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# Flyback Transformer Power Supply Design and Calculator

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When the switching transistor is turn on in a fly-back converter, the  primary winding of the transformer is energized, and no energy is transferred to the secondary windings. When the transistor is turned off the field collapses and the energy is transferred to the secondary windings.  This differs from a forward converter topology, where energy is transferred to the secondary windings when the switching transistor is turned on. You can tell the difference between the two topologies, by looking at the orientation of the dots on the secondary compared to the primary.  For the fly-back converter, the dot's are reversed, and for the forward converter the dots are aligned.

The calculator below calculates the number of turns, the inductance, and the wire gauge for the various windings of a discontinuous mode fly-back converter.

 Power Supply Specification: Frequency, F: (KHz) T: (uS) Diode Voltage Drop, Vd: (V) Transistor Voltage Drop, Vtran: (V) Efficiency: (%) Max Transistor Voltage, VDSMAX: (V) AL=L/N2: (uH/Turns^2) Voltage Primary, Vin: (V) Voltage Out 1, Vo1: (V) (Note that this must be positive, and feed back is derived from this winding) Current Out 1, Io1 (A) P1: (W) Optional Secondary Windings: Voltage Out 2, Vo2: (V) Current Out 2, Io2: (A) P2: (W) Voltage Out 3, Vo3: (V) Current Out 3, Io3: (A) P3: (W) Voltage Out 4, Vo4: (V) Current Out 4, Io4: (A) P4: (W) Transformer Result: Power In, Pin: (W) Turns Ratio Primary to Secondary Winding 1, Nps1: Charge Period, Tch: (uS) Discharge Period, Tdis: (uS) Dead Time Period, Tdt: (uS) Primary Inductance, L: (uH) Turns Primary, Np: (Turns) Turns Secondary 1, Ns1: (Turns) Turns Secondary 2, Ns2: (Turns) Turns Secondary 3, Ns3: (Turns) Turns Secondary 4, Ns4: (Turns) Peak Primary Current, Ip: (A) Primary RMS Current , Ipri(rms): (A) Primary Wire Diameter, Dp: (mils) Primary Wire Gauge, AWGp: AWG

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