Continuous Conduction Mode (CCM) is characterized by current flowing continuously in either the primary or the secondary windings.
Referring to the figure below, Continuous mode has two distinct intervals of operation:
Primary Switch turns on: From an initial step, current ramps up linearly in the transformer primary winding storing energy in the transformer. No current flows in the secondary.
Primary Switch turns off: Primary current falls to zero while the secondary current starts with a current step which is equal to the final value of the primary current reflected through the turns ratio. The secondary current ramps down supplying energy but does not reach zero before the TOPSwitch turns back on at the beginning of the next cycle.
Continuous mode reduces peak and RMS currents compared to the Discontinuous Conduction Mode (DCM) for the same output power.
This maximizes the power capability of the given TOPSwitch, increases efficiency (by reducing dissipation) and generates lower differential mode noise and higher common mode noise. However, continuous conduction mode may require larger transformer core size (increased primary inductance) and generate high frequency radiated EMI due to hard switching of the output diode and associated ringing.
Except for applications sensitive to high frequency noise (e.g. TV and VCR’s) Continuous Conduction Mode is recommended.
Note: TOPSwitch does not suffer from the loop stability problems that often deter designers from using CCM.
Primary current in a Flyback power supply
operating in continuous conduction mode (CCM).
Observe the characteristic front-end step.