In addition to the standard mechanical solutions , as well as monitoring the thermal behavior of electrical technology in order to stabilize the system and adjust the heat control . When the increase in high-brightness LED forward current is reduced package size , heat dissipation and also increases the potential catastrophic failure . Among the many LED applications, due to extreme high temperatures , you need a higher level of protection.

LED thermal foldback to reduce failures and avoid the common method for LED overheat and cause shortened life expectancy . This method uses a control signal is inversely proportional to temperature , the set temperature after the lower LED current breakpoint . This method can be achieved through a variety of ways . Here are two examples: a 100W 12W street light applications and a military flashlight application . Describes the differences between these two examples and their design process more complex systems with relatively simple systems.

Programme Background

In the tradition of using high-power LED lighting applications , require large LED heat sink to discharge released . LED itself does not heat , on the contrary , they are to conduct heat through a semiconductor junction . This conduction power (PD) is equal to the voltage before (VF) and forward current (IF) of the product. PD = VF × IF

In order to maintain a safe LED junction temperature must eliminate the conduction power . The need for thermal impedance of the system is analyzed in order to customize a radiator at rated power to ensure that the desired thermal characteristics.

A typical high power LED through its components, solder connections , printed circuit boards and heat sinks most of the power consumption . As shown in Figure 1 . Using this simple model calculation is quite simple. LED junction power dissipation (PD), must pass through the junction - ambient total thermal resistance (θJA) allocation , which is very similar to the current through the electrical resistance .


Temperature junction temperature (TJ) and the resulting ambient temperature (TA) difference between (TJ-TA) is equal to an electrical voltage ( Ohm's law of thermal equivalent ): TJ-TA = PD × θJA

θJA represents the sum of the following values ​​.
θJS: Thermal resistance junction to solder joints ;
θSH: tin solder to heat sink thermal resistance ;
θHA: tin solder to ambient thermal resistance.

θJS behalf of internal LED thermal resistance and representative of the printed circuit board θSH (PCB) dielectric and thermal resistance junction . Finally , θHA behalf heat sink thermal resistance , θJS value of the specified value LED manufacturer's data sheet, and is a simple function of the LED package . It may be in the range of 2 ~ 15 ℃ / W changes. If the connection to the radiator from the tin solder good ( including : multiple thermal vias , the amount of copper , good welding and thermal plastic may be used ), θSH basically negligible . This produces a very low θSH value less than 2 ℃ / W 's .

θHA remain unchanged because it depends more on the radiator surface area and thermal conductivity. In a standard FR4 ​​printed circuit board ( similar to LED 's size) , no external heat sink , just at the bottom of the copper layer , θHA value may be very large, more than 100 ℃ / W. By external heat sink shown in Figure 1, can reduce the thermal resistance to maintain the desired temperature difference (TJ-TA). ΘHA thermal design needs based on the following equation , choose the right radiator : Through this equation can be easily calculated , if the power to increase or reduce the allowable temperature difference , the necessary thermal resistance decreases, which is equivalent to the need for a greater radiator.

Practical applications, the system service life , due to the former presence of voltage and other electrical bias, LED power output will increase by 5 % to 10% forward . The temperature rise may be calculated based on the range of the worst-case expected TA value. In addition, the manufacturer 's specifications , usually TJ reduce the maximum allowed value , to ensure that the LED life and efficiency is not reduced. These tolerances forced us to improve the worst-case thermal design standards , increase 25% to 50% than the calibration.