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AN2067 Datasheet(PDF) 6 Page - STMicroelectronics |
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AN2067 Datasheet(HTML) 6 Page - STMicroelectronics |
6 / 25 page White LED power supply AN2067 6/25 DocID10973 Rev 2 2 White LED power supply 2.1 Schematic The power topology is an offline flyback, working at a fixed frequency of 66 kHz. The overall schematic is presented in Figure 2. 2.1.1 Primary section On the left side of the schematic, the fuse F1, inrush current limiter CTN1, input filter T1, followed by the rectifier BR1 and its bulk capacitance C3 can be found. R4, C4 and D4 build the RCD clamper, to discharge the leakage inductance of the transformer. D2, R2 and C5 are the rectifiers and filters of the primary auxiliary winding, used in forward mode (refer to Section 3). This generates a voltage supply from 21 V up to 80 V, proportional to turn ratio between main primary winding and auxiliary primary winding, and versus input voltage range (110 VAC up to 250 VAC). A serial voltage regulator is required to supply the VIPer53-E with the correct voltage (around 12 V). It is built with R14, DZ14, Q1 and C12. The VCE0 of the transistor must be higher than 80 V. This transistor may also dissipate 0.7 W when the input voltage is 250 VAC. The COMP pin filter is made of C8, R9 and C9. 2.1.2 Transformer A current generator may have a large output voltage variation, according to the output load. If the auxiliary winding is used in flyback mode, a wide voltage variation on auxiliary winding may happen, as it is proportional to the reflected voltage. So, it is used in forward mode in order to limit the voltage variation to supply the VIPer53-E. In order to guarantee the functionality even with low output voltage load, an auxiliary winding at secondary side has been added. This auxiliary winding is also used in forward mode. |
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