LIQUID COOLING OF BRIGHT LEDS FOR AUTOMOTIVE APPLICATIONS

This introduction paper is based on the paper "LIQUID COOLING OF BRIGHT LEDS FOR AUTOMOTIVE APPLICATIONS" published by "Not explicitly stated in the provided excerpt (likely EuroSimE Conference Proceedings, based on similar references)".

1. Overview:

• Title: LIQUID COOLING OF BRIGHT LEDS FOR AUTOMOTIVE APPLICATIONS
• Author: Yan Lai, Nicolás Cordero, Frank Barthel, Frank Tebbe, Jörg Kuhn, Robert Apfelbeck, and Dagmar Würtenberger
• Year of publication: Not explicitly stated in the provided excerpt (likely 2006 or later, based on references).
• Journal/academic society of publication: Not explicitly stated in the provided excerpt (likely EuroSimE Conference Proceedings, based on similar references).
• Keywords: LEDs, automotive headlights, thermal management, liquid cooling, active cooling, CFD, heat sink optimisation.

2. Abstract:

With the advances in the technology of materials based on GaN, high brightness white light emitting diodes (LEDs) have flourished over the past few years and have shown to be very promising in many new illumination applications such as outdoor illumination, task and decorative lighting as well as aircraft and automobile illuminations. The objective of this paper is to investigate an active liquid cooling solution of such LEDs in an application of automotive headlights. The thermal design from device to board to system level has been carried out in this research. Air cooling and passive liquid cooling methods are investigated and excluded as unsuitable, and therefore an active liquid cooling solution is selected. Several configurations of the active liquid cooling system are studied and optimisation work has been carried out to find an optimum thermal performance.

3. Introduction:

Light Emitting Diodes (LEDs) are increasingly utilized in automotive exterior lighting due to their small package size, styling flexibility, and superior performance. While common in brake lights and turn indicators, their application in forward lighting (headlamps) is still emerging. Current LEDs present a high-cost solution with insufficient lumen output for production vehicles, as legal requirements stipulate 750 lm per lamp, and bright LEDs average only 40 lm/W, necessitating more LEDs and higher driving powers.

As light output demands increase, so does the driving power and consequently, the heat generated. Effective thermal management of LED packaging is crucial as increased diode junction temperature reduces LED efficiency and causes shifts in emission wavelength. The LED operating temperature must be kept well below its maximum (e.g., < 125 °C) for optimal efficiency and stable color. This requires a comprehensive thermal solution addressing device, package, board, and system levels. This study utilizes commercially available bare die bright LEDs and employs Computational Fluid Dynamics (CFD) simulations, specifically with FloTherm software, to design and validate the thermal management solution.

4. Summary of the study:

Background of the research topic:

The increasing adoption of high-brightness LEDs in automotive applications, particularly for forward lighting, presents significant thermal management challenges. Higher light output demands lead to increased power consumption and heat generation, which can adversely affect LED performance, efficiency, and reliability if not managed effectively.

Status of previous research:

Previous efforts indicated that while LEDs are promising for vehicle forward lighting, they faced challenges like high cost and insufficient lumen output for standard production. The critical importance of maintaining LED junction temperatures below maximum limits (e.g., 125 °C) to ensure efficiency and prevent color shift was well-established. Thermal solutions were recognized as needing to be all-inclusive, covering device, package, board, and system levels, often employing CFD for design and analysis.

Purpose of the study:

The primary objective of this study was to investigate, design, and optimize an active liquid cooling solution for high-brightness LEDs intended for use in automotive headlights. The research aimed to develop a thermal management system that could effectively dissipate heat from the LEDs and maintain their junction temperatures within safe operating limits.

Core study:

The research systematically evaluated different cooling strategies and designed an active liquid cooling system.

1. Device to Board Level Design:
   • The chosen LED was a Cree XBright900 (900×900 µm blue light chip). The system comprised 15 LEDs on 5 boards (3 LEDs each).
   • LEDs were individually packaged with a phosphor layer for white light conversion. High thermal conductivity AlN (k=200 W/mK) was used for the package, achieving a thermal resistance < 2 °C/W between the LED and package bottom.
   • Packaged LEDs were mounted on an Insulated Metal Substrate (IMS) consisting of a 70 µm copper layer, a 75 µm dielectric layer (2.2 W/mK), and a 1 mm Al core board (Table 1).
2. Exclusion of Alternative Cooling Methods:
   • Air Cooling (Passive and Active): Passive air cooling with a heat sink behind the IMS board, within the headlamp enclosure, resulted in LED junction temperatures far exceeding 125°C (Figure 2). Active air cooling was deemed impractical due to space constraints, enclosure limitations, reliability concerns, cost, and assembly complexity.
   • Passive Liquid Cooling:
     • Passive closed-loop systems: While thermally capable, they require the heat exchanger to be above the heat source, which is incompatible with headlight design constraints (requiring heat exchanger below LED modules).
     • Heat pipe systems (loop heat pipe): These necessitate flexible heat pipes for adjustable LED boards, leading to prohibitively high costs (e.g., $1,000 per unit).
3. Selection and Design of Active Liquid Cooling:
   • Active liquid cooling was chosen as the most viable solution.
   • The system comprises a pump, cold plates (thermally connected to IMS boards), a reservoir, and a heat exchanger, connected by flexible hoses in a closed loop. Water with additives (antifreeze, anti-algae, anti-fungal) was selected as the coolant.
   • Several configurations were considered:
     • Parallel circuits: Thermally optimal but overly complex.
     • Series circuits (single loop): Higher pressure drop but manageable.
     • Chosen configuration (Figure 4): A liquid loop connecting all Low Beam (LB) cold plates in series, followed by High Beam (HB) cold plates in series, then to the heat exchanger. This offered a good balance of fewer hoses, shorter hose lengths, and simpler mounting, with a junction temperature difference of < 5°C between the first and last LEDs in the loop.
4. Thermal Optimisation:
   • Liquid Flow Optimisation: The relationship between nominal pump flow, actual flow, pressure drop, and LED temperature was analyzed (Figures 6, 7, 8). A pump with a nominal flow of 0.12 l/s and a nominal pressure head of 25 kPa was found to operate within its recommended range.
   • Heat Exchanger (Heat Sink) Optimisation: A pin-fin configuration was selected for the heat sink to reduce weight. Parameters like base thickness (t), pin height (h), pin length (l), pin width (w), and number of pins (Nx, Ny) were optimized iteratively (Figures 9, 10, 11).

5. Research Methodology

Research Design:

The research involved a comparative analysis of different cooling technologies (air, passive liquid, active liquid) for automotive LED headlights. It followed a multi-level thermal design approach, addressing thermal issues from the LED device to the package, board, and overall system. Once active liquid cooling was selected, various system configurations were studied, and the chosen configuration was then subjected to thermal optimisation.

Data Collection and Analysis Methods:

The primary method for data collection and analysis was thermal simulation using Computational Fluid Dynamics (CFD). The commercial CFD software FloTherm (version 6.1) was utilized to model the LED packages, IMS boards, heat sinks, and the complete liquid cooling system within the headlamp enclosure. These simulations provided temperature profiles, heat flow paths, and pressure drop characteristics, enabling the evaluation and optimisation of the thermal management solutions.

Research Topics and Scope:

The research focused on the thermal management of high-brightness white LEDs specifically for automotive headlight applications. The scope included:

• Thermal analysis at device, package, and board levels (e.g., LED chip, AlN package, IMS board).
• Evaluation of air cooling (passive and active) and passive liquid cooling (closed-loop, heat pipe) at the system level.
• Detailed design and analysis of an active liquid cooling system, including component selection (pump, cold plates, heat exchanger), coolant choice, and system configuration.
• Optimisation of the active liquid cooling system, focusing on liquid flow rates and heat sink design parameters to achieve optimal thermal performance while considering practical constraints like size, weight, and manufacturability.

6. Key Results:

Key Results:

• Passive air cooling was insufficient to maintain LED junction temperatures below the 125°C limit (Figure 2), and active air cooling was deemed impractical.
• Passive liquid cooling methods, such as closed-loop systems and heat pipes, were found to be unsuitable due to either design constraints (component placement) or excessive cost.
• Active liquid cooling was identified as the most suitable thermal management solution for the application.
• A specific active liquid cooling configuration was selected: a liquid loop connecting all Low Beam (LB) cold plates in series, followed by High Beam (HB) cold plates in series, and then to the heat exchanger (Figure 4). This design offered advantages in terms of a reduced number of hoses, shorter hose lengths, and simpler mounting. The junction temperature of the last set of LEDs in the loop was less than 5°C higher than those in the first set.
• Liquid flow optimisation determined that a pump with a nominal flow of 0.12 l/s and a nominal pressure head of 25 kPa would operate effectively within its recommended range for the designed cooling loop (Figures 6, 7, 8).
• The heat exchanger (heat sink) was optimized using a pin-fin design made of aluminium, with a total weight of less than 800 grams. The optimized dimensions and allowable design margins are:
  • Optimised design (dimensions in mm): t = 5, H > 30, h > 25, l = 4.5, w = 9, Nx=8, Ny=7.
  • Allowable design margins (< 1 °C increase in junction T, dimensions in mm): 3.5(Note: The optimised Nx=8 value is taken from the paper's summary on page 6; other sections and figures in the paper suggest an optimum Nx around 40.)

Figure Name List:

• Figure 1. Insulated Metal Substrate assembly. (a) AIN cup with wire-bonded LED, (b) Circuit layer, (c) Dielectric layer and (d) Aluminum substrate.
• Figure 2. Temperature profile across the headlight assembly for passive air cooling (T;=200°C).
• Figure 3. Cold plates and heat exchanger design and their hose connections.
• Figure 4. Active liquid cooling configuration—the liquid loop connects all the LB cold plates in series followed by the HB in series and then into the heat exchanger.
• Figure 5. Full model of active liquid cooling of complete low beam system inside the headlamp enclosure (shown in Figure 4).
• Figure 6. Calculated LED junction temperature (blue) and IMS board temperature (red) as a function of the nominal flow (with a nominal pressure head of 25 kPa)
• Figure 7. Calculated actual flow as a function of the nominal (zero pressure) flow of the pump.
• Figure 8. Pressure vs. flow characteristics for the liquid cooling loop and linear pump characteristics
• Figure 9. Heat sink parameters and dimensions
• Figure 10. LED temperature as a function of pin length and pin width. A) 3D view and B) Contour plot.
• Figure 11. LED temperature as the function of pin number in the X (diamonds) and Y (squares) directions.

7. Conclusion:

This paper demonstrates the procedure for selection and optimisation of an active liquid cooling solution for high brightness LEDs customised for novel headlight applications. It was found that air and passive liquid cooling was either insufficient to maintain LED junction temperature below its maximum allowable levels or unfeasible to realise in the actual application. While some of these solutions would be suitable from a purely thermal point of view, this is not the case when the optical and mechanical designs are taken into account. Therefore all aspects of the headlight design must be taken into account when seeking a suitable thermal management solution.

Therefore active liquid cooling is selected as the optimum cooling solution under these circumstances. Several different system structures of active liquid cooling are studied and compared in this paper. And thermal optimisations of the liquid flow and heat sink are carried out in order to maximise the thermal performance. During the search for the optimum thermal solution, thermal management is not the only factor to focus on; all related issues such as manufacturability and product specifications are also taken into account.

With the development of brighter white LEDs, the driving power required for a certain light output will be decreasing continuously in the future. Therefore heat dissipation will also decrease. With the reduced power requirements for the system and lower heat dissipation, the cooling solution can once again be simplified to only passive air-cooling.

8. References:

• [1] Pearson, T., Mounier, E., Eloy, J.C., Jourdan, D., "Solid-state lighting in the automobile: concept, market timing and performance,” LEDs Magazine, pp. 25-27, Apr. 2005.
• [2] Flomerics Ltd., FloThermTM 6.1 Instruction Manual, 2005.
• [3] Cree LED Lighting Website [Online]: http://www.cree.com.
• [4] Lai, Y. and Cordero, N., “Thermal management of bright LEDs for automotive applications," Proc. Of the 7th EuroSimE Conference, pp 390-394 (2006)
• [5] Stratford, J and Musters, A, “Insulated metal printed circuits – a user-friendly revolution in power design," Electronics Cooling, vol. 10, pp. 30-34, Nov. 2004.
• [6] Karimpourian, B. and Mahmoudi, J., “Some important considerations in heatsink design," Proc. of the 6th EuroSimE Conference, pp 406-413 (2005)

9. Copyright:

• This material is a paper by "Yan Lai, Nicolás Cordero, Frank Barthel, Frank Tebbe, Jörg Kuhn, Robert Apfelbeck, and Dagmar Würtenberger". Based on "LIQUID COOLING OF BRIGHT LEDS FOR AUTOMOTIVE APPLICATIONS".
• Source of the paper: Not provided in the paper.

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