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
