Hardware implementation of LED forward voltage measurement for junction temperature estimation

Abstract

LED junction overheating significantly decreases lifespan of LEDs. Colour and light intensity usually changes with temperature resulting in a reduced quality of light. Therefore, a thermal management is mandatory to prevent overheating. Even short temperature peaks have an influence on the LED lifespan and the light quality. LEDs and LED arrays are usually powered by LED drivers circuits based on several electronic components. Our approach is the extension of such drivers by measurement components and signal processing hardware to accurately detect the LED junction temperature. This information can be used by the driver to control current and therefore the temperature of the LED device. Focus of this paper is the measurement of the LED temperature based on the forward-voltage measurement of the LED. For data processing, i.e. the calculation of the related temperature, three different approaches are discussed based on implementation details and performance. Common to these approaches is the low to medium complexity of the algorithms which will be integrated into the control hardware, namely an ASIC. First, the algorithms are implemented using VHDL for coding on register transfer level and synthesizing for FPGA devices. All hardware devices have been integrated on PCB level including FPGA and step down converter to drive LEDs and to control the temperature of the LED chip. Determined temperatures are compared with temperature measurements via infrared camera of the LED to validate the accuracy of the temperature calculation. To include the temperature measurement into the LED driver the algorithms have to be part of the chip design. Therefore, the VHDL code is synthesized using Cadence Genus Synthesis. The synthesized VHDL code is than placed and routed using Cadence Encounter RTL compiler. Based on the resulting hardware complexity in therms of chip size and the achieved accuracy the most promising solution is selected for final implementation.