Forward Converter Design Calculator

Forward Converter Design Calculator (Single‑Switch / Two‑Transistor)

Topology: Single‑Switch w/ Reset Winding

Inputs

Worst case low line
0.85–0.95 typical
For Lout sizing
Used in Auto mode
Used in Manual mode
Only for Reset‑winding topology
Only for RCD clamp
Core max flux density
Core effective area

Key Results

Transformer & Power Stage

Notes:
  • Ideal CCM equations. Vout ≈ D · (VIN/n) − Vd where n = Np/Ns.
  • Reset‑winding duty limit: Dmax ≤ r/(1+r) (typ. r=1 → 0.5). Two‑transistor: Dmax ≈ 0.5. RCD clamp: user D‑limit via practical design (k).
  • Primary turns from flux: Np ≥ VIN_max · D / (BMAX · Ae · f) (Ae in m²). Rounds Np/Ns to integers and reports resulting duty & errors.
  • Validate with hardware: ringing/clamp, thermal, loop stability. Layout: tight primary loop, short secondary rectifier loop.

How to Use the Forward Converter Design Calculator

A practical guide for Single‑switch (reset winding / RCD clamp) and Two‑transistor forward topologies. It matches your calculator UI and is tuned for mobile.

0) Pick the Topology

  • Single‑switch + reset winding (energy reset via Nr): efficient, built‑in reset limit.
  • Single‑switch + RCD clamp (no reset winding): simpler magnetics, more loss in clamp.
  • Two‑transistor forward (TTF): lower switch stress, natural demag, ~50% max duty.

1) Global Inputs

  • VIN_min / VIN_max, VOUT, IOUT, η
  • fSW (kHz) and secondary VD (diode drop)
  • BMAX (T) and Ae (mm²) for core sizing
  • Ripple targets: ΔVout (mVpp) capacitive and ESR share (mVpp)
  • Margins: VDS +%, VRRM +%

2) Turns Ratio Mode

  • Auto: set a Target D @ VIN_min. The tool chooses n = Np/Ns accordingly.
  • Manual: enter n directly and the tool reports the resulting duties at VIN_min/max.
Ideal CCM: Vout ≈ D · (VIN / n) − VD. Rounding Np/Ns to integers changes D slightly.

3) Duty Limits

  • Reset winding: Dmax ≤ r/(1+r) where r = Nr/Np (r=1 → 0.5)
  • Two‑transistor: Dmax ≈ 0.5
  • RCD clamp: practical Dmax limited by clamp design (k) and loss; start near 0.5
Ensure D @ VIN_min stays below the allowed Dmax. If not: increase n, raise VIN_min, lower VOUT, or change topology.

4) Transformer & Turns

  • Np,min from flux: Np ≥ VIN,max·D / (BMAX·Ae·f) (Ae in m²)
  • Round to integer Np, compute Ns ≈ Np/n, and re‑check duties
  • Log the resulting n, D @ VIN_min, D @ VIN_max shown by the tool

5) Output Inductor (Lout)

  • Set ΔIL/IOUT (e.g., 30%)
  • At VIN_max, Vsec = VIN_max / n and D from the tool → size Lout
  • Check IL,pk, Irms for core and heating; low DCR helps efficiency

6) Rectifiers

  • Main diode VRRM ≈ Vsec,pk + Vout + VD
  • Freewheel diode VRRM ≈ Vout + VD
  • Current ratings: use tool’s Iavg per diode and allow margin; consider synchronous rectification for high current

7) Switch & Clamp

  • Single‑switch + reset: VDS ≈ VIN_max · (1 + Np/Nr)
  • Two‑transistor: VDS ≈ VIN_max per switch
  • RCD clamp: estimate with clamp factor k ≈ Vclamp/VIN_max → VDS ≈ VIN_max · (1 + k)
  • Apply your VDS margin, check Irms and Ipk, add snubber/RC damping as needed

8) Capacitors

  • Cout (capacitive ripple): ΔV ≈ ΔIL / (8 f C); ESR ≤ ESRmax from tool
  • Check ripple current rating; polymer/MLCC banks reduce ESR
  • Cin: tight to primary switch loop; adequate ripple rating

Quick Checklist

  • Topology chosen & duty limit understood
  • D @ VIN_min within Dmax
  • Transformer: Np, Ns rounded; flux within BMAX
  • Lout meets ripple target; Isat/Irms margins OK
  • Switch VDS & diode VRRM (with margins) cleared
  • Cout & ESR meet ripple targets; Cin placed well
  • Clamp/reset tuned; ringing acceptable
  • Bench at VIN_max hot and VIN_min cold
FAQ & Tips

Auto vs Manual turns?
Use Auto early to land near your target duty at VIN_min; switch to Manual if you need an exact ratio to hit standard turn counts or core windows.

My duty exceeds the limit after rounding.
Increase n slightly (fewer secondary turns), or reduce VOUT. Re‑check Np for flux.

RCD clamp is running warm.
Reduce leakage (better winding), lower D, or move to reset winding or two‑transistor. Tune RC for critical damping.

Multiple outputs?
Forward behaves like a buck on each secondary. Keep tight coupling; add post‑regulators or small preload for accuracy on less‑critical rails.

Copy‑Paste Mini Workflow

1) Pick topology (reset / RCD / two-transistor)
2) Enter VIN_min/max, VOUT, IOUT, η, fSW, BMAX, Ae; set ripple targets & margins
3) Choose turns mode: Auto (target D@VIN_min) or Manual (n)
4) Check duty vs Dmax; compute Np from flux; round Ns; re-check duties
5) Size Lout (ripple%), Cout & ESR; verify rectifier & switch stresses
6) Plan clamp/reset and snubber; build & validate on bench