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FDS2670 数据表(PDF) 2 Page - Fairchild Semiconductor |
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FDS2670 数据表(HTML) 2 Page - Fairchild Semiconductor |
2 / 12 page 2 Rev. A, June 2001 ©2001 Fairchild Semiconductor Corporation Figure 1. Simplified flyback regulator schematic. For 36- to 72-V input designs, the FDS3670, 100V MOSFET is the lowest voltage part that should be used. Even then, the drain voltage will have to be carefully controlled with some form of primary side clamp circuit. If the input supply can be expected to experience transient voltages on the order of 100V, the 150V FDS2570 should be considered. It is important to note, two devices would have to be connected in parallel to maintain the same power level that can be achieved with a single FDS3670. The off-state voltage is determined by the input voltage, output voltage, transformer turns ratio, transformer leakage inductance, and snubber/clamp circuit. Ignoring the effects of leakage inductance, the off voltage is equal to the input voltage plus the output voltage multiplied by the transformer turns ratio. As an example, assuming a 1:1 turns ratio and a 5V output, the off- state drain voltage is equal to Vin plus 5V. The effects of leakage inductance complicate the matter considerably. The energy stored in the leakage inductance is equal to LI2/2, where L is the leakage inductance and I is the peak primary current. This energy appears as a voltage impressed on the various stray capacitances around the circuit. The energy is equal to CV2/2 and, therefore, Vpk=Ipk(L/C) 1/2. The problem is complicated by the fact that most of the stray capacitances are nonlinear and vary greatly as a function of the instantaneous voltage applied. As such, an empirical approach to designing the snubbers and clamps is generally used. Figure 2 shows typical waveforms for the flyback regulator. Generally in a 48V application, the maximum useful power level for a flyback design will be around 20 to 30 watts. Above that, consider a single transistor forward converter as shown in Figure 3. Gate Q1 RTN C2 D1 T1 + C1 + U1 PWM GND VOUT VIN |
类似零件编号 - FDS2670 |
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类似说明 - FDS2670 |
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