Technical Information Site of Power Supply Design

2016.02.15 AC/DC

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

Design Method of PWM AC/DC Flyback Converters

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.

Discontinuous mode Continuous mode
Operation 3A_flyback_cont
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 ↑
Fast recovery type , cost↓ Requires a faster recovery type, cost ↑
Allowable power ↑, size ↑, cost ↑ Allowable power ↓, size ↓, cost ↓
Ripple current ↑, size ↑ Ripple current ↓, size ↓
Efficiency Switching loss ↓, efficiency ↑ Switching loss ↑, efficiency ↓

* Diode reverse recovery time

Applying a forward voltage to a PN junction diode causes a forward flow of the current. When a reverse voltage is rapidly applied in this condition, a reverse current not normally occurring for a period time flows. The length of time it takes for the normal condition to recover from this condition is referred to as reverse recovery time.

Key Points:

・Understand the differences between the continuous and discontinuous modes.

・Discontinuous mode is widely used in AC/DC converters.

Design Example for PWM Flyback Converter

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