Unlike Continuous mode, this mode has two or three distinct intervals of operation. Referring to the figure below:
Primary Switch turns on: Starting from zero, current ramps up linearly in the transformer primary winding and energy is stored in the transformer.
Primary Switch turns off: All of the energy stored in the transformer is released through the secondary circuit and into the load, as the secondary current ramps down completely to zero before the TOPSwitch (or other device) turns on again at the beginning of the next cycle.
There may be a ”dead time” between the secondary current reaching zero and the primary switch turning on again.
Discontinuous mode operation is preferred in applications sensitive to high frequency or video noise.
For conducted EMI, Discontinuous mode operation will generate differential mode noise (due to higher peak primary current), but it will also generate low common mode noise.
Discontinuous mode inherently has higher peak and RMS currents and is less efficient than Continuous mode and typically requires a smaller transformer (less primary inductance, LP).
Discontinuous offers slightly increased loop bandwidth due to a lack of right half plane zero. Bandwidth is generally increased over that of Continuous mode.
Primary current in a Flyback power supply operating in discontinuous conduction mode (DCM) Observe the current in a new cycle always starts out from zero.
Primary current in a Flyback power supply operating
in discontinuous conduction mode (DCM).
Observe the current in a new cycle always starts out from zero.