LEDs are experiencing increasing acceptance and growth across many market segments. Part of this growth is due to new, higher-power LEDs that can be packaged in increasingly small areas. However, increased power places greater importance on thermal management because of the negative effect that heat has on LED performance.
For typical LED devices, 70-80% of the input electrical power becomes waste heat. The waste heat, if not properly managed, can have a significant impact on LED device performance. Increasing device temperature is directly correlated to device life. A rule of thumb is that every 10°C increase in operating temperature over the maximum operating condition decreases device life by 50%.
It is clear that LEDs require thermal management solutions to ensure design longevity. As input power requirements increase and package sizes shrink, these thermal management solutions will be of greater importance to device performance and life.
Fortunately, there is a passive two-phase technology known as heat pipes that offer advanced heat transfer and heat-spreading capabilities for LEDs. By passive we mean the devices do not require any external power to operate. By two-phase we are referring to the working fluid being in both the liquid and vapor state simultaneously inside the heat pipe. The inherent advantages of heat pipes include:
- No input power to operate;
- Years of reliable operation;
- Silent operation.
Heat Pipe Fundamentals
Heat pipes are sealed vacuum devices constructed with a metal tube envelope. Inside the tube is a wick structure and a small amount of working fluid. Most applications are copper tube/copper wick and have water as the working fluid, but there are several other combinations of envelope materials, wick structures, and working fluids. To operate, a heat pipe must be connected to a hot end, or evaporator, and a cold end, or condenser, as can be seen in the schematic in Figure 1.
The difference in temperature between the hot evaporator end and the cold condenser end is the driving force. The heat from the evaporator causes the working fluid to vaporize. Pressure from the vapor pushes it to the cooler end where it condenses to the liquid state and is absorbed by the wick structure. The condensed liquid then returns to the evaporator by capillary force of the wick structure.
Heat pipes are freeze thaw tolerant and by controlling the amount of working fluid and selecting the appropriate wick structure, heat pipes can restart operation after freezing. Also, with a proper wick structure, heat pipes can operate against gravity. Typically heat pipes can transfer heat as much as ~8" against gravity, although gravity-aided operation, which is when the heat sink is above the heat source, is preferred. In terms of heat flux capabilities, heat pipes can operate with heat fluxes up to 50-75 W/cm2. Additionally, heat pipes can be bent and flattened to fit countless geometric shapes.
To understand the benefits of heat pipes it is helpful to examine several examples of how heat pipes may be used for LED thermal management. The first example is the remote sink.
The Remote Sink
In many lighting applications the LED device must fit in a fixed space to accommodate a variety of customer requirements which usually exclude thermal management considerations. A common example is a luminaire design where the ceiling or wall fixtures are based on a pre-existing design using non-LED technologies. These designs commonly have both restricted space for heat dissipation through conduction and limited air flow to remove heat via convection. In cases where there is space to remotely dissipate the heat, heat pipes can be used to transport the heat from the device to a heat sink located elsewhere. This is called the remote sink.
The remote sink solution has a heat pipe in direct contact with the LED device (or a PCB or similar component) at one end, which serves as the evaporator. At the other end the heat pipe is connected to the heat sink. A wall or other enclosure can be placed in between the LED and heat sink to separate the two. In Figure 2, a single heat pipe is used to transfer heat from the source on the bottom to the heat sink located above it. A thermal image on the right shows the effectiveness of the heat pipe at spreading heat to the remote sink.
Optimized Size Weight and Power (SWaP)
The second case, heat sink SWaP, uses the embedded heat pipe, or Hi-K (high thermal conductivity, K), in the heat sink assembly. It is well known that placing a discrete heat source on a large metal heat sink will produce large thermal gradients as the heat slowly conducts and dissipates heat to the external fins. Embedding heat pipes in the heat sink can increase the thermal conductivity from around 200 W/m-K to 500-1,200 W/m-K, offering the opportunity to reduce heat sink plate thickness and fin area. This approach can be im- plemented in a variety of LED applications, including large arrays and outdoor lighting as well as some down lighting applications.
Seen in Figure 3 is a heat sink size and weight analysis for cases with and without heat pipes. Total heat dissipation is 150W in both cases. The conventional metal heat sink is 12 inches long, weighs 9.6 lbs, and has a base thickness of 6-tenths of an inch. Introducing five heat pipes, three in close proximity to the heat source and another two a little further out for heat spreading, reduces the overall length to 10 inches and the weight to 6.3 lbs. The overall material reduction is nearly 35%.
PCB Level Heat Spreading
The third example of passive heat transfer technologies for LEDs is focused on improving heat spreading near the source of heat. It is advantageous to dissipate heat as close to the source as possible, which can be difficult as electrical isolation requirements must be satisfied for the device to function properly. Unfortunately, in many cases electrical isolation can only be achieved using materials that are thermally insulating, such as with FR4 boards.
Recent work has explored adding heat pipes to the structure of metal core printed circuit boards (MCPCB) to help spread heat right at the source. You can see in Figure 4 an example of heat pipes embedded into an MCPCB. Heat pipes are seen on the left, in close proximity to the circuitry on the opposite side. The circuit side is seen in the image to the right of the embedded heat pipes. Figure 5 shows a picture of a three-LED heat sink (right) and a thermal image of the heat sink during operation. The temperature scale in Figure 5 is only 10°C, with a 68°C maximum temperature, which demonstrates the heat spreading capability of this concept. Measurements have shown that the embedded heat pipes can reduce the heat-spreading resistance by 45% over the standard aluminum MCPCB and even 15% over a copper MCPCB, which boasts a valuable improvement, particularly so close to the heat source.
As LED devices become more accepted and more prevalent, customers are requiring higher performing products in smaller package sizes. This puts an increased burden of thermal management on LED device designers. Conventional metal heat sink solutions will continue to be implemented wherever practicable, but they are more frequently found to be insufficient for a variety of performance and size reasons.
Presented in this article are three examples of how heat pipes can be implemented to solve real LED thermal management issues. The remote heat sink demonstrated the heat transport capability of heat pipes. Heat can be effectively moved 8 inches away from the source against gravity, and even longer distances when gravity aided.
The second case focused on improving the thermal conductivity of an extrusion using embedded heat pipes. By reducing temperature gradients in the heat sink base, the size can be decreased — 35% in the example shown — with no change in heat sink performance.
Lastly, PCB level spreading offers thermal management solutions very close to the heat source, reducing heat spreading resistance by 45% over Aluminum Metal Core Printed Circuit Boards. Heat pipes have provided many successful examples of thermal management solutions in the broader electronics industry and can be applied to the challenging thermal issues of LED devices as well.
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