Digital Power Modules: The Convergence of Power and Body Electronics
Jul 1, 2005 12:00 PM
by Daniel D. Moore
Many applications are taking advantage of digital power modules to combine power and body electronics into a single module. The flexibility and scalability afforded by the DPMs also present worse-case scenarios for designers. Thus, designers must consider trade offs between cost and function /performance.
For the PDF version of this article, click here.
Historically, the vehicle architecture has separated body control functions, such as interior and exterior lighting, from power distribution. The body control modules and junction boxes (JB) have been customized for each vehicle's electrical content with little reuse between vehicle platforms. For example, there may be three JBs: one under the hood, one in the dash panel and one in the trunk while sprinkling body control modules throughout the vehicle. The next platform relocates both the JBs and body controllers while changing the functional content of each. This method of design does not take advantage of the reusability or the integration of body control functions and power distribution created by the digital power module (DPM).
OEMs are constantly adding higher levels of convenience, safety, telematics, diagnostics, and entertainment features to their vehicles. At the same time, microcontrollers and solid-state switching technologies are replacing many traditionally hydraulic, electrical-mechanical and mechanical systems. In this environment, the demand for the enabling technologies of power and data networks (PDN) in the vehicle has increased dramatically, while the pressures for reducing costs and limiting packaging space continue. The traditional wire harness provides the foundation for in-vehicle power and data distribution (high current circuits, low current analog inputs, digital inputs, outputs and communications). By adopting a more advanced DPM the OEMs can provide an enhanced driving experience. What must be kept in mind when designing a system is the balance of packaging, weight, reliability and cost.
Yazaki power and data solutions took shape with the first patents on electrical cables (1952), followed by instrumentation (1955), electrical connectors (1958), wire harnesses and power distribution boxes (1967). Yazaki North America introduced the first DPM (1993) with the union of the JB and chimes. Prior to this, the JB contained only power connectivity, fuses and relays in a serviceable module. The evolution of DPM has continued by combining JB functionality with body control functions and vehicle communications gateways via the inclusion of microcontrollers and solid-state drivers. This is necessary to support additional OEM-defined feature content in the automotive PDN.
The JB consists of fuses, relays and interconnects using stamped metal bus bars, insulation displacement connection (IDC), wire routing or high-current flex circuitry. Each interconnection system in the JB has its design limits. The IDC design allows for 10 A maximum per circuit and is flexible, allowing for quick design changes in circuit connections. Flex circuit overlays have similar current-carrying capabilities, but less flexible connectivity. The fret bus bar attachment allows for a low cost and up to 200 A current capabilities but is even less flexible for circuit routing design changes and is encumbered by long tooling lead times and cost. High-current, multilayered printed circuit boards (PCBs) with 6-ounce copper are flexible toward design changes and have allowed for the integration of electronics and high power. With increasing vehicle complexity, such as added electronic subsystem features, the JB alone does not provide the best systems solution due to size and the need for added performance within vehicles today and of the future. By combining multiple technologies such as an electronically controlled PCB, bus bar and IDC, trade offs can be made between flexibility, cost, current-carrying capacity and electronic functionality.
Yazaki has designed multiple versions of the JB/DPMs. One version has a separately serviceable, digital control board, and one contains the digital control board in the housing. The serviceable digital control board or front control module (FCM) is separated from the power section through a water-resistant, direct-connection system. This unit is sealed in a metal housing with a conformal-coated PCB. It incorporates a Gortex breather vent to avoid creating a differential pressure that can occur in the harsh under-hood environment. Internally, the FCM contains a 4-layer PCB and is populated with a 256k flash microprocessor, communication transceivers, support circuitry for digital inputs, analog inputs and low power solid-state outputs (Figure 1).
The integrated digital control board version of the DPM eliminated the potential for water intrusion between the FCM and the power board by combining them both in the housing. The water-resistant connector has been replaced with a soldered header connected directly to the power board (See Figure 2).
The power board consists of four layers with 6-ounce copper on the two outside layers. It has been populated with solid-state smart field-effect transistors (FETs) and zener diode surge suppression. Stitched, forked connector pins allow for relays, fuse links, mini fuses and fret bus bar connections to the PCB. A plastic insulation layer electrically isolates the bus bars from the power PCB and holds them in position within the housing enclosure. The bus bar layer provides high current connections to the power board. It also provides pass-through connections between connectors as well as connections to relays. This is all contained in an under-hood splash-resistant housing and cover (Figures 1 and 2).
Semiconductor and electronic capabilities have expanded dramatically to handle the automotive environment, which has allowed for the development of the DPM. Many more microcontrollers are available today to handle the 125 °C under-hood applications than ever before. Microcontrollers continue to increase in throughput, memory availability and I/O capabilities while dropping in price. Relays are becoming smaller and are being replaced by short-circuit-protected temperature FETs and Smart FETs. FETs have decreased in RDS(ON); therefore, they contribute less power dissipation and heating for continuous direct current ranges of less than 20 A while still being cost-effective. However, in the near term at least, DPMs will continue to be a marriage of solid-state and electromechanical components to obtain the optimal performance, size and cost point.
CURRENT CENTRALIZED DPM FEATURES
Functions that normally reside with body control modules have been integrated into the DPM. These functions include interior and exterior lighting, wipers, horn, washer fluid, A/C, clutch, fuel pump, starter relay, blower, centralized reverse polarity protection, load dump protection, load shedding, system diagnostics, and smart fusing. Any body control function can be integrated into the DPM. Examples include: alarms, security, window motors and heaters, locks, memory mirrors and seats, temperature sensing, keyless entry, heated and cooled seats, sunroof control and chimes.
The DPM is connected to the vehicle's communications network, and, therefore, the wiring for body-control input commands is minimized due to the benefit of multiplexing. The DPM provides gateway functionality between the vehicle's network protocols. Examples of a gateway would be a translation from Class II to GMLAN, or CANC to CANB. Software has allowed for fault-tolerant functionality using functional substitution and improved diagnostics. For example, if the stop lamp filament is open, the park lamp can be used as a substitute.
DISTRIBUTED DPM ARCHITECTURE
To optimize the wire harness design and the vehicle architecture, a distributed DPM requires the definition of the electrical hard-points and the specification of harness partitions and junctions. The main harness consists of power, ground and communications buses. Small sub-harnesses extend from the geographically located module to its respective loads. Smaller size, more electrical content, integration of body control and power distribution functions will make the distributed DPM more common. It will also enable bill of material reuse, common design and common manufacturing processes for reduced cost and improved reliability. A large portion of the designs in current vehicles can be converted from centralized JB/bused electrical centers (BECs) and body controllers to distributed DPMs (Figure 3).
ADVANTAGES OF DISTRIBUTED ARCHITECTURE
Wiring — We project a potential 15% wiring harness cost reduction while reducing the weight by an average of 10% to 20%.
Packaging — Small size modules will allow for flexible vehicle packaging.
Service — Elimination of many fuses and added diagnostics via the communications bus will minimize the service time required.
Customer — The elimination of fuse blow and replacement resulting in fewer service calls. As with the centralized DPM the capability of functional substitution and fault alert still exists.
The DPM's solid-state power switching with Smart FETs maintains a fault-tolerant functionality for specific subsystems while enabling circuit diagnosis through the use of a software interface. Flash memory allows for an increase in flexibility of functions with only software modifications. Flash and communications protocols also provide field reprogram capabilities for software updates. The DPM software is so versatile, modular and reusable, it provides commonization of feature content across various vehicle platforms within a particular OEM. This minimizes additional tooling cost and re-validation efforts. Maximum reuse and flexibility of DPMs can be accomplished with software and hardware population changes.
The DPM is designed with building blocks that can support either a centralized or a distributed architecture. The incorporation of body electronics allows for the integration of several modules, thereby reducing the vehicle packaging volume requirement. Yazaki has the capability to package in varying environments from the harsh under-hood to the more benign passenger compartment. Multiple, small distributed DPMs in select geographical locations allow for the body-control functions to reside near the application of the loads and inputs. This allows for a reduction in the total number of discrete modules and a reduced wire bundle size in the vehicle.
Choosing a packaging environment for the DPM always has trade offs between the packaging and the design. Under-hood placement requires a design that must be more robust for environmental requirements, and it must reduce the amount of power that is dissipated while maintaining safe operating temperature ranges for the semiconductors, packaging and terminals. Under-hood applications also make it necessary to design a package that can deal with water intrusion, chemicals and possibly immersion. This leads to a more complex and difficult design for thermal management and requires a selection of more expensive components. A distributed architecture benefits the vehicle by taking advantage of placing components where they are best suited for the functional location and environmental conditions while reducing the complexity of the wiring harness ( Table 1 ).
DESIGN AND ENGINEERING DEVELOPMENTAL TEST
Since we have the PDN knowledge to aid in the development of the vehicle architecture, we will map functional requirements into a specification-tracking matrix that will be used to cross-check each function and sub-function in the design and to ensure test coverage in software and hardware validation. Once the design inputs and outputs are established, engineering can begin with running simulations, developing circuit designs, custom software development, vibration, and thermal modeling before actual prototypes are built. This helps ensure a first-time pass in testing.
Based on vehicle CAD model data (UG-V4, CATIA-V5, IDEAS, CADDS-5, Solid Works, any of the AutoCAD products, and the neutral files such as STEP, and IGES) a mechanical sample for packaging studies can be developed within one to two weeks using stereo lithography assembly (SLA). Initial engineering level tests are used on small sample sizes to verify correct functionality of software, hardware, EMC and to update the models if required before design verification (DV) or process verification (PV) testing is started (Figure 4).
Functions embedded in DPMs have grown; therefore, a significant strategic and tactical challenge must be accepted to develop and validate software. To meet the technical challenge, the company has embraced multiple developmental technologies and infrastructures. We select from these based on product and customer requirements. Object-oriented modeling, state-based modeling, manual and auto code generation are all included in the software toolbox of skills to surmount complex technical software challenges.
For software and hardware validation, the company has developed a comprehensive functional auto-tester. The test systems can run through a suite of test functions including: I/O fault condition handling, diagnostics, communications handling and gateways. This provides user-friendly indication of pass/fail reports. If required, a vision system has also been incorporated to confirm selected functions.
DV AND PV TESTING
The Yazaki Test Center is located across from the North American headquarters in Canton, Mich., where the development team resides. Testing is done at the state-of-the-art, A2LA accredited environmental, mechanical and EMC testing facility. The close proximity of design and test engineers ensures the quick and thorough communication of testing requirements and issue resolution. Highly accelerated life testing (HALT) is done early during the development phases to quickly wring out vibration, humidity or thermal weaknesses. Design and test engineers review the customer requirements and develop test system specifications to provide the best coverage and to benefit from our custom-developed test systems.
Test plans can be optimized through the combined efforts of the customer validation engineers, release engineers, Yazaki's test engineers and design engineers. Examples for investigation could be: quantity of samples, monitoring type, realistic vs. worst-case loading and environmental conditions.
The opportunity to downsize wire by the use of smart fuses (smart and short circuit protected FETs) remains unexplored by the OEMs. Studies show consistent downsizing of one to two wire gauge sizes on circuits using smart fuses. The wire gauge can now be sized with the expected load, instead of the fuse current rating, which avoids inadvertent fuse blow. Conventionally, a fuse is 50% to 70% derated to avoid a nuisance blow. The wire is also sized to the fuse to ensure the fuse opens before the wire is damaged. A smart FET and wire can be optimized because the smart FET does not have the same temperature and derating issues as a fuse.
FUTURE TRENDS AND OPPORTUNITIES
Yazaki has a dedicated team of R&D engineers that are constantly benchmarking and developing new and innovative technologies to respond to ever-changing market needs. Investigations currently under way include: integration of digital and power board into one PCB; further integration of additional body functions; load management to help reduce charging and battery requirements while reducing fuel consumption; DPM packaging size reductions to allow easier placement in the vehicle; and reusable software, hardware and distributed DPMs with a master-slave communications (See Figure 5).
Yazaki is poised with our system expertise to assist OEMs in defining the topology and architecture. Using the collective vehicle design requirements, we will be able to optimize combinations of electronic body controls, power distribution, and gateways for either a centralized or totally distributed DPM.
ABOUT THE AUTHOR
Daniel D. Moore is product manager at Yazaki.
Back to Top