Working principle and brightness stability of LED light source

Since 2010, developed countries such as Europe, the United States, and Japan have begun to implement laws to ban incandescent bulbs. The reason for the laws is to restrict light sources with low luminous efficiency. Incandescent bulbs are banned, and the light sources that replace them include energy-saving bulbs, cold cathode tubes, LEDs, etc. However, if restrictions on harmful substances are taken into consideration in the future, LED light sources will become the best choice.

Working principle and characteristics of LED light source

Light-emitting diodes are diodes synthesized from semiconductor materials, consisting of PN interfaces. When a forward voltage is applied, electrons and holes combine to release energy in the form of photons, so they have luminous properties. The light source is generated within millimeters of the PN interface. The wavelength of the light depends on the characteristics of the material and has different luminous colors. Common ones are red, yellow, green, and blue light-emitting diodes. The brightness of the light-emitting diode can be adjusted by the size of the operating voltage (current). In a large range of operating currents, the brightness of the light-emitting diode increases with the increase of current.

LED lighting brightness stability

The brightness of LEDs varies with the current, and the voltage and current curves of manufactured LEDs are slightly different. Therefore, the brightness of LED lighting often cannot be stable with the change of power supply voltage. In order to maintain stable and consistent brightness, a constant current driver for LEDs is required. The constant current driver can make the LED work in a fixed current mode, so the brightness is highly stable. The constant current driver also allows the LED to work at a certain current for a long time, so that it maintains a longer life.

Kaiyu 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. T6316 has ±6% current accuracy and ±3% matching accuracy between channels. It can be used in lighting equipment such as street lamps and lamp tubes. 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 works at a constant current for a long time, its cross-voltage decreases slightly. 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, energy consumption is relatively increased, so the lighting system is designed with low energy consumption as the goal. As mentioned earlier, the voltage drop of the constant current driver is within 2V, which means that the design takes low energy consumption into consideration. If the voltage drop between the power supply end of the system and the voltage drop of the series LED exceeds 2V, it is necessary to consider using a voltage converter to achieve the low energy consumption goal, but still maintain the constant current working mode. The low-energy voltage converter works in a switching mode, and controls the switching cycle according to the feedback circuit to achieve a stable output voltage. However, in order to maintain the constant current working state of the LED, the feedback circuit controls the converter switching cycle with the output current.

Kaiyu Technology's T6322 is a step-down constant current LED driver. Its current is determined by an external resistor and can support up to 1.5A output current. It provides ±5% current accuracy and high power efficiency (low energy consumption) and high circuit adjustment capabilities.

Figure 1 compares the power conversion efficiency of the T6322 with other products. Figure 2 compares the line regulation capability of the T6322 with other products (low current fluctuation rate means high line regulation capability).

Figure 1

Figure 2

Among the LED lighting products currently flooding the market, their power input systems are divided into two categories: one is the front-end AC power input system plus the back-end constant current control module, such products include freezer light bars, indoor lamps, street lamps, table lamps, MR16, AR111, etc. The other is the AC power direct input system integrating AC/DC converters and constant current circuits, such products include E27 and GU10 bulb-type LED lamps, PAR lamps, T5 and T8 LED tubes, etc.

In addition to selecting a constant voltage power supply with better front-end efficiency, the first type of power supply design should also use a more efficient power supply design based on the product characteristics of the power module of the back-end constant power control. The advantage is that the front-end AC/DC power supply, such as switching power supplies, DC adapters and other constant voltage power supplies, has a high selection and many safety certification solutions, thus lowering the design threshold.

The second type of power supply design integrates AC/DC circuits and constant current circuits, so it can meet the design requirements of smaller spaces. However, from the perspective of power factor, power efficiency and safety regulations, the design is also difficult. Therefore, few such products in the market today can meet the standards of high luminous efficiency and high power factor (> 0.9).

Conclusion

At present, in order for LED lighting products to achieve high luminous efficiency and become the next generation of main light sources, the first consideration is the design of the power module, and the correct design architecture should be selected for different lighting products. In addition, the power control IC must also improve power efficiency, power factor, and reliability to develop products that meet the needs of LED lighting.