DCM Flyback Transformer & Wire Gauge Wire Size Calculator Tool

When you want to design a switching power supply by fixing the reflected voltage first, then this advanced calculator is exactly what we use to get a quick blueprint. It uses the discontinuous conduction mode method which means the transformer lets the magnetic energy drop completely to zero before the next cycle starts so let us look at the step-by-step guide so you can use it like a pro.

Step 1: Understand Your Basic Inputs

First, you need to look at your electrical requirements. Input Voltage (Vin) is the high DC voltage that comes after your rectifier filter capacitor, so when you are using standard 230V AC mains, then you should enter 310V DC here.

Target Reflected Voltage (Vor) is the voltage that reflects back from the secondary to the primary winding when the MOSFET turns off. If you set this between 90V and 120V, then it keeps the primary switch very safe from breaking down.

Output Voltage (Vout) is the clean DC level that you want for your load, so if you are making a 12V charger, then type 12. Output Power (Pout) is the total wattage of your load, which means if your circuit draws 5A at 12V, then you must enter 60W here.

Step 2: Enter Winding and Material Settings

Now we must look at the frequency and core details. Switching Frequency (kHz) is how fast your controller pulses every single second. Since running at 100 kHz is very common for standard power ICs, so you can leave it there.

Core Cross-Section Area (Ae) is the true thickness of your ferrite center leg in square millimeters, so look up the datasheet of the core you have. When you use a medium-sized core like an EE25, then it is usually around 40 to 42 but since this specific calculator is for a 60W load, then you might need a larger core like an EER28 or EE35 which can be around 80 to 120.

Max Flux Density (Bmax) should be around 200 mT, since this keeps the ferrite safely away from hard magnetic saturation. Now click calculate.

Step 3: Read the Winding Results

Let us analyze what the calculator outputs for you. Derived Operating Duty Cycle (D) tells you the exact percentage of time the switch stays closed, which is calculated automatically from your Vor choice.

Primary Turns (Np) and Secondary Turns (Ns) show the exact number of wire loops you must wrap around the plastic bobbin. Target Primary Inductance (Lp) is the critical value that you must measure with your LCR meter.

Since this is a DCM calculator, so you will need to grind or gap the center leg of the core until your meter matches this microhenry reading exactly.

Step 4: Check Peak Currents and Choose Your Wire Gauge

This calculator gives you Average Output Current (Iout), Peak Primary Current, and Peak Secondary Current. Since the current spikes up in a sharp triangle shape every cycle in DCM mode, so the peak values will look quite high but do not worry because that is completely normal for this topology.

The most helpful part is the Recommended Wire Size block because it gives you the absolute minimum copper cross-sectional area in square millimeters for both windings based on a safe current density of 4.5A per square millimeter.

When you get a value like 0.20 square millimeters for the primary, then you can look up a standard AWG wire gauge chart to find a matching wire thickness. If the secondary requires a thick area then remember that you can twist multiple thinner strands of wire together to match that total copper area so it is easier to wind around the bobbin.