How to design LED linear constant current drive circuit
LED is a cold light source with low operating voltage and high luminous efficiency. It is considered to be a new light source for 21st century lighting. However, at present, there are two key problems in the popularization and application of LED lighting equipment. One is that the price is high; the other is that the control circuit is unstable and the LED life is greatly reduced. According to statistics, at present, about 70% of failures of LED white light fixtures are power problems, about 20% are line and structural problems, and less than 10% are quality problems of LED single tubes, so the choice of power management scheme is Energy saving is also important, which requires the selection of the most suitable AC-DC driver in the drive circuit design. Therefore, reliable and low-cost control circuits are the premise for the popularization of LED lighting.
According to the electrical characteristics of the LED, the average forward current of the LED increases linearly with the increase of the forward voltage. The small variation of the forward voltage of the LED after the forward conduction will cause a large change in the LED current. And the current has a great influence on the LED junction temperature. Excessive current can easily cause the LED lamp bead temperature to rise and be damaged. In addition, it is known from the optical characteristics of the LED that as the forward current increases, the luminous flux of the LED increases, that is, the luminance increases. Therefore, in order to keep the brightness of the LED light constant, it is necessary to ensure the stability of the LED forward current. Therefore, designing a reasonable driving power source is very important for LED lighting fixtures.
This paper proposes an LED linear constant current driving circuit, which has the characteristics of low cost, simple structure, high efficiency, small size, etc. It is very suitable for driving power of indoor lighting LED lamps (such as LED fluorescent lamps).
1 LED linear constant current drive circuit
LED lights need to be connected in series or in parallel to work. The parallel voltage is used to drive multiple LEDs. Although the required voltage is lower, the brightness of each LED is different because the forward voltage drop of each LED is different. Different, unless a separate adjustment is used to ensure that each LED has the same brightness. Therefore, the parallel mode should ensure uniform brightness and be complicated to implement. The series connection can ensure that the current flowing through each LED is the same and the brightness is consistent, which is a commonly used structure.
When the series type driving mode is adopted, if one or several LEDs are broken and the circuit is broken (the short circuit has a small influence on the circuit and can be ignored), the circuit may be broken and cannot work normally. In order to avoid this defect, a voltage regulator tube can be connected in parallel with each other at both ends of the LED (as shown in Figure l). When an LED lamp bead breaks, the parallel voltage regulator tube is put into operation, ensuring the series connection. The lamp current does not change. It should be noted that the voltage regulation value of the Zener diode is higher than the conduction voltage of the LED. Otherwise, the parallel voltage regulator will shunt a part of the current and make the LED dark or not bright.
Figure 1 LED series drive circuit
In this paper, the series drive mode is adopted, and its LED linear constant current control circuit is shown in Figure 2.
Figure 2 LED linear constant current control circuit
In the figure, Vz1, Vz2, VQ1, VQ2, R1, and R2 form a linear constant current source, which ensures that the current flowing through each white LED is the same, and uniform brightness is obtained. The LED driving power supply adopts the direct current rectification and filtering of the commercial power to obtain the DC working voltage of the control LED, and does not need to be boosted or stepped down, so the power driving circuit is simple and the power supply efficiency is high. The LED used is a high-brightness white LED (operating voltage range: 3.0 to 3.2V), and 94 LED lamp beads are used to form an LED fluorescent lamp.
Next, the working principle analysis of the linear constant current source circuit is performed, and the circuit adopts a complementary constant current source structure at both ends, as shown in FIG.
Figure 3 Complementary two-terminal constant current source circuit
Transistor VQ1, voltage regulators Vz1 and R1 form a constant current source, which provides a stable operating current to Zener Z2, while transistor VQ2, Vz2 and R2 form another constant current source. The tube Vz1 has a stable operating current. Since the two constant current sources stabilize each other's Zener operating point, the stable voltages Vz1 and Vz2 and the total current flowing through the constant current unit are no longer changed, so that the operating current flowing through the LED can be kept constant.
The relationship between the current and voltage in the figure is as follows:
The total working current flowing through the LED, ILED is:
The LED lamp bead we use is 0.06 W LED and the working current is 20 mA. For the convenience of design, the selected circuit components are completely symmetrical. The parameters are:
Substituting the above parameters into equation (1) gives:
The constant current source circuit provides an operating current of 20 mA to meet the requirements of the lamp bead used.
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