Realize the thermal management strategy of long-term vehicle lighting
Insensitivity to vibration, long service life, high energy efficiency, and complete control of the light source-these are the key factors for LED application in the automotive field. Compared to incandescent light bulbs, LEDs are not sensitive to mechanical vibrations, and because of their easy-to-control characteristics, LEDs are a natural choice for smart car lighting systems in response to vehicle requirements and environmental conditions. However, driving LEDs to obtain high-efficiency light output requires current control independent of the power supply voltage.
The design of the LED system can be discussed from multiple angles. At the circuit board (PCB) level, one method is to first define the maximum temperature of the LED junction, because the high junction temperature can reduce the LED light emission, thereby reducing component efficiency. Printed circuits used in cars or trucks must be very reliable and highly durable, but they must also be cost-effective. In addition to considering the impact of the LED light source, the design of the PCB must also consider the impact of the driver. Material stress, electrostatic discharge, electric and magnetic fields, and radio frequency (RF) interference are all external factors that automotive electronics must face.
PCB thermal management
The main obstacle to energy saving and high power LED is to manage the heat generated by it. With the advancement of design technology, the need to protect components from heat accumulation has also increased, which has promoted on-board chip (COB) packaging, ceramic substrate-sub-mount technology (sub mount), and other thermal management standards for power LEDs Package development. High-power LEDs are small in size and require excellent heat dissipation performance to reduce the temperature of the chip, thereby improving efficiency.
The ability to manage thermal impedance throughout the product's life cycle (PLC) is critical to LED thermal management. In various high-temperature applications, the choice of package should properly consider the ability to dissipate heat. In particular, the square planar no-pin (QFN) package provides low-inductance characteristics for temperature-sensitive applications. On the other hand, the use of low temperature co-fired ceramic (LTCC) packages and substrates can ensure the reduction of dielectric loss, but most importantly, it can achieve a smaller component size and fewer interconnections, thereby reducing various passive parasitic parameters.
LED lighting design
Although thermal management cannot be ignored, each LED design must also meet the performance requirements and time-to-market limitations of the application. The most traditional thermal substrates-metal core PCB (MCPCB), aluminum oxide (Al2O3) and aluminum nitride (AlN), can meet all requirements to meet the market demand. Nano ceramic is a low-cost solution that can meet the market demand between 30w/mk and 170w/mk.
LED package must design a heat dissipation pad on the anode and cathode electrodes. As with other electronic components used in various fields, as the junction temperature increases by 10°C, the failure rate of LED packages will double. FR4 (flame retardant) material and composite epoxy material (CEMS) are perfect thermal insulators, with excellent thermal conductivity and good heat dissipation.
COB LEDs must dissipate 10W/cm2 of thermal power, which limits the choice of materials to AlN, Al2O3, and MCPCB. MCPCB uses a metal substrate as a heat sink. The metal core is usually composed of aluminum alloy. Thermal cladding dielectric material (TCLAD) is a metal dielectric coated with a copper layer on the surface, which has higher reliability, easy handling characteristics and excellent cost performance, making MCPCB with TCLAD an ideal alternative to traditional FR4 substrates .
Lighting driver
In addition to thermal management, LEDs also require driver ICs for optimal lighting performance. LEDs usually require a constant current to produce a consistent light output. The output voltage will depend on many parameters, such as the LED manufacturing process and the number of LEDs in series. Engineers must accurately predict the maximum output voltage to select the best regulator topology and the corresponding IC for their LED lighting applications.
The automotive environment is a challenge for integrating voltage regulators. The temperature range of the environment is very large, and it will also produce a high degree of transient and input interference. In addition, the power supply must be able to withstand loads and unload transients, although this battery-related phenomenon is usually managed by separate circuits (suppressors, terminals, and overvoltage protection). All switching regulators and drivers for automotive LED displays must comply with the AEC-Q100 standard.
Texas Instruments (TI)'s LMR23610ADDA is part of its Simple Switcher series. This is a synchronous buck converter in an 8-pin Power PAD package. It achieves simple control circuit compensation through peak current control. The input voltage range of the 36V/1-A synchronous buck regulator is 4.5V to 36V, and the application range is very wide. At a static current of 75μm, the LMR23610ADDA can be used in battery-powered systems; the ultra-low (2μm) shutdown current can further extend battery life. The precise enable input simplifies the control of the controller. The protection function prevents short-circuit damage and thermal shutdown due to excessive power dissipation (Figure 2).
Texas Instruments has also developed the TPS92692EVM-880 evaluation module (EVM) for fully assembled LED driver topologies. The circuit board can be configured as a boost or boost-battery topology to power a single string of LEDs in series. The use of low-offset rail-to-rail current sensing amplifiers and high-side current sensing achieves accurate closed-loop LED current regulation. The TPS92692EVM-880 can assist engineers to evaluate the operation and performance of TI TPS92692-Q1 and TPS92692 high-precision LED drivers (designed specifically for automotive lighting).
At the same time, ON Semiconductor provided the NCP3065 single-chip switching regulator, designed to provide constant current for high-brightness LEDs. The device has an input voltage of up to 40V, can operate at 12V AC voltage (VAC) or 12V DC voltage (VDC), and can be configured to support LED currents that exceed the 1.5A nominal switching current of the internal transistors. The NCP3065 can be configured to use a minimum number of external components for buck or boost conversion (Figure 3).
Infineon Technologies' Infineon Technologies (TLE4241GM IC) is an adjustable constant current driver designed to power LED arrays under harsh automotive environmental conditions to achieve consistent brightness and extend LED life. Its protection circuit prevents damage to the device under overload, short circuit, polarity reversal, or over temperature conditions (Figure 4).
The popularity of LED modules in vehicles has put forward new requirements for system hardware, including smaller component size, improved energy efficiency at the same or better thermal efficiency, to achieve high performance, connectivity and flexible support for multiple configurations. And keep precise control of LED lamp characteristics.