Flyback Converter
Design a flyback converter DC-DC power supply: calculate duty cycle, peak currents, and primary/secondary inductance from input voltage, output voltage, windings ratio, and switching frequency.
About This Calculator
The Flyback Converter Calculator helps power electronics engineers and hobbyists design isolated DC-DC flyback converters. Enter the input voltage, output voltage, rectifier forward voltage, windings ratio (Np/Ns), output current, and switching frequency to instantly compute the duty cycle, peak currents on both primary and secondary sides, and the primary/secondary inductance values.
The flyback converter is a buck-boost DC-DC topology with galvanic isolation, commonly used for low-power applications such as mobile chargers, AC-DC adapters, auxiliary power supplies, and LED drivers. Unlike forward converters, the flyback transformer acts as a coupled inductor that stores energy during the MOSFET on-time and releases it to the output during the off-time.
The core design equations are: Duty Cycle D = N × (Vout + Vrect) / (N × (Vout + Vrect) + Vin), secondary peak current Is,Peak = 2 × Iout / (1 − D), secondary inductance Ls = (Vout + Vrect) × (1 − D)² / (2 × Iout × fs), and primary-side values derived via the turns ratio: Ip,Peak = Is,Peak / N and Lp = Ls × N².
Flyback converters are widely used in India, the US, and the UK for power supplies up to 100-150W. Common applications include smartphone chargers (5V/2A-3A), laptop adapters (19V/3A), auxiliary power supplies for industrial controls, and isolated bias supplies for gate drivers. The topology excels where cost, simplicity, and isolation are priorities over high efficiency or high power density.
Key design considerations include selecting the correct windings ratio to optimize duty cycle around 50%, ensuring the switching MOSFET can handle the peak voltage stress (Vin + N × Vfb), choosing a core that avoids saturation at peak current, and selecting an output capacitor with adequate ripple current rating. This calculator assumes continuous conduction mode (CCM) operation for inductance calculations.
Frequently Asked Questions
What is a flyback converter?
A flyback converter is a DC-DC buck-boost converter with galvanic isolation between input and output. It uses a coupled inductor (flyback transformer) to store energy in the on-phase and release it to the output circuit in the off-phase, making it ideal for low-power isolated power supplies, chargers, and adapters.
How does a flyback converter work?
The flyback converter works by transferring energy in two phases. During the switch-on phase, the primary winding stores energy in the transformer core. When the switch turns off, the energy is released through the secondary winding to the output via the rectifier diode. This discontinuous energy transfer is what gives the flyback converter its name.
How do you calculate the duty cycle of a flyback converter?
The duty cycle D of a flyback converter in continuous conduction mode is calculated as D = N x (Vout + Vrect) / (N x (Vout + Vrect) + Vin) where N is the primary-to-secondary windings ratio, Vin is the input voltage, Vout is the output voltage, and Vrect is the diode forward voltage drop.
What is the flyback voltage?
The flyback voltage Vfb is the voltage reflected from the secondary side to the primary winding when the switch is off. It equals Vout + Vrect, the sum of the output voltage and the rectifier diode voltage drop. The flyback voltage determines the voltage stress on the switching MOSFET.
What are the disadvantages of a flyback converter?
The main disadvantages of a flyback converter are limited power capability (typically under 150W), high voltage stress on the switching transistor, lower efficiency compared to forward converters, higher output ripple, and electromagnetic interference. Despite these drawbacks, the flyback topology remains popular for cost-sensitive, low-power, and isolated applications worldwide.
How do you calculate peak current in a flyback converter?
The secondary-side peak current Is,Peak is calculated as 2 x Iout / (1 - D). The primary-side peak current is then Ip,Peak = Is,Peak / N. These peak currents determine the wire gauge and core saturation requirements for the transformer design.
Is this calculator free to use?
Yes, this flyback converter calculator is completely free to use with no registration or account required. You can share your calculation results via the shareable URL that saves all input parameters.
What is the difference between flyback and forward converters?
The key difference is that a flyback converter stores energy in the transformer core during the on-time and delivers it during the off-time, while a forward converter transfers energy directly from input to output during the on-time through a transformer with a reset winding. Flyback converters are simpler and cheaper but limited to lower power levels.