Evaluation of Digital Power Control Algorithms for Automotive LED Headlights using TMS320F28035 Microcontroller

Digital power management solutions offer some distinct advantages over their analog counterparts and are increasingly being employed by power electronics designers these days. The post-implementation flexibility of digital solutions is one of the major advantages. Increasing demands on functionalities like communication, remote fault monitoring and configurability have also fueled this transition from analog to digital control. The possibility of implementing non-linear control algorithms is a major advantage of these solutions. Highly integrated analog components, like analog to digital converters and comparators in modern microcontrollers, have enabled designers to decrease on-board component count and complexity. Digital power management solutions have limitations in performance when compared to analog designs, but rapid performance improvements in microcontrollers (MCUs) certainly create a bright future for digital power management. In this project we have investigated a digital power control implementation for switch mode DC-DC converters. The application under consideration is LEDbased automotive headlights. LEDs (light emitting diodes) have gained foothold in many lighting applications due to the decrease in cost per lumen. The automotive industry has previously employed LEDs in a number of lighting application, like back and interior lighting, and now consider a potential use in headlights. Moreover we wanted to prove the capabilities of the TMS320F28035 MCU that is designed for real-time control applications. The combination of two new avenues of digital power management and LED headlights has raised a few challenges that have been solved in this project. First, the characteristic behavior of LEDs is different from conventional incandescent bulbs. LEDs are controlled by maintaining a constant current through them rather than applying a constant voltage. Switch mode power stages intended to control LEDs inherently operate in voltage mode. Considerable modeling and implementation efforts are required to handle both these contrasting behaviors. Secondly, discrete-time models are required for MCU implementation. Existing control theory predominantly employs continuous-time models. These continuous-time models are required to be converted to discrete-time models to make use of existing models for digital implementation. Discrete-time models that are developed from scratch requires considerable effort. Third, existing testsetups used for analog designs can not be directly used for digital designs. Considerable analysis is required for these modifications. The first step in the design flow is modeling; we have to model converters for controlling current through LEDs. Two solutions are proposed: First, existing continuous-time voltage mode models for converters are used and modified to control LEDs. Later some further modifications are made to convert them to discrete time. Second, a new non-linear and discrete-time model is proposed for controlling LED current using inductor current feedback. Later, the developed model is implemented on TMS320F28035. This MCU contains two heterogeneous processor cores. A pre-implementation analysis is carried out to allocate hardware resources and system bandwidth to different software tasks. The control loop is implemented on a so-called control-law accelerator, as it is optimized for this purpose, while useful features like graphical user interface and diagnostics are implemented on C28x CPU. The initial bandwidth allocation to different software tasks is verified by doing measurements and re-allocation. The initial implementation of different software components is also optimized to enhance performance based on this post-implementation system analysis. Lastly, the implemented control algorithms are verified by performing frequency response measurements. Modifications are made to existing test-setups to suit them to needs of digital power control. Open loop and closed loop measurements are performed under different operating conditions. These measurements are compared with results from the model and used to lay down our final analysis. Recursive design approach is used at each design phase. Moreover previous design phases are revisited whenever necessary to optimize the implementation.