LEAF LED headlamps (3): Perform thermal fluid analysis simulation from basic design

In order to reduce the power consumption of LED headlamps, it is not enough to reduce the light loss of white LEDs by the light distribution control mentioned in the first round, and to obtain the desired light distribution characteristics in a few places. It must also be able to use white LEDs in an efficient state. The key to this is how to dissipate the heat generated by the white LED. In fact, the power of the input white LED is mostly turned into thermal energy. If this heat causes the temperature of the white LED to rise, the luminous efficiency is lowered, resulting in an increase in the amount of heat generation. As a result, there is a vicious circle in which the luminous efficiency is further lowered and the calorific value becomes larger.

Figure 1: Positioning of the headlight thermal design

Figure 2: Modeling LED headlamps

The city's optical industry used thermal fluid analysis simulation technology in the heat dissipation design of LED headlamps. It is not uncommon to use thermal fluid analysis in the headlamp heat dissipation design. However, this is the upstream design process, which is the heat resistance, convection, component layout and heat sink design of materials such as resin. This simulation technique is used in the basic design, thereby efficiently promoting the heat dissipation design.

The thermal design of the headlamps is applied to the product after the four major processes of basic design, detailed design, trial test and design change (Fig. 1). According to Kikuchi Kazuhiro, who is responsible for the heat dissipation design of Shiguang Industrial Co., Ltd. (a senior expert in the simulation department of the core engineering department of the development department, senior engineer of analytical technology), in the basic design, although the modeling of the simulation model is not difficult, the time spent on the work is not Not much, but the information obtained by calculations accounts for as much as 60% of the entire design (Figure 2). The LED headlamps achieved significant results by using analog technology in this basic design.

Can be applied to basic designs due to the ability to separate light and heat

So why is it possible to use simulation in the basic design this time? The answer lies in the flow characteristics of the heat in the LED headlamps. From the perspective of the heat dissipation path from the white LED, the flow path is:

LED chip as a heat source↓
Mounting substrate with LED chip package ↓
A heat spreader that diffuses heat from the mounting substrate to the entire package bottom surface ↓
Connection part of heat sink (heat sink, etc.) ↓
Heat sink (heat sink, etc.)
↓
External air can be said that the entire path from the white LED to the outside air is in series (Figure 3). The heat dissipation path of existing light sources such as halogen lamps and HID lamps is quite complicated. The reason is that the light source itself emits light and heat to the outside air at the same time. In other words, it cannot be reflected in a simple tandem state. Therefore, the heat dissipation model in the basic design is more complicated, and the simulation can only be used in the trial test and design change process.

Figure 3: White LEDs and heat dissipation paths for headlamps

The light from the LED is radiated from the surface to the outside air, and the heat is radiated from the back to the outer casing of the lamp, and the light is separated from the heat. Since the heat radiation of the LED surface is negligible, the heat flow path is as described above. Therefore, the modeling in the basic design becomes simple, and the thermal fluid analysis simulation can be performed. With the simulation effect, the thermal analysis of the LED headlamp achieved a calculation accuracy of ±2 °C, and the heat dissipation design was completed in only two days.

After using the simulation to design the heat sink and other parts and master the general shape and basic performance, Shiguang Industrial has trial-produced one product. Then, through the design of the optimal shape and the subtle part, the mass production product design confirming the heat dissipation performance under the final shape is entered. Due to the high calculation accuracy, it is also possible to use the product in the next time or later without trial production. This is the first time that Shiguang Industrial has mass-produced LED headlamps, so the actual products have been trial-produced. In addition, the thermal simulation in the basic design of the heat sink and the light control mirror is about two and a half hours, and the white LED is only one and a half hours to the heat sink. This time, the heat sink and other parts were divided into about 700,000 grids for calculation.

Reduce the thermal resistance of the heat sink and the connection

According to Shiguang Industrial, when designing the heat dissipation of LED headlamps, the design of the heat sink connection and the heat sink itself in the heat dissipation path were also discussed. The reason is that LED products are purchased from LED manufacturers in the state where LED chips and even heat spreaders are housed in white LED packages, and the headlamp manufacturers cannot change them.

The problem to be solved in the heat sink connection portion and the heat sink is how to reduce the thermal resistance of the connection portion and the heat sink itself. In order to reduce the thermal resistance of the connection, it is especially important to: (1) use clamps, springs and screws to maintain the pressure and uniform contact area of ​​the white LED on the heat sink; (2) consider using thermal grease and The thermal sheet is managed and its time and thickness are managed. In terms of heat sinks, it is necessary to combine the cooling fan without affecting the size and shape of the headlamp design. In this thermal design, the design premise is that 9W of the 10W power input to each white LED becomes heat. Since the maximum rating of the white LED is 130 ° C (the surface temperature of the white LED), it is designed to not reach 130 ° C for any application.

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