quetions in buck DC to DC converter

[newjie]

in the buck converter,when both PMOS and NMOS are shut down,there will be a ringing in the node LX,as in the enclosed picture
what is this effect?
and how to eliminate it?[/img]

 

[harrytrinh]

When both PMOS and NMOS shutdown, ideally there is no more energy in inductor but actually there is still little energy in inductor and this energy will make that ringing. To reduce it we can damp energy when both MOS are off.

 

[gevy]

NMOS must be open when current is negative .
You can use comparator and latch for disconnection NMOS current.
Also you can made dead time between PMOS ON and NMOS ON states.

 

[babbage]

there are two methods:
1. to add the snubber circuir in order to reduce this ring freq and amplify;
2. to short the inductor when both mos are off, then SW will be equal to output voltage;

Added after 2 minutes:

there is another 3 method like gevy said: to open the Nmos, but lost the efficiency; so I normally don't recommend this, but this method is so simple in all methods;

 

[newjie]

there are two methods:
1. to add the snubber circuir in order to reduce this ring freq and amplify;
2. to short the inductor when both mos are off, then SW will be equal to output voltage;

Added after 2 minutes:

there is another 3 method like gevy said: to open the Nmos, but lost the efficiency; so I normally don't recommend this, but this method is so simple in all methods;

turn off the NMOS to improve efficiency when the current is negetive,
if the feedback network out of the chip and there is no other pin for the output , the second method can't be used, so how to implement the 1st method?

 

[tom173]

May be you can search some doc about "anti-ring" ,I recommend the 1st method!