Accurate calculations and accurate data can help designers easily design lamps and luminaire that meet the expected requirements and have lower heatsink-related costs when calculating the heat generated by LED devices. The current method of power conversion efficiency commonly used there are many shortcomings, this article will introduce a simple and quickly calculate the heat of the new LED method.
Today, the white LED thermal analysis is still an unfinished science. Most manufacturers of LED lamps and luminaire rely only on inadequate, inaccurate or ambiguous data to determine LED device performance in the relevant application areas, which may lead to over-engineering of its heat sink design.
Power conversion efficiency (WPE) is currently used in the industry to calculate the power required to convert LEDs to optical radiation and the amount of heat actually generated by the LEDs. The drawback of WPE is that the results from the various LED devices in the same product category vary widely, making it difficult for fixture and luminaire manufacturers to compare LED products. Moreover, WPE usually has a great relationship with the operating environment. We will introduce a simple and clear, based on the radiation luminous efficiency (LER) of the LED heat calculation. The state-of-the-art, fluorescent-conversion white LEDs typically have a constant LER, so luminaire designers can use this formula to quickly estimate the heat generated by the LED device.
LED and heat dissipation
In thermal simulation experiments, LEDs are sometimes modeled as simple resistive heaters, and all the electric power going into the LED is assumed to be converted into heat and, in turn, dissipated from the illuminator. The problem with this assumption is that it is too conservative: a high-brightness fluorescent-to-white LED typically converts 30 percent of the incoming electrical power into light, while the sapphire LED's conversion power can be significantly more than 50 percent. Therefore, the total power required to heat the high-brightness LED is generally lower than the total electrical power entering the LED.
If this reduced heat capacity is improperly incorporated into the thermal simulation, the expected internal temperature of the luminaire will be too high, and thus a more complicated and costly heat sink design will be required. This is particularly important for applications that require a specific amount of heat (5W-10W) to dissipate from small PCBs and heat sinks, such as retrofit LED bulbs. To assess the cumulative thermal performance of a luminaire or luminaire, the designer must reasonably consider how much of the incoming electrical power will be converted to light and heat, respectively.
LED industry is now commonly used in the WPE method is defined as the total LED radiated power and the total power of the incoming LED ratio. Because 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 are quite different between 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.
Radiation luminous efficiency
In contrast, the thermal efficiency of LED applications using radiation efficiency (LER) than the WPE more convincing, the former can quantify the light source of the visible light efficiency. More specifically, the LER is defined as the total adapted luminous flux (lumens) of the light source divided by its total radiated power (Watts). The LER value of the LED can be obtained directly from the radiation spectrum power distribution (usually printed on the device data sheet), and unlike the WPE, the LER value does not appear to be significant due to the nominal flux and voltage or the actual drive current and connection temperature Variety. Given the known LER value, the total heat generation of the LED can be calculated by the following equation:
Wherein the diamond symbol represents the LER value, If represents the driving current, Vf represents the forward pressure under the operating condition, and? V represents the total luminous flux under the operating conditions. For example, for a fluorescent 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.
Current advances in the production of fluorescent conversion white LEDs allow for accurate color point control of the same product category, so LER values ??are more consistent. In fact, such as Philips Lumens (PhilipsLumileds) and other manufacturers to introduce the latest lighting-class LED (3rd order MacAdam oval) can achieve excellent color control. This allows the manufacturer to define a typical LER value for a particular CCT that can represent all LEDs in the same product category.
Philips is now starting 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 calorific value and luminous flux. This will help designers to more easily design to meet the expected requirements, the lower cost of heat sink-related lighting and lighting. In addition, the LER value makes it easier to compare the same class of LEDs from different manufacturers, increasing transparency and simplifying the LED specification process.