Flyback Converter Calculator

First-pass flyback design from the input range, output and switching frequency: turns ratio, reflected voltage, primary (magnetising) inductance, peak and RMS primary current, MOSFET drain voltage stress and output diode reverse voltage.

Flyback design equations (AN4137)

How the flyback numbers fit together

A flyback converter is a buck-boost whose inductor has two windings. While the MOSFET is on, the primary stores energy in the transformer’s magnetising inductance; when it turns off, the energy flows out of the secondary through the diode. Two choices set most of the design: the turns ratio, and how deep into continuous conduction the converter runs at low line.

Turns ratio. While the diode conducts, the output voltage (plus the diode drop) is reflected back to the primary multiplied by the turns ratio. Volt-second balance at minimum input and full load ties that reflected voltage to the maximum duty cycle: V_R = V_in,min·D/(1 − D). A higher V_R lowers the primary current but adds directly to the MOSFET’s drain voltage. In the example, 45% duty gives 81.8 V and a turns ratio of 6.545, and the drain sees about 557 V at high line with a 100 V spike allowance, so a 650 V MOSFET would be a usual choice; the diode must block about 69.3 V.

Inductance and the ripple factor. AN4137’s ripple factor K_RF compares the peak-to-peak primary current ripple with twice the average current during the on-time. At K_RF = 1 the current just reaches zero each cycle (the CCM/DCM boundary); smaller values run deeper into CCM with a larger inductance and a lower peak and RMS current. The note suggests lower values for universal input than for 230 V-only designs. Because the inductance is chosen at minimum input, the converter usually moves into DCM at high line; the page tells you which mode it is in at maximum input and gives the duty and peak current there.

What this page does not do. It does not choose a core, wind the transformer, size the clamp or snubber, or check the loop. Leakage inductance, which makes the drain spike, depends on the winding. The SMPS transformer calculator planned for this section will take the turns ratio and inductance on to the core and windings. For the MOSFET’s losses use the MOSFET loss calculator.

Safety. An offline flyback has a rectified mains bus of several hundred volts on the primary, and the bulk capacitor stays charged after the plug is pulled. The transformer’s insulation and creepage distances carry the safety isolation and are set by product safety standards, not by this arithmetic. Mains voltage can kill. This page does the arithmetic only; for anything connected to the supply, follow your local electrical code and have the work done or checked by a licensed electrician.

Frequently asked questions

Related calculators

References

1. Fairchild Semiconductor (now onsemi). Design Guidelines for Off-line Flyback Converters Using Fairchild Power Switch (FPS). Application note AN-4137, 2003. Ripple factor K_RF, L_m = (V_DC,min·D_max)² ÷ (2·P_in·f_s·K_RF), V_RO, turns ratio, drain and diode voltage stress.
2. Erickson RW, Maksimović D. Fundamentals of Power Electronics, 3rd ed. Springer, 2020. Ch. 2 (inductor volt-second and capacitor charge balance, ripple), Ch. 5 (the CCM–DCM boundary), Ch. 8, Table 8.2 (right-half-plane zero: D′²R/L for the boost, D′²R/(DL) for the buck-boost).
3. Mohan N, Undeland TM, Robbins WP. Power Electronics: Converters, Applications, and Design, 3rd ed. Wiley, 2003. Ch. 10, flyback converter.