>> Hi,
>> synchronous and non-synchronous are synonims for PWM and PFM?

Learning to use Gargoyle *WELL* is one of the best things you can do
to enhance your knowledge.
Looking at smps + synchronous will quickly show you that  the term
"synchronous rectifiier" is pertinent.
Following that up will allow you to answer most of what you need to
know about the basic subject, so that the questions you need to ask
will then be at a much higher level.

Gargoyle is your friend. Learn to use it well.

      +"a synchronous rectifier is"


Useful:
http://www.pdfgeni.com/book/Synchronous-rectifier-flyback-pdf.html

Awesome:
http://www.maxim-ic.com/app-notes/index.mvp/id/2031

"               To minimize this loss, switching regulators can use
Schottky diodes that have a relatively low forward-voltage drop and
good reverse recovery. For maximum efficiency, however, you can use a
MOSFET switch instead of the diode. This design is known as a
'synchronous rectifier' (see Figures 11, 12 and 13). The synchronous
rectifier switch is open when the main switch is closed, and the same
is true conversely. To prevent cross-conduction (both top and bottom
switches are on simultaneously), the switching scheme must be
break-before-make. Because of this, a diode is still required to
conduct during the interval between the opening of the main switch and
the closing of the synchronous-rectifier switch (dead time). When a
MOSFET is used as a synchronous switch, the current normally flows in
reverse (source to drain), and this allows the integrated body diode
to conduct current during the dead time. When the synchronous
rectifier switch closes, the current flows through the MOSFET channel.
Because of the very low-channel resistance for power MOSFETs, the
standard forward drop of the rectifying diode can be reduced to a few
millivolts. Synchronous rectification can provide efficiencies well
above 90%"

__

More than you knew you wanted
http://scholar.lib.vt.edu/theses/available/etd-173510281975580/unrestricted/chapter2.pdf

Understand this
http://www.fairchildsemi.com/ds/SG/SG6203.pdf

Good question
http://archive.electronicdesign.com/Files/29/14302/14302_01.pdf
    "What is a synchronous
rectifier?
A synchronous rectifier is a
circuit that emulates a diode,
allowing current to pass in one
direction but not the other
without the losses associated with
junction or Schottky devices. The
circuit comprises a pass-element
(most often a power MOSFET), a
sense element, a sense-signal
conditioner, and a driver.
How does a synchronous
rectifier work?
There are two broad techniques
to implement the synchronousrectifier
function. The first ..."

 R
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