In-Vehicle Electric Control Systems Using Intelligent Power Modules
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.
Comparison of 3-phase BLDC motor solution for in-vehicle system
In order to reduce the cost of in-vehicle 3-phase BLDC motor systems, a type without position detecting sensors (referred to as sensor-less type) can be used. In general a power MOSFET and a pre-driver IC with microcontroller is installed and the BDLC motor is controlled by software.
The printed circuit board normally has an average of 170 parts mounted upon it. By replacing this with a highly-integrated IPM, a specialist controller using the power MOSFET, pre-drivers and hard logic, together with a self-protection function (for safeguarding against short circuits, excess current, controlled power supply voltage drop and excess heat) can fit into one module, and thereby becomes a stand-alone unit.
The associated noise can be reduced. Also as software for driving the motor is not needed, the design period can be shortened. The system’s reliability also increases significantly.
Figure 3. BLDC Motor Driver Architectures
Normally, 3-phase BLDC motors are more expensive, compared with motors that have brushes. They also take up greater space and weigh more. High density IPMs that employ IMST allow space saving and the system cost can subsequently be reduced. For instance, ON Semiconductor’s standalone sensor-less type motor control IPM is described in the schematic shown below (see Figure 4). The sensor-less position detection logic and the motor controller IC (sequencer), 6 power MOSFETs, and peripheral parts are included. It responds to direct PWM controllers with synchronous rectification.
Figure 4. Standalone Sensor-Less Type Motor Control IPM Block Diagram
In this particular IPM example, the number of components has been reduced from the 170 normally needed to implement a discrete configuration to just 5. This is the result of being able to integrate various components through use of IMST.As a result, mounting weight can be reduced by 60% and board area curbed by 85%. When this IPM is used, a printed substrate is no longer required (enabling PCB-less implementation) and the IPM can fit within the motor’s footprint.
Figure 5. Discrete Solution with PCB vs. PCB-Less Solution (IPM)
Other benefits of IMST-based IPMs
The first additional benefit of IMST-based IPMs to be aware of is the high precision temperature detection that they offer. For discrete components, since the power transistor package and temperature detection circuit package are far apart, a delay in temperature detection time results. With an IMST-based IPM, as the power transistor, control circuit for temperature detection and protection circuit can be mounted on the same board. There is hence minimal error in temperature detection, making it possible to achieve high precision circuit operation.
Figure 6. Temperature Detection
The second benefit of IMST-based IPM’s is resilience to noise. Since lowering motor noise is an essential aspect of in-vehicle systems, design engineers need to reduce noise wherever possible. EMI noise poses a major problem at the design stage.
With an IMST-based IPM, there is a distributed capacitance through an insulating resin between the aluminum sheet portions and copper foil portions of the metal substrate. This works to effectively reduce switching noise of the power device. Comparative data with a discrete configuration is detailed in Figure 7.
Figure 7. Switching Noise Comparison
Further, as the parasitic inductance of the wiring layout and wire bonding on an IMST substrate can be reduced, noise can be lowered by inhibiting the high voltage surges that occur during switch state transition when a high-voltage, high-current device is hard-switched.
Figure 8. High Voltage Surges
The third benefit lies in superior heat dissipation of metal-based design.
This property is dependent on the insulation materials and thickness of the insulating layer. A thinner insulating layer can reduce thermal resistance. Figure 9 depicts the relationship between insulating layer thickness and thermal resistance values on multiple insulated metal substrates.
Figure 9. Thermal Resistance Figures
Future Developments
The number of motors used in today’s cars is between 50 and 60 for compact economy models and between 100 and 120 for luxury models, and this number 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.
To serve a diverse range of applications, in addition to pre-driver-only modules, the standalone system-on-IPMs introduced here and general-purpose platform-type solutions that set aside the development of motor control algorithms will also be needed.
Moreover with the advent of high-power EPS rapidly approaching, the development of technologies to further increase density of power modules equipped with current power devices and shunt resistances is underway. For IPM products a triumvirate of technological developments is key; package technologies including substrates and insulation materials, mounting technologies such as wire bonding and design technologies for power semiconductors and control circuits.
About the author
Matt Tyler is deputy general manager of Global Marketing, SSG, ON Semiconductor.
This article has been published originally on EE Times Automotive DesignLine
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