CFD Simulations Can Improve the Service Life of Automotive LEDs
Shawn Wasserman posted on September 16, 2016 |
FloEFD from Mentor Graphics enables design engineers to test automotive luminaire designs.

Diagram of an automotive LED lamp. (Image courtesy of Mentor Graphics and Luxeon Rebel.)

Diagram of an automotive LED lamp. (Image courtesy of Mentor Graphics and Luxeon Rebel.)

Designing an automotive LED luminary isn't as easy as replacing a xenon bulb for a chipset. For one thing, an LED isn't as replaceable as a xenon bulb. When the LED goes, you have to replace the entire headlight system.

Therefore, mistakes in the design process of an LED luminary can result in expensive costs to your customers or product recalls for your company.


When designing a luminary for an automotive LED lighting system, engineers will need to ensure that the light stays cool. Otherwise, they risk reducing the service life and luminous output of the lamp.

What Are the Design Challenges When Creating a Luminaire?

Graph showing that above 40°C, an LED’s luminous flux begins to drop off rapidly. (Image courtesy of Mentor Graphics.)

Graph showing that above 40°C, an LED’s luminous flux begins to drop off rapidly. (Image courtesy of Mentor Graphics.)

As the LED is powered by a constant current, the forward voltage is defined by the LED's properties and will decrease with increasing temperature.

This will affect the power dissipation, which, in turn, drops the LED's light emitting efficiency and increases the junction temperature.

“As LEDs get to higher temperatures, the light output as well as the color will change,” said Boris Marovic, technical manager at Mentor Graphics. “The color of a red LED, for example, can shift towards amber. Regulations define which color range a braking light can have and amber would be out of the range and therefore not allowed.”

One of the most influential design challenges when creating a luminaire is the size restrictions. The luminaire is limited by the design space where it will be installed but also by government regulations, which will limit the height of the lighting fixture. The shape of the heat sinks is key to a successful design, but if there isn’t enough room for all of those fins, then engineers still have some options.

“Since the size is limited, the designers often use fans to achieve forced cooling, actively blowing air over the heat sink,” explained Marovic. “With forced convection, the engineers can keep the heatsink smaller as more cooling air moves through it compared to natural convection at low air velocities.”

Engineers also have to combat the various climates in which the lights will operate. In warmer climates, the LED will naturally become hotter as the temperature gradient between the ambient temperature and luminaire is roughly constant. This translates into a hotter junction temperature.

“Often engineers implement temperature sensitive resistors that will automatically reduce the drive current to the LED, reducing their light output but also keeping them below the maximum temperature to prevent them from failing,” explained Marovic.

However, limiting the current to an LED to reduce its operational temperature is a clear stopgap measure. It is more effective to design a luminaire that will operate under temperatures that will limit the LED's performance in any climate.

“The main desire is, of course, to keep the LED at an acceptable temperature at which it provides sufficient light and does not degrade too fast in order to fulfill its lifetime requirements,” said Marovic. “But, on the other hand, the designer wants to keep the size and weight as well as the costs low.”

Engineers are tasked to optimize of heat dissipation for an LED luminary while maintaining an affordable bill of materials (BOM). Other design caveats engineers might have to deal with when designing a luminaire include:

  • Limiting the heat sink mass for lower fuel mileage and costs
  • Using lighter aluminum heat sinks despite copper’s higher thermal conductivity

Using computational fluid dynamics (CFD) simulation tools, like Mentor Graphics FloEFD, engineers can thermally test various luminaire designs to optimize the heat dissipation while ensuring these design criteria.

The Simulation Physics of Luminaires Thermal Management

When assessing the thermal management of a luminaire, engineers will need to look at the convection, conduction and radiation associated with the system.

Automotive luminaire CAD geometry and mesh geometry with thermal simulation overlay. (Image courtesy of Mentor Graphics.)
Automotive luminaire CAD geometry and mesh geometry with thermal simulation overlay. (Image courtesy of Mentor Graphics.)

Dealing with the conduction is easy; engineers simply need to know the material properties of the parts within the luminaire and the boundary conditions. The simulation software will typically handle the rest.

FloEFD can also be used to model the convective heat transfer within the luminaire. This is a bit more complicated than the conductive heat transfer as it will require a CFD analysis of the airflow within and around the headlight unit. During the CFD analysis, the software calculates the heat transfer coefficient.

As for the radiation, this will require the selection of the correct model.

The three radiation models available in Mentor Graphics FloEFD: Discrete Transfer Radiation Model (DTRM), Discrete Ordinates (DO) and Monte Carlo(MC) models. Radiation model selection is vital to an accurate simulation. (Image courtesy of Mentor Graphics.)

The three radiation models available in Mentor Graphics FloEFD: Discrete Transfer Radiation Model (DTRM), Discrete Ordinates (DO) and Monte Carlo(MC) models. Radiation model selection is vital to an accurate simulation. (Image courtesy of Mentor Graphics.)

“The selection of the radiation model is very important and determines to what extent and to which accuracy it can be considered,” warned Marovic. “The Monte Carlo radiation model is the most accurate for lighting applications.”

Since the luminaire will include lenses and reflectors, the radiation model chosen will also need to be one that has good optical focusing capabilities. If not, then design flaws can be introduced where hot spots can form as the sun beats down on the luminaire.

“Solar radiation can seriously damage luminaires. If the solar rays are focused onto components of the headlight, they can simply burn through, just like burning ants with a lens. These hot spots can burn through plastic components as it can reach several hundred degrees Celsius,” said Marovic. “At the same time, the lens will absorb a fraction of the energy shining through it, further heating it up. This is important for low melting temperature materials such as in plastic lenses.”

Simulation allows engineers to visualize these hot spots and the sources of radiation that causes them. This way, the engineer can adjust the reflectors and lenses so that the rays no longer meet at the hot spot.

The simulation will also have to model how the radiation travels: either as a band or individual rays. If a band model is chosen, then the parameters governing the radiation will be averaged within a specified wavelength band. Conversely, the parameters of each ray will remain separate in the ray model.

Due to the averaging of the band model, the results will be less accurate if multiple materials with different optical characteristics are used in the model. Therefore, if multiple materials are used, then the extra computational power to use a ray-based model might be in order.

FloEFD can incorporate other physical models into the simulation analysis of the luminaire. However, adding more physics, such as the buildup of moisture and ice, will increase the computational time. As a result, these more complex simulations might be best saved for when the design space has been narrowed down a bit with the initial thermal analysis.

The Benefits of Simulation Early in the Development of Automotive Luminaires

“Ideally, simulation should be done as early as possible in order to avoid late changes in the design that will increase the costs,” said Marovic.

Setting up an LED’s parameters in FloEFD. (Image courtesy of Mentor Graphics.)

Setting up an LED’s parameters in FloEFD. (Image courtesy of Mentor Graphics.)

“When luminaires are designed the initial CFD, calculations can be done on the first drafts of the design in order to estimate the temperatures and required materials that will influence the price of the system,” he added.

The simulations can also inform the engineer of the flowpath of the air and the heat sink performance that cool down the LED. These simulations can signify that the number of fins, pins, length, width, height or orientation of the heat sink is insufficient. Additionally, they can help engineers determine if a fan is needed to further cool down the headlights.

This simulation can alsoutilize the thermal and radiometric characterization results of T3Ster TeraLED, which determines the heat flow path through every LED layer from the junction to the base of the LED and to the heat sink. This results in improved accuracy when modeling the LED component in a CFD simulation, which in turn improves the accuracy of the results of the LED's performance by providing additional information.

As a result, engineers can use the simulations to estimate the hot lumen of the LED at its operating temperature. Therefore, these simulations will be critical when ensuring the warranty of the lights. As LED headlights are a lot more complicated to replace than traditional ones, this can save a company a lot of money.

To learn more read about the CFD technology by Mentor Graphics, read FloEFD's Technical Specifications.

 

Mentor Graphics has sponsored ENGINEERING.com to write this article. It has provided no editorial input. All opinions are mine. —Shawn Wasserman

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