“In recent years, high-brightness LED lighting is gradually replacing traditional light sources such as incandescent lamps and fluorescent lamps with the advantages of high light efficiency, long life, high reliability and no pollution. In some applications, it may be desirable to adjust the brightness of the light in certain situations to further save energy and provide comfortable lighting. Common dimming includes triac dimming, trailing edge dimming, ON/OFF dimming, remote control dimming, etc.The thyristor dimmer has been used in traditional incandescent lamps and other dimming lighting for a long time, and the installation cost is low without changing the wiring. The performance and specifications of various brands of thyristor dimmers are not much different, but they are directly applied There are still a series of problems in LED driving occasions
In recent years, high-brightness LED lighting is gradually replacing traditional light sources such as incandescent lamps and fluorescent lamps with the advantages of high light efficiency, long life, high reliability and no pollution. In some applications, it may be desirable to adjust the brightness of the light in certain situations to further save energy and provide comfortable lighting. Common dimming includes triac dimming, trailing edge dimming, ON/OFF dimming, remote control dimming, etc. The thyristor dimmer has been used in traditional incandescent lamps and other dimming lighting for a long time, and the installation cost is low without changing the wiring. The performance and specifications of various brands of thyristor dimmers are not much different, but they are directly applied There are still a series of problems in LED driving occasions.
1 Triac TRIAC dimming principle
The basic structure of most thyristor dimmers on the market is shown in Figure 1. The working principle is as follows: when the AC voltage is applied to both ends of the bidirectional thyristor TRIAC, since the RC charging circuit composed of Rt and Ct has a charging time, The voltage on the capacitor is charged from 0V, and there is a DIAC (two-way trigger diode, usually about 30V) in series with the driving pole of the TRIAC, so the TRIAC is reliably cut off. When the voltage on Ct rises to 30V, DIAC is triggered and turned on, and TRIAC is turned on reliably. At this time, the voltage at both ends of TRIAC instantly becomes zero, and Ct is rapidly discharged through Rt. When the voltage of Ct drops below 30V, DIAC is turned off. If the current passed by the TRIAC is greater than its holding current, it will continue to conduct, and if it is lower than its holding current, it will be cut off. The role of Inductor L and capacitor C is to reduce the rate of change of current and voltage to suppress electromagnetic interference EMI problems.
If the thyristor front-end dimmer is directly used to control the ordinary LED driver, the LED light will flicker, and it cannot achieve a wide range of dimming control. The reasons boil down to the following:
(1) The maintenance current problem of the thyristor. At present, the power levels of the thyristor dimmers on the market are different. The maintenance current is generally 7-75mA (the driving current is 7-100mA). The current flowing through the thyristor must be greater than this value to continue to conduct. , otherwise it will shut down by itself.
(2) Impedance matching problem. When the thyristor is turned on, the impedance of the thyristor and the driving circuit changes, and the driving circuit presents a capacitive impedance due to the existence of the differential mode filter capacitor, which has the problem of impedance matching with the thyristor dimmer. Therefore, it is generally necessary to use a smaller differential mode filter capacitor when designing a circuit.
(3) Impulse current problem. Due to the front-end chopping of the thyristor, the input voltage may always be near the peak value, the input filter capacitor will bear a large inrush current, and at the same time, the thyristor may be cut off unexpectedly, resulting in the continuous restart of the thyristor, so it is generally necessary to connect the input end of the driver. Series resistor to reduce shock.
(4) When the conduction angle is small, the LED will flicker. When the conduction angle of the thyristor is small, because the input voltage and current are both small at this time, the maintenance current is not enough or the chip power supply Vcc is not enough, and the circuit stops working, causing the LED to flicker.
2 A thyristor dimming LED driver power supply
The problem with linear dimming is that the human eye is very sensitive to subtle changes in light at low brightness; however, when it is brighter, due to the saturation of human vision, large changes in light are difficult to detect. And put forward a method to realize the nonlinear relationship between dimming signal and dimming output (such as exponential, square, etc.) using single chip programming, so that the dimming perceived by human eyes is a linear and stable process.
The circuit designed in this paper uses the RC charging and discharging circuit to realize this function.
Figure 2 is a block diagram of an LED drive circuit that uses a common pulse width modulation PWM chip combined with a peripheral circuit to build a thyristor dimming. The maintenance current compensation circuit controls the current flowing through the maintenance current compensation circuit by detecting the voltage at the R1 terminal (ie, the input current). When the input current is small, a large current flows through the maintenance current compensation circuit; when the input current is large, the maintenance current compensation circuit is turned off, and the maintenance current compensation ensures the maintenance current of the thyristor in the form of a constant current source. The dimming control circuit includes a comparator, an RC charge and discharge circuit and a gain circuit. In the experiment, a thyristor dimmer whose knob stroke is proportional to the chopping angle is selected, and its minimum conduction angle is about 30°.
According to Figure 2, the current reference obtained after the output of the RC charging and discharging circuit passes through the gain circuit is:
Where k is the gain, VC is the input voltage of the RC charging and discharging circuit, τ is the time coefficient of the RC, and θ is the conduction angle of the thyristor.
Then the output corresponding to the minimum conduction angle is zero, that is, the maximum value of the circuit output corresponds to the maximum value of the current reference:
From equations (1) and (2), the output current expression is shown in equation (3), and the relationship between the output current and the conduction angle of the thyristor under different RC time coefficients is shown in Figure 3a).
When the chopping angle is θ, the corresponding input power of the circuit is:
Where Vp is the peak value of the input voltage, and Rin is the equivalent input impedance.
Assuming that the conversion efficiency of the circuit is η, and the output power of the circuit is PO=IO·UO, the equivalent input impedance of the circuit can be obtained as shown in formula (5).
From the formula (5), the power factor of the circuit can be obtained as shown in the formula (6), and the relationship between the power factor and the conduction angle of the thyristor is shown in Figure 3b).
3 Experiments and results
According to the above analysis, this paper designs a thyristor dimming LED driver based on a flyback converter, the control chip is NCP1607; the input AC voltage is 220V, the maximum output power is 25W, and the maximum output current is 0.7A; 10 0.8W LED lights in series) are connected in parallel as the load; the RC time coefficient is selected as 0.5, and the gain is 0.2. The experimental waveform and working characteristic curve of the circuit are shown in Figure 4.
4a), b), and c) are the waveforms of impedance matching switch driving voltage VZ, input current Iin, and input voltage Vin when the conduction angle of the thyristor is 115°. The output current of the circuit is 470mA, and the power factor is 0.78. It can be seen from the figure that when the thyristor is turned on, the input current has an inrush current spike due to the differential mode filter capacitor at the input end of the driver, and when the input current is less than a certain value, the impedance matching switch is turned on to ensure the flow through The current of the thyristor is greater than its holding current.
For the output current curve corresponding to different conduction angles of the thyristor, in the actual debugging, the conduction angle of the thyristor is close to the full-load output after 150°. Figure 4e) is the cosφ curve of the corresponding circuit of the thyristor under different conduction angles.
This paper analyzes the problems existing in the existing thyristor dimmer used for LED driving, and according to the non-linear characteristics of the human eye’s response to light, a non-linear thyristor dimmer using ordinary PWM chips combined with peripheral circuits is designed. Optical LED drive circuit, the working characteristics of the circuit in the dimming process are analyzed, and the experimental results achieve 0-100% smooth and flicker-free dimming.