The intelligent control of interior and exterior lighting is gaining importance in automotive electronics; greater functionalities are incorporated in increasingly compact body control modules. Naturally, this evolution initiates corresponding technical challenges. An intelligent multiplexer would provide a solution to the ever increasing requirements for body control modules (BCMs) in terms of pulse width modulation (PWM) channels, diagnostics, and system reliability in automotive lighting applications.
Body control modules are electronic units that perform a wide array of tasks, including control, diagnostic, failure management of interior and exterior lighting of a vehicle, and power distribution for submodules. These devices also handle door lock, wipers, security, and access systems. Moreover, they fulfill gateway functions and serve as an information bridge between different communication networks within a car.
The modern BCM is undergoing an evolution. Letís have a look at some of the trends related to control and diagnostics of lighting functions.
PWM control of high side drivers
Electromechanical relays, which were used as actuators, are tending to be replaced by solid state switches, also called high side drivers (HSDs), in order to save cost and weight, and for enhanced diagnostics. The requirements for these HSDs are steadily increasing. Indeed, they must be controlled with pulse width modulation in order to compensate the variations of the battery voltage and maintain a constant brightness of LEDs and incandescent bulbs. This control also avoids the rapid degradation of incandescent bulbís filaments or the overheating of LEDs at high battery voltages.
Detailed and reliable diagnostics of the HSDs
The HSDs provide either a digital status or a current sense signal, which is proportional to the load current. Based on this information, the system must distinguish and report in a reliable way the different fault conditions: Overload, short circuit to ground or to the battery, overtemperature, open load in on-state, or open load in off-state.
Generalization of platform approach
BCMs must not only cover several equipment levels for a given car model, but they must also handle the specific requirements of different markets (Asia, Europe, America, etc). Furthermore, BCMs are shared between several car models. The main motivations behind this trend are the reusability of the modules, the reduction of the development and manufacturing costs and the improvement of the quality.
Considering these trends, one can realize that the development of a state-of-the-art BCM presents many challenges. Here we review some of them.High numbers of HSD with PWM control and synchronous diagnostics
Functionalities, which were once distributed between three BCMs for the front, rear, and interior areas of a vehicle, are now integrated in two modules or even a single module. It is common to see BCMs, which have more than 40 HSD channels for passenger cars and 80 channels for trucks. In order to prevent a high voltage drop caused by several bulbs simultaneous inrush currents and to improve the electromagnetic compatibility of the system, a phase shift between the PWM channels is also an advantage. Moreover, regular monitoring of the output currents of each activated load must be performed, in conjunction with the PWM control.
In addition, false error detections must be avoided, such as an overload detection caused by the high inrush current of a cold bulbís filament. The diagnostics should also filter out spurious fault conditions, which are caused for instance by ISO-specified pulse types
. The combination of PWM generation with phase shift and synchronous diagnostics for a high number of outputs results in a high microcontroller (MCU) workload.
Scalability of HSDs
In order to comply with functional safety requirements of ISO26262 (ASIL B), the supply voltage of the HSDs is generally split into two to four supply rails. Likewise, a multichannel HSD may not control more than one safety relevant load (low beam, brake light, position light, etc). In addition to these constraints, the optimization of the power stage size to the loads and the variants within a car platform might lead to a HSD partitioning with many single and dual channel devices. All in all, we need a scalable solution with low integration inside the HSDs, which fulfills the PWM control with synchronous diagnostics.Short circuit robustness with filtering of intermittent conditions
Device and system safety, failure tolerance, and robustness against anomalous load conditions are becoming more and more important, in line with the zero defect philosophy of the automotive industry. In particular short-circuit of HSD outputs to ground must not lead to a destruction of the faulty outputs.
Despite built-in protection, the standard HSDs degrade under short circuit conditions, due to thermo-mechanical stresses and electromigration effects. The longer the filter time, the higher the degradation of the HSD. Today, when the diagnostic is performed directly by the microcontroller, the sampling of the current sense or digital status of the HSDs is typically done with a period of 10 to 100 ms. On the other hand, filtering must be implemented to avoid reporting a "wrong failure" due to transient disturbances, before the validation of an actual failure.