Power Electronics & Isolated AC-DC Supply Design
Isolated Flyback Converter Duty Cycle & Vds Stress Formula
Calculate Coupled-Inductor Flyback Reflected Primary Voltage V_ref = (V_out + V_f)*(N_p/N_s), CCM Duty Cycle D = V_ref / (V_in + V_ref), Peak Primary MOSFET Drain-Source Stress V_ds_pk, and 80%-Derated Breakdown Headroom.
Add reflected secondary voltage AND leakage snubber clamp overshoot to Vin on Flyback MOSFETs!
Universal AC-DC USB-PD adapters and auxiliary industrial power supplies rely on the Isolated Flyback topology—a coupled magnetizing inductor that stores energy during primary MOSFET turn-on and transfers it across galvanic isolation during turn-off! Beginners often select a 400 V primary MOSFET for a 300 V rectified AC bus, forgetting that during MOSFET turn-off the secondary output voltage plus rectifier drop (V_out + V_f) reflects back across the primary-to-secondary turns ratio N_ps = N_p / N_s as V_ref = (V_out + V_f) * N_ps, PLUS leakage-inductance RCD snubber clamp overshoot V_clamp! In Continuous Conduction Mode (CCM), volt-second balance sets Duty Cycle D = V_ref / (V_in + V_ref) while Peak Drain-Source Voltage hits V_ds_pk = V_in + V_ref + V_clamp—which IPC-9592 derating requires keeping below 80% of MOSFET BV_DSS!
Verify in a power-electronics hardware review workbook—DC_Bus_Input V_in = 100 V (low-line rectified AC, B2), Regulated_Output V_out = 12.0 V (B3), Secondary_Diode_Drop V_f = 0.50 V (B4), Transformer_Turns_Ratio N_p/N_s = 8.0 (B5 -> Reflected Voltage V_ref = 12.5 * 8 = 100.00 V in E2), Leakage_Clamp_Overshoot V_clamp = 50 V (B6), and MOSFET_Rated_BVDSS = 650 V (B7 -> 80% derated ceiling = 520 V): Low-Line CCM Duty Cycle D is 100 / (100 + 100) = 0.5000 (50.0% in E3); Peak Drain Voltage is 100 + 100 + 50 = 250.00 V (E4); leaving +270.00 V below the 520 V derated ceiling (E5)!
| Isolated Flyback Metric (Vin=100V, Vout=12V, Vf=0.5V, Nps=8, Clamp=50V, 650V FET) | Formula | Verified Output |
|---|---|---|
| Reflected Primary Voltage V_ref | (V_out + V_f) * (N_p / N_s) | 100.00 V |
| CCM Steady-State Duty Cycle D | V_ref / (V_in + V_ref) | 0.5000 (0.2500 @ Vin=300V) |
| Peak MOSFET Drain Stress V_ds_pk | V_in + V_ref + V_clamp | 250.00 V (450.00 V @ Vin=300V) |
| IPC 80% BV_DSS Derating Margin | 0.80 * BV_DSS - V_ds_pk | +270.00 V (+70.00 V @ Vin=300V) |
Flyback Peak MOSFET Drain Voltage & 80% Derating Headroom Formula
Validate strictly positive V_in, V_out, turns ratio N_ps, BV_DSS, and non-negative diode drop and clamp overshoot!
=LET(v_in, B2, v_ot, B3, v_fd, B4, n_ps, B5, v_cl, B6, bv_d, B7, IF(OR(v_in <= 0, v_ot <= 0, v_fd < 0, n_ps <= 0, v_cl < 0, bv_d <= 0), "Invalid Flyback Specification", LET(v_rf, (v_ot + v_fd) * n_ps, v_pk, v_in + v_rf + v_cl, ROUND(0.8 * bv_d - v_pk, 2))))High-Line 265 VAC Rectified Stress Check (Set V_in = 375 V vs 300 V)
Raise DC_Bus_Input B2 to 300 V: Duty Cycle E3 drops to 0.2500, Peak Drain Stress E4 climbs to 450.00 V, and 80%-derated margin E5 narrows to +70.00 V!
=IF(ISTEXT(E2), E2, ROUND(E2 / (B2 + E2), 4))Try the example
Use the original values or change them to test your understanding.