Describe the considerations and general requirements for LED driver power design

　　According to the characteristics of LED driver power supply , the current rules of China's power grid and the requirements of exporting countries, we should pay attention to the following factors when designing LED driver power supply:

　　1. Driving mode. There are two main driving modes for LED driving power supply: one is direct constant current power supply, LEDs are operated in series or parallel. Its advantage is lower cost, but poor flexibility, and it is also necessary to solve the problem of a certain LED failure not affecting the operation of other LEDs. The other is a constant voltage source to supply multiple constant current sources, each constant current source to power each LED separately. This method has flexible combination, and the failure of one LED does not affect the operation of other LEDs, but the cost will be slightly higher. These two forms coexist for a period of time. The multi-channel constant current output power supply method will be better in terms of cost and performance. Perhaps it will be the mainstream direction in the future.

　　2. High reliability. Especially for the LED street lamp driver, which is installed at high altitude, it is inconvenient to maintain and the maintenance cost is high. Therefore, high reliability must be guaranteed. For example, the LED street lamp driver has high requirements for waterproof function, so the waterproof function of the LED waterproof power supply must meet the standard.

　　Figure 1: LED driver power supply design considerations

　　3. High efficiency. LED is an energy-saving product, and the efficiency of the driving power supply should be high. This is especially important for the structure where the power supply is installed in the lamp. Because the luminous efficiency of LED decreases as the temperature of LED increases, the heat dissipation of LED is very important. The high efficiency of the power supply means low power consumption, which reduces the heat generated in the lamp and reduces the temperature rise of the lamp. This is beneficial to delaying the light decay of LED. At the same time, it also meets the requirements of LED energy conservation and environmental protection. Energy conservation and environmental protection are important features and advantages of LED lighting, so as the heart of LED lighting, energy conservation and environmental protection are important factors that cannot be ignored.

　　4. High power factor. Power factor is the load requirement of the power grid. Generally, there is no mandatory index for electrical appliances below 70 watts. Although a low power factor of a single electrical appliance with low power has little impact on the power grid, when everyone turns on the lights at night, the same load is too concentrated, which will cause serious pollution to the power grid. For 30-40 watt LED driver power supplies, there will be certain index requirements for power factor in the future. Improving the power factor of LED driver power supplies below 70 watts is also a good opportunity for LED driver manufacturers.

　　5. Surge protection. The ability of LED to resist surge is relatively poor, especially the ability to resist reverse voltage. It is also important to strengthen protection in this regard. Some LED lights are installed outdoors, such as LED street lights. Due to the start-up of the grid load and the induction of lightning strikes, various surges will invade from the grid system, and some surges will cause damage to the LED. Therefore, the LED driver power supply must have the ability to suppress the intrusion of surges and protect the LED from being damaged. This is also an important indicator of outdoor LED lighting, and its importance in the future is self-evident.

　　6. Protection function. In addition to the conventional protection function of the power supply, it is best to add LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.

　　7. It must meet the requirements of safety regulations and electromagnetic compatibility. China has its own set of safety regulations, but when it comes to safety regulations, LED driver power supplies mainly refer to foreign standards. Because there are many exports, there are also requirements for electromagnetic compatibility. Meeting EU certification standards is generally a requirement for LED driver manufacturers.

　　8. The life of the driving power supply should be compatible with the life of the LED.

　　LED Driver General Requirements

　　Driving LEDs faces many challenges. For example, the forward voltage will change with changes in temperature and current, and the forward voltage of LEDs from different individuals, batches, and suppliers will also vary. In addition, the "color point" of the LED will also drift with changes in current and temperature.

　　In addition, multiple LEDs are usually used in applications, which involves the issue of how to arrange multiple LEDs. Among various arrangements, driving a single string of LEDs in series is preferred because this method provides excellent current matching performance regardless of how the forward voltage changes or how the output voltage (Vout) "drifts". Of course, users can also use other arrangements such as parallel, series-parallel combination and cross connection for applications that require "mutually matched" LED forward voltages and obtain other advantages. For example, in a cross connection, if one of the LEDs is open due to a fault, the drive current of only one LED in the circuit will be doubled, thereby minimizing the impact on the entire circuit.

　　Figure 2: Common LED arrangements

　　The arrangement of LEDs and the specifications of LED light sources determine the basic driver requirements. The main function of an LED driver is to limit the current flowing through the LED under a certain range of operating conditions, regardless of how the input and output voltages change. The basic operating circuit diagram of an LED driver is shown in Figure 2, where the so-called "isolation" means that there is no physical electrical connection between the AC line voltage and the LED (i.e., input and output). The most common method is to use a transformer for electrical isolation, while "non-isolation" does not use a high-frequency transformer for electrical isolation.

　　Figure 3: Basic operating circuit diagram of LED driver

　　It is worth mentioning that in LED lighting design, the two parts of the circuit, AC-DC power conversion and constant current drive, can be configured in different ways: 1) Integral configuration, that is, the two are integrated together and are both located in the lighting fixture. The advantages of this configuration include optimizing energy efficiency and simplifying installation; 2) Distributed configuration, that is, the two exist separately. This configuration simplifies safety considerations and increases flexibility.

　　Depending on the application requirements, LED drivers can operate with constant voltage (CV) output, that is, the output is a voltage clamped under a certain current range; they can also operate with constant current (CC) output, and the output design can strictly limit the current; they may also operate with constant current constant voltage (CCCV) output, that is, providing constant output power, so the forward voltage of the LED as a load determines its current.

　　In general, LED lighting design needs to consider the following factors:

　　. Output power: involves LED forward voltage range, current and LED arrangement, etc.

　　. Power supply: AC-DC power supply , DC-DC power supply, direct AC power drive

　　. Functional requirements: dimming requirements, dimming methods (analog, digital or multi-level), lighting control

　　. Other requirements: energy efficiency, power factor, size, cost, fault handling (protection characteristics), standards to be followed, reliability, etc.

　　. More considerations: mechanical connection, installation, repair/replacement, life cycle, logistics, etc.