Design flyback converters by computing turns ratio, duty cycle, magnetizing inductance, peak currents, and component stress for isolated DC-DC power supplies.
The flyback converter is one of the most widely used isolated switch-mode power supply topologies, found in phone chargers, LED drivers, telecom power modules, and countless other applications below about 150 W. It stores energy in a coupled inductor (often called a flyback transformer) during the on-time and releases it to the secondary during the off-time.
Designing a flyback converter requires balancing many interdependent parameters: turns ratio, duty cycle, magnetizing inductance, peak currents, and voltage stresses on the MOSFET and output diode. Getting any of these wrong can lead to excessive losses, component failure, or audible noise from the transformer.
This Flyback Converter Calculator automates the core design equations. Enter your input voltage, desired output, load current, switching frequency, and efficiency target, and the tool instantly computes the minimum inductance, ideal turns ratio, peak currents on both windings, voltage stresses, and a recommended output capacitor value. Use the operating mode selector to switch between continuous (CCM) and discontinuous (DCM) designs. Preset buttons provide common application profiles so you can explore trade-offs quickly.
Flyback converters are deceptively simple on paper but tricky to optimize. This calculator saves hours of spreadsheet iteration by solving all key design equations simultaneously and highlighting potential stress issues before you order components.
Turns Ratio: N = (Vin × Dmax) / ((Vout + Vf) × (1 − Dmax)) Duty Cycle: D = (Vout + Vf) / (Vin × N + Vout + Vf) Magnetizing Inductance: Lm = (Vin × D)² / (2 × Pin × fsw) Peak Primary Current: Ipk = 2 × Pin / (Vin × D) MOSFET Stress: Vds = Vin + (Vout + Vf) × N
Result: N = 1.412, Lm = 30.6 µH, Ipk = 1.96 A, Vds = 20.1 V
A 12 V to 5 V / 2 A flyback at 100 kHz needs a turns ratio of ~1.4, about 31 µH magnetizing inductance, and has a manageable 20 V MOSFET stress.
A flyback converter uses a coupled inductor to store energy when the primary switch is on and deliver it to the output when the switch turns off. Unlike a forward converter, the flyback does not need a separate output inductor, which keeps the circuit compact for low- to mid-power supplies.
The core must handle the peak flux density without saturating. Ferrite cores are common because they perform well at switching frequencies in the hundreds of kilohertz. Use the magnetizing inductance and peak current from this calculator to choose a core and gap that can tolerate the intended load.
Leakage inductance creates a voltage spike when the switch turns off, so a clamp network is still required in most real designs. The MOSFET stress shown here is a design estimate, not a complete worst-case value, so leave margin for spikes and component tolerances.
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In CCM the magnetizing current never reaches zero, giving lower peak currents. In DCM it drops to zero each cycle, simplifying control but increasing peak stresses.
Above ~150 W the peak currents and transformer size become impractical; forward, half-bridge, or full-bridge topologies are more efficient.
For most designs 40-50% is ideal. Exceeding 50% requires a more complex clamp and increases transformer reset time.
A higher turns ratio reduces MOSFET stress but increases secondary peak current, and vice versa. The design is a trade-off.
The output diode drop (0.3–0.7 V) directly affects duty cycle and reflected voltage calculations.
This calculator handles a single output. For multiple outputs, you would ratio the secondary turns proportionally.