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AN1621 Datasheet(PDF) 5 Page - STMicroelectronics |
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AN1621 Datasheet(HTML) 5 Page - STMicroelectronics |
5 / 14 page 5/14 AN1621 APPLICATION NOTE Figure 4. Output Current Limiting Characteristic with Foldback 2.6 Output Diode selection The reverse voltage of the output diodes is given by the formula: Vr = Vin max/n = 375/3.3= 114V, where n is the transformer turns ratio. For a correct functionality a ultra-fast recovery diode is requested, mainly to limit switching losses and EMI prob- lems. To calculate the conduction losses the following equation has been applied to the selected type, BYV52-200. For the single diode the formula is: P = 0.7 · I(AV) + 0.0075 · I 2 (RMS) that, rearranged to take into account both the diodes, becomes: P = 0.7 · IOMAX + 0.0075 · IO2MAX = 10.4W where IOMAX = 13A. Considering the TO247 package (1.2 °C/W total thermal resistance junction-heatsink), to ensure that the junc- tion temperature does not exceed 100 °C at 40 °C max. ambient temperature, the heatsink has to be dimen- sioned for about 4°C/W. 2.7 Power transformer design The forward transformer delivers energy from the primary to the secondary without any storage. The only consideration is with regards of the magnetising current, that has to be limited at a safety value far from core saturation. Our core selection is based on AP, area product, defined as AP = Aw · Ae. a: Output current limiting characteristic b: Schematic diagram of the modified 468 10 12 14 Io 0 5 10 15 20 25 Vo D96IN459 13 R8 3K D96IN460_mod L5991A VREF C16 ISEN 10T P1 R1 4 3K C2 C3 C4 L1 1 T D1 D5 D2 C6 R2 R11 1.2K 50T 5 FB 120K 2K hiccup limiting current thereshold. |
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