Hello, can some know what is called this valley in the PWM signal of buck converter power supply?
What is the cause of this valley in the PWM signal.

The circuit is also shown below.

 

It would help if you put the values on and the chip number..

 

It’s what it does when it goes into discontinuous mode.
When no current is flowing in the inductor it rings at the resonant frequency of the inductor and the output capacitance of the MOSFET.

 

It’s what it does when it goes into discontinuous mode.
When no current is flowing in the inductor it rings at the resonant frequency of the inductor and the output capacitance of the MOSFET.

Thank you very much for your reply.May I know what do you called that valley in the PWM signal? Is there a technical term on that?

 

Dead time.

 

It would help if you put the values on and the chip number..

It would help if you put the values on and the chip number..

here it is.

 

Dead time.

Thank you very much. Now I understand the technical term on that.

 

I would have only used the term "dead time" for a synchronous or half bridge converter where there are two MOSFETs, switched alternately; the dead time being the time during which neither is on.

 

I would have only used the term "dead time" for a synchronous or half bridge converter where there are two MOSFETs, switched alternately; the dead time being the time during which neither is on.

Thank you very much for your reply. It seems I have found the best technical term in the picture above. It is called Valley Switching or Quasiresonant Mode.

 

Thank you very much for your reply. It seems I have found the best technical term in the picture above. It is called Valley Switching or Quasiresonant Mode.

No, if that were the case, it would switch at the positive peak, so that the amount of stored energy in the parasitic capacitance was at its minimum.
It's just plain old ringing from an LC resonant circuit.

 

here it is.
View attachment 316274

Here is a snapshot of the circuit on the datasheet, your L1 component value is higher.. what is your input and output voltage and current?

 

More importantly, what is the output current.?
If your output current is less than 2Vt/L (where V is the output voltage, and t is the off-time) then it will be in discontinuous mode for all the time after t=L/(2V) and it will ring as soon as the inductor current reaches zero.