1 Introduction
After the development of LED lighting, due to its green, energy-saving and high-efficiency characteristics, it has gradually replaced traditional lighting sources such as incandescent fluorescent lamps and become a new generation of lighting sources in the 21st century. At present, LED dimming solutions existing on the market include thyristor dimming, pulse width modulation dimming, and segmented switch dimming. Due to the tight integration of thyristor dimming and incandescent fluorescent lamps, it is dominant in the market. Pulse width modulation is usually only applicable to dimming lamps because it cannot be compatible with thyristor leads, and segmented switch dimming cannot achieve continuous dimming. In summary, this paper combines the characteristics of DU8608 and the advantages of thyristor to complete the thyristor dimming LED driver.
2. Introduction to SCR
Thyristors are also known as thyristors. Since its inception in the 1950s, it has grown into a large family. Its main members are unidirectional thyristors, bidirectional thyristors, light-controlled thyristors, reverse-thyristing thyristors, turn-off thyristors, fast thyristors, and so on. The thyristor dimmer is a relic of the incandescent era.
The thyristor is equivalent to a controllable diode. When the control electrode is applied with a certain voltage, the cathode and the anode are turned on. The thyristor is a unidirectional thyristor and a two-way thyristor, and is three electrodes. The unidirectional thyristor has a cathode, an anode, and a control electrode. The two-way thyristor is equivalent to the reverse parallel connection of two single thyristors.
2.1. How does the thyristor work?
Caption: Front edge thyristor dimmer
Potentiometer R2 adjusts the phase angle of the thyristor. When VC2 exceeds the breakdown voltage of DIAC, the thyristor turns on at the leading edge of each AC voltage. When the thyristor current drops below its holding current (IH), the thyristor turns off and must wait until C2 is recharged in the next half cycle before turning it back on. The voltage and current in the filament of the bulb are closely related to the phase angle of the dimming signal, and the phase angle varies from 0 to 180 degrees.
Once the thyristor is triggered to conduct, it will continue to conduct until the AC voltage crosses zero. In order to avoid large current surges during startup, the thyristor needs to have a large current margin.
In addition, the trigger pulse should have sufficient amplitude and width to fully turn the thyristor on. In order to ensure that the thyristor can be reliably triggered 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 should be continued. When the anode current rises above the holding current, the thyristor will be turned off again because it is not fully turned on.
2.2. Advantages of SCR
Conventional incandescent and halogen lamps are typically dimmed with a thyristor. Because incandescent and halogen lamps are purely resistive devices, it does not require that the input voltage be a sine wave, because its current waveform is always the same as the voltage waveform, so regardless of how the voltage waveform deviates from the sine wave, just change the effective value of the input voltage. You can dim. The use of thyristors is to cut the sine wave of the alternating current to achieve the purpose of changing its effective value. The load is in series with the thyristor switch.
Changing the voltage division ratio of the variable resistor changes its conduction angle, thereby achieving the purpose of changing its effective value. Usually this potentiometer has a switch for switching lights. In addition to thyristors, there are transistor trailing edge dimming techniques.
3. Introduction of DU8608
DU8608 is a high precision step-down LED constant current power supply control chip mainly used in non-isolated LED constant current driving power system. The system operates in Inductor Current Continuous Mode (CCM). The patented TRUEC2 technology with unique closed-loop constant current control enables the LED power supply to maintain a constant current accuracy of less than 3% with wide input voltage, wide output voltage and wide inductance. The DU8608 features a source driver structure that allows system efficiency up to 95%. The DU8608 has built-in analog and PWM digital dimming. The DU8608 integrates various protection functions including output short circuit, open output, main inductor short circuit protection, sampling resistor open circuit, sampling resistor short circuit and over temperature protection. Thereby improving the reliability of the LED constant current power supply.
The DU8608 has the following features:
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 overcurrent, short circuit protection
Main inductor short circuit protection
Output overvoltage protection
Over temperature protection
Source driver
4. Experimental verification
4.1. Experimental template
The experimental sample 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 a point, the experimental parameters are as follows:
Input voltage: 220V/50Hz
Output voltage: 76V
Rated output current: 250mA
