T8 lamps are available in supermarkets, shopping malls, hospitals, factories, offices, schools, and hotels where lighting is required. The data shows that as of 2013, nearly 80% of the fluorescent lamps in China are still T8. Therefore, the design of the cost-effective T8/T5LED lamp driving solution is more conducive to the replacement of the original lamps.
For the application of T8/T5 lamps, Shenzhen Mingwei Electronics Co., Ltd. (referred to as Mingwei Electronics) has developed a variety of driver chips such as SM7307, SM7302, SM7330B and SM7332PB, which can realize all T-tube solutions from 8W to 48W. The following describes the T8/T5 solution in detail in combination with these products.
First, high performance constant current drive scheme SM7307/SM7302
The SM7307 and SM7302 are the same series of products for different powers. The SM7307 is suitable for the 8W to 18W T-tube solution, while the SM7302 is suitable for the 12W to 24W T-tube solution. Both are high-precision constant-current drive switches, integrated high-voltage power switch tubes, BUCK topology, without any compensation components, constant current accuracy less than ±3%; high-voltage self-starting and power supply patented technology, IC internal Power supply, no external auxiliary power supply; system efficiency can reach more than 90%, with multiple protections such as LED load opening and short circuit protection, high reliability, and system BOM cost-effective. The schematic diagram of the typical system of SM7307/SM7302 is shown in Figure 1.
The SM7307/SM7302 adopts the patented constant current control technology to limit the peak current of the power switch tube through the current sampling terminal CS, and control the closing of the power switch tube. The FB port detects the current zero crossing of the inductor and controls the opening of the switch tube, thus determining The high and low peak current values ​​flowing through the inductor achieve an output constant current.
The VDD capacitor is charged through the internal JFET through the DRAIN port, and the voltage regulator is used to implement the regulated power supply.
1.1 BOM
In the 9W to 18W (VOUT: 75V~150V, IOUT: 120mA) T8LED tube solution implemented by SM7307, the typical BOM of the drive power system (corresponding to the above circuit diagram) is shown in Table 1:
1.2 Test data
In the 180-264Vac input voltage range, the system's constant current accuracy is ±1.5%; the system average conversion efficiency is greater than 90%, and the test data under different input voltage and output load conditions are shown in Table 2:
1.3 PCB wiring precautions
The PCB size of this system is only 20*25mm, and the real shot of the circuit board is shown in Figure 2:
In the PCB design, in order to ensure the performance of the system, you need to pay attention to the following points:
(1) The GND grounding pin of the IC needs to be copper-plated to enhance the heat dissipation of the IC;
(2) Try to make the main loop (current loop) trace short, do not surround the FB and VDD pins in the main loop;
(3) It is recommended to use a plug-in resistor from the R2 resistor output to the FB terminal;
(4) When the system output capacitance value is the same, the electrolytic capacitor is preferred, and the secondary chip capacitor is used;
The design method of the SM7302 is the same as that of the SM7307, except that the power is greater than the power of the SM7307.
Second, high power factor, cost-effective T8/T5 program SM7330B/SM7332PB
The SM7330B and SM7332PB are both high power factor series products. The SM7330B is suitable for externally driven power tube solutions, while the SM7332PB is an internal integrated 2N60 power tube. Both are BUCK topologies, which operate in critical conduction mode. The constant current accuracy can reach ±5% and the PF value is greater than 0.9 in the full voltage range of 90Vac~265Vac. It has output switching and short-circuit protection characteristics. Reliability tests such as EFT, lightning surges, etc. The SM7330B output power can cover 9W to 48W. The typical working principle is shown in Figure 3. After the SM7330B is powered on, the sinusoidal half-wave voltage charges the capacitor of the VDD pin through the startup resistor. When the VDD voltage rises to the startup threshold voltage, The internal control circuit of the chip starts to work. When the output voltage is established, the VDD voltage is supplied by the output voltage through the diode.
The chip samples the inductor current and forms a closed-loop feedback network using an internal error amplifier to achieve high constant current accuracy and high load regulation.
The CS voltage and reference voltage are error amplified by a transconductance amplifier and integrated by an external COMP capacitor. The COMP terminal voltage controls the external power tube conduction time and adjusts the output current.
The SM7332PB integrates a 2N60 power tube with a maximum output power of 20W. Its typical output specification is 80V/180mA. The physical diagram and application schematic are shown in Figure 4 and Figure 5.
The SM7332PB works in the same way as the SM7330B and is no longer described. The typical electrical parameters of the SM7332PB are 80V/180mA. The basic electrical parameters, conversion efficiency and power factor are shown in Table 3, Table 4 and Table 5.
The T8/T5 drive solution designed with the SM73XX series chip is not only cost-effective but also capable of passing various reliability tests and EMC certification. It is the best choice for the T8/T5 drive solution.
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