Fluorescent lamps are popular in the world as a bright, soft and effective light source, 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.
As the fourth generation of new energy-saving light source, LED light source was used to make the light source of various lamps when it was born. The 0.06W white LED straw hat and piranha were first used on the LED strips of LED fluorescent lamps. The number of LED fluorescent tubes used varies from 280 to 360. Now a new generation of LED fluorescent light strips use from 0.06W to 1W, the color is pure white, white, warm white, cool white patch LED plane light source.
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.
The main technical features of PT4107: wide voltage input range from 18V to 450V, constant current output; frequency jitter to reduce electromagnetic interference, random source to modulate the oscillation frequency, so that the audio energy spectrum can be extended, and the expanded energy spectrum can be effectively reduced. Small in-band electromagnetic interference reduces system-level design difficulty; linear and PWM dimming can be used to support hundreds of 0.06W LED drive applications. The operating frequency is 25kHz-300kHz, which can be set by an external resistor.
The PT4107 package is shown in Figure 2. The functions of each pin are as follows:
1. GND chip ground terminal;
2. CS LED peak current sampling input;
3. LD linear dimming access terminal;
4. RI oscillation resistor access terminal;
5. ROTP over temperature protection setting terminal;
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;
Design full voltage 20W fluorescent lamp switch constant current source
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.
According to this concept, the schematic diagram of the constant current source of the full-voltage 20W fluorescent lamp switch is shown in Figure 4. Seen from the AC 220V, the AC mains inlet is connected to a 1A fuse FS1 and a surge-resistant negative temperature coefficient thermistor NTC, followed by an EMI filter consisting of L1, L2 and CX1. The BD1 is a rectified full bridge with four high voltage silicon diodes inside. C1, C2, R1, D1 to D3 form a passive power factor correction circuit. The PT4107 chip is powered by the electronic filter consisting of T1, D4, C4, and R2~R4. The input impedance of this filter is very high, and the output impedance is very small. After rectification, nearly 300V DC high voltage is stepped down to the PT4107 through this transistor. Vin provides a stable voltage of approximately 18 to 20V to ensure stable operation of the chip over the full voltage range.
This circuit is not as hot as the resistive step-down circuit of the previous scheme. The PWM control chip U1 (PT4107) and the power MOS transistor Q1, the ballast power inductor L3, and the freewheeling diode D5 form a step-down regulator circuit, and U1 collects the peak current on the current sampling resistors R6 to R9, and the internal logic is in a single cycle. Control the pulse duty cycle of the GATE pin signal for constant current control. The output constant current is combined with the freewheeling circuit of D5 and L3 to supply constant current to the LED light source. Changing the resistance of the resistors R6 to R9 can change the output current of the whole circuit, but D5 and L3 should also be changed accordingly. R5 is part of the chip oscillator circuit, which can be adjusted to adjust the oscillation frequency. The potentiometer RT is not used for dimming in this circuit, but is used to fine tune the current of the constant current source to make the circuit reach the design power. Due to the dispersibility of the device, the output current of each power board will be slightly different during mass production. This potentiometer can be used on the production line to adjust the output current of each power board. In order to ensure the stability of the power board has been adjusted, the turbine vortex trimmer potentiometer must be selected, and the glue is sealed after adjustment.
The parameters of this circuit are designed according to each series of 22 0.06W LEDs, a total of 15 series parallel, driving 330 60 mW white LED load, the current per string is 17.8 mA, and the design output is 36-80V/25OmA. If you change the number of LEDs, you need to correct the parameters of R6 ~ R9.
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 photograph of a T10 constant current source plate with 33 components mounted on a 235 x 25 x 0.8 mm epoxy single-sided printed board.
Key design and considerations
1. Anti-surge NTC.
The surge-resistant NTC uses a 300Ω/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
At the input end of the AC power supply, it is generally necessary to add a filter composed of a conjugate inductor, an X capacitor and a Y capacitor to increase the anti-EMI effect of the entire circuit, and to filter out the conducted interference signal and the radiation noise. This circuit uses a succinct way of conjugate inductor plus X capacitor, mainly due to the overall cost considerations, in terms of design principles that are sufficient. The X capacitor should be marked with the safety certification mark and the pressure AC275V. Its true DC withstand voltage is above 2,000V, and the appearance is mostly orange or blue. The conjugate inductor is two inductors of the same inductance wound around the same core. It is mainly used to suppress common mode interference, and the inductance is selected in the range of 10 to 30 mH. In order to reduce the volume and improve the filtering effect, it is preferred to use a product made of a magnetic core with a high magnetic permeability microcrystalline material, and the inductance should be selected as large as possible. Replacing a conjugate inductor with two identical inductors is also a cost-reducing method.
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 is increased from the original 75° to 105° to 30° to 150°; the conduction angle of the negative half-cycle input current is increased from the original 255° to 285° 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.
5. Buck regulator circuit
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+β) of the emitter current, it corresponds to a large capacitance with a capacitance of (1+β)C4 at the emitter. It is the principle of a 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. Please note that T1 should select the Vbceo500V 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 a high Q value, a large saturation current, and a small resistance. For a nominal 3.9mH inductor, the Q value should be greater than 90 in the 40kHz to 100kHz frequency range. 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 wirewound resistor of less than 2 ohms 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. The output current of the debugged power supply board becomes smaller.
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.
8. PT4107 switching frequency setting
The switching frequency of the PT4107 determines the size of the power inductor L3 and the input filter capacitors C1, C2, and C3. If the switching frequency is high, a smaller volume of inductors and capacitors can be used, but the switching losses of the Q1 MOSFETs will also increase, resulting in reduced efficiency. 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 = 500 K?.
9. MOSFET tube selection
MOSFET Q1 is the key component of this circuit's output. First, its RDS(ON) is small so that it consumes less power when it is working. In addition, its withstand voltage is high, so that high voltage surges are not easily broken down during work.
During each switching of the MOSFET, current spikes will inevitably occur on the sampling resistors R6 to R9. To avoid this, a 400ns sampling delay is set inside the chip. Therefore, the conventional RC filter can be omitted. During this delay time, the comparator will be inactive and will not control the output of the GATE pin.
10. Current sampling resistor
Resistors R6, R7, R8, and R9 are connected in parallel as sampling resistors, which can reduce the influence of resistance accuracy and temperature on the output current, and can easily change the resistance of one or several resistors to achieve the purpose of modifying the current. It is recommended to use SMD (1206) 1/4W resistor with a precision of one thousandth and a temperature coefficient of 50ppm. The total resistance setting and power selection of the current sampling resistors R6 to R9 are calculated based on the load current of the LED light source of the entire circuit.
R(6-9)=0.275/ILED
PR(6-9)=ILED2 x R(6-9)
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.
In early 2009, in order to reduce the carbon emissions of public lighting, the Japanese government forced enterprises to implement energy-saving and carbon-reduction policies. The demand for energy-saving lighting in Japanese offices gradually warmed up, and the promotion of LED fluorescent lamps was promoted, which promoted the production of LED fluorescent lamps in China. Therefore, with reference to this design circuit, the LED fluorescent lamp circuit suitable for AC110V has been widely used in production.
Note: This scheme can provide electrical schematic diagram, PCB board diagram and BOM.
Author: Yen Kuang
Senior engineer
China Resources Converse Technology (Shanghai) Co., Ltd.
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