Flyback Converter Calculator

What Is a Flyback Converter?

A flyback converter is an isolated buck-boost power supply. Energy is stored in a transformer during the switch-on phase and transferred to the output during the switch-off phase.

Flyback Design Formulas

$$D = \frac{N \cdot (V_{\rm out} + V_{\rm rect})}{(V_{\rm out} + V_{\rm rect}) \cdot N + V_{\rm in}}$$ $$I_{\rm s,Peak} = \frac{2}{1 - D} \cdot I_{\rm out}$$ $$L_s = \frac{V_{\rm out} + V_{\rm rect}}{2 I_{\rm out} f_s}(1 - D)^2$$ $$I_{\rm p,Peak} = \frac{I_{\rm s,Peak}}{N}, \quad L_p = L_s N^2$$

Example: 95 V input, 12 V output, 1 V rectifier drop, turns ratio 5.3, 4 A output, and 70 kHz switching gives about 42% duty cycle. Related tools: Buck Converter Calculator, Boost Converter Calculator, and Duty Cycle Calculator.

Frequently Asked Questions

What is the turns ratio N?

It is the primary-to-secondary winding ratio $N_p / N_s$ of the flyback transformer.

Why include rectifier voltage in the duty cycle?

The diode drop adds to the reflected output voltage, so the effective flyback voltage is $V_{\rm out} + V_{\rm rect}$.

What happens if duty cycle reaches 100%?

The converter cannot transfer energy to the output during the off-time. Keep duty cycle well below 50% in practical designs.

How is peak current related to output current?

Secondary peak current scales as $2 I_{\rm out} / (1 - D)$ for discontinuous boundary designs in this model.

Can I use this for step-up and step-down?

Yes. A flyback converter can step voltage up or down depending on turns ratio, duty cycle, and input/output voltages.

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What Is a Flyback Converter?

Flyback Design Formulas

Frequently Asked Questions

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