Flyback calculations


1. Minimum needed inductance and maximum turns ration
2. Check for continuous or discontinuous mode
3. On- and off-time
4. Voltages and currents on components
5. General calculations




1. Minimum needed inductance and maximum turns ration

    Average input voltage


    Average input current (primary side) at average input voltage


    1.1 Continuous conduction mode

    The calculation of the turns ratio is done with the minimum input voltage to stay below the maximum duty cycle (50 ... 60%).
    The calculation of the inductance is done with the average input voltage to achieve the desired mode at this voltage.

   
    Maximum turns ratio



    Duty cycle at average input voltage


    Inductor peak current at average input voltage


    Minimum inductance at average input voltage


  1.2 Transition mode

    The calculation of the turns ratio and the inductance is done with the average input voltage.

   
    Maximum turns ratio


  
    Minimum inductance at average input voltage


    1.3 Discontinuous conduction mode

    The calculation of the turns ratio is done with the minimum input voltage to stay below the maximum duty cycle (50 ... 60%).
    The calculation of the inductance is done with the minimum input voltage.

   
    Maximum turns ratio



    Inductor peak current at minimum input voltage


    Minimum inductance at minimum input voltage



2. Check for continuous or discontinuous mode

    It has to be checked if the flyback converter is working in continuous or discontinuous mode to use the proper formulas.


    Inductor peak current


    Equivalent inductor current (substitution for primary and secondary side current)


    Check for continuous or discontinuous mode




3. On- and off-time

    Depending on the mode (continuous or discontinuous) different formulas are needed.


    ton       - FET is in on-state (continuous and discontinuous mode)
               - Inductor current is rising

    toff       - FET is in off-state (
continuous and discontinuous mode)
               - Inductor current is falling

    tzero     - time slot when inductor current is zero
               - occurs only in discontinuous conduction mode


    Duty cycle



    On-time is continuous conduction mode


    Off-time in continuous condcution mode



    On-time is discontinuous conduction mode


    Off-time in discontinuous conduction mode



    Time slot when inductor current is zero (only in discontinuous condcution mode)



4. Voltages and currents on components

    Depending on the mode (continuous or discontinuous) different formulas are needed.

    4.1 Primary side

    Primary side current ripple (inductor primary side, FET)


    DC part of the input current on the primary side (inductor primary side, FET)


    Minimum current on the primary side (inductor primary side, FET)


    Maximum current on the primary side (inductor primary side, FET)


    Average current on the primary side (inductor primary side, FET)


    RMS current on the primary side (inductor primary side, FET)


    4.2 Secondary side

    Minimum current on the secondary side (inductor secondary side, diode)


    Maximum current on the secondary side (inductor secondary side, diode)


    Average current on the secondary side (inductor secondary side, diode)


    RMS current on the secondary side (inductor secondary side, diode)


    Secondary side inductance


    4.3 FET specific calculations

    Minimum FET voltage


    Maximum FET voltage


    FET voltage in discontinuous conduction mode when the inductor current is zero


    4.4 Inductor primary side specific calculations

    Minimum inductor (primary side) voltage


    Maximum inductor (primary side) voltage


    Inductor (primary side) voltage in discontinuous conduction mode when the inductor current is zero


    4.5 Inductor secondary side specific calculations

    Minimum inductor (secondary side) voltage


   
Maximum inductor (secondary side) voltage

    Inductor (primary side) voltage in discontinuous conduction mode when the inductor current is zero


    4.6 Diode specific calculations

    Minimum diode voltage


    Maximum diode voltage



    Diode voltage in discontinuous conduction mode when the inductor current is zero



5. General calculations

    Diode losses


    Output power



    Input power


    Input current