Technology Control: Analysis of LED Basic Driving Lighting Circuit Design

 　  LED is widely used in the industry because of its wide application and reasonable price. Whether it is holiday decorative lights or various lamps used in the home, LED drive circuits are used because they reduce energy consumption and can work stably for a long time. Today I will give you an extended introduction to LED lighting drive circuits from a practical LED circuit.

 　  Light-emitting diodes (LEDs) have become an alternative lighting technology today due to their high efficiency, energy saving, long life, and environmental protection, and are gradually being used in lighting. A key factor that has prompted people to pay attention to LED lighting technology is that it greatly reduces energy consumption and can achieve long-term reliable operation.

　　This article starts with the circuit using a constant current source. The main component of this circuit is the triode, which requires a withstand voltage of more than 400V and a power of more than 10W, such as MJE13003, MJE13005, etc., and a heat sink must be added. The filter capacitor C has a capacity of 4.7uF and a withstand voltage of more than 400V. The size of the light-emitting tube current is determined by the adjustment of R2. For the convenience of adjustment, the variable resistor can be adjusted and then replaced with a fixed resistor of the same resistance value. This circuit can carry as few as a dozen light-emitting tubes and as many as more than 90. Although it increases some costs, the effect is much better than the circuit that only uses resistors to limit current. Even if the voltage fluctuates greatly, the circuit can still keep the current constant, which is very beneficial to the life of the light-emitting tube. The current within this range can basically remain constant. The number of light-emitting tubes used in this circuit should not be too small. The fewer the number, the lower the efficiency. The total power consumption of this circuit is about 6W.

　　Here I would like to tell you whether it is better to use parallel connection or series connection for LED?

　　 The LEDs are connected in parallel or in series , which should be determined mainly based on the form and requirements of the power supply box circuit.

　　In the circuit of series connection, when one LED is disconnected, the whole string of LEDs will not light up; but when one LED is short-circuited, the other LEDs can still light up. In the circuit of parallel connection, when one LED is disconnected, the other LEDs can still light up; but when one LED is short-circuited, the power supply of the whole circuit will be short-circuited, so that not only the other LEDs cannot work normally, but also the power supply may be damaged. Therefore, the circuit of series connection is more advantageous in comparison.

　　The parallel connection method only requires a lower voltage to be applied across each LED , but a ballast resistor or current source is required to ensure that the brightness of each LED is consistent. If the bias current flowing through each LED is different, their brightness will also be different, resulting in uneven brightness of the entire light source. However, using a ballast resistor or current source to ensure consistent brightness of the LED will shorten the battery life. The series connection method can essentially ensure the consistency of the current flowing through each LED, but requires a high power supply voltage. When the LED is connected in parallel, since the total current of the circuit is the sum of the currents of each LED, the power supply is required to be able to supply a sufficiently large current.

　　In fact, strictly speaking, parallel or series connection methods have their own advantages and disadvantages. You need to consider many factors in practical use. In practical applications, LED arrays formed by series and parallel connection are often used, which can overcome or reduce the above-mentioned single LED open circuit or short circuit that causes the entire string of LEDs to not light up or the impact on the entire circuit and power supply. The so-called series-parallel connection is to first use a small number of LEDs in series and then connect a ballast resistor to form a branch, and then connect several branches in parallel to form a "branch group". In addition, the series-parallel-series form can also be used, that is, on the basis of the "branch group" that has been formed, several "branch groups" are connected in series to form the entire lamp circuit. This connection method not only reduces the impact of a LED failure, but also breaks down the ballast resistor into zero, turning several high-power resistors into dozens of low-power resistors, and changing from centralized installation to decentralized installation. This is not only conducive to resistor heat dissipation, but also can make the lamp design more compact.

　　First of all, we must consider the power drive of any circuit . Usually, a dedicated constant current source or driver chip is used to drive the LED, which is easily limited by factors such as size and cost. The most economical and practical method is to use a capacitor step-down power supply. Using it to drive low-power LEDs has the advantages of not being afraid of load short circuits and simple circuits, and one circuit can drive 1 to 70 low-power LEDs (however, the current impact when this power circuit is started, especially frequent startup, will damage the LED. Of course, appropriate protection can avoid this impact. In this regard, ON Semiconductor's NUD4700 LED shunt protection solution can be used. When the LED is working normally, the leakage current is only nearly 100 μA; when encountering transient or surge conditions, the LED will open the circuit. At this time, the shunt channel where the NUD4700 shunt protector is located is activated, and the voltage drop is only 1.0 V, which will minimize the impact on the circuit. This device uses a small, space-saving package and is designed for 1 W LEDs (rated current is 350 mA @ 3 V). If the heat dissipation is properly handled, it also supports operations with currents greater than 1 A.

　　The typical circuit of capacitor step-down power supply is shown in the figure

　　When checking the drive circuit, you should carefully check whether the circuit is connected incorrectly according to the circuit diagram, and pay special attention to checking whether the polarity of the rectifier bridge (the longer leg is the positive output, the opposite leg is the negative output, and the other two legs are the AC input) or the rectifier diode and the voltage regulator diode is correct (the end with a black or white line is the negative pole), and whether the three electrodes of the transistor or the voltage regulator integrated circuit are connected incorrectly.

　　C1 is a step-down capacitor (metallized polypropylene capacitor), and R1 provides a discharge circuit for C1. Capacitor C1 provides a constant working current for the entire circuit. Capacitor C2 is an electrolytic capacitor, and its withstand voltage value depends on the number of LEDs connected in series (about 1.5 times or more of its total voltage). Its main function is to suppress the voltage mutation caused by the moment of power-on, thereby reducing the impact of voltage shock on the life of the LED. R4 is the leakage resistor of capacitor C2, and its resistance value should be appropriately increased as the number of LEDs increases.

　　Since the capacitor step-down power supply is a non-isolated power supply, a large current will be generated at the moment of power-on, which is the so-called surge current. In addition, due to the influence of the external environment (such as lightning strikes), the power grid system will invade various surge signals, and some surges will cause damage to the LED. Therefore, it is necessary to provide thermistor protection, which mainly includes negative temperature coefficient thermistor protection (NTC thermistor, NTC is the abbreviation of Negative Temperature Coefficient) and positive temperature coefficient thermistor protection (PTC (Positive Temperature Coefficient)) and then transient voltage suppressor protection (Transient Voltage Suppressor, referred to as TVS)

　　Negative temperature coefficient means negative temperature coefficient, which generally refers to semiconductor materials or components with large negative temperature coefficient. The simplest and most effective way to limit surge current is to connect an NTC thermistor in series at the line input end.

　　When the positive temperature coefficient current passes through the PTC thermistor, it causes the temperature to rise, that is, the temperature of the heating element rises. When the temperature exceeds the Curie point, the resistance increases, thereby limiting the increase in current. As a result, the decrease in current causes the component temperature to decrease, and the decrease in resistance value increases the circuit current, and the component temperature rises again, and the cycle repeats.

　　Transient voltage suppressors are mainly used to quickly protect circuit components from overvoltage. When the two poles of the TVS tube are subjected to a reverse transient high-energy impact, it can change the high impedance between the two poles to a very low impedance at a speed of 10-12s, absorb high-energy surges, and clamp the voltage between the two poles to a predetermined value, protecting the components in the electronic circuit from damage caused by various surge pulses. (If you still have questions about the protection circuit, you can contact the author of this article, Xiao Yingquan, or contact QQ: 1529620855)

　　LED is becoming more and more popular in the industry, and the research on its basic circuit is expected to reach a higher level, so that the energy consumption, reliability and stability of LED can be greatly improved. Then there will be a huge application market in the future.