Optimized design and application of constant current LED power supply


At present, China is creating a resource-saving and environment-friendly society, and people are increasingly calling for the urban environment. According to statistics, global lighting energy accounts for about 20% of total electricity consumption. Green lighting is an important way to save energy and an important measure for the sustainable development of human society. In recent years, with the development of LED technology, LED products have been formally applied in many large-scale lighting projects, such as Beijing Olympic Village, Beijing Chang'an West Street, Xining Lishui River Covered Bridge, Shanghai Building, Shanghai Gaobao Financial Building, Qingdao Xinjiang China Resources Hotel, Shaanxi Wanbang Times Square, China Science and Technology Hall, Nanjing Water City, Guizhou Tongren Wa Yaohe Bridge and Wuhan Yangluo Yangtze River Bridge and many other important projects.

LED is a current control component that converts electrical energy into light energy directly through the current flowing through it. It is also called a photoelectric converter. Because it does not have friction loss and mechanical loss, it is more efficient than general light source in energy saving, but LED light source can not directly use grid voltage like ordinary light source. It must be equipped with a voltage conversion device to provide its rated value. The voltage and current can be used normally, that is, the LED driving power supply. However, the performance and conversion efficiency of various LED power supplies are different, so choosing a suitable and efficient LED driving power supply can truly reflect the high efficiency of LED light sources.

1 LED power supply basic working principle

The switching power supply is controlled by an isolation transformer, PFC (Power Factor Correction) control, and a constant current and voltage are output to drive the LED lamp. The overall block diagram of the circuit is shown in Figure 1.

For the main circuit part, the LED's ability to withstand surge is relatively poor, especially the anti-reverse voltage capability. It is very important to strengthen the protection of this aspect. If the LED power supply is used for street lights, it is necessary to strengthen the surge protection. Due to the start of the grid load and the induction of lightning strikes, various surges are invaded from the grid system, and some surges can cause LED damage. Therefore, the LED drive power supply should have the ability to suppress surge intrusion and protect the LED from damage. The EMI filter circuit mainly prevents harmonic interference on the power grid from entering the module and affects the normal operation of the control circuit. After the three-phase AC is rectified by the full bridge, it becomes a pulsating DC. Under the action of the filter capacitor and the inductor, the DC voltage is output. The main switch DC/AC circuit converts DC power into a high frequency pulse voltage at the secondary output of the transformer. The high frequency pulse output from the transformer is subjected to high frequency rectification, LC (passive) filtering and EMI filtering to output the DC power required by the LED street light. The PWM (Pulse Width Modulation) control circuit uses voltage-current dual-loop control to regulate the output voltage and limit the output current. The feedback network uses constant current and constant voltage devices and comparators. The feedback signal is optically coupled to the PFC device (this article uses ST company's L6562A). Due to the use of PFC devices, the module's power factor can be achieved above 0.95 [2]

The control circuit realizes the control of the inverter part by comparing with the preset reference voltage according to the sampling performed at the output end, thereby changing the output duty ratio of the switch tube to achieve stable output. In addition, according to the sampling and detection circuit, various protections for the whole machine can be realized by the control circuit.

2 LED power supply design ideas

2.1 Advantages of constant current drive mode

LED street lamps are low voltage, high current driving devices whose intensity of illumination is determined by the current flowing through the LED. Excessive current will cause the LED to attenuate. If the current is too weak, it will affect the LED's luminous intensity. In theory, the ideal LED driving method is to use constant voltage and constant current, but considering the increase of the cost of the driver, the constant current driving method is an ideal LED driving method. This LED driving method can avoid the current fluctuation caused by the change of the LED forward voltage, and the constant current stabilizes the brightness of the LED, which can ensure the safety of the high-power LED and achieve the desired luminous intensity.

2.2 EMI filter design

Since the switching power supply operates in an on-off state, there are many fast transients. It is an EMI source itself. It produces EMI signals with a wide frequency range and a certain amplitude. If this power supply is used directly in a digital device, the EMI signal generated by the device becomes more intense and complicated. Based on the working principle of the switching power supply, the design of the EMI filter for conduction interference suppression is discussed.

The EMI filter is a low-pass filter consisting of an inductor and a capacitor. It allows the low-frequency useful signal to pass smoothly and suppresses high-frequency interference. How can we suppress these high-frequency interference signals? It is nothing more than to contain the signal before it enters the device. That is, a so-called impedance mismatch is formed in the input circuit portion for high frequency interference. The more the mismatch, the better the attenuation achieved, and the better the insertion loss characteristics obtained. That is, if the internal resistance of the noise source is low impedance, the input impedance of the EMI filter to which it is connected should be high impedance (such as series inductance with large inductance); if the internal resistance of the noise source is high impedance, the EMI filter The input impedance should be low impedance (such as a large parallel capacitor). This principle is also designed to suppress switching power supply EMI filters that must be followed [3].

Conducted interference from almost all devices includes common mode interference and differential mode interference, and switching power supplies are no exception. Common mode interference is caused by the potential difference between the current-carrying conductor and the earth. It is characterized in that the noise voltages on the two lines are in the same direction, and the differential mode interference is due to the potential between the current-carrying conductors. The difference is that the noise voltage on the two lines is opposite to the potential. Usually, these two components of the interference voltage on the line are present at the same time. Due to the imbalance of the line impedance, the two components will change each other during transmission, and the situation is very complicated.

In actual use, since the components of the common mode and the differential mode in the noise generated by the device are different, the filter circuit used also changes, and the filter component can be appropriately increased or decreased. The adjustment of the specific circuit generally has to pass the EMI test to have satisfactory results. In addition, the grounding circuit must be well grounded when installing the filter circuit, and the input end and the output end should be well isolated, otherwise the filtering effect will not be achieved.