Today, thermal analysis of white LEDs is still an unfinished science. Most LED luminaire and luminaire manufacturers can only rely on insufficient, inaccurate or ambiguous data to determine the performance of LED devices in related applications, which can often lead to over-engineering their heat sink designs. Figure 1. Definition of LED Power conversion efficiency (WPE) methods are commonly used in the industry to calculate the power required to convert an LED into optical radiation, as well as the amount of heat actually produced by the LED. The drawback of WPE is that the results obtained between LED devices in the same product category vary widely, making it difficult for luminaire and luminaire manufacturers to compare LED products . Moreover, WPE usually has a lot to do with the operating environment. We will introduce a simple and clear calculation method based on radiant luminous efficiency (LER) for LED calorific value. The LER of the most advanced, fluorescent-converted white LEDs is generally kept constant, so illuminator designers can use this formula to quickly estimate the amount of heat generated by LED devices. Figure 2. Distribution of WPE and LER LED and heat dissipation In thermal simulation experiments, LEDs are sometimes modeled as simple resistive heaters, and all electrical power that enters the LED is assumed to be converted into heat and conversely dissipated from the illuminator. But there is a problem with this assumption, that is too conservative: high-brightness fluorescent-converted white LEDs typically convert 30% of incoming electrical power into light, while royal blue LEDs can convert power by much more than 50%. Therefore, the total power required to heat a high-brightness LED is typically lower than the total electrical power entering the LED. Figure 3. Comparison of WPE and LER If this reduced heat capacity is improperly incorporated into the thermal simulation, the expected illuminator internal temperature will be too high and a more complex and costly heat sink design will be required. This is especially important for applications that require specific heat (5W-10W) to be dissipated from small PCB boards and heat sinks, such as retrofit LED bulbs. To evaluate the cumulative thermal performance of a luminaire or luminaire, the designer must reasonably consider how much the incoming electrical power will be converted to light and heat, respectively. The WPE method commonly used in the LED industry today is defined as the ratio of the total radiated power of the LED to the total electrical power of the incoming LED. Since WPE is dependent 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 widely between different LED devices in the same product category. Thus for a particular LED product category, it is difficult to define a typical WPE value for different combinations of driver conditions, flux and voltage BIN. Radiated luminous efficiency In contrast, the thermal evaluation of LED applications uses radiant luminous efficiency (LER) to be more convincing than WPE, the former can quantify the visible light luminous efficiency of the light source. More specifically, LER is defined as the total accommodating flux (lumens) of the source divided by its total radiant power (watts). The LER value of the LED can be obtained directly from the radiation spectral power distribution (usually printed on the device's data sheet), and unlike WPE, the LER value is not significantly affected by the nominal flux and voltage or the actual drive current and connection temperature. Variety. When the LER value is known, the total calorific value of the LED can be calculated by the following formula: Wherein, the diamond symbol represents the LER value, If represents the drive current, Vf represents the forward pressure under the operating conditions, and Φv represents the total luminous flux under the operating conditions. For example, for a fluorescence conversion type white LED with a typical LER value of 300 lm/Wrad, assuming a driving current of 1000 mA, a luminous flux of 300 lm, and a forward voltage of 2.9 V, the total calorific value can be calculated according to the above formula. 1.9W. Advances in current fluorescent conversion white LED production processes have made precise color point control of LEDs of the same product category possible, so the LER values ​​are more consistent. In fact, the latest lighting-grade LEDs (3rd-order MacAdam ellipse) from manufacturers such as Philips Lumileds have achieved excellent color control. This allows the manufacturer to define a typical LER value for a particular CCT that can represent all LEDs of the same product category. Philips has now begun to use the LER value in the "LED System Calculator", which allows LED system designers to find key performance indicators for their final lighting applications, including system heat and luminosity. In this way, Philips can help designers more easily design luminaires and luminaires that meet the expected requirements and have lower heat sink related costs. In addition, the LER value makes it easier to compare LEDs of the same category from different manufacturers, increasing transparency and simplifying the LED specification process.
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