LED driver and related technical requirements

　　The LED drive circuit should ensure the constant current working characteristics of the LED, especially when the power supply voltage changes, it can still keep the LED working current stable.

　　1. Ensure that the LED load operating current is stable;

　　2. Avoid the LED drive current exceeding the maximum rated value, which will affect the working reliability of the LED load;

　　3. Obtain the required lumen output and ensure the consistency of the brightness and chromaticity of each LED.

　　At the same time, the LED driving circuit should maintain low power consumption, so that the working efficiency of the LED lighting system can be maintained at a high level.

　　Especially in dimming, LED can not only achieve 0~100% dimming, but also ensure high luminous efficiency in the whole dimming process without damaging the working life of LED, which is difficult for gas discharge lamps to achieve.

　　The damage ratio of relevant components in LED lamps is shown in Figure 1, among which the damage rate of LEDs is 10%, the control circuit is 7%, the installation of LED lamps

　　LED driver and related technical requirements

　　The main technical indicators of LED drive circuits are: maximum output power, allowable operating temperature range, transient on/off operating characteristics, power factor not less than 0.9, input and output voltage variation range, maximum allowable input voltage and current, total harmonic distortion (THD) of the drive power supply, etc.

　　More advanced LED drivers should be able to monitor and report all operating status parameters and intelligent control functions of the LED lighting system, such as the ability to implement VF values ​​of LEDs without grading, automatic detection and compensation for line voltage drops between LED drivers and LED lamps, optical feedback, automatic CCT control of white light LEDs , CCT control of multi-color LEDs, and other control functions.

　　The main factors affecting the reliability of LED lighting products

　　When using the LED driver circuit, attention should be paid to the application scenarios of the LED driver circuit, such as the purpose, installation method, environmental noise interference, correct use of the LED driver circuit and related technical support will help improve the working reliability of the LED driver power supply.

　　When using the LED drive circuit, you should also pay attention to the input voltage adaptation range, output voltage and output current variation range of the LED drive circuit, reasonably manage the heat of the LED and its drive circuit, and use the appropriate type of electrolytic capacitor for the LED drive circuit.

　　Pay attention to mechanical stress, vibration resistance, moisture resistance, and waterproofing of mechanical components of LED driver circuits. Pay attention to the impact of technical parameters such as light output, luminous color, and luminous angle of optical components and LED components on the use environment, and pay attention to the UV resistance/chemical corrosion resistance of LED driver circuits and the correct use of LED driver circuits.

　　Some of the main problems with current LED drive circuits are as follows:

　　1. LED driver circuits of average quality do not use closed-loop feedback control technology;

　　2. Few LED driver technology developments are based on complex intellectual property requirements;

　　3. Currently, only a few LED driver technologies use RDM (remote deployment management) technology to achieve remote control monitoring of the working status of LED lamps.

　　The impact of LED driving circuit on LED lighting system mainly includes direct impact and indirect impact.

　　The direct impact is mainly reflected in the cost of the LED driver power supply, the working efficiency of the driver power supply, anti-EMI working characteristics, working reliability, the regulation characteristics of the LED working current, the power factor (PF), the protection working characteristics of the driver power supply (such as overvoltage, overcurrent, overtemperature, etc.) and the working ripple current of the LED.

　　The indirect impact is mainly reflected in the working efficiency, working reliability, thermal management characteristics, working safety (such as output isolation or non-isolation) and system cost of the LED lighting system.

　　Common interferences in LED drive circuits

　　Common interferences in LED drive circuits include voltage sag, voltage surge, undervoltage, overvoltage, transient interference of oscillation waves, transient interference of lighting equipment, input surge current or surge voltage interference, common mode noise, differential mode noise, voltage imbalance, voltage distortion, etc. These interferences will affect the normal operation of the LED lighting drive circuit, and in severe cases may damage the LED lighting drive circuit. Therefore, when designing the LED drive circuit, the anti-interference performance of the drive circuit must be considered to ensure reliable operation of the drive circuit.

　　Working principle of LED constant voltage and constant current driver

　　LED linear constant current (CC) driver has the characteristics of simple circuit, small number of components and low EMI.

　　LEDs are operated in series to ensure that the operating current through each LED is consistent, while LED constant voltage (CV) driving LEDs in parallel cannot ensure that the operating current through each LED is consistent.

　　The power consumption of the linear LED driver circuit can be expressed by the formula (VIN–n×VF)×IF, where n represents the number of LEDs in the LED load string. In applications where the LED load current is equal to or greater than 350mA, the power tube in the linear LED driver circuit requires a heat sink, which increases the cost and size of the LED driver circuit.

　　(1) Working principle of LED constant voltage drive power supply

　　The working principle diagram of the LED load constant voltage driving power supply is shown in Figure 2. The output voltage value can be adjusted by adjusting the values ​​of the output sampling resistors RFB1 and RFB2. Since the color temperature of the LED light, the output lumens and the forward working current of the LED are related, in order to stabilize the LED light output, it is not appropriate to use the constant voltage LED driving working mode in practice.

　　(2) Working principle of LED constant current drive power supply

　　The working principle diagram of LED constant current drive power supply is shown in Figure 3. Stable LED load working current is beneficial to stabilizing the LED light color temperature and output lumens. Therefore, it is more beneficial to use constant current drive for LED load in practical application. In Figure 3, the LED load driving working current can be adjusted by adjusting the parameters of current sampling resistor RFB.

　　If the input supply voltage of the driver power supply is always greater than the output voltage, a more efficient step-down converter or Buck converter can be used to provide constant current power supply for the LED load. Buck converter has the advantages of high efficiency and small heat sink, but the circuit structure is more complex and the working noise is greater than that of the linear driver circuit. Now the switching operating frequency of the Buck converter can be made higher than 1 MHz or higher, so that the peripheral components of the Buck converter are small, and the volume of the Buck converter is much smaller than that of the linear driver power supply.

　　In practice, if the input DC voltage of the LED load driving power supply is lower than the working voltage of the LED load string, an output boost circuit can be used to power the LED load. The inductive output boost converter is very suitable for constant current LED driving applications with an output current greater than 350mA. At this time, the output voltage changes with the current of the LED load string.

　　If the DC input supply voltage of the LED load driving power source fluctuates within the range of the LED load string operating voltage, a Buck-boost, SEPIC, C'uk or flyback circuit can be used to drive the LED load.

　　The topologies and characteristics of commonly used LED drive circuits are shown in Table 1. In the formula, D represents the duty cycle of the switching conversion pulse.

　　references:

　　[1]Archenhold G.An Incredible Year for LED’s The story so far，Aston Science Park / SSLRC.2007 PPT

　　[2]Tuttle P C.White LED Chromaticity Control-The state of the Art, Transformations in Lighting .2011. DOE Solid-State Lighting R&D Workshop，CREE LED Lighting PPT

　　[3] Lu Qiusheng. LED drive circuit and related technical requirements. Today's Electronics Supplement, 2012(10):52-56

　　[4] Lu Qiusheng. SSL lighting technology and progress. Journal of Power Supply, 2011(1): 99-105