PWM dimming technology: how to optimize LED color stability

This article explores the method of providing dimming ( LED ) dimming, analyzing the effects of LED dimming on its long-term performance and the color stability of emitted light, and specifically discusses how to use a linear constant current regulator (CCR) and Digital transistors provide pulse width modulation (PWM) dimming.

PWM is the primary method to change the LED light output

How much is enough? The amount of luminescence required is usually determined by the environment and use. In the case of conference rooms, the lighting during a typical roundtable may be very bright; however, if a projection system is used to view the meeting materials, the light in the meeting room may be much darker; the lighting in the living room is usually adjusted to create the proper atmosphere. The backlight of the car's dashboard is usually adjusted automatically according to day or night driving.

Light sources have moved from incandescent lamps that are very easy to adjust to fluorescent lamps that require different levels of light in special circuits. LEDs are the latest generation of light sources in the industry, requiring new electronic circuits to change the light level. There are two ways to change the LED light output. The first method is to reduce the current flowing through the LED. The amount of light emitted by the LED is proportional to the current flowing through the LED. The second method is to provide pulse current to the LED. The light is proportional to the duty cycle.

LED dimming using a reduced current approach is similar to the dimming method used by most incandescent bulbs. This method of reducing current is acceptable in a small number of applications, but as the light level decreases, the color of the light emitted by the LED also varies according to the current. When the current is small, the chroma (Chromaticity) changes toward yellow. Figure 1 shows typical chromaticity changes at different currents.

Typical chromaticity changes when LEDs have different forward currents

Figure 1 Typical chromaticity changes of LEDs with different forward currents

The brightness of an incandescent bulb can be dimmed to less than 1% of full brightness by reducing the current because the incandescent lamp flows through the filament, causing the filament to heat up and illuminate. However, with the method of reducing the current, it is extremely difficult to adjust the brightness of the LED to less than 5% of the full brightness because the required voltage potential causes the tantalum node to break and begin to conduct electricity. The LED is a diode. As the junction breaks, it is like an electronic avalanche, and the resistance drops rapidly. It is very difficult to provide sufficient voltage to make the LED conduct electricity at very low currents. Figure 2 shows a plot of typical forward current for LEDs at different forward voltage (V) conditions.

Typical forward current of LEDs under different forward voltage conditions

Figure 2 Typical forward current of LED under different forward voltage conditions

The method of providing pulse current to LEDs, also known as PWM, has become the preferred method of changing the light level. The LEDs themselves are germanium components that turn on and off quickly, corresponding to the conduction and shutdown of current through them. The switching time is on the order of 100 nanoseconds (ns), which is equivalent to a maximum frequency of 10 MHz. Applications typically operate at frequencies from 100 Hz to 100 kHz. When the frequency is lower than 100 Hz, the human eye will observe the LED light flickering. When the frequency is between 500 Hz and 20 kHz, the circuit may generate audible noise. Dimming is achieved by turning the LED on for a certain period of a single switching cycle; and turning the LED off during the rest of the switching cycle. This turn-on-off period is called the duty cycle (D) and is expressed as the LED turn-on time (TON) divided by the entire switch/on-cycle time (TS) (Figure 3).

LED switching cycle diagram and duty cycle calculation formula

Figure 3 LED switching cycle diagram and work cycle calculation formula

With a constant current of IC, the working voltage of the IC is 12-24V. As long as the voltage is not lower than the rated voltage, the current of each led beacon will be consistent.It is made of high-brightness LED (LED is made of imported chip---high brightness, good electrical).

 No pressure drop can be achieved over a specific length and voltage.Suitable for urban lighting projects,home decoration, bars, dance halls, cars, indoor/outdoor advertising and interior and exterior contours of various high-end venues

Constant Current LED Strip

Constant Current LED Strip,5050 RGB LED Strip,Constant Current LED Strip Flexible,Bright Constant Current LED Strip

SHEN ZHEN SEL LIGHTING CO.,LTD , https://www.sellighting.com

Posted on