Under such circumstances, you may probably choose to use step-down (wide voltage) constant-current sources, like using 10V-30V constant-current power suppliers to realize the LED dimming technology. However, this kind of step-down constant-current source still has defects. When the forward input voltage has been adjusted into a certain low level, the load current of LEDs will be forced down to a very low level. This would dramatically increase the entire potential (voltage) of load (LED lights), even exceeding the normal working range of this step-down constant-current source (causing flicker). Working in the low brightness conditions for a long time will decline the efficiency of step-down constant-current sources and increase the working temperature in the mean time. Since the efficiency of step-down constant-current sources has directly relations with the potential drop ratio. The larger the potential drop ratio, the lower the efficiency, and more power will be consumed, causing obvious damages to power suppliers and the service life of lighting sources.

  Commonly used incandescent lamps and halogen lamps usually used SCR dimming technology. Since all of them are purely resistive devices, the input signal of these lamps are not necessarily sine waves like LED lighting devices required. The waveform of current will be indentified with the waveform of voltage forever. Thus regardless of the distortion of the input voltage waveform to a sine wave, changing the effective value of input voltage can realize the dimming effect.   However, the SCR will bring unexpected problems to LED dimming technology: LC filters in the input side will bring vibrations to SCR. This vibration has no effects on incandescent lamps, since the thermal inertia them hardly can be captured by human eyes. But this vibration will exert very obvious influences on LED power suppliers, causing audio noise and flickers. Besides, SCR dimming will also destroy the waveform of sine waves, reducing the power factor (normally less than 0.5). In order words, SCR dimming dramatically decline the lighting efficiency of LED lighting systems. Sometimes, SCR dimming technology will increase harmonic coefficients, generating non-sinusoidal waveforms, which will be a serious interference to single (EM), polluting the power grid. In serious conditions, the entire power grid will be failed.   Either step-down constant-current sources or SCR dimming sources are suitable for LED dimming technology. Some people may ask: what on earth type of dimming method is the most appropriate for this new lighting technology?

How to comprehend LED dimming technology? How to know it well and master it? In order to answer these questions, we need first understand the voltage-current characteristic of LEDs.

What is the voltage-current characteristic of LEDs? It is known by all that when voltage worked on PN junctions (typical construction of semiconductors), internal current will be varied with the vibration of input voltage. The characteristic curve can be cleared display on oscilloscope. A complete voltage-current curve includes the forward characteristic curve and the reverse characteristic curve. Typically, the reverse characteristic curve will be much steeper than the forward one. When input voltage surpassed a certain threshold value, the corresponding internal current will increase exponentially, and consequently, breakdown the entire LED PN junction. While the forward internal voltage is also determined by its forward internal current.

According to the voltage-current characteristic, we could conclude that LED dimming technology definitely cannot be realized through simply reducing input current or input voltage. In addition, LEDs’ sinusoidal waveform is different from the waveform of incandescent lamps, thus it is also impossible to vary the lighting effects (dimming effects) by changing their conduction angles.

For example, a LED lamp with an input voltage of 24VDC, with 8 pieces high-powered LED chips (1W/each) in series. Given that the forward current is about 350mA and the forward internal voltage for each LED chips supposed to be 3.3V, thus the total input voltage for the entire lighting sources (8 pieces LEDs connected in series) should be 26.4V. It is cleared that the load voltage (26.4V) is much higher than the input voltage (24V), thus here constant-current power suppliers should be applied. In order to realize the dimming effect, engineers have to reduce the internal current from 350mA to 100mA, causing the forward voltage in each LED chip reduced to only 2.8V. Then the total input voltage of the entire system is 22.4V (now the load voltage is smaller than the input voltage). So the constant-current is incompetent. That is the reason why many terminal users or buyers saw flickers of their LED lights. Theoretically, according to the working principles of LEDs, LED lighting applications will be free from flickers in ideal conditions.

Dimming is a commonly used technology in the lighting industry. For filament lamps, dimming technology can be easily realized on them in the case of low production costs. While LED dimming technology is much more difficult. For architects and ordinary families, they have no willing to lose the dimming function in the process of replacing traditional lamps with LED lighting products. In this article, we will give a small talk on LED dimming technology to provide some helpful information for someone needed them.

Power Factor (PF) is a rather important parameter among electrical appliances. A higher PF can effectively reduce the energy consumption of distribution network. Thus regulatory agencies around the world are striving to restrict the PF of their electrical appliances within a reasonable region. Energy Star Solid-state Lighting Efficiency Certificate is one famous example, in which the PF of residential electrical applications should be larger than 0.7 and the PF of commercial applications should be larger than 0.9.

There are three major methods for LED dimming technology. All of them realize the dimming function through changes of LED drives current. They also can be divided into simulated dimming and PWM dimming on the base of different circuit systems.

1)       Dimming LED lights by modulating the derive current. Brightness of LED chips is proportional with the LED drive current, therefore it is available for us to dim LED lights by varying the input current.

2)       Pulse Width Modulation (PWM). ① Convert the input current into square waves, increasing its pulse width; ② Adjust the constant lighting time by adjusting the pulse width, realizing the PF control and dimming at the same time. The dimming frequency is about 200Hz – 10KHz. Due to the visual lag of our eyes, flickers cannot be seen by us. PWM are good at improving cooling sections, which is the biggest advantage of it. However, it also has defects: the excessive input pulse may reduce the service life of LED chips. Currently, PWM is the most common method in LED dimming area.

3)       VID (voltage ID) method. The linear increase or decrease of drive current will have less effect on LED chips. This method is a simulation of power-on/power-off state, also named linear dimming method.