# In Quasi resonant flyback converter why doesn't MOSFET get immediate turn ON when Id becomes zero .

#### mishra87

Joined Jan 17, 2016
906

In above waveform,

What is Tw ???
why doesn't MOSFET get immediate turn ON when Id becomes zero.
It turns ON at 1st valley !!!.
What happens if mosfet gets turn ON at 1st valley ???
What happens if mosfet gets turn ON at 2nd valley ???

Thanks !!!

Last edited:

#### Jony130

Joined Feb 17, 2009
5,230
To reduce the power losses in the MOSFET. Do not forget about MOSFET's parasitics capacitances.
In the ideal case, we want zero current and zero voltage switching P = I*V = 0W.
But in QR we wait until the first 1st valley to turns-on the MOSFET to reduce the power dissipation when the MOSFET turns-on and discharge the MOSFET and the circuit parasitics capacitances between the drain and the source.

#### Papabravo

Joined Feb 24, 2006
16,137
AFAIK, a MOSFET will never turn ON or OFF immediately. There is a certain amount of gate charge that must be added or removed from the gate. The basic 1st order differential equation that governs an RC circuit tells us that this cannot happen instantaneously. It's just the nature of the thing.

#### mishra87

Joined Jan 17, 2016
906
To reduce the power losses in the MOSFET. Do not forget about MOSFET's parasitics capacitances.
In the ideal case, we want zero current and zero voltage switching P = I*V = 0W.
But in QR we wait until the first 1st valley to turns-on the MOSFET to reduce the power dissipation when the MOSFET turns-on and discharge the MOSFET and the circuit parasitics capacitances between the drain and the source.

I understood what you have written but still not satisfied the answer.
A bit more explanation needed !!!

#### Jony130

Joined Feb 17, 2009
5,230
What do you not understand?

#### mishra87

Joined Jan 17, 2016
906
AFAIK, a MOSFET will never turn ON or OFF immediately. There is a certain amount of gate charge that must be added or removed from the gate. The basic 1st order differential equation that governs an RC circuit tells us that this cannot happen instantaneously. It's just the nature of the thing.
Thank you so much !!!