Comprehensive analysis of LED spotlights (Part 1)

LED技术的快速发展，光效的不断提升，LED射灯技术也在快速发展。目前的LED射灯几乎是以替代传统卤钨灯为主。相比传统灯具，LED射灯在节能及长寿命方面具有显著的优势。随着LED射灯成本的降低，LED射灯的应用也逐步开始普及。《全面剖析LED射灯灯具》分为上下两集，上集笔者先从LED射灯的技术要求，常见的规格类型及结构给大家讲起。

(I) Technical requirements for LED spotlights

At present, LED spotlights are mainly used to replace traditional halogen lamps. The relevant safety requirements are mostly based on traditional lighting requirements, and a set of safety certification standards have been formed based on the characteristics of LED lamps. At present, in terms of LED spotlight certification, the main international certifications are European CE and North American UL, and voluntary certification (CQC) can be carried out in China.

CE certification includes two aspects: LVD and EMC. LVD is implemented in accordance with the EN60968 standard (to be upgraded to IEC 62560); EMC is implemented in accordance with EN55015, EN61547, EN61000-3-2 and EN61000-3-3 standards. In addition, LED lights usually need to refer to IEC 62471 for photobiological safety testing. In North America, UL safety and FCC electromagnetic compatibility certification are usually required. UL implementation standards are UL1993, UL8750 and UL1310; FCC implementation standards are FCC PART15 Subpart B. In China, voluntary certification (CQC) is used for LED spotlights, and the implementation standards are GB24906-2010 (safety) and GB 17743-2007 (electromagnetic compatibility).

In addition, there are many standards for the performance of LED spotlights. The United States has relevant requirements for ENERGY STAR; China has energy-saving certification requirements, and the implementation standard is CQC 3129-2010. These requirements all stipulate the color temperature, color rendering index, initial luminous flux, luminous efficacy, luminous flux maintenance rate, life, center light intensity, nominal power, power factor, product labeling, etc. of LED spotlights.

Calculation formula

The life of many electronic products is judged by whether they break down, but LED is a light source with a very long life and can be used for a long time without breaking down. However, the output of luminous flux will decay over time, so the industry usually defines the life of LED light sources by the time it takes for the light to decay to 70%, that is, L70. With the advancement of technology, the light decay of LEDs is getting slower and slower, and it is difficult to fully test the time it takes for the light to decay to 70%. Since the life of electronic products follows an exponential law, LEDs are usually aged for a shorter period of time. By sampling the light decay data, the exponential function is used to calculate the time it takes for the light to decay to 70%. The calculation formula is shown in the figure above.

LED spotlight specifications and types

● Common LED spotlight specifications and types

Usually, the luminous flux maintenance rate LM (Lumen Maintains) of LED aging is tested for 6000 hours. According to the exponential law, if LM>91.8% after 6000 hours, a lifespan of 25,000 hours can be claimed. If LM>94.1%, a lifespan of 35,000 hours can be claimed. However, the estimated lifespan time shall not exceed 6 times the test time.

Figure 1: Light decay index curve

Figure 2: LED spotlight polar coordinate light intensity distribution curve

The angle of an LED spotlight is generally defined by the beam angle of 50% peak light intensity. Figure 2 is a polar coordinate light intensity distribution curve of an LED spotlight. It can be seen that the angle corresponding to 50% light intensity is about ±20°, so the angle of this spotlight is 40°.

(II) Specifications and types of common LED spotlights

Figure 3: Different lamp holders (from left to right: E14, E17, E26, E27, GU10, GU5.3)

At present, LED spotlights are mainly used to replace traditional halogen lamps, so the external dimensions refer to the IEC 60630 standard, and the lamp holders refer to the IEC 60061-1 standard. According to the traditional spotlights, the current LED spotlights mainly include MR16, PAR16, PAR20, PAR30, and PAR38. Among them, MR16 usually uses GU5.3 lamp holders; PAR16 mainly uses GU10, E26 (America)/E27 (Europe and China), and E14 lamp holders; PAR20/PA30/PAR38 mainly use E26 and E27 lamp holders.

Brief Analysis of LED Spotlight Structure

(III) Brief analysis of the structure of common LED spotlights

Currently, all LED spotlights are self-ballasted, that is, the LED driver power supply is completely built inside the lamp body and can be used directly by connecting to the power supply.

Figure 4: Disassembly of LED spotlight (PAR16-GU10)

As shown in the spotlight disassembly diagram in Figure 4, the current LED spotlights are mainly composed of optical devices (lenses, reflective cups), LED light sources, heat sinks, driver power supplies, and lamp holders. However, due to the different materials, processes, and LED packaging methods used by different manufacturers, there are some differences in the design and processing of the LED spotlights.

1. LED light source and optical devices

Current LED spotlights are mainly made of multiple high-power LEDs and integrated packaged LEDs, as shown in FIG5 .

Figure 5: The first picture shows multiple high-power LEDs (the second and third pictures are integrated packaged LED spotlights)

When using multiple high-power LEDs, a circuit board is usually required to electrically connect the LEDs. This circuit board is mostly made of aluminum substrate (MCPCB); for some designs, fiberglass board (FR-4) is also used, but a specially designed heat dissipation pad is required. Then it is fixed to the heat sink of the lamp housing by screws or adhesives. For spotlights made of multiple high-power LEDs, the lens usually uses independent light distribution for each LED, and then combines them into a spot lens.

Spotlights made of integrated packaged LEDs do not require circuit boards, and the output line of the driving power supply can be directly connected to the LED lamp. They are also fixed to the heat sink of the lamp housing by screws or adhesives. The secondary optics of this type of spotlight usually uses a lens or a reflector cup for light distribution. The height of the lens and the reflector cup are relatively high, making it difficult to distribute light at small angles.

In addition, for spotlights with large light-emitting surfaces, most of them use a combination of multiple optical devices to complete the secondary light distribution design of the entire lamp, as shown in Figure 6.

Figure 6: Spotlights made from multiple optical devices

2. LED driver power supply

Most of the current LED spotlights are self-ballasted spotlights with built-in power supplies. The built-in LED driver is mainly implemented with a switching power supply, which is divided into isolated and non-isolated types. The primary and secondary of the isolated power supply form electrical isolation. When designing a spotlight, it is only necessary to fully protect the primary of the power supply from the casing or other parts that can be touched by the human body, while the secondary is usually a safe voltage and can be simply protected. This type of power supply is relatively safe and reliable, but requires a large placement space and its conversion efficiency is also low. Since the non-isolated power supply does not have electrical isolation between the primary and secondary, it requires more stringent protection isolation in structure, but this type of power supply is highly efficient and small in size.

Figure 7: Power glue-filled spotlight

In addition to the switching power supply, there are many other LED driving methods, but their safety and reliability are relatively low, and more efforts need to be made in structural design to meet relevant safety requirements. In addition, since the power supply is built-in, the temperature of the components is usually very high, which will directly affect the service life and stability of the LED spotlight. Therefore, many manufacturers use glue filling to improve the heat dissipation capacity of the power supply and improve the insulation between the power supply and the heat dissipation shell, as shown in Figure 7.

Due to the controllability of LED, various manufacturers have developed dimmable LED spotlights. The main existing dimming methods include thyristor dimming, PWM dimming, 0-10V dimming, DALI dimming, DMX51 dimming, power carrier dimming, etc., all of which change the brightness of the LED by controlling the driving current or voltage of the LED.

3. Lamp housing radiator

The lamp housing heat sink is the main place to consider the design capabilities of each manufacturer. Most manufacturers' heat sinks are made of aluminum, and the processing technology includes organic processing and mold forming. In terms of structural design, in order to increase the heat dissipation capacity of spotlights, manufacturers usually use fins, as shown in Figure 8.

Figure 8: LED spotlight heat sink with fins (left) and insert injection molded heat sink (right)

In addition, the lamp housing is also made of thermally conductive plastic or ceramic materials, both of which are molded. These two materials have good insulation properties, which can make the product have high electrical safety performance and facilitate the safe isolation of the internal LED driver. However, the thermal conductivity of these two materials is low, which is not conducive to heat conduction. They can be used for indoor products that do not require high heat conduction. In addition, plastic materials are also light in weight, which can greatly reduce the weight of LED spotlights.