Simplifying Automotive Electrification
With the release of this product, Fairchild brings to market a highly reliable, durable, efficient, and automotive qualified MOSFET inverter power stage in a compact and easily integrated package that is suitable for 12V accessories up to 150A or more depending on thermal conditions. Also under development are modules capable of handling even higher current for 12V applications, and a version populated with 80V MOSFETs suitable for 48V applications. These modules employ Fairchild’s PowerTrench® MOSFET technology and shielded gate PowerTrench® MOSFET technologies.
In this article, we discuss the features and benefits of the power module, and describe how it can be used to improve the system design for electrified automotive accessories.
Designers of power electronic systems must overcome many challenges to produce a durable, reliable, and efficient electric motor drive, especially for automotive applications where the environment and application conditions are severe and cost pressures are high. Rack mounted electric power steering systems, for example, may experience ambient temperatures in excess of 100˚C, high shock and vibration loads, exposure to petroleum products and salt water spray, while being required to deliver motor phase currents of 150A or more with a minimum of losses.
Key challenges arise at the interfaces between electrical, mechanical, and thermal domains. These include high current interconnections from the vehicle 12V battery to the inverter, high current interconnections from the inverter to the motor, stable mechanical interconnection of the inverter to its support, and an efficient thermal interconnection from the inverter power devices to the coolant, which in many instances is hot air or hot metal! The Fairchild APM addresses these challenges as described below.
Electrical Performance
The primary function of the inverter is to produce variable voltage, variable frequency ac power to drive an electric motor and, as such, must exhibit excellent electrical performance. The six MOSFETs comprising the inverter are of Fairchild’s 40V PowerTrench family, achieving typical die Rds(on) of 1.15 mW for high current, high efficiency operation. Integrated within the module is an RC snubber circuit from battery to ground, tightly coupled to the MOSFET bridge to improve EMI. A precision 0.5 mW shunt resistor for current sensing is included, providing current feedback for motor control and over-current protection. Also included is a temperature sensing NTC for monitoring the thermal state of the inverter.
The module is typically mounted directly to a motor housing surface, allowing the control PCB to connect only to the signal pins along one side of the module (refer to Figure 1). The power leads being separated to the opposite side of the module allow for the complete separation of control and power interfaces so that no high current traces are needed on the PCB, simplifying its design and manufacture. The thermally conductive direct bonded copper (DBC) construction provides 2500 Vrms electrical isolation between the mounting surface and the electrically active components.
With the power connections located along one side of the module and separated from the control PCB, it is feasible to design very low inductance power connections, giving additional design margin in the DC link filter and BVDSS rating of the MOSFETs. Similarly, with all high current conduction paths contained internal to the module, the resulting very low overall resistance from VBAT+ to GND enhances the efficiency of the system, allowing for high bus voltage utilization, maximizing the voltage available to the motor terminals. This provides the system designer with cost advantages in the design of the motor.
The MOSFET die inside the module has effectively integrated RC filter components that add almost no loss, so it can achieve highly effective EMI suppression, reduced voltage transients, and smoother switching transitions.
Mechanical Performance
In an inverter developed with discretely packaged components, such as TO-263 or MO-299 packages, there are more mechanical interfaces that must be handled by the inverter system designer. These include the MOSFET package to PCB, the PCB to isolated heat spreader, heat spreader to heat sink, and possibly heatsink to next level assembly. These mechanical interfaces are closely coupled to the thermal performance of the system, and most of them are incorporated into the FTCO3V455A1 construction, leaving only the module-to-heatsink interface to be handled by the inverter system designer. By following Fairchild recommendations for surface flatness and mounting screw torque (or clip forces), superior thermal and vibration performance can be achieved over the lifetime of the inverter.
Thermal Performance
The simplified mechanical interface design enabled by the use of the FTCO3V455A1 power module, when contrasted with six or more discrete MOSFET packaged parts mounted on a PCB or IMS, allows for very good thermal performance. Figure 2 shows the transient thermal impedance from junction to case and typical junction to heatsink, for all six MOSFETs. The junction to heatsink impedance assumes a 30 micron bondline thickness of a 2.1 W/(m-K) thermal interface material.
Fig. 2: Thermal impedance. For full resolution click here.
With this simple stack-up from heatsink to silicon, the excellent thermal performance allows for very high power density packaging of the full inverter. The volume of the APM is a trim 29 mm x 44 mm x 5 mm, and can result in a very compact inverter assembly on the order of 400 mL total volume, including relays, DC link filter components, control PCB, heatsink, and connectors.
About the author:
Dr. Roy Davis is the manager of Advanced Systems Design and Development in the Automotive Product Line of Fairchild Semiconductor Corporation, located in Ann Arbor, Michigan, USA. He joined Fairchild in 2006 after holding senior technical and management positions at AVL Powertrain, Ballard Power Systems Corporation, and Ford Motor Company. He has been involved in analysis, simulation, and design of power electronic and electric drive systems for automotive applications for more that 20 years.
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