What are the thermal management methods for automotive lighting?

Insensitivity to vibrations , long service life, high energy efficiency and the possibility of full control of the light source are key factors in the application of LEDs in the automotive field. Compared with incandescent bulbs, LEDs are insensitive to mechanical vibrations, and since intelligent automotive lighting systems need to comply with vehicle requirements and environmental conditions, the easy control of LEDs makes them a natural choice for this lighting system. However, driving LEDs to obtain efficient light output requires current control independent of the supply voltage.

LED system design can be approached from many angles. At the PCB level, one approach is to first define the maximum temperature of the LED junction, because high junction temperature reduces LED light emission, thereby reducing device 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 effects of the LED light source, the circuit board design must also consider the effects of the driver. Material stress , electrostatic discharge, electric and magnetic fields, and radio frequency interference are all external factors that automotive electronics must deal with.

PCB Thermal Management

The main obstacle to the realization of energy-saving high-power LEDs is the management of the heat they generate. As design technology advances, the need to protect devices from heat buildup is increasing, which in turn promotes the development of chip-on-board (COB), ceramic heat sinks, and other standard thermal management packaging solutions for power LEDs. High-power LEDs are small in size and require excellent heat dissipation performance to reduce the temperature of the chip and thus improve efficiency.

The ability to manage thermal impedance throughout the product lifecycle is critical for LED thermal management. In various high-temperature applications, the choice of package should take into account the ability to properly dissipate heat. In particular, the quad flat no-lead (QFN) package provides low inductive reactance characteristics for temperature-sensitive applications. On the other hand, LTCC packages and substrates guarantee reduced dielectric losses, but most importantly, allow for smaller device sizes, fewer interconnections, and thus reduced various passive parasitic parameters.

LED Design

While thermal management cannot be ignored, each LED design must also meet the performance requirements and time-to-market constraints of the application. The most traditional thermal substrates - metal core PCB (MCPCB), aluminum oxide (Al2O3) and aluminum nitride (AlN) - can meet all requirements as well as market needs. Nanoceramics are a low-cost solution that can meet market needs between 30 W/mk and 170 W/mk.

LED packages must be designed with thermal pads on the anode and cathode electrodes. Like other electronic devices used in various fields, the failure rate of LED packages doubles with every 10°C increase in junction temperature. FR4 (flame retardant) materials and composite epoxy materials (CEMS) are perfect thermal insulators with excellent thermal conductivity for good heat dissipation.

Chip-on-board (COB) LEDs (Figure 1) are rapidly gaining popularity in the market. COB LEDs must dissipate 10 W/cm2 of thermal power, limiting the choice of materials to AlN, Al2O3, and MCPCB. MCPCBs use a metal substrate as a heat sink. The metal core is usually composed of an aluminum alloy. Thermal CLAD (TCLAD) is a metal-based dielectric with a layer of copper on the surface. Higher reliability, easy handling, and excellent cost-performance make MCPCBs with TCLAD an excellent alternative to traditional FR4 substrates.

Lighting Driver

In addition to thermal management, LEDs also require driver ICs for optimal lighting performance. LEDs typically require a constant current to produce 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 corresponding IC for their LED lighting applications.

The automotive environment is a challenge for integrated regulators. The environment has a wide range of temperature variations and also generates large transients and input disturbances. In addition, the power supply must be able to withstand load and unload transients, although this battery -related phenomenon is usually managed by separate circuits (suppressors, terminations , and overvoltage protection). All switching regulators and drivers for LED displays in the automotive industry must meet the AEC- Q100 standard .

The Texas Instruments LMR23610ADDA, part of its Simple Switcher family, is a step-down synchronous converter in an 8-pin PowerPAD package that uses peak current control for simple control circuit compensation. The 36-V, 1-A synchronous buck regulator has an input voltage range of 4.5V to 36V, making it suitable for a wide range of applications. At 75-μA quiescent current, the LMR23610ADDA can be used in battery-powered systems; the ultra-low (2-μA) shutdown current further extends battery life. A precise enable input simplifies controller control. Protection features prevent 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 assembling a complete LED driver topology. The board can be configured as a boost or boost-to-battery topology to power a single string of LEDs in series. Accurate closed-loop LED current regulation is achieved using a low-offset rail-to-rail current sensing amplifier and high-side current sensing. The TPS92692EVM-880 helps engineers evaluate the operation and performance of TI 's TPS92692-Q1 and TPS92692 high-precision LED drivers designed for automotive lighting.

Meanwhile, ON Semiconductor also offers the NCP3065 monolithic switching regulator , designed to provide constant current for high-brightness LEDs. The device has an input voltage of up to 40 V, can operate at 12 V AC or 12 V DC, and can be configured to support LED currents exceeding the 1.5 A nominal switching current of the internal transistor . The NCP3065 can be configured for buck or boost conversion using a minimum number of external components (Figure 3).

The Infineon Technologies TLE4241GM integrated circuit is an adjustable constant-current driver designed to power LED arrays in harsh automotive environmental conditions, thereby achieving consistent brightness and extending LED life. Its protection circuitry prevents damage to the device in the event of overload, short circuit, polarity reversal, or overtemperature (Figure 4).

The proliferation of LED modules in vehicles places new demands on system hardware, including small component size, improved energy efficiency for high performance at the same or better thermal efficiency, architectures that connect and flexibly support multiple configurations, and maintain precise control of LED light characteristics.