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AN2528 Datasheet(PDF) 6 Page - STMicroelectronics |
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AN2528 Datasheet(HTML) 6 Page - STMicroelectronics |
6 / 21 page Flyback stage design AN2528 6/21 2 Flyback stage design Well known to all SMPS designers, the voltage stress on the device (power switch) is given by: Equation 1 where Vfl = flyback voltage = (Vout + VF, diode) * Np/Ns and Vspike is the over-voltage on the collector due caused by leakage inductance. This over-voltage is not limited by any clamping network in order to minimize as much as possible the solution cost using also the very large margin available which has been fixed to 200 V. Np is the number of turns on the primary side while Ns is the number of turns on the main output secondary winding. Now, taking into account a 300 V margin, the maximum flyback voltage that can be chosen is: Equation 2 After the calculation of the flyback voltage, we can proceed with the next step in the converter design. The turns ratio between primary and secondary side is calculated with the following formula: Equation 3 As a first approximation, since the turn-on of the device occurs immediately after the energy stored on the primary side, inductance is completely transferred to the secondary side: Equation 4 and Equation 5 Where Tonmax is the maximum on time, Treset is the time needed to demagnetize the transformer inductance and TS is the switching time. Combining the two previous formulas Tonmax results in: Equation 6 The next step is to calculate the peak current. The output power is set to 6 W and the desired transformer efficiency must be set by the designer (at least 80% in this case). Excluding the energy losses on the input diode bridge, on the power switch and on the secondary side rectifier, the following approximate formula can be used: V off V inmax V fl V spike – – = V fl BV V inmax – V spike – V min arg 1700 850 – 200 – 300 350V = – = – = N p N s ------- V fl V out V F diode , + ----------------------------------------- 350 14 1 + ---------------- 23.3 = = = V dcmin T onmax • V fl T reset • = T onmax T reset – T S = T onmax V fl T S • V dcmin V fl + ------------------------------- 14 µs ≅ = |
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