SMPS Flyback Transformer Calculator – Design by Target Duty Cycle

This flyback SMPS transformer calcuator tool uses the reflected voltage priority method that is strictly optimized for discontinuous conduction mode or DCM, so let us look at how we can use this specific layout step by step.

How to Use the Calculator

Step 1: Gather Your Basic Power Specs

Before touching the calculator, you need to know what your power supply is supposed to do, so let us fill in these basic electrical requirements first.

Minimum Input Voltage (V) is the lowest DC voltage your circuit will see under full load. When you are designing for a standard 12V battery system, then you can use 11V or 12V.

But when you are dealing with an offline 230V AC mains supply after rectification and filtering, then this usually drops to around 250V or 300V DC.

Output Voltage (V) is the exact DC voltage that you want at your load, such as 5V, 12V, or 24V. Output Current (A) is the maximum continuous current your load will draw.

Step 2: Set Your Target Duty Cycle

Instead of guessing abstract voltages, our calculator uses the professional volt-second balance method, so you tell it the maximum time you want the primary MOSFET to stay turned ON during each switching cycle.

Set Desired Duty Cycle (%) to 45% because it gives the perfect balance.

It leaves enough OFF-time for the energy to fully transfer to the secondary side, and it keeps the voltage stress on your primary MOSFET completely manageable.

Try to stay between 35% and 45%, but if you go above 45%, then the calculator will flash a warning reminding you to double-check if your PWM controller needs slope compensation to prevent subharmonic oscillations.

Step 3: Input Your Core and Frequency Variables

Now we can input the physical components you are working with. Switching Frequency (kHz) for most modern DIY flyback controllers like UC384X or standard 555-based setups run nicely between 50 kHz and 100 kHz.

Since higher frequencies mean a smaller transformer core, but higher switching losses, so we must balance this. Core Area Ae (mm²) can be found when you look up the datasheet of the ferrite core you are holding, since Ae is the cross-sectional area of the center leg. For example, a tiny EE16 core is around 19 mm², an EE25 is about 41 mm², and a beefy EFD30 can be around 58 mm².

Target Flux Density (mT) should be kept conservative to prevent your transformer from saturating and drawing massive current spikes. If you keep a target of 150 mT to 180 mT, then it is incredibly safe but do not exceed 200 mT here unless you want your circuit to run smoking hot.

Step 4: Fine-Tune Ripple and Efficiency

Ripple Current (%) defines your conduction mode so a value of 40% to 50% keeps the system cleanly in Continuous Conduction Mode (CCM).

If you increase this toward 100% or higher, then the inductor current will hit zero every cycle, pushing the system into Discontinuous Conduction Mode (DCM).

The calculator will automatically alert you when you cross this boundary. Estimated Efficiency (%) for a well-wound custom transformer has a very realistic, safe starting baseline around 80% to 85%.

Step 5: Hit Calculate and Analyze the Blueprint

Now that you can click the calculate button, the engine spits out your exact winding blueprint. Let us see what you need to look for. Primary Turns (Np) and Secondary Turns (Ns) are the exact integer turns you need to wind, since the calculator automatically rounds these up to real-world integers.

Primary Inductance (Lp) is the target value you need to hit when measuring your primary winding on an LCR meter. Approximate Air Gap is important because ferrite cores store their energy in the air gap, not the magnetic material itself.

If this value shows No Gap / Toroid, then your core has enough native reluctance. But if it shows a value like 0.250 mm, then you need to place a spacer shim of half that thickness between the outer legs of the core halves to achieve the target inductance.

When the air gap calculation hits larger than 1.5 mm then the calculator will flash an amber warning, which means your core is too small for the power you are demanding, so you must switch to a core with a larger Ae value.

Minimum MOSFET Rating tells you the absolute bare-minimum breakdown voltage Vds your primary switch needs, since it adds a crucial 50% safety buffer to absorb high-frequency leakage spikes. Minimum Diode Rating is the absolute minimum reverse voltage Vrrm rating required for your secondary output rectifier diode.

A Quick Troubleshooting list

When you are getting a Flux density approaching saturation warning, then you can increase your primary turns manually by lowering your target flux density input slightly, or you can use a larger core.

When you are getting an Entered DCM warning, then remember that this is not necessarily a bad thing because many low-power chargers operate completely in DCM. Now just keep in mind that your RMS currents will be slightly higher so your wire gauges might need to be a fraction thicker to handle the extra heating.