LED semiconductor light source characteristics and related thermal management

LED semiconductor light source features

Unlike incandescent lamps, conventional fluorescent lamps, and halogen lamps, LED semiconductor light sources are made of semiconductor materials. They are formed by a PN structure. Hole-to-electron pairs produce light, working in the positive direction of the PN junction, and the P region is positive ( Yang) pole, N zone is negative (female) pole. The LED semiconductor light source has small volume, high luminous efficiency, short response time and energy saving. In addition, it has features not found in traditional light sources:

1. Similar characteristics to general PN junction devices such as diodes:

The forward voltage must exceed a certain threshold to have current;

Both forward voltage and forward current are negative temperature coefficients, which decrease with increasing temperature;

In the reverse direction, no current does not work.

2. Like all semiconductor devices, their operating temperature is subject to the following factors:

The junction temperature must be kept below the rated value of 95 °C to 125 °C (depending on the light-emitting device), otherwise it will cause failure;

If the surface has a plastic lens, it will also be limited by the melting 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.

There are two possibilities for LED failure modes: Light degradation and Total failure. Light decay occurs when the emitted light drops to 50% of its initial value; in addition to exceeding the maximum allowable junction temperature, the overall failure occurs due to internal open circuit, which includes: between the chip and the lead frame, the chip and Between the bonding wires, and between the bonding wires and the lead frame. One of the causes of failure is that the LDE resin glass lens is overheated, softened, and the stress generated after cooling causes an internal open circuit.

It is important for users to understand these characteristics, especially their thermal characteristics. This can't help reminding me of the situation when transistors were replaced by electronic tubes in electronic circuits. Due to the sensitivity of transistors as semiconductor devices to temperature, at the beginning of the application, some engineers who used to be familiar with tube applications thought that transistors have many advantages. However, its reliability is not as good as that of electronic 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 stable operating points has made transistors and semiconductor integrated circuit technology become today. The core technology of electronic information technology. In the field of light source technology, LED application technology will also experience how to develop and avoid short.

Thermal Management Design and "Thermal Ohm's Law"

The purpose of the thermal management design is to:

· Ensure that the device is operating under the right conditions to achieve high reliability;

· Prevent driving under super-stress conditions and prolong the working life of LEDs;

• Operates at the maximum possible current to improve light output performance.

The main point of thermal management is to keep the LED operating temperature within a reasonable range through heat conduction and heat dissipation. Usually relying on heat conduction to direct the heat of the LED to the heat sink, and then dissipating the heat buried in the heat sink, this "guide" and "scatter" is very important, and the heat dissipation depends not only on conduction but also on conduction. Convection and radiation.

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

In analyzing current transmission, ohm establishes 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 resistance R. In the case of heat flow, there is a law of a form similar to its form: â–³T=Rth*Po is also referred to by some applicants as "thermal Ohm's law" (actually this law is independent of ohms).

Here Rth denotes the thermal resistance, which characterizes the resistance of heat flow. The unit is °C / W;

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

ΔT represents the temperature difference between two points in the middle of the heat flow transmission, that is, the temperature difference between the two points.

When detecting electronic circuits, we often use a multimeter to detect the potential and potential difference of the relevant node, that is, the voltage. In the detection of heat flow transmission, the temperature and temperature difference of the relevant nodes on the heat flow transmission path can be detected by using a thermometer, a thermocouple and an infrared camera.

In Ohm's law, the currents in the series circuit are equal everywhere, but the heat flow is not the case. At some points, the heat flow is blocked due to excessive thermal resistance, and heat is accumulated.

What can be detected and estimated by "Thermal Ohm's Law" are:

Similar to the establishment of an equivalent circuit in circuit analysis, an equivalent heat flow path map can also be established during heat flow analysis.

Detecting and estimating the LED junction temperature Tj;

Identify the heat dissipation effect between the relevant nodes, the size of the thermal resistance;

Evaluate the pros and cons of LED operation when using different material heat sinks.

There are several important temperature nodes in the heat flow analysis:

The junction temperature Tj of the chip PN junction should be less than the rating specified by the product to allow it to operate within a safe range.

Solder joint temperature Ts, that is, the temperature at the LED terminal and the pad of the base plate.

Radiator sheet and external environment interface temperature Ta

In order to dissipate the heat source LED and keep the junction temperature Tj at a reasonably safe value, in order to obtain the maximum forward current allowed by the device, the highest illuminating effect is the key.

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 an SMT package structure (SMD type) LED, and the preliminary experiment of the effect of using different bulk materials on the performance of the LED.

1. Equivalent heat flow diagram

Figure 1 and Figure 2 show the internal structure of the SMT package (ie SMD type) LED and the static equivalent thermal path.

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

The arrows in the figure refer to the heat flow transmission path.

Figure 2: Static equivalent heat path diagram of SMD type LED (Click the picture to view the original picture)

In this static equivalent thermal path, the internal thermal resistance is formed by four parts in series, namely 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 the specific application conditions. For example, if the LED is assembled on the 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 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°C is measured with a point thermometer, and the applied forward voltage U is 2.1V, and the forward current is 50mA. In the product data sheet, the thermal resistance value of LAE67B is found to be 130 oC /W, and it is assumed that the electric power is all converted into heat flow, calculated according to the "thermal Ohm's law":

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

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

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

It can be seen from the above analysis that the temperature has a great influence on the luminescence performance, life and reliability of the LED, and the influence of the heat dissipation effect on the performance of the LED is a subject of extensive discussion. This example is only a preliminary experiment.

In the experiment, for the same LED, aluminum and heat-conducting graphite heat sinks were respectively used for heat dissipation, and the same forward voltage was applied to record the forward current value, solder joint temperature and illuminance value. The experimental results show that the thermal resistance of the thermally conductive graphite material is much smaller than that of aluminum. After the LED is lit, the temperature of the graphite radiator at the lower current is heated at a faster rate. After a period of time, the temperature is slightly higher than the temperature of the aluminum plate radiator. It is also slightly higher, and the difference is gradually obvious under the conditions of long-time opening and large current operation.

Conclusion

This paper 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 attenuation of the LED lamp are related to temperature. The factors affecting are: the ambient temperature of the LED; the heat conduction channel between the LED junction and the outside; the energy released by the chip. Although the main point of the thermal management design and implementation is that the "heating" of the LED heat is reduced, the thermal resistance of each part is reduced. But it still involves many aspects:

Avoid external heat transfer to the LED junction point, so that the Ta temperature rises (such as separating the drive circuit from the LED circuit board);

LED pad design and assembly process, considering the factors of thermoelectric compatibility;

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

In view of the limited space, it will not be repeated. The thermal resistance of the package and the thermal resistance of the external heat sink are closely related to the thermal conductivity and assembly technology of the materials used. These are the hot topics of concern to the author and colleagues in the industry. We look forward to making new progress in this field.