White LED lighting has been widely recognized by everyone for its high efficiency, low power consumption, energy saving and environmental protection. In essence, an LED is a illuminable diode whose luminous intensity is proportional to the forward current passing through it and has a turn-on voltage. When the current is 20 mA, the forward voltage drop is generally 3 to 3.5 V. . In many cases, the luminous intensity of a single LED does not meet the needs of practical applications. It is also necessary to use multiple LEDs in series or in parallel, which requires a large voltage or current to drive, and different manufacturing processes, even different batches, LEDs There is a problem of performance mismatch, which also poses a problem for rational design drive. Therefore, although there are many types of original power supplies, they cannot directly supply power to the LEDs. This requires a boost or buck according to different needs, as well as a constant current or constant voltage drive.
1 Common LED driver working principle
1.1 Linear Regulator Driver
The earliest set of linear regulator drivers appeared in the 1970s, when the NPN tube was used as a voltage regulator device, as shown in Figure 1. This regulator requires 2Vbe between the input voltage and the output voltage. When the input voltage is lower than 2Vbe, the NPN tube becomes saturated and the regulator loses its regulation capability. In order to reduce the pressure difference, a combined regulator has appeared. As shown in Fig. 2, the base of the NPN tube is driven by a PNP tube, but the pressure difference is also close to 1 Vbe. In the mid-1980s, low-dropout linear regulators appeared on the market, as shown in Figure 3. Unlike the NPN regulator, the PNP regulator voltage difference is not a function of Vbe, but a function of the PNP tube Vce. This voltage value is much lower. As the manufacturing process matures, the PNP regulator voltage difference is less than 500. mV.
Linear regulator drivers have now evolved into the structure shown in Figure 4.
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