need help to understand flyback circuit

[Nora]

hello-
Can someone explain to me if I should use bjt or fet where the transistor is shown in flyback.jpg?
And also explain to me the polarity of the transformer. I mean why are the dots shown catty-corner. In the data sheet for this transformer, a step up example (also attached as step up.jpg) has the dots shown directly opposite.

When I hook this up as shown (driven by 555), my transformer gets very hot- no load besides cap.
Thanks!
N_N

 

[haibin]

at first, the BJT is switch on ,the current from Vin to L1\ BJT to ground, L1 voltage up+ down-,the L2 Voltage are up- down+,D can not conductor ,L2 no current, the transformer L1//L2 like a inductor only L1, the increase current flow L1 is Vin*ton/L1,the energy stored in the L1 or transformer;
the second , the BJT is switch off, at the time of BJT switch off,the current flow in L1 are not down to zero immediately, it charge the capacitor of the BJT Cce , when the voltage of Cce up to Vin+N*(Vo+Vf)(N is the rate of L1/L2 circle number,Vf is the D forward voltage),then the L1 voltage is up- down+,the L2 voltage is up+ down- and equal to (Vo+Vf),then the D is conduction,the current store in L1 are transmit to L2 and output to C and road,the energy stores in L1 or transformer are send to output.
when the BJT switch on and switch off cycle by cycle ,the energy can transmit to output from input,change the switch on time of one cycle(duty cycle control or PWM control),the energy be output can be controld.

 

[Awarapunshee]

Dots represent the same phase points.
Practically, when you wind a transformer the start point of one winding if consider a dot then, the start point of other winding will be a dot and vice versa.

Regards

 

[DonRose]

You can use a suitable FET for the switch in a flyback circuit. The reason the dots are catty cornered is this.
In a flyback when the switch is on energy is stored in the winding L1 The voltage will be Positive on the Dot ends with respect to the other end. The same is true on the L2 side. The diode D is revere biased and no current flows. When the turns off (FET of BJT) the energy is transfered L2 the polarity of the windings change and current flows in L2 through the diode charging the capacitor C. The on off of the switching transistor controls the Voltage on C assuming a load is applied.

I hope this simplified explanation helps

Regards

 

[permute]

Also, a flyback uses something which is really a coupled inductor. Basically, an inductor is made to store/release energy. A trasformer is designed to transform it from one V,I to another n*V,I/n.

A flyback uses both properties. It has a charging phase where energy is added to the core. when the switch is opened, the magnetic field breaks down, the voltages on the windings reverse. now the secondary can conduct.

the result is that L1,L2 are somewhat low for the flyback case, while a transformer would normally have high L1,L2 in order to reduce magnetization currents.