Tuesday, February 22, 2011

【 Weak current College 】 LED thermal solution analysis 】

Summary: LED circuit design in order to better address the thermal problem, LED and some high-power IC needs to aluminum base circuit.
LED aluminium baseboard design choices

LED circuit design in order to better address the thermal problem, LED and some high-power IC needs to aluminum base circuit.

Aluminum plate pcb by circuit layer (copper foil layer), thermal insulation and metal base. Circuit layer requires a lot of current-carrying capacity, which should use thicker copper foil, thickness 35 μ μ m m ~ 280; thermal insulation layer is the core technology of PCB aluminum plate, which is normally provided by special ceramic filled special polymer composition, thermal resistance is small, viscoelasticity fine, has the capacity of heat aging, able to withstand mechanical and thermal stress. IMS-H01, IMS-H02 and LED-0601, high-performance PCB aluminum plate heat insulating layer is precisely with this kind of technology that has very good thermal conductivity and high electrical insulation properties of strength; metal base is an aluminum plate of support structures that require a high thermal conductivity, typically aluminum or copper plate (which copper is able to provide better thermal conductivity), suitable for drilling, punching and cutting and other conventional machining. Technology requirements: gilt, gushes out the Tin, osp antioxidant, immersion gold, lead-free RoHS process, etc.

Base material: aluminium baseboard product features: small thin insulation, thermal resistance; no magnetic; heat; high mechanical strength product standard thickness: 0.8, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0mm copper foil thickness: 1.8um35um70um105um140um characteristics: high thermal resistance, electromagnetic shielding and high mechanical strength, excellent processing properties. Use: LED dedicated power hybrid IC (HIC).

Aluminum plate is the carry LED and thermal conduction, thermal devices or near the area, mainly concentrated heat conduction can choose high thermal conductivity of the material, such as the United States Bergquist plate; slow heat conduction or heat radiation General materials can be made. Price difference is large, Bergquist plates to produce finished products takes about 40 million square meters, General domestic materials to 1000 million square meters. LED General use voltage is not very high, select the thickness of the insulation voltage 1mil than 2000V.

Thermal reference design method:

Why thermal design?

High temperature on the impact of electronic products: insulation performance degradation; components damaged; materials for thermal aging; low melting point solder joint weld cracking and peeling.

Effect of temperature on components: in General, the lower the temperature resistance; high temperatures will reduce the service life of the capacitor; high temperature makes the transformer, choke coil insulation performance degradation, General transformer, choke coil allows the temperature is lower than 95C; temperature is too high it can cause changes in the structure of the solder alloy — IMC thickening, weld brittle, reduces mechanical strength; the rise in the junction temperature makes the transistor current amplification increases rapidly, resulting in an increase in the collecting electrode electric current, the junction temperature rise further, eventually leading to a component failure.

The purpose of thermal design

Control all electronic components within the product temperature, making it in the working environment conditions do not exceed the standards and norms of the maximum temperature. The maximum allowable temperature calculation should be based on the components of stress analysis, and product reliability requirements and are assigned to each of the components of the failure rate.

Thermal design is generally LED by fluid dynamics software simulation and do basic design.

Fluid flow resistance: due to the fluidviscosity and the influence of the solid border, make the fluid flow process subject to resistance, this resistance is called the flow resistance, can be divided into the frictional resistance and two local resistance.

Frictional resistance: in the border along the same region, fluid friction along the entire process of Sassafras resistance.

Local resistance: drastic changes in the boundaries of the region, such as section suddenly expanded or suddenly shrinks, elbow local location, is a fluid fluid state undergoing drastic changes and flow resistance.

Usually LED is the use of the natural thermal radiators, heat sink design for a three-step 1: design in accordance with the relevant constraints outline at. 2: according to the heatsink design standards on the radiator tooth thickness, tooth shape, tooth pitch, substrate thickness for optimization. 3: conduct check calculation.

Heat sink design method

Natural cooling heatsink design method

Considering the natural cooling temperature boundary layer thick, if tooth spacing is too small for two teeth of thermal boundary layer is easy to cross the influence of the tooth surface, convection, so in General, the recommended natural cooling heat sink teeth spacing greater than 12mm, if the heat sink teeth high below 10mm, tooth pitch ≥ 1.2 tooth height to determine heat sink teeth spacing.

Natural cooling radiator surface heat capacity is low, increase in heat-tooth surface corrugation does not have the effect of natural convection is too large, it is recommended that the heat surface of teeth without ripple teeth.

Natural convection radiator surface blackening treatment generally used to increase the cooling surface of the radiation factor, strengthening radiation heat.

Due to natural convection of heat balance reached for a longer time, so the natural convection radiator of substrate and tooth thickness should be sufficient to combat the impact of instantaneousheat load, recommends ¡ôduring above.

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