The basics of LED cooling and LED heat sinks

  Introduction

      In this document we give you a simple,

      straight forward approach how you can determine the correct LED heat sink for your new LED lighting design

      This is a simplified approach of the integral model and verification test has of course to be done,

      but it will give you enough insights to make sure both the functional integrity

      as well as the operational reliability of your design will meet the market expectations

 

  Your design – your parameters

       Each LED lighting design has it’s own specific parameters,

       so each designer needs to take these specifics in to account and need to define his specific starting points

       Some examples:

       Open air mounted spot light : 30°C

       Recessed ceiling downlighter : 50-55°C

       Automotive lighting : 45°C (design point only)

       Tlife = -40°C up to +85°C

            

       As example we take a LED COB module from the Citizen Citiled CLL020 series

       Model CLL020-1203A1-303M1A2

       Forward current If 360mA – forward voltage Vf 40.9Vdc – power 14.7W

       Luminous flux 1210lm – CCT 3000K

       Maximum case temperature Tc 100°C - Maximum junction temperature Tj 150°C

       Thermal resistance of the COB module Rj-c 2.6°C/W

       Citizen guarantees for this module a 50.000hr life time (conditional, 70% of remaining flux)

       Most LED COB module manufacturers just provide lifetime expectations under ideal conditions,

       like 25°C ambient temperature

       We calculate with 90% reliability on these for the maximum

       junction temperature we want in our design  – example below

       If a B10,L70 curve is available, we suggest you determine

       your required lifetime and read out the maximum junction

       temperature Tj related

       In this case we want to keep our junction temperature below 90% of the

       Tj max => Tj required < 150°C x 90% < 135°C

       The basics to do that is to understand the scheme at the right Each part of the design adds up some heat due to

       individual thermal resistances of each material – the adding up can be calculated as T = Pd x Rth

       In this case we have the thermal resistance of the Citizen LED COB module (Rj-c),

       the thermal resistance of a gap filler (thermal pad or grease) we want to place between the COB module and the heat sink (Rb),

       and the thermal resistance from our heat sink (Rh) which has to make that the total design stays below

       the maximum required junction temperature Tj

  

      If our led light is in a recessed environment I want to calculate with an ambient temperature Ta of 45°C

      Means the maximum temperature added in the total design is Tj – Ta = 135°C – 45°C = 90°C

      The total power to dissipate is of course lower than the total power the LED consumes

      Some part of the power becomes light – the more efficient your LED module,

      the bigger part of the total power will be transferred in to light, easy to verify if you compare the luminous flux to the power

      As a fist rule we use 80% of the total power to be dissipated (Pd)

      Pd = 14.7W x 80% = 11.76W

 

      Now we just define mathematically what would be the maximum thermal resistance our heat sink should have,

      or define the maximum raise in temperature our heat sink will create when dissipating Pd 11.76W

 

      Suppose we will use a phase change gap filler thichness 0.18mm (thermal pad which becomes fluid on first heating cycle)

      with a thermal resistance of 0.4°C/W

      Let’s see what we know already and what is missing

      Only thing missing now is the needed thermal resistance of the heat sink Rh

      Choose a heat sink with an Rth value of < 4.65°C

      OR (see notes below)

      Choose a heat sink which guarantees less than (99.7°C – 45°C) = 54.7°C

      heating at a dissipated power Pd of 11.76W

 

    

       After applying the thermal pad and the heat sink,

       verify the design Some LED COB module manufacturers foresee a thermal measurement point at the case

       Remember for this Citizen COB module Tc measurement max was 100°C

       Since we designed with some safety margins you should measure a temperature around 87-92°C

       －Some manufacturers give a single thermal resistance value Rth for the heat sink,

             independent on ambient temperature and power to dissipate

             Please be aware the Rth of the same heat sink will not be equal under all conditions

       －The approach made in this documents don’t take in effect that the heat spreading will become more and more difficult

             with the COB modules becoming smaller and smaller

             A COB LED module of 20x20mm for 20W or a COB LED module of 40x40mm for 20W can have a total different heat

             conduction towards the heat sink