Characteristics of LED semiconductor light sources and related thermal management

As a new type of semiconductor light source, LED has been increasingly valued by the industry. LED lamps 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 related thermal management (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.

Characteristics of LED semiconductor light sources

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):

The forward voltage must exceed a certain threshold for current to flow;

Both the forward voltage and the forward current have negative temperature coefficients and decrease as the temperature increases;

In reverse direction, 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℃~125℃ (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. When 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 failure causes is that the LDE resin glass lens is overheated, softened, and the stress generated after cooling causes an internal open circuit.

It is very important for users to understand these characteristics, especially their thermal characteristics. This reminds me of the time when transistors replaced electron tubes in electronic circuits. As semiconductor devices, transistors are sensitive to temperature. At the beginning of their application, some engineers and technicians who were familiar with the application of electron tubes believed that although transistors have many advantages, their reliability is 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 LEDs will also go through the process of how to make the best use of their strengths and avoid their weaknesses. Thermal management design and "Thermal Ohm's Law"

The goals of thermal management design are to:

Ensure that the device works under appropriate conditions to achieve high reliability;

Prevent driving under overstress conditions and extend the working life of LEDs;

Operate at the maximum possible current to improve light output performance.

The key point of thermal management is to keep the LED operating temperature 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. Heat dissipation depends not only on conduction but also on convection and radiation.

When conducting thermal management analysis, the basic law commonly used is the law of heat flow, the so-called "thermal Ohm's law".

When analyzing current transmission, Ohm established the well-known Ohm's law: U=R*I, where R is the resistance, I is the current, and U is the potential difference across the resistor R. When analyzing heat flow transmission, there is a law with a similar 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 stands for thermal resistance, which represents the resistance to heat flow. The unit is ℃ /W;

Po is the heat flow, that is, the heat transferred per unit time Po=Q (heat)/t (time), and its dimension is the same as power .

△T represents the temperature difference between two points during heat flow transmission, that is, the temperature difference in thermal resistance between these two points.

When testing electronic circuits, we often use a multimeter to test the potential and potential difference of related nodes, that is, voltage. When testing heat flow transmission, we can use a point thermometer, thermocouple, and infrared thermal imager to detect the temperature and temperature difference of related nodes on the heat flow transmission path.

In Ohm's law, the current in a series circuit is equal everywhere, but this is not the case for heat flow. At certain points, the heat flow 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 thermal flow analysis.

Detect and estimate 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 ensure that it operates within a safe range.

The soldering point temperature Ts is the temperature between the LED lead end and the base plate soldering pad.

The interface temperature between the radiator and the external environment Ta

The key is to dissipate the heat generated by the LED and keep the junction temperature Tj 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 Example

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 a preliminary experiment on the effect of different bulk materials on LED performance.

1. Equivalent heat flow diagram

Figures 1 and 2 are the internal structure diagram and static equivalent thermal circuit of an SMT package (i.e., SMD type) LED, respectively.

Figure 1: Internal structure of SMD LED (click on the image to view the original image)

The arrows in the figure indicate the heat flow transmission 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. External thermal resistance is determined by specific application conditions. For example, if the LED is assembled on a PCB , its external thermal resistance = pad thermal resistance + PCB thermal resistance.

Po is the heat flow, Tj is the junction temperature, Ts is the solder point temperature, and Ta is the ambient interface temperature.

2. Junction temperature detection and estimation:

For a LED with a certain (LAE67B) SMT package structure, the solder point temperature Ts = 70°C 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 value of LAE67B was found to be 130 oC /W in the product data sheet, and assuming that all electrical power is converted into heat flow, the calculation is based on the "thermal Ohm's law":

Tj=130130  ℃   /W *50mA*2.1V+70  ℃   =83.7  ℃

The actual junction temperature Tj is less than the maximum allowable junction temperature of 125°C, and the operation is safe.

3. Preliminary experiment on the effect of different heat sink materials on LED performance

From the above analysis, we can see that temperature has a great influence on the luminous performance, life and reliability of LEDs. The influence of heat dissipation on the performance of LEDs is a very extensive 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, the same forward voltage was applied, and the forward current value, solder point temperature and illumination value were recorded. The experimental results show that since 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, and 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, 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: ambient temperature of the LED; heat conduction channel between the LED junction and the outside; energy released by the chip, etc. Although the key point of thermal management design and implementation is to "conduct" and "dissipate" the heat of the 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, causing the temperature of Ta to rise (such as separating the drive circuit and LED circuit board);

The LED pad design and assembly process must consider thermal and electrical compatibility factors;

The most important thing is: the selection and assembly of the heat sink (including the assembly position and orientation), as well as the selection of new thermal conductive materials and heat sinks;

Due to limited space, I will not elaborate on this. However, 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 his colleagues in the industry are concerned about, and we look forward to making new progress in this field.