1. The purpose of LED heat dissipation design
When designing products that use LEDs, you must pay attention to the heat that may be generated
The usable temperature of the LED is defined by the junction temperature (Tj). If the junction temperature Tj is higher than the maximum value, it may cause significant brightness degradation, or even cause it
Other failures (such as dead light due to disconnection, etc.), so please do not use it when the maximum junction temperature Tj is exceeded.
In addition, lowering the junction temperature as much as possible can extend the life of the product.
For the above reasons, heat dissipation design is very important when using LEDs.
This guide will introduce some ideas about LED heat dissipation design.
2. Regarding the flow of LED heat
Regarding the heat generated by the LED, its flow direction can be illustrated in Figure 1.
The heat of the chip will be transferred to the surrounding air through the die-bonding, electrodes, tin in the soldering part, and circuit boards.
The heat emitted from the LED chip is expressed in terms of thermal resistance (℃/W), and the following two are the formulas for calculating the junction temperature Tj.
When using the thermal resistance Rthj-a from the LED chip to the surrounding air:
Tj = Ta + Rthj-a × W ①
・Ta: Ambient temperature (℃)
・Rthj-a: Thermal resistance from LED chip to surrounding air (℃/W)
・W: Input power (=IF×VF) (W)
(※IF: forward current (A), VF: forward voltage (V))
(2) When using the thermal resistance Rthj-s from the LED chip to the negative solder pin:
Tj = Ts ＋ Rthj-s × W ②
・Ts: The temperature of the negative welding leg (℃)
・Rthj-s: Thermal resistance from LED chip to Ts test point (℃/W)
・W: Input power (=IF×VF) (W)
(※IF: forward current (A), VF: forward voltage (V))
3. Tj calculation method
(To calculate the junction temperature Tj, the methods are divided into two categories: (A) Measure the Ts temperature and use it to calculate Tj; (B) VF measurement method. The following will explain in detail.
(A) Measure Ts temperature and calculate Tj
(1) As shown in Figure 3, connect the Ts temperature measurement point (negative electrode) of the LED to the thermocouple and light it, and then measure the Ts, If, and Vf values when it reaches the thermal equilibrium state.
※The Ts temperature measurement point and thermal resistance Rthj-s of each product are different. For details, please refer to our company's product specifications.
In order to minimize the effect of thermocouples on the release of heat, please try to use thinner thermocouples.
It is also recommended to connect the thermocouple to the temperature measuring point with solder.
4. LED related heat dissipation design
In product design, improve the heat dissipation capacity (reduce the overall thermal resistance,
It can effectively reduce the junction temperature Tj during operation.
The following suggestions can be referred to when designing.
(A) Selection of circuit board material
(B) Optimize the copper foil area on the circuit board
(C) Optimize the configuration of LEDs (LED pitch)
(D) Use heat sink
The following is a detailed description of each item.
(A) Selection of circuit board material
There are many types of circuit boards, as shown in Figure 6, roughly divided into resin, metal or ceramics.
(D) Use heat sink
Adding a heat sink to the back of the circuit board can improve the heat dissipation effect.
The following table 3 has the comparison test results of whether the heat sink is installed or not.
From the test results, it is also pointed out that because of the heat sink, Rthj-a and Tj are also reduced.
In addition, at the joint position between the bottom plate and the heat sink, double-sided stickers with high heat transfer ability, heat sink, or thermal paste can be added to make the heat dissipation effect.
Better... Figure 11 shows an example of a heat sink bonding method.
5. Summary
I hope that with the heat dissipation design cases introduced in this guide, you can use the LED's efficiency and improve reliability in product design more effectively.
Produce products with more stable performance.
