Heat dissipation design for high-power LEDs

In recent years, high-power LEDs have developed rapidly, with great improvements in structure and performance, increased production and reduced prices. Ultra-high-power white LEDs with a single power of 100W have also been developed. Compared with previous years, there has been great progress in luminous efficiency. For example, Edison's 20W white LEDs in previous years had a luminous flux of 700lm and a luminous efficiency of 35lm/W. The 100W white LED developed in 2007 had a luminous flux of 6000lm and a luminous efficiency of 60lm/W. For another example, the K2 white LED recently developed by Lumiled is compared with similar products in its I and III series as shown in Table 1. It can be seen from the table that the K2 white LED has made great improvements in luminous flux, maximum junction temperature, thermal resistance and outer dimensions. Cree's newly launched XLamp XR~E cold white LED has a luminous flux of 107~114lm at the highest brightness block QS of 350mA. These high-power LEDs with good performance have created conditions for the development of LED white light lighting fixtures.

In the past few years, various white light LED lighting fixtures were mainly made of low-power Φ5 white light LEDs. Such as 1-5W bulbs, 15-20W tube lamps, and 40-60W street lamps and projection lamps. These lamps use dozens to hundreds of Φ5 white light LEDs, with complex production processes, poor reliability, high failure rates, large shell sizes, and insufficient brightness. In order to improve the above shortcomings, high-power white light LEDs have been gradually used to replace Φ5 white light LEDs to design new lamps in recent years. For example, a street lamp made of 18 2W white light LEDs would require hundreds of Φ5 white light LEDs. In addition, a 1.25W K2 series white light LED can be used to make a strong light flashlight with a luminous flux of 65lm, and the irradiation distance can reach tens of meters. This is impossible if Φ5 white light LEDs are used.

Figure 1 Relationship between junction temperature TJ and relative light output

The price of high-power LED lamps is much higher than that of incandescent lamps, fluorescent lamps and energy-saving lamps, but its energy-saving effect and lifespan are much higher than other lamps. If LED lamps are used in all public places with large electricity consumption, such as street lamp systems, terminal halls, large department stores or supermarkets, and high-end hotel lobbies, the one-time investment is higher, but the long-term power-saving effect and economy are worth looking forward to.

At present, 1-3W high-power white light LEDs are mainly used as lighting lamps because of their high luminous efficiency, low price and flexible application.

Heat dissipation problem of high-power LED

LED is a photoelectric device. During its operation, only 15% to 25% of the electrical energy is converted into light energy, and the rest of the electrical energy is almost converted into heat energy, which increases the temperature of the LED. Heat dissipation is a big problem in high-power LEDs. For example, if the photoelectric conversion efficiency of a 10W white light LED is 20%, 8W of electrical energy is converted into heat energy. If no heat dissipation measures are taken, the core temperature of the high-power LED will rise rapidly. When its junction temperature (TJ) rises above the maximum allowable temperature (generally 150°C), the high-power LED will be damaged due to overheating. Therefore, in the design of high-power LED lamps, the most important design work is heat dissipation design.

In addition, in the heat dissipation calculation of general power devices (such as power IC), as long as the junction temperature is less than the maximum allowable junction temperature (generally 125℃), it is fine. However, in the heat dissipation design of high-power LEDs, the junction temperature TJ is required to be much lower than 125℃. The reason is that TJ has a great influence on the light output rate and life of the LED: the higher the TJ, the lower the light output rate of the LED and the shorter the life.

Figure 2 Internal structure of K2 series

Figure 1 is the relationship curve between junction temperature TJ and relative light output of K2 series white light LED. When TJ=25℃, the relative light output is 1; when TJ=70℃, the relative light output drops to 0.9; when TJ=115℃, it drops to 0.8.

Table 2 shows the relationship between the junction temperature TJ and the lifespan of high-power white light LEDs when the brightness decays by 70%, given by Edison (the lifespan of LEDs from different LED manufacturers is not the same, so it is for reference only).

Figure 3 Internal structure of NCCWO22

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It can be seen from Table 2 that when TJ=50℃, the life is 90,000 hours; when TJ=80℃, the life is reduced to 34,000 hours; when TJ=115℃, the life is only 13,300 hours. In the heat dissipation design, the maximum allowable junction temperature value TJmax should be proposed for TJ. The actual junction temperature value TJ should be less than or equal to the required TJmax, that is, TJ≤TJmax.

Figure 4 LED and PCB welding diagram

Heat dissipation path of high-power LEDs.

High-power LEDs attach great importance to heat dissipation in their structural design. Figure 2 shows the internal structure of Lumiled's K2 series, and Figure 3 shows the internal structure of NICHIA's NCCW022. From these two figures, we can see that there is a large metal heat dissipation pad under the tube core, which can transfer the heat of the tube core to the outside through the heat dissipation pad.

Figure 5 Double-layer copper clad heat dissipation structure

High-power LEDs are soldered on printed circuit boards (PCBs), as shown in Figure 4. The bottom surface of the heat sink is soldered to the copper-clad surface of the PCB, with the larger copper-clad layer serving as the heat dissipation surface. To improve heat dissipation efficiency, a double-layer copper-clad PCB is used, and its front and back diagrams are shown in Figure 5. This is the simplest heat dissipation structure.

Figure 6 Heat dissipation path diagram

Heat is dissipated from high temperature to low temperature. The main heat dissipation path of high-power LED is: die → heat dissipation pad → printed circuit board copper layer → printed circuit board → ambient air. If the junction temperature of the LED is TJ, the temperature of the ambient air is TA, and the temperature at the bottom of the heat dissipation pad is Tc (TJ＞Tc＞TA), the heat dissipation path is shown in Figure 6.

In the process of heat conduction, various materials have different thermal conductivity, that is, different thermal resistance. If the thermal resistance from the tube core to the bottom of the heat sink is RJC (the thermal resistance of the LED), the thermal resistance from the heat sink to the copper layer of the PCB surface is RCB, and the thermal resistance from the PCB to the ambient air is RBA, then the total thermal resistance RJA from the junction temperature TJ of the tube core to the air TA and the relationship between each thermal resistance is:

RJA=RJC+RCB+RBA

The unit of each thermal resistance is °C/W.

It can be understood this way: the smaller the thermal resistance, the better the thermal conductivity, that is, the better the heat dissipation performance.

If the LED heat sink and the copper layer of the PCB are soldered together by reflow soldering, then RCB = 0, and the above formula can be written as:

RJA=RJC+RBA

Heat dissipation calculation formula

If the junction temperature is TJ, the ambient temperature is TA, and the power consumption of the LED is PD, then the relationship between RJA and TJ, TA and PD is:

RJA=（TJ－TA）/PD (1)

The unit of PD is W. The relationship between PD, the forward voltage drop VF of the LED, and the forward current IF of the LED is:

PD=VF×IF (2)

If the temperature TC of the LED heat sink has been measured, equation (1) can be written as:

RJA=(TJ－TC)/PD+(TC－TA)/PD

Then RJC=(TJ－TC)/PD (3)

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RBA=(TC－TC)/PD (4)

In heat dissipation calculations, after a high-power LED is selected, its RJC value can be found from the data sheet; after the forward current IF of the LED is determined, PD can be calculated based on the VF of the LED; if the temperature TC has been measured, TJ can be calculated according to formula (3).

Before measuring TC, you need to make an experimental board (select a certain PCB and determine a certain area), solder the LED, input the IF current, and after it stabilizes, use a K-type thermocouple point thermometer to measure the LED's heat sink temperature TC.

In formula (4), TC and TA can be measured, PD can be calculated, and then the RBA value can be calculated.

If TJ is calculated, it can be substituted into formula (1) to obtain RJA.

This method of calculating TJ through testing is based on using a certain PCB and a certain heat dissipation area. If the calculated TJ is less than (or equal to) the required TJmax, it can be considered that the selected PCB and area are appropriate; if the calculated TJ is greater than the required TJmax, it is necessary to replace the PCB with a better heat dissipation performance or increase the heat dissipation area of ​​the PCB.

In addition, if the RJC value of the selected LED is too large, a high-power LED with better performance and smaller RJC value can be replaced in the design to meet the calculated TJ≤TJmax. This is explained in the calculation example.

Various PCBs

There are currently three types of PCBs used for heat dissipation with high-power LEDs: ordinary double-sided copper clad PCB (FR4), aluminum alloy-based copper clad PCB (MCPCB), and flexible film PCB glued to an aluminum alloy board.

The structure of MCPCB is shown in Figure 7. The thickness of each layer is shown in Table 3.

Figure 7 MCPCB structure diagram

The heat dissipation effect is related to the thickness of the copper layer and the metal layer and the thermal conductivity of the insulating medium. Generally, MCPCB with 35μm copper layer and 1.5mm aluminum alloy is used.

The structure of the flexible PCB bonded to the aluminum alloy plate is shown in Figure 8. The thickness dimensions of each layer generally used are shown in Table 4. 1-3W star-shaped LEDs use this structure.

MCPCBs using high thermal conductivity media have the best heat dissipation performance, but are more expensive.

Figure 8: Heat dissipation layer structure

Calculation Example

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Here we use part of the data from the TC measurement example of NICHIA as a calculation example. The known conditions are as follows:

LED: 3W white light LED, model MCCW022, RJC = 16℃/W. The measuring head of the K-type thermocouple spot thermometer is welded on the heat dissipation pad.

PCB test board: double-layer copper-clad board (40×40mm), t=1.6mm, welding side copper layer area 1180mm2, back side copper layer area 1600mm2.

LED working status: IF=500mA, VF= 3.97V.

According to Figure 9, use a K-type thermocouple point thermometer to measure TC, TC = 71°C. During the test, the ambient temperature TA = 25°C.

1. TJ calculation

TJ=RJC×PD+TC=RJC(IF×VF)+TC

TJ = 16°C/W (500mA × 3.97V)

+71℃=103℃

Figure 9 TC measurement location diagram

2. RBA calculation

RJA=（TC－TA）/PD

= (71℃－25℃)/1.99W

=23.1℃/W

3. RJA calculation

RJA=RJC+RBA

=16℃/W+23.1℃/W

=39.1℃/W

If the designed TJmax=90℃, the TJ calculated according to the above conditions cannot meet the design requirements. It is necessary to replace the PCB with better heat dissipation or increase the heat dissipation area, and test and calculate again until TJ≤TJmax is met.

Another method is that when the RJC value of the LED used is too large, if you replace it with a new similar product with RJC=9℃/W (VF=3.65V when IF=500mA), and other conditions remain unchanged, TJ is calculated as:

TJ=9℃/W（500mA×3.65V）+71℃

=87.4℃

There is some error in the 71℃ in the above calculation. A new 9℃/W LED should be soldered and the TC should be re-measured (the measured value is slightly smaller than 71℃). This has little effect on the calculation. After using the 9℃/W LED, there is no need to change the PCB material and area, and its TJ meets the design requirements.

Add heat sink on the back of PCB

If the calculated TJ is much larger than the TJmax required by the design, and the structure does not allow the area to be increased, you can consider gluing the back of the PCB to a "∪"-shaped aluminum profile (or aluminum stamping) or to a heat sink, as shown in Figure 10. These two methods are commonly used in the design of multiple high-power LED lamps. For example, in the above calculation example, a 10℃/W heat sink is attached to the back of the PCB with a calculated TJ=103℃, and its TJ drops to about 80℃.

Figure 10 “∪” shaped aluminum profile

It should be noted here that the above TC is measured at room temperature (generally 15-30°C). If the ambient temperature TA of the LED lamp is higher than room temperature, the actual TJ will be higher than the TJ calculated after measuring at room temperature, so this factor should be considered during design. If the test is conducted in a constant temperature box, it is best to adjust the temperature to the highest ambient temperature during use.

In addition, whether the PCB is installed horizontally or vertically, the heat dissipation conditions are different, which has a certain impact on the TC measurement. The shell material, size and presence of heat dissipation holes of the lamp also have an impact on the heat dissipation. Therefore, room should be left when designing.

Conclusion

Using a PCB with a certain heat dissipation area and a test board with LEDs installed, measuring TC when the LED is working and then calculating it for heat dissipation design is a simple and effective method, which can better design a heat dissipation structure (PCB material and area) that meets the junction temperature TJmax requirements.

This heat dissipation design method is not only applicable to high-power white light LED lighting fixtures, but also to high-power LED lamps with other luminous colors, such as warning lights, decorative lights, etc.