Forced Continuous Conduction Mode (FCCM)

[balamanikandan]

hi,

Advantages of FCCM:
At high loads this mode has the same efficiency as AUTO mode. Optimum load transient response
performance across the whole operating load range of the converter. Beneficial for EMI/EMC reasons to
ensure converter frequency is fixed.

Disadvantages:

Less efficient than other modes at low load because inductor current is sunk to ground by converter and converter is always switching.


Please explain me why it is less efficient at light loads??

 

[WimRFP]

Assuming you are discussing a buck converter were the freewheel diode is shorted by another mosfet (as in a synchronized rectifier). In fact this is a half bridge topology with an LC low pass filter to get the DC component.

When in discontinuous mode, there will be short bursts of current through mosfets and inductor, no current will flow back to the switches. Power is only dissipated during these bursts, during the off-period, there will be no dissipation. During the off period, energy for the load comes from the output capacitor.

when in FCCM, all time, even without load, you have the ripple currrent through the inductor, the current through the high side and low side switch, and of course switching loss in case of hard switching topology.

Because of this loss, the efficiency drops under light load.

 

[balamanikandan]

Hi,


Thanks for the response. But I could not understand. Please explain with some study material.

 

[WimRFP]

Hello,

Go to https://en.wikipedia.org/wiki/Buck_converter and go to the "synchronous rectification section" that has the low side switch that can pull current out of the inductor.