The high frequency switching power supply CC (constant current) control is a common method. However, while the performance is improved, the cost is greatly increased. Active or passive devices are connected in series with the LED lamp bead load to create a "constant voltage" effect on the line, so that the LED load passes through a constant current while the external wiring is subjected to a varying voltage. It is similar to the work of LDO (DropoutRegulato low dropout linear regulator). The driving method for realizing constant current and its development trend are described in detail below. Compared with the high-frequency switching power supply, the advantages of the linear high-voltage scheme are as follows: 1. The input electrolytic capacitor and the output capacitor are omitted, which is a circuit without a solution capacitor. The life of the electrolytic capacitor is the bottleneck of the power supply life, and the electrolytic capacitor is omitted, and the life of the driving power source is prolonged. Second, the circuit work mode, not working in the high frequency mode, eliminating the high frequency inductance, and no EMI problem, eliminating the EMC circuit. 3. Eliminate external components such as high-frequency inductors and further reduce costs. The first generation of RC linear control method is to use the capacitive reactance of a capacitor at a certain AC signal frequency to limit the maximum operating current. Capacitor buck is actually using capacitive reactance current limiting, and the capacitor actually Plays a role in limiting current and dynamically distributing the voltage across the capacitor and load. The circuit that resists buck is very simple, and the main advantage is low cost. The disadvantages are as follows: 1. When the input voltage fluctuates, the current fluctuates accordingly. 2. PF is low from the input side, the load is capacitive, the capacitor is resistant to high voltage, the selection is difficult, and the life is short. The second generation switch linear constant current control method is a constant current diode (CRD), a constant current transistor (CCT), and a MOSFET tube. According to the control method, it is a single switch control method. Advantages: 1. The LED lamp bead current is controllable and the brightness is controllable. 2. The lamp current is constant, and the lamp life is longer than that of the first generation. The short-circuit forward voltage drop is too high, the conduction time is too short, and the dark time is too long and the flashing is intense. The third-generation segmented switch linear control method developed a third-generation segmented control method for the shortcomings of the first-generation single-switch control. The principle of this control method is that a control chip detects the input AC voltage to provide a gate signal to several MOSFETs connected in series with the LED load. According to the input AC voltage level, the segmented open LED load, the market controls the third generation. The circuit proposes the following specific requirements: 1. Efficiency >0.9 2. PF>0.95 3. More than 90% thyristor dimmer compatibility 4. Switch dimming function 5. Built-in linear temperature compensation and over temperature protection Disadvantages The utilization of lamp beads is not high. The fourth-generation linearity is a method of controlling power, which is a series operation of low-voltage sub-strings in series and high-voltage sub-parallel connection. The "high-order segmentation" technology adopts a control scheme in which the N-type switch and the P-type switch cooperate with each other, and realizes a twelve-segment linear constant current drive with only a small cost cost, and the heat dissipation power is compared with the four-segment linearity. The constant current chip is reduced by 60%, and the power efficiency is up to a record 93%, providing a solid platform for the application of "dynamic configuration" and "capacitor filling" technologies. The "dynamic configuration" technology changes the series structure of the LED light string to a parallel structure when the full-wave rectification voltage is low, thereby solving the problem that the utilization rate of the LED wick is not high. The “capacitor-filling†technology solves the 100Hz strobo problem by applying a 1uF chip capacitor to power the LED string near the full-wave rectified zero voltage. The combination of “dynamic configuration†and “capacitor-filling†technology achieves a constant power supply output power during the full cycle of full-wave rectification, thus completely solving the problem of high-speed linear constant current scheme with deep strobe and low wick utilization. A solid platform. At the same time, the "power supply" linear high-voltage LED solution, due to its simple circuit, and its performance is gradually improved, the advantages of cost and reliability will make it a great development. In the future, China's LED market will have a clear hierarchical division. In response to the needs of consumers, a large number of corresponding products will emerge in the high, medium and low end.
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Linear high voltage LED driver scheme and its development trend
The rectified power frequency AC voltage, if this voltage is directly applied to the output LED, the problem is that constant current cannot be achieved, that is, the LED current is not constant throughout the power frequency cycle. 1. Unable to control the brightness. 2. LED lamp bead life is greatly shortened.