The arrangement of the LEDs and the specification of the LED source determine the basic driver requirements. The main function of the LED driver is to limit the current flowing through the LED under certain operating conditions, regardless of the input and output voltages. The most common is the use of transformers for electrical isolation. The paper discusses the factors that need to be considered in LED lighting design.
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First, the general requirements for LED drivers
Driving LEDs faces many challenges. For example, the forward voltage will change with temperature and current. The LED forward voltage of different individuals, different batches, and different suppliers will also vary. In addition, the color of the LED The point will also drift as the current and temperature change.
In addition, multiple LEDs are often used in applications, which involves the arrangement of multiple LEDs. Among the various arrangements, a single string of LEDs in series is preferred because it provides excellent current matching regardless of how the forward voltage changes and how the output voltage (Vout) “driftsâ€.
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 other advantages. For example, in a cross-connect, if one of the LEDs is open due to a fault, only one of the LEDs in the circuit will double the drive current, minimizing the impact on the overall circuit.
The arrangement of the LEDs and the specification of the LED source determine the basic driver requirements. The main function of the LED driver is to limit the current flowing through the LED under certain operating conditions, regardless of the input and output voltages. The basic working circuit diagram of the LED driver is shown in Figure 2. The so-called "isolation" means that there is no physical electrical connection between the AC line voltage and the LED (ie, input and output). The most common is to use a transformer for electrical isolation. "Non-isolated" does not use high-frequency transformers for electrical isolation.
It is worth mentioning that in the LED lighting design, the AC-DC power conversion and constant current drive can be configured in different configurations:
1) The integral configuration, that is, the fusion of the two, is located in the lighting fixture, the advantages of this configuration include optimizing energy efficiency and simplifying installation;
2) Distributed configuration, ie both exist separately, this configuration simplifies security considerations and increases flexibility.
LED driver can work with constant voltage (CV) output according to different application requirements, that is, the output voltage is clamped under a certain current range; it can also work with constant current (CC) output, and the output design can strictly limit the current; A constant current constant voltage (CCCV) output operation may be used, which provides a constant output power, so the current is determined as the forward voltage of the LED of the load.
In general, LED lighting design needs to consider the following factors:
Output power: related to LED forward voltage range, current and LED arrangement, etc.
Power supply: AC-DC power supply, DC-DC power supply, direct AC power supply
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, and reliability
More considerations: mechanical connection, installation, repair/replacement, life cycle, logistics, etc.
Second, the LED drive power topology
In LED lighting applications with AC-DC power supplies, the power conversion building blocks include discrete components such as diodes, switches (FETs), inductors and capacitors, and resistors to perform their functions, while pulse width modulation (PWM) regulators are used to control Power conversion.
The isolated AC-DC power conversion with a transformer usually included in the circuit includes topologies such as flyback, forward and half bridge. See Figure 3, where the flyback topology is the standard choice for medium and low power applications with power less than 30 W. The half-bridge structure is best suited to provide higher energy efficiency/power density. For transformers in isolated structures, the size of the transformer is related to the switching frequency, and most isolated LED drivers basically use "electronic" transformers.
In LED lighting applications using DC-DC power supplies, LED drivers can be used in the form of resistors, linear regulators, and switching regulators. See Figure 4 for a basic application. In the resistive drive mode, the forward current of the LED can be controlled by adjusting the current sense resistor in series with the LED. This drive mode is easy to design, low in cost, and has no electromagnetic compatibility (EMC) problem. The disadvantage is that it depends on the voltage and needs to be filtered. (binning) LED with low energy efficiency.
Linear regulators are also easy to design and have no EMC issues. They also support current regulation and fold back, and provide an external current set point. The power dissipation problem is insufficient, and the input voltage is always higher than the forward voltage. Voltage, and energy efficiency is not high. The switching regulator continuously controls the opening and closing of the switch (FET) through the PWM control module to control the flow of current.
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