What does it happen when we reach the breakdown voltage of a mosfet between the drain and source) , does it act like a zener and block overvoltage (it may heat a lot in this case) or does it break immediately?
A Mosfet has a big and powerful zener diode that will clamp the voltage and cause a lot of heat when it avalanches. The zener diode can also conduct a high current when it is forward-biased by the drain-source voltage going reversed.
Ok thanx this is good for me since it may occur that i get small high voltage transient (maybe +10V than the limit of the mosfet) and didn't wanted that the mosfet breakdown. they will absord it
They will only "absorb" it to an extent. If you know it's going to be tolerating this frequently then you should get a MOSFET that has been designed and rated for avalanche operation- and you sshould still have other devices to try and clamp the voltage to reduce stress on the MOSFET. Like flyback schottky diodes (placed across the inductive load for a unipolar driver, or anti-parallel placed across each MOSFET for bipolar drivers like an H-bridge. It's not good design practice to frequently rely on the breakdown to protect the MOSFET because it will only take so much.
If it's just a random voltage transient due to line noise...maybe a TVS diode or something.