Methods Of Converting Ac to Dc

The only way I know of changing ac to dc is rectification with diodes, But they are only 81% efficient. Transformers are used in power supplies and they also drop some efficiency. How are more efficient power supplies made? Like the new 80 Plus psu's. And what are other ways of efficiently converting AC to DC.

 

instead of using diodes, which have the forward voltage drop and therefore high losses with big currents, you can use MOSFETs to switch actively. use comparators for example to determine switching points-.

 

Efficiency is always the big challenge :-

This is a nice source .....

http://www.onsemi.com/pub_link/Collateral/SMPSRM-D.PDF

and this a dedicated web site

http://www.smps.us/smpsdesign.html

 

Thanks for the quick replies! Ill get to them as soon as I finish reading "Power to the people" (WIRED magazine)

 

I have a question. On page 18, figure 19, it says Passive Turn-ON, Passive turn-OFF, what is that?

 

Passive Turn-ON means that if you have a low drive signal applied to the controlling input MOSFET's gate the load is on. Passive Turn-OFF case means that with no drive applied to the controlling input MOSFET's gate the load is off.

 

well that was simple. Thanks alot!

 

Simple, but I think it misses the point.

PASSIVE TURN-ON CIRCUIT
Ron is a passive component. The turn-on of the output MOSFET is dependent on this resistor. Hence the term passive turn-on. The gate capacitance charges through Ron. If Ron is a high value, the transistor will turn on very slowly.

PASSIVE TURN-OFF CIRCUIT
Roff is a passive component. The turn-off of the output MOSFET is dependent on this resistor. Hence the term passive turn-off. The gate capacitance discharges through Roff. If Roff is a high value, the transistor will turn off very slowly.