A complete guide to the basics of LED lighting design

Discuss the LED light-emitting principle and main parameters

Understand LED driver classification and design solutions

Master the basics of LED heat dissipation solutions

In recent years, LED lighting has gradually been promoted and applied by all walks of life. The LED lamps that LED families can consume are all bulb-type LED lamps sold by major lighting manufacturers. Many companies have also developed fluorescent lamp-type LED lamps. LED lamps have different characteristics from traditional light sources such as incandescent lamps and fluorescent lamps. To make good LED lamps, relying solely on LED packaging cannot produce good lighting fixtures. In order to design better LED lighting fixtures, it is necessary to design the correct circuit design of LED that is different from traditional light sources.

This article introduces the design of LED lighting fixtures, including the LED light-emitting principle and main parameters, drive circuit, heat dissipation analysis and other aspects.

1. LED light emitting principle and main parameters

1. LED light emitting principle

LED is a type of electronic diode, and its main structure is PN junction. As shown in Figure 1, when voltage is applied to the two ends of the LED, the electrons will absorb energy and transfer to the valence electron band, and then release the absorbed energy. The released energy is light. The wavelength and color of the emitted light are determined by the potential difference of the semiconductor.
 

                        Figure 1 LED operation principle

LED has similar characteristics to general silicon diodes. Voltage is applied between the positive and negative electrodes. When the applied voltage reaches a critical value, current is generated in the LED and it begins to emit light. When the voltage exceeds this critical value, the current increases sharply. The critical voltage value of white LED is about 3.5V. The critical voltage values ​​of red, green, and blue LEDs are shown in Table 1. 
 
               Table 1 Overview of LED Lights

We use circuit simulation to illustrate the electronic characteristics of LEDs and drive circuits. Circuit simulation is a simple method of modeling electronic designs such as LEDs and resistors and simulating the operation of computational circuits on a computer. Here, Cadence's circuit simulation PSpice A/D is used. Philips Lighting's LUXEON series LX3-PW71 is also used.

First, use the circuit in Figure 2 to verify the electronic characteristics of the LED. To obtain the voltage-current characteristics, a DC analysis is required. 
 
             Figure 2 Circuit for verifying the voltage-current characteristics

The analysis results are shown in Figure 3. 
 
           Figure 3 Analysis results obtained based on PSpice A/D: voltage-current characteristics

The results of verifying the voltage-current characteristics of LXM3-PM71 show that the current is about 354mA at 3.0V.

2. Main parameters and characteristics of LED

Generally speaking, most LEDs used for lighting are white. LED lighting relies heavily on the invention of blue LED chips and the improvement of luminous efficiency.

There are two main ways to achieve white light LED. One is to use LED chips and phosphors, and the other is to use RGB 3-color LED chips. Currently, the first method is mainly used.

When using phosphor, yellow phosphor is usually coated on the blue LED chip. When the blue light emitted from the LED chip encounters the phosphor, part of the light is converted into yellow light. This part of the converted yellow light is mixed with the blue light to become white light. The color temperature of the white light LED can be controlled by adjusting the amount of phosphor, so the luminous color is determined during production and cannot be adjusted later.

At the same time, if blue light and yellow light are mixed, the color rendering is poor due to insufficient red and green components. In this way, the color rendering can be improved by mixing red and green phosphors in the blue LED chip or RGB phosphors in the ultraviolet LED chip.

The advantage of using RGB three-color LED chips is that RGB can adjust various chromaticities, so it can produce not only white light, but also other colors of light. However, as the number of LED chips used increases, the cost will also increase.

Three major characteristics of LED:

(1) Thermal characteristics

Although LEDs generate very little heat, they generate a lot of heat because they require multiple multi-watt LEDs in LED lighting. Although LEDs are highly efficient, they only operate at low currents. They are less efficient at high currents and temperatures.

In addition, phosphor-type LEDs lose energy when converting wavelengths, generating heat. Continuous high temperatures will reduce the life of LED chips, phosphors, and packaging resins. Therefore, in order to maintain the advantages of LEDs' "high efficiency" and "long life", the junction temperature of LEDs must be controlled.

(2) Characteristics of electricity

 LED power supplies are very different from incandescent lamps and fluorescent lamps. Incandescent lamps can be directly connected to 220V AC. Although fluorescent lamps have ballasts and conversion switches, they also use 220V AC. LED power supplies require a constant current of DC, so the 220V AC needs to be converted to DC. The low efficiency of the power supply will directly affect the efficiency of the entire lighting fixture, so improving the power supply efficiency is particularly important for improving the efficiency of LED lighting.

There are two main ways to adjust LED light. One is to change the constant current, and the other is to change the pulse modulation. LED emits light when electrons and holes recombine, and the light beam depends on the current. When the current is small, the light beam and current are basically proportional, but when the LED current increases, the heat increases, resulting in a lower light efficiency, and the light beam and current are no longer proportional.

In the method of changing the pulse duty cycle, due to the Talbot-Plateau effect (after repeatedly receiving instantaneous flashes, the human eye will feel the average brightness within the repetitive time), the brightness can be changed according to the pulse duty cycle.

(3) Optical properties

Compared with incandescent lamps and fluorescent lamps, one LED emits less light, so multiple LEDs are needed. At the same time, since the LED has a small light-emitting area and high brightness, it is easy for the human eye to get dizzy if it is directly viewed. In order to reduce the brightness, a diffusion plate is needed. However, if a diffusion plate is used, the light diverges in all directions, reducing the efficiency of the light. The
light distribution of LEDs, incandescent lamps, and fluorescent lamps is different. The so-called light distribution refers to the direction of the light source and the luminous intensity in each direction. Even if the light source has the same light beam, if the light distribution is different, the illumination distribution will be different. Sometimes the illumination of the place you want to illuminate decreases, while the illumination of the rest of the place increases.

To reduce light waste and control light distribution, lenses and reflectors are needed. LEDs have the advantages of small luminous area, hemispherical light radiation range, and rotational symmetry of light distribution. Together with lenses and reflectors, they can form a good light source.

Other properties of light sources include spectrum. The light spectrum of LED is concentrated in a very narrow range of specific wavelengths, and it does not emit infrared rays. Therefore, it is better to use LED when you do not want to heat up the irradiated object. However, LED itself will generate heat, so you need to be careful to prevent it from conducting heat. In addition, you need to pay attention to the color temperature of phosphor-type LEDs that changes with temperature.

2. Basic design of LED lighting drive circuit

The arrangement of LEDs and the specifications of LED light sources determine the basic driver requirements. The main function of LED drivers is to limit the current flowing through LEDs under a certain range of operating conditions, regardless of how the input and output voltages change. The basic operating circuit diagram of LED drivers is shown in Figure 4, where the so-called "isolation" means that there is no physical electrical connection between the AC line voltage and the LED (i.e., input and output). The most common method is to use a transformer for electrical isolation, while "non-isolation" does not use a high-frequency transformer for electrical isolation.

It is worth mentioning that in LED lighting design, the two parts of the AC-DC power conversion and constant current drive circuit can adopt different configurations:

(1) Integral configuration, where both are integrated and located inside the lighting fixture. The advantages of this configuration include optimizing energy efficiency and simplifying installation.

(2) Distributed configuration, that is, the two exist separately. This configuration simplifies safety considerations and increases flexibility. 
 
        Figure 4 Schematic diagram of the basic working circuit of the LED driver

LED driver classification and design

There are two types of typical LED drivers on the market today, namely linear drivers and switching drivers. The approximate application range is shown in Figure 5. For example, high-current applications with currents greater than 500mA use switching regulators, because linear drivers are limited by their own structure and cannot provide such a large current; in low-current applications with currents less than 200mA, linear regulators or separation regulators are usually used; and in medium-current applications between 200 and 500mA, both linear regulators and switching regulators can be used.

Switching regulators are highly efficient and provide excellent brightness control. Linear regulators are relatively simple in structure, easy to design, provide current regulation and overcurrent protection, and have no electromagnetic compatibility (EMC) issues. 
 
          Figure 5 LED driver classification

In low-current LED applications, although the resistor driver has a low cost and simple structure, the forward current of this driver is low under low voltage conditions, which will lead to insufficient LED brightness. In transient conditions such as load dump, the LED may be damaged. In addition, the resistor is an energy-consuming component, so the energy efficiency of the entire solution is low, as shown in Figure 6. 
 
            Figure 6 Comparison between resistor and linear driver

For example, in LED lighting applications using a DC-DC power supply, the LED drive methods that can be used include resistor type, linear regulator and switching regulator. The basic application diagram is shown in Figure 7. 
 
                

                 Figure 7 Common DC-DC LED driving methods

In the resistive drive mode, the forward current of the LED can be controlled by adjusting the current detection resistor in series with the LED. This drive mode is easy to design, low cost, and has no electromagnetic compatibility (EMC) problems. The disadvantages are that it depends on voltage, requires binning of LEDs, and has low energy efficiency.
 
Linear regulators are also easy to design and have no EMC problems. They also support current stabilization and overcurrent protection (fold back) and provide external current set points. The disadvantages are power dissipation problems, the input voltage must always be higher than the forward voltage, and the energy efficiency is not high. Switching regulators continuously control the on and off of switches (FETs) through PWM control modules, thereby controlling the flow of current.
 
Switching regulators have higher energy efficiency, are independent of voltage, and can control brightness. The disadvantages are relatively high cost, higher complexity, and electromagnetic interference (EMI) problems. Common topologies of LED DC-DC switching regulators include different types such as buck, boost, buck-boost, or single-ended primary inductor converter (SEPIC).

Among them, a buck structure is used when the minimum input voltage under all working conditions is greater than the maximum voltage of the LED string, such as using 24 Vdc to drive 6 LEDs in series; on the contrary, a boost structure is used when the maximum input voltage under all working conditions is less than the minimum output voltage, such as using 12 Vdc to drive 6 LEDs in series; and a buck-boost or SEPIC structure can be used when the input voltage and output voltage range overlap, such as using 12 Vdc or 12 Vac to drive 4 LEDs in series, but the cost and energy efficiency of this structure are the least ideal.

3. LED heat dissipation solution and foundation

In the design of LED lamps, if you only rely on LED packaging, you cannot make a good lighting fixture. This section mainly explains how to use the characteristics of LED design from the perspective of thermal analysis. Although incandescent lamps and fluorescent lamps have large energy losses, most of the energy is directly radiated through infrared rays, and the light source generates less heat; while LEDs, except for the energy consumed as visible light, all other energy is converted into heat. In addition, due to the small area of ​​LED packaging, the heat dissipation through convection and radiation is small, so a large amount of heat is accumulated.

Thermal solutions are simply solutions to various problems caused by heat. They mainly include:

1.Bending and cracking due to thermal expansion

Electronic devices are made of multiple parts, each of which is made of different materials and has different thermal expansion and contraction rates. Therefore, when various materials are combined together, they may bend or expand, and cracks may occur at the joints of the product due to excessive stress.

2. Operational problems of electronic circuits

Generally speaking, semiconductor components, which are heat sources, have a characteristic that when the temperature of semiconductor components in electronic equipment rises, the electrical impedance will decrease. This can easily lead to a vicious cycle of "temperature rise - impedance drop - current increase - heat increase - temperature rise", which can easily cause burnout.

3. Deterioration of material quality

Generally speaking, the materials used in electronic devices are easy to oxidize, and the higher the temperature, the faster the oxidation. If these materials are repeatedly subjected to high-temperature oxidation, their life will be shortened. At the same time, repeated heating, multiple expansion and multiple contraction of the materials will reduce the strength of the materials, thereby destroying the materials.

Thermal Solutions for LEDs

There are many ways to avoid heating of electronic equipment. For example, add a heat sink and place a fan that can provide cooling air around the heat source. The former is to increase the heat dissipation channel by increasing the heat dissipation area, and the latter is to prevent heat from gathering around the heat source. However, as shown in the general diagram of LED lamps in Figure 1, the LED package cannot be directly connected to the heat sink, and there is no place to install a fan. In addition, the internal power circuit board will also generate heat, so the heat dissipation problem of LED lamps can be said to be a very difficult problem. In this way, how to effectively use LED mounting materials and heat sinks becomes very important. 

 
            Figure 8 Overview of LED lights

So how can we effectively use LED mounting materials and heat sinks? First, we must understand the heat transfer path that generates heat.

The heat generated by the LED components moves to the circuit board through the package wires, and then dissipates through the heat sink. The same is true for the heat generated by the power supply circuit board, which is dissipated to the outside through the heat sink through the air around the circuit board and the filling material.

It is important to eliminate factors that hinder heat transfer in the heat transfer path in thermal solutions. For example, you can consider using materials with good thermal conductivity in the heat transfer path, expanding the cross-sectional area of ​​the path (for example, thick copper wire is easier to conduct heat than thin copper wire), and applying thermal conductive lubricant to ensure that there are no gaps in the connection parts of the product.

In addition, even if the heat conduction characteristics are improved through these methods, if the heat sink does not dissipate heat to the outside, a lot of heat will still accumulate inside. Therefore, the heat dissipation characteristics of the heat sink surface must also be improved. The typical method is to install several more heat sinks on the surface to expand the heat dissipation area of ​​the heat sink.

LED Applications

LED has the advantages of high luminous efficiency, long life, light weight, and no harmful substances. High luminous efficiency can increase the service life of the battery, which is very suitable for portable products. The service life of LED is 2 to 3 times that of ordinary incandescent lamps (some data show 40 times). A typical example of using LED because of its long life is traffic lights. In addition, since LED lights have a very fast response speed, they are also suitable for car brake lights. The design freedom of LED lamps is very high, and not only the brightness but also the color can be adjusted.