This paper analyzes the advantages and disadvantages of linear high voltage LED system without high frequency switching power supply. Aiming at a large number of linear high voltage drive chips emerging in the market, various types of schemes and their development trend are described. The key technology and market prospect of high voltage LED in the future are analyzed and discussed in detail.
One. High voltage LED
“High voltage led”, one is the low-power led provided by the LED manufacturer in series. As shown in the left figure of Figure 1, it is only one kind of integrated LED. The main difference between the integrated LED shown in the right figure and the former is that the former is all in series and the latter is in series and parallel. The feature of integrated LED is to cut the large chip into several small LEDs by slotting, then fill these grooves with insulating layer, and lay the wires connecting each LED according to the requirements of series and parallel connection.
Figure 1: high voltage LED
No matter what kind of “high voltage led”, the linear high voltage LED driving scheme discussed in this paper is the LED driving scheme with small current (theoretical less than 100mA) and high voltage.
Figure 2: LED load characteristics
The load characteristics of LED are shown in Figure 2. According to the load characteristics of LED, a controllable constant current source is required to control. If the rectified power frequency AC voltage is directly added to the output LED, the problem is that the constant current cannot be realized, that is, the current through the LED is not constant in the whole power frequency cycle. One. Brightness control cannot be realized. II. The service life of LED lamp beads is greatly shortened.
For high-frequency switching power supply, CC (constant current) control is a common method. However, with the improvement of performance, the cost is greatly increased.
The active or passive components are connected in series in the LED bulb load, so that the line has the effect of “constant current – voltage transformation”. In this way, the constant current passes through the LED load, and the external line bears the changing voltage. This is the working principle of LDO (low dropout regulator). This constant current driving mode and its development trend are introduced in detail below.
Compared with high-frequency switching power supply, the advantages of linear high-voltage scheme mainly include: first. It eliminates the input electrolytic capacitor and output capacitor, and is a circuit without electrolytic capacitor. The life of electrolytic capacitor is the bottleneck of power supply life. If electrolytic capacitor is omitted, the life of driving power supply is prolonged. II. The circuit works in the power frequency linear mode, not in the high-frequency mode, which eliminates the high-frequency inductance, no EMI problem and EMC circuit. 3、 The high-frequency inductor and other peripheral components are omitted to further reduce the cost.
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