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The spectrum of heat dissipation design of UV LED




1. The purpose of heat dissipation design
When designing products that use UV LEDs, you must pay attention to the heat that may be generated
The usable temperature of the UV 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 UV LEDs.
This guide will introduce some ideas about UV LED heat dissipation design.
2. Regarding the flow of UV LED heat
Regarding the heat generated by the UV LED, its flow direction can be illustrated in Figure 1.
, And the circuit board, etc. to the surrounding air.
The heat emitted from the UV 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 UV LED chip to the surrounding air:
 
Tj = Ta + Rthj-a × W ?
Okay: 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 ?
'Ts'
?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 UV 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.
,
It is also recommended to connect the thermocouple to the temperature measuring point with solder.
About thermal design
In product design, improve the heat dissipation capacity (reduce the overall thermal resistance,
'Tj'
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 UV LEDs (LED pitch)
(D) Use heat sink
 The following is a detailed description of each item.
'A'
There are many types of circuit boards, as shown in Figure 6, roughly divided into resin, metal or ceramics.
'D'
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.
, Add double-sided stickers with high heat transfer ability,, or thermal paste can make it 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 UV LED's efficiency and improve reliability in product design more effectively.
Produce products with more stable performance.

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