Basic SMPS Transformer Calculator (Using Ferrite E-Cores)

This calculator will help you to design a basic SMPS circuit by calculating the primary turns, secondary turns, and core selection based on the input voltage, the output voltage, the switching frequency and the power requirements.

Which Core Type Must be Used

We must always use ferrite core for this calculator. Prefer EE, ETD, or RM shapes. Use ferrite material like N87, 3C90, PC40 depending on your frequency. Then take Ae, Bmax, Lcore, Mur values from the core datasheet and put in the calculator.

If we use powdered iron cores or silicon steel cores here then calculation will go wrong because they are not suitable for high frequency flyback circuits.

You can use this calculator confidently only when we use correct high frequency ferrite core as per standard SMPS design practice.

How to Use the Calculator

Alright, here we are learning how to use this tool for designing an SMPS transformer.

This tool helps us calculate how many turns to wind on the transformer and other important things.

But we need to put the correct values inside it first.

Now let us see what each thing means and why we put specific values like 30 mm, 2000, 200 mT, and 100 mm².

Enter the Input Voltage (Example: 100V for AC mains rectified DC)

This is the DC voltage that goes into the transformer primary winding.

If we are using 100V AC mains then after rectifying, the DC voltage will be around 140V DC (because AC voltage after rectifying becomes AC × 1.414 minus diode losses).

This voltage is very important because it tells us how many turns we need on the primary winding of the transformer.

Enter the Output Voltage (Example 12V for low-voltage SMPS)

This is the voltage we want to get from the SMPS transformer after rectification.

If we are making a 12V SMPS then we put 12V here.

This number helps the calculator find out how many turns we need on the secondary winding.

Enter Switching Frequency (Example: 50 kHz for common flybacks)

This is how fast the SMPS is switching on and off in one second.

Example 50 kHz means it switches 50000 times per second.

Higher frequency (above 100 kHz) makes the transformer smaller but also creates more switching losses (more heat in MOSFET).

Lower frequency (below 20 kHz) means we need a bigger transformer which is also not good.

So 50 kHz is a good middle choice—not too big, not too much heating.

Enter Core Area (Example: 100 mm² for small ferrite cores)

This is the cross-sectional area of the core (the flat part inside the transformer where the magnetic field forms).

Measured in square millimeters (mm²).

Why 100 mm²?

Because it is a good size for small power supplies like 50W - 100W SMPS.

The core area decides how many turns we need for a given voltage.

V per turn = 4.

44 × f × Bmax × Acore

If we increase core area then we need fewer turns.

If we reduce core area then we need more turns which increases wire losses.

Common sizes:

50 mm² → Tiny transformers (small LED drivers).

100 mm² → Good for medium power SMPS (like battery chargers, adapters).

200+ mm² → Big transformers for high power (like 500W SMPS).

If the core area is too small then transformer will overheat and may not work well.

Enter Maximum Flux Density (Example: 200 mT for ferrite materials)

Flux density (Bmax) means how much magnetism the core can handle before it gets full (saturates).

Measured in milliteslas (mT).

Why 200 mT?

Ferrite cores used in SMPS can work well between 150 - 300 mT.

So, 200 mT is safe because:

It avoids saturation (which makes the transformer useless and causes high current draw).

It still allows good energy transfer.

If Bmax is too high like 400 mT then core saturates → Transformer stops working properly.

If Bmax is too low like 100 mT then we need more turns which means more wire resistance and losses.

Common Bmax values for different cores:

Ferrite cores (used in SMPS) → 150 - 300 mT.

Iron powder cores → 700 - 1000 mT (not used in high-frequency SMPS).

Silicon steel cores → 1000 - 1600 mT (used in normal 50Hz transformers).

So 200 mT is a good and safe choice for SMPS ferrite cores.

Enter Core Path Length (Example 30 mm for common cores)

This is the length of the magnetic path inside the core measured in millimeters (mm).

It is the total distance that the magnetism must travel inside the core.

Why 30 mm?

A common size for small and medium transformers.

If the path length is too long then magnetism gets weak and we need more turns which is bad.

If the path length is too short then flux gets too concentrated, causing saturation.

Typical values:

Small cores → 20 - 30 mm.

Larger cores → 40 - 60 mm.

So 30 mm is a balanced choice—not too big, not too small.

Core Permeability (default 2000 for ferrite cores)

Permeability (µ) tells how easily a material allows magnetism to pass through it.

Ferrite cores have permeability between 1000 and 5000.

Why 2000?

2000 is a good middle value—not too high, not too low.

If permeability is too high (5000 or more) then core stores less energy which is not good for SMPS transformers.

If permeability is too low (below 1000) then we need more turns which increases wire resistance and losses.

So 2000 is chosen because it gives good performance for SMPS transformers.

Conclusion:

So now you see, we put these values carefully.

If we choose the wrong values then transformer can overheat, stop working, or become inefficient.

By using the right values we get an efficient, compact and reliable SMPS transformer.