hi. friends.
this is a switch power supply circuit.I have two questions: 1. why the secondary part don't have diode?how to rectify? 2.what is the function of the capacitor near the resistor of the primary side?

 

why the secondary part don't have diode?how to rectify?

Because the AOZ7648 is a synchronous rectifier
http://www.aosmd.com/res/data_sheets/AOZ7648QI-06.pdf

2.what is the function of the capacitor near the resistor of the primary side?

It's part of the RCD suppression circuit to prevent overshoot of the primary voltage.

 

Because the AOZ7648 is a synchronous rectifier
http://www.aosmd.com/res/data_sheets/AOZ7648QI-06.pdf

It's part of the RCD suppression circuit to prevent overshoot of the primary voltage.

sorry I don't quite understand of your reply. I need more detailed explanation.
1.I think AOZ7648 is an ic that takes secondary circuit outputs and give feedback signals of TXN and TXP. how it works as a synchronous rectifier?
2. how's the capacitor do to make a RCD surprenssion?

 

1.I think AOZ7648 is an ic that takes secondary circuit outputs and give feedback signals of TXN and TXP. how it works as a synchronous rectifier?

It has some power pins labelled SOURCE and DRAIN, so they connect to an internal MOSFET which acts as a synchronous rectifier.

2. how's the capacitor do to make a RCD surprenssion?

See
http://www.ridleyengineering.com/images/phocadownload/12_ flyback_snubber_design.pdf

 

From the data sheet. Here is the diode.

 

From the data sheet. Here is the diode.
View attachment 244799
View attachment 244800

I think:
1. the equivalent diode should have positive end left side and negative end right side.because the source is grounded and voltage is added to drain.
2. the equivalent diode should be parrellel to the capacitor and register, not over.