LED and related thermal management

As a new type of semiconductor light source, LED has been increasingly valued by the industry. LED lights and backlight sources have been widely used in many fields. This article will introduce some characteristics of LED semiconductor light sources and the purpose and key points of thermal management, "Thermal Ohm's Law", heat flow transmission and node temperature detection and analysis methods, and preliminary experimental results of the comparison of the application of thermal conductive graphite and aluminum heat sinks for readers' reference

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Unlike incandescent lamps, traditional fluorescent lamps and halogen lamps, LED semiconductor light sources are made of semiconductor materials and consist of a PN junction. Hole-electron pairs recombine to generate light and work in the forward direction of the PN junction. The P region is the positive (anode) electrode and the N region is the negative (cathode) electrode. LED semiconductor light sources are small in size, high in luminous efficiency, short in response time and energy-saving. In addition, they have characteristics that traditional light sources do not have:
1. Similar characteristics to general PN junction devices (such as diodes):
forward voltage must exceed a certain threshold to have current; both
forward voltage and forward current have negative temperature coefficients and decrease with increasing temperature;
in reverse, there is no current and it does not work.
2. Like all semiconductor devices, its operating temperature is subject to the following factors:
the junction temperature must be kept below the rated value of 95 oC ~ 125 oC (depending on the light-emitting device), otherwise it will cause failure;
if there is a plastic lens on the surface, it will also be limited by the melting point temperature of the lens material;
the brightness of the LED is related to the forward current, and after the junction temperature exceeds a certain value, the forward current decreases and the brightness decreases.
Usually, there are two possible failure modes of LED: light degradation and total failure. Light degradation occurs when the emitted light drops to 50% of its initial value. In addition to total failure caused by exceeding the maximum allowable junction temperature, total failure also occurs due to internal open circuits, including: between the chip and the lead frame, between the chip and the bonding wire, and between the bonding wire and the lead frame. One of the reasons for failure is that the LDE resin glass lens is overheated, softened, and then cooled. The stress generated causes an internal open circuit.
It is very important for users to understand these characteristics, especially their thermal characteristics. This reminds me of the scene when transistors replaced electron tubes in electronic circuits. Due to the sensitivity of transistors as semiconductor devices to temperature, at the beginning of their application, some engineers and technicians who were familiar with the application of electron tubes in the past believed that although transistors had many advantages, their reliability was not as good as electron tubes. However, the power of new things is unstoppable. With the advancement of application technology, the use of temperature compensation and negative feedback to suppress temperature drift and stabilize the operating point has made transistors and semiconductor integrated circuit technology the core technology of today's electronic information technology. In the field of light source technology, the application technology of LED will also go through the process of how to make the best use of its strengths and avoid its weaknesses. Thermal

management design and "Thermal Ohm's Law"
The purpose of thermal management design is to ensure that the device works under appropriate conditions to achieve high reliability; prevent driving under overstress conditions to extend the working life of the LED; work at the maximum possible current to improve the light output performance. The key points of thermal management are: to keep the operating temperature of the LED within a reasonable range through heat conduction and heat dissipation. Usually, the heat of the LED is conducted to the heat sink by heat conduction, and then the heat "buried" in the heat sink is dissipated. This "conduction" and "dissipation" are very important and indispensable, and heat dissipation depends not only on conduction but also on convection and radiation. When conducting thermal management analysis, the commonly used basic law is the heat flow law, the so-called "thermal Ohm's law". When analyzing current transmission, Ohm established the well-known Ohm's law, namely: U=R*I, where R is the resistance, I is the current, and U is the potential difference across the resistor R. In the case of heat transfer, there is a law similar in form: △T=Rth*Po, which is also called "thermal Ohm's law" by some users (in fact, this law has nothing to do with Ohm). Here Rth represents thermal resistance, which characterizes the resistance to heat transfer. The unit is oC/W; Po is heat flow, that is, the amount of heat transferred per unit time Po=Q (heat)/t (time), and the dimension is the same as power. △T represents the temperature difference between two points in the heat transfer process, that is, the temperature difference on the thermal resistance between these two points. When testing electronic circuits, we often use a multimeter to detect the potential and potential difference of the relevant nodes, that is, the voltage. When testing heat transfer, point thermometers, thermocouples and infrared thermal imagers can be used to detect the temperature and temperature difference of the relevant nodes on the heat transfer path. In Ohm's law, the current in the series circuit is equal everywhere, but this is not the case for heat transfer. At some points, the heat transfer will be blocked due to excessive thermal resistance, causing heat accumulation. The following can be detected and estimated using "Thermal Ohm's Law": Similar to establishing an equivalent circuit in circuit analysis, an equivalent heat flow path diagram can also be established during heat flow analysis. Detect and estimate the LED junction temperature Tj; determine the heat dissipation effect and thermal resistance between related nodes; evaluate the quality of LED working conditions when using heat sinks of different materials. There are several important temperature nodes in heat flow analysis: the junction temperature Tj of the chip PN junction should be less than the rated value specified by the product to make it work within a safe range. The solder point temperature Ts is the temperature at the LED lead end and the base plate pad. The interface temperature Ta between the heat sink and the external environment should be able to dissipate the heat generated by the heat source LED, so that the junction temperature Tj is maintained at a reasonable and safe value, in order to obtain the maximum forward current If allowed by the device and obtain the highest luminous effect. Analysis Examples The three examples to be introduced here are: the establishment of a heat flow diagram, the calculation of the junction temperature Tj of a certain SMT packaging structure (SMD type) LED, and the preliminary experiment on the influence of different bulk materials on LED performance. 1. Equivalent heat flow diagram Figure 1 and Figure 2 are the internal structure diagram and static equivalent heat path of SMT package (i.e. SMD type) LED respectively.

Figure 1 Internal structure of SMD LED
The arrows in the figure indicate the heat transfer path.

Figure 2 Static equivalent thermal circuit diagram of SMD LED
In this static equivalent thermal circuit, the internal thermal resistance is composed of 4 parts in series, that is, internal thermal resistance = chip thermal resistance + chip bonding (attachment) thermal resistance + lead frame thermal resistance + solder joint thermal resistance. The external thermal resistance is determined by specific application conditions. For example, if the LED is assembled on a PCB board, its external thermal resistance = pad thermal resistance + PCB thermal resistance.
Po is the heat flow, Tj is the junction temperature, Ts is the solder joint temperature, and Ta is the ambient interface temperature.
2. Junction temperature detection and estimation:
For a certain (LAE67B) SMT package structure LED, the solder joint temperature Ts = 70 oC was measured with a spot thermometer.
At the same time, the external forward voltage U was measured to be 2.1V and the forward current was 50mA. The thermal resistance of LAE67B is 130 oC /W, and assuming that all electrical power is converted into heat flow, the calculation is based on "Thermal Ohm's Law":
Tj=130130 oC /W *50mA*2.1V+70 oC
=83.7 oC.
The actual junction temperature Tj is less than the maximum allowable junction temperature of 125oC, so it is safe to work.
3. Preliminary experiment on the effect of different heat sink materials on LED performance

From the above analysis, it can be seen that temperature has a great influence on the luminous performance, life and reliability of LED. The influence of heat dissipation effect on the performance of LED is a very wide-ranging research topic. This example is only a preliminary experiment.
In the experiment, for the same LED, aluminum and thermally conductive graphite heat sinks were used for heat dissipation, and the same forward voltage was applied to record the forward current value, solder point temperature and illumination value. The experimental results show that; because the thermal resistance of thermally conductive graphite material is much smaller than that of aluminum, the temperature Ta at the graphite heat sink rises quickly at a lower current after the LED is lit. After a period of equilibrium, it is slightly higher than the temperature at the aluminum plate heat sink. The former is also slightly brighter. When it is turned on for a long time and works at a larger current, the difference gradually becomes obvious.


Conclusion
This article introduces the importance, purpose requirements, design management points, detection and analysis methods and case analysis of thermal management design for LEDs. The overall failure and light loss of LED lamps are related to temperature. The influencing factors include: the ambient temperature of the LED; the heat conduction channel between the LED junction and the outside; the energy released by the chip, etc. Although the key point of thermal management design and implementation is to "conduct" and "dissipate" the heat of LED to reduce the thermal resistance of each part. But it still involves many aspects:
prevent external heat from being transferred to the LED junction point to increase the temperature of Ta (such as separating the drive circuit and the LED circuit board);
LED pad design and assembly process, to consider the factors of thermoelectric compatibility;
the most important is: the selection and assembly of heat sink (device) (including assembly position and orientation), also including the selection of new thermal conductive materials and heat sink;
due to limited space, no more details. The thermal resistance of the package and the thermal resistance of the external heat sink are closely related to the thermal conductivity of the materials used and the assembly technology. These are the hot topics that the author and colleagues in the industry are concerned about, and we look forward to making new progress in this field.