Thermal Effect of Electronic Components on LEDs: QLED

By Muhamad Moussa, Technical Marketing Engineer, Future Lighting Solutions

At the end of Q1 2008, Future Lighting Solutions released the first version of QLED. Being the first LED specific thermal design and optimization software available in the market, QLED’s main objectives are:

• To simplify the design and optimization of LED-based lighting systems
• To accelerate the design and optimization cycles
• To minimize cost and decrease time to market

This software package allows lighting designers to create LED-based lighting systems in a step-by-step fashion using design wizards. The process includes, but is not limited to, adding LEDs, circuit boards, heatsinks, enclosures and other integrated circuits (ICs) or components.

In general, when using Computer Aided Design (CAD), a certain level of error is introduced automatically due to modeling inaccuracies. The ideal case would be to model the LED-based lighting system exactly as it will be as a final product and including all details of the circuit board, fixture, etc. Unfortunately, this could be a very tedious exercise, especially when considering circuit boards that include dozens of components such as resistors, capacitors, ICs, inductors, etc. The tendency is to try to approximate the lighting system by only modeling the LEDs and ignoring other components in order to minimize time and cost. However, the amount of approximation must not compromise the designer’s target accuracy. The final objective for the designer is to develop and test a prototype and not to just develop an optimized simulation model.

A trade off between accuracy and time spent on developing the model must be made. The desire for simplicity will lead the user to model the LEDs, board and heatsink while ignoring other components on the board. Typically in lighting applications, the light engine (LEDs, board, heatsink) is enclosed within a fixture. This fixture could be completely sealed or with minimal openings for ventilation. As a result, all components in the light engine (in addition to the LEDs), contribute to the heat that is generated in the enclosed fixture. In most cases, this contribution will have a significant effect on the overall thermal behavior of the lighting system, and ignoring such components may cause modeling inaccuracies that should be avoided. An increase in the LED junction temperature of 30ºC or 40ºC could be expected, depending on the type of components and the amount of heat they generate.

When LEDs are placed in close proximity, they affect each other’s thermal behavior – i.e. the LED will be subject to the heat generated from its own die plus the heat generated from adjacent LEDs. Similarly, when the board includes other components, such as capacitors, resistors, ICs, etc. surrounding the LEDs, the thermal behavior of the LEDs will be altered. Each LED will be subject to the heat generated from its own die, adjacent LEDs and adjacent components.

In order to illustrate the effect of other components on the LED's junction temperature, 4 cases will be covered:

• Case 1 is a model that doesn’t take into account the presence of any components on the board other than the LEDs
• Case 2 is the same model but with one component placed at the center of the board
• Case 3 doubles the amount of dissipated power relative to case 2 but spreads the power across 4 components
• Case 4 is similar to case 3 except that the 4 components are placed on a separate board in the fixture

 

The model for the 4 cases represents a down light consisting of the following:

• 3mm thick plastic enclosure with no openings (no air can escape from the inside of the enclosure)
• Aluminum plate as a heatsink
• Standard MCPCB
• 4 LUXEON® K2 Cool-White LEDs driven at a forward current of 500mA

 

Case 1

In this case, the only components on the MCPCB are the LEDs as shown in Figure 1 that shows the 3D model constructed using QLED.

 

Having no components other than the LEDs on the board is the best case scenario where the only sources of heat are the 4 LED dice. Simulating the model gives an LED junction temperature of 114ºC.

Figure 2 illustrates the thermal distribution inside the plastic enclosure. Note that the plastic enclosure is still part of the model and the simulation; it is hidden to visually show the board and the LEDs.

 

The next step is to add a heat generating component to the board. The component is assumed to be an IC that is 6.5mm wide, 9mm long and 3mm thick.

 

Case 2

For this case, the IC is set to generate 1W of heat and it is added to the center of the board as shown in Figure 3.

 

While all conditions remain the same as case 1, the model is re-run to evaluate the effect of the extra component on the LED's junction temperature.

Figure 4 illustrates the LED's, IC and board thermal distribution. Clearly, the addition of the IC affected the overall system's thermal behavior. There was an increase of 7ºC in the LED's junction temperature to reach 121ºC. Having the extra component on the same board as the LEDs has contributed to the higher LED's junction temperature.

 

It is almost impossible to find a circuit board with 1 component on it given the space constraints in most applications. Generally, boards may contain 10 or more components for the LED driver circuit.

 

Case 3

In this case, four ICs instead of 1 are added to the board at several locations as shown in Figure 5. Each IC is set to generate 0.5W of heat (total of 2W of heat).

 

After simulation, the LED's junction temperature increased to 127ºC as shown in Figure 6. This represents a significant delta of 13ºC compared to case 1.

 

Increasing the number of components will further amplify the LED's junction temperature. As a result, the same model can transition from generating acceptable junction temperature (Tj = 114ºC) to giving higher junction temperature; note that the absolute maximum junction temperature for LUXEON® K2 Cool-White is 150ºC. Consequently, there exists a need to optimize the thermal design especially when the LED's lifetime is of critical importance.

 

Case 4

Case 4 places the same 0.5W ICs as in case 3, but on a separate FR-4 board, as shown in Figure 7. There is a gap between the MCPCB and the FR4 board.

 

When comparing to case 3, the junction temperature dropped by 12ºC to reach 115ºC as illustrated in Figure 8. Placing the ICs on a separate board performed very closely to case 1 in which no extra components were considered.

 

Conclusion

The effect of including heat generating components to the thermal simulation model has been demonstrated. As a result, it is very important to include such components in any thermal simulation. For preliminary analyses, not all components must be added to the model. However, for final evaluation and optimization runs, the user/designer should, at least, include major heat generating components in the final simulation.

The positioning of the components also plays a vital role in determining the LED's junction temperature. Having the components on the same board as the LEDs had a far greater effect when compared to placing the same components on a separate board. The user/designer can use such analyses as a guideline to choose the board layout and components locations to achieve the desired thermal behavior.

 

Obtaining QLED

QLED is available through Future Lighting Solutions (www.futurelightingsolutions.com/QLED). Users can download a trial version to evaluate the software or purchase an annual license.

 

About Future Lighting Solutions

Future Lighting Solutions is the leading provider of LED lighting components and solution support for lighting designers and OEMs interested in taking advantage of solid state lighting technology. Future Lighting Solutions provides LED lighting knowledge, resources, programs, partners, solutions and logistics support to promote the development of LED products and installations. The company is a division of Future Electronics, the 3rd largest electronic components distributor in the world. Both companies operate in 167 locations in 39 countries in the Americas, Europe and Asia. For more information, visit www.futurelightingsolutions.com.

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