As the title says, i want to invert AC to DC but i dont know wich diodes are suitable. The AC is 350v at 50khz.

Thanks

 

We need more information.
How much current does the diode need to conduct?

 

"Ultrafast" (meaning ultrafast reverse recovery time, usually abbreviated "trr") diodes are suitable. In many families of ultrafast diodes the reverse recovery time of the higher voltage parts (peak inverse volts, "PIV" rating) will typically be about 1.5 times that of the lower voltage parts. You will probably require diodes in the higher voltage range, depending on exactly how the circuit is made. You can probably find diodes with trr in the range of about 60-80 nanoseconds. Silicon carbide Schottky diodes, which have extremely short trr, are available, but they are a lot more expensive and probably not worth it unless the recovery current spike is intolerable for some reason.

 

We need more information.
How much current does the diode need to conduct?

Not much, only few mah, 10-20mah

UF Diodes like UF4007 until what frequency can work properly ?

 

UF4007 will be OK. I've used them at over 100 kHz. You can use them at even higher frequency but the power loss due to the reverse recovery becomes more significant at higher frequency. The UF400x series is quite popular so the diodes should be easy to obtain and inexpensive.

 

Just beware from where you purchase them, there are some that come from less reliable sources such as ebay, that seem to be more like wrongly coded 1n400x's. A member of my club was building a circuit that specific UF4007 so he bought some from an ebay supplier. The circuit would not function, and when I investigated for him and substituted a known good UF4007, everything was fine.
I performed a few tests on the rest he had, and they were all the same, either rejects, or wrongly (deliberately?) coded.

 

Ok i see, i am not going to buy them from ebay :-D