LED dimming technology
Why do you need to dim the lamps? As saying goes: By dimming, you can get a more comfortable environment in the light, and you can achieve energy saving and emission reduction.
How to achieve LED dimming? LED dimming currently has two ideas three ways: the first one is linear analog to adjust the LED current; the second is PWM digital dimming, which is to use the switching circuit in relation to the human eye recognition Work at a high enough frequency to change the average of the light output.
1: Linear dimming, that is, by adjusting the magnitude of the LED forward current, this method is a relatively primitive one.
Changing the current through the LED to change the brightness of the LED, the relationship between the LED output and the relative light intensity and forward current is almost positive linear. Figure 1. Dimming is performed by a variable resistor in the lighting circuit. The application is very simple and does not cause interference, but the principle of voltage division is that the electric energy is not completely used in the lamps (electrical appliances), and there is no efficiency at all. When dimming the light, the dimming resistor generates a large amount of heat due to excessive partial pressure, resulting in waste of energy and environmental degradation.
For a 3W high-power LED lamp post, the light output at 350mA is 100%, then the light output at 200mA is about 60%, and at 100mA is about 25%. Therefore, the adjustment of the brightness can be easily achieved by adjusting the current.
The disadvantages of this original dimming are also relatively large, mainly
1: adjusting the brightness by adjusting the forward current, and changing the spectrum and color temperature while adjusting the brightness. Because white LEDs are currently generated by blue LEDs to stimulate yellow phosphors, when the forward current is reduced, the brightness of the blue LEDs increases and the thickness of the yellow phosphors is not scaled down, causing color shifts. The human eye is very sensitive to color deviations. Disadvantage
2, LED is usually driven by DC-DC constant current driving power supply, while constant current driving source is usually divided into two types: boost type or buck type. The decrease of forward current also causes forward voltage. Reduced. Therefore, when the current is turned down, the forward voltage of the LED is also reduced. This will change the relationship between the supply voltage and the load voltage. Adjusting the current creates a serious problem that prevents the constant current source from working.
Seconds,Analog dimming: It can control silicon dimming and compare traditional analog dimming methods.
Common analog dimmings include two-way thyristor dimming, trailing edge dimming, ON/OFF dimming, and remote dimming. The thyristor dimmer has been used in dimming lighting such as traditional incandescent lamps for a long time, without changing the wiring, and the device cost is low. The performance and specifications of the brand thyristor dimmers are not much different, but they are directly applied in There are still a series of problems in LED driving applications.
The basic principle of thyristor dimming is shown in Figure 2:
This dimming method is relatively reliable and is based on the principle of thyristor dimming of ordinary incandescent lamps and halogen lamps. Because incandescent and halogen lamps are purely resistive devices, it does not require the input voltage to be a sine wave. The current waveform is always the same as the voltage waveform. No matter how the voltage waveform deviates from the sine wave, you can dim the light by changing the effective value of the input voltage.
The principle of the LED thyristor dimming circuit is as follows: Figure 3: When the AC voltage is added to the two ends of the triac TRIAC, the RC charging circuit composed of Rt and Ct has a charging time, and the voltage on the capacitor is charged from 0V. And the TRIAC drive series has a DIAC (bidirectional trigger diode, generally about 30V), so the TRIAC is reliably cut off. When the voltage on Ct rises to 30V, DIAC triggers conduction, TRIAC is reliably turned on, and the voltage across TRIAC instantly becomes zero, Ct is quickly discharged through Rt. When Ct voltage drops below 30V, DIAC is cut off. If the current passed by the TRIAC is greater than its holding current, it will continue to conduct. If it is lower than its holding current, it will be cut off. The function of the inductor L and the capacitor C is to reduce the rate of change of current and voltage to suppress electromagnetic interference EMI problems.