Options for Electric Power Steering Modules a Reliability Challenge

Electric and Electro-Hydraulic Power Steering Systems (EPS / EHPS) are gaining more and more momentum. They have substantial advantages over conventional hydraulic steering systems. In particular, EPS can improve comfort, safety and energy-efficiency of any car. The market share of these systems increases every year. The typical core component of an EPS system is a three phase brushless electric motor with power rating of up to one kilowatt. The drive system is highly dynamic and can draw more than 100 Amperes from a 12 Volt automotive power grid. EPS systems often are subjected to a 120°C ambient under-the-hood environment. Specific mission profiles (e.g. “driving school circuit”) and are a special strain for the thermal design of an EPS control unit. These factors describe a very demanding current and temperature profile for the PWM inverter power stage. Traditional discrete assemblies often exceed physical limitations and do not offer the reliability needed. In thermally dynamic applications, such as EPS, discrete semiconductors often suffer from limited silicon die size, direct Cu-bonding, CTE-mismatch and insufficient cooling. The bottleneck of discretes is not the silicon allowing junction temperatures of 175°C, but the bonding and joining technologies to connect and cool the MOSFET dies. Special semiconductor power modules, e.g. comprising of six transistors, currentand temperaturesense, can overcome these limitations. They provide the reliability needed to survive the whole automotive lifecycle of up to 20 years. A range of special module packages is suitable for EPS applications and will be described in this article. Three proven architectures are part of the solution matrix: Bare-die assemblies on ceramic substrates Cu-baseplate modules with bondable frame Transfer-molded modules The article analyzes the advantages and disadvantages of these designs. Different parameters, e.g. cost, thermal cycle-capability, robustness for assembly and integration into an ECU are discussed.