First, the general requirements for LED drivers
LED arrangement of the LED light source and determines the specification of the basic requirements of the drive. 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 1. 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 to electrically isolate. And "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) Integral configuration, that is, the two are fused together, all 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 security considerations and increases flexibility.
Second, how to choose the LED driver
Typical LED drivers on the market today include two types, linear drivers and switch drivers; the approximate range of application is shown in Figure 2. Switching regulators are used for high current applications with currents greater than 500 mA because linear actuators are limited to their own structural reasons. Provides such a large current; in low current applications with currents below 200mA, linear regulators or split regulators are typically used; in medium to 200mA to 500mA, linear regulators can be used. A switching regulator can be used.
Switching regulators are energy efficient and offer excellent brightness control. The linear regulator structure is simple, easy to design, provides steady current and overcurrent protection, and has no electromagnetic compatibility (EMC) problems.
In low-current LED applications, the resistive driver, although low in cost and simple in structure, has a low forward current under low voltage conditions, which causes LED brightness to be insufficient and under transient conditions such as load dump. The LED may be damaged; and the resistor is an energy consuming component, and the overall solution is less energy efficient, see Figure 3.
For example, in LED lighting applications using DC-DC power supplies, the LED driving methods that can be used are resistance type, linear regulators, and switching regulators. The basic application diagram is shown in Figure 4.
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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