Currently, the most common white LEDs are made by applying yellow phosphor to the blue LEDs. In addition, red, green, and blue light can be blended into each other to form a white LED. The development of LEDs is very fast, and now white LEDs from 100L/W to 120L/W can already be mass-produced.
However, the development of LED driver also has great challenges. Since LED itself is a linear component, the main problems currently faced are: First, the forward voltage changes with current and temperature; Second, different devices The forward voltage will vary; third, the "color point" will drift with current and temperature; fourth, the LED must operate within the specifications required to achieve reliable operation.
Figure 1 shows the basic structure of the LED driver , the left side is the input part, there are two important parts in the drive: one is power conversion, including non-isolated and isolated; the other is the driver, it will The input voltage is converted to a constant current to drive the LED. The main function of the driver is to limit the current within the operating conditions, regardless of input conditions and forward voltage changes. In addition to current limiting, we must also consider its efficiency, cost, size and many other factors when making drive products. In terms of efficiency, because the human visual system filters out current ripple, if the switching frequency reaches 100 Hz to 150 Hz, the "constant" current of the driver does not need to be a DC level, but a non-linear current can be used to drive the LED. This will not only improve efficiency, but also simplify the circuit.
Figure 1: Basic structure of the LED driver
Offline application
Typical off-line applications include electronic ballasts, fluorescent light substitutes, traffic lights, LED bulbs, street and parking lighting, building lighting, signs, and more.
Figure 2 is a circuit diagram of the ON Semiconductor NCP1014/28 off-line second-generation LED driver, in which the blue part is a constant current circuit, and the highest voltage clamping circuit is provided to ensure that the output voltage does not exceed the design voltage range, ensuring safety. . The NCP1014 is an off-line pulse width modulation (PWM) switching regulator with an integrated high voltage MOSFET that delivers up to 8 W of output power when powered by a universal AC line. Among them, the NCP1014 LED driver is a fully isolated AC-DC converter optimized for constant current applications. The NCP1014/NCP1028 are designed with a 350 mA/22 V DC transformer and a 700 mA/17V DC configuration. It should be noted that if another alternative transformer is used for the 230 V AC line, the converter can provide up to 19W (NCP1014) or 25W (NCP1028).
Figure 2: NCP1014/28 offline second generation LED driver
In some higher power applications, such as designs larger than 15W, general single-chip drivers have power limitations, so consider a discrete approach. In the application of Figure 3, a new MOSFET is added to the NCP1351 to create a driver that supports a constant current of 350 mA to 1 A. The NCP1351 is used in the design, which uses a variable frequency Controller with low switching frequency under light load or no load conditions, very low current consumption and high efficiency at startup. In addition, in the output section, a constant current circuit and a maximum voltage control circuit are also added.
Figure 3: Application example based on the NCP1351 20W universal input
Figure 4 is a constant current and constant voltage circuit using the NCP4300A. The circuit has the following advantages: the current sense voltage drop is only 20% of the simple discrete mode; the load current regulation accuracy is higher, less than 3%, and according to the component tolerance There will be differences; there are special voltage regulation and steady current loops, which can be easily designed by the designer; only need to be plugged when the maximum forward voltage exceeds 36 V, and the NCP4300A is regulated by a Zener device.
Figure 4: NCP4300 constant current and constant voltage feedback control for flyback converter
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