Teach you a thyristor dimming LED drive power circuit design

The basic structure of most triac dimmers on the market is shown in Figure 1. Their working principle is as follows: when the AC voltage is applied to both ends of the triac TRIAC, the RC charging circuit composed of Rt and Ct has a charging time. The voltage on the capacitor starts charging from 0V, and the driving pole of the TRIAC is connected in series with a DIAC (bidirectional trigger diode, usually about 30V), so the TRIAC is cut off reliably. When the voltage on Ct rises to 30V, DIAC triggers conduction, and TRIAC conducts 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 If the current flowing through the TRIAC is greater than its holding current, it will continue to conduct. If it is lower than its holding current, it will be turned off. The function of the 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 cutting-edge dimmer is directly used to control ordinary LED drivers, the LED lights will flicker, and wide-range dimming control cannot be achieved. The reasons boil down to this:

(1) The problem of maintaining current of silicon controlled thyristor. The power levels of thyristor dimmers currently on the market are different. The maintenance current is generally 7~75mA (the driving current is 7~100mA). The current flowing through the thyristor after it is turned on must be greater than this value before it can continue to conduct. , otherwise it will shut down by itself.

(2) Impedance matching problem. When the thyristor is turned on, the impedances of both the thyristor and the drive circuit change, and the drive circuit presents a capacitive impedance due to the presence of differential mode filter capacitors, which causes impedance matching problems with the thyristor dimmer. Therefore, it is generally necessary to use smaller differential mode filter capacitors when designing circuits.

(3) Inrush current problem. Since the input voltage may always be near the peak due to the cutting edge of the thyristor, the input filter capacitor will bear a large inrush current, and may also cause the thyristor to unexpectedly cut off, causing the thyristor to continuously restart. Therefore, it is generally necessary to connect the input voltage to the driver input end. Connect resistors in series to reduce impact.

(4) The LED will flicker when the conduction angle is small. When the conduction angle of the thyristor is small, because the input voltage and current are both small at this time, the maintaining current is insufficient or the chip power supply Vcc is insufficient, and the circuit stops working, causing the LED to flicker.

2 A thyristor dimming LED driving power supply

The problem with linear dimming is that the human eye is very sensitive to subtle changes in light at low brightness; while at brighter times, due to the saturation of human vision, larger changes in light are not easily detected. And a method of using microcontroller programming to realize the nonlinear relationship (such as exponential, square, etc.) between the dimming signal and the dimming output is proposed, so that the dimming perceived by the human eye is a linear and stable process.

The circuit designed in this article uses RC charge and discharge circuit to achieve this function.

Figure 2 is a block diagram of an LED drive circuit that uses an ordinary pulse width modulation PWM chip combined with peripheral circuits to build a thyristor dimming LED. The sustaining current compensation circuit controls the current flowing through the sustaining current compensation circuit by detecting the voltage at the R1 terminal (ie, the input current). When the input current is small, a larger 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, RC charge and discharge circuit and gain circuit. In the experiment, a thyristor dimmer whose knob stroke is proportional to the chopping angle was selected, and its minimum conduction angle is about 30°.

According to Figure 2, the current reference obtained by the output of the RC charge and discharge circuit after passing through the gain circuit is:

In the formula, k is the gain, VC is the input voltage of the RC charge and discharge 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 can be obtained as shown in equation (3). The relationship between the output current and the thyristor conduction angle under different RC time coefficients is shown in Figure 3a).

When the chopping angle is θ, the corresponding input power of the circuit is:

In the formula, 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 Equation (5).

From equation (5), we can get the power factor of the circuit as shown in equation (6). The relationship between the power factor and the conduction angle of the thyristor is shown in Figure 3b).

3 Experiments and results

Based on the above analysis, this article 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; with 3 strings (each A series of 10 0.8W LED lamps are connected in parallel as a load; the RC time coefficient is selected as 0.5, and the gain is 0.2. The experimental waveform and operating characteristic curve of the circuit are shown in Figure 4.

4a), b), and c) are the waveforms of the impedance matching switch driving voltage VZ, input current Iin, and input voltage Vin when the thyristor conduction angle 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 driver input end. When the input current is less than a certain value, the impedance matching switch is opened to ensure that the flow can The current of the silicon control is greater than its holding current.

These are the output current curves corresponding to different conduction angles of the thyristor. In actual debugging, the thyristor conduction angle is close to full load output after 150°. Figure 4e) shows the cosφ curve of the corresponding circuit of the thyristor at different conduction angles.