Hello, guys

I am trying to understand the basic concept of flyback transformer.I have read that The flyback converter is based on the buck-boost converter.with the switch realized using a MOSFET and diode.

Here is the picture of typical buck-boost converter

Next the inductor winding is constructed using two wires, with a 1:1 turns ratio.

Then It looks something like that

Next, the connections between the two windings are broken. One winding is used while the transistor Q1 conducts, while the other winding is used when diode D1 conducts.
Now look at the dotted terminals of both coils

After changing the position of the switch the dotted terminals of coils also changes.

My question is why this change happens from figure-3 to figure-4 ??

I know, Unlike the ideal transformer, current does not flow simultaneously in both windings of the flyback
transformer.

 

why this change happens from figure-3 to figure-4 ?

It's because moving the transistor caused a change in the polarity of the flyback generated voltage.
After Q1 is on for a period, causing the primary inductor current to increase, the transistor suddenly shuts off.
The inductive kick from the inductive current trying to continue to flow in the same direction causes a positive voltage at the transistor drain.
Since you want this positive voltage to also appear at the secondary diode anode, this means the transformer polarity at the transistor drain must be the same as at the diode anode, which the polarity dots show.

 

Also the mosfets are different in each case, P Channel for figure 3, N channel for figure 4. Also the gating is different for each mosfet. (See Vgs) properties of mosfets.

 

After changing the position of the switch the dotted terminals of coils also changes.

My question is why this change happens from figure-3 to figure-4 ??

First, it doesn't matter which end of the coil has the dot as long as it is consistent within a transformer.
In figure 3, the dots are at the end connected to the MOSFET and the diode. In this diagram moving both dots to the other end of the windings does not change the circuit at all.
The change to figure 4 includes two changes compared to figure 4. First the dots are both moved as above, then the switch and coil swap places.

 

Because Fig 3 & Fig 4 are "electrically" equivalent.

This allows us to analyze BOTH the input waveforms and output waveforms as overlapping positive Voltages and Currents.
This makes plotting and analyzing of the timing of the overlapping waveforms much easier to understand.

Otherwise ...
only the Primary waveforms would be the top ( positive half ) of the graph and
only the secondary waveforms would be in the bottom ( negative half ) of the graph.
Analyzing separated waveforms is more difficult.

When you plot the Current & Voltage in the Primary vs
the Current & Voltage in the Secondary you will understand why ...