Cooling high-power LEDs

Selecting a solution based on junction temperature ensures that the most critical parameter is identified and thermally managed

BY NORBERT ENGELBERTS
Advanced Thermal Solutions
Norwood, MA
http:/www.qats.com

Long used in instruments and computers as visual indicators for signal integrity and operations status, LEDs are ideal choices due to their high reliability, low power use, and little-to-no-maintenance needs. Recent market interest in LEDs is as lighting devices. However, as illumination becomes the focus, power consumption has risen dramatically.

Device heat fluxes rival those of CPUs and other semiconductor packages. Thus, the thermal management of LEDs has taken center stage for their successful implementation.

It is important to remember that an LED is not a high-temperature, filament-type lighting device. While a single LED is a cold and efficient light source, high-power LED applications, including arrays, need thermal management similar to other semiconductor devices.

Most LEDs are designed in surface-mount (SMT) or chip-on-board packages. In the new 1 to 8-W range of SMT power LED packages, the heat flux at the thermal interface can range from 5 to 20 W/cm2 .

These AlInGaP and InGaN semiconductors have physical properties and limits similar to other transistors or ASICs. While the heat of filament lights can be removed by infrared radiation, LEDs rely on conductive heat transfer for effective cooling.

As higher power is dissipated from LED leads and central thermal slugs, PCBs have changed to move this heat appropriately. Standard FR-4 PCBs can still be used for LEDs with up to 0.5 W of dissipation, but metallic substrates are required for higher levels.

A metal core PCB, also known as an insulated metal substrate (IMS) board, is often used underneath 1-W and larger devices. These boards typically have a 1.6-mm base layer of aluminum with a dielectric layer attached.

Copper traces and solder masks are added subsequently. The aluminum base allows the heat to move efficiently away from the LED to the system. But thermally dissipating PCBs are not always adequate or suitable for LED applications. Other cooling design choices are available, and it can be challenging to select the most appropriate and cost effective solution for a given application.

The cooling method

Cooling method and the optical lens are two parameters that play a pivotal role in the success of an LED. These factors affect the shape, size, and construction of the luminaire that comprises the lighting unit. Because long life and fail-safe operation are essential for any LED, the cooling process is uniquely critical.

An LED’s plastic body is not thermally conductive, and the device does not radiate heat. The only effective cooling method is to remove heat through the device bottom.

Therefore highly thermally-conductive materials are commonly used to take the heat from the LED’s back side (see Fig. 1 ). Depending on power dissipation and light emission uniformity, the cooling method can be passive (heat sink in natural convection), active (fan-sinks), or liquid cooling.

Fig. 1. Inside the Luxeon K2 power LED from Lumileds: LEDs commonly use highly thermally conductive materials to remove heat from the back.

With their basic, robust construction, LEDs can be used in environments ranging from ornamental to such critical illumination needs as automotive headlamps. Therefore their cooling systems must be designed with the ambient temperature and the specific end use in mind.

For example, a car’s headlamp with an under-the-hood temperature of 85 to 100°C and power dissipation of 42 to 90 W requires unique consideration for cooling and reliability. In other applications, to get the same light output as an incandescent lamp, the LED will often run on comparable power dissipation.

However, the LED’s maximum allowable junction temperature is limited to around 120° to 135°C (up to 185°C in recent developments). If we compare these limits to an incandescent lamp, which allows filament operating temperatures of 1,500 to 3,000°C, the thermal challenge for LEDs is the major obstacle to their successful implementation.

These thermal constraints typically need to be considered:

Tjunction LED max <120° to 185°C

Tjunction LED lifetime <100° to 110°C

LED power is 1 to 8 W

Light output is strongly dependent on temperature

Cooling options

The cooling options for LEDs range from simple natural convection in air to liquid cooling where a cold plate and liquid loop form the required cooling system. Because most market applications for LEDs shy away from liquid cooling, the focus of this article is on air cooling of LEDs.

Most LED lamps use familiar heat-sinking techniques. In some cases, the metal fixture of a luminaire can act as a heat sink, but the thermal requirements of its LEDs must be considered when designing the unit.

Increasing power density, a higher demand for light output, and space constraints are leading to more advanced cooling solutions. High-efficiency heat sinks, optimized for convection and radiation within a specific application, will become more and more important.

As with any semiconductor package, thermal resistance plays a significant role in the thermal management of LEDs. The highest thermal resistance in the heat transfer path is the junction-to-board thermal resistance (Rj-b ) of the package.

Spreading resistance is also an important issue. Thermally enhanced spreader materials, such as metal-core PCBs, cold plates, and vapor chambers for very high heat flux applications are viable systems to reduce spreading resistance.

Linear heat sinks are available specifically for LED strips. Round heat sinks are available specifically for round LED boards, which replace halogen light bulbs in applications such as spotlights and down lighting. The round heat sink has a special star-shaped fin design that maximizes surface area for more effective convection (air) and radiation cooling in the vertical mounting orientation.

Active thermal management systems can be used for high-flux power LED applications. These include water cooling, two-phase cooling, and fans. Although active cooling methods may not be energy-justifiable for LEDs, reasons for using them include ensuring lumen output or maintenance-free operation, or to meet specific wavelength requirements.

Analytical analysis

Analytical analysis is used to develop a first-order solution. This approach identifies the problem areas (components and system layout) and ascertains the magnitude of the problem (device junction temperature and required level of cooling). Some analyses can be performed quickly to get a handle on the scope of the problem.

Computational analysis

Computational analyses are used to develop second-order solutions to verify results from Step 1. The problem must be well understood in order to develop a model that accurately represents the problem.

CFD (computational fluid dynamics) will give a total 3-D picture of the problem. Both heat transfer and flow will be calculated. CFD is typically used to characterize the effect of spreading resistance within the PCB, the flow around the LED lamp, and the thermal performance and optimization of a heat sink.

Experimental analysis

The final product must be tested experimentally, whether for compliance or operation. For an LED-based application, the junction temperature is measured by the forward voltage characteristic. The LED has to be calibrated first with a 10-mA constant current source. During the operational test, the current measurement source is on all the time; then, after stabilization, the operational current is switched off.

After turning off the current, the drop in the forward voltage is measured. The thermal mass of the junction is small, which results in a fast cool-down time.

This temperature change occurs in less then 1 ms, so the forward voltage has to be measured in microseconds after the event. The forward voltage, together with the calibration curve, will give the junction temperature under operational conditions. This temperature must be within specifications for both maximum and typical ambient conditions. ■