Help me design an AC/DC converter with PFC

[dcj50105]

HELP! SOS!

Hello!everyone
I'm designing a AC/DC converter with PFC, because I'm fresh and absent of more practical experience, so I have to turn to everyone and hope to get your advice and suggestions.
The following is the speciafication of the converter,
The input voltage range:85Vac～264Vac; the output is 5V/22A; and the PFC is>0.95@230vac;and the higher the efficency is, the better it is
The PFC stage, I choose the Fairchild ic of FAN7527B, which is peak current mode,and it has a higher efficency in this section, but it is difficult to resolve the problems it produce; I'll try my best to resolve it.
The DC/DC stage is important for me, so I need more help in it, I choose the forward topology because of the limits of the converter's size,the PWM IC of uc3844 is used,and the transformer is PQ2625;and the frequency is aroud 100KHz,and the primary turn is 42 and the second one is 3,do you think it's the best? if not,can you optimize it for me?and high efficeny and low tempreture of the transformer is important.To get a higher efficency,I intent to choose synchronous rectifier,but I haven't used it before, so I want to get your-my teacher's help;do you think it is the best choice to use synchronous rectifier? if not ,can you tell me which one is the best? can you give me some relevant circuit schematic as a reference? Thanks a lot.
I'll be looking forward to your help!
Best regards!

 

[batdin]

Re: HELP! SOS!

Look,you should read a lot in order to make a decent switch-mode PS.
Here are some guidelines:
-Flyback is the right choise because of the wide input voltage range.
-At 22A/5V the sync rectifier is more than desirable.Be careful to choose FET with as low Rdson as possible.
-Be careful not to saturate the core of the pulse transformer.Choosing the right gap is important.
-There are some chips that provide most of the required circuitry.Check on TOPSWITCH.
-For rough calculations,use this:
**broken link removed**