LED driving technology and application for lighting

1 Introduction

"Low carbon" life is a concept currently advocated. For lighting, the application of LED is a concrete manifestation. LED has the advantages of environmental protection, energy saving, high efficiency, long life, safety and reliability. Therefore, it is necessary to understand the use conditions, working principles, driving methods and typical applications of LED.

2 LED Working Principle

To design a drive circuit, we must first understand its working principle. The brightness of an LED is mainly related to VF and IF. The volt-ampere characteristics of an LED are shown in Figure 1, where VF is the forward voltage drop of the LED and IF is the forward current. When the forward voltage exceeds the threshold (i.e., the turn-on voltage, about 1.7V in the figure), it can be approximately considered that IF is proportional to VF. As can be seen from the figure, the highest IF of an LED can reach 1A, while VF is usually 2V to 4V.

Figure 1 Relationship between VF and IF of LED

The forward voltage drop of LED varies in a wide range (up to 1V or more), and as can be seen from the VF-IF curve in the figure above, a small change in VF will cause a large change in IF, thereby causing a large change in brightness. Therefore, the luminous characteristics of LEDs are usually described as a function of current rather than a function of voltage. However, the output voltage of a general rectifier circuit will also change with the fluctuation of the grid voltage. Therefore, using a constant voltage source to drive cannot guarantee the consistency of LED brightness and affects the characteristics of the LED. Therefore, LED drivers usually use constant current sources.

3 LED driving technology

From the working principle of LED, we know that to keep the LED in the best brightness state, a constant current source is needed to drive it. The driving task is to maintain the constant current characteristics and keep the power consumption low. In order to meet the above requirements, the commonly used methods of controlling current are: controlling the current by adjusting the size of the current limiting resistor; adjusting the current by adjusting the reference voltage on the current limiting resistor; PWM modulation to achieve current control. The driving technology of LED is very similar to the technology used in switching power supplies. The LED driving circuit is a power conversion circuit, but it outputs a constant current instead of a constant voltage. In any case, a constant and average current must be output, and the ripple current must be controlled within a certain range.

⑴ Current limiting method

As shown in Figure 2, this is a traditional circuit. After the grid voltage is stepped down, rectified, and filtered, the LED is stabilized by current limiting through resistors. The fatal disadvantage of this circuit is that the power consumption on the resistor R directly affects the efficiency of the system. Coupled with the transformer loss, the system efficiency is about 50%. When the power supply voltage changes within the range of ±10%, the current flowing through the LED will change by ≥25%, and the power on the LED will change by more than 30%. The advantages of resistor current limiting are simple design, low cost, and no electromagnetic interference; however, the current will change with the change of VF, the efficiency is very low, and heat dissipation is difficult.

Figure 2 Current limiting method

(2) Voltage stabilization method

Figure 3 adds an integrated voltage regulator MC7809 to Figure 2, so that the output voltage is basically stable at 9V, and the current limiting resistor R can be very small, which will not cause the LED voltage to be unstable. However, the efficiency of this circuit is still low. Because the voltage drop on MC7809 and R1 still accounts for a large proportion, its efficiency is about 40%. This cannot be called an energy-saving lighting product. In order to achieve both stable operation of LEDs and high efficiency, low-power current limiting components and circuits should be used to improve system efficiency. The advantages of the linear voltage regulation method are simple structure, few external components, medium efficiency and low cost.

Figure 5 Typical application circuit of LED fluorescent lamp driver chip PT4207

⑵ ON Semiconductor LED driver chip

ON Semiconductor NCL30000 is an excellent product with low cost and good performance. This device integrates PFC, DC-DC conversion and LED driver, which is simple and easy to use. Its small SOIC package also facilitates compact design and low cost. The NCL30000 adopts a compact 8-pin surface mount package and uses a critical conduction mode flyback architecture to provide a high power factor greater than 0.95 over a wide AC input voltage range in a single-stage topology. Typical applications include LED driver power supplies, LED recessed lights, triac dimmable LED lamps and power factor correction constant voltage power supplies. Its typical application schematic is shown in Figure 6.

Figure 6 NCL30000 typical application schematic

⑶ National Semiconductor LED driver chip

Constant current buck type, Vin range: LM3405 3V -15V/LM3405A 3V-22V; reference current source: 200mV; operating frequency: 0.550/1.6MHZ; PWM dimming. Typical applications are: home lighting, street lighting, advertising decoration, etc. The schematic diagram of LM3405 typical application is shown in Figure 7.

Figure 7 LM3405 typical application schematic

⑷ Taiwan Dot-Crystal Technology LED driver chip

DM413 is a full-color driver chip with built-in grayscale generator and PWM pulse width modulation. It is designed for LED lighting, decoration, large-screen display and other applications. Maximum constant current output: 100mA; maximum output withstand voltage: 17V; maximum serial clock frequency: 20MHz; chip operating voltage: 3.3V--5.5V. DM413 has several signal input and output pins and multiple function setting pins. The signal is given to the corresponding input pin of DM413 through the input interface. The three output pins are used to connect LEDs, which can be assigned to three colors: R, G, and B. The same output pin is connected to the same color LED. According to the color matching requirements, each output pin can be connected to one or more LEDs. The controller output interface has three signal lines: a serial data output line, a clock signal output line and a latch signal line. These three lines are connected to the corresponding three pins of the driver chip respectively. Like the general serial shift mechanism, under the control of the clock signal, the serial data is shifted and stored inside the driver chip. When the data transmission and reception are completed, the controller sends a latch signal to the driver chip, causing the LED driver chip to latch the stored data and drive the LED to emit light according to the stored data. This method controls the LED to turn on and off quickly at a high frequency that the human eye cannot distinguish, and sets the proportion of on and off according to the data stored in the chip, that is, to achieve grayscale control. The typical application schematic diagram of DM413 is shown in Figure 8.

Figure 8 DM413 typical application schematic

5 Conclusion

From the above, it can be seen that LED needs components with stable current and voltage when working, but such components should have the characteristics of high partial pressure but low power consumption. Therefore, high-efficiency circuits such as capacitors, inductors or active switching circuits should be used as much as possible to ensure the high efficiency of the LED system. The use of series integrated constant power output circuit can keep the light output of the LED constant within a wide power supply range, but the efficiency of general IC circuits will be reduced. The use of active switching circuit can ensure constant power output when the power supply voltage changes greatly under high conversion efficiency. Most LED chips today can modulate the brightness from 0 to 100%, and can ensure that the light efficiency is maintained at a high level during the entire dimming process without damaging the life of the LED. At present, the two main solutions for light control are linear adjustment of the LED current (analog dimming) or switching the drive current back and forth from 0 to the target current value at a high frequency that cannot be detected by the naked eye (digital dimming). Using pulse width modulation (PWM) to set the cycle and duty cycle may be the simplest way to achieve digital dimming.

References

1. Wen Maoqiang, Electronic Engineering Album, A Complete Guide to LED Technology

2. LED lighting power supply design platform

3. Yan Zhongguang, Low-voltage drive technology for LED lamps, Electronic Quality, 2009, Issue 7

4. LED chip manufacturer manual

About the Author

Name: Yao Xiaoping, senior engineer, research direction is "applied electronic technology". ■