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.