1 Introduction
After the development of LED lighting, due to its own characteristics of green, energy saving and high efficiency, it has gradually begun to replace traditional lighting sources such as incandescent fluorescent lamps and become a new generation of lighting sources in the 21st century. LED dimming solutions currently on the market include thyristor dimming, pulse width modulation dimming, and segmented switch dimming. Due to the close integration of SCR dimming and incandescent fluorescent lamps, it has a dominant position in the market. Because pulse width modulation cannot be compatible with thyristor leads, it can usually only be applied to dimmable table lamps, while segmented switch dimming cannot achieve continuous dimming. In summary, this article combines the characteristics of DU8608 and the advantages of SCR to complete the SCR dimming LED driver.
2. Introduction of SCR
SCR is also called thyristor. Since its inception in the 1950s, it has developed into a large family. Its main members include unidirectional thyristors, bidirectional thyristors, light-controlled thyristors, reverse-conducting thyristors, turn-off thyristors, fast thyristors, and so on. SCR dimmers are relics of the incandescent lamp era.
The thyristor is equivalent to a controllable diode. When a certain voltage is applied to the control pole, the cathode and the anode are turned on. There are two types of thyristors, one-way thyristors and two-way thyristors, all with three electrodes. Unidirectional thyristor has cathode, anode and control pole. Bidirectional thyristor is equivalent to two single thyristors connected in reverse parallel.
2.1. Working principle of SCR
Caption: Frontier SCR dimmer
Potentiometer R2 adjusts the phase angle of the thyristor. When VC2 exceeds the breakdown voltage of DIAC, the thyristor will turn on at the leading edge of each AC voltage. When the thyristor current drops below its holding current (IH), the thyristor turns off, and it must wait until C2 is recharged in the next half cycle before turning on again. The voltage and current in the bulb filament are closely related to the phase angle of the dimming signal, and the phase angle varies from 0 degrees to 180 degrees.
Once the thyristor is triggered to turn on, it will continue to turn on until the AC voltage crosses zero. In order to avoid the large current impact when starting up, a large current margin is required when selecting SCR.
In addition, the trigger pulse should have sufficient amplitude and width to make the thyristor fully conductive. In order to ensure that the thyristor can trigger reliably under various conditions, the trigger voltage and current sent by the trigger circuit must be greater than the minimum value of the trigger voltage UGT of the thyristor and the IGT of the trigger current, and the minimum width of the trigger pulse must continue When the anode current rises above the maintenance current, otherwise the thyristor will turn off again because it is not fully turned on.
2.2. Advantages of SCR
Ordinary incandescent lamps and halogen lamps usually use thyristors for dimming. Because incandescent lamps and halogen lamps are purely resistive devices, it does not require the input voltage to be a sine wave, because its current waveform is always the same as the voltage waveform, so no matter how the voltage waveform deviates from the sine wave, as long as the effective value of the input voltage is changed, You can dim it. The use of SCR is to cut the sine wave of alternating current to achieve the purpose of changing its effective value. The load is in series with the thyristor switch.
Changing the variable voltage divider ratio can change its conduction angle, so as to achieve the purpose of changing its effective value. Usually this potentiometer has a switch for switching lights. In addition to thyristors, there are dimming techniques for the trailing edge of transistors.
3. Introduction of DU8608
DU8608 is a high-precision buck LED constant current power supply control chip mainly used in non-isolated LED constant current drive power supply system. The system works in continuous mode of inductor current (CCM). Using the unique closed-loop constant current control patent TRUEC2 technology, the LED power supply can maintain a constant current accuracy of less than 3% under a wide input voltage, a wide output voltage, and a wide inductance change. DU8608 adopts the source drive structure, which makes the system efficiency up to 95%. DU8608 has built-in analog and PWM digital dimming functions. DU8608 integrates various protection functions, including output short circuit, output open circuit, main inductor short circuit protection, sampling resistor open circuit, sampling resistor short circuit and over temperature protection. Thereby improving the reliability of LED constant current power supply.
DU8608 has the following characteristics:
TRUEC2 closed loop constant current control technology
3% system constant current accuracy
Analog dimming, PWM digital dimming
Sampling resistor open circuit, short circuit protection
Output over current and short circuit protection
Main inductance short circuit protection
Output overvoltage protection
Over temperature protection
Source driver
4. Experimental verification
4.1. Experimental model
The experimental model is shown in Figure 1 and Figure 2.
4.2. Experimental schematic
The experimental schematic diagram is shown in Figure 3.
4.3. Experimental results
Since the thyristor fixes the input voltage at one point, the parameters of this experiment are as follows:
Input voltage: 220V / 50Hz
Output voltage: 76V
Rated output current: 250mA
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