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

Thread Starter

mishra87

Joined Jan 17, 2016
857
upload_2018-6-6_22-50-42.png


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
5,186
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
14,689
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.
 

Thread Starter

mishra87

Joined Jan 17, 2016
857
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.
Thank you so much for your reply !!!

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

Thread Starter

mishra87

Joined Jan 17, 2016
857
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 !!!
 
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