Editor's Note: Fluorescent lamps are a popular source of light, soft and effective in the world, whether in homes, shops, offices, schools, supermarkets, hospitals, theaters, commercial freezers, advertising light boxes, subways, pedestrian tunnels, Civil air defense projects, night market lighting, etc., fluorescent lamps can be seen wherever lighting is required.
The traditional fluorescent fluorescent lamp has a poor power utilization rate: the additional ballast consumes a large amount of power, and the auxiliary high voltage is required when it is turned on; the mercury built in the fluorescent tube cannot be disposed of when it is discarded, and it becomes a pollution hazard to the environment. The fluorescent powder of the fluorescent tube contains a large amount of mercury (mercury) in the process of charging the fluorescent tube. Therefore, after the fluorescent tube is broken, the mercury vapor that is escaping is harmful to the human body. Authoritative data show that mercury vapours of 0.04 to 3 mg will cause chronic poisoning within 2 to 3 months, and 1.2 to 8.5 mg will induce acute mercury poisoning. If the amount reaches 20 mg, it will directly lead to animal death.
Energy saving is the biggest feature of LED fluorescent lamps. Taking the T8 fluorescent lamp as an example, the nominal 36W fluorescent fluorescent lamp (CFL) consumes 8W of additional ballasts, consumes 44W of electricity during operation, illuminates lumens to 420lm, and has a service life of 3,000 hours. The LED fluorescent lamp of the same specification, when operating, actually consumes only 16W, illuminates lumens to 550lm, and has a service life of 30,000 hours.
PWM LED Driver Controller PT4107
There are many kinds of LED light bar power supply driving schemes for LED fluorescent lamps. At present, non-isolated solutions are dominant because of their high efficiency, and PWM LED driving controllers are used for LED fluorescent lamp driving power.
The PT4107 is a typical PWM LED driver controller with an internal topology as shown in Figure 1.
The PT4107 is a high voltage buck PWM LED driver controller that can be used to clamp a rectified 110V or 220V AC voltage to 20V through an external resistor and an internal Zener diode. When the voltage on Vin exceeds the undervoltage lockout threshold of 18V, the chip begins to operate, driving the external MOSFET in a peak current controlled mode. A current sampling resistor is connected between the source of the external MOSFET and ground, and the voltage across the resistor is directly transferred to the CS terminal of the PT4107 chip. When the voltage at the CS terminal exceeds the internal current sampling threshold voltage, the drive signal at the GATE terminal is terminated and the external MOSFET is turned off. The threshold voltage can be set internally or by applying a voltage at the LD terminal. If a soft start is required, a capacitor can be shunted at the LD terminal to get the desired voltage rise rate and coincide with the LED current rise rate.
6. PWMD PWM dimming and enable input, the chip has a 100K pull-up resistor inside;
7. VIN chip power terminal;
8. GATE drives the external MOSFET gate;
Taking the AC 85V~245V full voltage input as an example, the PT4107 PWM LED driver controller is used as the main chip of the LED fluorescent lamp driving power supply, and an ideal application circuit scheme is designed (Fig. 3). The scheme consists of anti-surge protection, EMC filtering, full bridge rectification, passive power factor correction (PFC), buck regulator, PWM LED drive controller, and current spreading constant current circuit.
The arrangement of the PCB board is the key to making the product well, so the wiring of the PCB board should be designed according to the requirements of the power electronics specification. This circuit can be used for T10 and T8 fluorescent tubes. Because the space of the two tubes is different, the width of the two PCB boards will be different. It is necessary to reduce the height of all the parts so that the T10 and T8 tubes can be placed. Figure 5 is a photo of the T10 constant current source board with 33 components mounted on a 235 ??? 25 Ã… 0.8 mm epoxy single-sided printed board.
Key design and considerations
1. Anti-surge NTC.
The surge-resistant NTC uses a 300?/0.3A thermistor. If you change the output of this solution, such as increasing the current, the current of the NTC should also be selected to avoid over-current self-heating.
2. EMC filtering
3. Full bridge rectification
The full-bridge rectifier BD1 mainly performs AC/DC conversion, so it is necessary to give a safety margin of 1.5 coefficients. It is recommended to use 600V/1A.
4. Passive PFC
The current output of the ordinary bridge rectifier is pulsating DC, the current is discontinuous, the harmonic distortion is large, and the power factor is low. Therefore, a low-cost passive power factor compensation circuit needs to be added, as shown in FIG. 6. This circuit is called a balanced half-bridge compensation circuit. C1 and D1 form one arm of the half bridge, C2 and D2 form the other arm of the half bridge, and D3 and R form a charging connection path, which is compensated by the valley filling principle. The filter capacitors C1 and C2 are connected in series. The voltage on the capacitor is charged to half of the input voltage. Once the line voltage drops below half of the input voltage, diodes D1 and D2 are forward biased, causing C1 and C2 to start parallel discharge. Thus, the conduction angle of the positive half-cycle input current rises from the original 75 ??? to 105 ??? to 30 ??? to 150 ???; the conduction angle of the negative half cycle input current is from the original 255 ??? 285??? rose to 210??? to 330??? (Figure 7). The resistor R in series with D3 helps smooth the input current spikes and also improves the power factor by limiting the current flowing into capacitors C1 and C2. With this circuit, the power factor of the system is increased from 0.6 to 0.89. R has surge buffering and current limiting functions, so it should not be omitted.
The circuit that powers the PT4107 is a multi-capacitor ripple filter (Figure 8) that has the dual function of a capacitor-multiplying low-pass filter and a series regulator. Connect a capacitor C4 from the base of the emitter output to ground. Since the base current has only 1/(1+?) of the emitter current, it is equivalent to a capacitance value (1+???) at the emitter. The large capacitance of C4, this is the principle of the capacitance multiplying filter. If a Zener diode is connected between the base and ground, it is a simple series regulator that effectively eliminates high frequency switching ripple. Note that T1 selects the V bceo 500V of the bipolar transistor, Ic = 100mA. Zener diode D4 should use 20V, 1/4W small power regulator tube of any type.
6. Ballast power inductor
The ballast power inductor L3 and Q1 MOS tube, and the current sampling resistor connected in parallel with R6, R7, R8 and R9 are the three key components of the constant current output of this circuit. The ballast power inductor L3 requires an inductor with a high Q value, a large saturation current, and a resistance calibration of 3.9 mH. The Q value should be greater than 90 in the frequency range of 40 kHz to 100 kHz. The design should use a power inductor whose saturation current is twice the normal operating current. This circuit is designed to output 250mA, so choose 500mA. Use a power inductor with a winding resistance of less than 2 Ω and a high-quality power inductor with a Curie temperature greater than 400 oC. Once the inductor is saturated, the MOS tube, LED light source, and PWM control chip will burn out instantly. It is recommended to use the power inductor of high permeability microcrystalline material, which ensures that the constant current source works safely and reliably for a long time.
The L3 inductor should use the magnetic circuit closed inductor of the EE13 core or the EPC13 core with a lower height (Fig. 9). Most of the LED fluorescent lamps now use semi-aluminum and semi-PV plastic lamps to help the LED light source to dissipate heat. The magnetic circuit of the I-core inductor is open. When the power driver board of the I-core inductor is used to enter the semi-aluminum and semi-PV plastic tube, the metal aluminum can change its magnetic circuit. Debugged power supply board output current changes
7. Freewheeling diode
The freewheeling diode D5 must use a fast recovery diode, which must keep up with the switching period of the MOS transistor. If you use 1N4007 here, it will burn out at work. In addition, the current through the freewheeling diode should be 1.5 to 2 times the load current of the LED light source. This circuit should use a fast recovery diode of 1A.
The switching frequency of PT4107 determines the power inductor L3 and the input filter capacitors C1, C2, C3. If the switching frequency is high, a smaller volume of inductor and capacitor can be selected, but the switching loss of the Q1 MOSFET will also increase. The efficiency is declining. Therefore, for AC 220V power input, 50kHz ~ 100kHz is more suitable. The PT4107 switching frequency setting resistor R5 is calculated as follows. When F = 50 kHz, R5 = 500K ???.
9. MOSFET tube selection
11. Electrolytic capacitor
The LED light source is a long-life light source with a theoretical life of up to 50,000 hours. However, the application circuit design is unreasonable, the circuit components are not properly selected, and the LED light source is not well cooled, which will affect its service life. Especially in the drive power circuit, the electrolytic capacitor, which is the output filter of the AC/DC rectifier bridge, has a service life of less than 5,000 hours, which has become a roadblock for manufacturing long-life LED lamp technology. This circuit design uses C1, C2, C4, C5, C7 aluminum electrolytic capacitors. The life of aluminum electrolytic capacitors is also closely related to the ambient temperature. The increase in electrolyte temperature is accelerated by the increase in ambient temperature. For every 6 oC increase in ambient temperature, the life of electrolytic capacitors is reduced by half. The temperature inside the LED fluorescent tube is not easy to flow due to air. If the design of the power supply driving board is unreasonable, the temperature inside the tube will be relatively high, and the life of the electrolytic capacitor is greatly reduced. The choice of solid electrolytic capacitors may be one of the best ways to extend life, but it leads to increased costs.
The PT4107 can be used to design LED fluorescent lamp solutions with multiple 0.06W WLED light sources connected in series and parallel, with voltage input of AC 110V or AC 220V, T10, T8 and T5, as well as ceiling lamps, starry lights and field lighting for similar applications. Work lights, bulbs, etc., can also be designed with high-brightness 1W WLED light source in series with the load of LED garden lights, LED street lights, LED tunnel lights.
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