achieve a further reduction in inductance is to use several parallel connections to the DC link circuit.

Among other advantages for the user, the design features low overvoltages in the internal switches, which allows operation at relatively high DC link voltages and safe shutdown, even in the event of a short circuit. Smooth switching processes without oscillations ensure low switching losses and low disturbances in the release. Improved

semiconductors allow ever-higher power densities in small dimensions. The rated chip current of the 600V SKiM®93 is 900A, nearly twice that of standard modules. This current value also exceeds the upper limit of the permissible current of the main terminals of existing IGBT modules. The total resistance r _cc'-ee' (including contact resistance) of the wide and thick copper foil used in the SKiM® module is only 300 μΩ, which is only half the resistance of standard modules. The high contact force ensures low contact resistance. Nevertheless, the losses that occur are quickly dissipated via many short contacts to the cooled DCB surface and heat sink.

In the inverter, the highest current flows through the AC terminals. For this reason, the AC terminals are located at the lowest point of the sandwich structure, as this point offers the best cooling performance. The module is designed for an effective current of 600A at the AC output when the heat sink temperature is 70°C. This value is much higher than the expected continuous current (see Table 1). Even when the losses in the semiconductors are about 2000W, the temperature of the terminals can be kept below 125°C (see Figure 2).

3 DCB Layout

The design of the DCB and the location of the chips have a significant influence on the switching behavior and thermal resistance of the power semiconductors. Asymmetric component designs can easily lead to non-uniform current distribution of 10% or more. The total output current is limited by the component that produces the greatest power dissipation.

The voltage drop across the parasitic inductances leads to different switching speeds and oscillations between the paralleled chips. To ensure smooth and synchronous switching, the inductance must be as small as possible and, more importantly, the effect on all semiconductor chips must be the same. This is ensured by using an IGBT on each side and a freewheeling diode in the center. The current commutation path between the IGBT and the diode is as short as possible and has the same length for the top and bottom switches of the converter half-bridge (see Figure 3).

Figure 3 Current commutation paths between the IGBTs and freewheeling diodes of the top and bottom switches

Figure 4 shows the switching characteristics of a SKiM®63 module at 600A and 900V DC. The switching losses, overvoltage and di/dt are almost identical for the top and bottom IGBTs. This is not always the case, in fact, in most cases there are significant differences, which are caused by the different parasitic inductances in the current path.

Figure 4 Turn-off Ic (green), VCE (blue), VGE (brown) of the bottom IGBT at twice the rated current, at 600A 900V DC, 125°C

Likewise, good current sharing between paralleled chips is required to ensure that the capabilities of the components are fully utilized. The impedance of the +DC to –DC current path and the effect of the main current on the gate circuit must be the same for all chips.

The first condition is met by using a sandwich busbar system. The current communication magnetic field changes little from +DC to –DC. The individual inductances of the main terminals are coupled and therefore negligible. The impedance is the same for all paralleled chips.

The second requirement is also taken into account in the selected design. All IGBTs have the same gate-emitter voltage even under dynamic conditions. In an IGBT-diode-IGBT module, the voltage drops caused by di/dt cancel each other out, i.e. all transistors are affected in the same way by the voltage drop on the bond wire. The result is a good current distribution, even in the event of a short circuit.

[page]4 Thermal resistance R _th

The low conduction state voltage and the maximum junction temperature of 175°C allow very high rated currents. The rated current density can be greater than 2A/mm2. If the right chip size is selected, the best balance between rated current, cooling requirements and cost can be achieved.

Rth is a function of both chip size and the distance between chips. Oversized chips have large temperature gradients over the entire chip area and poor heat spreading within the module. Some chips with the same total area but a small distance between them have a lower _Rth . If the gap between the chips is small, the chips heat each other; likewise, the larger the chip spacing, the lower the thermal resistance. The SKiM® series offers the best compromise between maximum effective chip area and optimal thermal performance: chip areas between 60mm² and 8080mm² with a distance of 3mm between chips.

Crimp contacts on both sides of the chip prevent the DCB from bending. Helps reduce the thickness of the poorly conductive thermal paste to 20μm-30μm; modules with baseplates typically have a 80μm-100μm thick thermal paste. The ultra-thin sintered silver layer has good thermal conductivity and further reduces R _th compared to conventional solder layers .

5 Reliability

Conventional solutions using power modules with copper baseplates are not suitable for the extreme thermal cycling situations in automotive applications. Different thermal expansion coefficients cause strains in the connections between the materials. AlSiC baseplates (aluminum silicon carbide alloy) are a reliable alternative, but are relatively expensive. Press-fit modules without baseplates are another option. Unlike classic module designs, the low thermal resistance in these modules and the uniform heat spreading on the heat sink result in low temperature differences, even under active load cycling situations, which increases the service life of the module.

To improve the load cycle capability, even for very high junction temperatures, the SKiM® series uses low-temperature sintering technology to connect the chip and the DCB. Solder connections age due to load cycling, which increases the thermal resistance and ultimately leads to failure. The sintered connection is achieved using an ultra-thin layer of silver with excellent thermal conductivity. The melting point of silver is 900°C, which is significantly higher than the maximum junction temperature of the chip of 175°C. In lifetime performance tests, no fatigue of the joints was found (see Figure 5). Eliminating this potential failure mechanism increases the reliability of the entire system.

Figure 5 Comparison of soldered and sintered chips in thermal cycle test

Thanks to the use of press-fit and spring contact technology for connection and the elimination of baseplates with soldered chip connections, SKiM® modules are 100% solderless power modules. In addition, the modules have been optimized for best chip utilization and high output currents. Combined with a chip junction temperature of up to 175°C, this allows the design of compact inverters with unrivalled power density and thermal cycling capability.