Light-emitting diodes (LED), like any semiconductor, emit heat during their operation. This is because not all of electrical energy provided to a LED is converted to luminous energy (just about 30%). Thus, a significant portion of the electrical energy is converted to thermal energy which results in an increase in the temperature of the LED. As result the important electrical and optical parameters of LED will be changed as function of the operating temperature of device (LED fixture itself). It causes some negative effects in the LED fixture operating:
- increasing of load on related LED driver as result of decreasing of forward voltage drop of the LED;
- shifting of wavelength of emitted light (can cause orange LED lights to appear red or even white LED lights to appear bluish);
- loosing of efficiency in light output;
- thermally stressed LED junction may break down causing a state of complete thermal runaway and result a catastrophic failure (broken wire bonds, delaminating, internal solder joint detachment, damage to die-bond epoxy, and lens yellowing).
That is why it is highly necessary to keep the operating LED temperature (T*s – temperature of soldering point on PCB) below critical value (90-120) °C to run the LED’s which means obviously to apply cooling to LED fixture. The various combinations of methodologies, materials, devices for this cooling could be defined as Thermal Management of LED fixture which has become already the crucial area of LED fixture design. Accordingly, the main factor of LED fixture design involves the heat transfer away from LED engine (LED array on PCB, chip-on-board COB) in the most effective and fast way.
Physically, heat transfer process for LED cooling consists of two components: conduction and convection (radiation could be considered as negligible because of relatively low temperature). Thus we can consider heat transfer from hottest point (Ts) in the construction of LED fixture to coldest ambient point (Ta) as two stage process: thermal conduction through metal elements of fixture (PCB, heat sink, enclosure) and convective heat transfer by heat dissipation from solid surface (heat sink) to air.
Technically, these components could be represented by two values of thermal resistance accordingly:
- Rtk – thermal conduction component and
- Rth – thermal convection component and could be used in thermal design of LED fixture.
Theoretically these thermal resistances could be defined on the base of fundamental physics laws, Rtk from Fourier’s law and Rth from Newton’s law. However in practice it is difficult to calculate that values because of complexity of thermal path in real construction of LED fixture (at least you will be involved in thermal modeling and computing).
Meantime we will show in our samples of thermal design and projects that original methodology could be applied for analyzing and optimizing cooling system which is based on the processing of thermal transient function of LED fixture under test. In this way you can use relatively simple instrumentation and conduct experiments under condition regular workshop or “home lab”.
LED cooling can be realized in two different ways in principle: as combination of thermal conduction and natural convection thermal transfer without any additional source of energy (passive cooling); and as combination of thermal conduction and forced convection with some additional source of energy (active cooling). Passive cooling can be provided for LED fixtures with up to (15-20) W of input electrical power, for more powerful applications Active cooling is recommended to avoid loosing of efficiency. We will consider in details both of LED cooling technologies.