Isolated Flyback Converter Basics: What are Discontinuous Mode and Continuous Mode?

Switching power supplies operate in either continuous or discontinuous mode. Since the present design case involves the use of discontinuous mode operation, this article discusses the two modes. Their advantages and disadvantages are summarized in the table below. The waveforms in the [Operation] column indicate the current that flows in the primary and secondary windings in a transformer. The signs [↑] and [↓] assigned to the keywords denote [Increase] and [Decrease].

In the continuous mode operation, a reverse current flows during rectifying diode reverse recovery time (trr)* at switch-on time, and losses occur due to the reverse current. In low-voltage switching DC-DC converters, the reverse voltage on the rectifying diode is low, with an attendant decrease in the reverse current. As a general rule the continuous mode is used in favor of output ripple voltages.

By contrast, in AC-DC converters a large reverse current flows due to the high diode reverse voltage, resulting in substantial dropout. In many situations the discontinuous mode is employed to prevent occurrence of any reverse currents. However, since this approach entails an increase in peak current, for large loads the converter in some cases is operated in the continuous mode.

Although each approach has its own advantages and disadvantages, up to 60W generally the discontinuous mode may be appropriate. In other cases, the particular mode to be used is selected in consideration of the tolerated size of the specific transformer. In this design case, we selected the discontinuous mode to accommodate a 36W output requirement.

Comparison item	Discontinuous mode	Continuous mode
Operation	Presence of a zero current period between OFF and ON prevents a continuous current flow.	Current flows continuously, which turns on and off at the same frequency as the switching frequency.
Transformer	inductance ↓, size↓, cost↓	inductance ↑, size ↑, cost ↑
Rectifying Diode	Fast recovery type , cost↓	Requires a faster recovery type, cost ↑
Switching transistor	Allowable power ↑, size ↑, cost ↑	Allowable power ↓, size ↓, cost ↓
Output Capacitor	Ripple current ↑, size ↑	Ripple current ↓, size ↓
Efficiency	Switching loss ↓, efficiency ↑	Switching loss ↑, efficiency ↓