Flyback Converter Calculator | DCM Transformer Design

This advanced tool designs DCM Flyback Converters with multiple outputs. It calculates the primary inductance (L_p), transformer turns ratios (N_p:N_s), and RCD Snubber values.

Ideal for designing offline SMPS, LED drivers, or auxiliary power supplies with 12V/5V rails and bias windings.

System Specs

Configuration

Primary Side

Min V_in

Max V_in

Freq (f_sw)

kHz

Max Duty (D)

0-1

Efficiency (η)

0-1

Core A_e

mm²

Secondary Outputs

Rail	V_out	I_out	V_drop

Feedback Regulation Source

Bias / Aux Winding

V_aux

I_aux

Design Analysis

DCM Mode

Primary Inductance (Lp)

— µH

—Peak Current

—MOSFET Vds

—Input Power

Transformer Design

—N_pri Turns

—N_aux Turns

RCD Snubber Estimate

R: — C: — P: —

Output Winding Details

DCM vs CCM Flyback Mode

Choosing between Discontinuous Conduction Mode (DCM) and Continuous Conduction Mode (CCM) affects efficiency and stability.

Feature	DCM (Discontinuous)	CCM (Continuous)
Zero Current Switching	Yes (Turn-ON)	No (Hard Switching)
RHP Zero	High Freq (Easier Loop)	Low Freq (Complex Loop)
Peak Current	High (Higher stress)	Low (Lower stress)
Best For	Low Power (< 100W)	High Power (> 100W)

Physics Explained

Reflected Voltage (V_R)

The voltage reflected from the secondary back to the primary during the off-time. Higher V_R allows for a larger duty cycle but increases MOSFET voltage stress.

RCD Snubber

Leakage inductance causes voltage spikes when the switch turns off. An RCD clamping circuit dissipates this energy to protect the MOSFET.

Discontinuous Mode (DCM)

In DCM, the transformer fully demagnetizes (current hits zero) before the next cycle starts. This simplifies control loop stability but increases peak currents.

How to Use

Set Primary Specs

Enter the input voltage range and switching frequency. D_max is usually kept below 0.5 for DCM operation.

Configure Outputs

Add your output rails (e.g., 5V, 12V). Select which rail is Regulated (has the feedback loop); others will track based on turns ratio.

Review Transformer

Check the calculated turns ratios and primary inductance. Ensure the MOSFET Vds is within the rating of your switch (e.g., 600V/650V).