LED light source working principle and brightness stability

Since 2010, advanced countries such as Europe, the United States, and Japan have successively implemented the decree prohibiting incandescent bulbs. The reason for the decree is based on a light source that limits low luminous efficiency. Incandescent bulbs are disabled, and alternative sources include power-saving bulbs, cold cathode tubes, LEDs, and more. But in the future, if you consider the restrictions on harmful substances, LED light source will be the best choice.

LED light source working principle and characteristics

A light-emitting diode is a diode synthesized by a semiconductor material and is composed of a PN interface. When a forward voltage is applied, electrons and holes are combined to emit energy in the form of photons, and thus have luminescent characteristics. The light source is generated within a millimeter of the PN interface. The wavelength of the light depends on the characteristics of the material and has different light-emitting colors. Red, yellow, green and blue light-emitting diodes are common. The luminance of the LED can be adjusted by the magnitude of the operating voltage (current). In the large operating current range, the brightness of the LED increases with increasing current.

LED lighting brightness stability

The brightness of the LED varies with the current, and the voltage and current curves of the manufactured LED are slightly different. Therefore, the brightness of the LED illumination often cannot be stabilized with the variation of the power supply voltage. In order to maintain stable brightness, a light-emitting diode constant current driver is required. The constant current driver allows the LED to operate in a fixed current mode and thus has high brightness stability. The constant current driver also allows the LED to operate at a constant current for a long period of time, so that it can maintain a long life.

Kathy Technology's T6316 is a constant current driver. It is a constant current LED driver with 4 channels. The output current can be determined according to the external resistor. The T6316 has ±6% accuracy current and ±3% matching accuracy between channels, and can be used in lighting equipment such as street lamps and lamps. For energy saving considerations, the system design needs to consider the cross-voltage of the constant current driver between 0.5V and 2V. Since the LED operates at a constant current for a long time, its cross-voltage drops slightly, and this change also needs to be considered in the system design.

LED lighting energy saving considerations

The advantages of LED lighting are energy saving and safety, but due to constant current operation, the energy consumption is relatively increased. Therefore, the lighting system design targets low energy consumption. As mentioned above, the voltage drop of the constant current driver is less than 2V, that is, the design considering low power consumption. If the voltage of the power supply terminal of the system and the voltage drop of the series LED exceed 2V, it is necessary to consider the voltage converter to achieve the low energy consumption target. , but still maintain a constant current mode of operation. The low-energy voltage converter operates in a switching mode, and the switching circuit is controlled according to a feedback circuit to achieve a stable output voltage. However, in order to maintain the constant current operating state of the LED, the feedback circuit controls the converter switching period with the output current.

Kathy Technology T6322 is a step-down constant current LED driver whose current is determined by external resistors. It can support up to 1.5A output current, provide ±5% precision current and high power performance (low energy consumption) and high line adjustment capability. .

Figure 1 compares the power conversion efficiency of the T6322 with other products. Figure 2 is a comparison of the line adjustment capability of the T6322 with other products (low current rate of change indicates high line adjustment capability).

Among the currently flooding LED lighting products, their power input systems are of two types: one type of front end is an AC power input system plus a back-end constant current control module, such products include freezer light strips, indoor lamps, street lamps, Table lamp, MR16, AR111, etc. The other is an AC power direct input system that integrates AC/DC converters and constant current lines. These products include bulb-type LED lamps such as E27 and GU10, PAR lamps, T5 and T8 LED tubes.

In addition to the constant-voltage power supply with better front-end efficiency, the first-class power supply design uses a more efficient power supply design based on its product characteristics. The advantage is that the front-end AC/DC power supply, such as switching power supply, DC adapter and other constant voltage power supply, can be highly selective and has many safety certification schemes, thus reducing the design threshold.

The second type of power supply design integrates AC/DC circuits and constant current circuits, so it can meet the design of a small space, but from the power factor, power efficiency and safety considerations, the design also has some difficulties. Therefore, such products in the market today rarely achieve high luminous efficiency, high power factor (>0.9) standards.

At present, LED lighting products must achieve high luminous efficiency and become the next generation of main light sources. The primary consideration is the design of power modules, and the correct design architecture is selected for different lighting products. In addition, power control ICs must also improve power efficiency, power factor, and reliability to develop products that meet the needs of LED lighting.