The number of motors used in cars is expected to surpass 200 per car in 2015. As reduced size, weight, and cost will therefore become increasingly important, further integration will be critical.
Increasingly stringent fuel-efficiency regulations and growing concerns about environmental impact have meant that the mechanical systems within vehicles are being supplanted by electrical alternatives.
These enable far higher degrees of efficiency and operational performance to be delivered.
Use of DC motors, for example, is becoming more and more common in the design of in-vehicle control systems (other than the main motor) so that the vehicle's overall weight can be reduced and consequently improvements in fuel economy can be derived. In the case of power steering systems, changing from a hydraulic-actuated system to an electrical one can improve fuel efficiency levels by 3% to 5%.
For in-vehicle systems, brush-less DC (BLDC) motors are seeing wider proliferation as they offer much better reliability than conventional DC motors with brushes and commutators. BLDC motors are incorporated not only into the vehicle's electric power steering system (EPS) but also into its water pumps, oil pumps, fuel pumps, radiator fans, HVAC, seat fans, etc.
Requirements & considerations for in-vehicle motor control
Among the key requirements for in-vehicle motor control circuits are high-temperature operation.
The capability to operate at a Ta of 150 °C and a Tj of 170 °C is essential. High reliability of the entire control circuit in difficult environments is also required.
The more frequently discrete components need to be employed, the larger printed circuit boards will have to become - with increasing number of soldering works, which simultaneously lowers the reliability and increases weight, as well as posing difficulties when taking measures against heat and overall cost. Combatting noise, such as electro-magnetic interference (EMI) -- is also important.
As already outlined, BLDC motors are superior to motors with brushes in terms of their efficiency, maintainability, service life and safety, however they do have the disadvantages of being heavier and higher cost (because BLDC motors require larger mounting space due to inclusion of external control circuits).
As a result, there have been challenges when it comes to fully benefitting from the advantages of BLDC motors while simultaneously reducing the overall system size and weight, decreasing its cost, strengthening its reliability and taking measures against heat and noise.
Intelligent power modules for In-vehicle installation
The integration of advanced power semiconductors and peripheral components is a highly effective way of reducing size and weight to solve the issues faced when specifying BLDC motors for in-vehicle applications.
Different technologies and differently-shaped semiconductor components are mounted onto an insulated substrate and connected electrically.
This integration comes in many forms; from simple power modules consisting of multiple power devices (such as power MOSFETs and IGBTs), to intelligent power modules (IPMs) that incorporate power devices, their drive circuits (pre-drivers), protection circuits and so on.
There are typically two types of materials used for power module substrates.
These are ceramic-based materials suited to high power and metal-based materials (such as aluminum). As ceramic substrates are connected with metal sheets (like copper and aluminum), the machining of large substrates is difficult and there is a limit to integration that can be achieved. The metal-based Insulated Metal Substrate Technology (IMST) presents itself as a means of solving this issue.
IMST is an effective way to reduce size and weight that brings together semiconductor components as well as passive components and other parts of differing structures and integrates them into a single module. By covering aluminum sheets with an insulating layer, placing copper foil on top and etching the copper foil, the IMST structure allows single-layer wiring patterns to be freely customized.
Aluminum wire is used to make electrical connections (which are ultrasonically bonded) between transistors or IC/LSI bare chip devices and the copper foil pattern. Thicker copper foil patterns are designed for high current applications, with thinner patterns for low currents. The diameter of the aluminum wires is based on the rated current flowing through them and the purpose for which these wires are used (as jumper wires or grounding wires, for instance).
The insulating layer is made thinner when low thermal resistance is a priority and thicker when high voltages are important. As the base substrate is aluminum, thermal conductivity and electrical conductivity are intrinsically high. This allows the handling of large amounts of power and also offers electro-magnetic shielding effects.
Figure 1. IMST Cross Section
Normal IPMs mount their constituent components on separate circuit boards and as the structure is a frame with interconnected dots and lines, multiple functions cannot be implemented within that frame. With an IMST structure, however, the circuit is built on one circuit board and multiple functions can easily be implemented.
Figure 2. Structural Comparison between an IMST and a Typical IPM
For power modules or IPMs a number of power semiconductor manufacturers make products corresponding to the target application, and the number of MOSFETs/mounted discrete components or the base circuit board are not uniformly fixed.