Buck Converter PWM Signal

Thread Starter

julietsierra99

Joined Feb 26, 2024
5
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.
1708939292600.png
1708939632155.png
 

Ian0

Joined Aug 7, 2020
9,847
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.
 

Thread Starter

julietsierra99

Joined Feb 26, 2024
5
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?
 

Ian0

Joined Aug 7, 2020
9,847
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.
 

Thread Starter

julietsierra99

Joined Feb 26, 2024
5
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.
 

Ian0

Joined Aug 7, 2020
9,847
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.
 

Ian0

Joined Aug 7, 2020
9,847
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.
 
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