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Design strategies for car door modules

Posted: 02 Apr 2007 ?? ?Print Version ?Bookmark and Share

Keywords:automotive door module? electric door system design? ECU module for door module?

Many electromechanically-performed car functionssuch as the door applicationare now transferred into the electronic module. Power windows, electric mirrors and door locks controlled and driven by ECUs have almost become standard in almost all new cars.

In a broad sense, there are two different architectures for an electric door system: centralized and decentralized. A centralized system normally has one center module that controls and drives every load in all doors. Meanwhile, the decentralized system has multiple ECUs for different door functions.

Some vehicle models today still have a centralized architecture due to its lower material cost. However, this architecture is losing its popularity because of the following reasons: central module approach requires large amount of direct wiring, thus increasing the weight, cost in wiring and also the risk of cable short-circuit; a single module has limited capacity to embed all the needed features, such as comprehensive protection, diagnostic and communication interfaces; and a large-size module is cumbersome in assembly. These disadvantages have driven the electric door system architecture to the decentralized direction.

CAN and LIN are the two standard automotive protocols linking the decentralized ECU modules. Through the CAN/LIN interface, the major door functions can be realized in different ways. The most popular approach is the door zone module or door module. In this approach, the major function blocks of every doorsuch as door lock, mirror, window-lift and auxiliary lightingsare controlled and driven by an individual ECU. This ECU is normally located inside the door frame and can be built together with panel or window-lift. Such configuration is implemented in most mainstream models made by major European and U.S. OEMs such as Volkswagen, Audi, Daimler-Chrysler and GM.

The nature and characteristics of loads in a door module differ widely from one another. System designers are challenged to efficiently drive all the loads to meet both design specification and cost target.

In general, a door module consists of three major function blocks: MCU, system basis function block (including power supply, CAN/LIN communication and the circuit doing external event monitoring and signal sensing), and drivers and power stages interfacing with external loads.

A door module consists of three major function blocks: MCU, system basis function block, and drivers and power stages interfacing with external loads.

The types of load of a door module include DC electric motor, lamp, LED and heating coil. The diversity of the nature and characteristics of these loads requires different strategies, and should be developed to efficiently drive them.

Relay or smart devices?
For large loads like the window-lift motor, there are normally two different drive approaches. The first is a relay-based one, and the second is to use IC devices.

So far, the relay-based strategy is still being widely used due to its lower hardware cost, technical maturity and capability to handle the high current. However, some of its advantages are also well-recognized:

  • Relay solution needs more external components such as fuse, pre-driver (normally a Darlington pair) and freewheeling diode to form a basic circuit.

  • Relay does not offer protection for itself and for the application.

  • Relay does not have any diagnostic feature by itself. It needs external devices such as shunt resistor and op amp to do current sensing.

  • Pulse-width modulation (PWM) control is not possible in relay solution.

  • Relay is normally bulky and needs a fuse box.

  • Being a moving part, relay has less mechanical reliability.

  • Electrical lifetime is also limited when many on/off cycles are required for turn indicator.

  • The fuse-and-relay solution generates more heat due to higher losses in the relay coil and contact, in the fuse, and in the eventual current shunt resistor.

The advent of smart switches and bridges opens a door to solve these problems.

ASSP vs. standard chip
There are two approaches to drive the smaller loads such as mirror motors, lamps, LEDs, mirror heating coil or even door lock motor. The first approach is to use dedicated power ASSP, and the other one is to use discrete standard devices.

The convergence of requirements and strong incentive to minimize the system cost makes the platform concept more popular in the application areas such as door module. To satisfy such needs, a highly integrated power ASSP is suitable. It not only saves PCB area and pick-and-place costs, but also enhances the system quality and yield.

Compared with the dedicated power ASSP for door module, the multiple-standard chip solution is more flexible in system configuration, especially when some of the loads (e.g. mirror fold) are not required. Designers have more freedom to choose separate drivers to exactly match the characteristic of each load. These drivers can either be relay-based ones or smart devices.

Mirror heater coils and lamps in the door module can be switched by smart high-side switches. Infineon's Profet family can be used for such applications. A Profet device includes a vertically structured n-channel power transistor, a charge pump for high-side operation, and a logic circuit performing various protection and diagnostic functions. PWM capability meanwhile allows features such as "soft" start/stop of mirror heater or special effect for lighting control.

The different loads of a typical door module ECU ranging from some milliamps for LED to about 30A for a window lift motor require different drive strategies. Large loads like window-lift motor can be driven by a relay and fuse solution. An ASSP is a suitable device to realize a very compact PCB. For a more basic door module with only a few loads, discrete smart devices are the best trade-off between cost optimum and flexibility to cope with the different requirements.

- Chen Qi and Jean-Philippe Boeschlin
System Application Engineers
Infineon Technologies AG

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