Diving Deep Into Flyback Transformer Design

Tech Consultant Zach Peterson walks you through every step of designing a flyback transformer, from understanding the basics of coil inductances to selecting the right core and coil former. Whether you're working on a flyback converter project or looking to enhance your electronics design skills, this video is your ultimate resource.
0:00 Intro
0:38 Calculating Inductance
4:42 Determining Values
8:24 Primary Inductance
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[@abidinakdag658]

Great presentation. Thank you so much for your sharing

[@user-qp2ps1bk3b]

a very good video! Thank you

[@Yorumcu63]

Great video

[@AltiumAcademy]

Thanks so much!

[@nulldev42]

Awesome video and I'm looking forward to the results from the build. One question that is a bit off-topic, what is the name of the music track that you use for your openings? I'd love to hear more of it. Thanks!

[@josephbaker9932]

Could you verify that the dots on the windings are correct?

[@Zachariah-Peterson]

Good catch! If you watch the other video on this you'll see that the windings work out correctly, that was my mistake while doing this on the board.

[@hamidrezafallahian7752]

Hello, thank you very much for your good training🙏🙏🙏🙏🙏 Sorry, I had a question How did you calculate the NP/NS ratio?

[@Zachariah-Peterson]

The Np/Ns ratio is calculated based on the duty cycle of the switcher, the input and output voltages, and the voltage of the diode. You can see the formula where it is calculated at 4:50.

[@asidesigner8542]

Zack, please show how to use calculators for it like ExcellentIT, thanks for sharing🎉

[@alirezaabasi.]

Hi great work 👍 I know my question is kind of out of subject but for high frequency puls transformer like 300kHz how can l do the calculating?

[@Zachariah-Peterson]

The same method applies. I did this for 62 kHz.

[@victorlombardo2396]

Great video! I assume that for the calculus of critical inductance Ls, the D you are using is 50,as is the critical value? Otherwise I don't obtain the same inductance value, as if I use a D of 30 the formula is Ls= (3.3+0.5)*(1-0.3)^2/2*2.57*62000 which gives an inductance of 5.84 uH

[@Zachariah-Peterson]

Yes that Ls value was taken at the maximum available duty cycle as that gives you the true upper limit for your conduction mode across the entire range of duty cycle. If it is in DCM with 50% duty cycle then it is also in DCM with 30% duty cycle, so you should choose an inductance value that is based on the maximum duty cycle since that is the true lower limit for this conduction mode. I should have clarified this a bit better.

[@teddyjamilonatefreire8797]

Hello dear Zach, how do you set your output current (Iout)?

[@Zachariah-Peterson]

There are two ways to go about this. First is to approach is to set the primary side peak/average currents based on what your driver or switcher can handle. Once you have that number you can determine the DC output. The other approach is to set the output current to a target value (you are using this as a design target), and then find a switcher/driver and a transformer turns ratio that can reach that value.

[@nehayadav2862]

How you get the 19:1 turns ratio. I get the 13.53 After calculation.

[@Zachariah-Peterson]

At 9:26, you will see the inductances listed on the white board. Turns ratio is SQRT(820/2.25) = 19.1. That gives the 57:3 ratio I am targeting in the design.

[@abbasamir4878]

how u get peak current value 770mA and duty cycle value 50% as these are not given in data sheet ?

[@Zachariah-Peterson]

On page 4 of the datasheet: Aboslute maximum positive drain current single pulse, pulse max duration 25 μs is 770 mA On page 6, the typical maximum duty cycle ranges from 45% to 55%, I so I listed 50% since that is the midpoint.

[@abbasamir4878]

@@Zachariah-Peterson thanks

[@petersage5157]

I love that your design criteria are so much lower than the converter's maximum ratings. For power components, if you don't push them anywhere near their limits, they tend to last just a couple days longer than forever. Also, they're calling bobbins "coil formers" now?

[@Zachariah-Peterson]

Thanks, for power I try to get some derating, especially in this case where you might saturate the core and the vendor is not providing the best documentation on what the saturation flux/current will be. I used "coilformer" because that's what TDK calls it, so does Ferroxcube and a few others. But other vendors still call it a bobbin.

[@SatyajitRoy2048]

Dot represented for flyback is incorrect.

[@Zachariah-Peterson]

In haste on a whiteboard sometimes you put the dot in the wrong spot. But if you look at the design files for this design you'll see the circuit with the correct polarity.

[@klauskragelund8883]

I am pretty sure the formulas you are using are applied incorrectly. The formulas are for CCM, but it is stated that it will be running in DCM. In DCM it is not a volts seconds balance, it becomes an energy balance. If the load is reduced, the voltage goes to infinity.

[@Zachariah-Peterson]

The peak current equation, which was used for determining the secondary inductance limit, is for DCM. Rohm has a good article showing the formulas. Once you get the secondary inductance and the turns ratio you need to step the voltage, you can then get the primary inductance.