in a buck converter, initially, when switch is closed, then capacitor charges and stores energy. But when the switch is opened, then Inductor current flows through thr free wheeling diode, but what happens to the charge stored in the capacitor?

  • 1
    \$\begingroup\$ Charge isn't stored in a capacitor; energy is stored. \$\endgroup\$ Sep 8 '13 at 1:05

In a buck converter, the switch controls energy storage in the inductor. The average of the square wave applied to the filter will be the DC output level (12V @ 41.6% duty cycle = 5V average). The inductor acts as a current source to keep the output capacitor charged.

Depending on the load, switching frequency and inductor size, a fixed-frequency buck converter can operate in one of two modes. If the DC output current level is greater than half of the inductor current ramp, the converter is said to be in CCM (continuous conduction mode); if not, it's said to be in DCM (discontinuous conduction mode).

\$ I_o > \dfrac{1}{2} \cdot \dfrac{[V_i-V_o]\cdot T_{on}}{L} \$ or

\$ I_o > \dfrac{1}{2} \cdot \dfrac{-V_o\cdot T_{off}}{L} \$

In CCM, the inductor is sourcing current the entire time. The inductor current never goes to zero. The capacitor is always being charged by the inductor, so it never has to support the load by itself. The duty cycle will remain essentially fixed regardless of output current once in CCM.

enter image description here

In DCM, because the output current is less than half the inductor ramp current, the only way to regulate properly is to decrease duty cycle. This decrease in duty cycle leads to a third mode of operation, where the switch is off and the inductor has completely discharged.

enter image description here

(Some controllers will operate in what's called critical conduction mode, where instead of operating with fixed frequency and variable duty cycle, it operates with fixed duty cycle and adjusts the frequency to keep the converter exactly at the DCM/CCM threshold.)

  • \$\begingroup\$ Thanks, and looking at the explanation, CCM and DCM refers to the 2 states of opening of the switches right? \$\endgroup\$
    – Sherby
    Sep 8 '13 at 1:34
  • \$\begingroup\$ Not quite. See my edit. \$\endgroup\$ Sep 8 '13 at 1:57
  • \$\begingroup\$ @Madmanguruman: Can you please elaborate more details on CCM and DCM \$\endgroup\$
    – AKR
    Sep 8 '13 at 7:01
  • \$\begingroup\$ CCM means "Continuous Conduction Mode" When the switch is turned on current builds up in the inductor then when its turned off it decays. In CCM the current is not allowed to fall to zero before the switch is turned back on. in DCM (Discontinuous Conduction Mode) the current is is allowed to fall to zero. In CCM ignoring second order effects the output voltage is the the input voltage times the duty: this limits the minimum load. In DCM the duty is much more load dependant. \$\endgroup\$ Sep 8 '13 at 13:24

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