More reliable and efficient method for thermal analysis of LED

PHILPS (Philips LumiLEDs) of the lumens expert Dr. Mihir Kruger published an article about the calculation method of effective heat LED article recently described in detail about the process can increase the accuracy of thermal analysis of white LED and its simplified analysis. The following is the full text of Dr. Mihir Kruger's article.

Today, the thermal analysis of white LED is still an unfinished science. Most of the LED lamps and lighting device manufacturers can only rely on incomplete and inaccurate or vague data to determine the performance of the LED equipment in the relevant application, it may often lead to the excessive heat sink design engineering.

At present, the power conversion efficiency (WPE) method is usually used to calculate the power required for converting LED to light radiation, and the actual heat generated by LED. The WPE's flaw is that the results from each of the LED devices in the same product range vary widely, making it difficult for lamps and lighting manufacturers to compare LED products. Moreover, WPE is often associated with the operating environment. We will introduce a simple and straightforward method for calculating LED calorific value based on radiation luminous efficiency (LER). The most advanced and fluorescent conversion white LED LER generally remain constant, so the illuminator designer can use this formula to quickly estimate the heat generated by the LED device.

LED and heat dissipation

In thermal simulation experiments, the LED is sometimes modeled as a simple resistive heater, and all electrical power to enter the LED is assumed to translate into heat and, in turn, diverge from the illuminator. But this hypothesis has a problem, that is too conservative: high brightness fluorescence conversion white LED generally will 30% of the incoming electric power into light, and the blue LED conversion power can greatly exceed 50%. As a result, the total power required for high brightness LED heating is usually less than the total power delivered to the LED.

If this reduced heating capacity is improperly incorporated into the thermal simulation, the desired internal temperature of the luminaire will be too high, and a more complex and cost - effective design of the heat sink will be required. This is especially important for applications that require the divergence of specific heat (5W-10W) from small PCB plates and heat sinks, such as a modified LED bulb. To evaluate the total thermal performance of a luminaire or luminaire, the designer must consider reasonably the ratio of incoming power to light and heat.

Today, the WPE method commonly used in the LED industry is defined as the ratio of the total LED radiation power to the incoming LED total power. Since the WPE depends on the nominal flux and voltage of the LED and is a strong function of the actual drive current and the connection temperature, the results are different between the 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 drive conditions, flux, and voltage BIN combinations.

Luminous efficiency

In contrast, the use of radiance efficiency (LER) is more convincing than WPE in thermal evaluation of LED applications, where the former can quantify the visible light luminous efficiency of light sources. More specifically, the LER is defined as the total adaptive flux of light source (Liu Ming) divided by its total radiant power (Watt). The LED value of LER can be directly from the radiation spectral power distribution (usually printed on the device data sheet) acquisition, and unlike WPE, the LER value is not due to the nominal flux and voltage or the actual driving current and connected temperature changes. Given the value of LER, the total calorific value of LED can be calculated by the following formula:

PHILPS Lumileds: a more reliable and efficient method for thermal analysis and calculation of LED

Wherein, the diamond symbol represents the LER value, the If represents the drive current, and the Vf represents the forward pressure under operation conditions, and the phi V represents the total luminous flux under the operating conditions. For example, for a typical LER value for 300lm/Wrad phosphor converted white LED, assuming that the driving current is 1000mA, luminous flux of 300lm positive pressure is 2.9V, then according to the above formula can calculate the total calorific value is 1.9W.

The progress made in the current fluorescent white LED production process makes it possible to control the precise color point of the same product category LED, so the LER values are consistent. In fact, for example, PHILPS Lumileds (Philips) and other manufacturers launched the latest lighting level LED (3 steps MacAdam ellipse) has achieved excellent color control. This allows the manufacturer to define a typical LER value for all LED of a particular CCT that represents the same product category.

PHILPS has now started using the LER value in the LED system, which makes the calculator design business LED system can be a tool to find the key performance indicators for the final lighting applications, including heat and light emission system. As a result, PHILPS can help designers more easily design lamps and lighting that meet the expected requirements and lower costs associated with heat sinks. In addition, the use of LER values makes it easier to compare the same classes of LED in different manufacturers, improving transparency and simplifying the LED specification process.

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