Flyback Quasi Resonant Switching Frequency

Thread Starter


Joined Jul 24, 2019
Hi all,

As I know, the major difference between normal flyback converter & flyback quasi resonant converter is for flyback quasi resonant converter, the switching frequency is variable. This means that the switching frequency is based on the output load. When the load decrease, the switching frequency will increase, and when the load is increase, the switching frequency will decrease.

My question as below:
1. Since the switching frequency is always changing, how do I select the switching frequency as my initial value to start the transformer design?
2. Since the switching frequency is always changing, how do I know that my transformer is capable for that switching frequency range. Let say I choose a transformer A with value of 300uH @ 55kHz, and my switching frequency for quasi resonant converter is range from 35kHz to 130kHz. Will this transformer A is capable for that frequency range? As I know, as the frequency increase, the inductance will be smaller & as the frequency decrease, the inductance will be bigger. Is it means that at lower frequency, this transformer A (300uH,55kHz) will change its inductance value at lower frequency(suddenly will increase inductance) & high frequency(suddenly will decrease inductance)? Or this transformer A will have fix inductance value of 300uH at all frequency?



Joined Jun 22, 2012
The Reactance increases with frequency!! The Transformer will have a central frequency and a tolerance to operate within.


Joined Feb 14, 2020
Hi Mr. Deeplutan,

I will answer your questions, But first, Mr. Dodgydave is entirely correct in saying that the reactance of the transformer increases with frequency.

You raised several questions and they are:
(1) How do you select the switching frequency? Generally speaking the selection of switching frequency or range is selected to optimize the power supply and the transformer is then designed to meet the power supply requirements.

(2) If the power supply calls for a lower to some higher frequency range then you choose the transformers primary inductance which proportional to the ferrite core area and the number of turns. The flux density is inversely proportional the the core area and the number of turns. Generally you start the transformer design by choosing say the primary winding turns to satisfy the flux density you want at the highest winding voltage and lowest frequency. Primary inductance usually is not a factor except when you are designing a transformer for an LLC resonant topology. For most standard topologies you generally want a primary inductance to be as high as possible so you want the highest reactance possible. Usually that happens and you have no problem with excessive magnetizing current.

Specifically for your design which covers a frequency range of 35khz through 130khz you would choose a ferrite core, calculate the flux density for a given number of turns and peak primary voltage and you will most likely find if the core is ungapped then the primary inductance is higher than then 300uh mentioned. If you really need that value with some tolerance you achieve by gapping the core to lower the inductance to where you want it. And by the way, that inductance will mostly vary if the gap value changes.

You asked if the inductance value is a function of frequency, the answer is no, especially if you gap the core set with about 2 mils of insulation to make sure any DC current entering any of the windings is present. For example when rectifier diode forward characteristics are a little different in the real world.

If you can give more details about what you are designing than I can help by simulating you work and thus coming up with a transformer preliminary design if you wish.

I am a level 5 Senior Engineer, Designing hardware in the Power Electronics field since 1980. I do all the magnetic device designs also. So I can help if I get enough details to work with.

Hope this has cleared up some of your questions.

Best regards