1, LED basic knowledge
1, LED light-emitting principle
LED (Light Emitting Diode) is short for Light Emitting Diode. By gallium (Ga) and arsenic (AS), phosphorus (P) compounds made of diodes, when the electrons and holes compound can radiate visible light, and therefore can be used to make light-emitting diodes, in circuits and instruments as a light, or composed of text or digital display. Phosphorus gallium arsenide diodes emit red light, gallium phosphide diodes emit green light, silicon carbide diodes emit yellow light.
With the progress of science and technology, there can be similar to incandescent light color LED, which provides a long life for modern lighting, high efficiency lighting source, the most valuable is that the LED device has up to 50,000 to 100,000 hours of life, suitable for a variety of applications.
2, the characteristics of LED
LED light and natural light is different, its spectrum is not continuous, the lack of infrared part, so different from incandescent lamps, LED heat is not generated by radiation dispersion, but must be dispersed by conduction, which is also known as the reason why the LED cold light source.
LED has the following advantages:
Energy efficient: white LED is currently known as the most energy efficient white light source device. One thousand hours only a few degrees of power consumption (ordinary 60W incandescent lamp seventeen hours to consume 1 degree of electricity, ordinary 10W energy-saving lamps one hundred hours to consume 1 degree of electricity)
Ultra-long life: semiconductor chip light-emitting, no filament, no glass bubble, not afraid of vibration, not easy to break, the service life of up to 50,000 hours or more (ordinary incandescent lamps service life of only a few hundred hours, ordinary energy-saving lamps service life is only eight thousand hours)
Green: does not contain mercury and xenon and other harmful elements, conducive to recycling and utilization, and does not produce electromagnetic interference (ordinary lamps contain mercury and lead and other elements, energy-saving lamps in the electronic ballast will produce electromagnetic interference, and lamps are still present in the element of mercury)
Protection of eyesight: DC drive, no stroboscopic (ordinary lamps are AC drive, which will inevitably produce stroboscopic)
High light efficiency: small heat: 90% of the electrical energy into visible light (ordinary incandescent lamps 80% of the electrical energy into heat, only 20% of the electrical energy into light)
Large market potential: low-voltage, DC power supply, battery, solar power supply can be used in remote mountainous areas and field lighting and other power shortages, less power sites.
According to international authorities predict that 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 cheaper, technological progress will inevitably result in rapid growth in the overall capacity of the market. Technicians have been measured and found that in the same brightness, LED power consumption is only 1/10 of incandescent lamps, life is 100 times that of incandescent lamps. Due to the LED has energy saving, environmental protection, long life, small size and other advantages, experts say it is the history of human lighting following the incandescent and fluorescent lamps after another leap. According to the U.S. Department of Energy is expected, the complete renewal of traditional lighting devices will start in 2010. Japan, the United States, Europe, South Korea and other countries have officially launched the LED lighting strategy plan. According to reports, this year's World Expo held in Shanghai, China, many developed countries pavilions have been fully utilized LED as a lighting source. In our country, the application of LED is also in the vigorous development stage, but because our country has not mastered the core technology of high-power LED, the country is still in the import of semiconductor chips, domestic packaging stage, so from the price relative to foreign countries are still at a high level, the application is also slightly affected.
3, LED light-emitting diode electrical characteristics
LED is in the forward working state, in reaching its luminous voltage value, LED began to glow, continue to increase the voltage, LED luminous brightness increased dramatically, such as a slight increase in voltage, LED may be instantly burned, it can be seen that the LED belongs to a kind of current device, it is this characteristic, decided that the LED must be in a constant-current state in order to work safely, this is the LED device in the use of the main aspects of inconsistency with other light sources, the LED is not the same as other light sources. This is the use of LED devices and other light sources are not consistent with the main aspects of the key factors of the safe use of LED, the need for designers to pay full attention to the main points.
The figure above shows the power of 1W LED chip forward drive current and output brightness of the relationship, from which it can be seen that the drive current and lumen output in the range of 350mA shows a clear linear relationship. However, with the increase of the forward current, a non-linear relationship gradually emerges between the two. When the drive current exceeds the linear range, the efficiency of lumens produced per watt will decrease, while the number of lumens produced per watt will show an increasing trend in the linear range. When the drive current exceeds the linear range, it will lead to the output power from the LED to the form of heat, such heat will become a burden on the LED driver, increase the complexity of thermal design, and reduce the LED life. So for 1W power LED device, its forward drive current does not exceed 350mA. for the current commonly used Ф5 small power LED lamp beads, forward current does not exceed 20mA. for 3W high power LED chip, forward current should not exceed 700mA.
2, LED design basis
1, LED drive power design points
According to the characteristics of the LED, the safe use of constant-current drive, constant-current drive circuit is characterized by: the output current is constant, while the output DC voltage with the size of the load resistance varies within a certain range, the load resistance is small, the output voltage is low, the greater the load resistance, the higher the output voltage.
Currently on the market to LED as the main device of the finished product, most of the use of simple capacitance step-down mode, through the capacitor to achieve constant current effect, these products have a fatal shortcoming, that is, often in the moment of switching power supply, at once all the LED devices burned, the reason lies in the capacitor's charging and discharging characteristics caused by the capacitor is in the differential circuit capacitance, the light-emitting diode resistance, etc. Equivalent to the resistance in the differential circuit, the differential circuit resistance, the capacitor is in the differential circuit, the light-emitting diode resistance. Differential circuit resistance, differential circuit time parameter t = 0.7RC, then in t of the charge and discharge of the moment, the instantaneous current flowing through the LED is very large (may reach tens of times the normal current to hundreds of times), resulting in instantaneous permanent damage to the chip.
A high reliability LED driver circuit is shown below:
This circuit, C 1 can be omitted, in addition to U1 can be replaced by inexpensive ordinary diodes, the disadvantage of this circuit is that its efficiency is low, when using dozens (65 or less) more than the light-emitting diodes in series, the efficiency is higher. This circuit can also be simplified, the cancellation of U1 and C1, Q1 base directly connected to the lower end of R1 can be stabilized current for 0.6/R1 decision, when R1 take 30 ohms, the constant current is 20mA.
Theoretically LED are able to light-emitting diodes, but in fact the electrical properties of all LEDs are not the same, especially for electronic enthusiasts easy to get second-hand LEDs, each LED's threshold Vf are different, that is to say that the luminous intensity of the LED and the drive current is not identical, or a big difference, at this point the designer will need to analyze the actual situation to adopt the most reasonable Design program, in order to achieve the most effective reliability and economic benefits.
As the LED belongs to the current-type devices, strictly speaking can only be used in series operation, in parallel operation, must be connected to the light-emitting diode for handsome selection, regardless of the number of tubes connected in parallel, it has been proved that the tube voltage drop shall not be more than 0.005V, otherwise it is easy to cause damage. A few years ago, China's market appeared many by 7, 9 or even more LED parallel production of flashlights, once also a moment of glory, but since the emergence of a single tube 1W, 3W, 5W LED flashlight, this series of production of flashlights in the price of a straight line down to the point of almost no one asked for, I'm afraid that is not entirely the shape of the flashlight and so on, the key to the technology! Way backward, poor reliability is the key to the problem.
The most ideal LED drive circuit is the PWM constant current control mode switching power supply, mainly composed of four parts, input rectifier filter part, output rectifier filter part, PWM stabilized control part, switching energy conversion part. PWM switching stabilized basic working principle is in the case of the input voltage, internal parameters and external load changes, the control circuit is controlled by the control signal and the reference signal difference for closed-loop feedback, regulating the control signal and the reference signal. Difference between the controlled signal and the reference signal for closed-loop feedback, adjust the pulse width of the main circuit switching device conduction, so that the output current of the switching power supply is stabilized (i.e., constant current power supply). Power supply efficiency is extremely high, generally can do 80% ~ 90%, the output current is quite stable. Generally this circuit has perfect protection measures, is a highly reliable power supply. The disadvantage is that the cost is high.
2, LED thermal solutions
LED heat dissipation is also a key issue that designers must consider, LED light failure and LED operating time and operating current and LED operating temperature. The correct working condition of the LED working life can reach at least 50,000 hours or more, generally up to 100,000 or even hundreds of thousands is very easy, however, once the ambient temperature is too high, the working current is too large, will cause the LED's life expectancy drops dramatically, and even cause permanent damage. General small power LED applications can not have to consider the problem of heat dissipation, but the current operating current up to 60-150mA medium power LED has appeared, then the designer is required to have to consider the normal operation of the LED heat dissipation problems, well in the outer diameter of 8mm, the operating current of 150mA of a white LED pin is relatively thick, basically! Can use its thick pins for heat dissipation, but for higher power such as 1W and above 3W, 5W or even more than 10W LED must be added to the heat sink is considered a normal working environment.
Most electronics enthusiasts know that the PN junction semiconductor device is conduction, the junction voltage VF with the rise in ambient temperature and decline, that is: -2mv / ℃, said the negative temperature effect of the PN junction, the use of this feature can be made into a temperature sensor, but the characteristics of the light-emitting applications is a fatal flaw, directly affecting its luminous efficiency. Luminous brightness, luminous chromaticity. Common example, room temperature 25 ℃, select the LED best working current 20mA, when the ambient temperature rises to 85 ℃, the junction voltage VF decline, the working current increased sharply to 35-37mA, see Figure 2, the current curve Ⅰ, the temperature drops to -40 ℃, the junction voltage VF rises, the best working current will be reduced from 20mA 8-10mA., luminescent brightness is also with the reduction of the current and reduce the luminous intensity. Can not reach the required illumination of the place.
In order to avoid the above undesirable phenomenon, generally in the LED related products, usually need to use some measures. Such as LED mounted on the heat sink, or fan air cooling or LED using a constant-current source of power supply, will not be caused by the LED with the temperature rise in current increases, to prevent the PN junction of the malignant warming, or both methods and use. Practice has proved that for high-power LED lights (such as advertising background lights, street lights), the addition of heat sinks is indeed a proven measure. But the LED lights into the common people's home will encounter the following problems: heat sink, air-cooled can be set in an ordinary head space; using 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 entire system will depend on some kind of such as temperature rise, etc. "Short board".
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