First, the basic knowledge of LED

1. LED luminescence principle

LED (Light Emitting Diode) is an abbreviation for light-emitting diode. Diodes made of compounds of gallium (Ga), arsenic (AS) and phosphorus (P) can emit visible light when electrons and holes are combined, so they can be used to make light-emitting diodes, which can be used as indicators in circuits and instruments, or as text or digital displays. Gallium arsenide diodes glow red, gallium phosphide diodes glow green, and silicon carbide diodes glow yellow.

With the progress of science and technology, there is an LED that can emit a similar incandescent light color, which provides a long-life and high-efficiency lighting source for modern lighting, and the most valuable thing is that the LED device has a life of more than 50,000 to 100,000 hours, which is suitable for a variety of occasions.

2. Characteristics of LEDs

The light emitted by LEDs is different from natural light, its spectrum is not continuous, and the infrared part is missing, so unlike incandescent lamps, the heat generated by LEDs is not dissipated by radiation, but must be dissipated by conduction, which is why LEDs are called cold light sources.

LEDs have the following advantages:

High efficiency and energy saving: White LED is the most energy-efficient white light source device known at present. Only a few kilowatt-hours of electricity are consumed in 1,000 hours (ordinary 60W incandescent lamps consume 1 kilowatt-hour in 17 hours, and ordinary 10W energy-saving lamps consume 1 kilowatt-hour in 100 hours)

Ultra-long life: semiconductor chips are luminous, no filament, no glass bubbles, not afraid of vibration, not easy to break, and the service life can reach more than 50,000 hours (the service life of ordinary incandescent lamps is only hundreds of hours, and the service life of ordinary energy-saving lamps is only 8,000 hours)

Green and environmentally friendly: it does not contain harmful elements such as mercury and xenon, which is conducive to recycling and utilization, and does not produce electromagnetic interference (ordinary lamps contain elements such as mercury and lead, and electronic ballasts in energy-saving lamps will produce electromagnetic interference, and mercury elements are still present in the lamps)

Eyesight protection: DC drive, no flicker (ordinary lamps are AC driven, it will inevitably produce strobe)

High light efficiency: low heat generation: 90% of the electrical energy is converted into visible light (80% of the electrical energy of ordinary incandescent lamps is converted into heat energy, and only 20% of the electrical energy is converted into light energy)

The market potential is large: low-voltage, DC power supply, battery and solar power supply can be used, which can be used in remote mountainous areas and field lighting and other places lacking electricity and less electricity.

According to the prediction of international authoritative institutions, semiconductor light-emitting diode lighting will replace the existing traditional light source in the next 5-10 years, of course, this will depend on the white LED is cheaper, and scientific and technological progress will inevitably cause the rapid growth of the overall capacity of the market. Technicians have measured that at the same brightness, the power consumption of LED is only 1/10 of that of incandescent lamps, and the life span is 100 times that of incandescent lamps. Because LED has the advantages of energy saving, environmental protection, long life and small size, experts call it another leap in the history of human lighting after incandescent lamps and fluorescent lamps. According to the U.S. Department of Energy, a complete replacement of traditional lighting devices will begin in 2010. Japan, the United States, Europe, South Korea and other countries have officially launched the LED lighting strategic plan. According to reports, at the World Expo held in Shanghai, China this year, many developed country pavilions have fully used LED as a lighting source. In China, the application of LED is also in a vigorous stage of development, but because China has not mastered the core technology of high-power LED, the country is still in the stage of imported semiconductor chips and domestic packaging, so the price is still at a high level compared to foreign countries, and the application is slightly affected.

3. Electrical characteristics of LED light-emitting diodes

LED is in a positive working state, when it reaches its luminous voltage value, LED begins to emit light, when the voltage continues to increase, the luminous brightness of LED increases sharply, if the voltage is slightly increased, the LED may burn out instantly, it can be seen that LED belongs to a kind of current device, it is this characteristic that determines that the LED must work safely in a constant current state, which is the main aspect of the LED device in the use of other light sources is inconsistent, and it is also the key factor for the safe use of LED, Points that require the designer's full attention.

The figure above shows the relationship between the forward drive current and the output brightness of an LED chip with a power of 1W, and it can be seen that there is a clear linear relationship between the drive current and the lumen output in the range of 350mA. However, as the forward current increases, a nonlinear relationship gradually emerges between the two. When the drive current exceeds the linear range, the efficiency of the lumens per watt will decrease, while in the linear range, the number of lumens per watt will increase. When the drive current exceeds the linear range, it causes the output power to be converted from the LED to the form of thermal energy, which becomes a burden on the LED driver, increases the complexity of the thermal design, and reduces the lifetime of the LED. Therefore, for a 1W LED device, its forward drive current does not exceed 350mA. For the commonly used Ф5 low-power LED lamp beads, the forward current does not exceed 20mA. For 3W high-power LED chips, the forward current should not exceed 700mA.

Second, the basis of LED design

1. Key points of the design of LED driver power supply

According to the characteristics of LED, its safe use method is to use constant current drive, constant current drive circuit characteristics: the output current is constant, and the output DC voltage changes in a certain range with the size of the load resistance, the load resistance is small, the output voltage is low, the larger the load resistance value, the higher the output voltage.

At present, the finished products with LED as the main device on the market, most of them adopt a simple capacitor step-down method, and achieve constant current through the capacitor, this kind of product has a fatal shortcoming, that is, often at the moment of switching the power supply, all the LED devices are burned out at once, the reason is that the charge and discharge characteristics of the capacitor are caused, where the capacitor is in the capacitance of the differential circuit, the internal resistance of the light-emitting tube is equivalent to the resistance in the differential circuit, and the time parameter of the differential circuit t=0.7RC, then at the moment of charge and discharge of t, The instantaneous current flowing through the LED is extremely large (which can reach tens to hundreds of times the normal current), causing instantaneous and permanent damage to the chip.

A reliable LED driver circuit is shown below:

In this circuit, C1 can be omitted, and U1 can be replaced by an inexpensive ordinary diode, and the disadvantage of this circuit is that its efficiency is low, and it is more efficient when using dozens (less than 65) of light-emitting tubes in series. This circuit can also be simplified, cancel U1 and C1, Q1 base can be directly connected to the lower end of R1, the stable current is 0.6/R1 decision, when R1 takes 30 ohms, the constant current is 20mA.

Theoretically, LEDs are diodes that can emit light, and in fact, the electrical properties of all LEDs are not the same, especially for the second-hand LEDs that are easy to get by electronic enthusiasts, the threshold value Vf of each LED is different, which means that the luminous intensity of LED and the driving current are not exactly the same, or very different, at this time, the designer needs to analyze the actual situation and adopt the most reasonable design scheme, in order to achieve the most effective reliability and economic benefits.

Because LED is a current type device, strictly speaking, it can only be used in series work, in the parallel working state, the parallel light-emitting tube must be selected, regardless of the number of tubes in parallel, practice has proved that the voltage drop difference of the tube shall not exceed 0.005V, otherwise it is easy to cause damage. In the past few years, there have been many flashlights produced in parallel by 7, 9 or even more LEDs in the market in China, which were once prosperous, but since the emergence of a single tube of 1W, 3W, 5W LED flashlight, the flashlight produced in series has plummeted in price, to the point that almost no one cares, I am afraid it is not entirely the shape of the flashlight and other reasons, the key is that the technical mode is backward, and poor reliability is the key to the problem.

The most ideal LED drive circuit is the switching power supply of PWM constant current control mode, which is mainly composed of four parts, input rectifier filter part, output rectifier filter part, PWM stable current control part, and switching energy conversion part. The basic working principle of PWM switching current stabilization is that in the case of changes in input voltage, internal parameters and external load, the control circuit conducts closed-loop feedback through the difference between the controlled signal and the reference signal, and adjusts the pulse width of the main circuit switching device, so that the output current of the switching power supply is stable (i.e., constant current power supply). The power supply efficiency is extremely high, generally 80%~90%, and the output current is quite stable. Generally, this kind of circuit has perfect protection measures and is a high-reliability power supply. The disadvantage is that the cost is higher.

2. LED heat dissipation solutions

The heat dissipation of LEDs is also a key issue that designers must consider, and the light decay of LEDs is related to the working time and current of the LEDs, as well as the temperature at which the LEDs are operating. The working life of LED in the correct working state can reach at least 50,000 hours, and it is generally easy to reach 100,000 or even hundreds of thousands, but once the ambient temperature is too high and the working current is too large, it will cause the life of the LED to drop sharply, and even cause permanent damage. The general low-power LED application can not consider the problem of heat dissipation, but the current working current of 60-150mA of the medium power LED has appeared, then the designer has to consider the problem of heat dissipation in the normal operation of the LED, fortunately, the outer diameter of 8mm, the working current of 150mA of a kind of white LED pins are relatively thick, basically can use its thick pins for heat dissipation, but for larger power such as 1W and above 3W, LEDs of 5W or even more than 10W must be equipped with heat sinks to be considered a normal working environment.

The figure below shows the temperature vs. current curve of the LED LED when it is operating

Most electronics enthusiasts know that after the PN junction semiconductor device is conducted, the junction voltage VF decreases with the rise of the ambient temperature, that is: -2mv/°C, which is called the negative temperature effect of the PN junction, and the temperature sensor can be made by using this characteristic, but this characteristic is a fatal defect in the application of light, which directly affects its luminous efficiency. Luminous luminance, luminous chromaticity. For example, when the ambient temperature rises to 85 °C, the junction voltage VF decreases, and the working current increases sharply to 35-37mA, see Figure 2, current curve I., when the temperature drops to -40 °C, the junction voltage VF rises, and the optimal working current will decrease from 20mA to 8-10mA., and the luminous brightness also decreases with the decrease of current, which cannot reach the illumination required by the place.

In order to avoid the above-mentioned undesirable phenomena, some measures are usually required to be adopted in LED related products. If the LED is installed on the heat dissipation plate, or the fan is air-cooled or the LED adopts the power supply mode of constant current source, the current will not increase due to the increase of the LED with the temperature, and the PN junction will not be malignantly heated, or the two methods can be used together. Practice has proved that for high-power LED lights (such as advertising background lights, street lights), the installation of radiators is indeed an effective measure. However, when LED lights enter ordinary people's homes, they encounter the following problems: whether the heat dissipation plate and air cooling can be concentrated in the space of an ordinary lamp head; The use of integrated circuits or many components of the constant current source circuit, its life and reliability will not depend on the LED, according to the "barrel theory" the reliability of the whole system will depend on some such as temperature rise and other "shortcomings".

< there are many LED circuit design materials that can be found for reference now, so I will not repeat > here

LED as a light source, there is a very important parameter, that is, the color temperature of the LED, let's use the end of this special issue, but also to learn some knowledge about the color temperature of the light source~

The color temperature of the light source LED is different, the light color is also different, the LED color temperature is below 3300K, there is a stable atmosphere, a warm feeling; The LED color temperature is 3000-5000K for the middle LED color temperature, which has a refreshing feeling; The LED color temperature is above 5000K and has a cold feeling. Different light colors of different light sources make up the best environment.

Definition of LED color temperature

It is expressed by the absolute temperature K, that is, the standard black body is heated, and when the temperature rises to a certain extent, the color of the black body begins to be dark red-light red-orange-yellow-white-blue, and gradually changes, when the color of a light source and the black body is the same, we call the absolute temperature of the black body as the LED color temperature of the light source.

LED color temperature in different light source environments:

The following are the LED color temperatures used in common lighting fixtures

Halogen lamp 3000k

Tungsten lamp 2700k

High Pressure Sodium Lamp 1950-2250K

Candle light 2000k

Metal halide lamp 4000-4600k

Cool Camp Light 4000-5000K

High Pressure Mercury Lamp 3450-3750K

Warm fluorescent light 2500-3000k

Clear Sky 8000-8500k

Cloudy 6500-7500k

Summer midday sun 5500k

Afternoon daylight 4000k

Characteristics of LED color temperature

1. In high-latitude areas, the LED color temperature is higher, and the color seen is blue.

2. In low latitudes, the LED color temperature is low, and the color seen is reddish.

( <---- Low LED color temperature ------------------ High LED color temperature ----> )

3. During the day, the LED color temperature also changes, when the sunlight is obliquely shining, the energy is absorbed more by (clouds, air), so the LED color temperature is lower. When sunlight shines directly, the energy is absorbed less, so the LED color temperature is higher.

4. The sRGB color model for Windows is 6500 K as the standard LED color temperature, which is represented by D65.

5. The LED color temperature in the early morning is around 4400 K.

6. The color temperature of LEDs in the mountains is about 6000 K

LED lighting applications in various colors

1. White light has perfect color characteristics, but it can damage the vision that adapts to dark light, and it takes a certain amount of time to re-adapt after a certain light source is extinguished.

The LED color temperature is below 3300K, and the light color is reddish to give a warm feeling; There is a calm atmosphere, a warm feeling; When illuminated with a low LED color temperature light source, the red color can be more vivid.

The LED color temperature is in the middle of 3000-6000K, and there is no particularly obvious visual psychological effect in this tone, and there is a refreshing feeling; That's why it's called "neutral" LED color temperature. When illuminated by a medium LED color temperature light source, the blue color has a cool feeling.

The LED color temperature is more than 6000K, and the light color is blue, giving people a cold feeling, and when the high LED color temperature light source is used, the object has a cold feeling

2. Red light is often used for night vision. The red light does not cause your pupils to constrict too much and the eyes do not need to readjust to the darkness once the red light is extinguished. Red is also commonly used as a "safe" color in monochrome photo processing because it does not damage the negative being printed, and yellow light has some of the advantages of red and white light. Another advantage of yellow light is that when you read, you can reduce the reflection and glare of light that causes eye strain due to long readings.

3. Green light can also be used for night vision, and green light is also especially suitable for reading maps or charts at night. It is also less easily spotted by night vision equipment, so it is easy to detect by the human eye, and the green light is less bright than the red light.

4. Blue light can be used to read maps at night and is often favored by military personnel because it increases the level of contrast. It can also be used as a backstage working light color for cinemas and performances.

5. Blue-green light has similar night vision advantages of green light and blue light, but with the improvement of the color characteristics of blue-green light, some users like to use blue-green light for this reason.

6. Infrared red light is used with night vision equipment. Otherwise, infrared light cannot be seen with the human eye.

7. Ultraviolet light is usually used to identify whether banknotes are counterfeit or not, some UV LED illuminators are popular in nightclubs and parties, they are used to make fluorescent substances emit brighter light.

Comparison of LED color temperatures

1700 K: Match light

1850 K: Candle

2800 K: Tungsten lamp (incandescent)

3350 K: Studio "CP" light

3400 K: studio desk lamp, photographic floodlight (not flash), etc...

4100 K: Moonshine

5000 K: Daylight

5500 K: Average daylight, electronic flash (varies by manufacturer)

5770 K: Effective solar temperature

6420 K: Xenon arc lamp

6500 K: Daylight

9300 K: TV screen (analog)