Abstract

In this article, thermal management, measurement techniques of , as wl as MCPCB (metal core printed circuit board) characteristic will be presented. This paper introduces the background and importance of thermal management of LED lighting when designing lighting fixtures by LED light source.

Controlling the LED junction temperature

To extend LED life time by maintaining the lumen output and increases the reliability of LED lighting fixtures, Therefore, It is very important to control and monitor the junction temperature of LED chip when designing lighting appliances by LED.

However, the LED chip is sat on the substrate of LED package where it is not possible to do direct measurement. Therefore, estimating junction temperature becomes an important topic in designing with LED.

Fig.1 Junction temperature will affect the long term lumen maintenance of LED

Fig.2 Location of LED chip for Cree XLamp LED

Estimating the junction temperature

To estimate the junction temperature, designer should consider the solder point temperature of the LED, or the area closest to the LED as shown in Figure 3.

Once the soldering point temperature obtained through thermocouple, the junction temperature can be calculated.

Fig. 3 Measuring LED solder point temperature in XR LED

Example: XLamp LED driving by 350mA, forward voltage drop at 3.3V. The measured solder point temperature is 51.44C. The LED chip temperature can be calculated as:

LED power consumption: 3.3V x 350mA = 1.155W

Junction Temperature = Solder point temperature + thermal resistance x power

                     = 51.44 + 8C/W x 1.155

                     = 60.44C

Therefore, by this method, designer can design the required thermal resistance of heatsink and MCPCB by first pre-setting the junction temperature of LED. In fact the thermal management for most high power LED lighting application will require 3 major components as:

Thermal management system of LED = LED + MCPCB + Heatsink

The total thermal resistance is:

Metal Core Printed Circuit Board Characteristic:

MCPCB is the major component in LED lighting thermal management. The function of MCPCB is acting as a media to conduct the heat from high power LED chip to the heatsink and to the ambient. The lower the thermal resistance of the MCPCB, the higher the heat conductivity to the high power LED.

Figure 4 demonstrate the structure of MCPCB:

Fig.4 Internal structure of MCPCB

The base on the MCPCB is 1.5mm thick Aluminum. The 60um thermal adhesive can withstand up to 3KV insulation breakdown voltage. Typical thermal resistance of Arrow lighting MCPCB is 0.2C/W where makes it a perfect media for heat conduction.

Since 2007, Arrow light has developed a series of MCPCB which suitable for various high power LED for Cree. These include series and parallel connection of MCE, XPE, XRE, CLN6 in star board shape. In the meantime, Arrow lighting also introduces 3 in 1 MCPCB which suitable for MR16 and E27 retrofit lighting application. Table 1 demonstrates a summary showing common MCPCB part available by Arrow lighting.

Fig.5 1 panel of Cree XP LED PCPCB contains 20pcs of boards

Table 1. MCPCB available parts

Remarks: For XRE LED, it also includes thermal conductivity at 1.2W/K, 6 terminals as well as round shape MCPCB. Please consult Arrow business development specialist for more information.

For certain application, Arrow lighting will offer case by case circuit design for customers. Meanwhile, the MCPCB characteristic and specification can be modifided base on customer request.

References:

1) “XLamp LED thermal Management”, Cree Inc, 24/07/2007

2) “XRE LED MCPCB Specification”, Arrow lighting, 03/2008

3) “FAE training Thermal”, Cree Inc, 17/01/2008