100W Flyback Transformer (ER28/34, 100kHz) Full Design: How Every Parameter Is Set, from Spec to Prototype

Technical breakdown · Aug 2026 · Flyback design example

Flyback is the most widely used topology in low- and mid-power switch-mode power supplies. Turning a 100W / 12V adapter spec into a producible transformer involves trade-offs at every step—what turns ratio to pick, how to set the inductance, whether the turns will saturate, how to choose the wire gauge, and whether the temperature rise stays manageable. This article walks through each calculation with its basis; all data are sourced and can serve as an engineering checklist.

1. Design Spec (Starting Point)

2. Core Selection: ER28/34 Parameters and Basis

We use TDK PC95EER28-Z (IEC 62317 outline, PC95 material). PC95 keeps 410 mT saturation at 100°C with lower volumetric loss than PC40, making it a sound choice for efficient adapters. Its official key parameters:

All subsequent turns, saturation and core-loss calculations use this official parameter set.

3. Turns Ratio and Reflected Voltage

DCM flyback turns-ratio formula (boundary at min input, max duty):

Use standardized n=6.5; reflected voltage V_or=n·(V_o+V_f)=6.5×12.7≈82.6 V. Peak switch voltage V_{ds_max}=V_{in_max}+V_or=375+82.6≈457.6 V, so pick a 650 V MOSFET (e.g. FCP11N60) for margin.

4. Primary Inductance and Peak Current (DCM)

In DCM the primary current is a triangle bottoming at zero, so peak equals ΔI. From power balance:

• Peak current I_{p_pk} = 2·P_o / (η·V_{in_min}·D_max) = 2×100 / (0.9×100×0.45) = 4.94 A
• Primary inductance L_p = V_{in_min}·D_max / (f_s·I_{p_pk}) = 100×0.45 / (100k×4.94) ≈ 91 µH, use standard 90 µH

Note: in the DCM L_p formula, ΔI is the peak current (not half the ripple).

5. Turns Count and Saturation Check

By Faraday's law (design swing ΔB=0.25 T=2500 Gauss):

Secondary N_s=N_p/n=22/6.5=3.38, round up N_s=4, so actual ratio n_act=22/4=5.5. Re-check max duty:

Re-check peak flux density (true A_e=0.821, D_{max_act}=0.411):

0.228 T is well below the 0.25 T design swing and PC95 saturation of 410 mT@100°C—ample margin. Auxiliary winding for V_cc≈15 V: N_aux=N_s·(V_aux+V_f)/(V_o+V_f)=4×15.7/12.7≈4.94, take 5 turns (gives V_cc≈15.1 V).

6. Winding Design: Wire Gauge, Skin Effect, Structure

Current density J=4.5 A/mm². Primary RMS I_{p_rms}=I_{p_pk}·√(D_max/3)=4.94×√(0.411/3)=1.83 A → copper area 0.407 mm²; secondary I_{s_pk}=n·I_{p_pk}=5.5×4.94=27.2 A, I_{s_rms}=27.2×√((1−0.411)/3)=12.05 A → copper area 2.68 mm².

• Primary: skin depth at 100 kHz δ=0.233/√f_kHz=0.233 mm; a single Ø0.72 mm wire already exceeds 2δ, so use 4×Ø0.35 mm in parallel (total 0.385 mm², actual 4.75 A/mm²—acceptable).
• Secondary: 14×Ø0.5 mm in parallel (total 2.74 mm²), or copper foil 0.2×13.4 mm. Prefer foil on the high-current side to cut AC resistance.
• Aux: single Ø0.2 mm (≈20 mA) is enough.

Winding uses the standard two-layer primary (11 turns each) split layout (helps lower leakage and spread evenly): primary layer 1 → insulation → primary layer 2 → insulation → aux → insulation → secondary. At least 3 layers of 0.05 mm polyester tape + margin tape between primary and secondary.

7. Loss Calculation (Copper + Core)

Copper loss: primary MLT≈50 mm, 22 turns → 1.1 m, 4 parallel → R_p≈0.066 Ω (with AC factor F_r≈1.5), P_{cu_p}=1.83²×0.066×1.5≈0.33 W; secondary 4 turns → 0.2 m, 14 parallel → R_s≈0.0017 Ω (F_r≈1.3), P_{cu_s}=12.05²×0.0017×1.3≈0.32 W. Total copper ≈ 0.65 W.

Core loss: per TDK PC95, P_cv=280 kW/m³ at 100kHz/200mT/100°C; actual swing ΔB=0.228 T, extrapolated from 200 mT by Steinmetz (P_cv∝B^α, α≈2.5–2.7) gives P_cv≈395 kW/m³. Then:

Total magnetic loss P_trans≈0.65+2.07≈2.7 W.

8. System Efficiency and Temperature Rise

Semiconductor losses: switch ≈4 W, rectifier ≈2 W, other ≈1 W, plus magnetics 2.7 W → total loss ≈ 9.7 W, η=100/(100+9.7)≈91.1%, meeting the >90% target.

Temperature rise: use the surface thermal-resistance method for a sound estimate. ER28 magnetics in natural convection have ≈45–55 °C/W; 2.7 W loss implies ≈55–65 °C rise; with bobbin, pins and PCB copper helping, the core rise at full load typically lands 40–60 °C (core ≈80–100 °C), still within PC95 limits. The exact value must be measured; if too hot, drop to 65 kHz or move to PQ26/RM10 for more surface.

9. Safety Notes (IEC 62368-1)

Primary-secondary isolation is reinforced insulation. Per IEC 62368-1, at 250 V working voltage, pollution degree 2, material group IIIa, reinforced-insulation creepage requires ≥6.4 mm. In practice use triple-insulated wire (TIW) or margin tape + ≥3 layers of 0.05 mm tape between pri/sec, and form a margin so the creepage path ≥6.4 mm. Meets EN 61558 / IEC 62368 isolation.