Forward Converter Calculator
Forward Converter Calculator
Design a single-switch forward converter with a reset winding: turns ratio, duty cycle across the input range, output inductor and capacitor, magnetising current, the reset duty limit, MOSFET voltage stress and the ratings of all three diodes.
Forward converter power stage
36–72 V in (a telecom bus), 5 V at 10 A, 200 kHz, 42% maximum duty, 1:1 reset winding, 0.5 V diodes, 30% ripple, 50 mV output ripple, 500 µH magnetising inductance
Forward converter with a reset winding (CCM)
- Dmax, Dmin
- duty cycle at minimum and maximum input; Vin × D is constant in CCM
- Nr
- reset winding turns; Np = Nr gives the familiar 50% limit and 2 × Vin stress
- ΔI
- output inductor ripple, peak to peak, largest at maximum input
- Lm
- magnetising inductance of the primary
Worked example
36–72 V in (a telecom bus), 5 V at 10 A, 200 kHz, 42% maximum duty, 1:1 reset winding, 0.5 V diodes, 30% ripple, 50 mV output ripple, 500 µH magnetising inductance
Ns/Np = 5.5 ÷ (36 × 0.42) = 0.3638, so Np : Ns = 2.749 : 1
At 72 V the duty falls to 21.0%; reset limit 1/(1 + 1) = 50%
Lo = 5.5 × (1 − 0.21) ÷ (200,000 × 3) = 7.242 µH → 8.2 µH (E12)
Im = 36 × 0.42 ÷ (200,000 × 500 µH) = 151.2 mA; primary peak 4.334 A, RMS 2.413 A
MOSFET 72 × 2 = 144 V; forward and freewheel diodes 26.2 V each
Designing a forward converter
A forward converter is a buck converter with a transformer in it. While the MOSFET is on, the input appears across the primary, the secondary drives current through the forward diode into the output inductor, and energy passes straight through — unlike a flyback, whose transformer stores it. When the MOSFET turns off, the freewheel diode carries the inductor current exactly as in a buck. So the output stage is sized like a buck’s, with the secondary voltage Vin·Ns/Np in place of Vin: the buck converter designer explains the inductor and capacitor choices.
Resetting the core. The transformer’s magnetising current rises during every on-time and has to fall back to zero before the next, or the flux walks up cycle by cycle until the core saturates. This page models the classic reset winding: a third winding with a diode back to the input. When the MOSFET turns off, the magnetising current transfers to it and the primary is held at −Vin·Np/Nr until the current has gone. Resetting takes D·Nr/Np of the period, so the duty cycle is limited to Np/(Np + Nr): 50% with a 1:1 winding, and the MOSFET sees Vin(1 + Np/Nr), 144 V at 72 V in the example, plus a leakage spike. RCD clamps, resonant reset and the two-switch forward are the usual alternatives; their limits differ.
Turns ratio and duty cycle. The turns ratio is set at the lowest input and the largest duty you allow. As the input rises the controller reduces the duty in proportion, so the volt-seconds per cycle — and the magnetising current — stay the same across the range, while the output ripple is largest at the highest input. A simulation of the magnetising current, reset winding and output filter confirmed these figures, and showed the core failing to reset once the duty passes the limit.
Diodes. During the reset interval the secondary swings negative, so the forward diode must block Vin,max·Ns/Nr; the freewheel diode blocks Vin,max·Ns/Np during the on-time. Size the transformer’s core and wires with the SMPS transformer calculator, the MOSFET’s losses with the MOSFET loss calculator and the diodes’ with the diode power loss calculator.
Frequently asked questions
Why is a forward converter limited to 50% duty?
With a 1:1 reset winding the core resets at the same volts per turn it was magnetised at, so resetting takes as long as the on-time. The general limit is Np/(Np + Nr).
What voltage does the MOSFET see in a forward converter?
Vin × (1 + Np/Nr) during reset, so twice the maximum input with a 1:1 reset winding (144 V for a 72 V input), plus the leakage-inductance spike.
How do you choose the turns ratio of a forward converter?
Ns/Np = (Vout + Vf) ÷ (Vin,min × Dmax). For 5 V at 36–72 V with 42% maximum duty, Np : Ns = 2.749 : 1.
How do you size the output inductor of a forward converter?
As for a buck, at the maximum input: L = (Vout + Vf)(1 − Dmin) ÷ (f × ΔI). The example gives 7.242 µH.
Related calculators
References
- Choi H-S. Design Guidelines for Off-line Forward Converters Using Fairchild Power Switch (FPS). Fairchild Semiconductor (now onsemi) application note AN-4134, 2003. Reset-winding duty limit Dmax ≤ Np/(Np + Nr), drain stress VDC,max(1 + Np/Nr), turns ratio and output inductor.
- Mohan N, Undeland TM, Robbins WP. Power Electronics: Converters, Applications, and Design, 3rd ed. Wiley, 2003. Ch. 10 (forward converter with a demagnetising winding, D ≤ 1/(1 + N3/N1)).
- Erickson RW, Maksimović D. Fundamentals of Power Electronics, 3rd ed. Springer, 2020. Ch. 2 (volt-second and charge balance, the Ćuk converter example), Ch. 5 (discontinuous conduction).
- IEC 60063:2015. Preferred number series for resistors and capacitors. The E6 and E12 series used for the suggested standard parts.