Why does a forward converter need a reset winding?

During the on-time the magnetizing current in the transformer ramps up. Because the forward transformer is not a two-quadrant flyback, that flux must be reset to zero every cycle or the core will walk into saturation. A tertiary reset winding (or an active-clamp / RCD circuit) returns the magnetizing energy to the input and drives the magnetizing current back to zero during the off-time.

What is the maximum duty cycle of a forward converter?

With a reset winding of ratio Nr/Np the maximum duty is Dmax = 1/(1 + Nr/Np). For an equal-turns reset winding (Nr = Np) that gives Dmax = 0.5. If the duty exceeds Dmax the magnetizing current cannot fully reset during the off-time, the flux staircases up cycle by cycle, and the transformer saturates. This simulator flags that condition.

What is the difference between a forward and a flyback converter?

A flyback stores energy in the transformer during the on-time and delivers it during the off-time (Vo = n·Vs·D/(1−D), output capacitor only). A forward transfers energy directly during the on-time through a secondary rectifier into an output LC filter (Vo = n·Vs·D), needs a reset winding, and is used at somewhat higher power with lower output ripple.

What sets the switch voltage stress in a forward converter?

During reset the primary MOSFET blocks the input plus the reflected reset-winding voltage: Vds ≈ Vs·(1 + Np/Nr). For an equal-turns reset winding this is 2·Vs. A reset winding with fewer turns resets the core faster (higher Dmax) but raises the switch voltage stress — a core design trade-off shown live in the simulator.

Forward (Isolated) DC-DC Converter Simulator

Q: What is the output voltage of a forward converter?

In continuous conduction mode (CCM) the ideal output voltage of a forward converter is Vo = n·Vs·D, where n = Ns/Np is the secondary-to-primary turns ratio, Vs the DC input and D the duty cycle. Unlike the flyback, power is transferred directly to the output during the on-time, so it behaves like an isolated buck converter.

An advanced, physics-accurate simulator of the single-switch forward converter — an isolated, buck-derived SMPS that transfers power directly through the transformer during the on-time into an output LC filter. Sweep the duty cycle D, turns ratio n = Ns/Np, reset-winding ratio Nr/Np, magnetizing and filter values, add a real device model, and watch every waveform update on a real-time oscilloscope — validated live against Vo = n·Vs·D, with a core-reset visualizer, the maximum-duty limit Dmax = 1/(1+Nr/Np) and a transformer-saturation alarm, CCM/DCM detection, efficiency and a ripple-spectrum FFT.

Forward isolated DC-DC converter circuit diagram: DC input voltage Vs, single primary MOSFET switch, transformer with main and tertiary reset windings, secondary forward diode, freewheel diode and an output inductor-capacitor LC filter feeding a load resistor R with isolated output voltage Vo = n·Vs·D — Forward converter — power is transferred directly during the on-time through the secondary forward diode into the LC filter; a reset winding returns the magnetizing flux to zero each cycle. V_o = n·V_s·D.

Parameters

Duty cycle D 35%

Must stay below Dmax = 1/(1+Nr/Np) or the core saturates.

Input voltage V_s

Turns ratio n = N_s/N_p 0.30

Reset winding N_r/N_p 1.00

Fewer reset turns → higher Dmax but higher switch voltage.

Switching frequency 100 kHz

Transformer & output filter

Magnetizing inductance L_m 1000 µH

Output inductor L_o 100 µH

Output capacitor C_o 100 µF

Load resistance R 8 Ω

Output LC gives buck-like ripple: L_c = (1−D)·R/(2·f_sw) sets CCM/DCM.

Device & parasitics model

MOSFET R_ds(on) 0 mΩ

Diode V_f 0.0 V

Inductor DCR 0 mΩ

Capacitor ESR 0 mΩ

Ideal (all 0) gives Vo = n·Vs·D exactly; add parasitics to see the real drooped output and losses.

Sampling & display

Sampling rate 700 pts/cycle

Points plotted per switching period

Switching periods shown

Run speed (DSO) 1.0×

Show sample points

Presets

Waveforms to display

Waveforms — one steady-state switching period —

v_x secondary node Vo output i_Lo output inductor i_mag reset

LIVE

Core reset & duty limit (forward-specific)

Maximum duty D_max = 1/(1+Nr/Np)—

D = 35%

The magnetizing current ramps up during the on-time and is reset to zero by the reset winding during the off-time. Keep D below the red D_max mark, or the flux staircases up and the transformer saturates.

Efficiency & conduction-loss breakdown

Estimated efficiency (conduction losses)—

Power lost in each component, computed from the true integrated currents: MOSFET R_ds(on), diodes V_f, inductor DCR and capacitor ESR.

Ripple spectrum analysis

Harmonics shown 16

Output-voltage ripple / distortion (relative to |DC|)—

FFT of the output voltage — ripple harmonics at multiples of the switching frequency f_sw. Toggle to the inductor-current spectrum above.

Measurements

Live accuracy check — simulation vs closed-form theory

Output voltage

Inductor ripple

What is a forward converter?

A forward converter is an isolated, buck-derived DC-DC (SMPS) topology. Like a flyback it uses a transformer for isolation, but unlike a flyback it transfers energy directly to the output during the on-time rather than storing it. A single primary MOSFET drives the transformer; the secondary has a forward rectifier diode, a freewheel diode and an output LC filter — essentially an isolated buck stage. Because the transformer only carries flux in one direction, it needs a reset winding (or active-clamp) to return the magnetizing flux to zero every cycle.

How it works — the two switching states

Switch ON (0 → D·T): the primary sees V_s; the secondary forward diode conducts and applies n·Vs to the LC filter, so di_Lo/dt = (n·Vs − Vo)/Lo. Meanwhile the magnetizing current ramps up as di_m/dt = Vs/Lm.
Switch OFF (D·T → T): the forward diode blocks, the freewheel diode carries the inductor current (node = 0), and the reset winding drives the magnetizing current back to zero as di_m/dt = −Vs·(Np/Nr)/Lm.

Key equations

Vo = n · Vs · D (ideal, continuous conduction, n = Ns/Np)

D_max = 1/(1 + Nr/Np) · V_ds(off) = Vs·(1 + Np/Nr)

Δi_Lo = (n·Vs − Vo)·D/(Lo·f_sw) · L_c = (1 − D)·R/(2·f_sw)

This simulator does not plug numbers into these formulas — it numerically integrates the real forward switching circuit (secondary node, output inductor, capacitor and the magnetizing reset) to steady state, then measures V_o, the ripple and the losses and compares them to the equations above in the accuracy panel.

Core reset & the maximum duty cycle (topic-specific)

The magnetizing flux built up during the on-time must be reset every cycle. A tertiary reset winding of ratio N_r/N_p returns that energy to the input and forces the magnetizing current back to zero. The reset takes as long as the build-up scaled by N_r/N_p, so the duty cycle is limited to Dmax = 1/(1+Nr/Np) — for an equal-turns winding that is 0.5. Exceed it and the flux staircases up cycle after cycle until the core saturates. A reset winding with fewer turns resets faster (higher D_max) but makes the switch block a higher voltage Vs·(1+Np/Nr). The simulator's core-reset panel shows this limit live and raises a saturation alarm.

Continuous vs discontinuous conduction (CCM / DCM)

The output LC behaves like a buck: in CCM the inductor current never falls to zero and Vo = n·Vs·D holds; at light load or with a small inductor it enters DCM and the output rises above n·Vs·D. The boundary is the critical inductance Lc = (1−D)·R/(2·f_sw).

Advanced options in this simulator

Turns ratio & reset ratio: set the output voltage (n) and the reset behaviour / D_max / switch stress (N_r/N_p) independently.
Core-reset visualizer & saturation alarm: watch the magnetizing current reset and get warned when D exceeds D_max.
Output LC filter: buck-like ripple and CCM/DCM behaviour.
Device model: MOSFET R_ds(on), diode V_f, inductor DCR and capacitor ESR; the accuracy check stays locked to the ideal Vo.
Ripple spectrum: a real FFT of the output voltage or the inductor current at multiples of f_sw.
Export & capture: CSV data, a text report, or a PNG screenshot of the scope.

The six isolated DC-DC converters

Converter	Switches	Ideal Vo	Filter
Flyback	1	n·Vs·D/(1−D)	Output cap only
Forward	1	n·Vs·D	LC filter
Push-Pull	2 (center-tap)	2·n·Vs·D	LC filter
Half-Bridge	2 + split caps	n·Vs·D	LC filter
Full-Bridge	4	2·n·Vs·D	LC filter
Phase-Shifted FB	4 (ZVS)	2·n·Vs·D_eff	LC filter

Explore the others: Flyback, Push-Pull, Half-Bridge, Full-Bridge and Phase-Shifted Full-Bridge simulators. For the full theory see the Forward converter tutorial.

Applications

Telecom 48 V rectifier/bus converters, server and industrial supplies, medium-power isolated rails (roughly 50–500 W), and any application needing lower output ripple than a flyback with direct power transfer.

Frequently asked questions

What is the output voltage of a forward converter?

In CCM the ideal output is Vo = n·Vs·D with n = Ns/Np. It behaves like an isolated buck — power is transferred directly during the on-time into an LC filter.

Why does it need a reset winding?

The magnetizing flux built up during the on-time must be reset to zero every cycle. A reset winding returns that energy to the input; without it the core would walk into saturation.

What is the maximum duty cycle?

Dmax = 1/(1+Nr/Np). For an equal-turns reset winding that is 0.5. Above it the flux cannot reset and the transformer saturates — the simulator flags this.

How is a forward different from a flyback?

A flyback stores energy and delivers it during the off-time (output cap only, n·Vs·D/(1−D)). A forward transfers energy during the on-time through an LC filter (n·Vs·D) and needs a reset winding.

What sets the switch voltage stress?

During reset the MOSFET blocks Vs·(1+Np/Nr) — 2·Vs for an equal-turns reset winding. Fewer reset turns raise D_max but also the switch voltage.

Power4All · Forward Converter interactive simulator. All waveforms are produced by numerical integration of the actual switching circuit and validated against closed-form theory.