Advanced SMPS Flyback Transformer Calculator for High Power Converters

Let's say you are trying to make one flyback transformer for our project, but we are not knowing how many turns we should put, what thickness of wire we should use, what current will flow in primary and secondary, what is the duty cycle, and also how much inductance is needed in the primary winding.

That time if we try to do all calculations manually using formulas then it becomes very confusing and time wasting, and sometimes we are doing mistakes also.

So for solving this big headache and making everything automatic, we are using this flyback transformer calculator.

This tool is helping us to calculate full winding details of flyback type transformer which is used in smps circuits like adapter, charger, inverter or any dc to dc converter.

We just need to enter some simple values like input voltage, output voltage, watt, frequency, core area and permeability, then this calculator is giving us all other values automatically like turns, current, inductance, wire area, ripple, peak current and everything. So this is very helpful for beginners and also for experts who want to save time.

What is flyback transformer and why we are using calculator for that

So now we are knowing that this flyback transformer is not same like that normal ac to ac iron core type transformer. In flyback we are using that transformer not for passing energy directly but for storing that energy in magnetic core first when mosfet is switching on, and then we are dumping that energy into output side when mosfet is switching off. So because of that this transformer is actually not transformer, but that is coupled inductor type.

Now we are having one big problem, that we cannot calculate this kind of flyback design using simple v1/v2 = n1/n2 formula. That is because in flyback only one winding is working at one time.

So we need to design that core and winding using special magnetic formulas, otherwise it will saturate and blow.

For helping us in all this headache, we are making this calculator. This calculator is doing all the maths like flux, ampere turns, inductance, current, ripple, wire area and all. So we just need to enter some values and then we get all flyback transformer design data.

How we are using this calculator

First we are putting that input voltage which is going to primary side, that is vin. Then we are putting that output voltage which we want to get after secondary diode and capacitor, that is vout.

Then we are putting that output power in watts, like 50 watt or 100 watt. Then we put frequency in khz, like 50 or 100. After that we are taking ferrite core datasheet and putting core cross area ae in mm², maximum flux bmax in mt, core magnetic path length in mm, and core relative permeability mur.

Core Cross Section Area Ae mm2

This value means how fat the ferrite core middle leg is, where the copper winding actually sits, so now first thing we do is look at the ferrite core size, like EE16, EE19, EE25, EE33, ETD34, EER28, etc. Every ferrite core datasheet already gives Ae value in mm2, so if you have datasheet then no headache, you just copy paste that number here.

But if datasheet is not there, then we physically measure the center leg width and thickness using scale or caliper, multiply both values, so width mm × thickness mm gives Ae mm2.

For example if center leg is 10 mm wide and 5 mm thick, then Ae = 50 mm2, so we put 50 in the box.

Rule feeling is simple, low power like 10 to 20 W means Ae around 25 to 50 mm2, medium power 50 to 100 W means Ae around 70 to 150 mm2, big power means bigger core, so now that is how we estimate Ae.

Max Flux Density Bmax in mT

This mT value means how much magnetic stress we allow inside the ferrite, so ferrite gets angry if we push too much flux.

Normally ferrite saturation happens near 300 to 400 mT, but we never go there because heating and distortion comes.

So safe value is 180 to 220 mT, most flyback designs use 200 mT, that is why default value is often 200.

If frequency is high then we reduce it more, like 150 mT.

So if you are confused, just put 200 mT, it is safe, calm, peaceful for ferrite.

If when power is high and frequency is low then also 200 mT is good.

So this value is not calculated, it is chosen wisely.

Core Path Length Lcore mm

This is the total magnetic travel distance inside the ferrite, like how much road the magnetic field walks inside the core loop.

Again datasheet gives this value as le or lcore in mm.

If datasheet not there, then we roughly measure the magnetic loop length, which is center leg + both outer legs + yoke thicknesses, but we do not need extreme accuracy, flyback is forgiving.

Typical values are like 30 mm for small EE16, 50 to 70 mm for EE25 or EE33.

So if core looks small, then lcore around 30 to 40 mm, if core looks medium then 50 to 70 mm.

We just put that value here, no drama.

Core Relative Permeability Mur

This value tells how magnet friendly the ferrite material is, how easily it allows magnetic field to pass.

Ferrite material usually has Mur from 1500 to 3000.

Common power ferrites like N87, PC40, 3C90 all sit around 2000, that is why calculator already shows 2000.

So if you do not know material, just keep 2000, it works fine.

If datasheet says Mur = 2300 or 1800 then you can update it, otherwise leave default.

One important thing is when flyback has air gap, effective permeability reduces, but here we are still giving raw core Mur and air gap effect is indirectly handled by inductance and turns.

Summary In short

So in short Bro, Ae and Lcore we take from core size or datasheet, Bmax we choose safely like 200 mT, Mur we keep around 2000.

This calculator is not NASA rocket, it is practical flyback math, so small errors will not break anything.

If values are slightly wrong then turns count will change by one or two turns only, nothing dangerous.

What values we are getting from this calculator

Now this calculator is giving us full important data which we are needing to wind flyback transformer correctly. First it is giving us duty cycle, that is showing how much on time is needed for storing full energy. Then we are getting turns ratio between primary and secondary.

Then it is calculating how many turns we need to wind on primary side and how many turns on secondary side.

Then we are getting inductance value in microhenry which that primary is creating. After that we are getting output current based on output watt and voltage.

Then it is calculating ripple current, peak current on primary and secondary, and also rms current using triangle waveform formula. Finally it is calculating how much mm² wire area is needed for primary and secondary side, using 4.5 amp per mm² rule.

How this calculator is doing all calculation inside

Inside the code we are using that faraday’s law formula for finding number of primary turns like np = v / (4 × f × b × ae).

Then we are using that l = µ × n² × ae / lcore for calculating primary inductance. After that we are using power = voltage × current for finding output current.

Then ripple is taken as 40 percent of iout. After that peak current is calculated and then rms is calculated as ipeak / √3.

Then we divide rms current by 4.5 for getting safe wire cross-section in mm².

How to use this result in real transformer winding

So now when we get all result, we take our ferrite core and wind primary winding with number of turns shown in result, and then we wind secondary winding with given number of turns.

We use wire that is having equal or more cross-section area than the result. If current is more we use multi strand copper or litz wire.

Between primary and secondary winding we are keeping insulation like tape or mylar or kapton so that there is no spark or breakdown.

Then after that our flyback transformer is ready for testing.

Important notes and warnings

We must not increase bmax more than 220mt, otherwise core will saturate and short circuit will happen.

We must not reduce number of turns, otherwise same saturation will happen.

We must wind tightly and keep insulation between windings. Wire should not be thin otherwise it will get hot.

Ripple is taken as 40 percent in this code but we can change it inside script if we want.

Extra important info

This calculator is assuming that flyback is working in continuous mode. Diode voltage drop is fixed as 0.7v. Wire calculation is using 4.5a/mm² density which is safe for free air cooled transformers.