Analysis of the problem of selective computing to improve the heat dissipation of LED lamps

To this day, thermal analysis of white LEDs remains an unfinished science. Most LED lamp and luminaire manufacturers rely on inadequate, inaccurate or ambiguous data to determine the performance of their LED devices in their applications, which can often lead to over-engineering of their heat sink designs.

Figure 1. Definition of LED

The power conversion efficiency (WPE) method is commonly used in the industry to calculate the power required to convert an LED into optical radiation and the actual heat generated by the LED . The drawback of WPE is that the results obtained between individual LED devices in the same product category vary greatly, making it difficult for lamp and luminaire manufacturers to compare LED products. In addition, WPE is usually very dependent on the operating environment. We will introduce a simple and clear method to calculate the heat generated by LEDs based on the luminous efficiency of radiation (LER). The LER of state-of-the-art, phosphor-converted white LEDs is generally constant, so luminaire designers can use this formula to quickly estimate the heat generated by LED devices.

Figure 2. Distribution of WPE and LER LEDs and heat dissipation

In thermal simulations, LEDs are sometimes modeled as simple resistive heaters, and all electrical power entering the LED is assumed to be converted into heat and dissipated from the illuminator. But there is a problem with this assumption, which is that it is too conservative: high-brightness phosphor-converted white LEDs typically convert 30% of the incoming electrical power into light, while royal blue LEDs can convert well over 50%. Therefore, the total power required to generate heat for a high-brightness LED is usually less than the total electrical power entering the LED .

Figure 3. Comparison of WPE and LER

If this reduced heat capacity is not properly incorporated into thermal simulations, the expected internal temperature of the luminaire will be too high, requiring a more complex and costly heat sink design. This is particularly important for applications that need to dissipate a specific amount of heat (5W-10W) from a small PCB board and heat sink, such as retrofit LED bulbs. To evaluate the cumulative thermal performance of a lamp or luminaire, designers must consider what proportion of the incoming electrical power will be converted into light and heat, respectively.

The WPE method commonly used in the LED industry today is defined as the ratio of the total radiant power of the LED to the total electrical power delivered to the LED. Since WPE depends on the nominal flux and voltage of the LED and is a strong function of the actual drive current and connection temperature, the results vary greatly between different LED devices in the same product category. Therefore, it is difficult to define a typical WPE value for different driver conditions, flux and voltage BIN combinations for a specific LED product category.

Radiant luminous efficiency

In contrast, the luminous efficacy of radiation (LER) is more reliable than WPE when performing thermal evaluation of LED applications. It quantifies the visible light luminous efficiency of a light source. More specifically, LER is defined as the total adapted luminous flux (lumens) of a light source divided by its total radiant power (watts). The LER value of an LED can be obtained directly from the radiant spectral power distribution (usually printed on the device's data sheet), and unlike WPE, the LER value does not vary significantly with nominal flux and voltage or actual drive current and connection temperature. Knowing the LER value, the total heat generated by the LED can be calculated using the following formula:

The diamond symbol represents the LER value, If represents the driving current, Vf represents the forward voltage under the operating conditions, and Φv represents the total luminous flux under the operating conditions. For example, for a fluorescent conversion white LED with a typical LER value of 300lm/Wrad, assuming that its driving current is 1000mA, the luminous flux is 300lm, and the forward voltage is 2.9V, then according to the above formula, its total heat generation can be calculated to be 1.9W.

Current advances in the production of phosphor-converted white LEDs have made it possible to precisely control the color point of LEDs in the same product category, so the LER values ​​are more consistent. In fact, the latest lighting-class LEDs (3-step MacAdam ellipses) from manufacturers such as Philips Lumileds have achieved excellent color control. This allows manufacturers to define a typical LER value for a specific CCT that can represent all LEDs in the same product category.

Philips has now begun using LER values ​​in the " LED System Calculator", which allows LED system designers to find key performance indicators for their final lighting applications, including system heat and light output, through this tool. In this way, Philips can help designers more easily design lamps and luminaires that meet expected requirements and have lower heat sink-related costs. In addition, the use of LER values ​​makes it easier to compare the same category of LEDs from different manufacturers, improving transparency and simplifying the LED specification process.