Forward Converter Calculator (duty cycle, reset limit, output inductor ripple)

Understanding the forward converter calculator — concept and principle

This calculator works out the operating values of a single-switch forward converter with a reset winding: duty cycle at minimum and maximum input, the reset limit on duty cycle, switch and diode voltage stresses, output inductor ripple and peak currents. Power electronics designers use it to check a turns ratio and output inductor before building a prototype.

The worst-case duty cycle of 45.8 % stays below the 50 % reset limit, but the switch sees twice the input.

How it is calculated

Vout + V_F = Vin × (Ns/Np) × D, so D = (Np/Ns)(Vout + V_F) ÷ Vin

Reset limit: D ≤ Np ÷ (Np + Nr)

Switch stress = Vin,max × (1 + Np/Nr)

Secondary peak = freewheel diode voltage = Vin,max × Ns/Np; rectifier diode = Vin,max × Ns/Nr

Inductor ripple ΔI = (Vout + V_F)(1 − D) ÷ (Lo × f), at maximum input

Peak inductor current = Iout + ΔI/2; minimum Lo for continuous current = (Vout + V_F)(1 − D) ÷ (2 Iout f)

The principle

A forward converter is a buck converter with a transformer in it. While the switch is on, the transformer passes Vin × Ns/Np straight through to an LC filter; when it is off, the inductor current freewheels through a second diode. Unlike a flyback, it does not store load energy in the transformer, but the core's magnetizing flux must still be reset each cycle. A reset winding returns that energy to the input, and its volt-seconds limit the on-time.

Worked example

Default inputs: 36–72 V in, 5 V at 10 A, diode 0.5 V, Np/Ns = 3, Nr/Np = 1, 200 kHz, Lo = 10 µH.

D at 36 V = 3 × 5.5 ÷ 36 = 45.8%; at 72 V = 16.5 ÷ 72 = 22.9%.
Reset limit = 1 ÷ (1 + 1) = 50%, so 45.8% is allowed.
Switch stress = 72 × (1 + 1) = 144 V; secondary peak = 72 ÷ 3 = 24 V; both diodes see 24 V.
ΔI = 5.5 × (1 − 0.229) ÷ (10 µH × 200 kHz) = 2.12 A; peak = 10 + 1.06 = 11.06 A.
Minimum Lo = 1.06 µH; peak primary load current = 11.06 ÷ 3 = 3.687 A.

Each point is the calculator re-run with only “Rectifier diode forward drop (V)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Rectifier diode forward drop (V)	Duty cycle at minimum input (%)
0.125	42.7
0.25	43.8
0.375	44.8
0.5	45.8
0.625	46.9
0.75	47.9
1	50

How to read the result

Keep the worst-case duty cycle below the reset limit with some margin. A 1:1 reset winding doubles the input voltage across the switch, which drives the MOSFET rating. Smaller output inductors raise ripple and peak current; aim for ripple of a modest fraction of the load current.

Common mistakes

Forgetting the magnetizing current, which adds Vin × D ÷ (Lm f) to the primary peak.
Exceeding the reset limit at low input, which saturates the core.
Ignoring the leakage spike on top of the switch stress.

Keep in mind: This is a first-pass calculation. Real supplies need transformer, thermal, EMC and safety design by a qualified engineer.

Frequently asked questions

How is a forward converter different from a flyback?

A forward transfers energy while the switch is on and needs an output inductor; a flyback stores it in the transformer and releases it when off.

Why is duty cycle limited to 50% with a 1:1 reset winding?

The core needs as much time to reset as it had to magnetize.

Where is ripple largest?

At maximum input, where the duty cycle is lowest and the off-time longest.

Sources: D. Perreault, MIT OpenCourseWare 6.622 Power Electronics (Spring 2023), Lecture 13 "Isolated DC/DC"

Checked 9 Nov 2026. D. Perreault, MIT OCW 6.622 Power Electronics (Spring 2023), Lecture 13 “Isolated DC/DC”: forward conversion ratio V_out/V_in = (N₂/N₁)D and reset-winding limit D ≤ N₁/(N₁ + N₃). Output-inductor ripple and diode stresses: standard buck-stage derivations.

Your numbers