Hello Guys,
I have some questions in flyback simulation , appreciate your comment to improve my knowledge.

In reference to below waveform
1. Is flyback converter works in CCM mode or DCM mode ?
2. When MOSFET is turn ON there is always spike current , why so ?
3. During valley period of the switch, why no drain current ?

 

A schematic of what you are simulating would greatly improve the accuracy of our answers.

 

1. Is flyback converter works in CCM mode or DCM mode ?

In DCM mode.

When MOSFET is turn ON there is always spike current , why so ?

Probably due to the existence of a parasitic capacitance at the MOSFET drain.

. During valley period of the switch, why no drain current ?

You are in DCM mode, so when the current in the secondary side reaches 0A the drain parasitic capacitance starts to oscillate together with primary waining inductance. Because the drain voltage ( parasitic capacitance ) "wants to go back" to Vin.

 

Toff/Dead time/Ton
There is a time when nothing is happening. The FET is off and the Diodes are off. No real current is flowing.
The transformer/inductor is ringing at very close to zero current.

 

2. When MOSFET is turn ON there is always spike current , why so ?

If you are measuring the current using a resistor between the MOSFET source and ground, then the current to charge the gate capacitor also goes through that resistor. That current will be a spike that quickly decays.

 

2. When MOSFET is turn ON there is always spike current , why so ?

We need a schematic! All parts are not simple but complex. The MOSFET has capacitors inside. The transformer/inductor has capacitors inside, and leakage inductance. The diodes are complex, capacitors, reverse recovery time. When the MOSFET turns on all the capacitors need to be charged/discharged, and they will resonate with the inductor. Even the MOSFET has lead inductance that will resonate with the internal capacitors.

 

A schematic of what you are simulating would greatly improve the accuracy of our answers.

Hello,
Here is attached schematic.

 

When the transistor is suddenly switched on and there has been no current flowing, there is nothing to limit the current except the DC resistance of the circuit. as the current rises and the magnetization increases the current drops to a stable level. At least in the simulation. Then when the current suddenly switches off the collapsing field generates an oscillation.
Keep in mind that a "flyback" circuit is not a smooth linear amplifier.

 

Here is attached schematic.

Please RFB. I think it is connected wrong.