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LED fluorescent lamp drive circuit design

This paper introduces the characteristics of LED fluorescent lamp drive, designs a practical capacitor step – down LED fluorescent lamp drive circuit, and analyzes the selection principle of key component parameters. PSp ice simulation software was used to verify the feasibility of the circuit, and on this basis, the production of physical circuit, used as 12W T8 standard LED fluorescent power. Experiments show that the circuit is stable and reliable, low cost, suitable for a variety of low-power LED driver.

0 Introduction

Since the late 1960s, the use of phosphide on the gallium arsenide substrate invented the first visible red LED, with the semiconductor and LED packaging technology breakthrough, single chip red, green, blue, white LED power level And brightness has been improved. At present, high-power white LED has reached 70 lm / W. With the LED light efficiency, life and light color on the obvious increase in its display, backlight, decoration and lighting and other fields have great market potential. Although the LED application level continues to expand, life and light efficiency is higher than the traditional lighting, but the cost is relatively high, the proportion of LED light source cost in the total cost of up to 50% to 90%, in the lighting technology has yet to be improved, and high-power LED Cooling and light effects highlighted, which limits the LED in the field of lighting in the further application.

At present, low-power LED will be used in parallel with the LED, in series, and the use of the process as long as there is an LED short circuit or open circuit, will lead to small pieces or the entire LED off, affecting the lighting effect, so simple, inexpensive drive circuit Significance. According to the current status of LED fluorescent applications, the design of a simple, reliable 12W standard T8 LED fluorescent lamp drive circuit.

1 fluorescent lamp circuit design

1. 1 LED fluorescent lamp driver

The current low-power lighting products, widely used in two drive circuit form: constant current drive and regulator drive. The output current of the former circuit is constant, the output voltage changes with the load changes, and constant current drive is usually used constant current IC, the use of the IC to withstand the maximum voltage required to limit the number of LED use. The latter output voltage is fixed, the output current with the load (LED) increase or decrease the number of changes. Experiments show that, because the LED package in its forward voltage drop discrete value, and LED brightness output is proportional to its current, LED brightness consistency is poor, but by adding a series of appropriate resistance can make each string of LED brightness average, more suitable for Low-end lighting market.

1. 2 LED fluorescent lamp circuit design

LED fluorescent lamp drive circuit schematic diagram shown in Figure 1.

Figure 1 LED fluorescent lamp drive circuit

Figure 1 LED fluorescent lamp drive circuit

The circuit is driving a total of 140 white LED (low power), using 35 string 4 and the mode, the use of capacitive step-down drive.

Among them, C1, C4 in parallel with the same two capacitors, buck and limiting the role of four 1N4007 composition of the rectifier bridge on the input AC voltage rectifier; filter capacitor C3 used to filter out the rectified output voltage AC component , The voltage is more smooth; L1, C2 is used to filter out the high-frequency components in the output voltage; resistor R4 to provide a discharge circuit C3; SCR729210 one-way thyristor to protect the circuit, R3 is the current limiting resistor.

1. 2. 1 step-down capacitor selection

Because the current IO supplied to the load through the buck capacitor C is actually the charge and discharge current IC flowing through C. When the load current IO is less than the charge and discharge current IC of C, the excess current flows through the filter capacitor C2.

The following are the same as the “

Where Ui — input AC voltage RMS.

F – AC signal frequency.

Therefore, at least the required step-down capacitor value Cmin = 0. 928 μF is required for the 64 mA current IO consumed by the load. In addition, to ensure reliable operation of C, the withstand voltage should be chosen to be greater than twice the supply voltage, 684MF / 630 V capacitors work in parallel.

1. 2. 2 output rectifier and filter circuit

According to some literature, a single diode on the rectifier bridge to withstand the maximum voltage URM = √2Ui (Ui is the effective value of the input voltage) = 318. 4 V, therefore, the use of commonly used rectifier diode 1N4007 (URM = 1000 V, IF = 1 A).

RL C ≥ (3 ~ 5) T / 2, where RL is the load impedance, T is the input signal period (0. 02 s), available C ≥ 24.38 μF for the output terminal to obtain a smooth load voltage In principle, the larger the capacitance value, the smoother the output voltage, the smaller the ripple value. However, with the increase in capacitance, its volume also increases, taking into account the circuit to be installed in the ordinary T8 lamp, take the 33μF / 160 V electrolytic capacitor; at the same time to get a smoother output voltage, select L1 is 100 μH wire wound inductor, C1 is 0. 01μF ceramic capacitors.

1. 2. 3 Output protection circuit design

LED current used in the LED can not exceed its stability, long-term over load will not only increase the brightness (white LED in a large current under the saturation phenomenon, a significant reduction in luminous efficiency), but also shorten the LED life, the impact of LED lighting Circuit reliability. As the LED forward conduction, the forward voltage of the small changes will cause a significant change in LED current, therefore, need to set the output output protection circuit.

t8 tube led

The circuit is composed of VT1, R2, R3, VT1 uses MotorM2la MCR729210 (UDRM = 800 V, the control trigger voltage UGT = 0. 8 V, trigger current IGT = 10 mA) .R2 for the unidirectional thyristor to provide trigger bias Voltage, the choice of its resistance is essential, if the value is too large, the circuit in some of the unstable factors lead to LED current instantly larger, will lead to frequent trigger thyristor, protection circuit frequently work, resulting in circuit work is not normal ; If the resistance is too small, the LED may exceed its specifications to work under the stability of the protection circuit is not sensitive, will lead to LED life shortened. Since the LED branch current is 16 mA, then:

The following are the same as the “

To leave a certain margin, select R2 = 10Ω.

2 Simulation and experimental analysis

2. 1 pspice simulation of the fluorescent circuit

In order to verify the feasibility of the design, PSp ice software is used to simulate the circuit. The results are shown in Fig. The output voltage is 97 ± 2 2 V and the output current is 14. 7 ± 0. 3 mA, which verifies the feasibility of the circuit design when the circuit is stable at 210 ms. LED fluorescent lamp circuit output characteristics shown in Figure 2.

Figure 2 LED fluorescent lamp circuit output characteristics

Figure 2 LED fluorescent lamp circuit output characteristics.

To verify the function of the output protection circuit, perform parametric scanning analysis of the resistor R2, and then perform the performance analysis. The results are shown in Fig. It can be seen that when R2 increases from 1 Ω to 13. 889 Ω, the output current begins to decrease, indicating that the unidirectional thyristor is in the critical conduction state at this time; then, as R2 continues to increase, the output current drops sharply until R2 For 15. 119Ω, the output current stabilizes at about 5. 2 mA, indicating that the unidirectional thyristor has been fully turned on and the protection circuit is already active.

2. 2 test data

From the results of PSp ice simulation, the circuit design is feasible, so according to the circuit produced 12W fluorescent lamp. Standard lamp for the 1 190 mm × 30 mm, using two identical PCB circuit board (size 593 mm × 25 mm) docked, and the use of Huzhou Sheng spectrum SPL2000 light source light color integrated analysis system on the circuit related parameters taking the test. When the measured input voltage VAC is 220 V, the total current of the circuit is 127 mA, which is equivalent to 15. 85 mA for each path, 0.465 for the circuit power factor and 11. 4W for the circuit.

The relationship between the capacitance value of the buck capacitor C1 and the output current is shown in Fig. 4 (since the two capacitors are the same and only a single capacitance is shown in Fig. 4). It can be seen that with the increase of the capacitance value, the output current is almost linearly rising, to be increased to a certain value (about 20 mA), the protection circuit works, forcing the current through the LED branch down.

Figure 4 the relationship between the measured output current and the step-down capacitor

Figure 4 the relationship between the measured output current and the step-down capacitor

3 Conclusion

Capacitive step-down drive circuit designed 12 W standard T8 LED fluorescent lamp circuit. The circuit has a small size, low cost, by changing the buck capacitor can be used as a variety of LED lighting power. Although the power supply power factor is low, but particularly suitable for low-end lighting market applications. According to this circuit, by changing the buck capacitor value, produced a total of 1W, 4W, 8 W, 12 W and other lighting products.

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